Heat

Biyani's Think Tank

Concept based notes

Heat Transfer

(B.Tech)

Meenakshi yadav

Asst. Professor

Deptt. of Engineering

Biyani International Institute of Engineering and Technology

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Edition : 2013

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While every effort is taken to avoid errors or omissions in this Publication, any mistake or

omission that may have crept in is not intentional. It may be taken note of that neither the

publisher nor the author will be responsible for any damage or loss of any kind arising to

anyone in any manner on account of such errors and omissions.

Heat Transfer 3


Preface

I am glad to present this book, especially designed to serve the needs of the students. The book has been written keeping in mind the general weakness in understanding the

fundamental concepts of the topics. The book is self-explanatory and adopts the Teach

Yourself style. It is based on question-answer pattern. The language of book is quite easy and

understandable based on scientific approach.

Any further improvement in the contents of the book by making corrections, omission and

inclusion is keen to be achieved based on suggestions from the readers for which the author

shall be obliged.

I acknowledge special thanks to Mr. Rajeev Biyani, Chairman & Dr. Sanjay Biyani, Director

(Acad.) Biyani Group of Colleges, who are the backbones and main concept provider and also

have been constant source of motivation throughout this Endeavour. They played an active role

in coordinating the various stages of this Endeavour and spearheaded the publishing work.

I look forward to receiving valuable suggestions from professors of various educational

institutions, other faculty members and students for improvement of the quality of the book. The

reader may feel free to send in their comments and suggestions to the under mentioned

address.

Note: A feedback form is enclosed along with think tank. Kindly fill the feedback form

and submit it at the time of submitting to books of library, else NOC from

Library will not be given.

Author

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Unit 1

Q1 What do you mean by heat transfer?

Ans heat is defined in physics as the transfer of thermal energy across a well-defined boundary around a thermodynamic systemIt is a characteristic of a process and is never contained in matter. In engineering contexts, however, the term heat transfer has acquired a specific usage, despite its literal redundancy of the characterization of transfer. In these contexts, heat is taken as synonymous to thermal energy. This usage has its origin in the historical interpretation of heat as a fluid (caloric) that can be transferred by various causes, and that is also common in the language of laymen and everyday life.

Fundamental methods of heat transfer in engineering include conduction, convection, and radiation. Physical laws describe the behavior and characteristics of each of these methods. Real systems often exhibit a complicated combination of them.

The fundamental modes of heat transfer are:

conduction

The transfer of energy between objects that are in physical contact. Convection

Heat Transfer 5


The transfer of energy between an object and its environment, due to fluid motion. Radiation The transfer of energy to or from a body by means of the emission or absorption of electromagnetic radiation. CONDUCTION:- Conduction is heat TRANSFER by means of molecular agitation within a material without any motion of the material as a whole. If one end of a metal rod is at a higher temperture, then energy will be transferred down the rod toward the colder end because the higher speed particles will collide with the slower ones with a net transfer of energy to the slower ones. For heat transfer between two plane surfaces, such as heat loss through the wall of a house. CONVECTION:- Convection is heat transfer by mass motion of a fluid such as air or water when the heated fluid is caused to move away from the source of heat, carrying energy with it. Convection above a hot surface occurs because hot air expands, becomes less d ense, and rises Hot water is likewise less dense than cold water and rises, causing convection currents which transport energy. Convection can also lead to circulation in a liquid, as in the heating of a pot of water over a flame. Heated water expands and becomes more buoyant. Cooler, more dense water near the surface descends and patterns of circulation can be formed, though they will not be as regular as suggested in the drawing. RADIATION:-

Radiation energy emitted by matter as em waves due to the pool of thermal energy in all matter with a temperature above zero. Thermal radiation propagates without the presence of matter through the of space.

Thermal radiation is a direct result of the random movements of atoms and molecules in matter. Since these atoms and molecules are composed of charged particles their movement results in the emission of em radiations which carries energy away from the surface.

Unlike conductive and convective forms of heat transfer, thermal radiation can be concentrated in a small spot by using reflecting mirrors, which is exploited in solar power. For example, the sunlight reflected from mirrors heats the and during the day it can heat water to 285 C (545 F

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Q3 Explain Fouriers law of heat conduction?

Ans The law of heat conduction, also known as forier,s law, states that the time rate of ht through a material is proportional to the negative gradient in the temperature and to the area, at right angles to that gradient, through which the heat is flowing. We can state this law in two equivalent forms: the integral form, in which we look at the amount of energy flowing into or out of a body as a whole, and the differential form, in which we look at the flow rates or fluxes of energy locally.

Fourier's law is an empirical law based on observation. It states that the rate of heat flow dQ/dt, through a homogeneous solid is directly proportional to the area, A, of the section at right angles to the direction of heat flow, and to the temperature difference along the path of heat flow, dT/dx i.e.

Q4 Explain critical insulation thikness.

Ans:-

We know that heat loss fro the pipe is:-

Heat Transfer 7


If A, is increased, q will increase. When insulation is added to a pipe,

the outside surface area of the pipe will increase. This would indicate

an increased rate of heat transfer

The insulation material has a low thermal conductivity,

it reduces the conductive heat transfer

lowers the temperature difference between the outer surface

temperature of the insulation and the surrounding bulk fluid

temperature.

This contradiction indicates that there must be a critical thickness of

insulation.

