Fundamental understanding of phase change mechanism of refrigerant flow in HVAC pipe

Few years ago having an AC in vehicle was considered as a luxury, but these days AC comes as a standard fitment in all the vehicles. With the increasing competition, auto OEMs constantly striving to provide better comfort to passengers which has led to a lot of development in vehicle HVAC systems. Any unwanted noise created due to refrigerant fluid inside HVAC pipe can be annoying and also lead to increasing owner complaints. Predicting noise caused by refrigerant flow through HVAC pipes is more challenging and a time consuming process. This becomes altogether more important for OEM’s to predict this source of noise at very early stage of vehicle development. In this present investigation an attempt has been made to simulate the refrigerant flow through pipes using Computational Fluid Dynamics to observe liquid refrigerant R134a flashing phenomenon using multi-phase model in ANSYS Fluent. In a vehicle HVAC piping system the refrigerant flows under a certain operating condition and pipe packaging. When the vehicle is kept in idle condition there is a possibility that a local pressure drop may occur due to change in pipe configuration or change in operating conditions. This may lead to phase change and it can be one of the factor which causes noise and vibrations in the refrigerant pipe. A capillary tube having a pressure drop across the pipe has been considered in current CFD simulation. A two phase Eulerian- Eulerian multiphase flow is used as it is better suited to capture a thermal phase change phenomenon. Fluid properties have been assumed to vary as piecewise linear. A good correlation was obtained with available experimental data.