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

D. All of the above

4

# Depending on the radiating properties, body will be transparent when

P = 0, x = 0 and a = 1

P=1, x = 0, and a = 0

P = 0, T= 1, and a = 0

X = 0, a + p = 1

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 expression Q = ρ AT4 is called

Fourier equation

Stefan-Boltzmann equation

Newton Reichmann equation

Joseph-Stefan equation

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

6

9

27

81

4

Emissivity

Transmissivity

Reflectivity

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

Conduction

Convection

Scattering

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

# 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

Glass

Water

Plastic

Air

4

# Heat flows from one body to other when they have

Different heat contents

Different specific heat

Different atomic structure

Different temperatures

4

# The thickness of thermal and hydrodynamic boundary layer is equal if Prandtl number is

Equal to one

Greater than one

Less than one

Equal to Nusselt number

4

0.45

0.55

0.40

0.75

4

Solids

Liquids

Gases

None of these

4

Conduction

Convection

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

# The amount of heat flow through a body by conduction is

Directly proportional to the surface area of the body

Directly proportional to the temperature difference on the two faces of the body

Dependent upon the material of the body

All of the above

4

# A perfect black body is one which

Is black in colour

Reflects all heat

Absorbs heat radiations of all wave lengths falling on it

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 logarithmic mean temperature difference (tm) is given by (where Δt1 and Δt2 are temperature differences between the hot and cold fluids at entrance and exit)

tm = (Δt1 - Δt2)/ loge (Δt1/Δt2)

tm = loge (Δt1/Δt2)/ (Δt1 - Δt2)

tm = tm = (Δt1 - Δt2) loge (Δt1/Δt2)

tm = loge (Δt1 - Δt2)/ Δt1/Δt2

4

Black body

Grey body

Opaque body

White body

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

# Heat transfer by radiation mainly depends upon

Its temperature

Nature of the body

Kind and extent of its surface

All of the above

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

Conduction

Convection

None of these

4

Wien's law

Planck's law

Stefan's law

Fourier's law

4

# LMTD in case of counter flow heat exchanger as compared to parallel flow heat exchanger is

Higher

Lower

Same

Depends on the area of heat exchanger

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