Dr Alasdair Campbell


Senior Lecturer
M.A., M.Eng., Ph.D.
+44 (0)1483 682130
23 BC 02
9am - 5pm

Biography

Areas of specialism

Buoyant Flow; Thermal Explosion; Turbulent Plumes; Solar Ponds

My qualifications

2014
Graduate Certificate in Learning and Teaching
University of Surrey
2007
Ph.D. in Chemical Engineering
Pembroke College, University of Cambridge
2006
M.A.
Pembroke College, University of Cambridge
2003
B.A. (Hons.) and M.Eng. in Chemical Engineering
Pembroke College, University of Cambridge

Previous roles

02 July 2012 - 01 April 2015
Lecturer
Department of Chemical and Process Engineering, University of Surrey
01 October 2007 - 30 June 2012
Hertha Ayrton Research Fellow in Chemical Engineering
Girton College, Cambridge

Affiliations and memberships

AMIChemE
Associate Member of the Institution of Chemical Engineers
FHEA
Fellow of the Higher Education Academy
Member of the Combustion Institute (British Section)

Supervision

Completed postgraduate research projects I have supervised

My publications

Publications

Campbell AN, Cardoso SSS, Hayhurst AN (2007) A scaling and numerical analysis of the effects of natural convection when Sal'nikov's reaction: P ’ A ’ B occurs, together with diffusion and heat transfer in a spherical batch reactor, Chemical Journal of Armenia 60 (2) pp. 216-234
Sal'nikov's reaction: P’A’B involves a precursor, P, in two consecutive, first-order chemical reactions, yielding a final product B via an intermediate A. Partly as an academic exercise, but partly because of its relationship with cool flames, the situation is considered where the second step is faster than the first one, which is taken to be thermoneutral without an activation energy. The second step is assumed to have a significant activation energy, although it is exothermic. The reaction proceeds batchwise inside a spherical reactor, whose walls are held at a constant temperature, but do not participate chemically. Natural convection becomes important, once the temperature is high enough for the Rayleigh number (Ra) to reach ~ 103. The subsequent behaviour of the system depends on the interaction between convection, diffusion of heat and mass, and chemical kinetics. By examining the governing equations, we develop and evaluate scales for the characteristic velocity, the concentration of the intermediate A and the temperature rise during the progress of the reaction, for the two extreme cases when transport is dominated, in turn, by diffusion and then by natural convection. These scales depend on the characteristic timescales for the interacting phenomena of chemical reaction, diffusion and natural convection. Typically, the characteristic velocity in a relatively small reactor of radius 0.27 m is as large as 0.3 m s-1, when the temperature rise is 100 K near the centre of the vessel. These theoretical predictions from scaling are verified by full numerical simulations. Oscillations of both the temperature and the concentration of the intermediate, A, can occur and the conditions for their appearance are identified. Any accompanying flow field proves to be toroidal, with the fluid ascending close to the reactor's axis, but descending adjacent to its walls. In addition, the effects of variables, such as the initial temperature of the batch reactor and its contents, the pressure and also the size of the reactor are all assessed, together with a consideration of what happens when the reaction proceeds in the liquid phase. In this case, because of the different physical properties of a liquid and a gas, natural convection is more intense than in the gas-phase and is quite likely to lead to turbulence and good mixing.
An experimental investigation of heat exchange in a three-phase direct contact condenser was carried out using a 70-cm-high Perspex tube with a 4-cm inner diameter. The active direct contact condenser comprised 48 cm. Pentane vapour at three initial temperatures (40?,43.5?, and 47.5?) and water at a constant temperature (19?) were used as the dispersed and continuous phases, respectively, with different mass flow rate ratios. The results showed that the continuous phase outlet temperature increased with increasing mass flow rate ratio. On the contrary, the continuous phase temperature decreased with increases in the continuous mass flow rate. The initial temperature of the dispersed phase slightly affected the direct contact condenser output, which confirms a latent phase effect in this type of heat exchanger.
Liu TY, Campbell AN, Hayhurst AN, Cardoso SSS (2010) On the occurrence of thermal explosion in a reacting gas: The effects of natural convection and consumption of reactant, Combustion and Flame 157 (2) pp. 230-239
Whether or not a chemical reaction in a fluid leads to an explosion is shown to depend on four timescales: that for the chemical reaction to heat up the fluid containing the reactants and products, for heat conduction out of the reactor, for natural convection in the fluid, and finally for chemical reaction. This approach is developed for an irreversible, nth-order chemical reaction, A ’ B occurring exothermically in a closed spherical vessel, whose wall is held at a fixed temperature. These four timescales are expressed in terms of the physical and chemical parameters of the system. A new three-dimensional regime diagram is proposed, in which the three effects inhibiting explosion, viz. the consumption of reactant, and heat removal both by thermal conduction and by natural convection, appear separately. Numerical simulations are performed for laminar natural convection occurring, so that the development of temperature, composition and velocity throughout a reacting gas is computed for increasing times. The results are compared with previous experimental measurements in the gas phase for the decomposition of azomethane. The criterion for an explosion is considered in some detail; it appears that these systems explode if and when the maximum dimensionless rise in temperature exceeds a value close to 5. © 2009 The Combustion Institute.
When any exothermic chemical reaction occurs inside e.g., an unstirred spherical vessel, the heating effect of the reaction often induces temperature gradients and consequently natural convection. This work sets out to compare previously measured temperatures at different positions inside such a batch reactor with values computed numerically and analytically. It is the first such study for a reaction with an order greater than zero, occurring in a spherical reactor. The main reaction considered is the thermal decomposition of the gas, azomethane, which has often been used in experimental studies of thermal explosion. Other experimental results for the reaction between nitric oxide and oxygen, as well as between hydrogen and chlorine are also considered here. The measured temperatures at the centre of the vessel are first compared with analytical scales, derived by inspecting the governing equations. It is found that the temperature rise when diffusion is the dominant transport mechanism (i.e., at small values of the Rayleigh number) is directly proportional to the ratio of the characteristic timescales for diffusion and for the reaction. Similarly, when natural convection is the dominant transport mechanism, the temperature rise is proportional to the ratio of the timescales for convection and reaction. A numerical scheme was developed to simulate the thermal decomposition of azomethane vapour under the influence of natural convection, as well as the diffusion of both matter and heat (via thermal conduction). The results of these simulations are compared with the temperature profiles measured along the vertical axis of the reactor. There is excellent agreement between experimental and numerical results. This confirms the computational procedures. The simulations indicate that the hottest point in the reactor moves upwards above the centre of the vessel when Ra is increased. In fact, three distinct types of temperature profile occur, depending on the value of Ra. For low Ra, the temperature profile is approximately spherically symmetric, as expected. When Ra is increased, the symmetry in the temperature profile is disrupted by the flow produced by natural convection. In that case, the temperature profile becomes skewed, with the maximum occurring on the axis in the top half of the reactor. Thirdly, under some circumstances at high Ra, a large, sharp peak in the temperature profile near the top of the reactor is produced. This resembles a stabilised flame fr
Mahood HB, Campbell AN, Thorpe RB, Sharif AO (2015) Experimental measurements and theoretical prediction for the volumetric heat transfer coefficient of a three-phase direct contact condenser, International Communications in Heat and Mass Transfer 66 pp. 180-188
