8/2/2019 Chapter 1 (5th Semester) http://slidepdf.com/reader/full/chapter-1-5th-semester 1/59 Dr. Waqar A. Khan Associate Professor Department of Engineering Sciences National University of Sciences and Technology PN Engineering College, PNS Jauhar, Karachi Heat and Mass Transfer I (ME-312)
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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.
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.
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
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
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:
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.
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
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.
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. 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.
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.
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