Two Marks Hmt - Cs

7/24/2019 Two Marks Hmt - Cs

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HEAT AND MASS TRANSFER

UNIT ICONDUCTION

1. Define heat.

Heat is a form of energy which is following due to the temperature difference. Itis measured by the unit joules

2. What is heat transfer?

The transmission of heat energy from one region to another region due to changein temperature is called heat transfer.

3. Mention the significance of the study of heat transfer.

Estimate the rate of heat exchange between the system and itssurroundings.

Determines the temperature distribution in the system.

4. Give some examples of heat transfer in engineering. Internal combustion engines.

Refrigeration and air conditioning systems.

Heat treatment of metals.

Working of thermal equipments like steam generators, condensers, heatexchangers, and furnaces.

5. Give the different modes of heat transfer.The modes of heat transfer are,

a) Conduction

b) Convection

c) Radiation

6. Define Conduction.

It is defined as the mode of heat transfer from one part of a substance to anotherpart of the same substance or with a substance physical contact with it, without any

appreciable displacement of the molecules of the substance.

7. Define convectionConvection is defined as the mode of heat transfer with in a fluid by mixing one

portion of the fluid with another.

8. Define radiation.

Radiation is defined as the mode of heat transfer through space in the form ofelectromagnetic waves.

9. What is heat flux?Heat flux is the amount of heat transfer per unit area. It is denoted by q.

q = Q / AWhere, QAmount of heat transfer


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AArea perpendicular to the heat flow.

10. Explain Fouriers law of heat conduction.It states that the rate of heat conduction is proportional to the area perpendicular

to the direction of heat flow and to the temperature gradient along the direction.

dxdTKAQ

Where, KThermal Conductivity

11. Define Thermal conductivity.

It is a physical property of the substance and is defined as the ability of a

substance to conduct heat. It is denoted by K.

12. Define thermal resistance.

Thermal resistance is defined as the resistance offered by a substance for heat

flow. It is the reciprocal of thermal conduction and it is denoted by Rth.

13. Explain steady state heat conduction.

If the temperature at any particular point does not vary with time during the heattransfer, it is known as steady state heat transfer.

14. What is transient heat conduction?During heat conduction, if the temperature at any point varies with the time, it is

known as transient heat conduction.

15. Explain thermal diffusivity.

It is defined as the ratio of thermal conductivity to the thermal storage capacity ofthe material and it is denoted by .

PCp

K

16. What is critical thickness of insulation?

It is defined as the thickness of insulation at which the heat flow is maximum. Incase of cylinders and spheres it is called as critical radius of insulation.

17. Define Biot number.It is a dimensionless number and is defined as the ratio between surface

convective resistance and internal conduction resistance.

K

hLBi


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18. Define efficiency of fin.

The efficiency of fin is defined as the ratio of actual heat transferred by fin to the

maximum possible heat transferred if the entire fin area is maintained at basetemperature.

19. Define effectiveness of fin.The effectiveness of fin is defined as the ratio of heat transfer from the surfacewith fin to the heat transfer from the surface without fin.

20. What is lumped heat capacity analysis?In the analysis of Newtonian heating or cooling process, if the temperature

throughout the solid is considered to be uniform at a given time and the total heat

available within the system is considered as one lump, then it is called as lumped heat

capacity analysis.

21. Discuss the effect of temperature on thermal conductivity.

The temperature increases in the most of the solids and liquids decrease itsthermal conductivity, whereas in gases the increase in temperature increases the thermal

conductivity.

22. State the assumptions followed in Fouriers law of heat conduction.

Conduction takes place under steady state conduction.

The substance is homogeneous and isotropic throughput.

Heat flow is unidirectional.

Internal heat generation is negligible.

Constant temperature gradient and linear temperature profile.

23. What are the significance of Fouriers law? It can be applied for all substances irrespective of whether solid, liquid or

gas.

It is purely based on experimental results.

It helps to calculate the thermal conductivity of the substance throughwhich the heat is conducted.

It shows that the heat flow rate is in the direction of decreasing thetemperature and normal to an isotherm.

