4

In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the

Reynold's number

Grashoff's number

Reynold's number, Grashoff's number

Prandtl number, Grashoff's number

D. Prandtl number, Grashoff's number

4

1 : 1

2 : 1

1 : 2

4 : 1

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

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

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

Stanton number

Biot number

Peclet number

Grashoff number

4

In free convection heat transfer, Nusselt number is function of

Grashoff number and Reynold number

Grashoff number and Prandtl number

Prandtl number and Reynold number

Grashoff number, Prandtl number and Reynold number

4

Unit of thermal conductivity in S.I. units is

J/m² sec

J/m °K sec

W/m °K

Option (B) and (C) above

4

S.H/(S.H + L.H)

(S.H + L.H) /S.H

(L.H - S.H)/S.H

S.H/(L.H - S.H)

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

When α is absorptivity, ρ is reflectivity and τ is transmissivity, then for a diathermanous body,

α = 1, ρ = 0 and τ = 0

α = 0, ρ = 1 and τ = 0

α = 0, ρ = 0 and τ = 1

α + ρ = 1 and τ = 0

4

The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles caused by some mechanical means, is known as

Conduction

Free convection

Forced convection

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

Parallel flow

Counter flow

Cross flow

All of these

4

Total emissivity of polished silver compared to black body is

Same

Higher

More or less same

Very much lower

4

Joule sec is the unit of

Universal gas constant

Kinematic viscosity

Thermal conductivity

Planck's constant

4

All of these

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

Depending on the radiating properties, a body will be white 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 = 1 Where a = absorptivity, p = reflectivity, x = transmissivity

4

0

0.5

0.75

1

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

A grey body is one whose absorptivity

Varies with temperature

Varies with the wave length of incident ray

Varies with both

Does not vary with temperature and wave length of the incident ray

4

W/m²K

W/m²

W/mK

W/m

4

Solids

Liquids

Gases

None of these

4

The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body

Move actually

Do not move actually

Affect the intervening medium

Does not affect the intervening medium

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

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

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

Cork is a good insulator because it has

Free electrons

Atoms colliding frequency

Low density

Porous body

4

Thermal conductivity of air with rise in temperature

Increases

Decreases

Remain constant

May increase or decrease depending on temperature