摘要：

应用重正化群（RNG） k-ε 湍流模型对超临界CO ₂ 流体在内径为9 mm，有效受热长度为5.5 m，节距为32 mm，绕径为283 mm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CO ₂ 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入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。

Abstract:

Numerical analysis on heat transfer of supercritical CO ₂ in heated vertical helically coiled tubes is performed by renormalization group (RNG) k - ε turbulent model. The tube diameter, effective heated length, tube pitch, curvature diameter are 9 mm, 5.5 m, 32 mm and 283 mm, respectively. The influences of the mass flux, inlet pressure, heat flux and flow direction on heat transfer and pressure drop of supercritical CO ₂ are studied. Furthermore, the combined effects of variable physical properties, buoyancy and centrifugal force in different flow directions of helically coiled tubes on heat transfer are analyzed. It is found that the influence of the buoyancy on the supercritical fluid flow up and down in helically coiled tubes is not significant, while the influence of the buoyancy in the horizontal direction is great, especially when the ratio of the distance 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 cause the fluctuation of the horizontal heat transfer coefficient.

Key words: turbulent model, numerical analysis, supercritical fluids, heat transfer, buoyancy, centrifugal force, helically coiled tube

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