24. Mention the various factors affecting the thermal conductivity of material.

Pressure and temperature. Material structure

Moisture present

Material density

25. State the dependence of thermal conductivity on temperature.

K=K0(1+ T)


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Where, K0= Thermal conductivity at 0C

= temperature coefficient of thermal conductivity

26. Define thermal conduction resistance.

Thermal conduction resistance is defined as the resistance offered by thesubstance for heat flow.

27. Define thermal diffusivity of materials.The thermal diffusivity of materials is defined as the ratio of thermal conductivity

to the thermal storage capacity of the material.

28. What are the assumptions made in composite systems?

The adjacent layers are having the perfect physical contact with eachothers.

There is no temperature drop across the interface between the adjacentlayers.

29. Define thermal contact resistance.In composite systems, the contact surfaces touch only at discrete locations or

points because of the surface roughness and void space which are commonly filled withair. This imperfect contact reduces the actual heat transfer. Thus the resistance offered by

the surface due to imperfect contact is known as thermal resistance.

30. Give some examples for heat generation with internal heat generation.

Fuel rods in nuclear core.

Current carrying electrical conductors

Combustion process in IC engines.

Chemical process.

Drying and setting of cement concrete.

31. Define fin.

Fins are extended surfaces in order to increase the effective surface area for heat

convection.

32. What are the applications of fins?

Economisers used in steam power plants.

Automobile radiators

Cooling of IC engines.

Cooling of air compressors, electric motors and transformers.

33. Mention some common types of fin configurations available.

Rectangular fin

Triangular fin

Circular fin

Circumferential fin

Longitudinal fin


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Pin fin

34. What are the assumptions followed in the analysis of heat transfer through fins?

Steady state heat conduction.

No heat generation within the fin

Thermal conductivity of the fin materials is constant. Thermal contact resistance is negligible.

One dimensional heat conduction.

35. Mention the required conditions for fins to be effective.

Thermal conductivity of the fin materials should be large.

Convective heat transfer coefficient should be small.

Thickness of fin should be minimum.

36. Mention the applications of transient heat conduction.

Cooling of IC engine parts.

Heating and cooling of metals.

Cooling and freezing of food stuffs.

Heat treatment of metals by quenching.


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UNIT IICONVECTION

1. What is convection heat transfer?Convection is a mode of heat transfer in which the heat is transferred by the

means of an external media.

2. What are the types of convective heat transfer?a) Free convection

b) Forced convection

3. Explain forced convection.

In forced convection heat transfer, the movement of the fluid molecules over the

surface is created by some external means such as blowers, compressors, etc.

4. Explain free convection.

In this, the movement of the molecules over the surface is due to temperature

difference within the fluid.

5. Define convective heat transfer coefficient.

It is defined as the amount of heat transferred from a unit surface under unit

temperature difference between the surface and surrounding fluid.

6. What is local heat transfer coefficient?

The heat transfer coefficient at particular location is defined as local heat transfercoefficient.

7. What are the governing equations for convective heat transfer?

a) Continuity equationb) Momentum equation

c) Energy equation

8. What is the physical significance of Navier stokes equation?

Navier Stokes equation relates the inertia force, body force, pressure force and

viscous force in a fluid flow.

9. Define thermal boundary layer.

Thermal boundary layer is defined as the layer of the flowing fluid adjacent to the

boundary in which the temperature varies between the surface temperature and the freestream temperature.

10. Define displacement thickness.

It is defined as the distance measured perpendicular to the boundary, by which thefree stream flow is displaced on account of formation of boundary layer.


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11. Define momentum thickness.

It is defined as the distance from the boundary through which the total loss of

momentum per second is equal to if it were passing a stationary surface.

12. Define energy thickness.

It is defined as the distance measured normal to the boundary by which theboundary should be displaced to compensate for the loss in kinetic energy of the flowingfluid on account of boundary layer formation.

13. Define film temperature.Film temperature is defined as the temperature of the stable fluid film over the

surface and is equal to the arithmetic mean value of the surface temperature and the fluid

temperature.

14. Name the various dimensionless numbers.

Grashoff number

Rayleigh number Prandtl number

Nusselt number

15. State the mechanisms of convective heat transfer.

The heat transfer in convective heat transfer is due to the molecular movement ofthe fluid over the surface.

