New insights in twin screw expander performance for small scale ORC systems from 3D CFD analysis

Abstract

Twin screw expanders are widely used for power generation in small scale Organic Rankine Cycles (ORC). In order to increase the efficiency and maximize the generated power, an optimized design and appropriate operating conditions should be used. The use of Computational Fluid Dynamics (CFD) allows the analysis of the flow inside these machines which is impossible to investigate experimentally but which is influencing the performance of the expander. Some of the challenges when performing CFD analysis in these machines are the complexity of the rotors' motion and the properties of the refrigerant. In this paper a 3D CFD analysis of a twin screw expander is presented. The 3D blockstructured grid for the twin screw expander is constructed from the solution of Laplace problems in two-dimensional sections on an unstructured grid of the same geometry. During the transient calculations, grid nodes are moved while keeping the mesh topology. The properties of the refrigerant R245fa have been evaluated using the ideal gas Equation of State (EoS), the Aungier Redlich-Kwong EoS and the CoolProp. 3D CFD analysis of the screw expander showed that the difference in power output between the ideal gas EoS and the Aungier Redlich-Kwong EoS is 8% and between Aungier Redlich-Kwong EoS and CoolProp is negligible for operating conditions of interest. To investigate the performance of the expander, different pressure ratios and rotational speeds were studied for two different designs of the twin screw expander. The flow analysis inside the clearances that are forming leakage paths gives more insight in the performance of the expander. It is concluded that the biggest pressure drop is caused by a throttling loss at the inlet port and therefore an optimized design of the inlet port is necessary.

Development of a thermodynamic low order model for a twin screw expander with emphasis on pulsations in the inlet pipe

“…• The heat transfer between the working fluid and the rotor and between the casing and the ambient is not included 105 in the model, since it was also not included in the CFD model in [19]. This simplification resulted in good agreement between CFD and experiments in [22] for a compressor.…”

“…• If the chamber is connected to the outlet port, the pressure in the working chamber becomes equal to the discharge pressure.This assumption will affect the results (since it assumes that the pressure in the outlet port is constant) but this influence is small since the pressure in the outlet port is not affected by high pressure pulsations as is the case in the inlet port shown by CFD calculations in [19].…”

“…The validation of the Aungier Redlich-Kwong EoS for the range of operating conditions used in this study is presented in [19]. …”

“…For this the authors use the results of 3D CFD analysis of the twin screw expander presented in [19].…”

“…Mass flow rates through the clearances that are forming leakage paths have a significant effect on the performance of screw machines. Three different clearance gaps were evaluated in the CFD analysis performed by the authors in [19] namely the tip, the sealing and the blowhole leakage. These leakage paths are included in the developed 90 mathematical model and are shown in figure 1.…”