Conduction Flashcards
1
Q
In steady conduction for a material whose thermal conductivity decreases with increase in temperature, the temperature gradient at the low temperature side will be _____.
A
Lower
2
Q
A thick concrete slab exposed to flames on one side can be analysed using _____ model.
A
semi-infinite solid
3
Q
. For given fin volume as the length of the fin is decreased the heat flow will _____ and then _________.
A
increase and then decrease
4
Q
Heat conduction in insulators is mainly by _____.
A
lattice vibration
5
Q
For a given amount of heat conducted, higher the value of thermal conductivity _____ will be the temperature difference.
A
Lower
6
Q
When explicit numerical method is used for 2D temperature calculations in transient conduction the choice of nodal thickness Δx and time interval Δτ should satisfy the condition αΔτ/Δx2 < _____.
A
¼
7
Q
If heat flow across a plane in a given slab should increase the temperature gradient at the plane should _____.
A
Increase
8
Q
- To reduce error in temperature measurement of flowing fluids, the thermometer well length should be _____.
A
Longer
9
Q
- Thermal conductivity of materials (k) generally varies with _____.
A
Temperature
10
Q
- The ratio of heat flow by fin to the heat flow on the bare base areas is called _____ of the fin.
A
Effectiveness
11
Q
- For the same characteristic dimension, the shape of the object for shorter heating time is _____.
A
Sphere
12
Q
- The unit for thermal resistance in the SI system is _____.
A
K/W
13
Q
- The unit for convective heat transfer coefficient in the SI system is _____.
A
W/m^2K
14
Q
- In the SI system, the unit for thermal conductivity is _____.
A
W/mK
15
Q
- In a heat conducting slab of given material, with uniform heat generation the difference in temperature between the center plane and the surface is directly proportional to _____.
A
Square of thickness, heat generation rate
16
Q
- For lumped parameter model heat capacity of the piece should be _____.
A
Low
17
Q
- In radial heat conduction in a hollow sphere/cylinder, the temperature gradient at the outer surface will be _____ compared to the gradient at the inner surface.
A
Lower
18
Q
- For lumped parameter model volume/Area ratio should be _____.
A
Small
19
Q
- For lumped parameter model to be used the temperature gradient within the body should be _____.
A
Very Small
20
Q
- In transient conduction lumped capacity model can be used if _____.
A
Bi<0.1
21
Q
In the case of a hollow sphere, the temperature profile along the radius will be a _____.
A
Hyperbolic
22
Q
- Fin effectiveness is generally _____ than one.
A
Greater
23
Q
- For lumped parameter model the surface resistance should be _____ compared to the internal resistance.
A
Higher
24
Q
- Heat conduction in good conductors is largely by _____.
A
Free electron flow
25
23. With the same cross section two fins each of length L will dissipate _____ compared to a single fin of length 2L.
More heat
26
24. In the case of a hollow cylinder, the temperature profile along the radius will be _____.
logarithmic
27
26. In a heat conducting slab of given thickness, with uniform heat generation the difference in temperature between the center plane and surface is _____ proportional to the thermal conductivity of the material.
Inversely
28
28. In a conducting slab with uniform heat generation the temperature variation along the thickness is _____.
Parabolic
29
29. In fins the temperature gradient is highest at the _____.
Base
30
30. Thermal diffusivity of a material is indicative of _____ under transient conditions.
heat penetration
31
32. The pure metal whose thermal conductivity is highest at atmospheric temperatures is _____.
Silver
32
33. When conductivity varies linearly with temperature the conductivity at the _____ can be used in the calculation of heat flow using the general equations.
mean temperature
33
34. Convection resistance over a given surface area A is _____.
1/hA
34
35. With increase in temperature thermal conductivity of many liquids will _____.
Decreases
35
36. With increase in temperature thermal conductivity of good conductors will _____.
Decreases
36
31. With increase in temperature thermal conductivity of insulators will _____.
Increase
37
41. With increase in temperature thermal conductivity of gases in general will _____.
Increase
38
39. With increase in temperature thermal conductivity of water and glycerin will _____.
remain constant
39
39. With increase in temperature thermal conductivity of water and glycerin will _____.
remain constant
40
37. For a given material and temperature drop, if heat conducted is to be reduced the thickness should be _____.
Increased
41
38. In steady conduction for a material whose thermal conductivity increases with increase in temperature, the temperature gradient at the high temperature side will be _____.
lower
42
40. For a given thermocouple if the response time should be shortened then _____ should be increased.
convection coefficient
43
42. For a given fin configuration the heat flow is proportional to _____ power of h and k.
0.5
44
43. Conduction resistance due to a plane wall of thickness L of material with thermal conductivity k and area A is _____.
