heat transfer 2 marks

Sastra University, B Tech(Mech) Part Time

UMCE504 Heat Transfer

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Unit I

UNIT I o Introduction to heat, mass and momentum transfer, o Physical origins and rate equations, o Derivation of heat diffusion equation in Cartesian coordinates and cylindrical

coordinates o Heat diffusion equation in spherical coordinates (without derivation) o Boundary and Initial conditions, o Methods of solutions, o One dimensional steady conduction o Plane and composite walls cylinder sphere with dependent thermal conductivity o Critical Thickness of Insulation, o Conduction with Heat generation o Plane and composite walls cylinder sphere thermal and contact resistance. o Heat transfer from extended surfaces o Fins of uniform cross sectional area o Fin performance o Overall surface efficiency.

1 State Newtons law of cooling or convection law.

2 State and explain the fundamental equations for convective heat transfer per unit time. [ SU May/08 ]

Heat transfer from the moving fluid to solid surface is given by the equation

= ( ) This equation is referred to as Newtons law of cooling or convection law. Where

h = Local heat transfer coefficient in W/m2K

A = Surface Area in m2

Tw = Temperature of surface in K.

T = Temperature of fluid in K. 3 A hallow cylinder having inner and outer radii r1 and r2 respectively, is subjected to a

steady heat transfer resulting in constant surface temperatures T1 and T2 at r1 and r2. If the thermal conductivity can be expressed as K=K0(1+b). Obtain an expression for the heat transfer per unit length of the cylinder. [ SU May/08 ]

=

where k=k0(1+b)

4 Define fin efficiency. [ SU Nov/10 ] The efficiency of a fin is defined as the ratio of actual heat transferred to the

maximum possible heat transferred by the fin

=

UMCE 504 Heat Transfer

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5 What are the primary functions of fins? Define fin efficiency. [ SU May/08 ] The primary function of the fin is to increase the heat transfer rate by increasing

the surface of heat transfer.

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

=

6 Explain the variation in thermal conductivity of a non homogeneous material with

temperature and apparent bulk density. [ SU Nov/08 ] As a general rule, thermal conductivity(k) for a nonhomogeneous material increases both

with increasing temperature and increasing apparent bulk density.

Apprent bulk density, is the mass of the substance divided by the total volume occupied. This total volume includes the void volume, such as air pockets within the overall boundaries of the piece of material.

7 Define thermal conductivity? [ SU Nov/09 ] Thermal conductivity is defined as the ability of a substance to conduct heat.

8 Define thermal conductivity of material [ SU Nov/10 ] Thermal conductivity is defined as the ability of a substance to conduct heat.

9 Define conduction shape factor. [ SU Nov/10 ] The conduction heat transfer shape factor , S , is geometry dependent quantity

that can be used to calculate conduction heat transfer rate when multiplied by the medium thermal conductivity , k , and the temperature difference , T1 - T2 .

QQ==kkSS(())TT S being the conduction shape factor

Alternative

For steady heat conduction between two isothermal surfaces in 2D or 3D problems, the simplest way to present analytical solutions is by means of the so-called conduction shape factor S, defined by Q=kS()T.

In this the geometrical effects (S) is separated, from material effects (k), which appear mixed-up in the general equation Q=KA()T

10 Why is there a negative sign in the Fouriers law of heat conduction? [ SU Nov/09 ]

Fouriers law of heat conduction = !" #$ The negative sign indicates that the heat flows in a direction along which there is a decrease in temperature.

11 What is meant by lumped capacity? In a Newtonian heating or cooling process the temperature is considered to be uniform

at a given time. Such an analysis is called lumped parameter analysis. 12 What are the physical assumptions necessary for a lumped capacity unsteady state

analysis to apply? 1. Internal resistance is negligible.

2. Temperature throughout the solid is uniform for a given time. 3. At a given time t, convective heat loss from the body is equal to the decrease in

CCoo mmppii llee dd bbyy SS .. MMuurruu ggaannaa nntthh aamm

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internal energy of the solid.

13 Define the critical thickness of insulations for a cylinder. [ SU May/08 ] The radius of insulation for which the heat transfer is maximum, is called critical

radius of insulation, and the corresponding thickness is called critical thickness. Critical radius of insulation for a cylinder is given by the expression

rc = k/h rc = critical radius

k = thermal conductivity

h= heat coefficient

Critical thickness = rc r1 If the bare surface radius is less than the critical radius, application of insulation will result in increased heat loss. This will continue till the outer radius of insulation is equal to the critical radius. Further application of insulation will result in reduction of heat loss.

14 What is critical radius of insulation or critical thickness.

15 What is critical thickness of insulation and state any two applications of the same. Addition of insulating material on a surface does not reduce the amount of heat

transfer always. In fact under certain circumstances it actually increases the heat loss up to certain thickness of insulation. The radius of insulation for which the heat transfer is maximum is called critical radius of insulation and the corresponding thickness is called critical thickness. If the thickness is further increased the heat loss will be reduced.

% = Where k = thermal conductivity

H= heat transfer co-efficient.

16 Define heat transfer. Heat transfer can be defined as the transmission of energy from one region to another

due to temperature difference.

17 What are the modes of heat transfer? 1. Conduction

2. Convection 3. Radiation.

18 What is conduction? Heat conduction is a mechanism heat transfer from a region of high temperature to a

region of low temperature within a medium (solid, liquid, gases) or different medium in direct physical contact.

In conduction, energy exchange takes place by the kinematic motion or direct impact of molecules. Pure conduction is found only in solids.

