Conduction Flashcards
In steady conduction for a material whose thermal conductivity decreases with increase in temperature, the temperature gradient at the low temperature side will be _____.
Lower
A thick concrete slab exposed to flames on one side can be analysed using _____ model.
semi-infinite solid
. For given fin volume as the length of the fin is decreased the heat flow will _____ and then _________.
increase and then decrease
Heat conduction in insulators is mainly by _____.
lattice vibration
For a given amount of heat conducted, higher the value of thermal conductivity _____ will be the temperature difference.
Lower
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 < _____.
¼
If heat flow across a plane in a given slab should increase the temperature gradient at the plane should _____.
Increase
- To reduce error in temperature measurement of flowing fluids, the thermometer well length should be _____.
Longer
- Thermal conductivity of materials (k) generally varies with _____.
Temperature
- The ratio of heat flow by fin to the heat flow on the bare base areas is called _____ of the fin.
Effectiveness
- For the same characteristic dimension, the shape of the object for shorter heating time is _____.
Sphere
- The unit for thermal resistance in the SI system is _____.
K/W
- The unit for convective heat transfer coefficient in the SI system is _____.
W/m^2K
- In the SI system, the unit for thermal conductivity is _____.
W/mK
- 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 _____.
Square of thickness, heat generation rate
- For lumped parameter model heat capacity of the piece should be _____.
Low
- 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.
Lower
- For lumped parameter model volume/Area ratio should be _____.
Small
- For lumped parameter model to be used the temperature gradient within the body should be _____.
Very Small
- In transient conduction lumped capacity model can be used if _____.
Bi<0.1
In the case of a hollow sphere, the temperature profile along the radius will be a _____.
Hyperbolic
- Fin effectiveness is generally _____ than one.
Greater
- For lumped parameter model the surface resistance should be _____ compared to the internal resistance.
Higher
- Heat conduction in good conductors is largely by _____.
Free electron flow
- With the same cross section two fins each of length L will dissipate _____ compared to a single fin of length 2L.
More heat
- In the case of a hollow cylinder, the temperature profile along the radius will be _____.
logarithmic
- 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
- In a conducting slab with uniform heat generation the temperature variation along the thickness is _____.
Parabolic
- In fins the temperature gradient is highest at the _____.
Base
- Thermal diffusivity of a material is indicative of _____ under transient conditions.
heat penetration
- The pure metal whose thermal conductivity is highest at atmospheric temperatures is _____.
Silver
- 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
- Convection resistance over a given surface area A is _____.
1/hA
- With increase in temperature thermal conductivity of many liquids will _____.
Decreases
- With increase in temperature thermal conductivity of good conductors will _____.
Decreases
- With increase in temperature thermal conductivity of insulators will _____.
Increase
- With increase in temperature thermal conductivity of gases in general will _____.
Increase
- With increase in temperature thermal conductivity of water and glycerin will _____.
remain constant
- With increase in temperature thermal conductivity of water and glycerin will _____.
remain constant
- For a given material and temperature drop, if heat conducted is to be reduced the thickness should be _____.
Increased
- 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
- For a given thermocouple if the response time should be shortened then _____ should be increased.
convection coefficient
- For a given fin configuration the heat flow is proportional to _____ power of h and k.
0.5
- Conduction resistance due to a plane wall of thickness L of material with thermal conductivity k and area A is _____.
L/kA
- The temperature profile in a plane wall under steady conduction will be a straight line if thermal conductivity _____.
Is constant
- Along the length of a constant area fin the temperature gradient _____.
decreases
- The amount of heat flow through a body by conduction is
All of the other answers
- 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
- What is kinematics viscosity of a fluid?
dynamic viscosity per unit density of the fluid
- Thermal diffusivity is a
Physical property of a substance
- The Sherwood number is a convenient measure of
convective mass transfer coefficient
- The Peclet number in forced convection plays same role as
Rayleigh number (Ra) in natural convection
- 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
- 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
- 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
- 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
- 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
- What is the SI unit for absolute or dynamic viscosity (μ)?
Ns/m²
- The mechanical equivalent of heat is
1 cal = 4.186 J
- 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
- Which of the following property of air does not increase with rise in temperature?
Density
- 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
- In natural convection heat transfer, Nusselt number is function of
Grashof number and Prandtl number
- What should be the Biot number to assume the body at uniform temperature?
the Biot number should be less than 0.1
- Choose the correct statement or statements
the thermal conductivity of insulating solids increases with temperature
- Lumped parameter model can be used when
when all conditions are true
- 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
- Which of the following statement is wrong?
The thermal conductivity of solid metals increases with rise in temperature
- Boiling and condensation are:
A special case of the convection mechanism
- Unit of thermal conductivity in S.I. unit is:
W/mk
- 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∞)
- Thermal diffusivity of a substance is
All of the answers