Chapter 1 (5th Semester)

8/2/2019 Chapter 1 (5th Semester)

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Dr. Waqar A. Khan

Associate Professor Department of Engineering Sciences

National University of Sciences and TechnologyPN Engineering College, PNS Jauhar, Karachi

Heat and Mass Transfer I

(ME-312)



Heat Transfer By J. P. Holman

Momentum, Heat, and Mass Transfer ByC. O. Bennet & J. E. Myers

Principles of Heat Transfer By F. Krieth &W. Z. Black

Heat and Mass Transfer By F. P. Incroperaand D. P. DeWitt

Heat Transfer: A Practical Approach By

Yunus A Cengel

Books



Instructor: Dr. Waqar Ahmed Khan Subject: Heat and Mass Transfer I

Class/Term: 5th (Spring 2009) Code: ME-312

Text Book: Heat Transfer-A Practical ApproachAuthor: Yunus A. Cengel

Steady State HeatConduction .

Introduction, Thermodynamics and heat transfer. Modes of Heat transfer and applications.Brief description of Conduction, Convection and Radiation.Thermal conductivity –Mechanism of conduction in Solids,Liquids and Gases.

General Equation for conduction, Fourier ,Laplace and Poisson Equations.Heat Transfer and temperature distribution in Plane and Composite walls.Heat Transfer and temperature distribution in cylinders and spheresCritical radius of insulation.Heat Transfer and temperature in Planes, cylinders and spheres with internal heat generation.

CengelChapter 1

CengelChapter 2 &

3

6

12

ONE HOUR TEST NO 1

Forced Convectionbased on Boundary

Layer Theory.

Forced and Free Convection. Revision of concepts in fluid mechanics such as continuity, momentum, andenergy equations.Evaluation of heat transfer co-efficient-analytical and experimental.Dimensional analysis-application to forced and free convection.Thermal and momentum boundary layer.

Importance of Prandtl Number .Laminar flow over plates-derivation of expression for heat transfer co-efficient using cubic velocity and temperature profiles.

Turbulent flow over flat plates- introductory concepts of turbulence.Universal velocity profile.

CengelChapter 6

CengelChapter 7

6

6

Forced Convectionin Pipes

Empirical Relations for Pipe and Tube flow.Concept of Bulk Temperature. Concept of Hydraulic Diameter.Laminar and Turbulent flow in side tubes and empirical correlation.Use of Graetz Number.

CengelChapter 8

4

ONE HOUR TEST NO 2

Natural Convection Free Convection Principles.Free Convection on a vertical Flat Plate.Empirical Relations for Free Convection.

Free Convection from Vertical Planes and Cylinders for Isothermal and constant heat flux conditions.Free Convection from Horizontal Cylinders.Free Convection from Horizontal Planes for Isothermal andconstant heat flux conditions.

CengelChapter 9

6

Radiant Heat

Transfer Principlesand Applications

Electro-magnetic Radiation- fundamental concepts.

Interaction with surfaces. Blackbody-ideal absorber and ideal emitter. Emission of a black body.Radiometric curves- maximum emission.Greenhouse effect- visibility and colours.Emission in a specified wave band, and emission real body-gray body model. Kirchoffs Identity.Radiation Exchange in black bodies. Radiation intensities.Shape Factor and shape factor algebra.

Exchange between gray bodies. Radiosity and Irradiation. Network and numerical methods.

Cengel

Chapter 11Cengel

Chapter 12

10

10

ONE HOUR TEST NO 3



Assessment

Assignments (6) 10%

One-hour tests (3) 30%

Quizes (6) 10%

Final 50%



Exams

All exams will be closed books andclosed notes

Only one sheet of formulae will beallowed



PROBLEM-SOLVING TECHNIQUE

Step 1: Problem StatementIn your own words, briefly state the problem, the keyinformation given, and the quantities to be found.Step 2: Schematic

Draw a realistic sketch of the physical system involved,and list the relevant information on the figure.Step 3: AssumptionsState any appropriate assumptions made to simplify theproblem to make it possible to obtain a solution.Assume reasonable values for missing quantities thatare necessary.




Step 4: Physical LawsApply all the relevant basic physical laws and principles(such as the conservation of energy), and reduce themto their simplest form by utilizing the assumptions

made.

Step 5: PropertiesDetermine the unknown properties at known statesnecessary to solve the problem from property relationsor tables. List the properties separately, and indicatetheir source, if applicable.




