Unsteady turbulent thermal diffusion of two-dimensional surface discharge of heated water into a rectangular reservoir

Abstract

Thermally stratified unsteady flow caused by two-dimensional surface discharge of warm water into a rectangular reservoir is investigated. The basic objectives are to develop a measurement system for the unsteady flow phenomena, to study the interfacial mixing between a flowing layer of warm water and the underlying body of cold water and to accumulate experimental data to test computational turbulence models. In addition, it is also attempted to prove the uniqueness of the Richardson number as a controlling parameter for the thermal mixing driven by buoyancy. For this purpose, the same Richardson numbers are produced by different combinations of velocity, depth of the warm layer and the temperature difference between the incoming water and that in the reservoir. Mixing process at the thermal interface was visualized with thymol blue solution. Mean velocity field measurement is carried out by using NMR-CT(Nuclear Magnetic Resonance-Computerized Tomography). It detects quantitative flow image of any desired section in any direction of flow in short time. Test images of isothermal steady flow are compared with the velocity measurements by the Laser Doppler Velocimeter (LDV), which are found to be in good agreement. It is also proved that the NMR-CT phase encoded flow imaging technique is a very valuable tool to measure slowly-varying transient flow field. It is found that the Richardson number uniquely determines the integral structure of the flow field, and the buoyancy plays a major role to control the turbulent diffusion process. The results also show that the warm layer penetrates more rapidly into the cold layer at lower Richardson number because of strong turbulent diffusion and decrease of its stability, which is clearly verified by flow visualization using thymol blue solution. For lower Richardson number, surface velocity decays more rapidly along the downstream direction and temperature gradient near the interface is very small. The heat transfer across...