The thickness of insulation must be greater than the critical

thickness, so that the rate of heat loss is reduced as desired.

As the outside radius, ro, increases, then in the denominator, the

first term increases but the second term decreases.

Thus, there must be a critical radius, rc , that will allow

maximum rate of heat transfer, q.

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The critical radius, rc, can be obtained by differentiating and

setting the resulting equation equal to zero.

Ti,Tb, k, L, ro, ri are constant terms, therefore:

When outside radius becomes equal to critical radius, or ro = rc, we get,

Q5 Explain overall heat transfer coefficient ?

Ans:- The overall heat transfer coefficient is employed in calculating the rate of heat

transfer from one fluid at an average bulk temperature T1 through a solid surface to a second fluid at an average bulk temperature T2 (where T1 > T2). The defining equation is generally only applicable to an incremental element of heat

transfer surface dA for which the heat transfer rate is d , and the equation is strictly valid only at steady state conditions and negligible lateral heat transfer in the solid surface, conditions generally true enough in most practical applications. The defining equation is

Heat Transfer 9


(1)

where U is referenced to a specific surface (see below).

In the particular situation of heat transfer across a plane wall of uniform thickness, U is related to the individual film heat transfer coefficients, 1 and 2, of the two fluids by the equation

(2)

where w is the thickness of the wall and w is the thermal conductivity of the wall.

For the special but very important case of heat transfer through the wall of a plain round tube, the different heat transfer areas on the inside and outside surfaces of the tube need to be considered. Let dAi be the inside incremental area and dAo be the outside. Then (including fouling resistances Rfi and Rfo inside and out):

(4)

where Ui is termed the "overall heat transfer coefficient referenced to (or based on) the inside tube heat transfer area", and ri and ro the inside and outside radii of the tube.

Alternatively, the overall coefficient may be based on the outside heat transfer area, giving

(5)

where Uo is termed the "overall heat transfer coefficient based on the outside tube heat transfer area." Note that

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(6)

Heat Transfer 11


Unit 2

Q1 What do you mean by thermal boundary layer?

Ans Heat will flow between a wall and the fluid adjacent to it when a temperature gradient is established between the wall and the fluid. Near the wall the fluid velocity increases from zero at the wall to the bulk velocity, sometimes not too distant from the wall relative to the radius of curvature. Likewise, the temperature changes from that at the wall to that in the free stream. The result is that the fluid temperature adjacent to the wall is assumed to be equal to the surface temperature of the wall at the interface and is equal to the bulk fluid temperature at some point in the fluid. The distance over which the temperature change occurs is called the thermal boundary layer. A boundatr layer also is present if the fluid is flowing past the wall. The momentum (hydrodynamic) boundary layer and the thermal boundary layer can affect each other. The distances over which the velocity changes from zero to the free stream velocity and the temperature changes from the wall temperature to the free stream temperature are often different.

From a corrosion standpoint, the wall temperature or more specifically, the temperature at the wall-fluid boundary is the important parameter driving corrosion. The fluid temperature and even the average temperature in the wall could be vastly different. The two temperatures are related through an equation

of the form where Q is the heat transferred through the wall, A is the area through which the heat is transferred, Tfs is the free stream or bulk temperature, Tw is the wall temperature at the fluid-wall boundary, and h is the heat transfer coefficient. The science of heat transfer enables "h", the heat transfer coefficient, to be estimated from the fluid properties and fluid dynamics. Once the value of h is estimated, the interfacial temperature can be estimated (at least in principle)..

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Q2 Explain Buckingham Pi theorem.

Ans Buckingham theorem states that the total number of these relevant dimensional

parameters (n) can be grouped into n-m independent dimensionless groups. The

number m is usually equal to the minimum of independent dimensions required

to specify the dimensions of all relevant parameters.

Primary dimensions M(mass), L(length), t(time), and T(temperature).

Example: to describe the dimension of density r, we need M and L

[r]=[M/L3], [Dp]=[F/A]=[ma/A]=[ML/t2/L2]=[M/(Lt2)]

Similarly, [m]=[M/(Lt)], [V]=[L/t], [L]=[L], D=[L]

Therefore, there are a total of three (3) primary dimensions involved: M, L, and t.

We should be able to reduce the total number of the dimensional parameters to

(6-3)=3.

Now, we need to select a set of dimensional parameters that collectively they

includes all the primary dimensions. We will select three since we have three

primary dimensions involved in the problem.

We will select r, V and D for this example

Set up dimensionless P groups by combining the parameters selected previously

with the other parameters (such as Dp, m and L in the present example), one at a

time. Identify a total of n-m dimensionless P groups. You have to solve the

dimensional equations to make sure all P groups are dimensionless.The first

group: P1=raVbDcDp, a, b & c exponents are needed to non-dimensionalize the

group. In order to be dimensionless:

Heat Transfer 13


Q 3 Write Correlations for the heat transfer coefficient.

Ans Each flow geometry requires different correlations be used to obtain heat transfer coefficients. Initially, we will look at correlations for fluids flowing in conduits.

Most correlations will take the "Nusselt form":

The correlations that follow are limited to conduit flow without phase change. Different geometries, boiling, and condensation will be covered in later lectures. Frictional heating (viscous dissipation) is not included in these correlations. This should not be a problem, since this phenomena is typically neglected except for highly viscous flows or gases at high mach numbers.

