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4

# 40% of incident radiant energy on the surface of a thermally transparent body is reflected back. If the transmissivity of the body be 0.15, then the emissivity of surface is

0.45

0.55

0.40

0.75

A. 0.45

### Related Questions

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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

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4

# The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as

Kirchoffs law

Stefan's law

Wien' law

Planck's law

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4

# The unit of Stefan Boltzmann constant is

Watt/cm² °K

Watt/cm4 °K

Watt²/cm °K⁴

Watt/cm² °K⁴

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4

# Temperature of steam at around 540°C can be measured by

Thermometer

Thermistor

Thermocouple

None of these

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4

# Joule sec is the unit of

Universal gas constant

Kinematic viscosity

Thermal conductivity

Planck's constant

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4

# The expression Q = ρ AT4 is called

Fourier equation

Stefan-Boltzmann equation

Newton Reichmann equation

Joseph-Stefan equation

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4

# A designer chooses the values of fluid flow rates and specific heats in such a manner that the heat capacities of the two fluids are equal. A hot fluid enters the counter flow heat exchanger at 100° C and leaves at 60° C. A cold fluid enters the heat exchanger at 40° C. The mean temperature difference between the two fluids is

20°C

40°C

60°C

66.7°C

What is the correct answer?

4

# The value of Prandtl number for air is about

0.1

0.3

0.7

1.7

What is the correct answer?

4

# 40% of incident radiant energy on the surface of a thermally transparent body is reflected back. If the transmissivity of the body be 0.15, then the emissivity of surface is

0.45

0.55

0.40

0.75

What is the correct answer?

4

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

Melting of ice

Boiler furnaces

Condensation of steam in condenser

None of these

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4

# Moisture would find its way into insulation by vapour pressure unless it is prevented by

High thickness of insulation

High vapour pressure

Less thermal conductivity insulator

A vapour seal

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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

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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

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4

# The value of the wavelength for maximum emissive power is given by

Wien's law

Planck's law

Stefan's law

Fourier's law

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4

# Two long parallel surfaces each of emissivity 0.7 are maintained at different temperatures and accordingly have radiation heat exchange between them. It is desired to reduce 75% of the radiant heat transfer by inserting thin parallel shields of emissivity 1 on both sides. The number of shields should be

One

Two

Three

Four

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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

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4

# The amount of heat flow through a body by conduction is

Directly proportional to the surface area of the body

Directly proportional to the temperature difference on the two faces of the body

Dependent upon the material of the body

All of the above

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4

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

Thermal conductivity

Thermal diffusivity

Density

Dynamic viscosity

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4

# Cork is a good insulator because it has

Free electrons

Atoms colliding frequency

Low density

Porous body

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4

# A heat exchanger with heat transfer surface area A and overall heat transfer coefficient U handles two fluids of heat capacities Cmax and Cmin. The number of transfer units (NTU) used in the analysis of heat exchanger is specified as

A.Cmin/U

U/A.Cmin

A.U.Cmin

A.U/Cmin

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4

# Which of the following is a case of steady state heat transfer?

I.C. engine

Air preheaters

Heating of building in winter

None of the above

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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

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4

# Sensible heat factor is given by (where S.H. = Sensible heat, and L.H. = Latent heat)

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

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

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

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

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4

# Heat is transferred by all three modes of transfer, viz. conduction, convection and radiation in

Electric heater

Steam condenser

Boiler

Refrigerator condenser coils

What is the correct answer?

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

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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

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4

# In a heat exchanger with one fluid evaporating or condensing, the surface area required is least in

Parallel flow

Counter flow

Cross flow

All of these

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4

# An electric cable of aluminium conductor (k = 240 W/mK) is to be insulated with rubber (k = 0.15 W/mK). The cable is to be located in air (h = 6 W/m²). The critical thickness of insulation will be

25 mm

40 mm

160 mm

800 mm

What is the correct answer?

4

# Total emissivity of polished silver compared to black body is

Same

Higher

More or less same

Very much lower

What is the correct answer?

4