© 2015 Elsevier Ltd.The volumetric heat transfer coefficient of a three-phase direct contact heat transfer condenser has been investigated analytically and experimentally. The experiments were carried out utilising a column of 70. cm in total height and 4. cm inner diameter. The active column height throughout the experiments was taken to be equal to 48 cm. Vapour pentane with three different initial temperatures (40 °C, 43.5 °C and 47.5 °C) was used as a dispersed phase, while tap water at a constant temperature 19 °C was used as a continuous phase. The variation of the volumetric heat transfer coefficient along the height of the column was measured experimentally and predicted analytically. The effects of the initial dispersed phase temperature, the dispersed mass flow rate and the continuous mass flow rate on the volumetric heat transfer coefficient were tested. The results indicate that the volumetric heat transfer coefficient decreases upon moving up the column, while it increases with an increase in the mass flow rate of either the dispersed phase or the continuous phase. No considerable impact of the dispersed initial temperature on the volumetric heat transfer coefficient was observed under the experimental conditions considered here. Finally, an excellent agreement was achieved between the analytical model and the experimental results.
Sayer AH, Al-Hussaini H, Campbell AN (2016) New theoretical modelling of heat transfer in solar ponds, Solar Energy 125 pp. 207-218
© 2015 Elsevier Ltd.Solar energy has a promising future as one of the most important types of renewable energy. Solar ponds can be an effective way of capturing and storing this energy. A new theoretical model for a heat transfer in a salinity gradient solar pond has been developed. The model is based on the energy balance for each zone of the pond; three separate zones have been considered, namely the upper convective zone, the lower convective zones, as well as the non-convective zone. The upper and lower zones are considered to be well mixed, which means the temperatures in these zones are uniform. The model shows that the temperature in the storage zone can reach more than 90 °C during the summer season whereas it can be more than 50 °C in winter if the pond is located in the Middle East. In addition, the time dependent temperature for the three layers has been found. Furthermore, it is concluded that heat loss from the pond's surface occurs mainly by evaporation, in comparison to convection and radiation. Heat loss to the ground has been calculated by using three different equations. It was found that the perimeter of the pond has a significant effect on heat loss to the ground from a small pond, while its effect is small in the case of large pond. The validity of the model is tested against experimental data for several established ponds; good agreement is observed.
Liu TY, Campbell AN, Cardoso SSS, Hayhurst AN (2008) Effects of natural convection on thermal explosion in a closed vessel, Physical Chemistry Chemical Physics 10 (36) pp. 5521-5530
A new way of ascertaining whether or not a reacting mixture will explode uses just three timescales: that for chemical reaction to heat up the fluid containing the reactants and products, the timescale for heat conduction out of the reactor, and the timescale for natural convection in the fluid. This approach is developed for an nth order chemical reaction, A ’ B occurring exothermically in a spherical, batch reactor without significant consumption of A. The three timescales are expressed in terms of the physical and chemical parameters of the system. Numerical simulations are performed for laminar natural convection occurring; also, a theoretical relation is developed for turbulent flow. These theoretical and numerical results agree well with previous experimental measurements for the decomposition of azomethane in the gas phase. The new theory developed here is compared with Frank-Kamenetskii's classical criterion for explosion. This new treatment has the advantage of separating the two effects inhibiting explosion, viz. heat removal by thermal conduction and by natural convection. Also, the approach is easily generalised to more complex reactions and flow systems. © the Owner Societies.
Campbell A (2016) Investigating the Impact of Internal and External Convection on Thermal Explosion in a Spherical Vessel, Hazards: Engineering and Design - Paper 17 2016-January (161) IChemE
When any exothermic reaction proceeds in an unstirred vessel, natural convection may develop. The presence of this flow may alter the balance between heat generation and heat removal in the vessel, which ultimately governs whether the system will explode. Classical theories of thermal explosion have largely neglected the potentially important role that buoyant convection can play. More recently, numerical investigations of the effects of natural convection on thermal explosion have considered reactors where the temperature of the wall of the vessel is held constant. Thus, there is a physically unrealistic, infinitely fast heat transfer between the wall and the surrounding environment. In reality, there will be heat transfer resistances associated with conduction through the wall of the vessel and due to convective transfer from the surface arising from the local environmental conditions. These additional modes of heat transfer have the potential to fundamentally alter the rate of heat transfer from the hot zone in the vessel, to the cooler environment. This work presents a numerical study of thermal explosion in a spherical reactor under the influence of natural convection and external heat transfer, which neglects the effects of consumption of reactant. Simulations were performed to examine the changing behaviour of the system as the intensity of convection, as characterised by the Rayleigh number, and the importance of external heat transfer, as characterised by the Biot number, were varied. It was shown that the temporal development of the maximum temperature in the vessel was qualitatively similar as the Rayleigh and Biot numbers were varied. It has been shown that the maximum dimensionless temperature achieved in a non-explosive reaction on the explosion boundary increases from 1 in the well mixed limit to its constant wall temperature value asymptotically. For Bi Name of conference Hazards 26
Liu TY, Campbell AN, Hayhurst AN, Cardoso SSS (2009) Effects of natural convection and consumption of reactant on explosions,
Campbell AN, Cardoso SSS, Hayhurst AN (2007) A comparison of measured temperatures with those calculated numerically and analytically for an exothermic reaction inside a spherical batch reactor with natural convection,
Liu TY, Campbell AN, Cardoso SSS, Hayhurst AN (2009) The effects of natural convection and the consumption of reactant on the occurrence of thermal explosion in a reacting gas contained in a closed spherical vessel,
Sal'nikov's chemical reaction in its simplest form consists of two consecutive first-order steps, producing a product B from a precursor P via an active intermediate A, in P ’ A ’ B. The first step is assumed here to be thermoneutral, with zero activation energy, whilst the second step is exothermic and has a positive activation energy. These properties make this mechanism one of the simplest to display thermokinetic oscillations, as seen in e.g., cool flames or a batch reactor. We first consider a pure gas, P, undergoing Sal'nikov's reaction in a closed spherical vessel, whose walls are held at a constant temperature. Natural convection becomes significant once the temperature is high enough for the Rayleigh number (Ra) to reach
Mahood HB, Thorpe RB, Campbell AN, Sharif AO (2015) Experimental measurements and theoretical prediction for the transient characteristic of a two-phase two-component direct contact condenser, Applied Thermal Engineering 87 pp. 161-174
© 2015 Elsevier Ltd. All rights reserved.The transient characteristics of a three-phase direct contact condenser have been investigated experimentally, utilising a short Perspex column with 48 cm active height and 4 cm internal diameter, and theoretically. Two immiscible fluids, namely pentane vapour with varied initial temperature (41.5 °C, 43.5 °C and47.5 °C) and constant temperature (19 °C) tap water were exploited as the dispersed and the continuous phases respectively. A theoretical model to predict the continuous phase outlet temperature has been developed and solved numerically using MATLAB. The time dependent temperature distribution along the direct contact column was measured and the effect of the mass flow rate ratio and the initial dispersed phase temperature was studied. The experimental results showed that the continuous phase temperatures along the direct contact column increase with time. In addition, no significant effect of the dispersed phase initial temperature on the outlet temperature of the continuous phase was observed. This means that latent heat is dominant in such a direct contact process. Good agreement is achieved between the present theoretical model and the experimental data.
Campbell AN, Cardoso SSS, Hayhurst AN (2006) The behaviour of Sal'nikov's reaction, P ’ A ’ B, in a spherical batch reactor with the diffusion of heat and matter, Physical Chemistry Chemical Physics 8 (24) pp. 2866-2878