Unless otherwise specified, fluid properties should be evaluated at the "bulk average" temperature -- the arithmetic mean of the inlet and outlet temperatures:

0 0 0

3 2

1 2

[ ] [ ] [ ] [ ]

So that a 1 0, -3a b c-1 0 & b+2 0

Solved a -1, b -2, c 0.

pTherefore, the first group is

V

a b cM L ML M L tL t Lt

2 3

1 2 2 3 22

Use similar strategy, we can find the other two groups:

,

The functional relationship can be written as

p( , ) or ( , )

V

Therefore, the pressure drop in the pipe is a functi

L

VD D

Lf f

VD D

on of only two parameters:

the Reynolds number and the ratio between its length and diameter.

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Q4 Explain coorelation for turbulent flow. Ans

The historic equation for use in turbulent conduit flow is the Dittus-Boelter Correlation (MSH Eq. 12.32):

The exponent on the Prandtl number depends on the service -- 0.4 is used for heating and 0.3 for cooling. Different values are needed because of the variation of viscosity with temperature.

Heating and cooling effect the velocity profile of a flowing fluid differently because of the temperature dependence of viscosity. Heating usually makes the fluid near the wall less viscous, so the flow profile becomes more "plug-like." Cooling has the opposite effect, increasing the viscosity near the wall and impeding heat transfer. The effect is most pronounced for viscous flows with large wall -- bulk temperature differences.

Instead of using different exponents for heating and cooling, a direct correction for viscosity can be used. This takes the form of the ratio of the viscosity at the bulk fluid temperature to the viscosity at the wall temperature. The ratio is then raised to the 0.14 power.

When this is added, the result is the Seider-Tate Correlation (MSH Eq. 12.33), the correlation recommended for use in this class:

Seider-Tate applies to "normal" fluids in turbulent flow in long, straight pipes, so:

Heat Transfer 15


Multiplicative correction factors are available to adjust for the entrance/exit consequences of short tubes:

and for pipe curvature

If the conduit does not have a circular cross-section, the inside diameter should everywhere be replaced by the equivalent diameter

Q5 Write heat transfer correlations for free convection. Ans

In many cases it's convenient to have simple equations for estimation of heat transfer coefficients. Below is a collection of recommended correlations for single phase convective flow in different geometries as well as a few equations for heat transfer processes with change of phase. Note that all equations are for mean Nusselt numbers and mean heat transfer coefficients. 1 Forced Convection Flow Inside a Circular Tube

All properties at fluid bulk mean temperature (arithmetic mean of inlet and outlet temperature). Nusselt numbers Nu0 from sections 1-1 to 1-3 have to be corrected for

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temperature-dependent fluid properties according to section 1-4. 1-1 Thermally developing, hydrodynamically developed laminar flow (Re < 2300) Constant wall temperature:

(Hausen) Constant wall heat flux:

(Shah) 1-2 Simultaneously developing laminar flow (Re < 2300) Constant wall temperature:

(Stephan) Constant wall heat flux:

which is valid over the range 0.7 < Pr < 7 or if Re Pr D/L < 33 also for Pr > 7.

1-3 Fully developed turbulent and transition flow (Re > 2300) Constant wall heat flux:

(Petukhov, Gnielinski)

where Constant wall temperature: For fluids with Pr > 0.7 correlation for constant wall heat flux can be used with negligible error.

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1-4 Effects of property variation with temperature Liquids, laminar and turbulent flow:

Subscript w: at wall temperature, without subscript: at mean fluid temperature Gases, laminar flow:

Nu = Nu0 Gases, turbulent flow:

Temperatures in Kelvin

2 Forced Convection Flow Inside Concentric Annular Ducts, Turbulent (Re > 2300)

Dh = Do - Di

All properties at fluid bulk mean temperature (arithmetic mean of inlet and outlet temperature). Heat transfer at the inner wall, outer wall insulated:

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(Petukhov and Roizen) Heat transfer at the outer wall, inner wall insulated:

(Petukhov and Roizen) Heat transfer at both walls, same wall temperatures:

(Stephan) 3 Forced Convection Flow Inside Non-Circular Ducts, Turbulent (Re > 2300) Equations for circular tube with hydraulic diameter

4 Forced Convection Flow Across Single Circular Cylinders and Tube Bundles

D = cylinder diameter, um = free-stream velocity, all properties at fluid bulk mean temperature. Correction for temperature dependent fluid properties see section 4-4.

4-1 Smooth circular cylinder

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(Gnielinski)

where

Valid over the ranges 10 < Rel < 107 and 0.6 < Pr < 1000

4-2 Tube bundle

Transverse pitch ratio

Longitudinal pitch ratio

Void ratio for b > 1

for b < 1 Nu0,bundle = fANul,0 (Gnielinski)

Nul,0 according to section 4-1 with instead of Rel. Arrangement factor fA depends on tube bundle arrangement.

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In-line arrangement:

Staggered arrangement:

4-3 Finned tube bundle

Heat Transfer 21


In-line tube bundle arrangement:

(Paikert) Staggered tube bundle arrangement:

(Paikert) 4-4 Effects of property variation with temperature Liquids:

Subscript w: at wall temperature, without subscript: at mean fluid temperature. Gases:

Temperatures in Kelvin.