19 Define Convection. Convection is a process of heat transfer that will occur between a solid surface and a

fluid medium when they are at different temperatures. Convection is possible only in the


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presence of fluid medium.

20 Define radiation. The heat transfer from one body to another without any transmitting medium is known

as radiation. It is an electromagnetic wave phenomenon.

21 State Fouriers law of conduction. The rate of conduction is proportional to the area measured normal to the direction of

heat flow and to the temperature gradient in that direction.

" #$ = !" #$

where A- area in m2

dT/dx temperature gradient, K/m

k thermal conductivity , W/mk

22 Write down the three types of boundary conditions 1. Prescribed temperature

2. Prescribed heat flux. 3. Convection boundary conditions.

23 Define overall heat transfer co-efficient. The overall heat transfer by combined modes is usually expressed in terms of an overall

conductance or overall heat transfer coefficient. U.

Heat transfer, Q = UA T.

24 Define fins or extended surfaces. It is possible to increase the heat transfer rate by increasing the surface of heat

transfer. The surfaces used for increasing heat transfer are called extended surfaces or sometimes known as fins.

25 State the applications of fins. The main applications of fins are

1. Cooling of electronic components 2. Cooling of motor cycle engines. 3. Cooling of transformers 4. Cooling of small capacity compressors.

26 Define fin effectiveness. Fin effectiveness is the ratio of heat transfer with fin to that without fin.

& ''%()''** = ( (+,( 27 What is meant by steady state heat conduction?

If the temperature of a body does not vary with time, it is said to be in a steady state and that type of conduction is known as steady state heat conduction.

28 What is meant by transient heat conduction or unsteady state conduction? If the temperature of a body varies with time, it is said to be in a transient state and

that type of conduction is known as transient heat conduction or unsteady state


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conduction

29 What is Periodic heat flow. In periodic heat flow, the temperature varies on a regular basis:

Example : 1. Cylinder on an IC engine.

2. Surface of earth during a period of 24 hours.

30 What is non periodic heat flow. In non periodic heat flow, the temperature at any point within the system varies non

linearly with time.

Example: 1. Heating on an ingot in a furnace.

2. cooling of bars.

31 What is meant by Newtonian heating or cooling process? The process in which the internal resistance is assumed as negligible in comparison with

its surface resistance is known as Newtonian heating or cooling process.

32 What is meant by Semi-infinite solids? In a semi infinite solid, at any instant of time, there is always a point where the effect

of heating or cooling at one of its boundaries is not felt at all. At this point the temperature remains unchanged. In semi infinite solids, the boit number value is .

33 What is meant by infinite solid? A solid which extends itself infinitely in all directions of space is known as infinite solid.

In infinite solids, the biot number value is in between 0.1 and 100.

0.1 < Bi < 100.

34 Define Biot number? It is defined as the ratio of internal conductive resistance to the surface convective

resistance.

- = .('/ %+#,%()' '**(/%' 0,/%' %+%'%()' '**(/%'

- = % 35 What is the significance of Biot Number.

Biot number is used to find Lumped heat analysis, Semi infinite solids and Infinite solids.

If Bi < 0.1 Lumped heat analysis

Bi = Semi infinite solids.

0.1 < Bi < 100 infinite solids 36 Explain the significance of Fourier number.

It is defined as the ratio of characteristic body dimension to temperature wave penetration depth in time.

&+,' 1,23' = 4//%('*(% 3+#5 #2'*+ '26'/(,' /)' 6''(/(+ #'6( (2'

It signifies the degree of penetration of heating or cooling effect of a solid.

37 What are the factors affecting the thermal conductivity?


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1. Moisture 2. Density of material 3. Pressure 4. Temperature 5. Structure of material.

38 Explain the significance of thermal diffusivity? The physical significance of thermal diffusivity is that it tells us how fast heat is

propagated or it diffuses through a material during changes of temperature with time.

39 Write down the one dimensional, steady state conduction equation without internal heat generation.

778 = 9

40 Write down the steady state, two dimensional conduction equation without internal heat generation.

778 +

775 = 9

41 Write down the general equation for one dimensional steady state heat transfer in slab or plane wall without heat generation.

778 +

775 +

77; =

77(

42 Write down the general equation for one dimensional steady state heat transfer in slab or plane wall with heat generation.

778 +

775 +

77; +

=

77(

43 Define overall heat transfer co-efficient. The overall heat transfer by combined modes is usually expressed in terms of an overall

conductance or overall heat transfer coefficient U.

Heat transfer, Q = UA T.


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Unit II

UNIT II o Two dimensional steady state conduction- o Method of separation of variables. o Transient conduction o Lumped capacitance method. o Heisler chart o Introduction to convection o Average heat transfer coefficient- o Velocity and thermal boundary layers- o Laminar and turbulent flows. o Qualitative discussion of continuity, momentum and energy equations for two

dimensional flow. o External flow o Laminar and turbulent convection over flat plate cylinder sphere, o Flow across banks of tubes, o Internal flow o Turbulent flow in tubes o Non circular tubes o Concentric tube annulus

1 Define heat exchanger effectiveness. In which situations heat exchanger effectiveness method (NTU method) is advantageous in comparison to LMTD method in designing the heat exchanger. [ SU May/08 ]

=?@ A=B ==?>C= =DD = = /%(,/ '/( (/*'2/82,2 6+**3' '/( (/*'. If more than one of the inlet or outlet temperatures of the heat exchanger are unknown then the NTU method is used. Because it uses known parameters ( m,c,A,U). (NTU= Number of Transfer Units) = UA/Cmin . LMTD= Log Mean Temperature Difference.

