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Presentation1.pptx
1. Name:- Kunal Ramlal Chaudhari
Roll No:- 3368
Div./Batch:- C3
Sub:- Heat Transfer
Topics:-
1. Boiling Phenomena
2. Pool Boiling Curve
3. condensation and types of
condensation
Activity:- Make a presentation video on Convection
2. • 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
• The boiling heat transfer phenomenon may occurs in the following
forms:
1. Pool Boiling
2. Forced convection boiling
3. Sub-cooled boiling
4. Saturated boiling
1.Boiling Phenomena
3. 1. Pool Boiling
In this case the liquid above the hot surface is essentially stagnant and its motion near the surface is due to free
convection and mixing induced by bubble growth and detachment. The pool boiling occurs in steam boilers involving
natural convection
2. Forced convection boiling
This refers to a situation where the fluid motion is induced by external means (and also by free convection and bubble
induced mixing). The liquid is pumped and forced to flow.
3. Sub-cooled boiling
In this case the liquid temperature is below the saturation temperature and bubbles are formed in the vicinity of heat
surface. These bubbles after travelling a short path get condensed in the liquid which has a temperature less than the
boiling point.
4. Saturated boiling
the liquid temperature exceeds the saturation temperature. The vapour bubbles formed at the solid surface (liquid-solid
interface) are then propelled through the liquid by effects and eventually escape from a free surface
4. 2.Pool Boiling Curve
Pool Boiling :- In this case the liquid above the hot surface is essentially stagnant and its motion near the
surface is due to free convection and mixing induced by bubble growth and detachment. The pool boiling occurs
in steam boilers involving natural convection.
• Interface evaporation
• Nucleate Boiling
• Transition Boiling
• Film Boiling
5. Interface evaporation
• Interface evaporation (evaporation process with no bubble formation)
exists in region I, the free convection zone. Here the excess
temperature, Ate, is very small and = 5°C. In region the liquid near the
surface is superheated slightly, the convection currents circulate quid
and evaporation takes place at the liquid surface.
6. • The bubbles form at an increasing rate at an increasing number of nucleation
sites as we move along the boiling curve toward point C.
• Region A–B ─isolated bubbles.
• Region B–C ─ numerous continuous columns of vapor in the liquid.
• In region A–B the stirring and agitation caused by the entrainment of the
liquid to the heater surface is primarily responsible for the increased heat
transfer coefficient.
• In region A–B the large heat fluxes obtainable in this region are caused by the
combined effect of liquid entrainment and evaporation.
• After point B the heat flux increases at a lower rate with increasing DTexcess,
and reaches a maximum at point C.
• The heat flux at this point is called the critical (or maximum) heat flux, and is
of prime engineering importance.
Nucleate Boiling
7. • When DTexcess is increased past point C, the heat flux decreases.
• This is because a large fraction of the heater surface is covered by a
vapor film, which acts as an insulation.
• In the transition boiling regime, both nucleate and film boiling
partially occur.
Film Boiling
8. • Beyond Point D the heater surface is completely covered by a
continuous stable vapor film.
• The presence of a vapor film between the heater surface and the
liquid is responsible for the low heat transfer rates in the film boiling
region.
• The heat transfer rate increases with increasing excess temperature
due to radiation to the liquid.
Transition Boiling
9. • The condensation process is the reverse of boiling process. The
condensation sets in, whenever saturation vapour comes in contact
with a surface whose temperature is lower than the saturation
temperature corresponding to the vapour pressure.
• Two forms of condensation:
Film condensation.
Dropwise condensation.
3.Condensation and its types
10. • If the condensate tends to wet the surface and
thereby forms a liquid film, then the
condensation process is known as 'film
condensation’.
• The heat is transferred from the vapour to the
condensate formed on the surface by
'convection' and it is further transferred from the
condensate film to the cooling surface by the
conduction’.
• The heat transfer rates of film wise condensation
are lower than dropwise condensation.
Film condensation.
12. • In 'dropwise condensation' the vapour condenses
into small liquid droplets of various sizes which
fall down the surface in random fashion. The
drops form in cracks and pits on the surface, grow
in size, break away from the surface, knock off
other droplets.
• This type of condensation gives coefficient of heat
transfer generally 5 to 10 times larger than with
film condensation. Although dropwise
condensation would be preferred to film wise
condensation yet it is extremely difficult to
achieve or maintain. This is because surfaces
become 'wetted' after being exposed to
condensing vapour over a period of time.
Dropwise condensation.
