These 40 MCQs cover various aspects of Heat and Mass Transfer in Mechanical Engineering, including fundamental concepts, different modes of heat transfer, dimensionless numbers, and practical applications. They are designed to test knowledge in both theoretical and practical contexts.
1. What is conduction in the context of heat transfer?
a) Transfer of heat by the movement of fluid
b) Transfer of heat through electromagnetic waves
c) Transfer of heat through a solid medium
d) Transfer of heat by mixing of fluid
Answer:
c) Transfer of heat through a solid medium
Explanation:
Conduction is the process of heat transfer through a solid medium, from a region of high temperature to a region of low temperature without any movement of the medium itself.
2. What is the thermal conductivity of a material?
a) The rate at which heat is stored in a material
b) The rate at which a material transfers heat through convection
c) The ability of a material to transfer heat through conduction
d) The ability of a material to radiate heat
Answer:
c) The ability of a material to transfer heat through conduction
Explanation:
Thermal conductivity is a property of a material that indicates its ability to conduct heat. It is defined as the rate at which heat passes through a specified material.
3. What is convection in heat transfer?
a) Heat transfer in a solid
b) Heat transfer in a vacuum
c) Heat transfer due to the bulk movement of molecules within fluids
d) Heat transfer due to electromagnetic waves
Answer:
c) Heat transfer due to the bulk movement of molecules within fluids
Explanation:
Convection is the mode of heat transfer in fluids (liquids or gases) due to the bulk movement of molecules within the fluid.
4. What is radiation in the context of heat transfer?
a) Transfer of heat through direct contact
b) Transfer of heat by the movement of air or liquid
c) Transfer of heat through electromagnetic waves
d) Transfer of heat through a stationary medium
Answer:
c) Transfer of heat through electromagnetic waves
Explanation:
Radiation is a method of heat transfer that does not rely on any contact between the heat source and the heated object, as it is the transfer of energy through electromagnetic waves.
5. What is the Stefan-Boltzmann law?
a) The rate of heat transfer by conduction is proportional to the temperature gradient
b) The rate of heat transfer by convection is proportional to the temperature difference
c) The total energy radiated per unit surface area of a black body is proportional to the fourth power of the black body's temperature
d) The heat transfer through a material is directly proportional to the surface area and the temperature difference
Answer:
c) The total energy radiated per unit surface area of a black body is proportional to the fourth power of the black body's temperature
Explanation:
The Stefan-Boltzmann law states that the total energy radiated per unit surface area of a black body in unit time is directly proportional to the fourth power of the black body's absolute temperature.
6. What is a heat exchanger?
a) A device that produces heat for industrial processes
b) A device that converts heat into electrical energy
c) A device used for transferring heat between two or more fluids
d) A device that measures the amount of heat transfer
Answer:
c) A device used for transferring heat between two or more fluids
Explanation:
A heat exchanger is a system used to transfer heat between two or more fluids. The fluids can be separated by a solid wall to prevent mixing or they can be in direct contact.
7. What is the Prandtl number?
a) A dimensionless number relating the heat transfer by conduction and convection
b) A dimensionless number indicating the efficiency of a heat exchanger
c) A dimensionless number relating the rate of heat transfer to the rate of fluid flow
d) A dimensionless number that compares the thermal conductivity of a material to its electrical conductivity
Answer:
a) A dimensionless number relating the heat transfer by conduction and convection
Explanation:
The Prandtl number is a dimensionless number defined as the ratio of momentum diffusivity (kinematic viscosity) to thermal diffusivity. It is used in heat transfer calculations.
8. In heat transfer, what does the term 'laminar flow' refer to?
a) Turbulent fluid motion characterized by chaotic changes in pressure and flow velocity
b) Fluid particles moving in straight, parallel paths
c) Rapid, random, and chaotic movement of particles
d) The natural convective movement of fluids
Answer:
b) Fluid particles moving in straight, parallel paths
Explanation:
Laminar flow is characterized by smooth, constant fluid motion in which the fluid particles move in straight, parallel paths along the direction of flow.
9. What is Fourier's Law of Heat Conduction?
a) The rate of heat transfer through a material is proportional to the negative gradient of temperature and the area through which heat is transferred
b) The rate of heat transfer is proportional to the surface area and the temperature difference
c) The heat transfer in a fluid is proportional to the velocity of the fluid
d) The rate of radiant heat transfer is proportional to the emissivity of the surface
Answer:
a) The rate of heat transfer through a material is proportional to the negative gradient of temperature and the area through which heat is transferred
Explanation:
Fourier's Law of Heat Conduction states that the time rate of heat transfer through a material is proportional to the negative gradient in temperature and to the area, perpendicular to that gradient, through which the heat flows.
