Boiling and Condensation

Boiling

• Boiling is a liquid to vapor change process just like evaporation.
• Evaporation occurs at the liquid–vapor interface when the vapor pressure is less than the saturation pressure of the liquid at a given temperature. 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 temperature of the liquid.
• Evaporation involves no bubble formation or bubble motion.

Boiling and Evaporation Example

Classification of Boiling

1. Pool Boiling:  Boiling is called pool boiling when bulk fluid motion is absence. Fluid motion is due to natural convection and bubble-induced mixing.
2. Flow Boiling: Boiling is the presence of bulk fluid motion is called flow boiling (Forced Convection Boiling). Fluid motion is induced by external means such as pump, as well as by bubble-induced mixing. 
3. Subcooled Boiling: When the temperature of the liquid is below the saturation temperature.
4. Saturated Boiling: When the temperature of the liquid is equal to the saturation temperature.

The Boiling Curve

In a typical boiling curve, four different boiling regimes are observed: natural convection boiling, nucleate boiling, transition boiling, and film boiling depending on the excess temperature $ΔT_{excess} = T_s - T_{sat}$.

Boiling Curve of water at 1 atm

Natural Convection Boiling (to Point A)

• Liquid is slightly superheated in this case (a metastable condition) and evaporates when it rises to the free surface.
• Liquid motion is due to natural convection.

Nucleate Boiling (between Points A and C)

• Bubbles start forming at point A and increases number of nucleation sites as we move towards point C.
• Region A–B –– isolated bubbles are formed and heat flux rise sharply with increasing $ΔT_{excess}$.
• Region B–C –– Increasing number of nucleation sites causes bubble interactions and coalescence into jets and column. Heat flux increases at  lower rate and maximum at point C.

Transition Boiling(between Points C and D)

• When $ΔT_{excess}$ increases past point C, heat flux decreases because a large fraction of the heater surface is covered by a vapor film, which acts as an insulation.
• The transition boiling regime, which is also called the unstable film boiling regime.

Film Boiling (beyond Point D)

• At point D, where the heat flux reaches a minimum is called the Leidenfrost point.
• Heat transfer is by conduction and radiation across the vapor blanket, therefore,  heat transfer rate increases with increasing excess temperature.

Burnout Phenomenon

• A typical boiling process does not follow the boiling curve beyond point C.
• Any attempt to increase the heat flux beyond point C will cause the operation point on the boiling curve to jump suddenly from point C to point E.
• Surface temperature that corresponds to point E is beyond the melting point of most heater materials, and burnout occurs.
• Point C on the boiling curve is also called the burnout point, and the heat flux at this point the burnout heat flux.  

Condensation

Condensation occurs when the temperature of a vapor is reduced below its saturation temperature. 

Film condensation 

• The condensate wets the surface and forms a liquid film that slide down by gravity.

• This liquid wall serves as resistance to heat transfer. 

\[Re = \frac{(4δ) ρ_l u_m}{µ_l}\]

hydraulic diameter Dh = 4δ

• Re<30 ─ Laminar (wave-free),

• 30<Re<1800 ─ Wavy-laminar, 

• Re>1800 ─ Turbulent.

Dropwise condensation 

• The condensate forms droplets on the surface and surface covered by countless drops of varying size.

• Small droplets grow as a result of continued condensation and slide down when they reach a certain size.

• There is no film to resist heat transfer.

• Heat transfer rates in dropwise condensation can be achieved more than 10 times higher than in film condensation.

• Therefore, dropwise condensation is the preferred mode of condensation.

Dropwise condensation is achieved by-

• Adding a promoting chemical into the vapor (wax, fatty acid),
• Treating the surface with a promoter chemical,
• Coating the surface with a polymer such as teflon or a noble metal such as Au, Ag, Rh, Pd, Pt

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References:

Heat transfer: a practical approach- Cengel Y.A.

Fundamentals of Heat and Mass Transfer -Incropera