Study On The Effect Of Typical Vortex Struture On Wall Heat Transfer

Many engineering applications are concerned with the flow around bluff bodies,such as sturcture design,acoustic emissions,flow metering devices and other applications.In such studies,principal objectives have mainly been to control and weaken the effect of vortex in the wake of bluff bodies.But over the past 10 decades or so,people have found that vortex could enhance heat transfer in channel flow.It could be widely used in heat exchange systems such as the cooling of electronic circuits,air conditioning,high pressure turbine and so on.So many researchers are interested in this issue,as it leads to saving in energy and cost in heat exchanger system.Study on the effect of typical vortex structure on wall heat transfer in channel flow has been conducted in this paper.In the paper,effect of enhancing heat transfer by three types of vortex generators has been compared.These vortex generators has different cross sections which are circle,rectangle and rhombus.Be sure that blockage ratio is same in study.And Reynolds number changes from 100 to 40000.In laminar flow,vortex generators with rhombic sections could bring the best heat transfer performance,but in turbulence flow,it changes to the vortex generators with rectangular sections.And with the decrease of the width in one type of vortex generator,the heat transfer performance first increases and then decreases.So we could guess that there is the optimal solution.To design one type of vortex generator with outstanding heat transfer performance,the thesis carried on study from the perspective of vortex structure in the wake of vortex generator.We presented a method that could simulate vortex structure with the numerical method.In essence,the method could be realized by importing a special velocity profile to the computational domain according to the computing time.It could simulate two types of vortex streets,which are Karman Vortex Street and Reverse Karman Vortex Street.So the thesis emphatically studied the flow characteristic and the effect on enhancing heat transfer of these two types of vortex streets.In flow characteristic,Reverse Karman Vortex Street could maintain its structure in the channel flow except the influence by viscous dissipation which would reduce its vorticity.But,for the Karman Vortex Street,it would occur a particular phenomenon called “crisscross motions of vortices”.In the phenomenon,vortex blobs of different rows in the vortex street would cross the center line and exchange their positions at some distance downstream.And in the effect on enhancing heat transfer,Reverse Karman Vortex Street could improve the heat transfer by disrupting the thermal boundary layer and preventing the decay of the wall temperature gradient throughout the channel,and by enhancing the forced convection to carry off more quantity of heat from the wall.But,for the Karman Vortex Street,the corresponding heat transfer process is more complicated because of the “crisscross motions of vortices”.Firstly,it could induce larger vorticity near the wall.And the induced vortex could even break away from the wall under certain condition,which could enhance the heat transfer to a certain extent.On the other hand,after the “crisscross motions of vortices”,the structure of the vortex street transforms into similar to the Reverse Karman Vortex Street.So it could enhance heat transfer in the same way as the Reverse Karman Vortex Street after crisscross motions.Finally,this thesis proposed one type of vortex structure which has outstanding heat transfer performance with a given input kinetic energy.It is obtained in the Reverse Karman Street cases with zero background flow.And it may play a guiding role in the vortex generator design.