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4

Thermal diffusivity is

A. A dimensionless parameter

B. Function of temperature

C. Used as mathematical model

D. A physical property of the material

Correct Answer :

D. A physical property of the material


Related Questions

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A non-dimensional number generally associated with natural convection heat transfer is

A. Grashoff number

B. Nusselt number

C. Weber number

D. Prandtl number

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4

The rate of heat flow through a body is Q = [kA (T₁ - T₂)]/x. The term x/kA is known as

A. Thermal coefficient

B. Thermal resistance

C. Thermal conductivity

D. None of these

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4

In heat transfer, conductance equals conductivity (kcal/hr/sq.m/ °C/cm) divided by

A. Hr (time)

B. Sq. m (area)

C. °C (temperature)

D. K.cal (heat)

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4

The thickness of thermal and hydrodynamic boundary layer is equal if Prandtl number is

A. Equal to one

B. Greater than one

C. Less than one

D. Equal to Nusselt number

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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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The rate of energy transferred by convection to that by conduction is called

A. Stanton number

B. Nusselt number

C. Biot number

D. Peclet number

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4

Free convection flow depends on

A. Density

B. Coefficient of viscosity

C. Gravitational force

D. All of these

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4

A grey body is one whose absorptivity

A. Varies with temperature

B. Varies with wavelength of the incident ray

C. Is equal to its emissivity

D. Does not vary with temperature and. wavelength of the incident ray

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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

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

A. Absolute temperature

B. Square of temperature

C. Fourth power of absolute temperature

D. Fourth power of temperature

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The amount of radiation mainly depends upon the

A. Nature of the body

B. Temperature of the body

C. Type of surface of the body

D. All of these

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4

A steam pipe is to be insulated by two insulating materials put over each other. For best results

A. Better insulation should be put over pipe and better one over it

B. Inferior insulation should be put over pipe and better one over it

C. Both may be put in any order

D. Whether to put inferior OIL over pipe or the better one would depend on steam temperature

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4

Film coefficient is defined as the ratio of

A. Thermal conductivity to the equivalent thickness of the film of fluid

B. Temperature drop through the films of fluids to the thickness of film of fluids

C. Thickness of film of fluid to the thermal conductivity

D. Thickness of film of fluid to the temperature drop through the films of fluids

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4

Thermal conductivity of wood depends on

A. Moisture

B. Density

C. Temperature

D. All of the above

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According to Stefan Boltzmann law, the total radiation from a black body per second per unit area is directly proportional to the

A. Absolute temperature

B. Square of the absolute temperature

C. Cube of the absolute temperature

D. Fourth power of the absolute temperature

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4

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

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

B. P=1, x = 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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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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In case of liquids and gases, the heat transfer takes place according to

A. Conduction

B. Convection

C. Radiation

D. None of these

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When heat is transferred from one particle of hot body to another by actual motion of the heated particles, it is referred to as heat transfer by

A. Conduction

B. Convection

C. Radiation

D. Conduction and convection

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The product of Reynolds number and Prandtl number is known as

A. Stanton number

B. Biot number

C. Peclet number

D. Grashoff number

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4

Fourier's law of heat conduction is valid for

A. One dimensional cases only

B. Two dimensional cases only

C. Three dimensional cases only

D. Regular surfaces having non-uniform temperature gradients

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Stefan Boltzmann law is applicable for heat transfer by

A. Conduction

B. Convection

C. Radiation

D. Conduction and radiation combined

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Wiens law states that the wave length corresponding to ________ is proportional to the absolute temperature.

A. Minimum energy

B. Maximum energy

C. Both (A) and (B)

D. None of these

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4

Total heat is the heat required to

A. Change vapour into liquid

B. Change liquid into vapour

C. Increase the temperature of a liquid or vapour

D. Convert water into steam and superheat it

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The transfer of heat by molecular collision is known as

A. Conduction

B. Convection

C. Radiation

D. None of these

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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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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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Emissivity of a white polished body in comparison to a black body is

A. Higher

B. Lower

C. Same

D. Depends upon the shape of body

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Radiation is the process of heat transfer in which heat flows from a ________, in a straight line, without affecting the intervening medium.

A. Cold body to hot body

B. Hot body to cold body

C. Smaller body to larger body

D. Larger body to smaller body

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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