4

# Thermal conductivity of water ________ with rise in temperature.

Remain same

Decreases

Increases

May increase or decrease depending upon temperature

D. May increase or decrease depending upon 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

# 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

# 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

# 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

4

Solids

Liquids

Gases

None of these

4

# Heat flows from one body to other when they have

Different heat contents

Different specific heat

Different atomic structure

Different temperatures

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

# 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

# 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

# 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

# Thermal diffusivity is

A dimensionless parameter

Function of temperature

Used as mathematical model

A physical property of the material

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

# 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

Grashoff number

Biot number

Stanton number

Prandtl number

4

# When heat is transferred from hot body to cold body, in a straight line, without affecting the intervening medium, it is referred as heat transfer by

Conduction

Convection

Conduction and convection

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

# The heat transfer takes place according to

Zeroth law of thermodynamics

First law of thermodynamics

Second law of thermodynamics

Kirchhoff's law

4

Thermometer

Thermistor

Thermocouple

None of these

4

# Heat transfer in liquid and gases takes place by

Conduction

Convection

Conduction and convection

4

Steam

Solid ice

Melting ice

Water

4

Black body

Grey body

Opaque body

White body

4

# According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the

Absolute temperature

Square of the absolute temperature

Cube of the absolute temperature

Fourth power of the absolute temperature

4

# Heat transfer takes place as per

Zeroth law of thermodynamics

First law of thermodynamic

Second law of the thermodynamics

Kirchoff's law

4

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

Absolute temperature (T)

F

T

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

# The natural convection air cooled condensers are used in

Domestic refrigerators

Water coolers

Room air conditioners

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

Conduction

Convection

None of these

4

Wien's law

Planck's law

Stefan's law

Fourier's law