Boiling Heat Transfer

Ref: V.P. Carey, Liquid-Vapor Phase-Change Phenomena (Hemisphere, Washington, 1992)

Liquid-vapor phase change (Vaporization)Homogeneous bubble nucleationHeterogeneous bubble nucleation

ClassificationPool boilingForced convection boiling (Two-phase flow)

Subcooled boilingSaturated boiling

Formulation

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Phenomenological descriptionExcess temperature Latent heat hfg

Surface tension Buoyancy

Dimensionless parameters

Nusselt number

Anonymous number

Jacob number

Prandtl number

Bond number

1

Nukiyama’s experiment - Power-controlled boiling phenomena

Free convection boiling Nucleate boiling

Isolated bubblesJets and columns

Transition boilingUnstable (partial) film boilingAbsent in power-control mode

Film boilingAbove Leidenfrost pointVapor blanketRadiation heat transfer significant

Free convection boiling (c: 1/4~5/4)

c.f. Natural convection over a flat (hot) plate

Nucleate boilingBubble inception

Fluctuations of local molecular densityBubble nuclei

(unstable equilibrium)

2

Observation (a=1.2, b=1/3)

(d=5~6)

Rosenow’s Model (1952)Postulate: HT from surface to liquid Bubble formation Local agitation due to liquid flowing behind the wake of departing bubbles Enhanced HT

Experimental observation that the effect of subcooling on HT generally disappears with increasing heat flux

Characteristic velocities ,

Characteristic bubble size

Characteristic length

Rosenow’s bubble departure diameter

In Incropera, (r=1/3 and s=n)

Note that

3

Critical heat fluxZuber (1958)

Critical velocity for Helmholtz instability

Since ,

Minimum heat flux

Film boiling

Bromley (1950)

4

Two phase flow regimes

5