The behaviour of a simple chemical reaction, occurring with the release of heat in a closed batch reactor, is considered for the situation when matter and heat are transported only by diffusive processes; thus, the reacting fluid has negligible velocity, so that heat transfer is by thermal conduction. The reaction is Sal'nikov's, which consists of two, consecutive first-order steps, producing a product B, from a precursor P, via an active intermediate A, in P ’ A ’ B. The first of these steps is assumed to be thermoneutral, with zero activation energy, whilst the second is exothermic, with an appreciable activation energy. These features make Sal'nikov's reaction the simplest to display thermokinetic oscillations that characterise many, more complex schemes, e.g. cool flames in hydrocarbon combustion. This study involves identifying the regions of parameter space, in which these oscillations in the temperature and the concentration of the intermediate A occur, by means of numerical simulation. These regions are compared with previous analytical stability analyses in one-dimensional systems. It was found that oscillations occur over a much larger range of conditions in the case considered here, i.e. a reactor with spherical symmetry, than in the simple 1-D case, previously studied by Gray and Scott (P. Gray and S. K. Scott, Chemical Oscillations and Instabilities, Clarendon Press, Oxford, 1990, pp. 264-291, ref. 17). In addition, approximate analytical solutions for the temperature and concentration of A are presented for two limiting cases of non-oscillatory behaviour. These analytical solutions have been verified by comparison with full numerical solutions of the governing equations. © the Owner Societies 2006.
Campbell AN (2015) The effect of external heat transfer on thermal explosion in a spherical vessel with natural convection, Physical Chemistry Chemical Physics 17 (26) pp. 16894-16906
This journal is © the Owner Societies.When any exothermic reaction proceeds in an unstirred vessel, natural convection may develop. This flow can significantly alter the heat transfer from the reacting fluid to the environment and hence alter the balance between heat generation and heat loss, which determines whether or not the system will explode. Previous studies of the effects of natural convection on thermal explosion have considered reactors where the temperature of the wall of the reactor is held constant. This implies that there is infinitely fast heat transfer between the wall of the vessel and the surrounding environment. In reality, there will be heat transfer resistances associated with conduction through the wall of the reactor and from the wall to the environment. The existence of these additional heat transfer resistances may alter the rate of heat transfer from the hot region of the reactor to the environment and hence the stability of the reaction. This work presents an initial numerical study of thermal explosion in a spherical reactor under the influence of natural convection and external heat transfer, which neglects the effects of consumption of reactant. Simulations were performed to examine the changing behaviour of the system as the intensity of convection and the importance of external heat transfer were varied. It was shown that the temporal development of the maximum temperature in the reactor was qualitatively similar as the Rayleigh and Biot numbers were varied. Importantly, the maximum temperature in a stable system was shown to vary with Biot number. This has important consequences for the definitions used for thermal explosion in systems with significant reactant consumption. Additionally, regions of parameter space where explosions occurred were identified. It was shown that reducing the Biot number increases the likelihood of explosion and reduces the stabilising effect of natural convection. Finally, the results of the simulations were shown to compare favourably with analytical predictions in the classical limits of Semenov and Frank-Kamenetskii.
Mahood HB, Campbell AN, Thorpe RB, Sharif AO (2015) A new model for the drag coefficient of a swarm of condensing vapour-liquid bubbles in a third immiscible liquid phase, Chemical Engineering Science 131 pp. 76-83
© 2015 Elsevier Ltd.A semi-analytical model for the drag coefficient of a swarm of two-phase bubbles, condensing in direct contact with an immiscible sub-cooled liquid, has been developed. The analysis used a cellular model configuration, assuming potential (but not inviscid) flow around the reference two-phase bubble in the cell. The effect of the condensation ratio within the two-phase bubbles was included using an approximate relation. The drag coefficient for a wide range of Reynolds numbers (0.1. d. Re. d. 1000) has been found using the viscous dissipation integral method, and the effect of the liquid content within the two-phase bubble or the half opening angle (²), and the system void fraction (±) were examined. The drag coefficient has been found to increase with the condensation ratio and with the void fraction of the system. The present model agrees well with previously available experimental data and theoretical predictions for single bubbles or particles.
Campbell AN, Cardoso SSS, Hayhurst AN (2008) Oscillatory and nonoscillatory behavior of a simple model for cool flames, Sal'nikov's reaction, P ’ A ’ B, occurring in a spherical batch reactor with varying intensities of natural convection, Combustion and Flame 154 (1-2) pp. 122-142
When cool flames, or indeed any exothermic chemical reaction, occur in a fluid inside an unstirred vessel, the heat from the reaction often induces temperature gradients and consequently motion, i.e., natural convection. The intensity of the resulting flow is governed by the Rayleigh number (Ra). This work simulates numerically the behavior of Sal'nikov's reaction, P ’ A ’ B, under the influence of natural convection in an unstirred spherical reactor. This reaction is the simplest to exhibit the thermokinetic oscillations characterizing cool flames. The behavior of this system can be represented on a three-dimensional regime diagram, whose axes are ratios of the characteristic timescales (Ä) for chemical reaction, diffusion (of both heat and mass), and natural convection. Previous work has identified a region of oscillations on this diagram in the purely diffusive limit, when Ra = 0. This work extends this analysis to the general 3D space, where diffusion and natural convection are both important. A region in which oscillations are observed has been found for fixed values of the first-order rate constants for Sal'nikov's reaction. There is a distinct change in the shape of the region of oscillations around the critical value of Ra 500), the oscillations occur over a wider range of parameters than is the case for a diffusive system. The presence of natural convection also leads to various, more complex behaviors than are seen in the diffusive or well-mixed limits. A region in the regime diagram was found where the oscillations in temperature and the concentration of A have small amplitudes and a frequency that is quite different from those generated in a well-mixed system. It is possible that these oscillations are caused by natural convection, i.e., are not thermokinetic oscillations produced by the chemical reaction. It was also found that sometimes the oscillations in the temperature and the concentration of A are in phase; more generally they are in anti-phase. The evolution of nonoscillatory behavior with relatively small increases in temperature was found to be always fairly similar, regardless of the
Campbell AN, Cardoso SSS (2010) Turbulent plumes with internal generation of buoyancy by chemical reaction, Journal of Fluid Mechanics 655 pp. 122-151
Turbulent plumes, which are seen in a wide number of industrial and natural flows, have been extensively studied; however, very little attention has been paid to plumes which have an internal mechanism for changing buoyancy. Such plumes arise in e.g. industrial chimneys, where species can react and change the density of the plume material. These plumes with chemical reaction are the focus of this study. An integral model describing the behaviour of a plume undergoing a second-order chemical reaction between a component in the plume (A) and a component in the surrounding fluid (B), which alters the buoyancy flux, is considered. The behaviour of a reactive plume is shown to depend on four dimensionless groups: the volume and momentum fluxes at the source, the parameter 0 which indicates the additional buoyancy flux generated by the reaction and ³ which is a dimensionless rate of depletion of species B. Additionally, approximate analytical solutions are sought for a reactive plume rising from a point source of buoyancy when species B is in great excess. These analytical results show excellent agreement with numerical simulations. It is also shown that the behaviour of a reactive plume in the far field is equivalent to an inert plume issuing from a virtual source downstream of the real source, and the dependence of the location of the virtual source on 0 and ³ is discussed. The effects of varying the volume flux at the source and the Morton source parameter 0 are further investigated by solving the full governing equations numerically. These solutions indicate that is important in determining the buoyancy generated by the reaction, and the length scale over which this reaction occurs depends on ³ when ³ > 1. It is also shown that when the dimensionless buoyancy 0
