Evaporator, Microchannel heat exchanger, Refrigerant distribution, Infrared Thermography
Refrigerant distribution in the parallel tubes of a microchannel evaporator significantly affects its heat transfer performance, which can further affect the coefficient of performance of the whole air-conditioning or refrigeration system. This paper proposes an easy-to-implement quantification method using infrared thermography for the liquid refrigerant distribution in microchannel evaporators to update the original method developed by Li and Hrnjak (2015). Before the detailed discussion of the new quantification method, the effect of surface emissivity on the infrared thermography is investigated, and the calibration process of the infrared thermography is presented for a microchannel heat exchanger sample. Then, the updated quantification method is introduced in detail. The ε-NTU approach is clarified for the formula derivation. A new mathematical method is introduced for the determination of the transition between the two-phase region and the single-phase region. A facility with pump-driven two-phase refrigerant R134a has been built to demonstrate the updated quantification method for the liquid refrigerant distribution in a microchannel evaporator with vertical parallel tubes. The tests have been run at the conditions of 41.7 g/s refrigerant flow rate and 5 oC evaporation temperature with the evaporator inlet vapor quality of 0.15 and 0.25, respectively. The infrared images and the reduced liquid refrigerant mass flow rate distributions are presented to demonstrate the effectiveness of the updated quantification method.