Experimental study on supercritical heat transfer characteristics of CO2/R41 mixture in microchannel

Past researches have shown that CO2/HFC mixtures and microchannel heat exchanger could benefit solving the problems of high operating pressure and relatively low cycle efficiency. In this paper, the supercritical heat transfer characteristics of R41, CO2/R41 (20.5/79.5) and CO2/R41 (51.4/48.6) in square porous flat tube microchannel with the hydraulic diameter of 0.715 mm were experimentally studied. The experimental conditions were as follows: the operating pressure of 6.5-7.5 MPa, the mass flow rate of 150-300 kg/m(2) s, the heat flux of 3-9 kW/m(2), the fluid temperature of 20-90 degrees C. The results show that, similar to pure CO2, heat transfer coefficient (HTC) of mixtures also appears a peak value near the pseudocritical region. At higher operating pressure, the peak value is less and the change of HTC is gentle. Meanwhile, the inflection point temperature is high. When the bulk temperature is less than the inflection point temperature, HTC is higher at low operating pressure. When the bulk temperature is greater than the inflection point temperature, HTC is higher at high operating pressure. Peak value of HTC decreases and the inflection point temperature of mixtures becomes higher when the CO2 mass fraction is low. With the increase of R41 mass fraction, the change of HTC is gentle. The width of the peak distance in the pseudocritical region becomes wider. The model of Petrov and Popov is recommended as the prediction model for CO2/R41 mixtures. This research can serve as a database for the design of CO2/R41 mixtures microchannel heat exchangers.