4

# Wiens law states that the wave length corresponding to ________ is proportional to the absolute temperature.

Minimum energy

Maximum energy

Both (A) and (B)

None of these

B. Maximum energy

4

# Thermal diffusivity is

A dimensionless parameter

Function of temperature

Used as mathematical model

A physical property of the material

4

# In heat exchangers, degree of approach is defined as the difference between temperatures of

Cold water inlet and outlet

Hot medium inlet and outlet

Hot medium outlet and cold water inlet

Hot medium outlet and cold water outlet

4

h = k/ ρS

h = ρS/k

h = S/ρk

h = kρ/S

4

# The heat transfer takes place according to

Zeroth law of thermodynamics

First law of thermodynamics

Second law of thermodynamics

Kirchhoff's law

4

Conduction

Convection

None of these

4

# The most commonly used method for the design of duct size is the

Velocity reduction method

Equal friction method

Static regains method

Dual or double method

4

# Thermal diffusivity of a substance is

Directly proportional to thermal conductivity

Inversely proportional to density of substance

Inversely proportional to specific heat

All of the above

4

# Thermal conductivity of non-metallic amorphous solids with decrease in temperature

Increases

Decreases

Remain constant

May increase or decrease depending on temperature

4

Solids

Liquids

Gases

None of these

4

Liquids

Energy

Temperature

Entropy

4

Hr (time)

Sq. m (area)

°C (temperature)

K.cal (heat)

4

1 : 1

2 : 1

1 : 2

4 : 1

4

# The rate of heat flow through a body is Q = [kA (T₁ - T₂)]/x. The term x/kA is known as

Thermal coefficient

Thermal resistance

Thermal conductivity

None of these

4

# A composite slab has two layers of different materials with thermal conductivities k₁ and k₂. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be

k₁ k₂

(k₁ + k₂)

(k₁ + k₂)/ k₁ k₂

2 k₁ k₂/ (k₁ + k₂)

4

# Heat conducted through per unit area and unit thick face per unit time when temperature difference between opposite faces is unity, is called

Thermal resistance

Thermal coefficient

Thermal conductivity

4

# Film coefficient is defined as the ratio of

Thermal conductivity to the equivalent thickness of the film of fluid

Temperature drop through the films of fluids to the thickness of film of fluids

Thickness of film of fluid to the thermal conductivity

Thickness of film of fluid to the temperature drop through the films of fluids

4

# Cork is a good insulator because it has

Free electrons

Atoms colliding frequency

Low density

Porous body

4

6

9

27

81

4

# The heat transfer by conduction through a thick cylinder (Q) is given by (where T₁ = Higher temperature, T₂ = Lower temperature, r₁ = Inside radius, r₂ = Outside radius, l = Length of cylinder, and k = Thermal conductivity)

Q = [2πlk (T₁ - T₂)]/2.3 log (r₂/r₁)

Q = 2.3 log (r₂/r₁)/[2πlk (T₁ - T₂)]

Q = [2π (T₁ - T₂)]/2.3 lk log (r₂/r₁)

Q = = 2πlk/2.3 (T₁ - T₂) log (r₂/r₁)

4

Watt/cm² °K

Watt/cm4 °K

Watt²/cm °K⁴

Watt/cm² °K⁴

4

Iron

Concrete

Wood

4

# The use of heat exchangers is made in

Condensers and boilers in steam plants

Condensers and evaporators in refrigeration and air conditioning units

All of the above

4

# The heat transfer by conduction through a thick sphere is given by

Q = 2πkr1 r2 (T1 - T2)/ (r2 - r1)

Q = 4πkr1 r2 (T1 - T2)/ (r2 - r1)

Q = 6πkr1 r2 (T1 - T2)/ (r2 - r1)

Q = 8πkr1 r2 (T1 - T2)/ (r2 - r1)

4

# A steam pipe is to be insulated by two insulating materials put over each other. For best results

Better insulation should be put over pipe and better one over it

Inferior insulation should be put over pipe and better one over it

Both may be put in any order

Whether to put inferior OIL over pipe or the better one would depend on steam temperature

4

# Reynolds number is the ratio of

Energy transferred by convection to that by conduction

Kinematic viscosity to thermal diffusivity

Inertia force to viscous force

None of the above

4

Grashoff number

Biot number

Stanton number

Prandtl number

4

Stanton number

Nusselt number

Biot number

Peclet number

4

# According to Stefan's law, the total radiation from a black body per second per unit area is proportional to

Absolute temperature

T⁵

T

4

# The expression Q = ρ AT4 is called

Fourier equation

Stefan-Boltzmann equation

Newton Reichmann equation

Joseph-Stefan equation