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The ratio of surface convection resistance to the internal conduction resistance is known as

A. Grashoff number

B. Biot number

C. Stanton number

D. Prandtl number

Correct Answer :

B. Biot number


Related Questions

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The process of heat transfer from one particle of the body to another by the actual motion of the heated particles, is called

A. Conduction

B. Convection

C. Radiation

D. None of these

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4

The heat of sun reaches to us according to

A. Conduction

B. Convection

C. Radiation

D. None of these

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4

Fouling factor is used

A. In heat exchanger design as a safety factor

B. In case of Newtonian fluids

C. When a liquid exchanges heat with a gas

D. None of the above

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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 expected to have highest thermal conductivity?

A. Steam

B. Solid ice

C. Melting ice

D. Water

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When heat is transferred from hot body to cold body, in a straight line, without affecting the intervening medium, it is referred as heat transfer by

A. Conduction

B. Convection

C. Radiation

D. Conduction and convection

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4

Two balls of same material and finish have their diameters in the ratio of 2: 1 and both are heated to same temperature and allowed to cool by radiation. Rate of cooling by big ball as compared to smaller one will be in the ratio of

A. 1 : 1

B. 2 : 1

C. 1 : 2

D. 4 : 1

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4

Upto the critical radius of insulation,

A. Added insulation will increase heat loss

B. Added insulation will decrease heat loss

C. Convective heat loss will be less than conductive heat loss

D. Heat flux will decrease

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Log mean temperature difference in case of counter flow compared to parallel flow will be

A. Same

B. More

C. Less

D. Depends on other factors

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Heat flows from one body to other when they have

A. Different heat contents

B. Different specific heat

C. Different atomic structure

D. Different temperatures

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4

The highest thermal diffusivity is of

A. Iron

B. Lead

C. Concrete

D. Wood

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4

The most commonly used method for the design of duct size is the

A. Velocity reduction method

B. Equal friction method

C. Static regains method

D. Dual or double method

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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. Wien's law

B. Stefan's law

C. Kirchhoff's law

D. Planck's law

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4

Unit of thermal diffusivity is

A. m²/hr

B. m²/hr °C

C. kcal/m² hr

D. kcal/m. hr °C

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In convection heat transfer from hot flue gases to water tube, even though flow may be turbulent, a laminar flow region (boundary layer of film) exists close to the tube. The heat transfer through this film takes place by

A. Convection

B. Radiation

C. Conduction

D. Both convection and conduction

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The unit of overall coefficient of heat transfer is

A. kcal/m²

B. kcal/hr °C

C. kcal/m² hr °C

D. kcal/m hr °C

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The time constant of a thermocouple is

A. The time taken to attain the final temperature to be measured

B. The time taken to attain 50% of the value of initial temperature difference

C. The time taken to attain 63.2% of the value of initial temperature difference

D. Determined by the time taken to reach 100°C from 0°C

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Heat transfer by radiation mainly depends upon

A. Its temperature

B. Nature of the body

C. Kind and extent of its surface

D. All of the above

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4

Which of the following would lead to a reduction in thermal resistance?

A. In conduction, reduction in the thickness of the material and an increase in thermal conductivity.

B. In convection, stirring of the fluid and cleaning the heating surface.

C. In radiation, increasing the temperature and reducing the emissivity.

D. All of the above

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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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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

Thermal conductivity of air at room temperature in kcal/m hr °C is of the order of

A. 0.002

B. 0.02

C. 0.01

D. 0.1

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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

Unit of thermal conductivity in S.I. units is

A. J/m² sec

B. J/m °K sec

C. W/m °K

D. Option (B) and (C) above

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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

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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Heat transfer takes place as per

A. Zeroth law of thermodynamics

B. First law of thermodynamic

C. Second law of the thermodynamics

D. Kirchoff's law

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LMTD in case of counter flow heat exchanger as compared to parallel flow heat exchanger is

A. Higher

B. Lower

C. Same

D. Depends on the area of heat exchanger

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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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Fourier's law of heat conduction gives the heat flow for

A. Irregular surfaces

B. Nonuniform temperature surfaces

C. One dimensional cases only

D. Two dimensional cases only