L/kA
45
44. The temperature profile in a plane wall under steady conduction will be a straight line if thermal conductivity _____.
Is constant
46
45. Along the length of a constant area fin the temperature gradient _____.
decreases
47
1. The amount of heat flow through a body by conduction is
All of the other answers
48
2. When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by
Conduction and Convection
49
3. What is kinematics viscosity of a fluid?
dynamic viscosity per unit density of the fluid
50
4. Thermal diffusivity is a
Physical property of a substance
51
5. The Sherwood number is a convenient measure of
convective mass transfer coefficient
52
6. The Peclet number in forced convection plays same role as
Rayleigh number (Ra) in natural convection
53
7. Calculate the rate of heat flow through a composite wall of having three layers. The thicknesses of three layers are 0.5 m, 1 m and 0.2 m respectively. Thermal conductivity of these layers are 2.5 W/mK, 2 W/mK and 2.2 W/mK respectively. The temperature T1 and T4 are maintained at 1000 K and 200 K respectively. Take surface area of heat transfer 1 m2.
1011.4 W/m2
54
8. Suppose that a hot metal ball is immersed in cold water, then temperature distribution in the body depends upon
both thermal conductivity and convective heat transfer
| both A and B
55
9. What is the correct formula for the rate of heat transfer (q) through a plane wall of thickness s and surface are A, when temperature difference between two surfaces is ΔT ? The thermal conductivity is k.
q = (k A ΔT) / s
56
10. A composite wall of a furnace has 3 layers of equal thickness having thermal conductivities in the ratio of 1 : 2 : 4. What will be the temperature drop ratio across the three respective layers?
4: 2 : 1
57
11. What is the effect of convective heat transfer coefficient h on fin effectiveness?
fin is effective if the value of convective heat transfer coefficient h is small
58
12. What is the SI unit for absolute or dynamic viscosity (μ)?
Ns/m²
59
13. The mechanical equivalent of heat is
1 cal = 4.186 J
60
14. A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h₁, h₂ and h₃ respectively. The average heat transfer coefficient for the cube is
None of the answers
61
15. Which of the following property of air does not increase with rise in temperature?
Density
62
16. Thermal diffusivity of a substance is given by (where α = Thermal diffusivity, ρ = Density of substance, c = Specific heat, and k = Thermal conductivity)
α = k/ρc
63
17. In natural convection heat transfer, Nusselt number is function of
Grashof number and Prandtl number
64
18. What should be the Biot number to assume the body at uniform temperature?
the Biot number should be less than 0.1
65
19. Choose the correct statement or statements
the thermal conductivity of insulating solids increases with temperature
66
20. Lumped parameter model can be used when
when all conditions are true
67
21. What is the reason behind the fact that the thermal resistance of hollow cylinder for convection decreases with increase in the thickness of the insulation?
increase in surface area of the cylinder leads to more convective heat transfer
68
22. Which of the following statement is wrong?
The thermal conductivity of solid metals increases with rise in temperature
69
23. Boiling and condensation are:
A special case of the convection mechanism
70
24. Unit of thermal conductivity in S.I. unit is:
W/mk
71
25. What is the correct formula for the rate of heat transfer (q) from a surface of body of the area A to the surrounding fluid, when surface of the body is at temperature Ts and the surrounding fluid is at temperature T∞? Where, k = conductivity of the body and h = coefficient of convection
q = h A (Ts – T∞)
72
26. Thermal diffusivity of a substance is
All of the answers
73
27. What is the correct formula for the Nusselt number (Nu)?
Nu = (h L)/k
74
28. The Fourier number can be considered as
The dimensionless time in a conduction problem
75
29. For the composite cylinder as shown in figure, the thermal resistances of every layer from inside to outside of the cylinder are given as follows, R1 = ln (r1 / ri) / 2π(k1)L and R1 = ln (ro / r1) / 2π(k2)L. What will be the total thermal resistance of the composite cylinder?
R = R1 + R2
76
30. Consider a composite wall of three layers in series having thermal resistances R1, R2 and R3 respectively. The heat transfer takes place normal to the surface of the layers. How is the total thermal resistance of the composite system calculated?
