Abstract
A numerical simulation of global heat transfer process in three types of micro-channel heat exchangers was investigated in this paper: spiral double-pipe micro-channel heat exchanger (DPHE), cross-plate micro-channel heat exchanger (CPHE), and shell and tube micro-channel heat exchanger (STHE). The inner tubes of all three heat exchangers have a diameter of 1 mm and are charged with CO2 as refrigerant. A detailed analysis of the heat exchanger's global heat transfer process was carried out, which is entirely different for different structures. The heat transfer characteristics of supercritical CO2 were analyzed by considering the operating pressure, the refrigerant mass flux, the cooling water mass flux, and the heat exchanger refrigerant inlet temperature. The relationship between the CO2 heat transfer coefficient (hCO2) and CO2 bulk temperature (Tb,CO2) was analyzed in detail. The pseudo-critical temperature (Tpc) mainly determines where the peak CO2 heat transfer coefficient occurs. When Tb,CO2 < Tpc , the rise in Tb,CO2 is accompanied by an increase in the heat transfer coefficient, which reaches a maximum when Tb,CO2 is a little bit higher than the pseudo-critical temperature, the heat transfer coefficient curve begins to decline as Tb,CO2 continues to rise. Higher peak heat transfer coefficients can be achieved at higher pressures. Increased refrigerant mass flux always results in larger heat transfer coefficients. The influence of the refrigerant inlet temperature of the heat exchanger in the Tb,CO2 < Tpc region on the heat transfer coefficient is more significant than expected. In this study, different flow patterns on heat transfer due to different structures were compared. The best heat transfer was achieved using a spiral double-pipe micro-channel heat exchanger (DPHE). It consistently reaches the highest heat transfer and the lowest outlet temperature under the same operating conditions.
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