2 Define boundary layer thickness. [ SU Nov/10 ] The thickness of the boundary layer has been defined as the distance from the surface

at which the local velocity or temperature reaches 99% of the external velocity or temperature.

3 What is the difference between laminar and turbulent flow? [ SU Nov/09 ] In the laminar flow the fluid particles flow in the smooth and continuous path

where as in the turbulent flow the fluid particles move in zig-zag manner. 5 Name any two methods employed to solve the two dimensional steady state heat

conduction problems. [ SU Nov/09 ] Numerical, graphical and analog techniques are the methods employed to solve the

two dimensional steady state heat conduction problems. 6 What is the significance of Dimensional number?

Complete solution of all field problems is not possible for all cases. Under such


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instances, using dimensional analysis number of independent variables are either reduced or converted into another form

7 What are Heisler charts? In Heisler chart, the solutions for temperature distributions and heat flows in

plane walls, long cylinders and spheres with finite internal and surface resistance are presented. Heisler charts are nothing but a analytical solutions in the form of graphs.

8 What is meant by Lumped heat analysis? In a Newtonian heating or cooling process the temperature throughout the solid is

considered to be uniform at a given time. Such an analysis is called Lumped heat capacity analysis.

9 What is dimensional analysis?

Dimensional analysis is a mathematical method which makes use of the study of the dimensions for solving several engineering problems. This method can be applied to all types of fluid resistance, heat flow problems in fluid mechanics and thermodynamics.

10 State Buckingham theorem.

Buckingham theorem states as follows: if there are n variables in a dimensionally homogeneous equation and if these contain m fundamental dimensions, then the variables are arranged into (n ) dimensionless terms. These dimensionless terms are called terms.

11 What are all the advantages of dimensional analysis?

1. It expresses the functional relationship between the variables in dimensional terms.

2. It enables getting up a theoretical solution in a simplified dimensionless form. 3. The results of one series of tests can be applied to a large number of other

similar problems with the help of dimensional analysis.

12 What are all the limitations of dimensional analysis?

1. The complete information is not provided by dimensional analysis. It only indicates that there is some relationship between the parameters.

2. No information is given about the internal mechanism of physical phenomenon. 3. Dimensional analysis does not give any clue regarding the selection of variables.

13 Define Reynolds number (Re)

It is defined as the ratio of inertia force to viscous force

G' = .'(/ &+%'H*%+,* &+%'

14 Define Prandtl number (Pr)


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It is the ratio of the momentum diffusivity to the thermal diffusivity.

I = J+2'(,2 #,*)(5'2/ #,*)(5

15 Define Nusselt Number (Nu)

It is defined as the ratio of the heat flow by convection process under an unit temperature gradient to the heat flow rate by conduction under an unit temperature gradient through stationary thickness(L) of meter

1, = K%+)'%(+K%+#,%(+

16 Define Grashof Number (Gr)

It is defined as the ratio of product of inertia force and buoyancy force to the square of viscous force.

L = .'(/ +%' -+,5/%5 +%' (H*%+,* +%') 17 Define Stanton Number (St)

It is the ratio of Nusselt number to the product of Reynolds number and Prandtl number.

0( = 1,G' I 18 What is meant by Newtonion and non-newtonion fluids?

The fluids which obey the Newtons law of viscosity are called Newtonion fluids and those which do not obey are called non-newtonion fluids.

19 What is meant by laminar flow and turbulent flow?

Laminar flow: Laminar flow is sometimes called stream line flow. In this type of flow, the fluid moves in layers and each layer remain in an orderly sequence without mixing with each other.

Turbulent flow: In addition to the laminar type of flow, a distinct irregular flow is frequency observed in nature. This type of flow is called turbulent flow. The path of any individual particle is zig-zag and irregular. Fig. shows the instantaneous velocity in laminar and turbulent flow.

Fig???

20 What is hydrodynamic boundary layer?

In hydrodynamic boundary layer, velocity of the fluid is less than 99% of free


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stream velocity.

21 What is thermal boundary layer

In thermal boundary layer, temperature of the fluid is less than 99% of free stream temperature.

22 Define convection.

Convection is a process of heat transfer that will occur between a solid surface and a fluid medium when they are at different temperatures.

23 What is meant by free or natural convection?

If the fluid motion is produced due to change in density resulting from temperature gradients, the mode of heat transfer is said to be free or natural convection.

24 What is forced convection?

If the fluid motion is artificially created by means of an external force like a blower or fan, that type of heat transfer is known as forced convection.

25 What are the dimensionless parameters used in forced convection?

1. Reynolds number (Re) 2. Nusselt number (Nu) 3. Prandtl number (Pr)

26 Define boundary layer thickness.

The thickness of the boundary layer has been defined as the distance from the surface at which the local velocity or temperature reaches 99% if the external velocity or temperature.

27 Indicate the concept or significance of boundary layer.

In the boundary layer concept the flow field over a body is divided into two region.

A thin region neat the body called the boundary layer where the velocity and the temperature gradients are large.

The region outside the boundary layer where the velocity and the temperature gradients are very nearly equal to their free stream values.

28 Sketch the boundary layer development of a flow indicating the regions.


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29 Define displacement thickness

The displacement thickness is the distance, measured perpendicular to the boundary, by which the free stream is displaced on account of formation of boundary layer.

30 Define momentum thickness

The momentum thickness is defined as the distance through which the total loss of momentum per second be equal to if it were passing a stationary plate.

31 Define energy thickness.

The energy thickness can be defined as the distance, measured perpendicular to the boundary of the solid body, by which the boundary should be displaced to compensate for the reduction in kinetic energy of the flowing fluid on account of boundary layer formation.