Campbell AN, Cardoso SSS, Hayhurst AN (2005) A scaling analysis of the effects of natural convection on an oscillatory reaction in a batch reactor, IChemE
Sal'nikov's chemical reaction is very simple; it consists of two consecutive first-order steps, yielding a product B from a precursor P via an active intermediate A, in P’A’B. The first of these steps is assumed here to be thermoneutral, with zero activation energy, whilst the second step is taken to be exothermic with a positive activation energy. These properties make this reaction one of the simplest to display thermokinetic oscillations, such as characterise cool flames. This study considers Sal'nikov's reaction occurring batchwise in the gas-phase in a closed spherical reactor, whose wall is held at a constant temperature. Natural convection becomes significant once the temperature in the reactor has risen sufficiently for the Rayleigh number to reach
Mahood H, Thorpe R, Campbell A, Sharif A (2015) Effect of Various Parameters on the Temperature Measurements In a Three-Phase Direct Contact Condenser, International Journal of Thermal Technologies 5 (1) pp. 23-27
Mahood HB, Campbell AN, Thorpe RB, Sharif AO (2015) Heat transfer efficiency and capital cost evaluation of a three-phase direct contact heat exchanger for the utilisation of low-grade energy sources, Energy Conversion and Management 106 pp. 101-109
© 2015 Elsevier Ltd. All rights reserved.Low-grade energy cycles for power generation require efficient heat transfer equipment. Using a three-phase direct contact heat exchanger instead of a surface type exchanger, such as a shell and tube heat exchanger, potentially makes the process more efficient and economic. This is because of its ability to work with a very low temperature driving force, as well as its low cost of construction. In this study, an experimental investigation of the heat transfer efficiency, and hence cost, of a three-phase direct contact condenser has been carried out utilising a short Perspex tube of 70 cm total height and 4 cm internal diameter. Only 48 cm was used for the direct contact condensation. Pentane vapour with three different initial temperatures (40 °C, 43.5 °C and 47.5 °C) was contacted with water with an inlet temperature of 19 °C. In line with previous studies, the ratio of the fluid flow rates was shown to have a controlling effect on the exchanger. Specifically, the heat transfer efficiency increased virtually linearly with this ratio, with higher efficiencies also being observed with higher flow rates of the continuous phase. The effect of the initial temperature of the dispersed phase was shown to have a lower order impact than flow rate ratio. The capital cost of the direct contact condenser was estimated and it was found to be less than the corresponding surface condenser (shell and tube condenser) by 30 times. An optimum value of the continuous phase flow rate was observed at which the cost of the condenser is at a minimum.
Mahood HB, Campbell AN, Sharif AO, Thorpe RB (2016) Heat transfer measurement in a three-phase direct-contact condenser under flooding conditions, Applied Thermal Engineering 95 pp. 106-114
© 2015 Elsevier Ltd. All rights reserved.The transient temperature distribution and volumetric heat transfer coefficient during the inception of flooding in a three-phase bubble type direct contact condenser have been experimentally investigated. The flooding mechanism and the factors affecting the onset of flooding of the three-phase direct contact column are not considered. A short Perspex column of 70 cm total height and 4 cm internal diameter utilising two immiscible fluids was studied. Pentane vapour with initial temperatures of 40 °C, 43.5 °C and 47.5 °C was the dispersed phase and tap water at a constant temperature (19 °C) was the continuous phase. Only 48 cm of the column was used as the active height and different mass flow rates of both phases were used. The experimental results showed that the instantaneous temperature distribution along the direct contact column tends to be uniform when the direct contact column is working under flooding conditions. Furthermore, the volumetric heat transfer coefficient increases as the dispersed mass flow rate is increased towards the flooding limit and remains constant along the column height. In addition, the dispersed phase mass flow rate that leads to flooding increased with increasing mass flow rate of the continuous phase. The initial temperature of the dispersed phase did not have a considerable effect on the flooding inception limit under the present experimental conditions.
Campbell AN, Cardoso SSS, Hayhurst AN (2005) A scaling analysis of Sal'nikov's reaction, P ’ A ’ B, in the presence of natural convection and the diffusion of heat and matter, Proceedings of the Royal Society A: Mathematical, Physical and Engineering Sciences 461 (2059) pp. 1999-2020
Sal'nikov's chemical reaction is very simple; it consists of two consecutive first-order steps, producing a product B from a precursor P via an active intermediate A, in P ’ A ’ B. The first step is assumed to be thermoneutral, with zero activation energy, while the second step is exothermic and has a positive activation energy. These properties make this mechanism one of the simplest to display thermokinetic oscillations, as seen in cool flames. We consider a pure gas, P, undergoing Sal'nikov's reaction in a closed spherical vessel, whose walls are held at a constant temperature. Natural convection becomes significant once the temperature is high enough for the Rayleigh number to reach approximately 103. The subsequent behaviour of the system depends on the interaction between convection, diffusion of heat and mass, and chemical kinetics. By examining the governing equations, we develop and evaluate scales for the characteristic velocity, concentration of the intermediate A and the temperature rise during the progress of the reaction. These scales depend on the characteristic time-scales for the interacting phenomena of chemical reaction, diffusion and natural convection. Our theoretical predictions are verified by full numerical simulation for the two limiting cases when transport is dominated, respectively, by diffusion and natural convection. © 2005 The Royal Society.
Ivleva TP, Merzhanov AG, Rumanov EN, Vaganova NI, Campbell AN, Hayhurst AN (2011) When do chemical reactions promote mixing?, Chemical Engineering Journal 168 (1) pp. 1-14
This paper reviews various situations, in which a chemical reaction promotes the mixing (of reactants and products) in an unstirred reactor. One example is an exothermic chemical reaction, which of course increases the local temperature of an unstirred reacting fluid and hence decreases the density. This can produce natural convection. Thus if the walls of the reactor are cooled, there is often toroidal motion in the reacting mixture and consequently enhanced mixing. Of course, the flow field depends on the shape of vessel, but usually natural convection moves fluid up the middle of the vessel and downwards near the cooler walls. Such convective motion influences, in turn, the temperature field and consequently local rates of reaction and heat release. In a large vessel, the velocities associated with natural convection can be large enough for turbulence to arise and so considerably improve mixing. A second example is so-called "critical mixing"; this occurs when a substance is close to its critical point and large fluctuations of density, temperature, concentration, etc., can occur. These fluctuations sometimes lead to intense mixing, likely to develop into turbulence. Similar features are manifested by a continuously stirred tank reactor (CSTR) approaching a bifurcation point (critical chaos), consideration of which requires a review of Russian work on the dynamics of a CSTR. Next, a chemical reaction with several steps might behave chaotically. Chaotic behaviour in time implies the generation of spatial inhomogeneities, which can promote mixing. In contrast to driven mixing or stirring, the scales for this "self-mixing" are, as a rule, much smaller than the reactor. This latter property is important for encouraging mixing. Finally, the important cases of a liquid or fluidised bed being mixed by bubbles produced, e.g. by chemical reaction, is briefly considered. © 2011 Elsevier B.V.
Abbassi Monjezi A, Mahood H, Campbell A (2017) Regeneration of dimethyl ether as a draw solute in forward osmosis by utilising thermal energy from a solar pond, Desalination 415 pp. 104-114 Elsevier
