Pool boiling phase conversion while

fluid is at rest. It is common in day

today life. There are empirical correlations

to find the heat transfer during pool

boiling. Boiling a heat transfer process in

which liquid changes to vapor at the

saturation temperature. In this, there is

latent heat transfer . The latent heat is very

high. Thus, rate of heat transfer is quiet

high. This heat transfer is 10 to 20 times

than the convective heat transfer without

phase change. When there is no phase

change, heat transfer is sensible heat


  1. List the various types of boiling.

There are four types of boiling namely

(i)  Pool boiling or Nucleate boiling

(ii)  Forced convection boiling

(iii)  Sub-cooled boiling

(iv)  Saturated boiling

  1. Discuss pool boiling or nucleate boiling?

The liquid is at rest and is in contact with the heated surface.  Motion of the liquid is due to density difference. This is due to Buoyancy effect on heating. Bubble formation and bubble detachment causes fluid motion.  Phase change takes place at saturation temperature. The hot surface temperature is higher than the saturation temperature.

  1. Q. Explain forced convection boiling?

 Pump causes motion of the fluid.  Phase chase takes place at saturation temperature. In this, rate of heat transfer is still higher. It takes place in boiler tubes.

  1. Discuss sub-cooled boiling?

The liquid, in contact with the hot surface, changes into vapor. The remaining liquid sub-cools. It is at a temperature lower than the saturation temperature. Therefore, the bubble collapses as it tends to go up. Vapors do not reach the free surface.  It is case of sub-cooled boiling.

  1. Explain saturated boiling?

When the bulk of the liquid at saturation temperature, the bubbles  forms at the hot surface. These bubbles rise and reach up to the free surface of the liquid.  It is a case of saturated boiling.

  1. What is excess temperature?

The difference between temperature of the hot surface and the saturation temperature at the prevailing pressure.

ΔTexcess=ΔTe=TSurface –TSaturation

  1. What is a critical heat flux? Explain is its importance.

It is the maximum heat flux during nucleate or pool boiling. It is burn out point.

Importance: If the temperature of the surface not controlled as per critical flux, the film boiling will start. Heat flux to boiling will decrease. The surface temperature will go on increasing and ultimately the metal will melt.

3. What are the two separate processes of nucleate boiling?

  1. Sub-cooled boiling

The vapor forms at the hot surface. It starts rising in the liquid. But it condenses before reaching the free surface of the liquid.

  1. Saturated boiling

With the increase in excess temperature, the vapor forms at the hot surface. The vapor rises through the liquid due to buoyancy effect and leaves the free surface.

4. List the parameters involved in boiling and condensation.

  1.  Latent heat

  2.  Surface tension

  3.  Surface characteristics

  4.  Properties of the two phase systems.

  5.  Constant temperature during phase change

  6.  High heat transfer coefficients due to latent heat

  7.  Small temperature differences as excess temperature.

Q. Explain the mechanism of bubble formation and collapse in pool boiling.

When excess temperature (Tsur –Tsat) is > 5 0C and <100C, vapor formation starts from the heated surface due to absorbing of latent heat. But the liquid above the bubble is not so hot and hence bubble collapse as it tries to rise. It is in pool boiling since liquid is at rest.

Q. Write note on the critical diameter of a bubble.

Critical diameter of a bubble is the maximum diameter of a bubble. It is given by an equation given below:

dc  = Cβ( σlv / σls) ( σlv /(g(ρL —ρv))0.5

Where C = constant = 0.0148 for a water bubble

β=angle formed by a bubble with the horizontal surface = 300 =  for a water bubble

σlv = surface tension between liquid and vapor      N/m

σls  = Surface tension between liquid and solid surface        N/m

dc = maximum or critical diameter of a bubble

ρ = density,   kg/m3

Nucleate boiling correlations

There are five Pool boiling correlations.

(i) General correlation for no phase change q=hA ΔTe

Where h is heat transfer coefficient for nucleate boiling

(ii) ROSENHOW correlation for Nucleate boiling

Q= µlhfg (g(ρl–ρv)/σ)0.5 (CplΔTe/(CslhfgPrln))3

Where n=1 for water

n=1.7 for other fluids

Csl  = 0.013 for water copper

Csl  = 0.060 for water brass

 (iii)  Jacob relation for nucleate boiling over a horizontal plate

Nu = 0.16 (Gr. Pr) 0.33

(iv)  Jacob relation for nucleate boiling over a vertical plate

Nu= 0.61 (Gr. Pr) 0.25


                q.”chf = 0.18 hfg ρV0.5 [σg (ρL–ρV)]0.25



Ans. During boiling amount of vapor is increasing and it is a two phase phenomenon. Relations are available for single phase flow heat transfer.

Chen correlation gives the combination of single phase liquid convection and nucleate boiling heat flux

Therefore q’’ = hc(Tsur—Tl)+ hnb(Tsur—Tsat)=h’(Tsur—Tsat) =heq  (Tsur—Tsat)

Nucleate boiling heat flux

q= hnb(Tsur—Tsat) )

Single phase liquid convection

q= h(Tsur-Tl).

where h’ is single heat transfer coefficient for forced convection

heq   can be directly calculated by modified Dittus- Boelter equation

i.e. h’ =heq=0.023 (ks/D)F Rel0.8 Prl0.4

where F is a multiplier or correction factor for  two phase flow

Remember Dittus-Boelter equation is for liquid flow only

F=2.35(1/x+ 0.213)0.736   if 1/x >0.1

F=1                                      if 1/x ≤ 0.1

Q.   What are film boiling correlations?

(v)  Correlation for film boiling

q=hfA ΔTe

Where hf is heat transfer coefficient for film boiling

hf = hconv4/3 +( hrad)1/3

where hconv from Nusselt Eq for free or forced convection

hrad = 5.67 x 10-8 Є(T4super  — T4sat)/(Tsuper –Tsat)

Nu= havD/kv

= c((g(ρl–ρv)h’fg D3)/(νvkv(Tsur—Tsat))1/4

Where    c=0.62 for cylinders

c=0.67 for spheres

Q. Factors does the shape and size of bubble depend on the following three factors:

  1. Nature of the heating surface: thermal conductivity of the surface

  2. Condition of the heating surface: rough or smooth

  3. Surface tension of the liquid at the solid liquid interface. Surface tension is the wetting capability.

(a) When surface tension is less or low wettability. Small but more number of bubbles are readily form and rise up.

(b) If the surface tension is medium type, less wettability. Large size and less number of bubbles will form. These require more buoyant force for detachment.

( c) If the surface tension is high, very very less wettability. Bubbles become still larger size. These are unable to detach from the surface.

Q.   What are various parameters for bubbles formation?

Ans. Critical diameter of bubble = dc

dc = C β(σlv/σls)( σlv/(g(ρlv)))0.5

where σlv is surface tension between liquid and vapor

σls is surface tension between liquid and solid surface

C is a constant and for water its value is 0.0148

β is the angle formed as show in figure.

dc is the critical diameter of the bubble.

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