5 Forced Convection Flow over a Flat Plate

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All properties at mean film temperature Laminar boundary layer, constant wall temperature:

(Pohlhausen) valid for ReL < 2105, 0.6 < Pr < 10

Turbulent boundary layer along the whole plate, constant wall temperature:

(Petukhov)

Boundary layer with laminar-turbulent transition:

(Gnielinski)

Heat Transfer 23


Unit 3 Q1 Boiling phenomenon in fluids. Ans Boiling is the formation of vapor bubbles at the heating surface. These bubbles

form at nucleation sites whose number and location depend upon the surface

roughness or cavities, fluid properties, and operating conditions. The boiling heat

transfer coefficient is very sensitive to the temperature difference between the

surface and the liquid. Inaddition, the heat transfer coefficient is affected by the

local vapor-liquid mixture ratios and velocities, which are a function of the

vaporizer design and operating con-ditions. The complex interaction of all these

variables makes the accurate prediction of a boiling coefficient virtually

impossible, but in large commercial vaporizers the two-phase flow heat transfer

becomes controlling and reducesthe number of variables.

Nucleate boiling is a type of boiling that takes place when the surface

temperature is hotter than the saturated fluid temperature by a certain amount

but where the heat flux is below the critical heat flux For water, as shown in the

graph below, nucleate boiling occurs when the surface temperature is higher than

the saturation temp (TS) by between 4 C (7.2 F) to 30 C (54 F). The critical heat

flux is the peak on the curve between nucleate boiling and transition boiling.

When a liquid is in contact with a surface maintained at a temperature above the saturation temperature of the liquid, boiling will eventually occur at that liquid-solid interface. Conventionally, based on the relative bulk motion of the body of a liquid to the heating surface, the boiling is divided into two categories; pool boiling and convective boiling.

Pool boiling is the process in which the heating surface is submerged in a large body of stagnant liquid. The relative motion of the vapor produced and the surrounding liquid near the heating surface is due primarily to the buoyancy effect of the vapor. Nevertheless, the body of the liquid as a whole is essentially at rest. Though the study on the boiling process can be traced back to as early as the eighteen century (the observation of the vapor film in the boiling of liquid over the heating surface by Leiden in 1756), the extensive study on the effect of the very large difference in the temperature of the heating surface and the liquid, , was first done by Nukiyama (1934). However, it was the experiment by Farber

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and Scorah (1948) that gave the complete picture of the heat transfer rate in the pool boiling process as a function of . Applying the Newton's law of cooling,

, the heat transfer coefficient, h, was used to characterize the pool boiling process over a range of by Farber and Scorah as illustrated by the boiling curve in given fig.

Farber and Scorah conducted their experiments by heating the water at various pressures with a heated cylindrical wire submerged horizontally under the water level. From the results, they divided the boiling curve into 6 regions based on the observable patterns of vapor production. Region I, is so small that the vapor is produced by the evaporation of the liquid into gas nuclei on the exposed surface of the liquid. Region II, is large enough that additional small bubbles are produced along the heating surface but later condense in the region above the superheated liquid. Region III, is enough to sustain "nucleate boiling", with the creation of the bubbles such that they depart and rise through the liquid regardless of the condensation rate. Region IV, an unstable film of vapor was formed over the heating surface, and oscillates due to the variable presence of the film. In this region, the heat transfer rate decreases due to the increased presence of the vapor film. Region V, the film becomes stable and the heat transfer rate reaches a minimum point. In Region VI, the is very large, and "film boiling" is stable such that the radiation through the film becomes significant and thus increases the heat transfer rate with the increasing .

This behavior as described above occurred when the temperature of the wire was

the controlled parameter, . If the power is the controlled variable then the increase in the power (or heat flux, q") in Region III results in a jump in the wire surface temperature to a point in Region VI, This point of transition is known as the critical heat flux and occurs due to hydrodynamic fluid instabilities as discussed later. This results in the stable vapor film being formed, and the wire surface temperature increases as the heat transfer resistance increases for a fixed input power. If the power is now decreased, the vapor film remains stable in Region VI and the decreases to the minimum point for film boiling within Region V. At this point the vapor film becomes unstable and it collapses, with "nucleate boiling" becoming the mode of energy transfer. Thus, one passes quickly through Region IV and III to a lower wire surface temperature. This "hysteresis" behavior is always seen when the power (or heat flux) is the controlled parameter.

Heat Transfer 25


Q2 Explain the whole process of pool boiling.

Ans

Boiling is associated with transformation of liquid to vapor by heating The formation of bubbles stir the fluid and breaks the boundary layers thereby increasing the heat transfer coefficient It differs from vaporization in the sense that it is associated with the formation

of bubbles The bubbles are normally formed on the surface scratches. The bubbles do not appear till the wall is heated in excess of the saturation temperature, called wall superheat. The excess temperature required for the onset of formation of bubbles

decreases with increase in the size of surface scratches One of the main interest is the prediction of heat transfer coefficient The results are summarized in what is known as Boiling Curve. Free convection region TSat< 5 oC (single phase) Vapor formed at the free surface Onset of nucleation Tsat~ 5 oC Bubbles nucleate, grow and detach from the surface

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Increase of wall superheat leads to more vigorous nucleation and rapid increase

in heat transfer As the superheat is increased, the vapor formation become vigorous, it blankets

the surface and the heat transfer decreases. This turn around point is called the Critical Heat Flux or Boiling crisis As the superheat is increased, more blanketing causes the heat transfer to drop, till the entire heated surface is blanketed Now the radiation heat transfer also starts playing a role and eventually, the heat transfer starts increasing due to increase convection and radiation heat transfer The second turnaround point is called Leidenfrostpoint or rewetting point. The heat transfer beyond this point is called film boiling

Q 3 Explain critical heat flux in boiling.

