The Experimental Investigation of Buoyant Flows in Inclined Differentially Heated Cavities

Abstract

ABSTRACT:Buoyant flows are present in nature and also in many engineering applications,from domestic heating to the cooling of nuclear power plants. This experimental studyfocuses on the effects of angle of inclination on buoyancy-driven flows inside tall,rectangular, differentially-heated cavities. The objective is to produce detailed localflow and thermal data, which will advance our understanding of the flow physics andalso provide CFD validation data. It considers a 2.18m × 0.52m × 0.0762m cavity,resulting in an aspect ratio of 28.6, with its two opposing long walls maintained atconstant but different temperatures, while all the remaining walls are thermallyinsulated. The Rayleigh number, based on the temperature difference and spacing of thelong sides, is 0.86 x 106 for most cases and the working fluid is air (Prandtl number0.71).Experimental data for the flow and the thermal fields, using laser Doppleranemometry and Chromel-Alumel thermocouple traverses respectively, are presentedfor the cavity inclined at 60o and 15o to the horizontal, for both stable (the hot surfacebeing the upper surface) and unstable (the hot surface the lower one) orientations. The15o stable case is investigated at a higher Rayleigh number of 1.54 x106 and someadditional data for the 15o unstable case are also presented at this high value of Rayleighnumber. Comparisons with the measurements of Betts and Bokhari [1], for the samecavity at the vertical position, are also included.For moderate angles of inclination, the flow is two-dimensional and the effectsof inclination are primarily confined to the fluctuating fields. For large angles ofinclination, the flow becomes three-dimensional. In the unstable 15o angle of inclinationcase, a set of four longitudinal vortices are formed over the entire length of the cavity,with four counter-rotating re-circulation cells within the cross-section parallel to thethermally active walls. The enhanced mixing at 15o unstable inclination leads touniform temperature in the cavity core and thus only minor deviations from twodimensionalityin the thermal field. A modest rise in Rayleigh number, in the 15ounstable case, does not affect the mean motion, but causes an increase in the normalisedturbulence intensities, which in turn leads to a more uniform temperature within thecavity core and a practically two-dimensional thermal field. The stable 15o angle ofinclination case, surprisingly, leads to the formation of two longitudinal vortices andtwo re-circulation cells. The lack of mixing, in the 15o stable case, leads to morenoticeable three-dimensional thermal field. The thesis includes a full set of flow andthermal predictions and also spectral analysis of thermal fluctuations, which show asignificant effect of the angle of inclination on both the power density level and theranges of frequencies involved.

Date of Award	1 Aug 2011
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Hector Iacovides (Supervisor), Dennis Cooper (Supervisor) & Timothy Craft (Supervisor)