Step 6: CalculationsSubstitute the known quantities into the simplifiedrelations and perform the calculations to determine theunknowns. Pay particular attention to the units and unit

cancellations, and remember that a dimensional quantitywithout a unit is meaningless.

Step 7: Reasoning, Verification, and DiscussionCheck to make sure that the results obtained arereasonable and intuitive, and verify the validity of thequestionable assumptions.



Practice Questions and Problems

Practice Questions for Quiz 1:

Q. 1.44C – 1.61C (Yunus Cengel)

Practice Problems for Test 1:

Q. 1.62, 1.63,1.65, 1.67-1.71, 1.73, 1.74,1.76-1.79, 1.82, 1.83, 1.91-1.93 (Yunus Cengel)

All solved examples of Chapter 1 (Cengel andInropera)



Introduction Thermodynamics and heat transfer Modes of heat transfer and applications Brief description of conduction, convection and radiation Thermal conductivity

Mechanism of conduction in solids, liquids and gases General equations for conduction Heat transfer and

temperature distribution in plane and composite walls Heat transfer and temperature distribution in cylinders and

spheres with insulation Critical thickness of insulation Heat transfer and temperature in planes cylinders and

spheres with internal heat generation

Ch. 1 Steady State Heat Conduction



Thermal Sciences



Thermodynamics

deals with the amount of heat transfer as a system

undergoes a process from one equilibrium state toanother, and makes no reference to how long theprocess will take.

But

in engineering, we are often interested in the rate of heattransfer, which is the topic of the science of heat transfer

Thermodynamics and Heat Transfer



A cold canned drink left in a room warms up and

A warm canned drink left in a refrigerator cools down.

This is accomplished by the transfer of energy from thewarm medium to the cold one.

The energy transfer is always from the higher

temperature medium to the lower temperature one,

The energy transfer stops when the two mediums reachthe same temperature.




Thermodynamics energy exists in various forms:

Mechanical , Chemical, Nuclear, Heat,…….

We are primarily interested in heat, which is the form of

energy that can be transferred from one system to

another as a result of temperature difference.

The science that deals with the determination of therates of such energy transfers is heat transfer.







Thermodynamics deals withequilibrium states and changes fromone equilibrium state to another.

Heat transfer deals with systems thatlack thermal equilibrium, and thus itis a nonequilibrium phenomenon.

The study of heat transfer can not bebased on the principles of thermodynamics alone.




The laws of thermodynamics laythe framework for the science of heat transfer.

The first law requires that the rateof energy transfer into a system beequal to the rate of increase of theenergy of that system.

The second law requires that heatbe transferred in the direction of decreasing temperature




The basic requirement for heat transfer is the presenceof a temperature difference.

There can be no net heat transfer between two mediums

that are at the same temperature.

Heat Transfer: Temperature DifferenceFluid Flow : Pressure Difference

Electric Current Flow: Voltage Difference





Application Areas of Heat Transfer





Conduction: T ransfer of energy from the more energeticparticles of a substance to the adjacent, less energeticones as a result of interactions between the particles.

Convection is the mode of heat transfer between a solidsurface and the adjacent liquid or gas that is in motion,and it involves the combined effects of conduction andfluid motion.

Radiation is the energy emitted by matter in the form of electromagnetic waves (or photons) as a resultof the changes in the electronic configurations of the

atoms or molecules.

Modes of Heat Transfer



Conduction can take place in solids,liquids, or gases.

In solids, it is due to the combination

of vibrations of the molecules in alattice and the energy transport byfree electrons.

In gases and liquids, conduction isdue to the collisions and diffusion of the molecules during their random

motion.

Conduction Heat Transfer



Conduction Heat Transfer

The rate of heat conduction througha medium depends on the geometry of the medium, thickness,

material of the medium, and temperature difference acrossmedium.

Fourier’s law of heat conduction









Thermal Conductivity














The Cost of Heat Loss through a Roof

Assumptions

1 Steady operatingconditions exist duringthe entire night since thesurface temperatures of the roof remain constant

at the specified values.

2 Constant properties canbe used for the roof.



The Cost of Heat Loss through a Roof



Example

Determination of thermal conductivity

To measure the effective thermal conductivity of anopaque honeycomb material for an aircraft wall, aspherical shell of inner radius 26 cm and outer radius 34cm was constructed and a 100 W electric bulb placed in

the center. At steady state, the temperatures of the inner and outer surfaces were measured to be 339 K and 311K respectively. What is the effective conductivity of thematerial?

