Natural convection in a horizontal concentric cylindrical annulus with the outer wall temperature higher than the inner wall temperature has received wide engineering applications due to its good thermal insulation effect.The thermal insulation and cooling effect of the horizontal concentric cylindrical annuli is greatly affected by the fluid flow and heat transfer in the annuli,and there is currently a lack of research on the natural convection heat transfer characteristics in the horizontal concentric cylindrical annuli with the outer wall temperature higher than the inner wall temperature.A numerical study of natural convection and heat transfer in horizontal concentric cylindrical annuli with the outer wall temperature higher than the inner wall temperature is performed to investigate the gap ratio and Rayleigh number effect on flow and heat transfer characteristics of air(Pr=0.7).The gap ratio(a ratio of the inner diameter to the gap width of the annulus,namely δ)in the range of 0.1 to 14 and the Rayleigh number over several orders of magnitude(103 ≤Ra≤ 107)are considered.The computed flow patterns and temperature fields are shown by means of streamlines and isotherms,respectively,and the effect of gap ratio and Rayleigh number on the average Nusselt number of the inner wall of a cylindrical annulus is also presented.The results show that a two-dimensional stable unicellular flow pattern occurs at low Rayleigh numbers throughout the whole range of gap ratios,and four different flow patterns are also observed,which are a two-dimensional funnel oscillatory flow,a three-dimensional spiral flow,a two-dimensional multicellular flow and a two-dimensional multicellular oscillatory flow.These four flow patterns appear at relatively high Rayleigh numbers depending on the gap ratio.According to the distribution characteristics of flow patterns,the gap ratio can be divided into three parts,namely,wide gap(δ=0.1-2),moderate gap(δ=2-8)and narrow gap(δ=8-14).In each part,correlating equations of the average Nusselt number to the gap ratio and the Rayleigh number are proposed with the coefficient of determination(R2)greater than 0.990 2.
horizontal concentric cylindrical annulinumerical simulationnatural convectionflow patternsheat transfer
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