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

C. All bodies above absolute zero emit radiation

4

Glass

Water

Plastic

Air

4

# Sensible heat is the heat required to

Change vapour into liquid

Change liquid into vapour

Increase the temperature of a liquid of vapour

Convert water into steam and superheat it

4

# Absorptivity of a body will be equal to its emissivity

At all temperatures

At one particular temperature

When system is under thermal equilibrium

At critical 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

# Unit of thermal conductivity in S.I. units is

J/m² sec

J/m °K sec

W/m °K

Option (B) and (C) above

4

# The concept of overall coefficient of heat transfer is used in heat transfer problems of

Conduction

Convection

Conduction and convection

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

Liquids

Energy

Temperature

Entropy

4

h = k/ ρS

h = ρS/k

h = S/ρk

h = kρ/S

4

Minimum energy

Maximum energy

Both (A) and (B)

None of these

4

# The thermal diffusivities for solids are generally

Less than those for gases

Less than those for liquids

More than those for liquids and gases

More or less same as for liquids and gases

4

S.H/(S.H + L.H)

(S.H + L.H) /S.H

(L.H - S.H)/S.H

S.H/(L.H - S.H)

4

Absorptive power

Emissive power

Absorptivity

Emissivity

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

Aluminium

Steel

Brass

Copper

4

# Fourier's law of heat conduction gives the heat flow for

Irregular surfaces

Nonuniform temperature surfaces

One dimensional cases only

Two dimensional cases only

4

Stanton number

Nusselt number

Biot number

Peclet number

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

# Depending on the radiating properties, a body will be black when

P = 0, x = 0 and a = 1

P= 1, T = 0 and a = 0

P = 0, x = 1 and a = 0

X = 0, a + p = 0 Where a = absorptivity, p = reflectivity, X = transmissivity.

4

# Emissivity of a white polished body in comparison to a black body is

Higher

Lower

Same

Depends upon the shape of body

4

# The expression Q = ρ AT4 is called

Fourier equation

Stefan-Boltzmann equation

Newton Reichmann equation

Joseph-Stefan equation

4

Reflected

Refracted

Transmitted

Absorbed

4

# Thermal conductivity of air with rise in temperature

Increases

Decreases

Remain constant

May increase or decrease depending on temperature

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

# Thermal conductivity of water ________ with rise in temperature.

Remain same

Decreases

Increases

May increase or decrease depending upon temperature

4

# A grey body is one whose absorptivity

Varies with temperature

Varies with wavelength of the incident ray

Is equal to its emissivity

Does not vary with temperature and. wavelength of the incident ray

4

# Which of the following would lead to a reduction in thermal resistance?

In conduction, reduction in the thickness of the material and an increase in thermal conductivity.

In convection, stirring of the fluid and cleaning the heating surface.

In radiation, increasing the temperature and reducing the emissivity.

All of the above

4

# The natural convection air cooled condensers are used in

Domestic refrigerators

Water coolers

Room air conditioners

All of these

4

Kirchoffs law

Stefan's law

Wien' law

Planck's law