4

# Thermal conductivity of a material may be defined as the

Quantity of heat flowing in one second through one cm cube of material when opposite faces ^re maintained at a temperature difference of 1°C

Quantity of heat flowing in one second through a slab of the material of area one cm square, thickness 1 cm when its faces differ in temperature by 1°C

Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces

All of the above

D. All of the above

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

h = k/ ρS

h = ρS/k

h = S/ρk

h = kρ/S

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

Stanton number

Nusselt number

Biot number

Peclet number

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

Pb = pa - pv

Pb = pa + pv

Pb = pa × pv

Pb = pa/pv

4

# Film coefficient is defined as Inside diameter of tube

Equivalent thickness of film

Thermal conductivity Equivalent thickness of film Specific heat × Viscosity

Thermal conductivity Molecular diffusivity of momentum Thermal diffusivity

Film coefficient × Inside diameter Thermal conductivity

4

Temperature

Wave length

Physical nature

All of the above

4

Kirchhoff's law

Stefan's law

Wines law

Planck's law

4

Watt/cm² °K

Watt/cm4 °K

Watt²/cm °K⁴

Watt/cm² °K⁴

4

# The rate of heat flow through a body is Q = [kA (T₁ - T₂)]/x. The term x/kA is known as

Thermal coefficient

Thermal resistance

Thermal conductivity

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

Hr (time)

Sq. m (area)

°C (temperature)

K.cal (heat)

4

Parallel flow

Counter flow

Cross flow

All of these

4

# Fouling factor is used

In heat exchanger design as a safety factor

In case of Newtonian fluids

When a liquid exchanges heat with a gas

None of the above

4

Composition

Density

Porosity

All of the above

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

Reflected

Refracted

Transmitted

Absorbed

4

# Thermal diffusivity is a

Function of temperature

Physical property of a substance

Dimensionless parameter

All of these

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

Temperature

Thickness

Area

Time

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

# 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

Conduction

Convection

4

# In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by

Convection

Conduction

Both convection and conduction

4

# Thermal conductivity of water in general with rise in temperature

Increases

Decreases

Remain constant

May increase or decrease depending on temperature

4

# Cork is a good insulator because it has

Free electrons

Atoms colliding frequency

Low density

Porous body

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

# Two plates spaced 150 mm apart are maintained at 1000°C and 70°C. The heat transfer will take place mainly by

Convection

Forced convection

Free convection