Presentation on theme: "Chapter 10 Sections 10.6 through 10.11"— Presentation transcript:
1 Chapter 10 Sections 10.6 through 10.11 CondensationChapter 10Sections 10.6 through 10.11
2 General Considerations Heat transfer to a surface occurs by condensation when the surface temperatureis less than the saturation temperature of an adjoining vapor.Film CondensationEntire surface is covered by thecondensate, which flowscontinuously from the surfaceand provides a resistance to heattransfer between the vapor and thesurface.Thermal resistance is reduced through use of short vertical surfacesand horizontal cylinders.Characteristic of clean, uncontaminated surfaces.Dropwise CondensationSurface is covered by drops ranging froma few micrometers to agglomerations visibleto the naked eye.
3 General Considerations (cont). Thermal resistance is greatly reduced due to absence of a continuous film.Surface coatings may be applied to inhibit wetting and stimulatedropwise condensation.
4 Film Condensation: Vertical Plates Film Condensation on a Vertical PlateDistinguishing FeaturesThickness and flow rate ofcondensate increase with increasing xGenerally, the vapor is superheatedand may be part of a mixturethat includes noncondensibles.A shear stress at the liquid/vaporinterface induces a velocity gradientin the vapor, as well as the liquid.Nusselt Analysis for Laminar FlowAssumptions:A pure vapor atNegligible shear stress at liquid/vapor interface.
5 Vertical Plates (cont) Negligible advection in the film.Hence, the steady-state x-momentumand energy equations for the film areThe boundary layer approximation, may be applied to the film.Hence,Solutions to momentum and energy equationsFilm thickness:
6 Vertical Plates (cont) Flow rate per unit width:Average Nusselt Number:Total heat transfer and condensation rates:
7 Vertical Plates (cont) Effects of Turbulence:Transition may occur in the film and three flow regimes may be identifiedand delineated in terms of a Reynolds number defined as
9 Vertical Plates (cont) Calculation procedure:Assume a particular flow regime and use the corresponding expression for(Eq , or 10.39) to determineIf value of is consistent with assumption, proceed to determination ofIf value of is inconsistent with the assumption, recompute its valueusing a different expression for and proceed to determination of
10 Film Condensation: Radial Systems Film Condensation on Radial SystemsA single tube or sphere:Tube: C =0.729Sphere: C=0.826
11 Film Condensation: Radial Systems (cont). A vertical tier of N tubes:Why does decrease with increasing N?How is heat transfer affected if the continuous sheets (c) breakdown and thecondensate drips from tube to tube (d)?What other effects influence heat transfer?
12 Film Condensation: Internal Flow Film Condensation for a Vapor Flow in a Horizontal TubeIf vapor flow rate is small, condensate flow is circumferential and axial:For larger vapor velocities, flow is principallyin the axial direction and characterized bytwo-phase annular conditions.
13 Dropwise Condensation Steam condensation on copper surfaces:
14 Problem: Condensation on a Vertical Plate Problem a,b: Condensation and heat rates per unit width for saturatedsteam at 1 atm on one side of a vertical plate at 54˚C if(a) the plate height is 2.5m and (b) the height is halved.SCHEMATIC:
15 Problem: Condensation on a Vertical Plate (cont)
16 Problem: Condensation on a Vertical Plate (cont)
17 Problem: Condensation on a Vertical Plate (cont) COMMENT: