传热学第六章（英文）ppt课件

1、2021/1/31,1,CHAPTER 6 Experimental Correlations of Single Phase Convection,2021/1/31,2,6.1 Similarity Principle and Dimensional Analysis,1. Similarity of Physical Phenomenon,If two physical phenomena have the same dimensionless forms of governing differential equations and boundary conditions, they

2、are similar phenomena,similar conditions,Must be the same kind phenomena,Physical quantities are proportional to corresponding physical quantities,reduce experiment times obtain general law,The spacial distribution of physical quantities are similar at the same time in unsteady state,Same differenti

3、al equations,describe similar physical quantities with uniform dimensionless field,2. Content of Similarity Principle,1）equivalence of same characteristic parameter,2）number and relation of sililar characteristic parameter,theorem：m=n-r n: number of independent physical quantities in equation r: num

4、ber of basic dimensions in equation m: number of dimensionless parameters in correlation,3）necessary and sufficient conditions of similar phenomena,same determined characteristic numbers composed by known quantities are equal,condition of unique solution are similar,initial condition boundary condit

5、ion geometry condition physical condition,2021/1/31,5,3. Derivation of Similar Characteristic Parameter,1）similarity method,nondimensionalization of parameters,charateristic parameter equation,6,introducing proportional coefficient,charateristic parameters of convection in common use：Re, Pr, Nu, Gr,

6、2021/1/31,7,2）dimensional analytical method,basic dimensions,e.g. forced convection in a pipe,kg、s、m、K、mol、A、cd,time,length,mass,temperature,n=7 r=4,2021/1/31,9,set up three dimensionless parameters,2021/1/31,10,Similarly, for other cases,Natural convection heat transfer,Mixed convection heat transf

7、er,For forced convection heat transfer,6.2 Application of Similarity Principle,1. Guidance of Experiment Arrangement and Data Processing,individual experiment results can represent entire group,common form of experimental correlation （characteristic parameter equation,or,C, n, m are determined by ex

8、periments,2021/1/31,12,1）determine,2021/1/31,13,2）determine,2021/1/31,14,2. Guidance of Simulation Experiment,simulation experiment: experiment to study the physical process in an actual device with a reduced scale model,Adopt similarity principle, only require some critical conditions satisfying si

9、milarity principle, such as B.C, G.C, Re, Pr etc, other conditions can be approximate,2021/1/31,15,在一台缩小成为实物1/8的模型中，用20oC的空气来模拟实物中平均温度为200oC空气的加热过程。实物中空气的平均流速为6.03m/s，问模型中的流速应为若干？若模型中的平均表面传热系数为195W/(m2K)，求相应实物中的值。在这一实验中，模型与实物中流体的Pr数并不严格相等，你认为这样的模化试验有无实用价值,例题,2021/1/31,16,解,2021/1/31,17,3、应用特征数方程应注意问

10、题,1）特征长度应按该准则式规定的方式选取 （2）特征速度应按规定方式计算 （3）定性温度应按该准则式规定的方式选取 （4）准则方程不能任意推广到得到该方程的实验参数范围以外使用,3. Accuracy of Experiment Correlations,convection heat transfer is a very complex phenomenon,calculation error derived from restriction of determination condition,continuous renewal, better accuracy,refinedcalc

11、ulations should employ specific correlations,2021/1/31,19,6.3 Forced-convection Correlations of Internal Flow,1. Characteristics of Internal Forced-convection Flow and Heat Transfer,two flow regimes：laminar flow turbulent flow,Rec=2300,Re2300 laminar flow,2300Re10000 transitional flow,Re10000 turbul

12、ent flow,external flow: free developing of B.L internal flow: passage wall impose restrictions on developing of B.L,entrance region fully developed region,entry length of laminar flow,turbulent flow, if , entry length has no impact on average h,2021/1/31,21,typical thermal boundary (T.B.) conditions

13、,uniform heat flux,uniform wall temperature,2021/1/31,22,mean fluid temperature and mean temperature between fluid and wall,mean temperature difference in equation of Newtons Cooling Law,For uniform heat flux，fully developed region,For uniform wall temperature，local temperature difference at cross s

14、ection changes continuously,referencetemperature is the mean temperature of fluid at cross section (or inlet , outlet cross section,2. Forced Convection Turbulent Flow Correlations in Tubes,conventional fluid （Pr0.6,Dittus Boelter equation,characteristic length：internal diameter（or hydraulicdiameter

