Study On Flow Characteristics And Mechanism Of Heat Transfer Enhancement Of The Double Cylinders Near The Wall In Transition Flow

In recent years,with the rapid development of modern industry,energy shortage has gradually become a worldwide environmental problem.Saving energy and improve energy efficiency were supported by the world.In energy consumption structure,industry energy consumption held the majority shares,so the industry is the key area to save energy or improve energy efficiency.And heat exchanger is one of the most widely used industrial equipments.To improve thermal efficiency of the heat exchanger plays an important role on improving the production efficiency of industrial equipment as well as saving energy.On the other hand,the current heat exchanger is developing towards the direction of compact structure and miniaturization,which will cause the flow field out of the pipe changes from the turbulent flow state to transition flow state.But the flow and heat transfer characteristics in transition flow state has not been clear enough to guide the design of small compact heat exchanger effectively.This study's purpose is to clarify flow characteristics and heat transfer enhancement mechanism in the transition flow state.Prophase study of single circular cylinder,in-line double circular cylinders,offset double columns circular cylinders,three columns circular cylinders and a single circular cylinder near wall have been carried on the related research.Based on the previous studies,this article focus on the numerical simulation and related experiments of in-line double circular cylinders near the wall,which is in order to interpret the effect of in-line double circular cylinders on the flow characteristics and heat transfer enhancement mechanism of the wall.This article is one part of the whole subject and aims to expand the previous study.This study aims to the model of in-line double circular cylinders near the wall among incompressible two-dimensional unsteady flow in transition flow.The control equations include continuity equation(the mass conservation equation),energy conservation equation and momentum conservation equation.In finite-differencing the governing equations,a central finite-difference scheme was used for the diffusion terms,the third-order upwind scheme(QUICK)was applied for the convection terms of the momentum equations and the implicit difference method was used in time terms.ADI was used to solve the differential equations,SIMPLE was used to fix pressure field.PIV experiment,HT experiments and visualization experiments were carried out to verify the result of numerical simulation.Following two results can be concluded through the comparative study and analysis on single circular cylinder near the wall and in-line double circular cylinders near the wall:(1)On the one hand,insert cylinder near the wall can make the fluid between the cylinder and the wall forced to accelerate,and it is good to the heat transfer between the fluid and the wall;on the other hand,insert cylinder near the wall can make the flow field to fluctuate,the wash effect through the criss-cross motion makes heat transfer enhanced;(2)Compared with inserting single circular cylinder near the wall,inserting in-line double circular cylinders near the wall can effectively improve the heat transfer coefficient of the whole wall surface and optimize the wall heat transfer effect obviously.It can be found through the study on different Reynolds number Rein transition flow: the local Nusselt number of the wall increases with the increase of Re in transition flow and this also means that Reynolds number Re plays an advanced influence on the enhancement of heat transfer in the wall.Following two results can be concluded through the study with different horizontal spacing between the two cylinders(Ps)and spacing between the cylinder and the wall(C):(1)Ps has significant influence on the enhancement of heat transfer of the wall: Ps should not be too small,otherwise the back cylinder will have rectifying effect on the front cylinder,which is not good to the heat exchange;Ps also should not be too large,if the Ps is too large,it is hard to achieve the superposition effect of washing effect and fluid forced acceleration,which can promote the wall heat transfer enhancement;(2)C has significant influence on the enhancement of heat transfer of the wall: the enhancement of heat transfer of the wall decreases with the increase of C/D.Finally conclusions are as following: the cylinder number,Reynolds number and geometric parameters are important parameters on flow and heat transfer characteristics of the wall.In the design and application of heat exchanger used in air conditioning,appropriate Reynolds number and geometric parameters should be chosen to improve heat transfer efficiency and optimize the heat transfer performance of the heat exchanger.