Investigation of two-phase heat transfer coefficients of cryogenic nitrogen in 160-μm and 65-μm microchannels

In this study, the two-phase microheat transfer characteristics of cryogenic two-phase nitrogen were investigated. This basic information is required for the sequential design of mixed-refrigerant microcryocoolers. The effective heat exchanger must be designed with a hydraulic diameter (D_h) of less than 100 µm. The experimental setup was developed using microchannels with D_h values of 160 µm and 65 µm. The two-phase heat transfer coefficient was evaluated under different qualities (x), mass fluxes (G = 50–150 kg/m²s), and heat fluxes (q’’=5–15 kW/m²). The two-phase heat transfer coefficient values ranged from 5000 to 20000 W/m²K. The measured heat transfer coefficient values were compared with the predicted values from existing heat transfer correlations derived by Liu and Winterton. The mean percentage error (MPE) was as low as 25.6 %. Moreover, the correlation derived by Karayiannis and Mahmoud, which was derived for ambient-temperature applications, was found suitable for predicting previous cryogenic microchannel literature data (D_h: 300–10000 µm). When applying these findings to mixed refrigerants in microchannels, it is crucial to account for the reduced influence of mass flux on nucleate boiling and the dominance of thin-film evaporation under varying heat flux conditions in confined microscale geometries.