Ans Critical heat flux (CHF) in pool boiling is an interesting phenomenon. As this indicates, if one controls the input heat flux, there comes a point where as the heat flux is increased further the heater surface temperature undergoes a drastic increase. This increase originally was not well understood. Kutateladze (1951) offered the analogy that this large abrupt temperature increase was caused by a change in the surface geometry of the two phases. In fact, Kutateladze first empirically correlated this phenomenon as analogous to a gas blowing up through a heated porous plate cooled by water above it. At a certain gas

volumetric flowrate (or superficial velocity, ) the liquid ceases to contact the heated surface and the gas forms a continuous barrier. Kutateladze concluded

Heat Transfer 27


this by measuring the increasing electrical resistance between the plate and water as a function of the increase gas flowrate. Thus, pool boiling CHF may be thought of as the point where nucleate boiling goes through a flow regime transition to film boiling with a continuous vapor film separating the heater and the liquid. More generally, one may say CHF is the condition where the vapor generated by nucleate boiling becomes so large that it prevents the liquid from reaching and rewetting the surface.

Consider this final physical picture of the critical heat flux, , where the liquid is prevented from reaching the heater surface by the flow of vapor generated by boiling,

where is that critical superficial velocity preventing the liquid flow. A simple

force balance on the liquid as droplets, , is given by

where is assumed to determined by the characteristic Taylor wavelength (Equ 5.11), which results in a velocity of

Combining these relations one obtains a general expression for CHF in pool boiling

where the constant, Co, is found to be in the range of 0.12 to 0.18; e.g., Zuber

(1958) theoretically estimated , Kutateladze (1951) correlated data for Co = 0.13, and Lienhard (1976) correlated data for Co = 0.15.

All of these previous discussions focused on the case where the liquid pool was at its saturation temperature. If the stagnant pool is maintained at a temperature below saturation, subcooled, the vapor bubbles can condense before they get very far from the heater surface. Thus, the heater power can go into directly heating the liquid and actual vapor superficial velocity is decreased; thus increasing the

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allowable heat flux before CHF occurs. Ivey and Morris (1962) correlated this

subcooling effect as a multiplicative correlation to ,

where is the degree of subcooling in the liquid.

The final point to emphasize is the location of the CHF point on the pool boiling curve of . Critical heat flux appears as a horizontal line of the pool boiling curve and its intersection with the nucleate boiling curve indicates the temperature at which CHF occurs.

Q 4 What do you mean by condensation? Also explain its types.

Ans Condensation implies transformation of vapor back to liquid. Vapour may condense onto a cooled surface in two distinct modes known as filmwise and dropwise. For the same temperature difference between the vapour and the surface, dropwise condensation is several more times effective than filmwise. However it involves special surface finishes or treatment in order to maintain dropwise condensation and for this reason, though desirable, it seldom occurs in real plant operation.

Heat Transfer 29


The process of dropwise condensation is enhanced by the special water cooled condenser surface finish that prevents wetting of the surface. Condensation then occurs in droplets which grow and fall under gravity. These falling droplets wipe the surface clean ready for more droplets to form. This continuous cleaning puts the water cooled surface in direct contact with the vapour.

The duplicate filmwise condenser is not specially treated and allows condensation to form as a film. This effectively grows and runs down the condenser gaining thickness as it falls. The film effectively acts as a resistance to heat transfer, as heat must be conducted through this film to the internal cooling water.

Thermocouples are fitted to the surfaces of both condensers allowing the direct comparison of surface temperatures in both filmwise and dropwise condensation.

There are basically two mechanisms for condensation 1.Drop Condensation 2. filmwise condensation

Film Condensation:-

Entire surface is covered by the condensate, which flows continuously from the surface and provides a resistance to heat transfer between the vapor and the surface.Thermal resistance is reduced through use of short vertical surfaces and horizontal cylinders.

Dropwise Condensation

Surface is covered by drops ranging from a few micrometers to agglomerations visible to the naked eye.Thermal resistance is greatly reduced due to absence of a continuous film.Surface coatings may be applied to inhibit wetting and stimulate dropwise condensation.

Q5 Explain forced convection boiling.

Ans FORCED CONVECTION BOILING:-

Boiling is most often understood as a phase transition from a liquid to a vapor

state involving the appearance of vapor bubbles on a hot surface. In this respect,

forced convection boiling and pool boiling have much in common. However,

forced convection imparts a number of specific features to the conditions of

bubble production and breakaway into the bulk of the liquid. The structures of

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vapor-liquid mixtures resulting from boiling and mixing of liquid and vapor

phases also differ appreciably from each other.

Forced Convection Boiling Modes

Single phase convection

Subcooled Flow Boiling - Nucleation begins as Twall becomes Tsat

Saturated Film Boiling The thickness of bubble region increases and core of the liquid reaches saturation and bubbly flow begins.As the volume fraction of vapor increases N dividual bubble coalesces to form slugs of vapor. The liquid then forms a film which move along the inner surface in annular flow .

Mist flow till all liquid is converted into vapor.

