Abstract: Under constant heat flux conditions,an experimental investigation was conducted on heat transfer to carbon dioxide flowing upward through a helically coiled tube with inner diameter 9 mm,coil diameter 283 mm and coil pitch 32 mm.The operating pressure was maintained at 8 MPa for all the tests. The mass flux and heat flux were in the range of 0-650 kg·m^-2·s^-1 and 0-50 kW·m^-2,respectively.It is found that the inner wall temperatures and heat transfer coefficients are determined by the combined effects of physical property variation,buoyancy force and centrifugal force.The Buoyancy number can qualitatively stand for the relative importance of physical property variation and buoyancy effect.When Bo>8×10^-7,the natural convection becomes predominant,and heat transfer rate is obviously enhanced because of the buoyancy induced secondary flow.Due to the combined effects of buoyancy force and centrifugal force,the site with the lowest inner wall temperature and the highest heat transfer coefficient is located in the region 90°<ø<180°.When the buoyancy force takes advantage over the centrifugal force,the heat transfer coefficient in the bottom region is larger than that in the outer region. When the centrifugal force becomes dominant,the heat transfer coefficient in the bottom region is smaller than that in the outer region.Based on the 2346 data obtained in the present experimental study,two correlations of Nusselt number suitable for different temperature regions is proposed.It is verified that at least ninety percent of experimental data fall within 20% deviation predicted by the proposed correlations.

Key words: supercritical CO₂, helically coiled tube, mixed convection, pseudo-critical temperature, buoyancy force

摘要： 在恒热流条件下,对超临界压力CO₂在内径为9 mm,绕径为283 mm,节距为32 mm的螺旋管内垂直上升混合对流的传热特性进行了实验研究,实验参数范围为:进口压力8 MPa、质量流速0～650 kg·m^-2·s^-1、内壁热负荷0～50 kW·m^-2。研究发现:受热螺旋管内超临界压力CO₂的壁温及传热特性由变物性、浮升力及离心力的耦合作用共同支配,变物性及浮升力影响的相对大小可用Buoyancy数定性表征,当Bo>8×10^-7时,自然对流占主导作用,浮升力作用引起强烈的二次流效应,显著强化传热;在浮升力和离心力共同作用下,截面周向温度最低点出现在外下侧区域,且当浮升力作用占优时,底部区域的传热系数大于外侧,当离心力作用占优时,底部区域的传热系数小于外侧。基于本实验获取的2346个数据点,得出了计算Nu实验关联式,90%以上的实验值与拟合公式计算值偏差在±20%以内。

关键词: 超临界压力CO₂, 螺旋管, 混合对流, 准临界温度, 浮升力