Study On Characteristics Of Flow And Heat Transfer Of Circular Cylinders In Transitional Flow State

With the rapid development of science and economy,our country has become the largest air-conditioning producer and consumer in the world.However,since conventional refrigerants of air-condition(HCFCs)can cause ozone depletion and greenhouse effect,looking for alternative refrigerants zero ODP(ozone depletion potential)and low GWP(global warming potential)have become the common responsibility of global counterparts in a coming period.R290 and R32 are trend to be used all over the world and domestic also gradually tend to use these two alternative refrigerants.These two refrigerants are slightly flammable,so it is necessary to reduce the diameter of tube bundles for small filling volume.The decrease of the tube diameter also leads to the transitional flow state in which the mechanisms of flow and heat transfer outside tube bundles,especially the effect of heat transfer enhancement of the periodical fluctuation of wake on other tubes,has not been clarified.Thus,it is significant to study this mechanism of heat transfer enhancement of tube bundles in transitional flow state.Considering that the study is focus on the characteristics of flow and heat transfer in the air side and the influence of refrigerants in tube is not taken into account,the circular cylinder array was selected as the research object.The circular cylinder array includes the single cylinder,two circular cylinders in tandem,circular cylinders array with the in-line and staggered arrangements,two circular cylinders of different diameters in tandem.A combined method using numerical simulation and visualization experiment was adopted.For the numerical simulation,the compound grid system method that cylindrical coordinate was used around circular cylinder array was established to accurately simulate the information around circular cylinders.For the experimental method,an open-loop recirculating water tunnel was designed and established,a two-dimensional particle image velocity measurement system(PIV)calculating velocity field by the cross correlation algorithm in Insight4 G was applied.Based on the validation of flow visualization,the effects of Reynolds number,transverse and longitudinal center distance between circular cylinders,as well as ratio of the different diameters on the characteristics of flow and heat transfer on the walls of circular cylinders were analyzed and some conclusions were summarized as follows:(1)Flow patterns of circular cylinder arrays in transitional flow were distinguished and the characteristics of flow and heat transfer of that are analyzed.One of them is the model of two circular cylinders in tandem as a typical,including three flow patterns.(a)The wake of the upstream cylinder shows the laminar flow state and the shear layer of the wake attaches to the wall of the downstream cylinder.This leads that the flow between two cylinders shows a closed low-speed and backflow movement,making a difficult to exchanger heat with the mainstream flow.This behavior deteriorates the heat transfer of the back face of the upstream cylinder and the front face of the downstream cylinder.The wake of the downstream cylinder shows a fluctuating state and the vortex motion shed from the wall brings the cold fluid to the rear stagnation point to enhance the heat transfer here.(b)The wake of the upstream and downstream cylinder shows the laminar flow state.Compared with state(a),the wake of the downstream cylinder is laminar,which reduce the heat transfer of the rear stagnation point of the downstream cylinder.(c)The wake of the upstream cylinder shows the fluctuating state and periodically impact on forward stagnation point of the downstream cylinder to enhance the heat transfer here.The fluctuation in the wake of the upstream cylinder strengthens that of the downstream cylinder.This leads that separation point is earlier and amplitude of the wake increases,which enhance the heat transfer of the rear stagnation point of the downstream cylinder.The flow pattern seriously restricts the convective heat transfer characteristics of circular cylinders.Therefore,the summary of the flow pattern provides a guide role for the tube bundle design in air-condition heat exchanger.(2)The mechanism that the wake of the upstream cylinder periodically affects the wall of the downstream cylinder was revealed.When the shear layer in the wake of the upstream cylinder rolls up vortex,the clockwise or counterclockwise motions are guided.This leads that cold fluid in mainstream flow is brought into the region in the center of wake and hot fluid in the center of wake is brought into the mainstream,which strengthen heat mixing between the wake and the mainstream flow.This behavior increases recovery of velocity and temperature in the wake,which increase the effective Reynolds number and the temperature difference between the approaching flow and the wall.In addition,when the positive and negative vortex motions in the wake are alternately close to the downstream cylinder as time goes on,whether the positive or negative vortex,the cold fluid in the mainstream flow is imported into one side and hot fluid is exported from other side in the front face of the downstream cylinder.The way of heat transfer increases the mixing between the wall and the mainstream flow.The heat transfer on the wall of the downstream cylinder is enhanced through this way.(3)The influence of accelerating effect between the upstream cylinders array on the downstream cylinders array was revealed.The fluid past upstream cylinder array is accelerated due to rectified action.For in-line arrangement cylinder array,compared with single in-line cylinders,the accelerated fluid increases the effective Reynolds number of the wake of upstream cylinders and the strength of vortex shedding.This strengthens the recovery of the velocity and temperature in the wake and enhances the heat transfer on the wall.For staggered arrangement cylinders array,under short center distance,the accelerated fluid directly dashes against the area near the forward stagnation point of downstream cylinders.The velocity and temperature boundary of the area between downstream cylinder arrays are thinned due to rectified action.Under long center distance,the accelerated fluid decreases the velocity of the approaching flow of the downstream cylinder due to the effect of the wake fluctuation of upstream cylinders,which decreases the heat transfer performance near the rear stagnation point.Also,the wake fluctuation increases the wake fluctuation of downstream cylinders,which enhances the heat transfer near the rear stagnation point.