The vapor is then heated by forced convection

Heat Transfer 31


Unit 4

Q 1 Explain Radiation Heat Transfer.

Ans Radiation heat transfer is concerned with the exchange of thermal radiation

energy between two or more bodies. Thermal radiation is defined as

electromagnetic radiation in the wavelength range of 0.1 to 100 microns (which

encompasses the visible light regime), and arises as a result of a temperature

difference between 2 bodies.

Radiation heat transfer must account for both incoming and outgoing thermal radiation.

Incoming radiation can be either absorbed, reflected, or transmitted. This decomposition can be expressed by the relative fractions,

Since most solid bodies are opaque to thermal radiation, we can ignore the transmission

component and write,

To account for a body's outgoing radiation (or its emissive power, defined as the heat flux

per unit time), one makes a comparison to a perfect body who emits as much thermal

radiation as possible. Such an object is known as a blackbody, and the ratio of the actual

emissive power E to the emissive power of a blackbody is defined as the surface

emissivity ,

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No medium need exist between the two bodies for heat transfer to take place (as is needed by conduction and convection). Rather, the intermediaries are photons which travel at the speed of light.

The heat transferred into or out of an object by thermal radiation is a function of several components. These include its surface reflectivity, emissivity, surface area, temperature, and geometric orientation with respect to other thermally participating objects. In turn, an object's surface reflectivity and emissivity is a function of its surface conditions (roughness, finish, etc.) and composition.

Absorption and Emissivity

By stating that a body's surface emissivity is equal to its absorption fraction, binds

incoming and outgoing radiation into a useful dependent relationship,

Heat Transfer 33


Q .2 Explain the concept of Basic concepts of radiation from a surface: black body Radiation

Ans Heat transfer through radiation takes place in form of electromagnetic waves mainly in the infrared region. Radiation emitted by a body is a consequence of thermal agitation of its composing molecules. Radiation heat transfer can be described by a reference to the so-called 'black body'.

The Black Body

A black body is defined as a body that absorbs all radiation that falls on its surface. Actual black bodies don't exist in nature - though its characteristics are approximated by a hole in a box filled with highly absorptive material. The emission spectrum of such a black body was first fully described by Max Planck.

A black body is a hypothetic body that completely absorbs all wavelengths of thermal radiation incident on it. Such bodies do not reflect light, and therefore appear black if their temperatures are low enough so as not to be self-luminous. All blackbodies heated to a given temperature emit thermal radiation.

The radiation energy per unit time from a blackbody is proportional to the fourth power of the absolute zero and can be expressed with Stefan-Boltzmann Law as

q = T4 A (1)

where

q = heat transfer per unit time (W)

= 5.6703 10-8 (W/m2K4) - The Stefan-Boltzmann Constant

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T = absolute temperature Kelvin (K)

A = area of the emitting body (m2)

The Stefan-Boltzmann Constant in Imperial Units

= 5.6703 10-8 (W/m2K4)\

Q 02 Explain the different laws of radiation.

Ans The average or bulk properties of electromagnetic radiation interacting with matter are systematized in a simple set of rules called radiation laws. These laws apply when the radiating body is what physicists call a blackbody radiator. Generally, blackbody conditions apply when the radiator has very weak interaction with the surrounding environment and can be considered to be in a state of equilibrium. Although stars do not satisfy perfectly the conditions to be blackbody radiators, they do to a sufficiently good approximation that it is useful to view stars as approximate blackbody radiators.

Planck Radiation Law

The primary law governing blackbody radiation is the Planck Radiation Law,

which governs the intensity of radiation emitted by unit surface area into a fixed

direction (solid angle) from the blackbody as a function of wavelength for a fixed

temperature. The Planck Law can be expressed through the following equation.

The behavior is illustrated in the figure shown above. The Planck Law gives a distribution that peaks at a certain wavelength, the peak shifts to shorter wavelengths for higher temperatures, and the area under the curve grows rapidly with increasing temperature.

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The Wien and Stefan-Boltzmann Laws

The behavior of blackbody radiation is described by the Planck Law, but we can

derive from the Planck Law two other radiation laws that are very useful. The

Wien Displacement Law, and the Stefan-Boltzmann Law are illustrated in the

following equations.

The Wien Law gives the wavelength of the peak of the radiation distribution,

while the Stefan-Boltzmann Law gives the total energy being emitted at all

wavelengths by the blackbody (which is the area under the Planck Law curve).

Thus, the Wien Law explains the shift of the peak to shorter wavelengths as the

temperature increases, while the Stefan-Boltzmann Law explains the growth in

the height of the curve as the temperature increases. Notice that this growth is

very abrupt, since it varies as the fourth power of the temperature.

The following figure illustrates the Wien law in action for three different stars of quite different surface temperature. The strong shift of the spectrum to shorter wavelengths with increasing temperatures is apparent in this illustration.

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For convenience in plotting these distributions have been normalized to unity at the respective peaks; by the Stefan-Boltzmann Law, the area under the peak for the hot star Spica is in reality 2094 times the area under the peak for the cool star Antares.

Q 3 Explain the Radiation heat exchange between surfaces ,the view factor. Ans

In general, for any two objects in space, a given object 1 radiates to object 2, and to other places as well, as shown in

Radiation between two bodies

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Radiation between two arbitrary surfaces

We want a general expression for energy interchange between two surfaces at different temperatures. This is given by the radiation shape factor or view factor,

. For the situation in given fig.

