Natural convection heat transfer enhancement in mercury with gas injection and in the presence of a transverse magnetic field.

摘要

A natural convection heat transfer experiment using a vertical plate held at constant heat flux in mercury with gas injection and a transverse magnetic field was performed. The experiment was conducted with heat fluxes up to 16 kW/m;Measurements at low heat flux, where the flow was mostly laminar, indicated that gas injection enhanced the heat transfer coefficient two to three times. At high heat flux, where the flow was mostly turbulent and strong stratification was present, the observed enhancement was less pronounced. The enhancement mechanism at low heat flux was attributed to the bubbles which populate and induce turbulence inside a thick laminar thermal boundary layer. At high heat flux, however, the bubbles reside mostly outside a thin turbulent thermal boundary layer and act to suppress the stratification.;In the presence of a transverse magnetic field, the single-phase data at low heat flux agreed with the theoretical results of past investigations, while, at high heat flux, the heat transfer coefficient did not decrease as much in contrast with the same theory. This was attributed to the increasing influence of three-dimensional effects in turbulent magneto-fluid-mechanic natural convection. With the addition of gas injection at low heat flux, a magnetic field intensity of 0.07 Tesla decreased the heat transfer coefficient six-fold in contrast to the enhanced value observed with injection. At a field intensity of 0.35 Tesla, the reduction was fifteen-fold. At high heat flux, equivalent field intensities reduced the heat transfer coefficient slightly at 0.07 Tesla and two-fold at 0.35 Tesla. These results were attributed to the suppression of both thermally and bubble-induced fluid motions at low heat flux while mainly the thermally-induced turbulence was suppressed at high heat flux. These trends were further explained in terms of the changes in the bubble size and rise velocity with magnetic field intensity.;The influence of thermal stratification in natural convection was additionally investigated. A new stratified Nusselt number, sNu, incorporating the stratification parameter, S, was introduced through theoretical arguments and shown to successfully correlate both present and past stratified natural convection data.;Some theoretical considerations have also been forwarded to analyze our and other available data of the effect of the aspect ratio in natural convection in an enclosure.