Abstract:

To obtain the heat transfer law of supercritical CO₂ under high heat flow and low flow rate, the experimental study on convective heat transfer of supercritical CO₂ in a small circular tube with an inner diameter of 2 mm was carried out, and the effects of variable properties, buoyancy and thermal acceleration were discussed. The operating parameters were as follows: system pressure p = 7.6—8.4 MPa, mass flux G = 400—500 kg/(m²?s), heat flux q = 0—200 kW/m², fluid temperature T _b = 20—62℃, and Reynolds number Re = 1.23×10⁴—4.3×10⁴. The dimensionless parameters of Gr/Re ² and Kv were adopted to represent the buoyancy effect and thermal acceleration, respectively. The results show that the buoyancy effect is significant (Gr/Re ² > 10^-3) under high heat flux and low mass flux conditions, and the heat transfer coefficients at top wall are always lower than those at bottom wall for a certain section. The buoyancy effect is the main factor that cause heat transfer deteriorate, while the thermal acceleration criterion is insignificant (Kv < 8.5×10^-7) and thus can be neglected. The experimental data were compared with typical heat transfer empirical correlations, in which the Liao-Zhao correlation fit well with the experimental data.

Key words: supercritical CO₂, heat transfer, flow, buoyancy, empirical correlation

摘要：

为获取高热流、低流速条件下超临界CO₂的传热规律，开展了超临界CO₂在内径2 mm水平小圆管内对流传热试验研究，并重点探讨了变物性、浮升力和热加速等效应对传热过程的影响。试验参数范围：系统压力7.6~8.4 MPa，质量流速400~500 kg/(m²?s)，热通量0~200 kW/m²，流体温度20~60℃，Reynolds数1.2×10⁴~4.3×10⁴。分别采用Gr/Re ²和Kv作为浮升力效应和热加速效应的判别因子。结果显示，在高热流低流速工况下，浮升力效应显著（Gr/Re ² > 10^-3），同一个截面处的上壁面传热系数始终小于下壁面传热系数。浮升力效应是高热流低流速工况下传热恶化的主要诱发因素。试验中热加速因子较小（Kv < 8.5×10^-7），其效应可以忽略。将试验数据与典型的传热经验关联式作对比，结果表明Liao-Zhao关联式的计算结果与试验结果最吻合。

关键词: 超临界二氧化碳, 传热, 流动, 浮升力, 关联式