非均匀热流下水平管内混合对流大涡模拟研究

Study on the large eddy simulation of mixed convection in a non-uniformly heated horizontal pipe

针对以槽式太阳能集热器为背景的高密度、高度非均匀热流下水平管内的混合对流换热问题,采用大涡模拟方法,研究了热流密度非均匀性对水平管内混合对流瞬态涡结构、脉动强度、湍流热通量及局部平均壁温的影响;揭示了非均匀热流下自然对流对管内湍流特性的影响规律;提出了适用于不同热边界条件下管内混合对流换热的强化措施。结果表明:均匀热流时,自然对流会抑制管顶部的湍流脉动,使流动层流化,造成传热能力局部恶化;非均匀热流时,随着自然对流的增强,近壁面速度脉动强度先减小后增大,二次流逐渐增强,换热能力逐渐提高,故管内换热能力受湍流脉动与二次流协同影响;在自然对流影响下,均匀加热时管顶部可采用针对层流的强化换热措施,非均匀加热时需着重提高管底部高热流区域的湍流脉动与涡强度。

The effects of heat flux non-uniformity on the transient vortex structure, fluctuation intensity, turbulent heat flux and local mean wall temperature of mixed convection in a horizontal tube heated by highly non-uniform high heat flux were investigated by large eddy simulation method, which was based on parabolic trough solar collector. The influences of natural convection on turbulent characteristics in the tube under non-uniform heat flux were studied. The heat transfer enhancement measures suitable for mixed convection in the tube at various thermal boundary conditions were proposed. The results show that under uniform heat flux, natural convection restrains turbulent fluctuation at the top of the tube and causes laminar fluidization, which results in local deterioration of heat transfer capability. Under non-uniform heat flux, with the increase of natural convection, the velocity fluctuation intensity near the wall decreases first and then increases and the secondary flow increases gradually, which leads to the heat transfer capacity increases. Therefore, the heat transfer capacity in the tube is affected by turbulent fluctuation and secondary flow synergistically. Under the influence of natural convection, heat transfer enhancement for laminar flow can be adopted at the top of the uniform heated tube, and the turbulent fluctuation and vortex intensity in the high heat flux area at the bottom of the tube should be enhanced when the tube is heated non-uniformly.