AN EXPERIMENTAL STUDY OF TRANSITION TO TURBULENCE IN VERTICAL MIXED CONVECTION FLOWS AND ANALYSES OF LAMINAR MIXED CONVECTION IN PLUMES AND IN WALL PLUMES (HEAT TRANSFER, FLUID MECHANICS, NATURAL).

摘要

Mixed convection or combined forced and natural convection commonly arises in nature and in technological applications. In this dissertation attention is focussed on the effects of small free stream velocities on a natural convection boundary layer. Both the free stream velocity and the buoyancy force are taken to be in the same direction. This configuration is called aiding mixed convection. In Chapters I - III, mixed convection adjacent to a vertical uniform flux surface is considered. Chapters III and IV are on laminar mixed convection from line source plumes and wall plumes, respectively.;In Chapter II, predictive correlations for heat transfer coefficients are given. These are for laminar and turbulent flow regimes. The Nusselt number correlation in each case, is shown to be obtainable by a combination of the correlations for forced and natural convection. The mixed convection parameter is seen to be different in laminar and turbulent mixed convection. However, in each, it is shown to be proportional to the ratio of applicable characteristic velocity scales in forced and natural convection.;In Chapter III, a numerical simulation of variable property effects in laminar mixed convection is presented. Fluids considered are air and water. Box scheme was used in the solution. The results indicate that variable property effects are small, in both the fluids, for the conditions considered.;In Chapter IV, laminar mixed convection from a line source plume is considered. The method of matched asymptotic expansion was used. The effects of non-boundary layer corrections and of mixed convection effects are considered simultaneously. Results indicate that the non-boundary layer corrections contribute significantly.;In Chapter I, an experimental study is reported of transition to turbulence. Quantitative predictive criteria for the beginning and end of transition are given. The data are seen to indicate the enhanced stability of mixed convection. This stabilization is shown to be in accord with the well known point of inflection criterion.;In Chapter V, laminar mixed convection from wall plumes is analyzed. The method of matched asymptotic expansions was used. The results reveal several subtle and interesting features of wall-bounded mixed convection flows. The imposed thermal condition at the surface is shown to exert a strong influence on the wall shear stress.