(R1 + R2 + R3)
77
31. In steady state conduction with variable thermal conductivity if the conductivity decreases along the flow direction, then the temperature gradient along the flow direction will become
steeper
78
33. The thermal diffusivities for solids are generally
More than those for liquids and gases
79
34. The unit of Stefan-Boltzmann constant is
W/(m² K4)
80
35. The Nusselt number is a convenient measure of
convective heat transfer coefficient
81
36. The Biot number is given by
the ratio of internal conduction resistance to the external convection resistance
82
37. The thermal gradient in a heat generating cylinder under steady conduction, at half the radius location will be
One half of that at surface
83
38. The current in the electrical system is analogous to
the heat flow in the heat transfer system
84
39. Heat transfer deals with the rate of
energy transfer
85
40. The materials which have low thermal conductivity are called as
thermal insulators
86
41. Consider a fluid of height H, flowing on a stationary plate as shown in figure. The top most layer of the fluid is moving with velocity U0. Which of the following condition is correct for the given condition?
The shear stress (τ) is directly proportional to (U0 / H)
87
42. In free convection heat transfer, transition from laminar to turbulent flow is governed by the critical value of the
Prandtl number, Grashof number
88
43. The Sherwood number is
Mass diffusivity to molecular diffusivity
89
44. What is lumped heat capacity analysis?
the analysis of a system in which it is assumed to be at
48.The molecular diffusivity D
Increases as the temperature increases
49. Which of the following fluid can be considered as an ideal fluid?
non-viscous fluid
50. What does the Nusselt modulus or Nusselt number physically mean?
the ratio of the temperature gradient at the surface [(∂T/∂y)y=0] to the reference temperature gradient [(Tw – T∞)/L]
51. When the layers of a composite wall are in series and the thermal resistance of every individual layer is given then the total thermal resistance of the composite wall is given by:
adding both the thermal resistances
52. For effective working of fins, the thickness of the fins should be
small
53. The heat transfer process between body and its ambient is governed by an Internal Conductive Resistance (ICR) and an External Convective Resistance (ECR). The body can be considered to be a lumped heat capacity system is
ICR is negligible
54. Heat conducted through per unit area and unit thick face per unit time when temperature difference between opposite faces is unity, is called
thermal conductivity
55. The value of Prandtl number for air is about
0.7
uniform temperature
90
45. How is the rate of heat transfer from an infinite fin given? where h = convective heat transfer coefficient, P = perimeter of the fin, k = thermal conductivity of fin , A = cross-sectional area of the fin. Ts= Base temperature of the fin, T∞ = Temperature of surrounding fluid.
q = √(hPkA) (Ts – T,)
91
The heat transfer is largely governed by conduction from the surface of the fluid, when:
the velocity of the fluid is small
92
47. Sensible heat is the heat required to
Increase the temperature of a liquid of vapour
93
58. What is the relation between the absolute viscosities of liquid and gas?
μ(liquid) >> μ(gas)
94
59. What is the ratio of the buoyancy forces to the viscous forces acting on a fluid called?
Grashof number (Gr)
95
61. Consider that heat transfer is taking place through a fin having circular cross-sectional area, one dimensionally as shown in figure.
sum of rate of heat transfer by conduction out of the element (x+dx) and heat transfer by convection from the surface between x to (x+dx)
96
62. In the lumped parameter model, the temperature variation is
exponential with time
97
63. What does a composite wall mean?
all of the answers
98
64. Unit of thermal diffusivity is
m²/s
99
65. What is the correct formula for the Biot number?
hL/k
100
66. In a long fin if the convection coefficient is increased with other parameters maintained constant
The temperature drop will be faster along the length
101
67. What is the formula for the thermal resistance for convection at the outer surface of hollow cylinder? Where, h = convection heat transfer coefficient, L = length of hollow cylinder, r = outer radius of the cylinder
R = 1 / (2 π h r L)
102
70. Stefan Boltzmann law is applicable for heat transfer by
Radiation
103
71. The concept of overall coefficient of heat transfer is used in heat transfer problems of
Conduction and convection
104
72. Thermal conductivity of liquids with rise in temperature normally
Decreases
105
73. Viscosity of a fluid can be defined as
flow resistance offered by the fluid
106
74. The ratio of internal conduction resistance to the surface convection resistance is known as:
Biot number
107
75. Which of the following arrangements of fins is more effective for the same base area? 1. Large number of closely spaced fins; 2. Small number of thick fins
Arrangement 1
108
76. In the process of heat transfer through extended surfaces or fins, the entire surface area is at
different temperatures
109
78. When heat is transferred from hot body to cold body, in a straight line, without affecting the intervening medium, it is referred as heat transfer by
Radiation
110
79. Heat transfer in liquid and gases takes place by
Convection
111
81. In a given fin configuration, the increase in conductivity will
Increase the total heat flow
112
82. Which among the following has lowest thermal conductivity among the others?
Water
113
83. Thermal conductivity of solid metals with rise in temperature normally
Decreases
114
84. The rate of energy transferred by convection to that by conduction is called
Nusselt number
115
85. Prandtl number is the ratio of
kinematic viscosity (ν) to thermal diffusivity
116
87. Conduction is a process of heat transfer
From one particle of the body to another without the actual motion of the particles
117
88. What is the temperature gradient in the conduction heat transfer?
change in temperature per unit change in distance in the direction of heat flow
118
89. The Lewis number is
Thermal diffusivity to molecular diffusivity
119
90. For conduction through a spherical wall with constant thermal conductivity and with inner side temperature greater than outer wall temperature, (one dimensional heat transfer), what is the type of temperature distribution?