Given: Spherical shell containing a 100 W heat source.Required: Thermal conductivity of shell material.Assumptions:

• Steady state • Spherical symmetry, T=T(r)



Example





Convection Heat Transfer

The rate of heat convection

depends on the surface area, nature of fluid, properties of fluid,

bulk fluid velocity, temp. difference.

( ) [ ]∞

∞

= −

= −

=

=

=

2

con c W

c o

W

Q h A T T where

Wattsh convective Heat transfer co efficient in

m C

A surface area

T plate surface Temperature

T free stream fluid Temperature

NEWTON’S LAW OF COOLING



Convection Heat Transfer Coefficients





Free Convection

Heated air rises,cools, then falls



Free Convection

What if coils were atthe bottom?



Natural Convection

Air above warmer ground rises Inversion Layer: Air near

ground is more dense

than air higher up.



Natural Convection

Very hot, low-densityair is buoyed upward,

carrying thermalenergy with it.





Measuring Convection Heat Transfer Coefficient

Assumptions1 Steady operating conditions2 Negligible radiation heat transfer Analysis

For steady operating conditions, the rate of heat lossfrom the wire will equal the rate of heat generation inthe wire as a result of resistance heating.



Radiation Heat Transfer

Thermal radiation is the form of radiation emitted by

bodies because of their temperature.

It differs from other forms of electromagnetic radiationsuch as x-rays, gamma rays, microwaves, radio waves,and television waves that are not related to temperature.

All bodies at a temperature above absolute zero emit

thermal radiation.

Unlike conduction and convection, the transfer of energy by radiation does not require the presence of an

intervening medium.



Radiation Heat Transfer

Maximum rate of radiation emitted from a surface at an

absolute temperature Ts (in K or R) is given by theStefan–Boltzmann law as

Radiation emitted by real surfaces is less than theradiation emitted by a blackbody at the sametemperature, and is expressed as





Ex: Radiation Effect on Thermal Comfort

Consider a person standing in

a room maintained at 22°C atall times. The inner surfaces of the walls, floors, and theceiling of the house are

observed to be at an averagetemperature of 10°C in winter and 25°C in summer.Determine the rate of radiation

heat transfer between thisperson and the surrounding surfaces if the exposedsurface area and the average outer surface temperatureof the person are 1.4 m2 and 30°C, respectively.




Assumptions

1 Steady operating conditions exist.2 Heat transfer by convection is not considered.3 The person is completely surrounded by the interior surfaces of the room.

4 The surrounding surfaces are at a uniform temperature

Properties The emissivity of a person is 0.95Analysis The net rates of radiation heat transfer from thebody to the surrounding walls, ceiling, and floor in winter and summer are








Q



Practice Questions

Q. A1200-W iron is left on the ironing board with its base exposed to the air.

About 90 percent of the heat generated in the iron is dissipated through its basewhose surface area is 150 cm2, and the remaining 10 percent through other surfaces. Assuming the heat transfer from the surface to be uniform, determine(a) the amount of heat the iron dissipates during a 2-hour period, in kWh,(b) the heat flux on the surface of the iron base, in W/m2, and

(c ) the total cost of the electrical energy consumed during this 2-hour period.Take the unit cost of electricity to be $0.07/kWh.

Iron

1200 W





P ti Q ti



Practice Questions

Q. A transistor with a height of 0.4 cm and a diameter of 0.6 cm is mounted ona circuit board. The transistor is cooled by air flowing over it with an average

heat transfer coefficient of 30 W/m2 · °C. If the air temperature is 55°C and thetransistor case temperature is not to exceed 70°C, determine the amount of power this transistor can dissipate safely. Disregard any heat transfer from thetransistor base.

Air,

55°C

Power

transistor

P ti Q ti



Practice Questions

2

2

2 4 2

/ 4(0.6 cm)(0.4 cm) (0.6 cm) / 4

1.037 cm 1.037 10 m

s A DL Dπ π

π π

−

= +

= +

= = ×

2 -4 2

( )

(30 W/m . C)(1.037 10 m )(70 55) C

s sQ hA T T ∞

= −

= ° × − °

= 0.047 W

&

Assumptions:

1 Steady operating conditions exist.2 Heat transfer by radiation is disregarded.3 The convection heat transfer coefficient is constant and uniform over thesurface.4 Heat transfer from the base of the transistor is negligible.

Then the rate of heat transfer from the power transistor at specified conditions is

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Chapter 1 (5th Semester)