= fraction of energy leaving 1 which reaches 2

= fraction of energy leaving 2 which reaches 1

, are functions of geometry only

For body 1, we know that is the emissive power of a black body, so the energy

leaving body 1 is . The energy leaving body 1 and arriving (and being

absorbed) at body 2 is . The energy leaving body 2 and being

absorbed at body 1 is . The net energy interchange from body 1 to body 2 is

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(4)

Suppose both surfaces are at the same temperature so there is no net heat exchange. If

so,

but also . Thus

Equation (4) is the shape factor reciprocity relation. The net heat exchange between the two surfaces is

Example: Concentric cylinders or concentric spheres

Radiation heat transfer for concentric cylinders or spheres

The net heat transfer from surface 1 to surface 2 is

We know that , i.e., that all of the energy emitted by 1 gets to 2. Thus

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This can be used to find the net heat transfer from 2 to 1.

View factors for other configurations can be found analytically or numerically. Shape factors are given in textbooks and reports (they are tabulated somewhat like Laplace transforms), and examples of the analytical forms and numerical values of shape factors for some basic engineering configurations

Q 4 Explain the heat exchange between gray bodies.

Ans

Gray Bodies and Emissivity Coefficients

For objects other than ideal blackbodies ('gray bodies') the Stefan-Boltzmann Law can be expressed as

q = T4 A (2)

where

= emissivity of the object (one for a black body)

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For the gray body the incident radiation (also called irradiation) is partlyreflected, absorbed or transmitted.

The emissivity coefficient lies in the range 0 > < 1 depending on the type of material and the temperature of the surface. The emissivity of some common materials

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Unit 5

Q .1 What do u mean by heat exchanger. Explain one of its type.

Ans Heat exchangers are devices used to transfer heat energy from one fluid to

another. Typical heat exchangers experienced by us in our daily lives include

condensers and evaporators used in air conditioning units and refrigerators.

Boilers and condensers in thermal power plants are examples of large industrial

heat exchangers. There are heat exchangers in our automobiles in the form of

radiator sand oil coolers. Heat exchangers are also abundant in chemical and

process industries. There is a wide variety of heat exchangers for diverse kinds of

uses, hence the construction also would differ widely. However, in spite of the

variety, most heat exchangers can be classified into some common types based on

some fundamental design concepts. We will consider only the more common

types here for discussing some analysis and design methodologies.

Basic Heat Exchanger Flow Arrangements Two basic flow arrangements are as shown in Figure. Parallel and counter flow provide alternative arrangements for certain specialized applications. In parallel flow both the hot and cold streams enter the heat exchanger at the same end and travel to the opposite end in parallel streams. Energy is transferred along the length from the hot to the cold fluid so the outlet temperatures asymptotically approach each other. In a counter flow arrangement, the two streams enter at opposite ends of the heat exchanger and flow in parallel but opposite directions.

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Temperatures within the two streams tend to approach one another in a nearly linearly fashion resulting in a much more uniform heating pattern. Shown below the heat exchangers are representations of the axial temperature profiles for each. Parallel flow results in rapid initial rates of heat exchange near the entrance, but heat transfer rates rapidly decrease as the temperatures of the two streams approach one another. This leads to higher energy loss during heat exchange. Counter flow provides for relatively uniform temperature differences and, consequently, lead toward relatively uniform heat rates throughout the length of the unit.

Q 2 Explain construction of shell and tube type heat exchanger.

Ans Shell and tube heat exchangers in their various construction modifications are

probably the most widespread and commonly used basic heat exchanger

configuration in the process industries. The reasons for this general acceptance

are several. The shell and tube heat exchanger provides a comparatively large

ratio of heat transfer area to volume and weight. It provides this surface in a form

which is relatively easy to construct in a wide range of sizes and which is

mechanically rugged enough to withstand normal shop fabrication stresses,

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shipping and field erection stresses, and normal operating conditions. There are

many modifications of the basic configuration, which can be used to solve special

problems. The shell and tube exchanger can be reasonably easily cleaned, and

those components most subject to failure - gaskets and tubes can be easily

replaced.

There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U-tubes.

In nuclear power plants called , large heat exchangers calledsteam generator are two-phase, shell-and-tube heat exchangers which typically have U-tubes. They are used to boil water recycled from a surface condenser into steam to drive a turbine to produce power. Most shell-and-tube heat exchangers are either 1, 2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the tubes passes through the fluid in the shell. In a single pass heat exchanger, the fluid goes in one end of each tube and out the other.

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Surface condensers in power plants are often 1-pass straight-tube heat exchangers (see surface condenser for diagram). Two and four pass designs are common because the fluid can enter and exit on the same side. This makes construction much simpler.

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There are often baffles directing flow through the shell side so the fluid does not take a short cut through the shell side leaving ineffective low flow volumes. These are generally attached to the tube bundle rather than the shell in order that the bundle is still removable for maintenance.

Counter current heat exchangers are most efficient because they allow the highest lmtd between the hot and cold streams. Many companies however do not use single pass heat exchangers because they can break easily in addition to being more expensive to build. Often multiple heat exchangers can be used to simulate the counter current flow of a single large exchanger.

Q 3 Explain construction different types of evaporator..

Ans The evaporator is kind of heat transfer apparatuses where the heat transfer is

done by forced convection or natural convection. And its an important

component of refrigeration system and air conditioning system.

