Effects of Oil and Saturation Temperature on Developing Two-Phase Flow of R134a in an Air-Conditioning System

The structure of two-phase flow plays a key role in the performance of air-conditioning components. Specifically, evaporators with parallel channels are known to suffer from maldistribution of the two refrigerant phases caused by phase separation (Bowers et al., 2006 and Fei et al., 2002). This is known to be influenced by the characteristics of the two-phase flow entering the manifolds or distributors of such heat exchangers. In an effort to understand how these flows develop without the influence of other parameters the authors have investigated the characteristics of this kind of flow development in detail for pure R134a at a saturation temperature of approximately 25 degrees C. (Bowers and Hrnjak, 2008 and Bowers and Hrnjak, 2009) However, in actual air-conditioning systems oil is added to the system to ensure proper lubrication of the compressor and the saturation temperature at the entrance of the evaporator is typically lower. For this reason, this paper presents an initial study of flow development in a real automotive air-conditioning system, showing how some of these characteristics are altered by the change in the above mentioned parameters. This system was designed to study both two-phase flow development between the expansion device and the evaporator using a 700mm long 8.7mm transparent PVC tube between the expansion valve and the evaporator. The condenser and the compressor were commercially available automotive components, the expansion device was an electronically controlled expansion valve, the evaporator was a modified automotive microchannel evaporator, also an internal heat exchanger was added to the system to protect the compressor from liquid slugging. The lubricant used in this system was ND-8, a polyalkylene glycol (PAG) with a viscosity rating of 46 centistokes at 40 degrees C. This oil is miscible with liquid R134a at the temperatures examined in the course of this study.