关键词: 超临界CO₂, R41, 混合物, 对流, 换热, 小通道

Abstract:

An experiment was conducted to study the heat transfer characteristics of supercritical pure R41, mixtures of CO₂/R41 (20.5/79.5), and CO₂/R41 (51.4/48.6) cooled in a horizontal smooth mini-channel tube with inner-diameter of 2 mm. Mass flow rate was in the range of 400 to 800 kg·m^-2·s^-1, pressure changed from 6.0 to 8.0 MPa,heat flux ranged from 12 to 48 kW·m^-2 and bulk temperature varied from 20℃ to 80℃. The maximum heat transfer coefficient (α) of the three fluids increased with the CO₂ mass fraction. Additionally, α of pure R41 is smaller than that of the mixtures. However, α variation tendency of the mixtures is the same as that of pure R41. α of the three working fluids are ranging from 2 kW·m^-2·K^-1 to 25 kW·m^-2·K^-1 under the present test condition. The influence of pressure is significant. The closer of the pressure approaches the critical pressure, the higher of the maximum α is. The heat flux has little influence on α when the bulk temperature is far away from the critical temperature. However for the bulk temperature near the critical temperature, α increases with the decrease of heat flux. The experimental results are compared with prediction values calculated by twelve correlations. It is concluded that four correlations predicting with good precisions, and the errors are within ±30%. Nevertheless, the prediction errors increase with CO₂ mass fraction.

Key words: supercritical CO₂, R41, mixture, convection, heat transfer, mini-channel