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Boiling and Condensation, The bubbles form at an increasing rate at an…
Boiling and Condensation
BOILING HEAT TRANSFER
BOILING
Occurs at the solid-liquid interface when a liquid is brought into contact with a surface maintained at a temperature sufficiently above the saturation of the liquid
EVAPORATION
Occurs at the liquid-vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a given temperature.
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POOL BOILING
NATURAL CONVECTION
- Bubbles do not form on the heating surface until the liquid is heated a few degrees above the saturation temperature (about 2 to 6°C for water)
- The fluid motion in this mode of boiling is governed by natural convection currents.
- Heat transfer from the heating surface to the fluid is by natural convection.
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HEAT TRANSFER CORRELATIONS
- Boiling regimes differ considerably in their character
- In the natural convection boiling regime heat transfer rates can be accurately determined using natural convection relations.
CONDENSATION
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FILM CONDENSATION
- 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.
DROPWISE CONDENSATION
- The condensed vapor forms droplets on the surface
- No liquid film to resist heat transfer.
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- The bubbles form at an increasing rate at an increasing number of nucleation sites as we move along the boiling curve toward point C.
- In region A–B the stirring and agitation caused by the entrainment of the liquid to the heater surface is primarily responsible for the increased heat transfer coefficient.
- In region A–B the large heat fluxes obtainable in this region are caused by the combined effect of liquid entrainment and evaporation.
- After point B the heat flux increases at a lower rate with increasing ΔTexcess, and reaches a maximum at point C.
- The heat flux at this point is called the critical (or maximum) heat flux
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- liquid film starts forming at the top of the plate and flows downward under the influence of gravity.
- 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.
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- Beyond Point D the heater surface is completely covered by a continuous stable vapor film.
- Point D, where the heat flux reaches a minimum is called the Leidenfrost point.
- The presence of a vapor film between the heater surface and the liquid is responsible for the low heat transfer rates in the film boiling region.
- The heat transfer rate increases with increasing excess temperature due to radiation to the liquid.
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- absence of bulk fluid flow
- any motion of the fluid is due to the natural convection currents and the motion of the bubbles under the influence of buoyancy
- with presence of bulk fluid flow
- in flow boiling, the fluid is forced to move a heated pipe or over a surface by external means such as pump
- When ΔTexcess is increased past point C, the heat flux decreases.
- This is because a large fraction of the heater surface is covered by a vapor film, which acts as an insulation.
- When the power applied to the heated surface exceeded the value at point c, the surface temperature increased suddenly to point E
- when the power is reduced gradually starting form point E, the cooling curve will drop in excess temperature
The rate of heat transfer strongly depends on the nature of nucleation and the type and the condition of the heated surface.
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- The CHF is independent of the fluid–heating surface combination, as well as the viscosity, thermal conductivity, and the specific heat of the liquid.
- The CHF increases with pressure up to about one-third of the critical pressure, and then starts to decrease and becomes zero at the critical pressure.
- The CHF is proportional to hfg, and large maximum heat fluxes can be obtained using fluids with a large enthalpy of vaporization, such as water.
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- Minimum heat flux, which occurs at the Leidenfrost point, is of practical interest since it represents the lower limit for the heat flux in the film boiling regime.
- Zuber derived the following expression for the minimum heat flux for a large horizontal plate
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- It is observed to underpredict heat transfer because it does not take into account the effects of the nonlinear temperature profile in the liquid film and the cooling of the liquid below the saturation temperature.
