4

# The natural convection air cooled condensers are used in

Domestic refrigerators

Water coolers

Room air conditioners

All of these

A. Domestic refrigerators

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

4

# The automobile radiator is a heat exchanger of

Parallel flow type

Counter flow type

Cross flow type

Regenerator type

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

# A steam pipe is to be lined with two layers of insulating materials of different thermal conductivities. For the minimum heat transfer,

The better insulation must be put inside

The better insulation must be put outside

One could place either insulation on either side

One should take into account the steam temperature before deciding as to which insulation is put where

4

Liquids

Energy

Temperature

Entropy

4

Reflected

Refracted

Transmitted

Absorbed

4

Grashoff number

Biot number

Stanton number

Prandtl number

4

One

Two

Three

Four

4

# Cork is a good insulator because it has

Free electrons

Atoms colliding frequency

Low density

Porous body

4

Stanton number

Biot number

Peclet number

Grashoff number

4

Wien's law

Planck's law

Stefan's law

Fourier's law

4

m²/hr

m²/hr °C

kcal/m² hr

kcal/m. hr °C

4

# Thermal diffusivity of a substance is

Directly proportional to the thermal conductivity

Inversely proportional to density of substance

Inversely proportional to specific heat

All of the above

4

0.1

0.23

0.42

0.51

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

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

Absolute temperature (T)

F

T

4

Watt/cm² °K

Watt/cm4 °K

Watt²/cm °K⁴

Watt/cm² °K⁴

4

W/m²K

W/m²

W/mK

W/m

4

Same

Less

Greater

None of these

4

# The heat is transferred by conduction, convection and radiation in

Melting of ice

Boiler furnaces

Condensation of steam in condenser

None of these

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 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

# Metals are good conductors of heat because

Their atoms collide frequently

Their atoms are relatively far apart

They contain free electrons

They have high density

4

# According to Stefan Boltzmann law, ideal radiators emit radiant energy at a rate proportional to

Absolute temperature

Square of temperature

Fourth power of absolute temperature

Fourth power of temperature

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

# 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

# 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

Parallel flow

Counter flow

Cross flow

All of these

4

# According to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a

Grey body

Brilliant white polished body

Red hot body

Black body