4

Conduction

Convection

None of these

B. Convection

4

# Log mean temperature difference in case of counter flow compared to parallel flow will be

Same

More

Less

Depends on other factors

4

-1/3

-2/3

1

-1

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

h = k/ ρS

h = ρS/k

h = S/ρk

h = kρ/S

4

# 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

Convection

Conduction and convection

4

Iron

Concrete

Wood

4

# Pick up the wrong case. Heat flowing from one side to other depends directly on

Face area

Time

Thickness

Temperature difference

4

Stanton number

Biot number

Peclet number

Grashoff number

4

6

9

27

81

4

Black body

Grey body

Opaque body

White body

4

# The amount of radiation mainly depends upon the

Nature of the body

Temperature of the body

Type of surface of the body

All of these

4

# A grey body is one whose absorptivity

Varies with temperature

Varies with the wave length of incident ray

Varies with both

Does not vary with temperature and wave length of the incident ray

4

Thermometer

Thermistor

Thermocouple

None of these

4

# The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles due to difference of density caused by temperature of the particle is known as

Conduction

Free convection

Forced convection

4

# The insulation ability of an insulator with the presence of moisture would

Increase

Decrease

Remain unaffected

May increase/decrease depending on temperature and thickness of insulation

4

0.002

0.02

0.01

0.1

4

# According to Prevost theory of heat exchange

It is impossible to transfer heat from low temperature source to t high temperature source

Heat transfer by radiation requires no medium

All bodies above absolute zero emit radiation

Heat transfer in most of the cases takes place by combination of conduction, convection and radiation

4

# Total heat is the heat required to

Change vapour into liquid

Change liquid into vapour

Increase the temperature of a liquid or vapour

Convert water into steam and superheat it

4

# Thermal conductivity of solid metals with rise in temperature normally

Increases

Decreases

Remain constant

May increase or decrease depending on temperature

4

# Thermal conductivity of water in general with rise in temperature

Increases

Decreases

Remain constant

May increase or decrease depending on temperature

4

# According of Kirchhoff's law

Radiant heat is proportional to fourth power of absolute temperature

Emissive power depends on temperature

Emissive power and absorptivity are constant for all bodies

Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body.

4

0

0.5

0.75

1

4

# The thermal diffusivities for gases are generally

More than those for liquids

Less than those for liquids

More than those for solids

Dependent on the viscosity

4

Stanton number

Nusselt number

Biot number

Peclet number

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 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

# The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles caused by some mechanical means, is known as

Conduction

Free convection

Forced convection

4

# Which of the following statement is correct?

A grey body is one which absorbs all radiations incident on it.

At thermal equilibrium, the emissivity and absorptivity are same.

The energy absorbed by a body to the total energy falling on it, is called emissivity.

A perfect body is one which is black in colour.

4

# Metals are good conductors of heat because

Their atoms collide frequently

Their atoms are relatively far apart

They contain free electrons

They have high density