10. What is the Nusselt number in heat transfer?
a) A dimensionless number that measures the effectiveness of natural convection
b) A dimensionless number that indicates the ratio of convective to conductive heat transfer
c) A dimensionless number that measures the efficiency of a heat exchanger
d) A dimensionless number that relates the pressure drop to the flow rate in a pipe
Answer:
b) A dimensionless number that indicates the ratio of convective to conductive heat transfer
Explanation:
The Nusselt number is a dimensionless number which provides a measure of the convective heat transfer occurring at a surface, relative to conductive heat transfer across the fluid.
11. What is black body radiation?
a) Radiation emitted by a black-colored object
b) Radiation that occurs only in a vacuum
c) Radiation emitted by a body that absorbs all radiation incident on it
d) The solar radiation absorbed by the earth's surface
Answer:
c) Radiation emitted by a body that absorbs all radiation incident on it
Explanation:
Black body radiation refers to the theoretical or model radiation emitted by a perfect black body. A black body absorbs all incident electromagnetic radiation, regardless of frequency or angle of incidence.
12. What is the primary mechanism of heat transfer in solids?
a) Convection
b) Radiation
c) Conduction
d) Advection
Answer:
c) Conduction
Explanation:
In solids, heat transfer primarily occurs through conduction, where thermal energy is transferred from higher to lower temperature regions within the solid.
13. What is 'thermal diffusivity'?
a) The ability of a material to conduct heat
b) The rate at which heat is generated per unit volume
c) The measure of a material's ability to conduct thermal energy relative to its ability to store thermal energy
d) The rate at which a material radiates heat
Answer:
c) The measure of a material's ability to conduct thermal energy relative to its ability to store thermal energy
Explanation:
Thermal diffusivity is a measure of how quickly a material can conduct thermal energy compared to its ability to store thermal energy. It is a combination of the material's thermal conductivity, density, and specific heat capacity.
14. In heat transfer, what is 'forced convection'?
a) Natural movement of heat in a fluid due to temperature differences
b) Movement of heat in a fluid driven by external means such as a fan or pump
c) Heat transfer in a solid
d) Heat transfer by electromagnetic radiation
Answer:
b) Movement of heat in a fluid driven by external means such as a fan or pump
Explanation:
Forced convection occurs when fluid motion is generated by external means (like a fan, pump, or wind), which enhances the heat transfer rate compared to natural convection.
15. What is the difference between sensible heat and latent heat?
a) Sensible heat is related to temperature change, while latent heat is associated with phase change
b) Sensible heat is the total heat content, while latent heat is the usable heat
c) Sensible heat can be measured directly, while latent heat cannot be measured
d) Sensible heat is heat transferred by radiation, while latent heat is transferred by conduction
Answer:
a) Sensible heat is related to temperature change, while latent heat is associated with phase change
Explanation:
Sensible heat refers to heat added or removed from a substance that results in a temperature change. Latent heat is the heat added or removed that results in a phase change (like melting or vaporization) without changing the temperature.
16. What is a dimensionless parameter in heat transfer that is a function of temperature and pressure?
a) Prandtl number
b) Grashof number
c) Reynolds number
d) Rayleigh number
Answer:
d) Rayleigh number
Explanation:
The Rayleigh number in heat transfer is a dimensionless parameter that is a function of temperature and pressure. It is used in the study of natural convection and is the product of the Grashof and Prandtl numbers.
17. What is 'heat flux'?
a) The total amount of heat transferred
b) The rate of heat transfer per unit area
c) The temperature gradient within a material
d) The velocity of heat transfer
Answer:
b) The rate of heat transfer per unit area
Explanation:
Heat flux is defined as the rate of heat transfer per unit area, usually measured in Watts per square meter. It represents the quantity of heat passing through a unit area in a given time.
18. What does the Grashof number signify in heat transfer?
a) Ratio of convective to conductive heat transfer
b) Ratio of inertial forces to viscous forces
c) Importance of natural convection compared to conduction
d) Effectiveness of a heat exchanger
Answer:
c) Importance of natural convection compared to conduction
Explanation:
The Grashof number in heat transfer is a dimensionless number that indicates the relative importance of natural convection compared to conduction. A high Grashof number suggests that natural convection is significant.
19. In the context of mass transfer, what is diffusion?
a) Movement of particles due to temperature difference
b) Transfer of particles from a region of higher concentration to a region of lower concentration
c) Movement of particles in a fluid due to pressure difference
d) Transfer of particles due to electromagnetic field
Answer:
b) Transfer of particles from a region of higher concentration to a region of lower concentration
Explanation:
Diffusion in mass transfer refers to the movement of particles (molecules, atoms, ions) from a region of higher concentration to a region of lower concentration, driven by the concentration gradient.