16. Illustrate the importance of Navier strokes equation?

Navier stroke equation relates the inertia force, body force, pressure force and

viscous force in a fluid flow.

17. Define velocity boundary layer.

Velocity boundary layer is defined as the layer of the flowing fluid adjacent to theboundary. In the velocity boundary layer the velocity of the flowing fluid over the surface

is zero and increases with the increase in distance from the surface.

18. Define thickness of velocity boundary layer.

The thickness of velocity boundary layer is defined as the thickness up to which

there will be velocity gradient, above which the velocity of the fluid is equal to the freestream velocity.

19. Define the thickness of thermal boundary layer.

The thickness of boundary layer is defined as the thickness up to which there willbe temperature gradient, above which the temperature of the fluid is equal to the free

stream velocity.

20. Mention some of the characteristics of a boundary layer.


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Thickness of the boundary layer increases as distance from leading edgeincreases.

Thickness of the boundary layer is zero at the leading edge and maximumat the trailing edge.

The increase in velocity of the fluid reduces the boundary layer thickness.

The increase in kinematic viscosity increases the boundary layerthickness.

21. Define the skin friction coefficient.Skin friction coefficient is defined as the ratio of surface shear stress to the flow

energy or dynamic head.

22. What is the application of skin friction coefficient?The skin friction coefficient is highly useful in the calculation of surface frictional

drag.

23. Give the comparison between laminar boundary layer and turbulent boundary layer. In a laminar boundary layer the fluid motion is highly ordered, wherein

turbulent boundary layer it is highly irregular.

The thickness of laminar boundary layer is less than that of turbulentboundary layer.

The velocity profile in laminar boundary layer is parabolic in nature,whereas it is a flatter in turbulent boundary layer.

24. Briefly explain fully developed flow.In any flow of fluid through circular pipes, the boundary layer developed on the

surface increases with the increase in length. The continuous increase in thickness will

make the boundary layer to merge at a point on the centre line of the pipe which resultsan invariant velocity profile in the flow direction. Such flow is known as fully developed

flow.

25. Define hydraulic diameter.Hydraulic diameter is defined as the diameter of the cylinder equivalent for non-

circular tubes in such a way that heat transfer from both the tubes should be same.

26. Define bulk temperature.Bulk temperature is defined as the enthalpy average temperature of the entire

mass of the substance. It is the representative parameter of the total flow of energy at a

particular location.

27. Define Reynolds number and mention its significance.

Reynolds number is defined as the ratio between inertia force to viscous force.The values of the Reynolds number determine whether the flow is laminar or turbulent.

The increase in Reynolds number increases the heat transfer rate and vice versa.


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28. Define Prandtl number.

Prandtl number is defined as the ratio between momentum diffusivity and thermaldiffusivity.

29. Define Nusselt number.

Nusselt number is defined as the ratio between convective heat transfer

coefficient and thermal resistance.

30. Mention the application of Von Karman momentum equation.

Von Karman momentum equation is used to calculate the frictional drag on

smooth flat surface for both laminar and turbulent boundary layers.

31. Define average heat transfer coefficient.

The convective heat transfer coefficient over the entire surface is known as

average heat transfer coefficient.

32. Define eddy diffusivity of heat transfer.

Eddy diffusivity for heat transfer is defined as the transport of heat energy by

small amounts of fluid known eddies.

33. Briefly explain Moodys diagram.

Moodys diagram is a graph drawn between Reynolds number and friction factorfor various values of relative roughness of the surface. By using the graph, the friction

factor values can be obtained for forced convection values.

34. What do you mean by critical Reynolds number.The value of the Reynolds number at which the transition of flow takes place

from laminar to turbulent is known as critical Reynolds number

35. What do you meant by Reynolds analogy?

Reynolds analogy is a method determines the skin friction coefficient over the flat

surface and it expresses the relationship between fluid friction and heat transfer for

laminar flow over the flat plate.

36. What do you mean by Colburn analogy?

Colburn analogy is a method to determine the skin friction coefficient for thelaminar flow over the flat surface having the Prandtl number value other than unity.

