Heat Transfer Characteristics of Carbon Dioxide at Super-critical Pressure in Minutia-channels

对管内层流强制对流冷却过程中二氧化碳在超临界压力下的传热系数和压降特性进行了数值研究，得出以下结论：(1)由于物理性质的变化，轴向速度和温度的径向分布在轴向上不断发生变形。(2)局部换热系数在体温接近拟临界温度时出现峰值。(3)平均换热系数在平均体温接近拟临界时达到最大值。当系统压力接近临界压力时，平均换热系数的最大值增大。(4)在接近临界压力的情况下，随着平均体温接近准临界温度，总压降略有增大。动量变化占总压降的比例随着热量的增加而增加。实验研究了超临界二氧化碳在水平微通道内冷却条件下的局部和平均换热和压降特性。在长500毫米、宽20毫米、厚2毫米的铝制测试截面上，有10个内径为1.31毫米的多孔挤压圆形通道，在7.4至8.5MPa的压力范围、22至53℃的温度范围、0.8至9 kW的热流密度m^<-2>s，以及113.7至418.6kg m^<-2>s的质量速度范围内进行了测试。主要结论如下：(1)在近临界区域换热显著增强，在相应的伪临界温度处换热系数最大；(2)系统压力、质量速度和CO_2温度对换热性能都有显著影响，尤其是在近临界区域。最大换热系数随压力的增大而减小。质量速度越大，传热系数越高。(3)文献中可用的相关性偏离实验数据。基于实验数据，建立了超临界二氧化碳在冷却条件下水平微通道内强制强制对流换热的Dittus-Boelter关系式。少

Heat transfer coefficient and pressure drop characteristics of carbon dioxide at supercritical pressure were investigated numerically for the in-tube laminar forced convective cooling process and the following conclusions can be drawn : (1) The radial distributions of axial velocity and temperature deform continuously in the axial direction due to the change of physical properties. (2) The local heat transfer coefficients show peak values as the bulk temperatures approach the pseudo-critical temperature. (3) The average heat transfer coefficient shows a maximum value when the average bulk temperature is nearly pseudo-critical. The maximum value of average heat transfer coefficient increases when the system pressure approaches the critical pressure. (4) The total pressure drop increases slightly as the average bulk temperature approaches to pseudo-critical temperature for the case of near critical pressure. The proportion of momentum change to total pressure drop increases as the heat f … More lux increases.Both the local and average heat transfer and pressure drop characteristics of supercritical carbon dioxide in horizontal mini channels in a cooling condition were investigated experimentally. An aluminum test section (500 mm long, 20mm wide and 2mm thick), which has 10 multi-port extruded circular channels with each 1.31mm inner diameter, was tested under pressures ranging from 7.4 to 8.5MPa, temperatures from 22 to 53℃, heat fluxes from 0.8 to 9 kW m^<-2>, and mass velocities from 113.7 to 418.6kg m^<-2>s. The main conclusions are as follows : (1) Heat transfer is enhanced greatly in the near-critical region, with a maximum heat transfer coefficient obtained at the corresponding pseudocritical temperature. (2) The system pressure, the mass velocity, and the temperature of CO_2 all have significant effects on heat transfer performance, especially in the near-critical region. The maximum heat transfer coefficient decreases as the pressure increases. The larger the mass velocity, the higher the heat transfer coefficient. (3) Correlations available in the literature deviate the experimental data. Based on the experimental data, a new Dittus-Boelter type correlation was developed for forced forced convective heat transfer of supercritical carbon dioxide in horizontal mini channels in cooling case. Less