4

# A non-dimensional number generally associated with natural convection heat transfer is

Grashoff number

Nusselt number

Weber number

Prandtl number

A. Grashoff number

4

# Thermal conductivity of air with rise in temperature

Increases

Decreases

Remain constant

May increase or decrease depending on temperature

4

# Heat is transferred by all three modes of transfer, viz. conduction, convection and radiation in

Electric heater

Steam condenser

Boiler

Refrigerator condenser coils

4

0.1

0.3

0.7

1.7

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

# The natural convection air cooled condensers are used in

Domestic refrigerators

Water coolers

Room air conditioners

All of these

4

Steam

Solid ice

Melting ice

Water

4

# Thermal conductivity of solid metals with rise in temperature normally

Increases

Decreases

Remain constant

May increase or decrease depending on temperature

4

Watt/cm² °K

Watt/cm4 °K

Watt²/cm °K⁴

Watt/cm² °K⁴

4

# In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the

Reynold's number

Grashoff's number

Reynold's number, Grashoff's number

Prandtl number, Grashoff's number

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

Moisture

Density

Temperature

All of the above

4

# The amount of radiation mainly depends on

Nature of body

Temperature of body

Type of surface of body

All of the above

4

# The total radiation from a black body per second per unit area is ________ fourth power of the absolute temperature. This statement is known as Stefan Boltzmann law.

Equal to

Directly proportional to

Inversely proportional to

None of these

4

Conduction

Convection

4

Kirchhoff's law

Stefan's law

Wines law

Planck's law

4

# Joule sec is the unit of

Universal gas constant

Kinematic viscosity

Thermal conductivity

Planck's constant

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

# The automobile radiator is a heat exchanger of

Parallel flow type

Counter flow type

Cross flow type

Regenerator type

4

0.1

0.23

0.42

0.51

4

RN = hl/k

RN = μ cp/k

RN = ρ V l /μ

RN = V²/t.cp

4

Same

Less

Greater

None of these

4

# Thermal conductivity of water ________ with rise in temperature.

Remain same

Decreases

Increases

May increase or decrease depending upon temperature

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

Pb = pa - pv

Pb = pa + pv

Pb = pa × pv

Pb = pa/pv

4

# Heat transfer in liquid and gases takes place by

Conduction

Convection

Conduction and convection

4

2 TR

4 TR

8 TR

10 TR

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

Wien's law

Planck's law

Stefan's law

Fourier's law

4

# Planck's law holds good for

Black bodies

Polished bodies

All coloured bodies

All of the above