In this research, a one-way fluid-thermal-solid numerical coupling model of a scroll expander for a waste heat recovery system was developed and used to investigate the deformation of the scroll pair. The pressure and thermal loads were firstly calculated by a CFD model, and the surface pressure and body temperature distributions of the scroll members were used as boundary conditions in the FEM model to obtain the deformation distributions of the scroll parts. Three time instants that may have significant adverse impacts on the maximum forces were selected to determine the most critical time for the occurrence of the maximum deformation of the scroll wraps. The results showed that the deformations induced by inertial force only occurred at the orbiting scroll tail, whereas the deformations in other regions negligible. At the time instants of t/T equaled to 13/15 and 1, the deformations induced by pressure loads had the opposite direction compared to that of the thermal loads and thus the two deformations canceled each other out and the coupling deformations decreased. The deformations induced by pressure loads were less significant than the thermal loads, therefore the coupling deformation was dominated by the thermal loads. The results also confirmed that the critical time at t/T equaled to 7/20 for the occurrence of the largest deformation resulted from the maximum axial forces that were exerted on the fixed scroll.
Scroll expanders are currently attracting interest for integration in small scale organic Rankine cycle (ORC) waste
heat recovery applications and have been subject to significant research over the last two decades. The most common
geometrical design uses a scroll profile generated by the involute of a circle with a constant wall thickness. A major
disadvantage of this approach is that the increase of the geometric expansion ratio is constrained, since it is accompanied
with a large increase in the scroll profile length and is associated with a decreased efficiency. In this paper, the
published literature related to scroll expander geometry is reviewed. Investigations regarding the influence of varying
scroll geometrical parameters on the performance of scroll expanders with a constant wall thickness are first examined.
The use of variable wall thicknesses and their effects on the performance are then considered. Finally, the impact of
scroll expander geometries using unconventional scroll profiles and scroll tip shape variations on the performance is
discussed and summarised. The major conclusion to be drawn from this review is that scroll expanders with variable
wall thickness scrolls should be further designed and developed. It is possible to increase the geometric expansion ratio
without increasing the length of the scroll profiles. CFD simulations are a promising tool to illustrate and understand
the non-uniform and asymmetric inner flow and temperature fields. The related benefits could lead to scroll devices
with variable wall thickness not only improving the performance of organic Rankine cycle (ORC) systems but also
opening a broad new field of applications such as refrigeration cycles and other power cycles where a high pressure
ratio is preferred.
With the increasing pressure of fossil fuel consumption and pollutions from vehicles powered by internal combustion engines, much attention has been attracted for hybrid and electric vehicles. With this background, an increasing demand for compact and high power density engines is being developed for the purpose of hybrid vehicles. In this paper, the design of a novel opposed rotary piston engine was investigated. In comparison with conventional reciprocating engines, this design has no crank connecting rods and intake/exhaust valves, and the operation cycle takes 360° crank angle to complete but similar to a four stroke cycle. 3D and 1D simulations were conducted to analyse the in-cylinder flow and evaluate the engine performance. The simulation results indicated the air velocity was very high at the end of intake stroke due to the lack of intake valves. The opposed rotary piston engine had a higher fraction of constant volumetric combustion that yielded to less heat loss, which contributed to a higher power output per combustion cycle than a reciprocating engine at low engine speed. The estimated minimum brake specific fuel consumption and maximum power density were 240 g/(kW·h) and approximately 80 kW/L, respectively.
The built-in volume ratio of variable wall thickness scroll expanders can be increased without increasing the number of scroll turns and the expander size in contrast to constant wall thickness expanders. CFD models for these novel scroll-type designs are presented in this research paper. The validation, verification and the findings have proven consistency with the theory of small scale ORC scroll expanders. The performance analysis indicates that the optimum performance point was reached at a pressure ratio of 3.5. The decrease of radial clearance from 200¼m to 75¼m had a significant effect on the isentropic efficiency and the specific power output, with the isentropic efficiency significantly increasing from 31.9% up to 53.9%. Based on the second-law analysis, it is found that exergy of 336.5W (75¼m) and 864.2W (200¼m) were destroyed during the expansion processes. Furthermore, characteristic pressure imbalances were observed in the expansion chambers. The studies also reveal that the large-scale vortices, generated during the suction process, were completely dissipated in the expansion chambers at a crank angle of 600°. Analysis of the pressure-volume diagram shows that variable wall thickness scroll expanders with built-in volume ratios above 4.5 could fully expand the working fluid to the defined outlet pressure.
Higher efficiencies and more compact designs in spite of larger expansion ratios are associated with variable wall thickness scroll expander geometries. However, the literature for these innovative scroll designs is mainly limited to theoretical studies since the research and development of scroll expanders is still in the early stages. Considering the potential benefits of less overall leakage areas, shorter residence time of the gas and less time for leakages and heat transfer, the scroll machine with variable wall thickness could be a promising candidate to further improve the efficiency and power output in organic Rankine cycle systems. It has also the opportunity of opening up new application fields such as hybrid vehicle powertrain systems in which power cycles with high pressure ratio are needed. The main aims of this PhD research were to investigate variable and constant wall thickness scroll expanders for small scale organic Rankine cycle systems using three-dimensional and transient CFD simulations. The aerodynamic performances are compared to examine the features of the novel variable wall thickness scroll expander design. This is followed by the investigation of spark-ignition scroll engines and their performance by means of a heat release rate analysis and CFD based combustion modelling tools. The evaluation of the CFD simulations of variable wall thickness scroll expanders reveals that the geometrical effects of varying wall thicknesses did not affect the characteristic scroll machine operation. The validation, verification and the findings had proven consistency with the theory of scroll expanders. The optimum performance was achieved at a pressure ratio of 3.5 regardless of the rotational speed. The decrease of radial clearance from 200¼m to 75¼m had a positive effect on isentropic efficiency and specific power output. The isentropic efficiency at the optimum performance point was significantly improved by 22% from 31.9% to 53.9%. It is also found that the lower number of working chambers resulted in a shorter gas residence time, associated with less time for flank leakages, in comparison to the constant wall thickness scroll expander. Thus, the fluid friction was reduced, converting less kinetic energy into enthalpy. The large-scale swirls were completely dissipated in the expansion chambers of the variable wall thickness scroll expander at the crank angle of 600°, in contrast to 672°in the expansion chambers of the constant wall thickness design. In addition, the shorter scroll profile length of the variable wall thickness scroll expander generated lower average radial and axial gas forces. Moreover, higher pressure gradients between individual working chambers contributed to a higher peak of the tangential gas moment but at the expense of higher transient radial and axial gas force and tangential gas moment variations. More significant pressure drops occurred along the local radial clearance reducing the isentropic efficiencies in spite of the shift towards higher pressure ratio. The heat release rate analysis reveals that a more thorough expansion was achieved by employing the scroll engine in the Miller/Atkinson cycle instead of the conventional Otto cycle. The highest power output of 44.5kW was achieved for a compression ratio of 10.1:1 and an expansion ratio of 17.8:1 (V=4.62dm^3) at a rotational speed of n=3000rpm. The thermal efficiency followed the same trend reaching a peak value of 43.1% but for a lower compression ratio of 8.2:1. The evaluation of the CFD based combustion model results shows that the third combustion cycle was technically not feasible because the entire domain was filled with burned gas due to the lack of flame quenching. No steady-state solution was achieved and all the results are therefore hypothetical.