37. Mention the most commonly used tube arrangements in tube banks.

Inline arrangement

Staggered arrangement

38. Define longitudinal pitch in tube banks.


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Longitudinal pitch is defined as the centre to centre distance between two adjacent

tubes in flow direction.

39. Define transverse pitch in tube banks.

Transverse pitch is defined as the centre to centre distance between the adjacent

tubes in a direction normal to the flow.

40. Mention the various factors influence the natural convection heat transfer.

Temperature difference between the surface and the surrounding fluid.

Density of fluid.

Thermal expansion of the flowing fluid.

Buoyancy force acting on the fluid.


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UNIT IIIPHASE CHANGE HEAT TRANSFER AND HEAT EXCHANGERS

1. What are the different modes of condensation?a) Film condensation

b) Drop wise condensation

2. Explain film condensation.It is a condensation process in which the condensate formed over the surface is in

the form of continuous film and it covers the entire surface.

3. Explain drop wise condensation.

It is a condensation process in which the vapour condenses in to small liquid

droplets of different sizes which moves down over the surface in a random manner.

4. What are the different types of boiling processes?

a) Pool boiling

b) Flow boiling or forced convection boiling

5. What are the modes of pool boiling?

Sub cooled or local boiling

Saturated or bulk boiling

Nucleate boiling

Film boiling

6. What is pool boiling?

If heat is added to the liquid from a submerged solid surface in a boiling process,

it is known as pool boiling.

7. What is sub cooled boiling?

It is defined as the pool boiling process in which the temperature of the liquid isbelow its saturation temperature.

8. Define saturated boiling.

It is one of the pool boiling process in which the temperature of the liquid is equalto or higher than its saturation temperature.

9. Explain nucleate boiling.It is one of the pool boiling process in which the bubbles formed over the surface

condense in the liquid without reaching the free surface of the liquid.

10. Mention the different regimes of flow boiling.a) Local boilingbubble region

b) Bulk boiling

c) Dry wall

11. What is a heat exchanger?


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It is equipment which transfers heat from hot substance to cool substance.

12. Give the types of heat exchangers.a) Parallel flow heat exchangers

b) Counter flow heat exchangers

c) Cross flow heat exchangers

13. Classify heat exchangers based on the type of contact.

a) Direct contact type heat exchangers

b) Indirect contact type heat exchangers

14. What is a cross flow heat exchanger?

In this type of heat exchanger the hot and cold fluids are flowing in the directions

perpendicular to each other.

15. What is a condenser?

It is a type of heat exchanger in which the temperature of the hot fluid is unalteredand the cold fluid temperature is increased.

16. Briefly explain boiling heat transfer and mention some of its applications.

Boiling is a process of phase change from liquid to vapour, the heat transferduring the boiling process is known as boiling heat transfer. Boiling heat transfer finds

wide applications in the following fields.

Cooling of nuclear reactor core

Boilers in steam power plants

Heat exchangers in refineries and process industries.

Evaporators in refrigeration and air conditioning systems.

17. Briefly explain condensation heat transfer with some examples.

Condensation is a process of phase change from vapour to liquid, the heat transferduring the condensation process is known as condensation heat transfer. It finds wide

application in the condensers used in steam power plants and refrigeration systems.

18. What are the advantages of boiling and condensation heat transfer process?

Highly heat transfer rate is achieved with small temperature difference.

Heat transfer occurs without influencing the fluid temperature.

19. Way the phase change heat transfer process is much more complex than the normal

convection process?The phase change heat transfer process is much more complex than the normal

convection process because of the following reasons,

Involvement of latent heat effects.

Presence of effects due to surface tension

Surface characteristics and other properties of two phase systems.20. Define film boiling.


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Film boiling is a process in which the bubbles formed so rapidly and blanketed

the surface by the vapour film. The vapour film over the surface reduces the heat

exchange between the liquid and the surface; this is because of low thermal conductivityof the vapour.

21. Define unstable film boiling.Unstable film boiling is defined as the film boiling process in which the vapourfilm is formed over the surface is not stable and collapses and reforms rapidly. This type

of boiling process occurs if the excess temperature is in the range between 50- 500C.

