This
post covers the unit wise 2 marks questions with answers of Unit 2 Convection, 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 II Convection
Part- A (2 Marks with Answers)
1. What is meant by free or natural
convection? Give examples.
If the fluid motion is produced due to
change in density resulting from temperature gradients, the mode of heat
transfer is said to be free or natural convection.
Ex:
(i) Ice Melting
(Heat
move to the ice from air-This causes the melting from a solid to liquid)
(ii) Steaming cup of hot tea
(The
steam is showing heat being transferred in the air)
2. What is forced convection? Give
examples.
If
the fluid motion is artificially created by means of an external force like a
blower or fan, that type of heat transfer is known as forced convection.
Ex:
Steam turbines, Air conditioning, Heat exchangers, Car radiator using fluid
3. What are the dimensionless parameters
used in forced convection.
(i) Reynolds number (Re)
(ii) Nusselt number (Nu)
(iii) Prandtl number (Pr)
4. What are the dimensionless parameters
used in free convection.
(i)
Grashof number (Gr)
(ii) Nusselt number (Nu)
(iii) Prandtl number (Pr)
5. Indicate the concept or
significance of boundary layer.
(i) A thin region near the body called the
boundary layer where the velocity and the temperature gradients are large.
(ii) The region outside the boundary layer
where the velocity and the temperature gradients are very nearly equal to their
free stream values.
6. Define critical Reynolds number.
What is the typical value for flow over a flat plate and flow through a pipe?
The Reynolds number at which flow changes
from laminar to turbulent is called critical Reynolds number.
For plate, it is 5 * 105
For Pipe it is 2300.
7. What is hydrodynamic and thermal
boundary layer?
The velocity of the fluid is less than 99%
of free stream velocity, it is said to be “hydro dynamic boundary layer”.
The temperature of the fluid is less than
99% of free stream temperature, it is said to be “thermal boundary layer”.
8. Define Reynolds number & Prandtl
number.
Reynolds number (Re) is defined as the
ratio of inertia force to the viscous force.
Prandtl
number (Pr) is defined as the ratio of the momentum diffusivity to the
thermal diffusivity.
9. Define Nussselt number &
Groshof number.
Nusselt
number (Nu) is defined as the ration of the heat
flow by convection process under an unit temperature gradient to the heat flow
rate by conduction under an unit temperature gradient through a stationary
thickness (L) of mater.
Groshof number (Gr)
is defined as the ratio of product of inertia force and buyoncy force to the
square of the viscous force.
10. How does laminar flow differ
from turbulent flow?
Laminar
flow
Laminar flow sometimes called stream line flow.
In this type of flow, the fluid moves in a
layers and each fluid particle follows a smooth continuous path.
The fluid particles in each layer remain
in orderly sequence without mixing with each other.
Turbulent
flow
In addition to the laminar type of flow a distinct
irregular flow is frequently observed in nature.
This type of flow is called turbulent
flow.
The path of any individual particle is zig-zag
and irregular.
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