32 An electrically heated plate dissipates heat by convection at a rate of 8000 W/m2 into the ambient air at 25C. if the surface of the hot plate is at 125C, calculate the heat transfer coefficient for convection between the plate and air.

Heat transfer Q = h A (Tw - T)

8000 = h x 1 x (125 25)

h = 8000 / 100

h = 800 W/m2K


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Unit III

UNIT III o Free convection o Laminar and turbulent free convection on vertical, horizontal and inclined

surfaces- cylinder - sphere o Boiling and condensation, Pool boiling Boiling curve o Modes of pool boiling o Pool boiling correlations. o Laminar and turbulent film condensation on vertical and inclined plane surfaces o Dropwise condensation. o Heat exchangers classification o Overall heat transfer coefficient o Log mean temperature difference o Parallel flow and counter flow heat exchangers o Multi pass and cross flow heat exchangers o Effectiveness-NTU method.

1 What are the primary functions of fins? Define fin efficiency. [ SU May/08 ] The primary function of the fin is to increase the heat transfer rate by

increasing the surface of heat transfer.

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

=

2 Show a boiling curve for typical pool boiling indicating the six different regions. [ SU

May/08 ]

B


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3 What is Excess temperature in boiling?

Excess temperature is measured from the boiling point of the fluid. T= Ts -Tsat.

4 Differentiate between film condensation and drop wise condensation. Also compare the heat transfer rates in both cases. [ SU Nov/08 ]

The filmwise condensation wets the solid surface, spreads out and forms a continuous film over the entire surface. Film condensation occurs when a vapour is free from impurities.

In dropwise condensation, the vapour condenses into small liquid droplets of various sizes which fall down the surface in a random fashion.

Heat transfer rates in dropwise condensation is 10 times higher than filmwise


condensation. 5 With a neat sketch define Log

heat exchanger. Also state the important assumptions made in defining the LMTD. [ SU Nov/08 ]

In heat exchanger applications, the inlet and outlet temperatures are commonly specified base on the fluid in the tubes. The temperature change that takes across the heat exchanger from the entrance to the exit is not linear. A precise temperature change between two fluids across the heat exchanger is best represented by the log mean temperature difference (LMTD or DTlm), defined below .

The assumptions are made while defining the LMTD are:1. Flow is steady 2. The overall heat transfer co3. The specific heats of both fluids are constant.4. The mass flow rates of both fluids are constant.5. Axial conduction along the tube is negligible.6. The change is kinetic and potential energies of the fluids are negligible.

6 What are the two modes in which condensation can take place on a cooling surface? [ SU Nov/09 ]

There are two modes of condensation:

1. Filmwise condensation2. Dropwise condensation

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With a neat sketch define Log-Mean temperature difference applicable to parallel flow exchanger. Also state the important assumptions made in defining the LMTD. [ SU

In heat exchanger applications, the inlet and outlet temperatures are commonly

specified base on the fluid in the tubes. The temperature change that takes across the heat exchanger from the entrance to the exit is not linear. A precise temperature change between two fluids across the heat exchanger is best represented by the log mean temperature difference (LMTD or DTlm), defined below .

2 O

ade while defining the LMTD are:

The overall heat transfer co-efficient is constant. The specific heats of both fluids are constant. The mass flow rates of both fluids are constant.

conduction along the tube is negligible. The change is kinetic and potential energies of the fluids are negligible.

What are the two modes in which condensation can take place on a cooling surface? [ SU

There are two modes of condensation:

Filmwise condensation Dropwise condensation

Mean temperature difference applicable to parallel flow exchanger. Also state the important assumptions made in defining the LMTD. [ SU

In heat exchanger applications, the inlet and outlet temperatures are commonly

specified base on the fluid in the tubes. The temperature change that takes place across the heat exchanger from the entrance to the exit is not linear. A precise temperature change between two fluids across the heat exchanger is best represented by the log mean temperature difference (LMTD or DTlm), defined below .

The change is kinetic and potential energies of the fluids are negligible.

What are the two modes in which condensation can take place on a cooling surface? [ SU


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7 What is the difference between parallel flow and counter flow heat exchanger? [ SU Nov/09 ]

In the parallel flow heat exchanger the hot and cold fluids move in the same direction where as in the counter flow heat exchanger the hot and cold fluids move in the opposite direction.

8 Define effectiveness of heat exchanger. [ SU Nov/09 ] The effectiveness of heat exchanger is defined as the ratio of actual heat transfer to

the maximum possible heat transfer.

P'%()''** = "?>QO @=> >B D=BRQ STDDUO= @=> >B D=B

P'%()''** = 9 What are the modes of heat transfer present in boiler? [ SU Nov/10 ] Conduction and Convection.

10 What is fouling? What is its effect on the heat exchanger? [ SU Nov/10 ] The heat exchangers surfaces becomes fouled with scale or deposits after the

prolonged use. This is called fouling. The fouling reduces the value of overall heat transfer co-efficient.

11 Define Recuperators. Show the sketch of a shell and tube heat exchanger with multiple tubes, two passes and baffles. [ SU Nov/08 ]

Recuperators heat exchanger is the one in which the hot and cold fluid do not come into contact with each other but are separated by a tube wall or a surface.

For fig refer page 3.70 fig 3.9 12 What is the relation between the Newtonian law of cooling and convective heat transfer

coefficient? [ SU Nov/10 ] Newtons law of cooling

Heat transfer by convection is given by Newtons law of cooling

Q =h A [ TS T]

A - Area exposed to heat transfer in m2

H Heat transfer co-efficient in W/m2K

TS - Temperature of the surface in K

T - Temperature of the fluid in K. 13 Define boiling The change of phase from liquid to vapour state is known as boilint

14 What is meant by condensation?

The change of phase from vapour to liquid state is known as condensation.