22. Define stable film boiling.Stable film boiling is defined as the film boiling process in which the vapour film

formed over the surface is stable and the surface is completely covered by the vapour

blanket.

23. Mention the various regimes of pool boiling.

Interface evaporation by natural convection Nucleate boiling

o Bubbles condense in superheated liquid

o Bubbles rise to liquid surface.

Film boiling

o Unstable film

o Stable film

o Radiation dominates

24. Define critical heat flux of burnout point.Critical heat flux or burnout point is defined as the point of maximum heat flux on

the boiling curve at which the transition initiates from nucleate boiling to film boiling.The point is also called as boiling crisis.

25. What are the factors influences the critical diameter of bubble?

Surface tension between liquid and vapour.

Surface tension between liquid and solid surface.

Surface tension between vapour and solid surface.

Buoyancy force

Angle formed by the bubble with the surface.

26. What are the factors affecting nucleate boiling?

Surface roughness, materials and shape of heating surface Physical and thermal properties of fluid.

Pressure of the systems.

Mechanical agitation of the fluid.


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27. Distinguish between film wise and drop wise condensation.

S. No Film condensation Drop condensation

1. Condensate film is continuous and

completely covers the surface

Condensate is in the form of droplets

2 Heat transfer rate is high Heat transfer rate is low

3 It occurs only on the clean anduncontaminated surfaces

It occurs on the contaminated uncleanedsurfaces

28. Define heat exchanger and give some examples.Heat exchanger is defined as an equivalent which transfers heat from a hot

substance to cool substance. Some of the examples are

Boilers and condensers in stream powerplant

Intercoolers, air preheaters and regenerators

Condensers and evaporators in refrigeration systems.

Radiators in automobiles.

Oil coolers in IC engines.

29. How the heat exchangers are classified based on the design and construction?

Concentric tubes

Shell and tubes

Multiple shell and tube passes heat exchangers

Compact heat exchangers

30. How the heat exchangers are classified based on the physical state of fluids?

Condensers

Evaporators

31. Briefly explain regenerators.

In regenerators, the hot and cold fluids are allowed to pass alternatively throughthe space containing solid matrix. This solid matrix acts as source and sinks alternativelyfor heat flow. Examples for regenerators are

Heat engines and gas turbines

Open health furnaces

Air preheaters for blast furnaces.

32. Briefly explain recuperators.

In recuperators, the hot and cold fluids are exchanging heat continuously on both

sides of the separating wall. This type is best suitable where the mixing of two fluids isnot permitted. Examples for recuperators

Radiation in automobiles

Air preheaters, economizers and superheaters

Oil coolers in IC engines and intercoolers

Condensers and evaporators in refrigeration units.


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UNIT IVRADIATION

1. What is radiation heat transfer?Transfer of heat energy from one place to another place by means of

electromagnetic waves is called radiation heat transfer.

2. State Stefan Boltzmann law of radiation.It states that the energy emitted from the surface is directly proportional to the

fourth power of its absolute temperature.

3. What are the two types of radiation intensity?

a) Spectral radiation intensity

b) Total radiation intensity

4. What are the two basic theory of radiation heat transfer?

a) Maxwells classical electromagnetic theory

b) Max planks theory

5. Explain emissive power.

It is defined as the energy emitted per unit time from a unit surface area. It is

expressed as W/m2.

6. Define reflectivity.

Reflectivity is defined as the ratio of total energy reflected from the surface to thetotal energy incident upon the surface.

7. What is a black body?

Black body is defines as an ideal body which absorb all the incident rays and alsocompletely emits all the absorbed energy.

8. Define grey body.It is defined as a real body which does not absorb the all incident rays and also

does not completely emit the absorbed energy.

9. Define radiosity.Radiosity is defined as the total amount of heat energy leaving the surface which

includes the reflected energy and the emitted energy.

10. Sate Kirchoffs law.

It states that for all substances which are in thermal equilibrium with its

surroundings the ratio of total power to absorptivity is constant.

11. Define irradiation.

Irradiation is defined as the amount of total radiation incident upon a surface per

unit time per unit area.