Utilisation of solar thermal energy in forward osmosis (FO) can provide an attractive method for seawater desalination. This study presents a novel process for the regeneration of dimethyl ether (DME) as a draw solute in FO using thermal energy from a solar pond. The location considered for this process is Chabahar (Iran) which benefits from a very high solar irradiance and access to an abundance of seawater from the Sea of Oman making it an ideal location for the proposed process. The average daily volume of desalinated water produced using this process coupled to a solar pond of 10,000 m2 was determined. It is indicated that a solar pond of such moderate size can drive a forward osmosis plant to provide 5,210 m3 of freshwater in the first two years of operation in Chabahar. The proposed process provides freshwater at varying rates throughout the year and benefits from a very low electricity consumption rate of 0.46 kWh per cubic metre of desalinated water offering a viable option for solar desalination. Considering that there are vast uninhabited coastal areas particularly in the Middle East and North Africa (MENA) region, the proposed method can contribute towards addressing the growing potable water scarcity.
This study presents a novel heat extraction method, which can be operated in batch or continuous, modes for salinity gradient solar ponds. A comparison between the performance of two solar ponds of the same size (10,000 m2) in Adana (Turkey) and Ahvaz (Iran) is also presented. The heat extraction method entails brine removal from the non-convective zone (NCZ) as well as the heat storage zone (HSZ). The presented model incorporates the heat losses from the bottom and surface of the pond as well as the cooling effect imposed as a consequence of the replacement of extracted brine from each layer, and the supply of freshwater to the surface of the pond to maintain its inventory. The model can be employed to predict the performance of solar ponds of various dimensions for any given location. It was established that the pond modelled for Ahvaz would perform 30% better than the pond in Adana in both batch and continuous heat extraction modes, predominantly due to the higher quantities of solar energy reaching the surface of the pond and the higher air temperatures throughout the year at this location. The quantities of heat provided in the first year of operation from the ponds in Adana and Ahvaz in batch mode extraction are 2.8 x 106 MJ and 4.0 x 106 MJ, respectively. These values are approximately three times higher than those from the continuous mode of heat extraction due to the larger volume of withdrawal in the batch mode. Using the proposed heat extraction method in batch mode, 85% of the total heat is removed from the HSZ while this is just over 50% for the continuous mode indicating the better energy storage performance of the batch mode. Both heat extraction modes offer an efficient mechanism of stabilising a temperature gradient throughout the pond with the aim of insulating the HSZ for heat storage. This is carried out by designating brine removal thresholds of 70 °C, 80 °C and 90 °C within the NCZ and 95 °C in the HSZ. It is also demonstrated that the requirement for the supply of freshwater to the surface of solar ponds is significantly dependant on the wind velocity at each location and is unaffected by the mode of heat extraction.
Mahood H, Campbell A, Thorpe R, Sharif A (2017) Measuring the overall volumetric heat transfer coefficient in a vapour-liquid-liquid three-phase direct contact heat exchanger, Heat Transfer Engineering 39 (3) pp. 208-216 Taylor & Francis
An experimental investigation of the volumetric heat transfer coefficient in a three-phase direct contact condenser was carried out. A 75-cm long cylindrical Perspex column with a 4 cm diameter was used. Only 48 cm of the column was utilised as the active direct contact condensation height. Pentane vapour at three different initial temperatures (40°C, 43.5°C and 47.5°C), with differing mass flow rates, and tap water at a constant initial temperature (19°C) with five different mass flow rates were employed as the dispersed phase and the continuous phases, respectively. The results showed that the volumetric heat transfer coefficient increased with increasing mass flow rate ratio (variable dispersed phase mass flow rate per constant continuous phase mass flow rate), the continuous phase mass flow rate and holdup ratio. An optimal value of the continuous phase mass flow rate is shown for an individual dispersed phase mass flow rates. This value increases with increasing vapour (dispersed) phase mass flow rate. Furthermore, it was observed that the initial driving temperature difference had no effect on the volumetric heat transfer coefficient. While, the temperature gained by the continuous phase has a considerable effect.
Baqir A, Mahood H, Campbell AN, Griffiths A (2016) Measuring the Average Volumetric Heat Transfer Coefficient of a Liquid-Liquid-Vapour Direct Contact Heat Exchanger, Applied Thermal Engineering 103 pp. 47-55 Elsevier
The average volumetric heat transfer coefficient in a spray column liquid-liquid-vapour direct contact evaporator has been experimentally investigated. The experiments were carried out utilising a cylindrical Perspex tube of diameter 10 cm and height and 150 cm. Saturated liquid n-pentane and warm water at 45oC were used as the dispersed and continuous phases, respectively. Three different dispersed flow rates (10,15 and 20 L/h) and four different continuous phase flow rates (10, 20, 30 and 40 LDh) were used in the study. The effect of different parameters, such as the initial drop size, continuous and dispersed phase flow rates and sparger configuration, on the average volumetric heat transfer coefficient in the evaporator was studied. The results showed that the average volumetric heat transfer coefficient was reduced as the initial drop size increased. Also, both the continuous phase and the dispersed phase flow rates have a significant positive impact on the average volumetric heat transfer coefficient.
Solar ponds offer an effective way to collect and store incident solar radiation, making them an attractive alternative to photovoltaic systems for applications which require low-grade heat to operate. If these ponds are to be implemented successfully, then a more complete understanding of the mechanisms and phenomena governing their behaviour is required. Evaporation has been shown previously to be the dominant mode of heat loss from the pond surface, and the fresh water that would need to be added to maintain the pond?s inventory could potentially add significantly to operating costs. To this end, an experimental unit was constructed to examine and observe the behaviour of a salinity gradient solar pond (SGSP) before and after covering the pond with a thin layer (0.5 cm) of paraffin, with the aim of eliminating evaporation. The unit was run for 71 days in Nasiriyah, Iraq. This is the first study to attempt to completely eliminate the harmful effects of evaporation on solar pond performance using a liquid layer. The layer successfully eliminated the significant evaporation observed from the uncovered pond and crucially, while the salinity gradient through the non-convective zone remained substantially intact over the course of the study, the temperature profile became approximately uniform throughout the entire pond after about 50 days. This behaviour has significant implications for the construction of the pond, as it may mean that if evaporation can be largely suppressed, the salinity gradient may not be necessary for the pond to capture and efficiently store heat. Furthermore, the effects on evaporation of different climatic factors such as relative humidity, wind speed, ambient temperature and solar radiation were considered by analysing data measured on-site and longer-term meteorological data. The results showed that ambient temperature, solar radiation and humidity have a significant correlation with the evaporation rate; and their impact varies seasonally. A more comprehensive multiple regression analysis showed that ambient temperature has the highest impact on evaporation, while the effect of the incident solar radiation is insignificant. Such insights are vital in the design and siting of solar ponds, and can be used to minimise evaporative losses.
Baqir A, Mahood H, Hameed M, Campbell AN (2016) Heat Transfer Measurement in a Three-Phase Spray Column Direct Contact Heat Exchanger for Utilisation in Energy Recovery from Low-Grade Sources, Energy Conversion and Management 126 pp. 342-351 Elsevier
Energy recovery from low-grade energy resources requires an efficient thermal conversion system to be economically viable. The use of a liquid-liquid-vapour direct contact heat exchanger in such processes could be suitable due to their high thermal efficiency and low cost in comparison to a surface type heat exchanger. In this paper, the local volumetric heat transfer coefficient (U_v ) and the active height (H_v ) of a spray column three-phase direct contact heat exchanger (evaporator) have been investigated experimentally. The heat exchanger comprised a cylindrical Perspex tube of 100 cm height and 10 cm diameter. Liquid pentane at its saturation temperature and warm water at 45 ? were used as the dispersed phase and the continuous phase respectively. Three different dispersed phase flow rates (10, 15 and 20 l/h) and four different continuous phase flow rates (10, 20, 30 and 40 l/h) were used throughout the experiments. In addition, three different sparger configurations (7, 19 and 36 nozzles) with two different nozzle diameters (1 and 1.25 mm) were tested. The results showed that the local volumetric heat transfer coefficient (U_v ) along the column decreases with height. An increase in both the continuous and dispersed phase flow rates had a positive effect on U_v, while an increase in the number of nozzles in the sparger caused Uv to decrease. The active height was significantly affected by the dispersed and continuous phase flow rates, the sparger configuration and the temperature driving force in terms of the Jacob number.