15 Give the application of boiling and condensation.

Boiling and condensation process finds wide applications as mentioned below.

1. Thermal and nuclear power plant 2. Refrigerating system 3. Process of heating and cooling


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4. Air conditioning system

16 What is meant by pool boiling?

If heat is added to a liquid from a submerged solid surface, the boiling process is referred to as pool boiling. In this case the liquid above the hot surface is essentially stagnant and its motion near the surface is due to free convection and mixing induced by bubble growth and detachment.

17 What are the modes of condensation?

There are two modes of condensation

1. Filmwise condensation 2. Dropwise condensation

18 What is meant by Filmwise condensation?

The liquid condensate wets the solid surface, spreads out and forms a continuous film over the entire surface is known as film wise condensation.

19 What is meant by Dropwise condensation?

In dropwise condensation, the vapour condenses into small liquid droplets of various sizes which fall down the surface in a random fashion.

20 Give the merits of dropwise condensation.

In dropwise condensation, a large portion of the area of the plate is directly exposed to vapour. The heat transfer rate in dropwise condensation is 10 times higher than in film condensation.

21 Write the force balance equation on a volume element for filmwise condensation on a vertical plane surface.

VWXVYW =

1[\

V]V^

_^[\

Where

Bx- Body force in x direction `a`b = Pressure gradient

22 Draw different regions of boiling and what is Nucleate boiling?

Nucleate boiling exists in regions II and III. The nucleate boiling begins at region. II. As the excess temperature is further increased, bubbles are formed more rapidly and rapid evaporation takes place. This is indicated in region III. Nucleate boiling exists upto T= 50 C.


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23 What is heat exchanger?

A heat exchanger is defined as an equipment which transfers the heat from a hot fluid to a cold fluid.

24 What are the types of heat exchangers?

The types of heat exchangers are as follows

1. Direct contact heat exchangers 2. Indirect contact heat exchangers 3. Surface heat exchangers 4. Parallel flow heat exchangers. 5. Counter flow heat exchanger 6. Cross flow heat exchanger 7. Shell and tube heat exchangers 8. Compact heat exchanger

25 What is meant by direct heat exchanger (or) open heat exchanger?

In direct contact heat exchanger, the heat exchanger takes place by direct mixing of hot and cold fluids.

26 What is meant by indirect contact heat exchanger?

In this type of heat exchangers, the transfer of heat between two fluids could be carried out by transmission through a wall which separates the two fluids.

27 What is meant by regenerators?

In this type of heat exchangers, hot and cold fluids flow alternately through the same space.

28 What is meant by recuperators or surface heat exchangers?

This is the most common type of heat exchanger in which the hot and cold fluid do not come into direct contact with each other but are separated by a tube wall or a surface.

29 What is meant by parallel flow heat exchanger?

In this type of heat exchanger, hot and cold fluids move in the same direction.

30 What is meant by counter flow heat exchanger?

In this type of heat exchanger, hot and cold fluids move in parallel but opposite direction.

31 What is meant by cross flow heat exchanger?

In this type of heat exchanger, hot and cold fluids move at right angles to each other.

32 What is meant by shell and tube heat exchanger?

In this type of heat exchanger, one of the fluids move through a bundle of tubes enclosed by a shell. The other fluid is forced through the shell and it moves over the outside surface of the tubes.

33 What is meant by compact heat exchanger? Give examples.

There are many special purpose heat exchangers called compact heat exchangers. They are generally employed when convective heat transfer co-efficient associated with one


of the fluids is much smaller than that associated with the other fluid.

Examples Car radiators, Gas turbine heat exchangers.

34 What is meant by LMTD?

We know that the temperature difference between the hot and cold fluids in the heat exchange varies from point to point. In addition various modes of heat transfer are involved. Therefore based on concept of appropriate mean temperature difference, also called logarithmic mean temperature difference, the total heat transfer rate in the heat exchanger is expressed as

Q =U A ( T)m where U Overall heat transfer co

A Area in m2

(T)m Logarithmic mean temperature differ

35 What are the assumptions made in LMTD method?

1. Velocity is constant throughout the exchanger.2. The system is adiabatic; heat exchange takes place only between the two fluids.3. The temperatures of both fluids are constant over a given

be represented by bulk temperatures.4. The specific heats of the fluids are constant.

36 What is mean by fouling factor?

We know, the surfaces of a heat exchangers do not remain clean after it has been in use for some time. The surfaces become fouled with scaling or deposits. The effect of these deposits affecting the value of overall heat transfer cotaken care of by introducing an additional thermal resistance called the fouling resistance.

37 What is meant by Effectiveness of heat exchanger?

The heat exchanger effectiveness is defined as the ratio of actual heat transfer to the maximum possible heat transfer.

Effectiveness

cddeffghiejekk

38 Sketch the temperature variations in parallel flow heat exchanger

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radiators, Gas turbine heat exchangers.

We know that the temperature difference between the hot and cold fluids in the heat exchange varies from point to point. In addition various modes of heat transfer are involved. Therefore based on concept of appropriate mean temperature difference,

lled logarithmic mean temperature difference, the total heat transfer rate in the heat exchanger is expressed as

Overall heat transfer co-efficient, W/m2 K

Logarithmic mean temperature difference.

What are the assumptions made in LMTD method?