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23. Enumerate the effects of surface temperature on emissivity

The total emissivity of metals decreases with increase in temperature for ashorter wavelength

The spectral emissivity for non-conductors decreases with the increase intemperature.

24. Enumerate the effects of variation in wavelength on emission.

The emissivity of the conducting materials decreases with the increase inwavelength.

The emissivity of the non-conducting materials changed randomly with

the increase in wavelength

25. Define radiation shape factor.Radiation shape factor is defined as the fraction of radiative heat energy diffused

from one surface strikes directly the other surface with or without any intervening

surfaces. This is also called as geometric configuration factor, angle factor and view

factor.

26. What is meant by irradiation?Irradiation is defined as the amount of total radiation incident upon a surface per

unit time per unit area. It is measured by W/m2

27. Illustrate surface resistance.

Surface resistance is defined as the resistance to heat flow due to the surface

nature and convective heat transfer coefficient.

28. Illustrate define space resistance.Space resistance is defined as the resistance to heat flow due to the distance

between the geometry of the radiating surfaces.

29. Differentiate the radiation from gases and the radiation from solids.

The gases will emit or absorb radiation only between a very narrow rangesor bands of wavelength, whereas solids radiative at all wavelengths over

the entire spectra.

In solids the absorption of radiation takes place within a small distance,whereas in an absorbing gas, the intensity of radiation decreases with the

increase in length of passage through the gas volume.

30. Mention some of the applications of simultaneous heat exchange due to radiation and

convection.

Heat loss from the pipe passing through a room.

The flow of hot combustible products leaving from the IC engines.

31. What is the effect of increase in surface roughness?


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Generally, the increase in surface roughness increases the emissivity.

32. Define monochromatic emissive power.

Monochromatic emissive power is defined as the rate of heat energy radiated froman unit area of the surface per unit wave length. It is also called as spectral emissive

power.

33. Define absorption and absorptivity.

Absorption is defined as the process of absorbing the incident rays. Absorptivity

is defined as the ratio of energy absorbed to the energy incident on the surface.

34. Define reflectivity.

Reflectivity is defined as the ratio of total energy reflected from the surface to the total

energy incident upon the surfaces.

35. Define transmissivity.

Transmissivity is defined as the ratio of total energy transmitted through the

surface to the total energy incident upon the surface.

36. Briefly explain about reciprocity theorem.

The reciprocity theorem is used in the evaluation of radiation shape factor,according to the theorem,

A1F1-2 = A2F2-1

Where, A1 A2 = Surface areas

F1-2= Shape factor between surface 1 and 2.

F2-1

= Shape factor between surface 2 and 1.


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UNIT VMASS TRANSFER

1. Explain mass transfer.When a system consisting of two or more components whose concentrations vary

from point to point, there is a natural tendency for the transport of different species from

high concentration region to low concentration region, such process of transfer of mass asa result of species concentration difference in a mixture is called mass transfer.

2. What are the different modes of mass transfer?

a) Diffusion mass transferb) Convective mass transfer

c) Mass transfer due to chemical reactions.

3. Explain diffusion mass transfer.Diffusion mass transfer is defined as the mass transfer by random molecular

motion due to concentration difference, similar to conduction heat transfer.

4. Define convective mass transfer.

It is defined as the mass transfer from a surface whenever the concentration of

some species at the surface differs from its concentration on the flowing fluid over the

surface similar to convective heat transfer.

5. What is mass concentration?

It is the ratio of mass of individual substance to the total volume of the mixture.

6. What is molecular concentration?

It is the ratio of moles of individual component to the total moles of the mixture.

7. What is mass diffusion velocity?

Mass diffusion velocity of a particular component in a multi component mixture

is the velocity of that particular component relative to the mass average velocity of themixture.

8. Define mass transfer flux.

Mass transfer flux is defined as the amount of a particular component in either

mass or mole units which crosses a given area per unit time.

9. State Ficks law of diffusion.

Ficks law of diffusion states that the transfer of species in the direction x isdirectly proportional to the concentration gradient in that particular direction.

10. Define diffusion coefficient.

It is defined as the ability to diffuse one component into another component.

