Abstract:

The convective heat transfer characteristics of supercritical CO₂ were investigated experimentally in different heated mini-channels under horizontal flow condition. The results showed that the convective heat transfer coefficients of supercritical CO₂ increased significantly with the increase of mass flow rate. But the convective heat transfer coefficients decreased obviously with the increase of heating power and inlet temperature. However, the trends of convective heat transfer coefficient under the pressure condition were totally different. When the fluid temperature was below the pseudo-critical temperature, the convective heat transfer coefficient of supercritical CO₂ increased significantly with the loss of pressure. If the fluid temperature was higher than the pseudo-critical temperature, the convective heat transfer coefficient of supercritical CO₂ increased significantly with the increase of pressure. The convection heat transfer coefficient of supercritical CO₂ significantly rose with the reduction of diameter of mini-channel when the different experimental parameters (pressure, mass flow rate, heating power, and inlet temperature) held constant, respectively. Compared with the convective heat transfer coefficient when the diameter of mini-channel was 1mm, the variation of different experimental parameters had no significant influence on the minimum and maximum of relative heat transfer enhancement rates of supercritical CO₂ when the diameters of heated tube were 0.75mm and 0.5mm, respectively. If the buoyancy effect would be ignored in different heated mini-channels. The variation trend of convective heat transfer coefficient could be explained reasonably by the drastic change of thermal physical properties of supercritical CO₂ near the pseudo-critical region.

Key words: supercritical, horizontal flow, different tube diameter, convection heat transfer, buoyancy

摘要：

针对水平流动方向上超临界CO₂在不同小流道加热管内的对流换热特性进行了详细实验研究。结果表明，当加热管的管径尺寸保持恒定时，超临界CO₂的对流换热系数均随着质量流量的增大而显著增大，但随着加热功率的增大以及进口温度的升高而明显减小。然而，超临界CO₂的对流换热系数随着压力的变化趋势却有所不同。当流体温度低于假临界温度时，超临界CO₂的对流换热系数随着压力的降低而显著增大；当流体温度高于假临界温度时，超临界CO₂的对流换热系数随着压力的升高而显著增大。不同实验参数（压力、质量流量、加热功率以及进口温度）独自保持恒定时，超临界CO₂的对流换热系数均随着管径尺寸的减小而显著增大。相较于管径尺寸为1mm时的对流换热系数而言，不同实验参数的独立变化对超临界CO₂在管径尺寸分别为0.75mm以及0.5mm时的最小和最大相对传热增强率并无显著影响。若忽略不同小流道加热管内浮升力效应的影响，对流换热系数的变化规律可通过超临界CO₂的热物理性质在其假临界区域发生剧烈变化的现象进行合理解释。

关键词: 超临界, 水平流动, 不同管径, 对流传热, 浮升力