Velocity is constant throughout the exchanger. The system is adiabatic; heat exchange takes place only between the two fluids.The temperatures of both fluids are constant over a given be represented by bulk temperatures. The specific heats of the fluids are constant.

What is mean by fouling factor?

We know, the surfaces of a heat exchangers do not remain clean after it has been in use surfaces become fouled with scaling or deposits. The effect of

these deposits affecting the value of overall heat transfer co-efficient. This effect is taken care of by introducing an additional thermal resistance called the fouling

meant by Effectiveness of heat exchanger?

The heat exchanger effectiveness is defined as the ratio of actual heat transfer to the maximum possible heat transfer.

cddeffghiejekk(l) = mfgno\ peog gqojkdeqroshsns ]tkkhu\e peog gqojkdeq

cddeffghiejekkl vvso^ Sketch the temperature variations in parallel flow heat exchanger


We know that the temperature difference between the hot and cold fluids in the heat exchange varies from point to point. In addition various modes of heat transfer are involved. Therefore based on concept of appropriate mean temperature difference,

lled logarithmic mean temperature difference, the total heat transfer rate in

The system is adiabatic; heat exchange takes place only between the two fluids. The temperatures of both fluids are constant over a given cross-section and can

We know, the surfaces of a heat exchangers do not remain clean after it has been in use surfaces become fouled with scaling or deposits. The effect of

efficient. This effect is taken care of by introducing an additional thermal resistance called the fouling

The heat exchanger effectiveness is defined as the ratio of actual heat transfer to the

gqojkdeqgqojkdeq

Sketch the temperature variations in parallel flow heat exchanger


39 Sketch the temperature variations in counter flow heat exchanger

40 Sketch the eveporation of the fluid inside the the pipe line flow.

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Sketch the temperature variations in counter flow heat exchanger

Sketch the eveporation of the fluid inside the the pipe line flow.

Sketch the temperature variations in counter flow heat exchanger

Sketch the eveporation of the fluid inside the the pipe line flow.


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Unit IV

UNIT IV o Thermal Radiation: Fundamental concepts o Black body radiation o Plancks distribution o Wiens displacement law o Stefan-Boltzmann law o Lamberts law o Surface emission and absorption o Reflection and transmission o Kirchhoffs law Gray surface. o View factor relations o Hottels crossed string method o Black Body radiation exchange o Radiation exchange between diffuse gray surfaces in an enclosure o Network method o Radiation shields. o Introduction to gas radiation.

1 Define absorptivity, reflectivity and transmissivity as applicable to radiations. [ SU May/08 ]

3*+6()(5 = G/#/(+ /3*+3'#.%#'( /#/(+

G''%()(5 = G/#/(+ ''%('#.%#'( /#/(+

/*2**)(5 = G/#/(+ (/*2(('#.%#'( /#/(+

2 Explain Kirchoffs law of radiation. [ SU May/08 ] This law states that the ratio of total emissive power to the absorptivity is constant for

all surfaces which are in thermal equilibrium with the surroundings. This can be written as cwxw =

cWxW =

cyxy

3 Explain the influence of temperature and pressure in the thermal conductivity of gases. [ SU May/08 ]

Thermal conductivity of a gas increases with increasing temperature, but is essentially independent of pressure for pressures close to atmospheric.

4 State and explain the fundamental equation for radiant heat transfer [ SU Nov/08 ] Stefan Boltzmann law : The emissive power of a black body is proportional to the

fourth power of absolute temperature.


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Eb = T4

5 Define thermal diffusivity. What is its unit in SI system. [ SU Nov/08 ] Thermal diffusivity is nothing but how fast heat is diffused through a material during

changes of temperature with time. It is the ration between thermal conductivity to volumetric heat capacity.

= z{|} where:

k : thermal conductivity (SI units: W/mK) : density (kg/m) cp : specific heat capacity (J/kgK)

Another way

In heat transfer analysis, thermal diffusivity (symbol: ) is the thermal conductivity divided by the volumetric heat capacity. It has the SI unit of m/s.

6 What is emissive power? [ SU Nov/10 ] The emissive power is defined as the total amount of radiation emitted by a body per unit

time and unit area.

It is expressed in W/m2

7 Define total emissive power. What are the factors on which total emissive power of a surface depend upon? [ SU Nov/08 ]

Total emissive power is the power emitted by unit surface area of a black body.

RT = T4

The power emitted by a black body changes with the fourth power of the absolute temperature of the body. This relationship is called the Stefan Boltzmann Law and is denoted by

P = AT4

Where A stands for the area of the black body.

stands for Stefan Boltzmann constant equal to 5.67 X 10-8 W/m2K4

8 Differentiate between specular and diffuse surface. [ SU Nov/08 ] A specular reflection is one similar to the reflection of light from a mirror. Light is

reflected diffusely from a wall painted with a flat textured paint. Specular reflections can cause an image to appear on a diffuse surface. Diffuse reflections will cause a uniform glow in all directions, relative to the intensity of the light that hit the surface

9 State Lamberts law. [ SU Nov/09 ] Lamberts law states that the total emissive power Eb from a radiating plane

surface in any direction is directly proportional to the cosine of the angle of emission.

P3 ?TD ~

10 State Wiens displacement law [ SU Nov/10 ] The Wiens law gives the relationship between temperature and wave length

corresponding to the maximum spectral emissive power of the black body at that


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temperature.

2/8 = 4 Where C3 = 2.9 x 10-3 (radiation constant) b= 2.9 10-3 mk

11 What is the significance of Boit Number. [ SU Nov/09 ] The ratio of internal conduction resistance to the surface convection resistance is

known as Biot number.

