Heat Transfer MCQ’s – Lump System Analysis
1 - Question
According to lumped system analysis, solid possesses thermal conductivity that is
a) Infinitely large
b) Infinitely small
d) 50% small
Explanation: Solutions to the many of the transient heat flow problems are obtained by the lumped system parameter analysis.
2 - Question
The temperature and rate of heat conduction are undoubtedly dependent on
a) Time coordinates
b) Space coordinates
c) Mass coordinates
d) Both time and space coordinates
Explanation: It should depend on both time coordinates and space coordinates.
3 - Question
Glass spheres of 2 mm radius and at 500 degree Celsius are to be cooled by exposing them to an air stream at 25 degree Celsius. Find the minimum time required for cooling to a temperature of 60 degree Celsius. Assume the following property values Density = 2250 kg/m3 Specific heat = 850 J/kg K Conductivity = 1.5 W/m K
a) 13.78 seconds
b) 14.78 seconds
c) 15.78 seconds
d) 16.78 seconds
Explanation: t – t a/t I – t a = exponential (- h A T/p V c).
4 - Question
Which is true regarding lumped system analysis? (i) Conductive resistance = 0 (ii) Convective resistance = 0 (iii) Thermal conductivity = 0 (iv) Thermal conductivity = infinity Identify the correct statements
a) i and ii
b) i, ii and iv
c) i and iv
d) ii and iv
Explanation: Solids have infinite thermal conductivities. It implies that internal conductance resistance is very low.
5 - Question
Which of the following is an example of lump system analysis?
a) Heating or cooling of fine thermocouple wire due to change in ambient temperature
b) Heating of an ingot in an furnace
c) Cooling of bars
d) Cooling of metal billets in steel works
Explanation: Others are the examples of non-periodic variation.
6 - Question
What is the criterion for the applicability of lump system analysis?
a) Mean length
b) Normal length
c) Characteristics length
d) Mass no
Explanation: The first set in establishing criteria for the applicability of lump system analysis is to define a characteristics length.
7 - Question
What is the value of characteristics length for cylinder?
Explanation: π R2 L/2 π R L = R/2.
8 - Question
During heat treatment, cylindrical pieces of 25 mm diameter, 30 mm height and at 30 degree Celsius are placed in a furnace at 750 degree Celsius with convective coefficient 80 W/m2 degree. Find the value of biot number if thermal conductivity is 40 W/m degree
Explanation: For a cylindrical piece, the characteristic linear dimension is, l = volume/surface area = .00441 m. So, biot number = hl/k = -.00882.
9 - Question
What is the value of characteristics length for sphere?
Explanation: 4/3 π R3/4 π R2 = R/3.
10 - Question
What is the value of characteristics length for cube?
Explanation: L3/6 L2 = L/6.