Numerical Investigation on Deteriorated Heat Transfer of Supercritical Water Flowing Upward in Tubes with Variable Cross-sectional Geometries

To mitigate Deteriorated Heat Transfer (DHT) of Supercritical Water (SCW) upward flow, tubes with variable cross-sectional geometries are numerically investigated. Three types of variable cross-sectional geometries: converging channel, diverging channel, and periodic geometries, are modelled. Results of wall temperature in the smooth channel are compared with experimental data, and good agreements are obtained. The wall temperature effects of convergent and divergent channels of various proportions and periodic geometries of various amplitudes are investigated. Thermo-Hydraulic Performance Evaluation Criterion (PEC) is proposed to evaluate the performance of the studied geometries using dimensionless parameters Nu/Nu0, f/f0, and PEC = (Nu/Nu0)/(f/f0)1/3. The results indicated that the convergent channels suppress and delay DHT downstream, while opposite effects are observed for divergent channels. Periodic geometries, which have alternating convergent and divergent sections, suppress DHT with minimal pressure drop. 1st temperature peak is reduced by 200 K with 1 mm of outlet radius reduction for the convergent channel. Average PEC is enhanced by 60% with 0.5 mm A of periodic geometry. It is discovered that the convergent section can suppress DHT, and the mitigation effect is proportional to the size of convergence. It is concluded that, since the DHT caused by buoyancy-induced flow laminarization has flattened the cross-sectional velocity profile, the convergent section reverts the laminarization to the normal velocity profile by the nozzle acceleration effect. Nozzle acceleration also increases Turbulent Kinetic Energy (TKE) in the boundary layer which further enhances turbulent heat transfer.为缓解超临界水（Supercritical Water，SCW）向上流动时的恶化传热（Deteriorated Heat Transfer，DHT）问题，本文对具有变截面几何结构的换热管开展了数值研究。本文共建模三类变截面几何结构：收敛通道、扩张通道及周期性几何结构。首先将光滑通道的壁面温度计算结果与实验数据进行对比，二者吻合良好。随后分别研究了不同比例的收敛、扩张通道，以及不同振幅的周期性几何结构对壁面温度的影响。本文提出热工水力性能评价准则（Thermo-Hydraulic Performance Evaluation Criterion，PEC），采用无量纲参数努塞尔数比（Nu/Nu₀）、摩擦阻力比（f/f₀）以及PEC=(Nu/Nu₀)/(f/f₀)^(1/3)对所研究的几何结构性能进行评价。结果表明：收敛通道可抑制并延缓下游的恶化传热，而扩张通道则会产生相反效果；兼具交替收敛与扩张段的周期性几何结构可在压降增幅极小的前提下抑制恶化传热。以收敛通道为例，当出口半径缩减1mm时，第一温度峰值可降低200K；当周期性几何结构的振幅A取0.5mm时，平均PEC提升60%。研究发现，收敛段可有效抑制恶化传热，且缓解效果与收敛程度正相关。进一步分析表明，由浮力诱导流动层流化导致的恶化传热会使截面速度分布趋于平坦，而收敛段可通过喷嘴加速效应使流动脱离层流化状态，恢复至正常速度分布；同时，喷嘴加速效应还可提升边界层内的湍流动能（Turbulent Kinetic Energy，TKE），进一步强化湍流传热。