化工原理英文教材 传热 无相变传热Heat transfer to fluids without phase change课件.ppt

1、化工原理化工原理PrinciplesofChemicalIndustryHeat transfer to fluids without phase changeRegimes of heat transfer in fluidsA fluid being heated or cooled may be flowing in different flow patterns.Also,the fluid may be flowing in forced or natural convection.At ordinary velocities the heat generated from flui

2、d friction is negligible in comparison with the heat transferred between the fluids.Because the situations of flow at the entrance to a tube differs from those well downstream from the entrance,the velocity field and associated temperature field may depend on the distance from the tube entranceThe p

3、roperties of the fluid-viscosity,thermal conductivity,specific heat,and density are important parameters in heat transfer.Each of these,especially viscosity,is temperature-dependent.Heat transfer by forced convection in turbulent flowPerhaps the most important situation in heat transfer is the heat

4、flow in a stream of fluid in turbulent flow.Since the rate of heat transfer is greater in turbulent flow than in laminar flow,most equipment is operated in the turbulent range.A dimensional analysis of the heat flow to a fluid in turbulent flow through a straight pipe yields dimensionless relations.

5、(12-27),pchddufk The three groups in Eq(12-27)are recognized as the Nusselt(Nu),Reynolds(Re),and Prandtl(Pr)numbers respectively.The Nusselt number for heat transfer from a fluid to a pipe or from a pipe to a fluid equals the film coefficient multiplied by d/kThe film coefficient h is the average va

6、lue over the length of the pipehdNuk Prandtl number Pr is the ratio of the diffusivity of momentum/to the thermal diffusivity k/cpPrpck The Prandtl number of a gas is usually close to 1（0.69 for air,1.06 for steam).The Prandtl number of gases is almost independent of temperature because the viscosit

7、y and thermal conductivity both increase with temperature at about the same rate.Empirical equationFor heat transfer to and from fluids that follow the power-law relation,the dimensionless relation becomesTo use the dimensionless relation,the constant c and index m,n must be known.()()pmnchdduckk A

8、recognized empirical correlation,for long tubes with sharp-edged entrances,is the Dittus-Boelter equation Where n is 0.4 when the fluid is being heated and 0.3 when it is being cooled.0.80.023RePrnihdNuk A better relationship for turbulent flow is known as the Sieder-Tate equation (12-32)0.81/30.140

9、.023RePr()wNu Equation(12-32)should not be used for Reynolds numbers below 6000 or for molten metals,which have abnormally low Prandtl number.Effect of tube lengthNear the tube entrance,where the temperature gradients are still forming,the local coefficient hx is greater than h for fully developed f

10、low.In entrance,hx is quite large,but hx value drops rapidly toward h in a comparatively short length of tube.Average value of hi in turbulent flow.Since the temperature of the fluid changes from one end of the tube to the other and fluid properties ,cp and k are all function of temperature,the loca

11、l value of hi also varies from point to point along the tube.The relation of local heat transfer coefficient hi and long tube h is as followsWhen L approaches infinite,hi is close to the h of long tube.7.0)(1/LDhhiFor laminar flow,the relation of Nu and Pr and Re is (12.25)3/1Pr)(ReANu For gases the

12、 effect of temperature on hi is small.The increase in conductivity and heat capacity with temperature offset the rise in viscosity,giving a slight increase in hi.For liquids the effect of temperature is much greater than for gases because of the rapid decrease in viscosity with rising temperature.Th

13、e effects of k,cp,and in Eq(12-36)all act in the same direction,but the increase in hi with temperature is due mainly to the effect of temperature on viscosity.In practice,an average value of hi is calculated and used as a constant in calculating the overall coefficient U.the average value of hi is

14、computed by evaluating the fluid properties k,cp,and at average fluid temperature,defined as the arithmetic mean between the inlet and outlet temperatures.Estimation of wall temperature tw The estimation of tw requires an iterative calculation based on the resistance equation111mowiooooimitTttddbUhk

15、 dh d To determine tw the wall resistance can usually be neglected11miooiittdUh dSubstituting Uo,gives (12-38)111iimiioohttdhh d Cross sections other than circular To use Eq(12-30)for cross section other than circular it is only necessary to replace the diameter in both Reynolds and Nusselt number b

16、y the equivalent diameter de.de is defined as 4 times the hydraulic radius rH.The method is the same as that used in calculating friction loss.Heat transfer in transition region between laminar and turbulent flow Equation(12-32)applies only for Reynolds numbers greater than 6000.The range of Reynold

17、s numbers between 2100 and 6000 is called the transition region,and no simple equation applies here.A graphical method therefore is used.The method is based on a common plot of the Colburn j factor versus Re,with lines of constant value of L/D The heat transfer coefficient can be calculated by follo

