Numerical Study On The Flow Structure And Its Effect On Heat Transfer In The Flow Over A Surface-mounted Rib

The surface-mounted rib is widely used for heat transfer enhancement in engineering systems,such as heat exchangers,combustors and gas turbine engines et al.Meanwhile,the separating-reattaching flow over the rib is a basic flow in fluid dynamics.Previous studies generally focused on the study of forward-or backward-facing step.As a typical and more complex separating-reattaching flow,however,the understanding of the physical mechanism is not comprehensive.In the present work,the flow structure and its effect on heat transfer in the flow over a surface-mounted rib are studied using large-eddy simulation(LES).The main purpose is to gain further insight into the physical mechanism of the evolution of flow structures,unsteadiness of the flow field and the heat transfer.The main work and the corresponding results are as following:Firstly,the unsteadiness of flow field with laminar separation at low Reynolds number(Reh=Uoh/v 1000)is numerically studied.Based on the spectral analysis,two obvious frequency peaks are found.One is at Sth=0.361,which is related to the Kelvin-Helmholtz(K-H)instability of shear layer,and the other is at Sth=0.18,which is thought to be subjected to the vortex pairing.And this process is further confirmed by the flow visualization through the iso-surface of pressure fluctuation and Q-criterion.Secondly,a systematically study on the characteristics of flow structure and unsteadiness is performed under turbulent separation at a moderate Reynolds number(Reh=3000).The phenomenon of periodical vortex shedding and the corresponding flapping of the shear layer and recirculation bubble are observed through conditional averaging technique.Based on the spectral analysis,two lowe frequencies(Sth?0.06 and 0.09)except for the K-H instability(Sth?0.4)are also found in the recirculation region.By POD analysis of LES data,regions of shifted positive and negative values are found travelling downstream,which indicates the advection of large-scale vortical structures.In addition,the POD modes indicate that the streamwise vortical structures dominate in the recirculation bubble near the wall,while the spanwise vortical structures are observed to exist in the shear layer.All above observations lead to the conclusion that the flapping mode(Sth?0.09)found in the shear layer is induced by the spanwise vortical structures and that the pumping mode(Sth?0.06)of the recirculation bubble is caused by the streamwise structures near the wall.Thirdly,a numerical study of the manipulated flow and heat transfer over a surface-mounted rib with five different frequencies is performed using LES.It is found that when the excitation frequency is comparable to the instability of the vortex pairing in the shear layer(Sth=0.2),the mean reattachment zone can be significantly reduced by 22.5%.In addition,the heat transfer is mostly augmented with the excitation frequencies of Sth=0.05 and 0.2,the corresponding Nusselt number is increased by 20%compared to the unperturbed case.By analysis of flow structures and frequency spectral,the processes of vortex shedding and pairing can be induced when the exited frequencies are Sth=0.2 and 0.4;while the excitation frequencies of Sth=0.05 is believed to induce large scale structures in the recirculation region with a larger influence scope.Besides,the Stanton number and the coefficient of friction are applied to study the dissimilarity of heat and momentum transport.Detailed instantaneous flow structures and spectral analysis suppose that spanwise vortices(large turbulent coherent structures)shed behind the rib contribute to the dissimilarity by transporting the hot fluid to the center flow and carrying the cold fluid to the bottom wall.The phase-averaged iso-surface of Q-criteria confirms above dissimilarity mechanism.Lastly,the flow field and heat transfer of the ribbed channel with different shapes of ribs at a high Reynolds number(Re=UbDh/v=20000)are numerically studied.By comparing the flow and temperature fields,the heat transfer is found locally deteriorated immediately behind the attached and middle slit ribs.When the ribs arc slit at bottom,the jet flow through the crack can eliminate the local hot spots and enhance the temperature uniformity.By the studying of averaged Nusselt number and thermal performance,however,the solid rib shows the best performance while the middle slit rib shows the worst.This is because the flow through slit prevents the reattachment of the separated shear layer,and the corresponding turbulent intensity is weaken in the region near the wall.