Heat-Transfer : Boiling and Condensation

 Flow Boiling

• Boiling is called flow boiling in the presence of bulk fluid flow.
• In flow boiling, the fluid is forced to move in a heated pipe or over a surface by external mean such as a pump.

 Subcooled Boiling

• When the temperature of the main body of the liquid is below the saturation temperature.

 Saturated Boiling

• When the temperature of the liquid is equal to the saturation temperature.

1. BOILING REGIMES IN POOL BOILING

• Boiling takes different forms, depending on the DT_excess =T_s-T_sat.

Figure: Typical Boiling Curve for water at 1 atm

   Natural Convection (to Point A on the Boiling Curve)

• 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 liquid is slightly superheated in this case (metastable state).

• 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.

   Nucleate Boiling

• The bubbles form at an increasing rate at an increasing number of nucleation sites as we move along the boiling curve toward point C.
• Region A–B ─isolated bubbles.
• Region B–C ─ numerous continuous columns of vapor in the liquid.
• 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 DT_excess, and reaches a maximum at point C.
• The heat flux at this point is called the critical (or maximum) heat flux, and is of prime engineering importance.

  Transition Boiling

• When DT_excess 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.
• In the transition boiling regime, both nucleate and film boiling partially occur.

   Film Boiling

• Beyond Point D the heater surface is completely covered by a continuous stable vapour 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.

   Burnout Phenomenon

• A typical boiling process does not follow the boiling curve beyond point C.
• When the power applied to the heated surface exceeded the value at point C even slightly, the surface temperature increased suddenly to point E.

Figure:The actual boiling curve obtained with heated platinum wire in water as the heat flux is increased and then decreased

• When the power is reduced gradually starting from point E the cooling curve experiences a sudden drop in excess temperature when point D is reached.

   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.
• Therefore, modification that enhances nucleation on the heating surface will also enhance heat transfer in nucleate boiling.
• Irregularities on the heating surface, including roughness and dirt, serve as additional nucleation sites during boiling.
• The effect of surface roughness is observed to decay with time.
• 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.

CONDENSATION

• Condensation occurs when the temperature of a vapor is reduced below its saturation temperature.
• Only condensation on solid surfaces is considered in this chapter.

   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.

Figure: Film Condensation

   Dropwise condensation

• 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 than with film condensation can be achieved.
• Small droplets grow as a result of continued condensation, coalesce into large droplets, and slide down when they reach a certain size.
• Large heat transfer coefficients enable designers to achieve a specified heat transfer rate with a smaller surface area.
• The challenge in dropwise condensation is not to achieve it but rather to sustain it for prolonged periods of time.

Figure: Dropwise Condensation

FILM CONDENSATION ON A VERTICAL PLATE

• Liquid film starts forming at the top of the plate and flows downward under the influence of gravity.
• d increases in the flow direction x
• Heat in the amount h_fg is released during condensation and is transferred through the film to the plate surface.
• T_s must be below the saturation temperature for condensation.
• The temperature of the condensate is T_sat at the interface and decreases gradually to T_s 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 Reynolds number increases in the flow direction.

Figure: Flow regimes during film condensation on a vertical plate.

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