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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

A. 0.45

B. 0.55

C. 0.40

D. 0.75

Correct Answer :

A. 0.45


Related Questions

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4

Thermal diffusivity of a substance is

A. Directly proportional to the thermal conductivity

B. Inversely proportional to density of substance

C. Inversely proportional to specific heat

D. 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

A. Kirchoffs law

B. Stefan's law

C. Wien' law

D. Planck's law

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4

The unit of Stefan Boltzmann constant is

A. Watt/cm² °K

B. Watt/cm4 °K

C. Watt²/cm °K⁴

D. Watt/cm² °K⁴

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4

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

A. Thermometer

B. Thermistor

C. Thermocouple

D. None of these

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4

Joule sec is the unit of

A. Universal gas constant

B. Kinematic viscosity

C. Thermal conductivity

D. Planck's constant

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The expression Q = ρ AT4 is called

A. Fourier equation

B. Stefan-Boltzmann equation

C. Newton Reichmann equation

D. Joseph-Stefan equation

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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

A. 20°C

B. 40°C

C. 60°C

D. 66.7°C

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4

The value of Prandtl number for air is about

A. 0.1

B. 0.3

C. 0.7

D. 1.7

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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

A. 0.45

B. 0.55

C. 0.40

D. 0.75

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4

The heat is transferred by conduction, convection and radiation in

A. Melting of ice

B. Boiler furnaces

C. Condensation of steam in condenser

D. None of these

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4

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

A. High thickness of insulation

B. High vapour pressure

C. Less thermal conductivity insulator

D. 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)

A. tm = (Δt1 - Δt2)/ loge (Δt1/Δt2)

B. tm = loge (Δt1/Δt2)/ (Δt1 - Δt2)

C. tm = tm = (Δt1 - Δt2) loge (Δt1/Δt2)

D. tm = loge (Δt1 - Δt2)/ Δt1/Δt2

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4

Depending on the radiating properties, body will be transparent when

A. P = 0, x = 0 and a = 1

B. P=1, x = 0, and a = 0

C. P = 0, T= 1, and a = 0

D. X = 0, a + p = 1

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4

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

A. Wien's law

B. Planck's law

C. Stefan's law

D. 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

A. One

B. Two

C. Three

D. Four

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4

In counter flow heat exchangers

A. Both the fluids at inlet (of heat exchanger where hot fluid enters) are in their coldest state

B. Both the fluids at inlet are in their hottest state

C. Both the fluids at exit are in their hottest state

D. 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

A. Directly proportional to the surface area of the body

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

C. Dependent upon the material of the body

D. All of the above

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4

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

A. Thermal conductivity

B. Thermal diffusivity

C. Density

D. Dynamic viscosity

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4

Cork is a good insulator because it has

A. Free electrons

B. Atoms colliding frequency

C. Low density

D. Porous body

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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. A.Cmin/U

B. U/A.Cmin

C. A.U.Cmin

D. A.U/Cmin

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4

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

A. I.C. engine

B. Air preheaters

C. Heating of building in winter

D. None of the above

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4

Thermal conductivity of a material may be defined as the

A. 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

B. 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

C. Heat conducted in unit time across unit area through unit thickness when a temperature difference of unity is maintained between opposite faces

D. 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)

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

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

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

D. 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

A. Electric heater

B. Steam condenser

C. Boiler

D. Refrigerator condenser coils

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4

Depending on the radiating properties, a body will be white when

A. P = 0, x = 0 and a = 1

B. P=1, T = 0 and a = 0

C. P = 0, x = 1 and a = 0

D. 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

A. Thermal resistance

B. Thermal coefficient

C. Temperature gradient

D. Thermal conductivity

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4

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

A. Parallel flow

B. Counter flow

C. Cross flow

D. 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

A. 25 mm

B. 40 mm

C. 160 mm

D. 800 mm

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4

Total emissivity of polished silver compared to black body is

A. Same

B. Higher

C. More or less same

D. Very much lower

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The radiation emitted by a black body is known as

A. Black radiation

B. Full radiation

C. Total radiation

D. All of these