Hyperbolic
120
91. Heat transfer takes place as per
Second law of the thermodynamics
121
92. A fin of uniform cross-section A and perimeter P has a base temperature Ts and it is exposed to fluid having temperature T∞. How is the temperature distribution in the fin given? Where, m = √(hP/kA), h = convective heat transfer coefficient, k = thermal conductivity of fin , T = Temperature of the fin at the distance x from the base of fine, and the constants C1 and C2 depend on the boundary conditions
(T – T∞)/(Ts – T∞) = C1emx + C2e– mx
122
93. The ratio of Nusselt number and the product of Reynolds number and Prandtl number is equal to
Stanton number
123
Two plates spaced 150 mm apart are maintained at 1000°C and 70°C. The heat transfer will take place mainly by
Radiation
124
96. Pick up the wrong case. Heat flowing from one side to other depends directly on
Thickness
125
97. The Grashof number is
Buoyancy forces to viscous forces
126
98. What is the general heat conduction equation which gives the temperature distribution and conduction heat flow in an isotropic solid? Where, q̇ = rate of heat generation , k = thermal conductivity, α = (k/ρc) thermal diffusivity, ρ = density of the element, c = specific heat of the element
(∂2T/∂x2) + (∂2T/∂y2) + (∂2T/∂z2) + (q̇/k) = (1/α)(∂T/∂t)
127
99. Temperature at the end tip of the fin having uniform cross-sectional area is
Minimum
128
100. What is the formula for the thermal resistance (R) of a hollow cylinder of internal radius Ri, outer radius Ro and the length L? Thermal conductivity of the cylinder material is k. The heat flows from inner side to outer side.
R = ln (Ro / Ri) / (2πkL)
129
101. The Schmidt number in mass transfer plays the same role as
Prandtl number (Pr) in convection heat transfer
130
102. Consider a composite wall with two layers combined in parallel and heat is transferred through wall as shown in figure. The thermal resistances of two layers are R1 and R2 respectively. How is the total thermal resistance (R) of the wall calculated?
R = R1R2 / (R1 + R2)
131
103.What is the correct formula for the shear stress (τ) in a flowing fluid according to the Newton's law of viscosity? Where, μ = absolute viscosity of the flowing fluid, (du/dy) = change in velocity of a layer per unit change in distance of that layer from the surface, A = area of the surface on which the fluid is flowing
(τ) = μ(du/dy)
132
104. A hollow cylinder of length of 1m, inner radius (Ri) 0.01m and outer radius (Ro) 0.05m has a thermal conductivity (k) 20 W/mK. The inside surface temperature is at 500K. What is the outside surface temperature of the cylinder when the rate of radial heat transfer is 3.13 kW?
460 K
133
105. In a long fin if the parameter m = (hP/kA) increases, other parameters being maintained constant then
The temperature drop along the length will be steeper
134
106. The product of Reynolds number and Prandtl number is known as
Peclet number
135
107. What is the effect of change in outer radius of the hollow cylinder on the thermal resistance of convection?
the thermal resistance of convection decreases with increase in outer radius of the hollow cylinder
136
109. The parameters of a very long fin are: diameter of the fin d = 2 cm, thermal conductivity k = 200 W/mK, convective heat transfer coefficient h = 12 W/m2K, base temperature of the fin Ts = 500 °C, the air temperature T∞ = 50 °C. Calculate the rate of heat transfer from the fin.
97.94 W
137
110. The most effective way to reduce the temperature drop in a heat generating solid is to
reduce the linear dimension
138
112. The surface temperatures of a plate with thickness of 0.06 m are 100°C and 40°C. The thermal conductivity (k) of wall is 350 W/mK. What is the rate of heat transfer through the plate in kW/m2?
