Experimental Study on Heat Transfer Performance of Supercritical CO2-water in Complex Mini-channels

The printed circuit heat exchanger (PCHE) is one of the promising candidates for supercritical CO2 Brayton cycle, its performance is important to the safety and economy of the next generation nuclear technology. In this paper, an experimental study on heat transfer performance of supercritical CO2-water in zigzag mini-channels was conducted to explore the heat transfer characteristics in the pre-cooler channels of a supercritical CO2 Brayton cycle. The experimental pressure of the supercritical CO2 is 7.5-9.0 MPa, the temperature is 50-90 ℃, and the flow rate is 300-600 kg/(m2·s). The test section has a diameter of 2.0 mm, a pitch of 7.24 mm, and a bend angle of 40°. By arranging multiple temperature measurement points, the temperature distribution data along the complex mini channels were obtained. The influence of different operating conditions on the logarithmic mean temperature difference and overall heat transfer coefficient along the mini channels was studied, and the effects of overall method and segmented method on the calculation results were further evaluated. The results show that the overall heat transfer coefficient exhibits a relatively high level near the pseudo-critical region. Meanwhile, due to the pronounced enhancement effect of the buffer layer on the near-critical temperature, the peak value of the overall heat transfer coefficient corresponds to a slightly higher CO2 bulk temperature than the near-critical temperature. Under the given operating conditions, increasing the CO2 inlet temperature has a small effect on the overall heat transfer coefficient of the heat exchanger. With the increase of inlet pressure, the peak value of overall heat transfer coefficient decreases, the peak values of the overall heat transfer coefficient corresponding to 7.5, 8.0, and 9.0 MPa are 3 828.3, 3 466.0, and 3 281.8 W/(m2·℃) respectively. However, the overall trend gradually becomes less sensitive to the changes in bulk temperature and exhibits higher heat transfer capability in the gas-like region. Increasing the inlet mass flow rate can significantly improve the overall heat transfer capacity of the heat exchanger. When the inlet mass flux increases from 311.72 kg/(m2·s) to 495.37 kg/(m2·s), the maximum value of the overall heat transfer coefficient increases from 3 305 W/(m2·℃) to 4 193 W/(m2·℃), which is a 26.87% improvement. Besides, in the heat transfer conditions near the pseudocritical region, the deviation between the overall heat transfer coefficients calculated by the overall method and the segmented method is significant, while in the heat transfer conditions across the pseudocritical region, due to the compensating effect of the s-shaped logarithmic mean temperature difference, the deviation in the calculation results is significantly reduced.