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.

A. P = 0, x = 0 and a = 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

# 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

4

Temperature

Wave length

Physical nature

All of the above

4

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

Face area

Time

Thickness

Temperature difference

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

20°C

40°C

60°C

66.7°C

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

# 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

# According to Wien's law, the wavelength corresponding to maximum energy is proportion to

Absolute temperature (T)

F

T

4

# A cube at high temperature is immersed in a constant temperature bath. It loses heat from its top, bottom and side surfaces with heat transfer coefficients of h₁, h₂ and h₃ respectively. The average heat transfer coefficient for the cube is

h₁ + h₂ + h₃

(h₁.h₂.h₃)1/3

1/h₁ + 1/h₂ + 1/h₃

None of these

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 expression Q = ρ AT4 is called

Fourier equation

Stefan-Boltzmann equation

Newton Reichmann equation

Joseph-Stefan equation

4

Steam

Solid ice

Melting ice

Water

4

Solids

Liquids

Gases

None of these

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

Absorptive power

Emissive power

Absorptivity

Emissivity

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

# Which of the following statement is wrong?

The heat transfer in liquid and gases takes place according to convection.

The amount of heat flow through a body is dependent upon the material of the body.

The thermal conductivity of solid metals increases with rise in temperature

Logarithmic mean temperature difference is not equal to the arithmetic mean temperature difference.

4

# Free convection flow depends on

Density

Coefficient of viscosity

Gravitational force

All of these

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

# 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

Kirchhoff's law

Stefan's law

Wines law

Planck's law

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

Absorptive power

Emissive power

Emissivity

None of these

4

# In counter flow heat exchangers

Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state

Both the fluids at inlet are in their hottest state

Both the fluids at exit are in their hottest state

One fluid is in hottest state and other in coldest state at inlet

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

# Radiation is the process of heat transfer in which heat flows from a ________, in a straight line, without affecting the intervening medium.

Cold body to hot body

Hot body to cold body

Smaller body to larger body

Larger body to smaller body