350
139
113. Why is the negative sign introduced in the equation of Fourier's law of heat conduction? q = – kA(dT/dx)
because heat is transferred from higher temperature to lower temperature
140
114. The Grashof number in natural convection plays the same role as
Reynolds number (Re) in forced convection
141
115. In a sphere under steady state conduction with uniform heat generation, the temperature gradient at half the radius location will be
one half of that at the surface
142
116. Aluminium is used as a fin material because
it has higher thermal conductivity
143
117. Which is the correct temperature profile in a homogeneous flat plate having thickness 2L with uniform heat generation for steady-state heat conduction? Where, x is a distance from the heat generation point.
Plane wall with uniform heat generation option a (ghose mohaddab)
144
118. A non-dimensional number generally associated with natural convection heat transfer is
Grashof number
145
119. A plane wall of thickness 2L has a uniform volumetric heat source q (W/m3 ). It is exposed to a fluid at temperature Tf at both the ends (x = ± L). The surface temperature Ts of the wall under steady-state condition (where h and k have their usual meanings) is given by
Ts = Tf + qL/h
146
120. Consider a plane wall of area A, having a layer of insulation on it. What will happen to the thermal resistance for convection of wall if the thickness of the insulation is increased?
the thermal resistance for convection remains same with increase in thickness of insulation
147
121. The value of thermal conductivity k depends upon
the material through which the heat is transferred
148
122. Reynolds number is the ratio of
Inertial forces to viscous forces
149
123. For a given sectional area of fin if the circumference is increased by adopting different geometric shape, then
The temperature variation along the fin length will be steeper
150
124. Fin effectiveness will be increased more by
higher thermal conductivity
151
125. Unit of thermal conductivity in S.I. units is
W/(m K)
152
126. [M L–1 t–2] is the dimensional formula for
Pressure
153
127. For a given volume of material for use in a pin fin
as length is increased heat flow will increase and after some length will decrease
154
131. Semi infinite model can be adopted when
all of answers
155
F/T: In a slab of material with variable conductivity, with conductivity decreasing with temperature the surface temperatures are 200°C and 100°C. The mid plane temperature will be higher than 150°C.
False
156
F/T: For transient conduction analysis of very small objects lumped capacitance method is used.
True
157
F/T: In three dimensional steady state conduction with uniformly spaced nodes the temperature at a node will be one sixth of the sum of the adjacent nodal temperatures.
True
158
F/T: Higher the value of Biot number slower will be the cooling of a solid.
True
159
F/T: To reduce the time constant of a thermocouple its characteristic linear dimension (V/A) should be reduced.
True
160
F/T: To reduce the time constant of a thermocouple its characteristic linear dimension (V/A) should be reduced.
True
161
F/T: Lumped capacity model can be used in the analysis of transient heat conduction if Biot number is greater than one.
False
162
F/T: Lumped parameter model can be used if the internal conduction resistance is high compared to the surface convection resistance.
False
163
F/T: To reduce the time constant of a thermocouple, the convection coefficient over its surface should be reduced.
False
164
F/T: The temperatures gradient at the inner surface will be steeper compared to that at the outer surface in radial heat conduction in a hollow cylinder.
True
165
F/T: In a hollow sphere with radial conduction, the mid plane temperature will be higher than the mean of surface temperatures.
False
166
F/T: Fins are more useful with liquids than with gases.
False
167
F/T: Fins for the same flow should be longer if the thermal conductivity of the material is increased.
False
168
F/T: In good conductors lattice vibration contributes more for heat conduction.
False
169
F/T: n a hollow cylinder, the temperature variation with radius will be linear.
False
170
F/T: For identical fins of different materials the tip to base temperature difference will be lower if the thermal conductivity is lower.
False
171
F/T: For the same amount of heat conduction through a slab, as thickness increases the temperature gradient should increase.
False
172
F/T: In mass transfer studies the function of Sherwood number is similar to Prandtl number in heat transfer studies.
False
173
F/T: In a slab conducting heat the surface temperatures are 200 and 100°C. The mid plane temperature will be 150°C if k is constant.
True
174
F/T: Convection coefficient is a material property.
False
175
F/T: In a hollow cylinder with radial conduction the mid plane temperature will be lower than the mean of surface temperatures.
True
176
F/T: Fins effectiveness is generally greater than one.
True
177
F/T: Thermal conductivity of water decreases with increase in temperature.
False
178
F/T: Lewis number is used to predict mass transfer rates using heat transfer rates at similar conditions.
True
179
F/T: Schmidt number replaces Nusselt number in convective mass transfer studies.
False
180
F/T: A solid of poor conductivity exposed for a short period to surface convection can be analysed as semi infinite solid.
True
181
F/T: Thermal conductivity is a material property.
True
182
F/T: A slab will cool the fastest compared to a long cylinder or sphere of the same characteristic dimensions when exposed to the same convection conditions.
False
