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

C. Conduction

4

# According of Kirchhoff's law

Radiant heat is proportional to fourth power of absolute temperature

Emissive power depends on temperature

Emissive power and absorptivity are constant for all bodies

Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body.

4

h = k/ ρS

h = ρS/k

h = S/ρk

h = kρ/S

4

Conduction

Convection

Scattering

4

All of these

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

Emissivity

Transmissivity

Reflectivity

4

Grashoff number

Nusselt number

Weber number

Prandtl number

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

# Which of the following property of air does not increase with rise in temperature?

Thermal conductivity

Thermal diffusivity

Density

Dynamic viscosity

4

# Upto the critical radius of insulation,

Added insulation will increase heat loss

Added insulation will decrease heat loss

Convective heat loss will be less than conductive heat loss

Heat flux will decrease

4

# Thermal diffusivity is

A dimensionless parameter

Function of temperature

Used as mathematical model

A physical property of the material

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 heat transfer from a hot body to a cold body is directly proportional to the surface area and difference of temperatures between the two bodies. This statement is called

First law of thermodynamics

Newton's law of cooling

Newton's law of heating

Stefan's law

4

Aluminium

Steel

Brass

Copper

4

Moisture

Density

Temperature

All of the above

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

# The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles due to difference of density caused by temperature of the particle is known as

Conduction

Free convection

Forced convection

4

# Thermal diffusivity of a substance is

Directly proportional to thermal conductivity

Inversely proportional to density of substance

Inversely proportional to specific heat

All of the above

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

# Fourier's law of heat conduction is valid for

One dimensional cases only

Two dimensional cases only

Three dimensional cases only

Regular surfaces having non-uniform temperature gradients

4

Stanton number

Biot number

Peclet number

Grashoff number

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

4

# Free convection flow depends on

Density

Coefficient of viscosity

Gravitational force

All of these

4

2 TR

4 TR

8 TR

10 TR

4

# Heat transfer in liquid and gases takes place by

Conduction

Convection

Conduction and convection

4

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

Conduction

Convection

Conduction and convection

4

# The critical temperature is the temperature

Below which a gas does not obey gas laws

Above which a gas may explode

Below which a gas is always liquefied

Above which a gas will never liquefied

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

# The natural convection air cooled condensers are used in

Domestic refrigerators

Water coolers

Room air conditioners

All of these

4

# Thermal conductivity of air with rise in temperature

Increases

Decreases

Remain constant

May increase or decrease depending on temperature