Flow Characterization in the Upper Cavity of a Rotary Compressor

Abstract

As hundreds of millions of Air conditioning (AC) systems are produced each year, and many of them use rotary compressors as the heat pump, optimizing the flow inside the rotary compressor to improve its reliability and efficiency becomes a key issue of the manufactures. Since the invention of the rotary compressor, its internal flow has been studied numerically with real models. However, a rotary compressor’s internal flow can be extremely complicated due to the complex internal structures’ geometry and high-speed moving parts, making it difficult to interpret the result by CFD simulation and repeat the simulation in different models. In our experiments for observing lubricant oil droplets above the rotor/stator in a rotary compressor, droplets’ movement reveals that two major effects control the gas flow in the compressor’s upper cavity. One is the swirling jet produced by the high-speed rotating rotor with no-slip condition on its sidewall. The other one is the rotating disk effect induced by the top of the high-speed rotating rotor. Either of them has been studied individually in different areas. For example, the swirling jet is often used in combustors while the rotating disk is applied in the viscous pump. However, the coupling of these two effects in the rotary compressor with different velocity ranges, size scales, and fluid properties has not been studied according to our best knowledge. In our simulation, a model that only consists of a simplified rotor, simplified stator, sidewall, and discharge tube (outlet) is built. Thus, the effect by small parts, such as the balance block and coils, is excluded. The rotor is set to rotate at 30, 60, and 90 Hz. Uniform velocity calculated with the theoretical flow rate and ambient pressure conditions are given at the inlet (rotor/stator clearance) and outlet, respectively. No-slip conditions are defined at other walls. Steady-state K-ω SST turbulence models are applied, and the cases are computed with OpenFoam. The CFD results show an inner recirculation zone above the rotor that creates a downward velocity component above the rotor and an outer circulation zone above the stator. The CFD result meets the observation of the droplets’ movement above the rotor/stator. With the CFD results and the experiment’s observations, we propose the model of the oil droplet’s path in the rotary compressor’s upper cavity, which can help reduce the exhausted lubricant oil droplets from the compressor.