_htg nsueq = Internal conduction resistanceSurface convection resistance Biot number is used to find Lumped heat analysis, semi infinite solids and infinite solids.

12 What is the difference between white body & black body? [ SU Nov/09 ] A black body absorbs all incident radiation, regardless of wavelength and direction

whereas the white body reflects all radiation. 13 What is meant by gray body? [ SU Nov/10 ] The gray body absorbs some percentage of incident radiation. The emissive power of

gray body is always less than that of the black body.

The black body absorbs all incident radiation. The emissive power of black body is one.

14 Define Opaque body.

Opaque body when no incident radiation is transmitted through the body, it is called an opaque body.

15 Define white body.

White body. If all the incident radiation falling on the body are reflected, it is called a white body.

16 Differentiate between Secular reflection and Diffused reflection Secular reflection occurs from a surface such as a mirror, which is very smooth. In

secular reflection, the angle of reflection is equal to the angle of incidence.

Diffuse reflection occurs when the surface is rough and it reflects in all direction.

17 Define Radiation.

The heat transfer from one body to another without any transmitting medium is known is radiation. It is an electromagnetic wave phenomenon.

18 Define emissive power (Eb)

The emissive power is defined as the total amount of radiation emitted by a body per unit time and unit area. It is expressed in W/m2.

19 Define mano chromatric emissive power. [ Eb]

The energy emitted by the surface at a given length per unit time per unit area in all directions is known as monochromatic emissive power.

20 What is meant by absorptivity?

Absorptivity is defined as the ratio between radiation absorbed and incident radiation.


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muktq]ghihgY, (x) = ohoghtj qed\efgeojkdeqjfhejg qohoghtj 21 What is meant by reflectivity?

Reflectivity is defined as the ratio of radiation reflected to the incident radiation.

ed\efghihgY() = ohoghtj qed\efgejfhejg qohoghtj 22 What is meant by transmissivity?

Transmissivity is defined as the ratio of radiation transmitted to the incident radiation.

qojkshkkhihgY, () = ohoghtj gqojkshggejfhejg qohoghtj 23 What is black body?

Black body is an ideal surface having the following properties.

1. A block body absorbs all incident radiation, regardless of wave length and direction.

2. For a prescribed temperature and wave length, no surface can emit more energy than black body.

24 State Plancks distribution law.

The relationship between the monochromatic emissive power of a black body and wave length of a radiation at a particular temperature is given by the following expression, by Planck.

c = w

e 1

Where Eb = Monochromatic emissive power W/m2

= Wave length m

C1 = 0.374 x 10-15 W m2

C2 = 14.4 x 10 3 mk

25 State Stefan-Boltzmann law

The emissive power of a black body is proportional to the fourth power of absolute temperature.

Eb T4

Eb = T4

Eb = Emissive power, W/ m2

= Stefan-Boltzmann constant = 5.67 x 10-8 W/m2K4

T = Temperature , K

26 Define emissivity.

It is defined as the ability of the surface of a body to radiate heat. It is also defined as the ratio of emissive power of any body to the emissive power of a black body of equal temperature.


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cshkkhihgY() = cc 27 What is meant by gray body?

If a body absorbs a definite percentage of incident radiation irrespective of their wave length, the body is known as gray body. The emissive power of a gray body is always less than that of black body.

28 State Kirchoffs law of radiation.

This law states that the ratio of total emissive power to the absorptivity is constant for all surfaces which are in thermal equilibrium with the surrounding. This can be written as

cwxw =

cWxW =

cyxy

It also states that the emissivity of the body is always equal to its absorptivity when the body remains in thermal equilibrium with its surroundings.

xw = c w ; xW = c W 29 Define intensity of radiation [Ib]

It is defined as the rate of energy leaving a space in a given direction per unit solid angle per unit area of the emitting surface normal to the mean direction in space.

= cy 30 State Lamberts cosine law.

It states that the total emissive power Eb from a radiating plane surface in any direction proportional to the cosine of the angle of emission.

Eb cos 31 What is the purpose of radiation shield?

Radiation shields constructed from low emissivity (high reflective) materials. It is used ro reduce the net radiation transfer between two surfaces.

32 Define irradiation [G]

It is defined as the total radiation incident upon a surface per unit time per unit area. It is expressed in W/m2.

33 What is radiosity[J]

It is used to indicate the total radiation leaving a surface per unit time per unit area. It is expressed in W/m2.

34 What are the assumptions made to calculate radiation exchange between the surfaces ?

1. All surfaces are considered to be either black or gray. 2. Radiation and reflection process are assumed to be diffuse. 3. The absorptivity of a surface is taken equal to its emissivity and independent of

temperature of the source of the incident radiation.


35 What is meant by shape factor and mention its physical significance.

The shape factor is defined as The fraction of the radiative energy that is diffused from one surface element and strikes the other surface directly with no intervening reflections. It is represented by Fif. Other names for radiation shape factor are view factor, angle factor and configuration factor. The shape factor is sued in the analysis oradiative heat exchange between two surfaces.

36 The heat transfer by radiation takes place by means of ____________________

Electromagnetic waves.

37 A perfect black body is one which ____________________________

Absorbs heat radiation of all

38 Two plates spaced 150 mm apart are maintained at 1000will take place mainly by ______________________.

Radiation.

39 According to Stefan-Boltzmann law, ideal radiators emit radiant energy at a rate proportional to _______________________.

Fourth power of absolute temperature.

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

Radiation.

41 The amount of radiation mainly depends on ______________

Nature of body, temperature of b ody and type of surface of body.