Evaporation process is rejection of water (or other liquids) by concentrating the

solution. The required time for this process can by shortened by increasing the

surface area, the solution is exposed to it, or by exposing the solution to heating

to a higher temperature.

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Types of evaporators

1.Natural circulation type a) Vertical short tube or Calandria evaporator b) Long tube vertical (LTV) rising film typec) Long tube vertical (LTV) falling film type 2 .Fo r ce d C ir cul a t i o n ty pe

Calandria Evaporator

It has a vertical tube bundle consisted of short tubes (usually less than six feet)integral with the shell. This is called a calandria. There is a vapour space abovethe tube bundle. The calandria is of annular construction i.e. there is an opencylindrical region at the center called the downcomer .. Feed is supplied througha nozzle above the upper tube sheet and steam to the shell or the steam chest of the calandria. Inonce through operation(useful for heat sensitive material), thefeed liquor passes through the tubes only once,releases the vapour and leavesthe unit as thick liquor i.e. all the evaporation isaccomplished in a single pass.T h e r a t io o f v apo ur iz at io n to f e ed i s l i mi te d i n s i ng l e p a ss u n i t s ; t h u s t he se evaporators are well adapted tomultiple effect operation. Inrecirculation typeof operation, a pool of liquid is held within the equipment. Incoming feed mixeswith the liquid from the pool and the mixture passes through the

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tubes. Thesolution is heated and partly vapourized in the tubes. The vapour-liquid mixtureflows upthrough the tubes due to the prevailing density gradient. Vapourliquidd i se nga ge me nt o ccur s a b ove t h e u p per t ube sh ee t .U nev a porate d l i q ui dd i sch arge d f ro m t he t u be s r e t u r n s t o t h e p o o l d o w n t h r o u g h t h e c e n t r a l downcomer. Thus acontinuous natural recirculation of the solution occurs. Thickproduct liquor iswithdrawn from the bottom through a thick liquor outlet pipe. Thesteam condensate leavesthrough a drain nozzle connected to asteam trap. Ableed or vent line is provided in the shell for the release of the noncondensablein steam. The vapour leaves through an outlet at the top of theevaporator bodythrough anentrainment separator or mist eliminator which arrests the liquiddroplets in the vapour. They are used to concentratea variety of solutions acommon example is concentration of the sugarsolution. However, these are notsuitable for solutions in which precipitation orsalting out a solid may occur. Thenatural circulation velocity in theevaporator is not sufficient to keep the solidparticles in suspension. Theproblem may be overcome by installing an agitator in the downcomer pipe to increase the circulation rate.

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LTV RISING-FILM EVAPORATOR

A long tube vertical rising film evaporator consists of a long vertical tube bundlefitted within a shell. The shell is projected into a larger diameter chamber or vapour head at the top. liquor is fed into the bottom liquor chamber and entersthe tube bundle at the bottom. It is heated with condensing steam or any other suitable heat-transfer medium flowing outside the tubes and rises upwards due todensity gradient. For cold feed, the lower portion of the tubes is used to preheatthe liquor to its boiling point. Vaporization then begins at that height within thet u be s wher e t he l i qu or tem p erat ur e e xcee ds t h e bo i l in g t em per at ure a t t he prevailing pressure. As the liquor climbs up the inside of the tubes, the liquor undergoes vigorous boiling and additional vapor is generated and the velocity of the liquid-vapor mixture increases. The outlet mixture impinges upon a deflector,mounted above the top tubesheet of the heat exchanger, where gross, initial separation of the liquid from the vapor occurs.

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Long Tube Vertical Falling Film Evaporator

It consists of a long vertical tube bundle heated by condensing steam or anyother hot liquid on the shell side. Liquor is fed into the top liquor chamber of theheat exchanger where it is distributed to each tube and flows down the inner walls of the tubes as thin film in once through mode. The liquor accelerates invelocity as it descends inside the tubes because of the gravity and drag of thevapor generated by boiling. Liquid is separated from the vapor in the bottomliquor chamber of the heat exchanger and with a skirt-type baffle in the vapor body. Concentrated liquor is discharged from the bottom liquor chamber andcone bottom of the vapor body. The vapor body can be provided either as aseparate component (fig 3) or as an integral component of the heat exchanger,similar to that shown in fig 2, except the heat exchanger would be located abovethe va por bo dy in t he f a l l ing -f i l m co n f i gur at ion . Th e v apo ur co l l e c te d i n th e vapour body goes to an entrainment separator installed in the upper portion of the vapor body to reduce liquid entrained with the vapor to a minimum.The falling-film evaporator is particularly useful in applications where the drivingforce in temperature difference between the heat-transfer medium and the liquidis small (T's of less than 150F). The retention time for liquor in this evaporator is less than that for a rising-film evaporator. The combination of short liquidretention time and the ability to operate at a low Delta-T makes the falling-filmevaporator ideal for concentrating the most heat-sensitive materials. Highh ea t - t ra n sfer coe f f i c i ent s are a t ta i ne d i n fa l l i ng - f i l m eva por ator s wh en a continuous film of liquid, preferably at its boiling point, flows down the inside tubewall with a vapor core in the tube center. For some applications, however, it isnecessary to supplement an insufficient quantity of feed liquor with product liquor pumped to the top liquor chamber to avoid vapor blanketing of the inside tube surface

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