Experimental and analytical study of heat transfer and mixing in thermally stratified buoyant flows

Abstract

Unsteady natural convection in a nonuniformly stratified, finite depth, layer of water heated from below has been studied. Laboratory experiments were performed using a Mach-Zehnder interferometer as a diagnostic tool for measuring temperature and a shadowgraph technique for flow visualization. A 1-dim. model in conjunction with a two-differential equation k- ϵ turbulence model is used to predict the dynamics of the mixed layer which develops when the thermally stratified fluid is heated from below. Good agreement between data and predictions have been obtained using the model. However, the entrainment processes at the interface between the mixed layer and the stable region as well as turbulence in the interfacial layer must be better understood for more realistic modeling of turbulent natural convection in nonuniformly stratified fluids. Unsteady natural convection heat transfer and mixing which occur when either a thermally stratified layer of liquid is cooled by air flow over the free surface or a layer of liquid is simultaneously heated by an external radiation source and cooled by air flow over the free surface has been studied both experimentally and analytically. Temperature measurements were made using a Mach-Zehnder interferometer, thermocouples and a thermistor. The flow structure in the fluid was visualized using a black-blue dye and “fish-scales” as tracers, and also by shadowgraphic techniques. An unsteady 1-dim. model has been developed for predicting the temperature structure in water irradiated by an external source and simultaneously cooled by air flow over the surface. Turbulent mixing was calculated using the k- ϵ model of turbulence. The results obtained show that deposition of radiation in water plays an important role on buoyancy and wind shear induced mixing processes in the surface layers during simultaneous heating by radiation and cooling by convection. The mixed layer depth was ovcrprcdicted by the turbulence model because it docs not adequately simulate the entrainment process in the intcrfacial layer between the mixed and stable regions.