SHORT QUESTION ANSWERS-POOL BOILING       Q.Ans Convection         Q. A. Boiling Cond          Q.A. Fins        Q.A.Conduction-1


 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

12.   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

 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 also called the wetting capability.

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

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

( c) If the surface tension is high, very very less wettability, bubbles becomes still larger size and are unable to detach from the surface.

10.   What are various parameters for bubbles formed?

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.