Numerical simulation of two-phase flow in the second header of MAC condenser

Phase separation circuiting have been proved in the past to effectively improve the performance of mobile air conditioning (MAC) condensers. In the vertical second header of the condenser, liquid separates from vapor mainly due to gravity, leaving vapor-rich flow with higher heat transfer coefficient to go into the upper passes. The condenser effectiveness is improved in this way. However, separation is usually not perfect, expressed through the separation efficiency (η_l and η_v). This paper presents the numerical study of phase separation phenomena in the second header. The Euler-Euler method of Computational Fluid Dynamics (CFD) is used. Simulations are conducted for two-phase refrigerant R-134a for MAC application. Inlet mass flow rate is simulated at values of 16 g-s^-1, 20 g-s^-1, and 30 g-s^-1 for 21 inlet microchannel tubes, which is the same 1^st-pass tube number as of a real separation condenser. Corresponding mass fluxes are 166 kg-m^-2-s^-1, 207 kg-m^-2-s^-1, and 311 kg-m^-2-s^-1. Inlet quality is simulated to be 0.16 to 0.21, which was covered by past flow separation efficiency experiments. The CFD simulation is compared to the experimental results with flow visualization and the results of a simple mechanistic model based on empirical correlations. Results agree well with each other, which provides a backup for the physical understanding of two-phase flow inside the second header. Conclusion can be drawn that liquid separation efficiency η_l decreases with increasing η_v following a step-wise decreasing trend. Pressure and streamline profiles inside the header are revealed. Reversed flow can be seen on the cross section of the header.