Experimental and numerical investigation of turbulent convective heat transfer deterioration of supercritical water in vertical tube

摘要

The heat transfer phenomenon at supercritical pressure is different from the normal pressure due to the strong variation of thermal physical properties near the pseudo-critical point. The strong change of density leads to a great buoyancy effect at low flow rate, and flow acceleration at high flow rate, both of which influence heat transfer characteristics significantly. In this paper an experimental study is performed to analysis the heat transfer behavior of supercritical water. The test section is a vertically 10-mm ID tube with uniformly heating. Heat transfer deterioration phenomenon caused by the buoyancy effect and the acceleration effect is identified. At the same time, the heat transfer character of upward and downward flow is numerical studied by an in-house CFD- code developed by Shanghai Jiao Tong University. An improved k-ε-k_t-ε_t model is proposed and installed in this code. The wall temperature, especially near the heat transfer deterioration, is well predicted by this new model, compared with the traditional turbulence models. It is shown that the buoyancy effect causes shear stress and radial flow velocity redistribution which leads to heat transfer deterioration and recovery. Moreover, buoyancy can influence the turbulent kinetic energy dramatically, which will affect heat transfer capacity. In the downward flow case with high flow rate, the effect of buoyancy on heat transfer is not so obvious. However, as found in numerical study, the heat transfer deterioration caused by flow acceleration can occur both in upward and downward flow.