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Lecturer:•Dr. Hala Mahmoud

rerequ s te ourses:•Thermodynamics & Fluid Mechanics

Ref. Text• Yunus A. Cengel, Heat Transfer – A Practical Approach”International. 3rd Ed. New York: McGraw‐Hill, 2003•Frank P. Incropera and D. P. Dewitt, “ Fundamental of  heat  and 

mass transfer” 7 th ED.  JOHN  WILEY  & SONS,  2011

Objectives•To understand the basic principles of heat transfer,•To be able to analyze the thermal applications and equipments(Temperature distribution & Heat rate).

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•   Introduction to heat transfer•   Steady ‐state one dimensional conduction heat transfer 

•   One dimensional conduction heat transfer  with heat 

•   Steady  conduction heat transfer in two‐dimensions 

•   Transient Heat Conduction 

•   Fundamentals of  Convection Heat Transfer •   Forced Convection relations. •   Free Convection relations•   Mixed convection heat transfer

 ns

 an

 ex en e

 sur ace

•   Heat transfer  with phase change•   Heat exchanger

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Grading

Grading will be based on the following distribution: Written examination (Final)  90  marksMid‐Term Examination  20  marks 

 Assignments/class  work  10  marksOral exam 30  marks

HINTS

*Data book

  will

 be

 given

 and

  will

 be

 used

 along

 the

 term

 and

 Final

 Exam,

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Basic  

Concepts 

of  

Heat  

Transfer  

In this chapter  we  will learn W at is  eat trans er How is heat transferred Relevance and importance

Thermodynamics:   Energy  can be transferred between a system and its surroundings.   A  system interacts  with its surroundings by  exchanging  work and heat

 

  Does not give information about:   Rates at which energy is transferred   Mechanisms through with energy is transferred 

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Example: 

Design 

of  

a 

container A  closed container filled  with hot coffee is in a room  whose air and 

 walls are at a fixed temperature. Identify  all heat transfer processes that contribute to cooling of  the coffee. Comment on features that 

.

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Conduction Of HeatThrough A Material

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THERMAL CONDUCTIVITY

The rate of  heat transfer through a unit thickness of  the 

Kinetic theory : thermal conductivity  of gases is proportional to the square root of  the absolute temperature and inversely  proportional to the square root of  the molar mass.

independent of  pressure in a  wide range

In liquids decreases  with increasing temperature and also 

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In solids depends upon lattice  vibrational  waves and the free 

flow electrons

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THERMAL 

DIFFUSIVITY

Thermal diffusivity  is a material property   which represents how fast heat diffuses through a material.

The ratio of  heat conducted through a material to the heat stored per unit  volume

PC 

k 

stored heat 

conducted heat 

     

 

    Thermal diffusivity 

Specific heatP

C 

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g 

or 

ow  P   arge

 erma

 us v y 

The larger the thermal diffusivity, the faster the propagation of  heat into the 

medium.  A  small  value of  thermal diffusivity  means that heat is mostly  absorbed by  the material and a small amount of  heat  will be conducted further.

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CONVECTION

  onvectionis the transfer ofea y e ac ua

movement of the warmedmatter.

Convection is the transfer of

liquid by movement of

currents.

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CONVECTION

Heated air

rises, cools,

then falls

Hot water

rises, cools,

and falls

 Air near heater is

replaced by cooler

air, and

the cycle repeats

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CONVECTION

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SEA 

BREEZE 

AND 

LAND 

BREEZE

SEA BREEZE LAND BREEZE

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2. ConvectionEnergy transfer  by random molecular  motion (as in 

conduction) 

 plus 

bulk  

(macroscopic) 

motion 

of  

the 

 fluid.  Convection: transport by  random motion of  molecules and by  

.

Forced convection: Caused by  external means Natural (free) convection: flow induced by  buoyancy  forces, 

arising from density  differences arising from temperature 

 variations in the fluid

The above cases involve sensible heat (internal energy) of  the fluid Latent heat exchange is associated  with phase changes – boiling 

and condensation.

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RADIATIONSunlight is a form of

radiation that is radiatedthrough space to our planetwithout the aid of fluids orsolids.

Since there are no fluids in

s ace convection is notresponsible for transferring theheat. Thus, radiation bringsheat to our planet.

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RADIATION

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RADIATION AMONG OBJECTS

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