Numerical study of R134a liquid-vapor flow in a vertical header for phase separation with low inlet quality

Abstract

• CFD simulation is conducted in an intermediate header for flow separation in a microchannel condenser. • The 3-D CFD model is validated by experiments and compared to a 1-D model. • The liquid separation efficiency decreases with higher vapor separation efficiency. • The inlet tube intrusions make the other half of the header the “free-flow” region. • Void fraction and pressure profiles are also presented in the header. The separation circuitry has been proven in the past to improve the performance of microchannel condensers. In the vertical second header of the condenser, liquid separates from vapor mainly due to gravity. However, separation is usually not perfect, expressed through the separation efficiency. This study presents the phase separation result in the second header calculated by the Euler-Euler method of Computational Fluid Dynamics (CFD). Simulations are conducted for two-phase refrigerant R134a flow in the second header with 21 microchannel tubes in the 1st pass. The inlet mass flux to the second header (through the microchannels of the 1st pass) in the simulation is 166 kg m − 2 s − 1 , 207 kg m − 2 s − 1 , and 311 kg m − 2 s − 1 . The inlet quality is 0.13 to 0.21. The results agree well with the experimental results with flow visualization and the results of a simpler 1-D numerical model. Results show that the liquid separation efficiency decreases as the vapor separation efficiency increases, following a linear trend in the experimental range. The void fraction result shows liquid mainly flows in the half of the header without microchannel tube intrusions. The velocity profile in the header is presented and reverse flow is identified on the exit planes of the inlet section connecting to the 2nd-upper pass and the 2nd-lower pass. The pressure profile in the header is also revealed and it indicates that the 1-D pressure assumption may still apply to two-phase flow in a header.