Question  set 141

Steady and Unsteady Heat Transfer

1.      Regarding one dimensional heat transfer, choose the correct statement.
a) Steady – f (x), Unsteady – f (x, t)
b) Steady – f (x, t), Unsteady – f (x)
c) Steady – f (x, y, t), Unsteady – f (x)
d) Steady – f (y, z), Unsteady – f (y)
Answer: a
Explanation: In case of one dimensional heat flow steady state is a function of x coordinate only while unsteady state is a function of x coordinate and time only.

2.      Which statement is true regarding steady state condition?
a) There is a variation in temperature in the course of time
b) Heat exchange is constant
c) It is a function of space and time coordinates
d) Internal energy of the system changes
Answer: b
Explanation: Heat influx is always equal to heat efflux. It is a function of space coordinates only.

3.      Which of the following is an example of steady state heat transfer?
a) Boilers and turbines
b) Cooling of I.C engine
c) Chilling effect of cold wind on a warm body
d) Electric bulb cools down by the surrounding atmosphere
Answer: d
Explanation: System is a perfect black body.

4.      Heat transfer in a long, hollow cylinder which is maintained at uniform but different temperatures on its inner and outer surfaces may be assumed to be taking place in which direction?
a) Axial only
b) Unpredictable
c) Radial only
d) No heat transfer takes place
Answer: c
Explanation: Ambient temperature is uniform on the periphery of cylinder and temperature is uniform. So it takes place in the radial direction only.

5.      Heat transfer takes place according to which law?
a) Newton’s law of cooling
b) Second law of thermodynamics
c) Newton’s second law of motion
d) First law of thermodynamics
Answer: b
Explanation: Second law states about heat transfer between source and sink.

6.      Heat transfer takes place in liquids and gases is essentially due to
a) Radiation
b) Conduction
c) Convection
d) Conduction as well as convection
Answer: c
Explanation: Convection is a process by which thermal energy is transferred between solid and fluid flowing through it.

7.      The appropriate rate equation for convective heat transfer between a surface and adjacent fluid is prescribed by
a) Newton’s first law
b) Wein’s displacement law
c) Kirchhoff’s law
d) Newton’s law of cooling
Answer: d
Explanation: The rate equation used to describe the mechanism of convection is called Newton’s law of cooling when the solid surface is cooled by the fluid.

8.      Identify the wrong statement
a) The process of heat transfer is an irreversible process
b) For heat exchange, a temperature gradient must exist
c) A material medium is not necessary for heat transmission
d) Heat flow doesn’t depend on temperature
Answer: d
Explanation: Heat flows from higher to lower temperature.

9.      During a cold winter season, a person prefers to sit near a fire. Which of the following modes of heat transfer provides him the maximum heat?
a) Conduction from the fire
b) If it is near the fire, convection sounds good
c) Convection and radiation together
d) Radiation will provide quick warmth
Answer: d
Explanation: Heat transfer by radiation can occur between two bodies even when they are separated by a medium colder than both of them.

10.  Most unsteady heat flow occurs
a) Through the walls of the refrigerator
b) During annealing of castings
c) Through the walls of the furnace
d) Through lagged pipe carrying steam
Answer: b
Explanation: Under steady state condition, with time there is a change in temperature i.e. temperature field is a function of space and time.