20. What is a fin in heat transfer?
a) A device to measure temperature
b) A type of heat exchanger
c) An extended surface to enhance heat transfer through increased area
d) A component used in heat engines
Answer:
c) An extended surface to enhance heat transfer through increased area
Explanation:
A fin is an extended surface used in heat transfer applications to increase the surface area available for heat exchange, thereby improving the rate of heat transfer.
21. What principle does a heat pipe operate on?
a) Capillary action and phase change
b) Electromagnetic induction
c) Mechanical pumping of fluid
d) Forced convection
Answer:
a) Capillary action and phase change
Explanation:
A heat pipe operates on the principle of capillary action and phase change. It efficiently transfers heat through the evaporation and condensation of a working fluid within a capillary structure.
22. In radiative heat transfer, what does emissivity represent?
a) The ability of a surface to reflect radiation
b) The ability of a surface to absorb radiation
c) The ability of a surface to emit radiation relative to a black body
d) The temperature of a radiating body
Answer:
c) The ability of a surface to emit radiation relative to a black body
Explanation:
Emissivity is a measure of a material's ability to emit thermal radiation compared to an ideal black body, which is the perfect emitter of radiation.
23. What is the driving force for mass transfer in distillation?
a) Temperature gradient
b) Concentration gradient
c) Pressure gradient
d) Electrical potential gradient
Answer:
b) Concentration gradient
Explanation:
In distillation, the driving force for mass transfer is the concentration gradient. The process separates components based on differences in their volatilities in a boiling liquid mixture.
24. What is the overall heat transfer coefficient?
a) The sum of all individual heat transfer coefficients
b) The ratio of total heat transfer to the product of area and temperature difference
c) A measure of a system's ability to transfer heat
d) The average heat transfer coefficient over a surface
Answer:
b) The ratio of total heat transfer to the product of area and temperature difference
Explanation:
The overall heat transfer coefficient is a measure that represents the total heat transfer occurring in a system, per unit area per unit temperature difference. It incorporates all modes of heat transfer (conduction, convection, and radiation).
25. What is the primary mechanism of heat transfer in a vacuum?
a) Conduction
b) Convection
c) Radiation
d) Advection
Answer:
c) Radiation
Explanation:
In a vacuum, where there are no molecules to conduct or convect heat, radiation is the primary mechanism of heat transfer. Heat is transferred by electromagnetic waves in a vacuum.
26. In the context of heat exchangers, what is 'cross-flow'?
a) Fluids flow perpendicular to each other
b) Fluids flow in the same direction
c) Fluids flow in opposite directions
d) Fluids mix and flow together
Answer:
a) Fluids flow perpendicular to each other
Explanation:
In cross-flow heat exchangers, the two fluids flow perpendicular to each other, which allows for effective heat exchange between the fluids.
27. What is the function of baffles in a shell and tube heat exchanger?
a) To increase the flow rate of fluids
b) To prevent mixing of the two fluids
c) To direct the flow and increase heat transfer efficiency
d) To reduce the pressure drop in the heat exchanger
Answer:
c) To direct the flow and increase heat transfer efficiency
Explanation:
Baffles in a shell and tube heat exchanger direct the flow of fluid and increase the turbulence, which enhances the heat transfer efficiency by disrupting the boundary layer on the tube surfaces.
28. What is the primary purpose of insulation in heat transfer applications?
a) To increase heat transfer
b) To reduce heat loss or gain
c) To absorb heat
d) To conduct heat
Answer:
b) To reduce heat loss or gain
Explanation:
The primary purpose of insulation in heat transfer applications is to reduce heat loss or gain by providing a barrier to heat flow, thereby improving energy efficiency.
29. What does a high Reynolds number indicate in a heat transfer context?
a) Laminar flow
b) Turbulent flow
c) Low fluid velocity
d) High viscosity
Answer:
b) Turbulent flow
Explanation:
A high Reynolds number in heat transfer typically indicates turbulent flow, where the fluid motion is chaotic and mixes efficiently, enhancing heat transfer compared to laminar flow.
30. What is 'boiling heat transfer'?
a) Transfer of heat through a solid medium
b) Transfer of heat from a hot surface to a boiling liquid
c) Transfer of heat in a refrigeration cycle
d) Transfer of heat by the movement of air
Answer:
b) Transfer of heat from a hot surface to a boiling liquid
Explanation:
Boiling heat transfer occurs when heat is transferred from a solid surface to a liquid in contact with it, causing the liquid to boil. This process involves both latent heat (due to phase change) and sensible heat transfer.
