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4

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

A. Grashoff number

B. Nusselt number

C. Weber number

D. Prandtl number

Correct Answer :

A. Grashoff number


Related Questions

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4

Thermal conductivity of air with rise in temperature

A. Increases

B. Decreases

C. Remain constant

D. May increase or decrease depending on temperature

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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

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

Which of the following statement is correct?

A. A grey body is one which absorbs all radiations incident on it.

B. At thermal equilibrium, the emissivity and absorptivity are same.

C. The energy absorbed by a body to the total energy falling on it, is called emissivity.

D. A perfect body is one which is black in colour.

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4

The natural convection air cooled condensers are used in

A. Domestic refrigerators

B. Water coolers

C. Room air conditioners

D. All of these

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4

Which of the following is expected to have highest thermal conductivity?

A. Steam

B. Solid ice

C. Melting ice

D. Water

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4

Thermal conductivity of solid metals with rise in temperature normally

A. Increases

B. Decreases

C. Remain constant

D. May increase or decrease depending on temperature

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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

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

A. Reynold's number

B. Grashoff's number

C. Reynold's number, Grashoff's number

D. Prandtl number, Grashoff's number

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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

Thermal conductivity of wood depends on

A. Moisture

B. Density

C. Temperature

D. All of the above

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4

The amount of radiation mainly depends on

A. Nature of body

B. Temperature of body

C. Type of surface of body

D. All of the above

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4

The total radiation from a black body per second per unit area is ________ fourth power of the absolute temperature. This statement is known as Stefan Boltzmann law.

A. Equal to

B. Directly proportional to

C. Inversely proportional to

D. None of these

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4

Stefan Boltzmann law is applicable for heat transfer by

A. Conduction

B. Convection

C. Radiation

D. Conduction and radiation combined

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4

The value of the wave length for maximum emissive power is given by

A. Kirchhoff's law

B. Stefan's law

C. Wines law

D. Planck's law

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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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4

A grey body is one whose absorptivity

A. Varies with temperature

B. Varies with the wave length of incident ray

C. Varies with both

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

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4

The automobile radiator is a heat exchanger of

A. Parallel flow type

B. Counter flow type

C. Cross flow type

D. Regenerator type

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4

Thermal conductivity of water at 20°C is of the order of

A. 0.1

B. 0.23

C. 0.42

D. 0.51

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4

Reynolds number (RN) is given by (where h = Film coefficient, l = Linear dimension, V = Velocity of fluid, k = Thermal conductivity, t = Temperature, ρ = Density of fluid, cp = Specific heat at constant pressure, and μ = Coefficient of absolute viscosity)

A. RN = hl/k

B. RN = μ cp/k

C. RN = ρ V l /μ

D. RN = V²/t.cp

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4

In counter current flow heat exchanger, the logarithmic temperature difference between the fluids is ________ as compared to parallel flow heat exchanger.

A. Same

B. Less

C. Greater

D. None of these

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4

Thermal conductivity of water ________ with rise in temperature.

A. Remain same

B. Decreases

C. Increases

D. May increase or decrease depending upon temperature

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4

The thermal diffusivities for gases are generally

A. More than those for liquids

B. Less than those for liquids

C. More than those for solids

D. Dependent on the viscosity

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4

According to Dalton's law of partial pressures, (where pb = Barometric pressure, pa = Partial pressure of dry air, and pv = Partial pressure of water vapour)

A. Pb = pa - pv

B. Pb = pa + pv

C. Pb = pa × pv

D. Pb = pa/pv

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4

Heat transfer in liquid and gases takes place by

A. Conduction

B. Convection

C. Radiation

D. Conduction and convection

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4

An ordinary passenger aircraft requires a cooling system of capacity.

A. 2 TR

B. 4 TR

C. 8 TR

D. 10 TR

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4

A composite slab has two layers of different materials with thermal conductivities k₁ and k₂. If each layer has the same thickness, then the equivalent thermal conductivity of the slab will be

A. k₁ k₂

B. (k₁ + k₂)

C. (k₁ + k₂)/ k₁ k₂

D. 2 k₁ k₂/ (k₁ + k₂)

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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

Planck's law holds good for

A. Black bodies

B. Polished bodies

C. All coloured bodies

D. All of the above

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4

Which of the following has least value of conductivity?

A. Glass

B. Water

C. Plastic

D. Air