Study On Flow And Heat Transfer Characteristics Of Three Offset Cylinder Arrays In Transition Flow

The stronger requirements of environmental protection cause all countries commit to control the emissions of greenhouse gases and protect the ozone layer.Transitional alternative refrigerants tend to be used widely in our air-conditioning industry,such as R32?R290.Their own shortcoming,flammability,so safety factors should be taken into account during the use of these two kinds alternative refrigerants.One of the main safety measures is to reduce the fill amount of these refrigerants in heat exchanger tubes.Then the tube diameter of the air conditioning heat exchanger is required to be smaller.As a result,the minimized heat exchanger is widely studied.The flow state besides the tubes becomes transitional flow.However,flow and heat transfer mechanisms about transitional flow are still not clear.This study's purpose is to find out the flow and heat transfer mechanisms of tubes and fins used in minimized heat exchanger.In previous work,the single cylinder,double cylinders and offset two row cylinder arrays have been researched.As a part of this project,this study is based on the previous researches and it also expands for deeper results.It simulated the three offset cylinder arrays' characteristics in aspect of flow and heat transfer,and then summarized the mechanism and compared it with the precious work.By Fortran program,this paper simulated the three offset cylinder arrays model based on the compound grid system.The mass conservation equation,momentum conservation equation and energy conservation equation are the governing equations.Three order upwind scheme was applied in the convective term,the diffusion term applied the central difference method,and ADI procedure was used at each time step to deal with finite difference and pressure correction,SIMPLE method was used to correct the pressure so as to satisfy the continuity equation.In order to research the flow and heat transfer characteristics,the flow field and temperature field of the three offset cylinder arrays model with different cylinders pitches in X direction(Ps)and Y direction(Pt)under different Reynolds(150?200)are studied.The results show that heat transfer characteristics of this model depend upon Ps and Pt noticeably.(1)When Ps is particular,pressure difference ?P decreases with the increase of the Pt.When Pt is increased,negative effects on the fluid becomes weaker.(2)When Ps is increased slightly,the value of Nu decreases with the increase of Pt.This phenomenon's existing is because of the rectifying action.The fluid velocity between the adjacent pipes increases and that makes the boundary layer thinner.As a result,the heat transfer of the downstream cylinders is enhanced.On the other hand,the instability of the tail flow of upstream cylinders lead to the fluctuation of the flow,which has a positive effect on the heat transfer of the downstream cylinders.(3)When Ps gets larger,the curve of Nu raises firstly and then declines.While the adjacent temperature is reduced,the heat transfer enhancement of downstream cylinders is achieved.Some parameters and the flow field have been analyzed,as well as the thermal field.Affecting factors on the instability of flow have been found and influence factors on heat transfer of offset three-row cylinders were summarized.These findings and conclusions will provide reference for the development of small diameter heat exchanger.