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
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 PAB. 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
© 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.
© 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.
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.
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.
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.
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.
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.
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.
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.
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
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.
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.
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.
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.
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%.
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.