大学精品课件：chapter 9(Heat Transfer.J.P.Holman ).ppt

1、Chapter 9 condensation and boiling heat transfer,9-1 introduction,The heat transfer convection we discussed before is considered homogeneous single-phase systems. For convection associated with a change of phase of fluid .usually we deal with two conditions.,condensation,boiling,9-2 condensation hea

2、t-transfer phenomena,When temperature of the plate below the saturation temperature of the vapor, with the action of the gravity . The condensate form and flow down the surface .,Filmwise condensation : the liquid can wet the surface , a smooth film formed ;the thickness of the film on the surface g

3、rows as it goes down the plate , there is a gradient exists in the film. The film also presents thermal resistance to heat transfer.,Dropwise condensation: the liquid cant wet the surface, droplets formed on the surface , the surface is not completely covered by the liquid , and when droplets goes d

4、own ,they can take many droplets in their way with them , so dropwise condensation has no film barrier for heat flow , and its heat transfer rates may be as much as 10 times higher than film condensation.,Dropwise condensation is extremely difficult to maintain , general success of dropwise condensa

5、tion havent been realized.,Film condensation on a vertical flat plate Assume :,viscous shear of the vapor on the film is negligible at the edge with vapor; pure vapor , constant property .,Only convection in the film , the temperature distribution inside the film between the wall and vapor is linear

6、;,The state of the film is still . and the thickness of film is very small , the velocity of the fluid is very slow.,simplification,Reference temperature hfg should be evaluated at Tg, other properties at film temperature For superheated vapor use h-h as substitution of the hfg,Applicability Pr0.5 c

7、T /hfg 1.0,For superheated vapor use h-h as substitution of the hfg,For vertical plate of unit depth,For vertical tube,Critical Reynolds number : the criterion for determining whether the flow is laminar or turbulent .approximately:1800,Some factors that can affect the condensation,The effect of non

8、condensable gases Resistance of mass transfer from vapor to the film The presence of noncondensable gas reduces the partial pressure of vapor , so does the heat transfer coefficient and heat flow, There is an example 1% noncondensable gas would reduce the heat transfer coefficient by 60%,The effect

9、of vapor velocity One of the approximation made in the Nusselts theory , fractional drag between the film and the vapor is negligible. the cases valid when the velocity of the vapor is substantial When the u is upward , the , h ； u is downward , ,the film may break ,h .,Principle of condensation hea

10、t transfer enhancement : decrease the thickness of the film,The superheat vapor. The subcooling degree of the film and nonlinear distribution of the temperature in the film. Tube bank .,9-3 the condensation number,9-4film condensation inside horizontal tubes,Self-learn !,9-5 boiling heat transfer,Wh

11、en a surface is exposed to a liquid and is maintained at a temperature above the saturation temperature of the liquid the boiling may occur .,pool boiling : When the surface is submerged below a free surface of a liquid .,Forced convection boiling ： When a liquid is forced through a channel or over

12、a surface maintained at a temperature greater than the saturation temperature of the liquid.,Subcooled (local) boiling : The temperature of the liquid is below the saturation temperature of the liquid .,Saturated(bulk)boiling : The liquid is maintained at the saturation temperature of the liquid,Typ

13、ical pool boiling curve at atmosphere pressure,Bubbles begin to form and are dissipated in the liquid Nucleate boiling Individual bubble regime,region I,region II,Continuous column regime Film (Transition) boiling regime Stable film boiling regime Radiation,region III,region IV,region V,region VI,fo

14、r the same heat flux , a small increase in t at point C ,h decreases , equilibrium may be reestablished only at point E . the increase in the temperature may reaches above 1000 C , this temperature usually exceeds the melting temperature of the metal , and then burnout for the heater. Point C is cal

15、led burnout point , the heat flux at this point is called critical heat flux,Critical heat flux,Departure from nucleate boiling (DNB),tltv,bubbles loss heat and reduce in size,bubbles absorb heat and reduce in size,Bubble dynamics,The place where a bubble is initially formed is called Nucleation sit

16、e,the stable condition for bubble,heat balance tl= tv,force balance taking a hemisphere as control volume,From these discussions we see that the cavities in the surface(usually trapping gas ) can satisfy the conditions for the nucleation sites .,Surface texture,Liquid level,Gravity,Subcooling degree

17、,Noncondensable gases,Affecting Factors,Boiling in the pipes,examples,Rohsenow correlation,Forced convection boiling for full developed nucleate boiling calculation,Peak heat flux for nucleate pool boiling,Zuber correlation,Other correlation and the applicable conditions are shown in the books,Stabl

18、e film boiling Bromley correlation(stable film boiling region on horizontal tubes),Total heat transfer coefficient (hr is the radiation heat transfer coefficient) Where hr is the radiation heat transfer coefficient , Reference temperature,9-6 simplified relations for boiling heat transfer with water,Selflearn !,the heat pipe,tgcavszhjdbvnm f,High heat-transfer coefficient Low temperature difference Reliability Adjustable heat flux Temperature control(space) Heat transfer(High-power electronic components),The characteristics of heat pipe,Application,The end !,Thanks,