Solar energy is likely to be the energy of the future; solar ponds, especially salinity gradient solar ponds (SGSPs), facilitate simple and cost-effective thermal energy storage. Research on maximising their potential is of particular relevance to developing countries, which often have an abundance of solar energy and a critical need for increased power supplies. For this research, a theoretical model for heat transfer in a SGSP was developed to study the energy balance in the three separate zones: the upper convective zone (UCZ), lower convective zone or storage zone (LCZ) and non-convective zone (NCZ). The model showed that the LCZ temperature could reach more than 90 °C in summer and more than 50 °C in winter, in a pond in the Middle East. It was also concluded that surface heat loss occurred mainly by evaporation.
The new model was also used to examine the feasibility of a second type of solar pond, the gel pond; this offers solutions to some of the SGSP?s challenges, but presents other difficulties relating to cost and labour.
To verify the theoretical results of the SGSP, a small experimental pond was constructed and operated for 71 days in Nasiriyah, Iraq. It was observed that adding a thin surface layer (0.5 cm) of paraffin eliminated the significant evaporation seen in the uncovered pond. Further analysis of the evaporation rate showed a significant correlation with temperature, solar radiation and humidity. Crucially, it was also noted that while the salinity gradient in the NCZ remained substantially intact, the temperature profile became approximately uniform throughout the pond after about 50 days.
Analytical formulae to describe the concentrations and temperatures of the UCZ and LCZ were derived. The results achieved and comparisons with the experimental data showed that these equations can be used to compute both concentrations and temperatures.
Renewables offer the best opportunity to reduce greenhouse gases and introduce sustainable and desirable solutions to the world?s increasing demand for energy. Solar ponds are a simple, low-priced and efficient way to collect and store incident solar radiation; they have enormous capacity and huge unrealised potential. The most effective and widely-used type is the salinity gradient solar pond (SGSP), which can provide long-term storage, large capacity, and sufficient year-round power for a wide range of domestic, industrial and commercial purposes. It is essential to understand the best forms of construction and maintenance, including how to overcome the most significant challenge, that of establishing the different layers of water with their varying levels of saline concentration, as well as how to maintain these levels for optimal performance. This study investigates how to predict variations in salt concentrations over the pond?s lifetime, in order to maintain maximum performance. The paper derives analytical formulae to calculate how concentrations in the upper and lower convective zones (UCZ and LCZ) change over time. It also explores the different responses of ponds with vertical walls and inclined walls. The computed concentrations were compared with actual measurements from a small experimental pond, which had a surface area of 1 m2 and a total depth of 1 m, comprising UCZ, non-convective zone (NCZ) and LCZ with depths of 0.1, 0.5, and 0.4 m respectively. The results were also compared with the results of previous experiments from established ponds. An acceptable agreement was achieved. The results illustrated that the derived formulae can be used to estimate salt concentrations in the UCZ and LCZ. The outcomes also show that the inclination of the walls affects concentration levels in the UCZ, while its impact on the LCZ concentration is slight. The findings of this study could form the basis of future research, which could investigate other factors affecting salt concentration in the layers of a SGSP, such as wind speed, temperature and the timing of salt injections to the LCZ.
Sayer A, Al-Hussaini H, Campbell A (2017) New comprehensive investigation on the feasibility of the gel solar pond, and a comparison with the salinity gradient solar pond, Applied Thermal Engineering 130 pp. 672-683 Elsevier
Solar energy is increasingly being exploited to supply energy for many purposes. This paper explores the feasibility of gel solar ponds as a source of renewables, using theoretical evaluation. This could be of critical future utilization in areas such as desalination, where the gel solar pond could in effect be a means to deliver fresh water in the Middle East and other regions where water scarcity is predicted to become an increasingly critical issue to resolve. This study explores all aspects of the gel solar pond?s functioning, including optimal thicknesses for its different layers, and explores its strengths and weaknesses. In this study; temperature profiles in the upper convective zone (UCZ) and lower convective zone (LCZ) of a gel pond are investigated. The impact of the thickness of the pond?s layers on the temperatures of these zones was also investigated. The cost of the gel pond was calculated and compared with that of the salinity gradient solar pond (SGSP) for a particular application, the multi-effect desalination (MED), which is frequently used to desalinate sea water. The results showed that the gel pond could supply thermal energy to applications requiring low-grade temperatures, and that temperatures in the LCZ of the gel pond could reach values similar to those achieved in the SGSP. Varying the thicknesses of the gel layer and the LCZ affects the temperature of the storage zone. The optimal thickness of the upper water layer and the gel layer was found to be 0.05 and 0.9 m respectively, while the optimal thickness of the storage zone depends on the particular application for which the pond is being used in each case. The results also show that a gel pond normally costs more than a SGSP. This study illustrates that gel solar ponds can offer solutions to some of the challenges posed by the SGSP; however, difficulties relating to cost and labour decrease their potential exploitation. Gel ponds can be seen as a viable alternative to SGSPs only if cheap and environmentally friendly polymers are used to form the gel layer.
Sayer Asaad, Monjezi AA, Campbell Alasdair (2017) Behaviour of a salinity gradient solar pond during two years and the impact of zonal thickness variation on its performance, Applied Thermal Engineering 130 pp. 1191-1198 Elsevier
The interest in solar energy has increased substantially as a consequence of greenhouse gas
emissions that result from the combustion of fossil fuels in power generation processes. Solar
energy is likely to be the energy of the future and solar ponds, in particular, salinity gradient
solar ponds (SGSP), facilitate simple and cost-effective thermal energy collection and
storage. In this study; the influence of varying the thicknesses of the zones present in a
salinity gradient solar pond on the temperatures of the upper convective zone (UCZ) and the
lower convective zone (LCZ) is investigated. The study finds that thickness variation of the
zones within the pond has a considerable impact on the temperature of the LCZ while it has a
small effect on the temperature of the UCZ. The optimal thicknesses of the UCZ and the
non-convective zone (NCZ) have been found to be 0.2 and 2 m respectively. The results also
show that the type of application plays a substantial role in determining the depth of the LCZ,
and that temperature of this zone varies with the rate of heat extraction. A period of no heat
extraction is required to allow the pond to warm up and the length of this period depends on
the depth of the LCZ, the type of application coupled with the pond, and the rate of heat
extraction. It was found that the SGSP could be deeper with less surface area, and still
suitable for applications that require low-grade heat. These findings could form the basis of
future studies regarding the performance and financial viability of the overall depth of
SGSPs.
Mahood Hameed B., Campbell Alasdair, Baqir Ali Sh., Sharif Adel, Thorpe Rex (2017) Convective Heat Transfer Measurements in a Vapour-Liquid-Liquid Three-Phase Direct Contact Heat Exchanger, Heat and Mass Transfer 54 (6) pp. 1697-1705 Springer Verlag