15、,referencetemperature,range of application,suitable for low temperature difference between fluid and wall,heating n=0.4 cooling n=0.3,turbulent flow region,2021/1/31,24,Corrections for cases below,1）correction of variable properties impact,a: exponent change with heating or cooling,b: temperature di

16、fference (T.Diff.) beyond application range, introducing T.Diff. correction coeff., n=0.4,Heating gas,Cooling gas,Heating liquid,Cooling liquid,2021/1/31,25,2）impact of entrance region,correction factor of entrance effect,3）noncircular tubes (rectangle, ellipse, triangle,Adopt hydraulic diameter as

17、characteristic length,cross area,Wetting perimeter,Total length of pipe or tube,2021/1/31,26,Gnielinski equation,liquid,gas,application range：Ref=2300106, Prf=0.6105,also available for transition region,2021/1/31,27,Correction of twice circulation in helically coiled tube,liquid,gas,2021/1/31,28,liq

18、uid metal,for uniform heat flux,for uniform wall temperature,application range: Ref=3.61039.05105, Pef=102104,application range: Pef=102104,in which,2021/1/31,29,characteristic length：internal diameter ( or hydraulic diameter,referencetemperature,application range,turbulent regime in actual devices,

19、2021/1/31,30,Comments of table 6-2.3.4,For the same shape of cross section, Nu of uniform heat flux is always greater than that of uniform wall temperature. For uniform cross section, Nu of fully developed flow is independent of Re; When taking hydraulic diameter as characteristic length, Nu of full

20、y developed flows are different in different cross section shape of tubes, hydraulic diameter is just a geometrical parameter.,3. Forced Convection Laminar Flow Correlations in Tubes,Sieder-Tate equation,laminar entrance regime in actual devices,characteristic length：internal diameter ( or hydraulic

21、 diameter,referencetemperature,application range,2021/1/31,32,作业：水在管内受迫流动，质量流量 ， 管内径 ，,求（1） 流体平均温度 时,2）进口流体,3,2021/1/31,33,characteristic：separation of B.L,1. Experiment Results of Fluid Flowing across a Single Tube,change of hx along outer surface of the tube,separation,turbulent,separation,6.4 For

22、ced-convection Correlations of External Flow,2021/1/31,34,convection H.T. coeff. correlation on circular tube surface,characteristic length：outer diameter,t=15.5980，tw=211046,qualitativetemperature,application range,2021/1/31,35,Churchill Bernstein correlation,RePr 0.2,characteristic length：outer di

23、ameter,referencetemperature,application range,2021/1/31,36,Characteristic length,qualitativetemperature,gas convection H.T. coeff. correlation on noncircular tube surface,2021/1/31,37,free stream temperature,3.5Re7.6104， 0.71Pr380,2. Experiment Results of Fluid Flowing across a Sphere,characteristic

24、 length：outer diameter,referencetemperature,application range,2021/1/31,impact of tube bank arrangement mode on convection heat transfer,fluid flowing across tube bank is a common mode in heat exchanger Tube arrangements in a bank. (a) Aligned. (b) Staggered,impact factors of tube bank on convection

25、 heat transfer,Re, Pr Arrangement Row number Tube pitch,umax,3. Experiment Results of Fluid Flowing across Tube Bank,Tube arrangements in a bank. (a) Aligned. (b) Staggered,2021/1/31,39,Experimental correlations,characteristic length：diameter,characteristic velocity,reference temperature,2021/1/31,4

26、0,Choice of C、m,2021/1/31,41,correlation of Zhkauskas,convection heat transfer correlation of fluid flowing across aligned tube bank （16 rows,2021/1/31,42,2021/1/31,43,6.5 Natural Convection Correlation in Large Space and Confined Space,1. Characteristics of Natural Convection Heat Transfer,distribu

27、tion of velocity and temperature in B.L,laminar flow turbulent flow,Thermal resistance depends on thickness of B.L. hx is almost constant in turbulent flow on surface,Boundary layer development on a heated vertical plate,a) Velocity boundary layer. (b) Thermal boundary layer,2.Correlation of Natural

28、 Convection H.T and Similar Characteristic Number,correlation of natural convection,expansion coefficient,temperature difference,Grashof number,equivalent to Re in forced convection,gas,liquid: from tables,Rayleigh number,3. Experimental Correlation of natural convection H.T in Large Space,natural c

29、onvection in large space and enclosures,natural convection in large space： thermal B.L can develop freely,natural convection in enclosures: the development of thermal B.L is disturbed, the flowing of fluid is under restriction,Thermal B.L formation in an air interlayer between two hot vertical wall,characteristic length： l or d,liquid state correction,correction factor,nat