42 The heat transfer equation Q

Stefan-Boltzmann equation.

43 Discuss the radiation characteristics of carbon dioxide and water vapour.

The CO2 and H2O both absorb and emit radiation over certain wavelength regions called absorption bands.

The radiation in these gases is a volume phenomenon.

The emissivity of CO2 and the emissivity of H2O at a particular temperature increases with partial pressure and mean beam length.

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What is meant by shape factor and mention its physical significance.

The shape factor is defined as The fraction of the radiative energy that is diffused one surface element and strikes the other surface directly with no intervening

reflections. It is represented by Fif. Other names for radiation shape factor are view factor, angle factor and configuration factor. The shape factor is sued in the analysis oradiative heat exchange between two surfaces.

The heat transfer by radiation takes place by means of ____________________

A perfect black body is one which ____________________________

Absorbs heat radiation of all wavelengths falling on it.

Two plates spaced 150 mm apart are maintained at 1000C and 70will take place mainly by ______________________.

Boltzmann law, ideal radiators emit radiant energy at a rate proportional to _______________________.

Fourth power of absolute temperature.

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

The amount of radiation mainly depends on ______________


equation Q = AT4 is known as __________________

Boltzmann equation.

Discuss the radiation characteristics of carbon dioxide and water vapour.

The CO2 and H2O both absorb and emit radiation over certain wavelength regions called

The radiation in these gases is a volume phenomenon.

The emissivity of CO2 and the emissivity of H2O at a particular temperature increases with partial pressure and mean beam length.

Compiled by S.Muruganantham

[email protected]

What is meant by shape factor and mention its physical significance.

The shape factor is defined as The fraction of the radiative energy that is diffused one surface element and strikes the other surface directly with no intervening

reflections. It is represented by Fif. Other names for radiation shape factor are view factor, angle factor and configuration factor. The shape factor is sued in the analysis of

The heat transfer by radiation takes place by means of ____________________

A perfect black body is one which ____________________________

C and 70C. The heat transfer

Boltzmann law, ideal radiators emit radiant energy at a rate

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


is known as __________________

Discuss the radiation characteristics of carbon dioxide and water vapour.

The CO2 and H2O both absorb and emit radiation over certain wavelength regions called

The emissivity of CO2 and the emissivity of H2O at a particular temperature increases

Compiled by S.Muruganantham


Page 27 of 27


Page 28 of 28

Some more questions

1. What is a Heat Exchanger? 2 How heat exchangers are Classified? 3 Give examples of non mixing type heat exchangers. 4 Sketch temperature distribution graph for condensers & evaporators. 5 What is overall heat transfer coefficient in a heat exchanger? 6 What is LMTD? 7 What is effectiveness of a heat exchanger? 8 Discuss the advantage of NTU method over the LMTD method. 9 What are the assumptions made during LMTD analysis? 10 What are the factors are involved in designing a heat exchangers? 11 In what way Boiling & Condensation differs from other types of heat exchange? 12 What is Excess temperature in boiling? 13 What is meant by sub cooled or local boiling? 14 What is Nucleate boiling? 15 Give expression for heat transfer coefficient in Nucleate boiling. 16 What is flow boiling? 17 What is meant by condensation? 18 Draw heat flux curve for various regions of flow boiling. 19 Define Film wise condensation. 20 Define Drop wise condensation. 21 How is the Reynolds number in condensation defined? 22 State and explain the Fourier's law of heat conduction 23 Define the efficiency and effectiveness of a fin. 24 Distinguish the heat transfer by conduction and convection. 25 What is the role of extended surfaces in heat transfer applications? 26 What do you understand by Log Mean Area? State its significance. 27 What is lumped heat analysis? 28 Write about transient heat conduction. 29 Write expression for variation of thermal conductivity with temperature. 30 What is an error function? 31 What is the difference between the free convection and forced convection? 32 Define heat flux and thermal diffusivity. 33 Distinguish between Grashoff number and Nussetl's number. 34 What are the types of fins? Which one is more effective'? 35 A thin plate 1 meter long and 1 meter wide is placed in an air stream moving with

velocity of 0.25 m/s. Determine the type of flow over the plate. 36 Distinguish between natural and forced convection. 37 Give the physical significance of Nusselt number and Prandtl number. 38 Define the Nusselt number and Prandtl number. 39 Illustrate the hydrodynamic boundary layer thickness for the flow over the flat plate in

the forced convection. 40 A turbulent flow over a solid surface facilitates more heat transfer when compared to a

laminar flow. Comment on the statement. 41 Give at least two practical examples for free convective heat transfer. 42 Explain Reynolds Analogy for laminar flow. 43 Define intensity of radiation.


Page 29 of 29

44 Define radiation intensity and radiation shape factor. 45 Define irradiation and radiosity. 46 Define the terms absorptivity and transmittivity of radiation. 47 State the reciprocity theorem of the radiation shape factors. 48 Define radiation shape factor. 49 Define black body in radiative heat transfer. 50 What is Fouling factor'? 51 Define effectiveness as a function of NTU of heat exchanger for counter flow type. 52 Define overall heat transfer coefficient. 53 Define "LMTD". 54 Define the effectiveness of the heat exchanger. 55 What is ablative heat transfer? 56 Draw the temperature profile across a cooled rocket combustion chamber wall. 57 Why does an aerospace vehicle flying at high supersonic Mach number experience

aerodynamic heating in atmosphere? 58 What is meant by recuperator? 59 What is shape resistance .How it influences on over all heat transfer coefficient? 60 How is ablation used for high speed cooling? 61 Define Transpiration Cooling?

heat transfer 2 marks

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