| Heat exchangers play an important role in industrial application,which are the main equipment to ensure the normal operation of the machine,and the study to improve the ability of haet transfer becomes an important job for reserchers.Disclosing advanced method to enhance heat transfer is the way to solve these problems.Based on the engineering practice background,this thesis investigates the flow and heat transfer in twisted tubes(twisted elliptical tube,twisted rectangular tube and the circular tube with twisted tape insert)with numerical method.It is found secondary flow induced in twisted tubes is efficient heat transfer enhancement techaniques.However,there are no parameters to specify the intensity of secondary flow in twisted elliptical tube and twisted rectangular tube,which will seriously affect the usage in engineer practice.Moreover,the mechanism of heat transfer enhancement in twisted tubes is not effectively disclosed.For these problems,the thesis investigates the flow and heat transfer in twisted tube,and the main results are as follows:(1)The parameter of Sw is established in this thesis to specify the intensity of secondary flow in twisted tubes.It is a dimensionless parameter,and flow condition and geometric condition(twist ratio?and the ratio of the length of semi-minor axis to the length of semi-major axis)are both included in this parameter.The simulation to solve flow field can be ignored when Sw is used.It makes up for the deficiency of other parameters to describe the intensity of secondary flow in twisted tubes,and it is convenient for engineering application.(2)Three different kinds of parameters can be used to specify the intensity of secondary flow in twisted tube,Se,Sw and Re R,these parameters have three kinds physical meaning,but there exist correlations to express the value of the three parameters.Secondary flow in ellipitical tube produce heat transfer enhancement of the tube.At Re=800,?=3,b/a=0.6,under uniform wall temperature boundary condition(UWHF),the parameter of Nus,TWI/Nus,STRis1.35,under uniform wall heat flux boundary condition(UWHF)the parameter of Nus,TWI/Nus,STRis1.37.Morever,the fraction factor will increase.The average convective heat transfer ability and fraction factor of twisted elliptical tube can be regarded as the summation of convective heat transfer intensity induced in straight oval tube and convective heat transfer intensity induced by secondary flow,this indirectly revealed the mechanism of heat transfer enhancement in twisted elliptical tube.(3)For periodically fully developed laminar fluid flow and heat transfer in straight and twisted elliptical tube,Nusselt number is not uniformly distributed on the tube wall.In straight elliptical tube the ratio of the maximum value of local Nusselt number to the minimum value of local Nusselt number,Numax/Numin,is 5.66/1.72 under UWT,and it is 7.41/2.71 under UWHF.With the same configuration,in twisted elliptical tube,this value is 10.04/1.07 under UWT and8.90/3.21 under UWHF.In straight elliptical tube large local Nusselt number appears on the wall surface facing at semi-minor axis,small value is located on the wall surface facing at semi-major axis.In twisted elliptical tube these special positions rotated clockwise with about 22.5°and 37°under UWHF and UWT,respectively.The local Nusselt number in straight elliptical tube is larger under UWHF than that under UWT everywhere.The value Nulocal,UWHF/Nulocal,UWTis greater than 1 at every position on the wall.For twisted elliptical tube,the local Nusselt number under UWHF is not always larger than that under UWT.Thus,the value of Nulocal,UWHF/Nulocal,UWT is smaller than 1 at some positions on the wall.Under UWHF and UWT twisting of elliptical tube greatly enhances convective heat transfer on large region but also deteriorates convective heat transfer on the other small regions.In twisted elliptical tube the characteristics of velocity and its gradient contributions are dependent on regions.Combination of velocity and its gradient contributions has larger value under UWT than under UWHF result in local Nusselt number under UWT larger than that under UWHF in some regions.In twisted elliptical tube velocity and its gradient contributions have large value in most regions under UWHF produces large circumferentially averaged Nusselt number than under UWT.(4)This thesis has investigated the periodically fully developed laminar and turblent flow and heat transfer in twisted rectangular tube,the numerical results obtained in the thesis are cheked with the reference results in smooth tube.It is founded that Numand f both increase with the increase of b/a and Re,and the decrease of twist ratio?,thus twisted rectagalar tube can efficiently enhance convective heat transfer.And the formular of Sw in twisted rectangular tube is obtained.Se can be used to specify the intensity of secondary flow in twisted rectangular tube.Num,TWIand f TWI can be written as a summation of secondary flow term and a constant,the constant is the value of straight tube,the fomular is similar when use different secondary flow parameters.This method can be used to reveal the mechanism in twisted tubes.(5)The parameter of Sw can be used to specify the intensity of secondary flow in circular tube with twisted tape insert.From the method to establish Sw,it is found the secondary flow induced by a twisted tube,the twisted tube can be epsicify by twisted ratio and the ratio of side length.Se and Re R can be used to specify all secondary flow becaused these two parameters are obtained by the local information of flow field.(6)Based on heat flux convective transport equation,it is found the summation of velocity and velocity gradient contributions to the transport of heat flux increase in twisted tubes,twisted elliptical tube,twisted rectangular tube and circular tube with twisted tape insert,and produce heat transfer enhancement.For twisted elliptical tube and circular tube with twisted tape insert,secondary flow enlarges the contributions of velocity and its gradient to the transport of q?.That is when(We-?+Wc-?)/?T increases,|q?/?T|will be enlarged,and the process of heat transfer in the?direction is enhanced,then more heat will be transferred from the wall surfaces with z=constant,with such way the heat transfer is enhanced.For the case of twisted rectangular tube,heat flux is obtained from the wall surface in+?direction,+?direction,-?direction and–?direction,which induces(We-?+Wc-?)/?T and(We-?+Wc-?)/?T increase,both promotes the transport of q?and q?,and much more heat will be transferred from the wall surfaces with z=constant,and produce heat transfer enhacment. |
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