Please enable JavaScript.

Coggle requires JavaScript to display documents.

Boiling & Condesation - Coggle Diagram

- - - - Boiling regimes differ considerably in their character
  - - - with the absence of bulk fluid flow
      - ΔT excess=Ts-Tsat
      - Enhancement of Heat Transfer in Pool Boiling
        
        The rate of heat transfer in the nucleate boiling regime strongly depends on the number of active nucleation sites on the surface, and the rate of bubble formation at each site.
        
        , modification that enhances nucleation on the heating surface will also enhance heat transfer in nucleate boiling.
        
        Surfaces that provide enhanced heat transfer in nucleate boiling permanently are being manufactured and are available in the market.
        
        Heat transfer can be enhanced by a factor of up to 10 during nucleate boiling, and the
        
        critical heat flux by a factor of 3
    - - with the presence of bulk fluid flow
    - - liquid is below the saturation temperature
    - - liquid is equal to the saturation temperature
    - - At high surface temperatures (typically above 300°C), heat transfer across the vapor film by radiation becomes significant and needs to be considered.
- - - - The condensate wets the surface and forms a liquid film.
        
        The surface is blanketed by a liquid film which serves as a resistance to heat transfer.
      - Film Condensation on a Vertical Plate
        
        liquid film starts forming at the top of the plate and flows downward under the influence of gravity.
        
        δ increases in the flow direction x
        Heat in the amount hfg is released during condensation and is transferred through the film to the plate surface.
        
        Ts must be below the saturation temperature for condensation
        
        The temperature of the condensate is Tsat at the interface and decreases gradually to Ts at the wall.
        
        Vertical Plate ─ Flow Regimes
        
        The dimensionless parameter controlling the transition between regimes is the Reynolds number defined as:
        
        Three prime flow regimes:
        
        Re<30 ─ Laminar (wave-free),
        
        30<Re<1800 ─ Wavy-laminar,
        
        Re>1800 ─ Turbulent
    - - The condensed vapor forms droplets on the surface
        
        The droplets slide down when they reach a certain size.
        
        No liquid film to resist heat transfer.
        
        As a result, heat transfer rates that are more than 10 times larger
      - Heat Transfer Correlations for Film Condensation ─ Vertical wall
        
        Both the plate and the vapor are maintained at constant temperatures of Ts and Tsat, respectively, and the temperature across the liquid film varies linearly.
        
        Heat transfer across the liquid film is by pure conduction
        
        The velocity of the vapor is low (or zero) so that it exerts no drag on the condensate (no viscous shear on the liquid–vapor interface
        
        The flow of the condensate is laminar (Re<30) and the properties of the liquid are constant.
        
        The acceleration of the condensate layer is negligible
    - - The waves at the liquid–vapor interface tend to increase heat transfer
        
        Knowledge is based on experimental studies
        
        The increase in heat transfer due to the wave effect is, on average, about 20 percent, but it can exceed 50 percent
        
        Based on his experimental studies, Kutateladze (1963) recommended the following relation