Design study of microchannel heat exchanger headers using experimentally validated multiphase flow CFD simulation

Microchannel heat exchangers (MCHXs) are compact, light-weight, and are capable of providing high heat transfer performance. However, one of the major challenges with MCHXs is the maldistribution of two-phase refrigerant from the header into the microchannel tubes. This issue is especially prevalent at the inlet header in evaporator applications, which has two-phase refrigerant getting distributed into the MCHX tubes. Recent advances in multiphase flow CFD have made it possible to visualize and study the detailed multiphase flow behavior inside the MCHX header. The Hybrid Eulerian Multiphase (Hybrid EMP) model is used in this paper to perform a two-phase flow simulation of the refrigerant distribution. This model has the combined benefits of both the Conventional EMP model and the Volume of Fluid (VOF) model, where the former detects the dispersed phases while the latter captures the phase separation interfaces. The simulation domain is divided into 3 regimes. Two of the regimes use dispersed phase modeling, with one phase dispersed in another and vice versa. The third regime employs a large-scale interface approach to capture the separation between the two phases. The application of the Hybrid EMP approach is discussed in this paper, thoroughly explaining the developed CFD modeling including a sensitivity analysis of the sub-models and parameters. The model is experimentally validated using thermal infrared images. The flow behavior is discussed and the causes responsible for the maldistribution are identified. A design study is carried out for two types of vertical headers- a Tube-Insertion header and a Loop header, which has a loop added to the former type. The Loop header is found to show superior distribution performance. To understand the effects of the parameters critical to improving the distribution, a sensitivity analysis is performed for parameters like the tube-insertion depth, the inlet mass flow rate and the header loop dimensions. Overall, this paper is aimed at providing a comprehensive simulation approach of the header maldistribution problem in MCHXs.