Condensation and Boiling Heat Transfer

Condensation and Boiling Heat TransferDr. Senthilmurugan S. Department of Chemical Engineering IIT Guwahati - CL204 - Part 131Condensation DefinitionA phase is a region of space (a thermodynamic system), throughout which all physical properties of a material are essentially uniform.A phase transition is the transformation of a thermodynamic system from one phase or state of matter to another one by heat transfer.Condensation is the change of the physical state of matter from gas phase into liquid phase, and is the reverse of vaporizationMarch 27, 2016 | Slide #Modes of CondensationDirect / Indirect / homogeneous condensationDirect-contact condensation refers to condensation of vapor (bubbles or a vapor stream) in a subcooled liquid or condensation on liquid droplets entrained in the vaporIndirect / surface condensation: Heat transfer without mixing the vapor and liquid phaseHomogeneous condensation: If vapor temperature falls below its saturation temperature, condensation can occur in the bulk vapor. This phenomenon is called homogeneous condensation Indirect / surface (Dropwise / filmwise) condensationDropwise condensation: liquid does not wet the surface, droplets are formed.Filmwise condensation: liquid wets the surface, smooth film is formed. The surface is blanked by the film, which grows in thickness as it moves down the plate.

DropwiseFilmwise

Direct

HomogeneousVapourMarch 27, 2016 | Slide #Modes of Condensation

Direct-contactDropwise /HomogeneousfilmwiseMarch 27, 2016 | Slide #Dropwise / Filmwise Condensation video

March 27, 2016 | Slide #Dropwise vs Filmwise CondensationWhen vapour is exposed to plate below the saturation temperature of the vapor, condensate will form on the surface and under the action of gravity will flow down the plate. If the liquid wets the surface, a smooth film is formed, and the process is called film condensation the liquid does not wet the surface, droplets are formed that fall down the surface in some random fashion. This process is called dropwise condensation.The heat-transfer rates in dropwise condensation may be as much as 10 times higher than in film condensation due to film thermal resistance to heat transfer but in dropwise condensation a large portion of the area of the plate is directly exposed to the vapor dropwise condensation would be preferred to film condensation, However extremely difficult to maintain dropwise condensation since most surfaces become wetted after exposure to a condensing vapor over an extended period of time Special coatings can enable dropwise condensation for specific applications but not in generic March 27, 2016 | Slide #Filmwise CondensationFilm condensation has Three distinct regions:1. Laminar region, near the top, the film is relatively thin (Ref < 30)2. Wavy region, The film becomes thick enough to show the signs of transition (300.5 and cT/hfg 1.0.March 27, 2016 | Slide #For Vertical, Inclined Surfaces and Horizontal tubesHeat flux calculationLaminar region, (Ref < 30), Wavy region, (30