Computational Investigation of a Multiphase Turbo Expander for Heat Pumps and Refrigeration Cycles

The main goal of this study is to design and develop an innovative turbo expander for two-phase fluids, in order to substitute the lamination valve in the Heat Pump (HP) inverse cycle. Thereby enhancing the overall performance by recovering mechanical work to reduce the HP compressor requirements. Major challenge in such configurations, is the reliable operation of the expander when a phase change occurs across it, from a purely liquid flow to a mainly vapor -by volume- flow with liquid droplets.

To investigate the phase change, a modelling approach is adopted which is routinely applied to fuel-flash modelling, where the phase change deviates strongly from equilibrium. The Homogeneous Relaxation Model (HRM) is employed, which follows an Eulerian approach. Following this computational approach, the detailed flow profile in the proposed Tesla turbo-expander is investigated. Two different expander designs are considered, one working with water and the other with butane (R600). The expander performance is evaluated for rotational speeds up to 16’000 RPM.

The proposed computational model is firstly validated against experimental results available in the literature. A sensitivity analysis of the phase change model relaxation parameter is performed, to assess its impact on the final solution. While the relaxation time constant decreases, the phase change process initiates further upstream, producing closer agreement to the experiments.

Results on the turbo-expander under investigation, showed that the presence of a dense cloud of liquid droplets produces a significant pressure drop on the turbine rotor, postponing the phase change, especially at high RPM. High volume-fraction of liquid was predicted to penetrate deeper inside the rotor above 2’000 RPM for the water and above 16’000 RPM for the butane expander. The resulting lower liquid flow velocities at the inlet of the rotor are expected to significantly degrade the performance of the turbine. Finally, liquid droplet diameter was also found to impact this behavior. Larger droplets were found to be more strongly influenced by the centrifugal forces generated in the rotor, concentrating the liquid fraction and increasing the back pressure on the nozzle. Thus it was determined from this study that critical point is to remove as much liquid as possible from the flow before it enters the rotor.