18、wing equation10.1432RePr4wdNuLHeating and cooling of fluids in forced convection outside tubes The mechanism of heat flow in forced convection outside tubes differs from that of flow inside tubes.The local value of heat-transfer coefficient varies from point to point around circumference in forced c

19、onvection outside tube.In Fig12.5,the local value of the Nusselt number is plotted radially for all points around circumference of the tube.Nuis maximum at the front and back of the tube and a minimum at the sides.In practice,the variations in the local coefficient are often no importance,and averag

20、e values based on the entire circumference are used.fluids flowing normal to a single tube The variables affecting the coefficient of heat transfer to a fluid in forced convection outside a tube are Do,the outside diameter of the tube;cp,and k,the specific heat,the viscosity,and thermal conductivity

21、,respectively,of the fluid;and G,the mass velocity.Dimensional analysis givesNusselt number is only a function of the Reynolds number.,poooch DD GfkkThe experimental data for air are plotted in this way in Fig12.6For heating and cooling liquids flowing normal to single cylinders the following equati

22、on is used0.30.520.350.56pfooofffch DD GkkNatural convection Consider a hot,vertical plate in contact with the air in a room.The density of the heated air immediately adjacent to the plate is less than that of the unheated air at a distance from the plate,and the buoyancy of the hot air causes an un

23、balance between the vertical layers of air of differing density.Temperature difference between the surface of plate and the air causes a heat transfer.Natural convection in liquid follows the same pattern.The buoyancy of heated liquid layers near a hot surface generates convection currents just as i

24、n gases.For single horizontal cylinders,the heat transfer coefficient can be correlated by equation containing three dimensionless groupsNu=f(Pr,Gr)Gr:Grashof numberPr:Prandtl number(12-67)The coefficient of thermal expansion is a property of fluid322,pfofoffcDg thDfkk Fig 12.8 shows a relationship,

25、which satisfactorily correlates experimental data for heat transfer from a single horizontal cylinder to liquids or gases For magnitudes of log Gr Pr of 4 or more,the line of Fig 12.8 follows closely the empirical equation0.250.53PrfNuGrNatural convection to air from vertical shapes and horizontal p

26、lates Equations for heat transfer in natural convectionbetween fluids and solids of definite geometric shape are of the form (12-73)Values of the constants b and n for various conditions are given in Table 12.4322npffffcLg thLbkk A double pipe heat exchanger is used to condense the saturated toluene

27、 vapor(2000kg/h)into saturated liquid.The condensation temperature and latent heat of toluene are 110 oC and 363kJ/kg,respectively.The cold water at 20 oC(inlet temperature)and 5000kg/h goes through the pipe(di=50 mm)fully turbulently.If the individual heat transfer coefficient hi of water side is 2

28、100 w/(m2 K),and heat resistances of pipe wall as well as toluene side are much larger than that of water side(this means both resistances can be ignored),find:Outlet temperature of cold water,in oC.Pipe length of exchanger.In order for mass flow rate of toluene to be double,if the mass flow rate of

29、 cold water at the same inlet temperature(20 oC)is double,what is the pipe length of new exchanger to be required?Solution:Heat balance q=m1=m2Cp(Tcb-Tca)2000363=50004.19(Tcb-20)(1)Outlet temperature of cold water Tcb=54.65oC(2)U=h(from the problem)T1=110-54.65=55.35,T2=110-20=90T=(T1+T2)/2=72.68(si

30、nce T2/T12)L=q/(UdT)=20003631000/3600/(21000.0572.68)=8.42m(3)q=2qm1=2m2Cp(Tcb-Tca)Outlet temperature of cold water Tcb=54.65oCT=(T1+T2)/2=72.68Fully developed turbulent flow,hRe0.8m0.8u0.8 h/h=20.8,h=1.74hq=1.74hdL T=2m1q=hdL T=m1L/L=2/1.74 so L=28.42/1.74=9.68mA single pass(1-1)shell-tube exchange

31、r is made of many 252.5 mm tubes.Organic solution,u=0.5m/s,m(mass flow rate)=15000kg/h,Cp=1.76 kJ/kg.oC,=858 kg/m3,passes through the tube.The temperature changes from 20 to 50 oC.The saturated vapor at 130 oC condenses to the saturated water,which goes through the shell.The individual heat transfer

32、 coefficients hi and ho in the pipe and shell are 700 and is 10000 W/m2 oC,respectively.The thermal conductivity k of pipe wall is 45 W/m.oC.If the heat loss and resistances of fouling can be ignored,find(1)Overall heat transfer coefficient Uo.（based on outside tube area）and LMTD.(2)Heat transfer area,number of pipes and length of pipes.