Abstract: The heat transfer and flow characteristics of trapezoidal tab with and without clearance are analyzed by Realizable k-ε turbulent model.The effects of clearance location and flow direction on the thermal performance of rectangular channel are studied to understand the mechanism of heat transfer enhancement induced by all kinds of vortices behind the tab.The mechanism of heat transfer enhancement is understood from the analysis on parameters,such as vorticity magnitude,streamlines,velocity profiles,pressure variations and turbulence intensity.The results show that Nusselt number and friction factor of backward flow are 21.7% and 25% higher than that of forward flow,respectively.The tab with inside clearance shows higher heat transfer coefficient and higher friction drag,while the tab with outside clearance exhibits lower heat transfer coefficient and lower form drag.The overall performance analysis for rectangular channel with tab shows that tab with or without clearance is competitive with other conventional turbulators,especially in its low power consumption.At lower Reynolds number(10000<Re<14000),the overall performance for backward flow is about 6.7% higher than that for forward flow. However,at higher Reynolds number(14000<Re<20000),the tab with clearance shows better overall performance.Due to Kelvin-Helmholtz instability,the hairpin vortex induced by the free shear layer shed from the tip of tab generates high turbulence intensity,which enhances fluid mixing.The pressure difference generated by the high-momentum fluid in the bulk region and the low-momentum fluid in the wake of tab produces two longitudinal vortex pairs named as primary counter-rotating vortex pairs(CVP) and secondary CVP.The reverse vortices are formed by recirculation induced by adverse pressure gradient behind the tab.Longitudinal vortices intensify the convective transportation between the base wall and flow center.Thus,the heat transfer intensification is mainly due to the counter-rotating longitudinal vortices and hairpin vortex,which produces a swirling motion.The two inflections of velocity profiles are caused by the hairpin vortex and reverse vortex,separately.

Key words: trapezoidal tab, vortex generator, hairpin vortex, longitudinal vortex, heat transfer enhancement

摘要： 利用Realizable k-ε湍流模型对带缺口的梯形扰流片进行流动和传热特性的数值模拟，研究了梯形扰流片的缺口位置及流动方式对矩形通道内流场以及传热的影响，同时通过对涡量、流线、流速分布、压力变化、湍流强度等的分析，揭示了扰流片强化传热的机理。结果表明，逆流时Nusselt数比顺流时提高了21.7%，同时摩擦因子也提高了25%。顺流时内侧缺口绕流片提高了传热系数的同时也增加了摩擦阻力，而外侧缺口的绕流片降低了传热系数同时也降低了形状阻力。研究发现较低Reynolds数下(10000<Re<14000)，逆流体现了较好的综合性能，但较高Reynolds数下(14000<Re<20000)，带缺口的绕流片则表现更好的综合性能。由于Kelvin-Helmholtz不稳定性导致了绕流片顶端后缘产生自由剪切层并诱发了发夹涡；绕流片的前后压差导致了流场内流体的旋流运动，形成了两个纵向涡；扰流片背面的逆压梯度产生了回流并形成回流涡。纵向涡强化了壁面与流动中心的对流传递过程，发夹涡则强化了主流区的流动混合，两种涡的共同作用加速了壁面的热量交换，实现了强化传热。

关键词: 梯形片, 涡流发生器, 发夹涡, 纵向涡, 强化传热