This
post covers the unit wise 2 marks questions with answers of Unit III Phase
change heat transfer and Heat Exchangers, repeatedly asking from Anna
University, Chennai, for the subject ME6502- Heat and Mass Transfer which is
the one of the important subjects of Mechanical Engineering students whom were
studying V semester.
Unit
III Phase change heat transfer and Heat Exchangers
Part- A (2 Marks with Answers)
1.
Define boiling and condensation.
Boiling
The change of phase
from liquid to vapor state is known as boiling.
Condensation
The change of phase
from vapor to liquid state is known as Condensation.
2.
Give the application of boiling and condensation.
(i) Thermal and nuclear
power plant
(ii)
Refrigeration systems
(iii) Air
conditioning systems
(iv) Process
of heating and cooling
3.
What is meant by pool boiling?
If heat is added to a liquid from a
submerged solid surface, the boiling process is referred to as 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.
4. What is meant by film wise
condensation?
The liquid condensate wets the solid
surface, spreads out and forms a continuous film over the entire surface is known
as film wise condensation.
5.
What is meant by drop wise condensation? Give its merits.
The vapor condensation, the vapor
condenses into small liquid droplets of various sizes which fall down the
surface in a random fashion.
Merits:
A
large portion of the area of the plate is directly exposed to vapor. The heat
transfer rate in drop wise condensation is 10 times higher than in film
condensation.
6. What is heat exchanger? What are
the types of heat exchangers?
A heat exchanger is defined as equipment
which transfers the heat from a hot fluid to cold fluid.
Types
(i)
Direct contact heat exchangers
(ii) Direct contact heat exchangers
(iii) Surface heat exchangers
(iv) Parallel flow heat exchangers
(v) Counter flow heat exchangers
(vi) Cross flow heat exchangers
(vii) Shell and tube heat exchangers
(viii) Compact heat exchangers
7. What do you meant by parallel
and counter flow heat exchangers?
Parallel
flow heat exchangers:
In this type of heat
exchanger, hot and cold fluids move in the same direction.
Counter
flow heat exchangers:
In this type of heat
exchanger, hot and cold fluids move in parallel but opposite directions.
8.
Write short notes on compact heat exchangers.
There are many special purpose heat
exchangers called compact heat exchangers.
They are generally employed when
convective heat transfer coefficient associated with one of the fluids is much
smaller than that associated with the other fluid.
9.
What is meant by LMTD?
We know that the
temperature difference between the hot and cold fluids in the heat exchanger
varies from point to point.
In
addition various modes of heat transfer are involved.
Therefore
based on the concept of appropriate mean temperature difference, also called
logarithmic mean temperature difference, the total heat transfer rate in the
heat exchanger is expressed as
Q=UA ΔTm
Where
U-Overall heat transfer coefficient in W/m2K
A- Area in m2
ΔTm – LMTD
10. What is meant by fouling
factor?
We know, the surfaces of heat exchangers
do not remain clean after it has been in use for some time.
The surfaces become fouled with scaling or
deposits.
The effect of these deposits affecting the
value of overall heat transfer coefficient.
This effect is taken care of by
introducing an additional thermal resistance called the fouling resistance.
11. What is meant by heat exchanger
effectiveness?
It is defined as the ratio of actual heat
transfer to the maximum possible heat transfer.
Effectiveness = Q/Q max
12.
What is meant by NTU? Give its expression.
Number
of Transfer Unit (NTU) is a method which is used to determine the inlet or exit
temperatures of heat exchangers.
NTU=
UA / Cmin
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