第七章 传热学沸腾对流换热9.ppt

Nine ChapterBoiling And CondensationBoiling processCondensation processSolid-liquid interfaceChange in phase of a liquidCondensationBoilingConvection heat transferWithout influencing fluid temperaturePhase changeLatent heat Surface tensionDensity differenceThree parameters9.1 Dimensionless Parameters in Boiling and Condensation(1)(2)(3)(1)Jakob number is ratio of the sensible energy to the latent energy.(2)Bond number is the ratio of the gravitational body force to the surface tension force.(3)Unnamed parameter represents the buoyancy-induced fluid motion on heat transfer.9.2 Boiling ModesWhen evaporation occurs at a solid-liquid interface,it termed boiling.The process occurs when the temperature of the surface Ts exceeds the saturation temperature corresponding to the liquid pressure Tsat.Boing may occur under various conditions.(1)Pool boiling(by free convection).(2)Forced-convection boiling(by external means,and free convection.Boiling may also be classified according to subcooled or saturated.(1)Subcooled boiling(TlTsat).(1)Pool BoilingCPDBAFree convectionNucleateBoiling regimes transitionFilm 530120101000Critical heat fluxLeidenfrost point,Free convection boiling Insufficient vapor in contact with the liquid phase.Free convection fluid motion.Nucleate boiling A-B:Isolated bubbles form at nucleation sites and separate from the surface.Most of the heat exchange is through direct transfer from surface to liquid in motion at the surface.B-C:the vapor escapes as jets or columns.Transition boilingBubble formation is so rapid that a vapor film or blanket begins to form on the surface.Conditions may oscillate between film and nucleate boiling.Film BoilingThe surface is completely covered by a vapor blanket.Heat transfer from the durface to liquid occurs by conduction through the vapor.Radiation through vapor film become significant.9.3 Pool Boiling Correlations(1)Nucleate Pool Boiling(2)Critical Heat Flux for Nucleate Pool Boiling(3)Minimum Heat Flux(4)Film Pool Boiling9.4 Forced-Convection Boiling9.5 Condensation:Physical MechanismsVapor temperatureSaturation temperatureCondensationCool surfaceVaporb Latent energyConsider only surface condensationFilm Drop Figure 9.1 Modes of condensationFilm condensation is generally characteristic of clean,uncontaminated surface.However,if the surface is coated with a substance that inhibits wetting,it is possible to maintain dropwise condensation.9.6 Laminar Film Condensation on a Vertical PlateygLiquid and vapor thermal boundary layersLiquid and vapor velocity boundary layersLiquid,l vapor,v xAssumptions:1.Laminar flow and constant properties are assumed for the liquid film.2.Pure vapor,uniform temperature Tsat,heat transfer occur by condensation no conduction at interface.3.The shear stress at interface is negligible.4.Momentum and energy transfer by advection in the film are negligible.Liquid thermal boundary layersLiquid velocity boundary layersLiquid,l vapor,v xMathematical modelIn vapor,u=v=0The condensation mass flow rate per unit width(x)may be obtainedThe rate of heat transfer across the interface must equal the that to the surface.