化工四大化學(xué)(專業(yè))流體力學(xué)(課堂講義)

1、1CHAPTER 13Heat Transfer to Fluids with Phase Change 2Content n13.1 Heat Transfer from Condensing Vapors n13.2 Heat Transfer to Boiling Liquids 3nheat transfer accompanied by phase change include: Heat Transfer from Condensing Vapors ( Vapor-Liquid(drop) phase change) Heat Transfer to Boiling Liquid

2、s (Liquid-Vapor(bubble) phase change) Crystallization (Chapter 27) (Liquid-Solid(crystal) phase change)413.1 Heat Transfer from Condensing Vaporsn1. Dropwise and film-type condensationn2. Coefficients for film-type condensation n3. Practical use of Nusselt equations n 4. Condensation of superheated

3、vaporsn 5. Condensation of mixed vaporsn6. Main factors that influenced the condensation process5Condensation is met in unit operations:n evaporation (蒸發(fā))n distillation（蒸餾）n drying （干燥）The condensing vapor may consist of : a single substance a mixture of condensable and noncondensable substances a m

4、ixture of two or more condensable vapors 61. Dropwise and film-type condensationTwo condensing ways on a cold surface Dropwise滴狀film type膜狀vapor/steamvapor/steamCold wallCold wallDirection of heat flow72. Coefficients for film-type condensationFirst derived by Nusselt.Basic assumption:(1)vapor and l

5、iquid at the outside boundary of the liquid layer are in thermodynamic equilibrium (2)condensate flows downward in laminar flow(3)liquid velocity at the wall is zero 8Basic assumption:(4)liquid velocity at the outside of the film is not influenced by vapor velocity(5)temperatures of the wall and the

6、 vapor are constant (6)vapor superheat is neglected (7)the condensate is assumed to leave the tube at the condensing temperature(8)physical properties of the liquid are taken at the mean film temperature 9nVertical tubesThe heat flows through the condensate film solely by conduction.solely by conduc

7、tionfxkh (13.1)Local coefficient10nFIGURE 13.1 Film thickness and local coefficients, descending film of condensate. 11Therefore, local coefficient at a distance L from the top of the vertical surface is can be found from equation for flat plate Eq.(4.50)3/123 gff(13.2)3/123fffxgkh(13.3)Liquid loadi

8、ngfxkh 12When outside a tube , dLTddLDTmddATdqhooooox(13.5)where = heat of vaporization = local flow rate of condensate m The average coefficient h for the entire tube is defined by(13.6)oTboToTooTTLTLDmTAqh3/12334fbffgkh(13.9)After derivation13Compare the local coefficient and average coefficient3/

9、123fffxgkh(13.3)3/12334fbffgkh(13.9)Local coefficientAverage coefficientFor a vertical tube, when condensate is flowing outside the tube in laminar flow, the average coefficient is 4/3 times the local coefficient at the bottom of the tube. 143/12334fbffgkh(13.9)Equation (13.9) can be rearranged3/13/

10、13/1232Re47. 1447. 1fbfffgkh(13.10)4/123943. 0foffLTgkh(13.12)Applied conditionsLaminar flow, Re1200or15For wavy laminar and turbulent flow, empirical equation2/13 . 18 . 0644. 03/122)PrRe1082. 5(RegkhuNfff(13.13)FIGURE 13.2 (p383)16Single horizontal tube3/13/1232451. 1ffffgkh4/123729. 0fooffDTgkh(1

11、3.14)(13.15) is the condensate loading per unit length of tube Outside diameter of pipe173. Practical use of Nusselt equations(13.17) For a stack of horizontal tubes , eq.(13.15) becomes4/123729. 0fooffNDTNgkhN = number of tubes in stack 1n2n3n4n5n18Comparison between film condensing on a horizontal

12、 tube and on a vertical tube4/123943. 0foffLTgkhhorizontal tubevertical tube4/123729. 0fooffDTgkh4/14/13 . 1729. 0943. 0LDLDhhoohvooDDL85. 23 . 14hvhh In general, the coefficient of a film condensing on a horizontal tube is larger than that on a vertical tube.If 194. Condensation of superheated vapo

13、rsnThe heat of superheat is usually small in comparison with the latent heat.nThe equipment can be considered as two sections, one a desuperheater and the other a condenser. nSuperheat can be eliminated more economically by injection of a spray of liquid directly into the superheated vapor, 205. Con

14、densation of mixed vaporsIf the vapor contains two or more volatile components (unless it is an azeotropic mixture), the condensation temperature is no longer constant at a given pressure. 216. Main factors that influenced the condensation processnKey factors for film-type condensation are film thickness and its flow condition. 224/123943. 0foffLTgkh4/123729. 0fooffNDTNgkhVertical tube horizontal tube(1) Temperature difference