螺旋管内超临界CO_2流动方向对换热的影响

Flow direction effect on heat transfer of supercritical CO_2 in helically coiled tube

应用重正化群（RNG）,Pc湍流模型对超临界CO2流体在内径为9mm，有效受热长度为5．5m，节距为32mm，绕径为283mm的竖直螺旋管中的加热过程开展了数值模拟。研究了质量流速、进口压力、热通量以及不同流向对超临界COz换热和压降的影响，并进一步分析变物性、浮升力和离心力在不同流动方向上对螺旋管中换热的耦合作用。结果表明：浮升力对超临界流体在螺旋管向上流动与向下流动的影响差别不大，而对水平流动的影响较大尤其是当流体在螺旋管的截面到人口截面的距离与管径之比为150～350之间（即临界温度附近）时，由于变物性、浮升力和离心力的耦合作用，导致水平流动方向上换热系数的震荡。

Numerical analysis on heat transfer of supercritical 002 in heated vertical helically coiled tubes is performed byrenormalization group （RNG） k-E turbulent model. The tube diameter, effective heated length, tube pitch, curvature diame-ter are 9 mm, 5.5 m, 32 mm and 283 mm, respectively. The influences of the mass flux, inlet pressure, heat flux and flowdirection on heat transfer and pressure drop of supercritical CO2 are studied. Furthermore, the combined effects of variablephysical properties, buoyancy and centrifugal force in different flow directions of helically coiled tubes on heat transfer areanalyzed. It is found that the influence of the buoyancy on the supercrJtical fluid flow up and down in helically coiled tubes isnot significant, while the influence of the buoyancy in the horizontal direction is great, especially when the ratio of the dis-tance which a cross-section of helically coiled tube to the entrance section and the pipe diameteris between 150 and 350（near the critical temperature）. The variable physical properties, the coupling effect of buoyancy and centrifugal force causethe fluctuation of the horizontal heat transfer coefficient.