Energy usage is increasing around the world due to the continued development of technology, and population growth. Solar energy is a promising low-grade energy resource that can be harvested and utilised in different applications, such solar heater systems, which are used in both domestic and industrial settings. However, the implementation of an efficient energy conversion system or heat exchanger would enhance such low-grade energy processes. The direct contact heat exchanger could be the right choice due to its ability to efficiently transfer significant amounts of heat, simple design, and low cost. In this work, the heat transfer associated with the direct contact condensation of pentane vapour bubbles in a three-phase direct contact condenser is investigated experimentally. Such a condenser could be used in a cycle with a solar water heater and heat recovery systems. The experiments on the steady state operation of the three-phase direct contact condenser were carried out using a short Perspex tube of 70 cm in total height and an internal diameter of 4 cm. Only a height of 48 cm was active as the direct contact condenser. Pentane vapour, (the dispersed phase) with three different initial temperatures (40?,43.5? and 47.5?) was directly contacted with water (the continuous phase) at 19?. The experimental results showed that the total heat transfer rate per unit volume along the direct contact condenser gradually decreased upon moving higher up the condenser. Additionally, the heat transfer rate increases with increasing mass flow rate ratio, but no significant effect on the heat transfer rate of varying the initial temperature of the dispersed phase was seen. Furthermore, both the outlet temperature of the continuous phase and the void fraction were positively correlated with the total heat transfer rate per unit volume, with no considerable effect of the initial temperature difference between the dispersed and continuous phases.
Salinity gradient solar ponds can be used to store heat by trapping solar radiation. The heat can then be employed to drive various industrial applications that require low-grade heat. In this study, a comprehensive finite difference transient model has been developed incorporating many processes that affect the performance of a solar pond to predict the hourly temperature distribution.
A novel heat extraction method for salinity gradient solar ponds is then proposed. This method can be operated in batch or continuous modes. A comparison between the performance of two solar ponds of the same size (10,000 m2) in Adana (Turkey) and Ahvaz (Iran) is also presented. The heat extraction method entails brine removal from the Non-Convective Zone (NCZ) as well as the HSZ. The presented model incorporates the heat losses from the bottom and surface of the pond as well as the cooling effect imposed as a consequence of the replacement of extracted brine from each layer, and the supply of freshwater to the surface of the pond to maintain its inventory. The model can be employed to predict the performance of solar ponds of various dimensions for any given location.
In the final part of this study, utilisation of solar thermal energy from salinity gradient solar ponds in forward osmosis (FO) is investigated. This study will present two novel processes for the regeneration of dimethyl ether (DME) and ammonium bicarbonate as a draw solutes in FO using thermal energy provided from a solar pond.
The average daily volume of desalinated water produced using these processes and a solar pond of 10,000 m2 was determined. It is indicated that, a solar pond of such moderate size can drive a forward osmosis plant to provide a total of 5,210 m3 of potable water in the first two years of operation in the location considered in this study (Chabahar) if DME is used as the draw solute. The proposed process can provide freshwater at varying rates throughout the year and benefits from a very low electricity consumption rate of 0.46 kWh per cubic metre of desalinated water presenting a viable option for solar desalination. In case of ammonium bicarbonate, the product water contains small quantities of ammonia ions making it unsuitable for drinking purposes. Given that there are vast uninhabited coastal areas in many countries, particularly in the MENA region where there are high solar radiation rates, this method can contribute towards addressing the growing water scarcity.
Anagnostopoulos A, Campbell A, Arellano-Garcia H (2017) Modelling of the Thermal Performance of SGSP using COMSOL Multiphysics, Proceedings of the 27th European Symposium on Computer Aided Process Engineering ? ESCAPE 27 40C pp. 2575-2560 Elsevier Science Ltd (Hersteller)
Novel renewable energy sources are necessary to counter the current environmental crisis. The largest source of renewable energy is the sun. One possible application of solar energy is the harvesting and storage of low temperature thermal heat ( The new models were compared with experimental data from two different test sites, concerning mainly the temperature at the lower convective zone (LCZ) and the upper convective zone (UCZ). The 3D model was proven to be the most accurate with the 1D model being the least. Furthermore, the general radiative heat transfer equation, with an isotropic scattering phase function, solved using the discrete ordinates method was proven to give a satisfactory accuracy in terms of radiation in semi-transparent media.
Mahdi Mustafa S., Mahood Hameed B., Khadom Anees A., Campbell Alasdair N., Hasan Mohanad, Sharif Adel O. (2019) Experimental investigation of the thermal performance of a helical coil latent heat thermal energy storage for solar energy applications, Thermal Science and Engineering Progress 10 pp. 287-298 Elsevier
Thermal performance of a Latent Heat helical coil Thermal Energy Storage (LHTS) was investigated experimentally for both phases; melting and solidification processes. Paraffin wax (type P56-58) and tap water were used as a Phase Change Material (PCM), and a Heat Transfer Fluid (HTF), respectively. The paraffin wax (PCM) thermos-physical properties were determined experimentally. To simulate the solar energy conditions, three different initial temperatures (70/°C, 75/°C and 80/°C) and flow rates (1/L/min, 3/L/min and 5/L/min) of the HTF were tested throughout the PCM melting experiments, while the temperature of HTF was only 30/°C with the same flow rates for solidification process. The storage was completely insulated to reduce the heat losses. The PCM temperature during the melting and solidification processes was measured with time using 16 K-type calibrated thermocouples distributed along the PCM axially and radially. The experimental results showed that contrary to the solidification process, the melting was a superior in the helical coil LHTS under different operational conditions. Axial and radial melting fronts were noticed during the PCM melting process which considerably shortened the melting time under the effect of convection and a shape like a pyramid is formed at the core of the storage. Initial temperature of heat transfer fluid (HTF) was significantly affected the melting process and the increased of it from 70/°C to 75/°C and from 75/°C to 80/°C resulted in shortening the total melting time by about 34.5% and 27.2% respectively. An optimum HTF flow rate was observed during the melting process and it was found to be 3/L/min under the operational conditions of the present experiments. Contrary, the flow rate of HTF was insignificant during the solidification process. The initial temperature of HTF was slightly affected the effectiveness of the melting process. In spite of the efficiency of the melting process, enhancement of the solidification in the coiled LHTS is necessary in order to use the process in the thermal applications of solar energy.
Mahood Hameed B., Baqir Ali Sh., Yousif Al-Dunainawi, Khadom Anees A., Campbell Alasdair (2019) Direct contact evaporation of a single two-phase bubble in a flowing immiscible liquid medium. Part I: two-phase bubble size, Heat and Mass Transfer
The evaporation of a single n-pentane drop in another warm flowing liquid (water) medium has been studied experimentally. A Perspex column with an internal diameter of 10 cm and height of 150 cm was used throughout the experiments. N-pentane liquid at its saturation temperature and warm flowing water with flow rate of 10, 20, 30 and 40 L/h were employed as the dispersed and continuous phases, respectively. The active height of the continuous phase in the column (i.e. the level of the continuous phase in the column) covered only 100 cm of the column?s height. A Photron FASTCAM high-speed camera (~ 65,000 f/s) was used to film the evaporation of the drop, while AutoCAD was used to analyse the images from the camera. The diameter ratio (diameter of growing two-phase bubble to initial drop diameter) of the two-phase bubble formed because of the evaporation of the pentane drop in direct contact with the water was measured. Also, the vaporisation ratio (x), the open angle of vapour (²), the total height for complete evaporation and the total evaporation time were measured. The effects of the continuous phase flow rate and the temperature difference between the contacting phases, in terms of Jakob number (Ja), on the measured parameters were investigated. Furthermore, a statistical model to fit the experimental data was developed. The experimental results showed that the diameter of the two-phase bubble is strongly influenced by varying the continuous phase flow rate. The final size of the evaporated vapour bubble was unaffected by the flow rate of the continuous phase, while both the total height required for complete evaporation and hence the time required was significantly influenced. A similar impact was observed for the vaporisation ratio and the open angle of vapour.
