| The ribbed channels have a wide range of applications due to their high efficiency,reliability and economy,such as cooling channels in turbine blades,solar heaters,and electronic components.The ribs with different inclined angles not only increase the heat transfer area but also produce secondary flow.There are many factors affecting heat transfer enhancement in ribbed channels,including rib inclination,rib spacing,rib size,rib shape and so on.In this paper,numerical calculation is used to study the heat transfer enhancement and flow characteristics of rough ribs with different inclination angles in rectangular channels,and the causes of heat transfer enhancement with inclined ribs are analyzed from the point of view of secondary flow mechanism.The contribution of transverse secondary flow and longitudinal secondary flow to heat transfer enhancement is compared.Firstly,a three-dimensional rectangular channel model is established and compared with the known experimental data,and a suitable turbulence model is selected.Three-dimensional rectangular channel model is established by Solidworks,structured hexahedral mesh is created by ICEM CFD,five turbulence models are compared with experimental data by FLUENT.And the Realizabl k-?model which is the closest to the experimental values is selected to carry out subsequent calculation.Secondly,the heat transfer and flow characteristics under different geometric parameters are analyzed by changing the geometric parameters in the channel.Thirty-three rectangular channel models are obtained by changing the rib spacing to rib height ratio,the rib width to rib height ratio,the inclination angle of the rib and the cross-section shape of the rib.Based on the Nussel number Nu and resistance coefficient f,analysis of the influence of different geometric parameters on heat transfer and flow resistance in Reynolds number Re=800020000.The results are further verified by analyzing the temperature field,velocity field and turbulent kinetic energy.Thirdly,the secondary flow intensity is introduced to analyze the relationship between the secondary flow intensity and heat transfer.The absolute vortex flux and secondary flow intensity are defined by CFD-Post.Analysis of the effects of smooth passage and geometric parameters to the secondary flow intensity.The transverse secondary flow intensity is compared with the longitudinal secondary flow intensity,and the corresponding relationship between the secondary flow intensity and heat transfer is compared.Finally,the following ribbed channel heat transfer results are obtained.1.Different angles of rough ribs are set in the rectangular channel to enhance heat transfer,but the resistance coefficient is also increased accordingly.2.For the rib spacing to rib height ratio,and the rib width to rib height ratio,Nu/Nu0 and f/f0 decrease with the increase of Reynolds number Re.For the inclined angleαof the rib,with the increase of Reynolds number Re,the ratios of Nu/Nu0 of 30°,45°,60°,75°inclined ribs decrease.With the increase of Reynolds number Re,the ratios of f/f0 of inclined ribs increase.Both Nu/Nu0 and f/f0 of the 90°ribs are reduced.3.For square ribs,rectangular ribs,isosceles triangle ribs and boot-shaped ribs with different inclination angles,45°ribs have the highest heat transfer coefficient and resistance coefficient,and 75°and 90°ribs have lower heat transfer coefficient and resistance coefficient.4.Under the same parameters,the change trend of the secondary flow intensity is the same as that of the Nussel number.In the scope of this study,different rib shapes have the same change rules of secondary flow intensity.Both transverse and longitudinal secondary flow have a certain contribution to heat transfer,and the longitudinal secondary flow has a greater contribution.The secondary flow intensity can be used to characterize the heat transfer characteristics,and there is a certain relationship between the secondary flow intensity and the Nussel number. |
