Question  set 140

Modes of Heat Transfer

 

1.      The literature of heat transfer generally recognizes distinct modes of heat transfer. How many modes are there?
a) One
b) Two
c) Three
d) Four
Answer: c
Explanation: There are three modes of heat transfer i.e. radiation, convection and conduction.

 

2.      Consider system A at uniform temperature t and system B at another uniform temperature T (t > T). Let the two systems be brought into contact and be thermally insulated from their surroundings but not from each other. Energy will flow from system A to system B because of

a) Temperature difference
b) Energy difference
c) Mass difference
d) Volumetric difference
Answer: a

Explanation: Greater the temperature imbalance the higher would be the rate of energy transfer.

 

3.      An oil cooler in a high performance engine has an outside surface area 0.12 m2 and a surface temperature of 65 degree Celsius. At any intermediate time air moves over the surface of the cooler at a temperature of 30 degree Celsius and gives rise to a surface coefficient equal to 45.4 W/ m 2 K. Find out the heat transfer rate?
a) 238.43 W
b) 190.68 W
c) 543.67 W
d) 675.98 W
Answer: b
Explanation: Q = (T2 – T1) A h = 0.12 (65-30) 45.4 = 190.68 W.

 

 

 

4.      Unit of the rate of heat transfer is
a) Joule
b) Newton
c) Pascal
d) Watt
Answer: d
Explanation: Unit of heat transfer is Joule but the rate of heat transfer is joule per second i.e. watt.

 

5.      Convective heat transfer coefficient doesn’t depend on
a) Surface area
b) Space
c) Time
d) Orientation of solid surface
Answer: a
Explanation: It is denoted by h and is dependent on space, time, geometry, orientation of solid surface.

 

 

 

6.      The rate equation used to describe the mechanism of convection is called Newton’s law of cooling. So rate of heat flow by convection doesn’t depend on
a) Convective heat transfer coefficient
b) Surface area through which heat flows
c) Time
d) Temperature potential difference
Answer: c
Explanation: It is directly proportional to all of above except time.

 

7.      How many types of convection process are there?
a) One
b) Three
c) Four
d) Two
Answer: b
Explanation: Forced, natural and mixed convection.

 

8.      Thermal conductivity is maximum for which substance
a) Silver
b) Ice
c) Aluminum
d) Diamond
Answer: d
Explanation: Thermal conductivity of diamond is 2300 W/m K.

 

9.      A radiator in a domestic heating system operates at a surface temperature of 60 degree Celsius. Calculate the heat flux at the surface of the radiator if it behaves as a black body
a) 697.2 W/m2
b) 786.9 W/m2
c) 324.7 W/m2
d) 592.1 W/m2
Answer: a
Explanation: As, q = Q/A = 5.67 * 10-8 (273+60)4 = 697.2.

 

10.  Which of the following is an example of forced convection?
a) Chilling effect of cold wind on a warm body
b) Flow of water in condenser tubes
c) Cooling of billets in the atmosphere
d) Heat exchange on cold and warm pipes
Answer: b
Explanation: In forced convection, the flow of fluid is caused by a pump, fan or by atmospheric winds.