31. What is the thermal boundary layer in convection?
a) The layer of fluid where temperature is uniform
b) The layer of fluid at the surface where heat transfer occurs
c) The layer of fluid that does not move
d) The layer of fluid with maximum velocity
Answer:
b) The layer of fluid at the surface where heat transfer occurs
Explanation:
The thermal boundary layer in convection is the layer of fluid in the immediate vicinity of a heated (or cooled) surface where heat transfer between the surface and the fluid occurs and where temperature gradients exist.
32. In heat transfer, what is 'entrainment' in the context of a jet or plume?
a) The mixing of surrounding fluid into the flow of the jet or plume
b) The expulsion of fluid from the jet or plume
c) The velocity profile of the jet or plume
d) The temperature change within the jet or plume
Answer:
a) The mixing of surrounding fluid into the flow of the jet or plume
Explanation:
Entrainment in heat transfer refers to the process of surrounding fluid being drawn into the flow of a jet or plume, which is significant in applications like ventilation and cooling.
33. What is a thermocouple?
a) A device used to measure pressure
b) A device used to control temperature
c) A device that converts thermal energy into electrical energy
d) A device used to measure temperature
Answer:
d) A device used to measure temperature
Explanation:
A thermocouple is a temperature measuring device consisting of two dissimilar metal wires joined at one end. It measures temperature by producing a voltage that changes with temperature.
34. What is 'mass diffusivity'?
a) The ability of a material to absorb moisture
b) The measure of a material's ability to conduct mass
c) The rate at which particles mix due to random motion
d) The rate of mass flow in a fluid
Answer:
c) The rate at which particles mix due to random motion
Explanation:
Mass diffusivity is a property that measures the rate at which particles (atoms, molecules, etc.) mix with each other in a medium due to random molecular motion, influenced by the concentration gradient.
35. In a cooling tower, what is the primary mode of heat transfer?
a) Conduction
b) Forced convection
c) Radiation
d) Evaporative cooling
Answer:
d) Evaporative cooling
Explanation:
In a cooling tower, the primary mode of heat transfer is evaporative cooling, where water is cooled as it comes into contact with air, and some of the water evaporates, removing heat from the remaining water.
36. What is a grey body in the context of radiation heat transfer?
a) A body that reflects all incident radiation
b) A body that emits radiation uniformly in all wavelengths
c) A body that absorbs all incident radiation
d) A body that emits less radiation than a black body at the same temperature
Answer:
d) A body that emits less radiation than a black body at the same temperature
Explanation:
A grey body is an idealized emitter and absorber of radiation, which emits less radiation than a black body (perfect emitter) at the same temperature. It has a constant emissivity less than 1 across all wavelengths.
37. What is the primary function of fins in heat transfer applications?
a) To decrease the temperature of the base object
b) To increase the rate of heat transfer by increasing the surface area
c) To act as insulators
d) To direct fluid flow
Answer:
b) To increase the rate of heat transfer by increasing the surface area
Explanation:
Fins are used in heat transfer applications to enhance the heat transfer rate. They work by increasing the surface area available for heat exchange, thereby improving heat dissipation or absorption efficiency.
38. What is meant by 'critical radius of insulation'?
a) The minimum radius at which insulation starts to be effective
b) The radius at which the heat loss from an insulated pipe is maximum
c) The radius of insulation at which the heat loss is minimized
d) The optimal radius of insulation for cost-effectiveness
Answer:
b) The radius at which the heat loss from an insulated pipe is maximum
Explanation:
The critical radius of insulation is a concept in heat transfer where, for cylindrical and spherical bodies, there is a specific radius of insulation at which the heat loss is maximum. Below this radius, adding insulation actually increases the heat loss.
39. In mass transfer, what is the Sherwood number?
a) A dimensionless number comparing convective mass transfer to diffusive mass transfer
b) A dimensionless number indicating the rate of heat transfer in a fluid
c) A dimensionless number representing the effectiveness of a heat exchanger
d) A dimensionless number that compares the rate of heat transfer to the rate of fluid flow
Answer:
a) A dimensionless number comparing convective mass transfer to diffusive mass transfer
Explanation:
The Sherwood number in mass transfer is a dimensionless number used in mass transfer operations and is analogous to the Nusselt number in heat transfer. It compares the convective mass transfer to the rate of mass transfer by diffusion.
40. What is the primary difference between heat transfer by conduction in solids and fluids?
a) In fluids, conduction is always accompanied by convection
b) Conduction does not occur in fluids
c) Conduction in solids involves molecular movement
d) In solids, conduction is less efficient than in fluids
Answer:
a) In fluids, conduction is always accompanied by convection
Explanation:
In fluids (liquids and gases), heat transfer by conduction is usually accompanied by convection due to the fluid's ability to move and circulate, which is not the case in solids where molecules are fixed in place.