Mahdi Mustafa S., Mahood Hameed B., Hasan Ahmed F., Khadom Anees A., Campbell Alasdair N. (2019) Numerical study on the effect of the location of the phase change material in a concentric double pipe latent heat thermal energy storage unit, Thermal Science and Engineering Progress 11 pp. 40-49 Elsevier
Latent heat thermal energy storage (LHTS) represents an intriguing solution to the problem of variability of supply that exists in solar thermal power systems. One such storage system consists of a double pipe, where a phase change material (PCM) is enclosed in either the central pipe, or the annulus surrounding it. The heat transfer fluid fills the other void in the system. Whether the PCM is used in the central pipe or the annulus could, potentially, significantly alter the thermal performance of the system. Thus, a comparison between the PCM mounted in the annulus (case A) and the inner tube (case B) was conducted numerically, to investigate the advantages and disadvantages of each case with regard to the melting and solidification performance. A horizontal double pipe latent heat thermal energy storage device was considered. The numerical simulation solved the transient balance equations in a two-dimensional system. The enthalpy-porosity method was used to simulate the phase change and the Boussinesq approximation, which accounts for the small changes in density that drive natural convection, was applied. The effect of the initial temperature of the heat transfer surface (HTS) on the sensible and latent heat changes of the model PCM, RT-50, was tested for both the melting and solidification processes. Aiming to assess the differences in the storage performance, the average PCM temperature, the liquid fraction, and the average velocity of both the melting and solidification processes were examined. The results of the simulation showed that for both cases, convection was dominant after only a short period of the melting process. The melting time was significantly different in the two cases, i.e. it was shorter in case B than case A by almost 50%. Furthermore, an increase in the temperature of the HTS by 5/°C notably affected the melting time of both cases by as much as 20%. This effect was more pronounced in case B, which had a melting time which was 41% shorter than case A. Finally, the results revealed that the solidification process in case A was more rapid than case B with the total solidification time of case A being lower by 43.4%.
Mahdi Mustafa S., Mahood Hameed B., Hasan Ahmed F., Khadom Anees A., Campbell Alasdair N. (2019) Numerical study on the effect of the location of the phase change material in a concentric double pipe latent heat thermal energy storage unit, Thermal Science and Engineering Progress 11 pp. 40-49 Elsevier
Latent heat thermal energy storage (LHTS) represents an intriguing solution to the problem of variability of supply that exists in solar thermal power systems. One such storage system consists of a double pipe, where a phase change material (PCM) is enclosed in either the central pipe, or the annulus surrounding it. The heat transfer fluid fills the other void in the system. Whether the PCM is used in the central pipe or the annulus could, potentially, significantly alter the thermal performance of the system. Thus, a comparison between the PCM mounted in the annulus (case A) and the inner tube (case B) was conducted numerically, to investigate the advantages and disadvantages of each case with regard to the melting and solidification performance. A horizontal double pipe latent heat thermal energy storage device was considered. The numerical simulation solved the transient balance equations in a two-dimensional system. The enthalpy-porosity method was used to si-
mulate the phase change and the Boussinesq approximation, which accounts for the small changes in density that drive natural convection, was applied. The effect of the initial temperature of the heat transfer surface (HTS) on the sensible and latent heat changes of the model PCM, RT-50, was tested for both the melting and solidification processes. Aiming to assess the differences in the storage performance, the average PCM temperature, the liquid fraction, and the average velocity of both the melting and solidification processes were examined. The results of the simulation showed that for both cases, convection was dominant after only a short period of the melting process. The melting time was significantly different in the two cases, i.e. it was shorter in case B than case A by almost 50%. Furthermore, an increase in the temperature of the HTS by 5 °C notably affected the melting time of both cases by as much as 20%. This effect was more pronounced in case B, which had a melting time which was 41% shorter than case A. Finally, the results revealed that the solidification process in case A was more rapid than case B with the total solidification time of case A being lower by 43.4%.
Cerebrovascular and neurodegenerative diseases are clinical conditions increasingly observed in geriatric populations. A common symptom of these pathologies is represented
by the increased difficulty with food manipulation and intake. Developing food products,
dietary supplements and oral medications for this set of patients requires an improved understanding of the interplay between bolus rheology, tongue coordination and lubrication
of the oral cavity and their effect on the resulting ease of swallowing.
This study employs an in vitro model experiment to elucidate the role of bolus rheology
and to describe the oral swallowing dynamics in presence of suspended particles, to mimic
swallowing of solid oral dosage forms.
The model was first used to test viscoelastic liquids to validate the hypothesis that elasticity can contribute to a smoother bolus flow in the transition between the oral and the
pharyngeal phase of swallowing. In this respect, the experimental results confirmed the
effectiveness of thin viscoelastic liquids. These consistently led to a measurable reduction
in bolus fragmentation at the ejection from the in vitro oral cavity without however significantly delaying the overall oral transit time. Conversely, model thick elastic liquids
noticeably increased the measured oral transit times and led to an increased quantity of
post-swallow residues. These results suggests the existence of an optimum range of rheological properties in vitro to secure the best balance between bolus fragmentation and
bolus velocity. This finding, if confirmed in vivo could help designing novel products with
elastic properties for a better management of swallowing disorders.
Attention was then dedicated to the study of the alterations to the in vitro swallowing
dynamics resulting from the presence of suspended tablets and multiparticulates. This
topic was developed in relation to the peculiar needs of paediatric and geriatric populations
that require a high dose flexibility and often show a reduced acceptance towards large solid
oral dosage forms. Based on the theoretical results reported for simpler peristaltic flow, the
study aimed at quantifying the additional pumping effort required for swallowing single
and multiple tablets as a function of their physical attributes. in vitro tests confirmed
that model-tablets with a larger cross section in the direction of swallowing consistently
delayed the bolus flow. The viscosity of the suspending vehicle was also an important
factor: thicker liquid media were able to ensure a smoother flow in vitro, even for large
solid oral dosage forms. This finding confirms the effectiveness of thickened liquids as
suspending vehicles for the oral administration of tablets and capsules.
The importance of the suspending vehicle rheology was further highlighted with a combined in vitro and sensory study that considered the swallow-ability of placebo multiparticulates. The in vitro model provided a higher discrimination ability among the different
formulations. This helped to clarify the results obtained from the 30 untrained healthy
volunteers recruited for the sensory tests. Whilst confirming its utility, the study also
pointed out some of the limitations of the in vitro model that consistently over-predicted
the viscosity for smooth swallowing, compared to the in vivo data. This led to the development of a novel experimental setup, inspired by the functionality of the tongue, and
capable of tackling a significant number of the limitations observed for previous in vitro
studies of swallowing. This three dimensional model, can handle liquid boli spanning a
wide range of consistencies and the soft robotic actuation can also be tailored to provide
insights on the role of poor tongue coordination. The availability of this model allows to
greatly extend the kinematic and dynamic comparison with clinical data and can allow
for a more in-depth investigation of the role of oral lubrication in the bolus transport.