CH302 Heat Transfer Op. Handouts Chemical Engineering Department, IT

- 1 - Heat Transfer with Change of Phase

Lecture 13 Heat transfer with change of Phase

The most important processes which are associated with change of phase

(I) Boiling

(II) Condensation

(III) Melting or Solidification

(IV) Sublimation

Nu and Pr are familiar for single phase convection, here some new dimention less

parameters are Jakob number Ja and Bond nuimber Bo are used.

Jakob number Ja = CpΔt/λ is the ratio of the maximum sensible energy absorbed to

the latent heat.

Bond number Bo = g(ρl - ρv)L2/ σ, is the ratio of the gravitational body force to the

surface tension force

Phenomena of Boiling: The boiling heat transfer process may occur in the following forms or different regimes of

boiling are as follows:

(A) According to applied forces:

( I ) Pool boiling

( II ) Forced Convection boiling

(B) According to Temperature:

( I ) Sub cooled or local boiling

( II ) Saturated or boiling with net evaporation

The first two forms of boiling depends upon the presence of bulk fluid motion while the

last two forms depends on the bulk liquid temperature.

Pool Boiling (Free Convection Boiling)

Forced Convection Boiling

Sub cooled or Local Boiling

Saturation boiling or boiling with net evaporation:

Reference: Engineering Heat Transfer By R.Prakash & C.D.Gupta

CH302 Heat Transfer Op. Handouts Chemical Engineering Department, IT

- 2 - Heat Transfer with Change of Phase

Lecture 14 Regimes of pool boiling: - The phenomena of pool boiling is characterized by different regimes

- Nukiyama, in 1934 actually distinguished the existence of various regimes of boiling.

Brew and Miller extended the theory further in 1937.

- Consider the case of pool boiling of water, Over a heated flat plate or hot wire

submerged in a pool or water, at tsat. After getting the datas, the heat flux (q/A) is plotted

against the excess temperature Δte = (tW – tsat).

The boiling phenomena is divided in three types of boiling which takes place in six

regimes as shown in the table

Sr. No.

Type of Boiling Regime No.

Type of Regime

1 Interface Evaporation 1 Free convection boiling

2 Nucleate boiling 2

3

Bubbles form but condense as they rise.

Bubbles rise to interface causing rapid

evaporation

3 Film Boiling 4

5

6

Partial nucleate boiling with unstable film

Stable film boiling

Radiation Predominant

( I ) Interface Evaporation: (II) Nucleate Boiling (III ) Film Boiling Critical heat flux point/ Burnout point/Boiling crisis point

Leiden frost point: Point D on the boiling curve, where the heat flux becomes

minimum due to the film resistance, is called Leiden frost point. Here heat is a

minimum qD = qmin., and the surface is completely covered by a vapour blanket. Heat

Transfer from the surface to the liquid occurs by conduction through the stable vapour

film.

Reference: Engineering Heat Transfer By R.Prakash & C.D.Gupta

CH302 Heat Transfer Op. Handouts Chemical Engineering Department, IT

- 3 - Heat Transfer with Change of Phase

Lecture 15 Nucleate boiling The nucleate boiling is of maximum engineering importance. It involves two processes.

( a ) The nucleation or formation of bubbles

( b ) The subsequent growth and motion of these bubbles

Factors affecting nucleate boiling: The nucleate boiling is affected by the following factors:

1. Material, Shape and condition of heating surface.

2. Liquid properties

3. Pressure

4. Mechanical agitation

( a ) Nucleation - Three typical shapes of vapor bubbles may be observed as shown in the figure below

( I ) Unwetted Surface ( II ) Partially wetted surface

(III ) Totally wetted surface Bubble growth and collapse

Bubble Growth and Motion:

Reference: Engineering Heat Transfer By R.Prakash & C.D.Gupta

CH302 Heat Transfer Op. Handouts Chemical Engineering Department, IT

- 4 - Heat Transfer with Change of Phase

Lecture 16 Condensation Phenomena: - Condensation is reverse of boiling

There are two types of condensation.

( I ) Filmwise Condensation

( II )Dropwise condensation

( I ) Film Condensation:

( II ) Dropwise condensation: Difference between Drop- wise Condensation and Film- wise Condensation

Reference: Engineering Heat Transfer By R.Prakash & C.D.Gupta