Heat and Mass Transfer Questions and Answers Pdf

1. According of Kirchhoff's law

A. Radiant heat is proportional to fourth power of absolute temperature

B. Emissive power depends on temperature

C. Emissive power and absorptivity are constant for all bodies

D. Ratio of emissive power to absorptive power for all bodies is same and is equal to the emissive power of a perfectly black body

Answer: Option D

Solution:

Kirchhoff's law states that the emissivity and the absorptivity of a surface at a given temperature and

wavelength are equal. 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 Kirchhoff's law

Answer & Solution

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2. Thermal conductivity of wood depends on

A. Moisture

B. Density

C. Temperature

D. All of the above

Answer: Option D

Solution:

Thermal conductivity is a material property.

It will not differ with the dimensions of a material, but it is dependent on the temperature, the density and the moisture content of the material. The thermal conductivity of a material depends on its temperature, density and moisture content.

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3. The rate of energy emission from unit surface area through unit solid angle, along a normal to the surface, is known as

A. Emissivity

B. Transmissivity

C. Reflectivity

D. Intensity of radiation

Answer: Option D

Solution:

The intensity of radiation is defined as the rate of emission of radiation in a given direction from a surface per

unit solid angle and per unit projected area of a radiating surface on a plane perpendicular to the direction of

radiation.

4. Which of the following is the case of heat transfer by radiation?

A. Blast furnace

B. Heating of building

C. Cooling of parts in furnace

D. Heat received by a person from fireplace

Answer: Option D

Solution:

Radiation refers to the emission of energy in rays or waves. Heat moves through space as energy waves. It is

the type of heat one feels when sitting in front of a fireplace or around a campfire.

5. The ratio of the energy absorbed by the body to total energy falling on it is called

A. Absorptive power

B. Emissive power

C. Absorptivity

D. Emissivity

Answer: Option A

Solution:

Absorptive power is a measure of heat absorbed by an object. When heat is incident on the surface an object, a part of it is absorbed and the remaining part is reflected. Usually the absorbed radiation is converted to thermal energy, increasing the object's temperature.

Heat and Mass Transfer Questions and Answers Pdf

7. 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

Answer: Option A

Solution:

Counter flow heat exchangers are inherently more efficient than parallel flow heat exchangers because they

create a more uniform temperature difference between the fluids, over the entire length of the fluid path.

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8. The automobile radiator is a heat exchanger of

A. Parallel flow type

B. Counter flow type

C. Cross flow type

D. Regenerator type

Answer: Option C

Solution:

The automobile radiator is a heat exchanger of Cross flow type

A radiator is a type of heat exchanger. It is designed to transfer heat from the hot coolant that flows through it to the air blown through it by the fan.

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9. 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

Answer: Option D

Solution:

A grey body is a body (object) that emits radiation at each wavelength in a constant ratio less than unity to that emitted by a black body at the same temperatures. A grey body is a physical body whose absorptivity of a surface does not vary with variation in temperature and wavelength of the incident radiation.

10. 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

Answer: Option D

Solution:

In free convection heat transfer transition from laminar to turbulent flow is governed by the critical value of the Prandtl number, Grashoff's number

Heat and Mass Transfer Questions and Answers Pdf

11. The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body

A. Move actually

B. Do not move actually

C. Affect the intervening medium

D. Does not affect the intervening medium

Answer: Option B

Solution:

The process of heat transfer from one particle of the body to another is called conduction, when the particles of the body Do not move actually

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12. The thermal diffusivities for solids are generally

A. Less than those for gases

B. Less than those for liquids

C. More than those for liquids and gases

D. More or less same as for liquids and gases

Answer: Option D

Solution:

The thermal diffusivity for solids is generally less than those for liquids and gases.

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13. 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

Answer: Option D

Solution:

Heat transfer through radiation takes place in form of electromagnetic waves mainly in the infrared region.

It depends on nature of the body, its temperature and kind & extent of its surface.

14. The transfer of heat by molecular collision is known as

A. Conduction

B. Convection

C. Radiation

D. None of these

Answer: Option B

Solution:

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal

energy along. This type of heat transfer is called convection.

Answer & Solution

15. According to Kirchoff's law, the ratio of emissive power to absorptivity for all bodies is equal to the emissive power of a

A. Grey body

B. Brilliant white polished body

C. Red hot body

D. Black body

Answer: Option D

Solution:

Kirchhoff's law states that the emissivity and the absorptivity of a surface at a given temperature and

wavelength are equal. 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 Kirchhoff's law.

Heat and Mass Transfer Questions and Answers Pdf

16. 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

Answer: Option A

Solution:

a + p + T = 1

a + 0.40 + 0.15 = 1 (p = 0.40, T = 0.15)

a = 1 - 0.55

a = 0.45

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17. The heat transfer takes place according to

A. Zeroth law of thermodynamics

B. First law of thermodynamics

C. Second law of thermodynamics

D. Kirchhoff's law

Answer: Option C

Solution:

Heat transfer takes place as per Second law of the thermodynamics it states that Heat transfer occurs spontaneously from higher- to lower-temperature bodies but never spontaneously in the reverse direction.

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18. In case of liquids and gases, the heat transfer takes place according to

A. Conduction

B. Convection

C. Radiation

D. None of these

Answer: Option B

Solution:

When a fluid, such as air or a liquid, is heated and then travels away from the source, it carries the thermal

energy along. This type of heat transfer is called convection.

19. 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

Answer: Option D

Solution:

Opaque body are those body where the atoms are closely packed and the light cannot pass through. Whereas

a light may or may not pass through a black body depending whether it is opaque or transparent.

Depending on the radiating properties, a body will be opaque when x = 0, a + p = 1.

20. 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

Answer: Option B

Solution:

The larger the LMTD more heat is transferred.

In parallel flow heat exchangers both hot fluid and cold fluid flow in same direction. Counter flow heat exchanger: In counter flow heat exchangers both hot fluid and cold fluid flow in opposite direction.

Heat and Mass Transfer Questions and Answers Pdf

21. 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

Answer: Option C

Solution:

Conduction transfers heat via direct molecular collision. An area of greater kinetic energy will transfer thermal

energy to an area with lower kinetic energy. Higher-speed particles will collide with slower speed particles. The slower-speed particles will increase in kinetic energy as a result. Conduction is the most common form of heat transfer and occurs via physical contact.

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22. Wien’s 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

Answer: Option B

Solution:

Wien's law formula

The equation describing Wien's law is very simple:

where,

• is the aforementioned peak wavelength of light

• T is an absolute temperature of a black body

• b = 2.8977729 mm K is the Wien's displacement constant

Although the relation between wavelength and frequency of electromagnetic waves is fairly simple ( × f = c),

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23. 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

Answer: Option D

Solution:

The amount of radiation mainly depends on Nature of body ,Temperature of body , Type of surface of body

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Heat and Mass Transfer Questions and Answers Pdf