11. What is convective mass transfer coefficient?

It is the ability to transfer mass from the surface due to convection. It is defined asthe ratio between the amount of molar flux transfer and the concentration difference.


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12. Define Schmidt number.

Schmidt number is defined as the ratio of momentum diffusivity to the diffusioncoefficient.

13. Define Sherwood number.Sherwood number is defined as the ratio of diffusion resistance to the convectionresistance.

14. Mention some of the examples for mass transfer.

Diffusion of smoke through all chimneys into the environment.

Humidification of air in a cooling tower.

Evaporation of petrol in carburetor of a petrol engine.

Dissolution of salt added to water

Drying and evaporation processes.

15. What is mass fraction?

Mass fraction of a particular component in a multi component mixture is definedas the ratio of mass concentration of that particular component to the total concentration

of the mixture.

16. Define mole fraction.Mole fraction of a component in a multi component mixture is defined as the ratio

of number of moles of a component to the total number of moles of the mixture.

17. What is the total mass concentration of a binary mixture?

Total mass concentration of a binary mixture is the sum of the mass concentrationof individual components present in the mixture.

18. What is the total number of moles in a binary mixture?

The total number of moles in a binary mixture is the sum of the number of molesof the individual components present in the mixture.

19. What is the total mass of a binary mixture?

The total mass of a binary mixture is the sum of masses of the individual

components present in the mixture.

20. What is mass average velocity?

Mass average velocity of a multi component mixture is the sum of the molar

velocities of individual component present in the mixture.


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21. What is molar average velocity?

Molar average velocity of a multi component mixture is the sum of the molarvelocities of individual components present in a mixture.

22. What is mass diffusion velocity?Mass diffusion velocity of a particular component in a multi component mixtureis the velocity of that particular component relative to the mass average velocity of the

mixture.

23. Illustrate molar diffusion velocity?

Molar diffusion velocity of a particular component in a multi component mixture

is the velocity of that particular component relative to the molar average velocity of the

mixture.

24. Define mass transfer flux.

Mass transfer flux is defined as the amount of a particular component in eithermass or mole units which crosses a given area per unit time.

25. Define steady state diffusion of mass transfer?

In any diffusion mass transfer process, if the concentration at any particular point

does not vary with respect to time, it is called as steady state diffusion mass transfer.

26. What is unsteady state diffusion mass transfer?

In any diffusion mass transfer, if the concentration at any particular point varies

with respect to time, it is called as unsteady state diffusion mass transfer.

27. What are the limitations of Ficks law of diffusion?

It is valid only when the diffusion occurs only due to concentrationgradient and not due to temperature gradient, pressure gradient and any

other external forces.

It is valid only when the mass or molar fluxes are measured relative to theco ordinates moving with some average velocity and not to the fixed co-ordinates.

28. Compare mass diffusivities of liquids with mass diffusivities of gases.The mass diffusivities of liquids are comparatively smaller than those for the

gases. This is because of high molecular density of the liquids.

29. Define steady state equi-molar counter diffusion.equi-molar counter diffusion between two species A and B present in a binary

mixture is defined as an isothermal diffusion process in which each molecule of species

A is replaced by each molecule of species B and vice versa.


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30. What is diffusion resistance?

Diffusion resistance is the resistance offered by the mixture for the mass transferdue to diffusion.

31. What is concentration boundary layer?Concentration boundary layer occurs in convective mass transfer situations and isdefined as the region in which the concentration species varies from the concentration

value at the surface to the free stream concentration.

32. Define the thickness of concentration boundary layer.

The thickness of concentration boundary layer is defined as the thickness up to

which there will be concentration gradient and above which the species concentration is

equal to the free stream concentration.

33. What is the effect of turbulence in mass transfer rate?

Normally, the turbulence of the flowing fluid over the surface increases the masstransfer rate.

34. Define Lewis number?

Lewis number is defined as the ratio between concentration diffusivity anddiffusion coefficient.

35. What is the convective mass transfer coefficient?

Convective mass transfer coefficient is the ability to transfer mass from the

surface due to convection. It is defined as the ratio between the amount of molar flux

transfer and the concentration difference.

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