Modelling refrigerant distribution in minichannel evaporators

This thesis is concerned with numerical modelling of flow distribution in a minichannel evaporator for air-conditioning. The study investigates the impact of non-uniform airflow and non-uniform distribution of the liquid and vapour phases in the inlet manifold on the refrigerant mass flow distribution and on the cooling capacity of the evaporator. A one dimensional, steady state model of a minichannel evaporator is used for the study. An evaporator consisting of two multiport minichannels in parallel is used as a test case and two different refrigerants, R134a and R744 (CO2), are applied in the numerical experiments using the test case evaporator. The results show that the reduction in cooling capacity due to non-uniform airflow and non-uniform liquid and vapour distribution is generally larger when using R134a than when using CO2 as refrigerant. Comparing the capacity reductions with reductions of the area covered by refrigerant in a two-phase condition shows that the capacity decreases significantly more than the two-phase area when imposing a non-uniform airflow. On the other hand the reductions in capacity and in two-phase area are almost equal when imposing a non-uniform distribution of the liquid and vapour in the inlet manifold. Combining non-uniform airflow and non-uniform liquid and vapour distribution shows that a non-uniform airflow distribution to some degree can be compensated by a suitable liquid and vapour distribution. Controlling the superheat out of the individual channels to be equal, results in a cooling capacity very close to the optimum. A sensitivity study considering parameter changes shows that the course of the pressure gradient in the channel is significant, considering the magnitude of the capacity reductions due to non-uniform liquid and vapour distribution and non-uniform airflow. It is found that a large pressure gradient in the first part of the channel is beneficial.