Question  set 170

Lump System Analysis

1.     According to lumped system analysis, solid possesses thermal conductivity that is
a) Infinitely large
b) Infinitely small
c) Moderate
d) 50% small
Answer: a
Explanation: Solutions to the many of the transient heat flow problems are obtained by the lumped system parameter analysis.

2.     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
Answer: d
Explanation: It should depend on both time coordinates and space coordinates.

3.     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
Answer: b
Explanation: t – t _a/t _I – t _a = exponential (- h A T/p V c).

4.     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
Answer: c
Explanation: Solids have infinite thermal conductivities. It implies that internal conductance resistance is very low.

5.     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
Answer: a
Explanation: Others are the examples of non-periodic variation

6.     What is the criterion for the applicability of lump system analysis?
a) Mean length
b) Normal length
c) Characteristics length
d) Mass no
Answer: c
Explanation: The first set in establishing criteria for the applicability of lump system analysis is to define a characteristics length.

7.     What is the value of characteristics length for cylinder?
a) R/5
b) R/4
c) R/3
d) R/2
Answer: d
Explanation: π R² L/2 π R L = R/2.

8.     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
a) 0.0082
b) 0.0072
c) 0.0062
d) 0.0052
Answer: a
Explanation: For a cylindrical piece, the characteristic linear dimension is, l = volume/surface area = .00441 m. So, biot number = hl/k = -.00882.

9.     What is the value of characteristics length for sphere?
a) R/2
b) R/3
c) R/4
d) R/5
Answer: b
Explanation: 4/3 π R³/4 π R² = R/3.

10. What is the value of characteristics length for cube?
a) L/3
b) L/4
c) L/5
d) L/6
Answer: d
Explanation: L³/6 L² = L/6.