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.