Dr. Mohamed Reda Salem - Bu shoubra/mechanical... · Dr. Mohamed Reda Salem ... J.P. Holman “Heat Transfer”,10th Edition, 2010. ... So, heat transfer is due to the combination

Dr. Mohamed Reda SalemLecturer at Faculty of Engineering at Shoubra

Benha University, Egypt

[email protected][email protected]

https://www.facebook.com/Dr.Mohamed.Reda.Salem

Heat Transfer and Industrial Furnaces

Code : MEP2932014/2015

GRADING POLICYTotal Degrees : 125

Assignments/Class Work : 20%Oral Exam : 20%Final Exam : 60%

REFERENCES Course Notes:

Course notes prepared by instructor.

Essential Books (Text Books)

Theodore L. Bergman, Adrienne S. Lavine, Frank P.Incropera, and David P. Dewitt, “Fundamentals of Heat andMass Transfer”, 7th Edition, John Wiley & Sons, Inc., 2011.

J.P. Holman “Heat Transfer”, 10th Edition, 2010.

Yunus A. Cengel, “Heat Transfer: A Practical Approach”, 2nd

Edition, July 2002.

Chapter (1)

Fundamentals of Heat Transfer

Lecture (1)

Topics to be Covered in Lecture (1) Heat Transfer Concept

Modes of Heat Transfer

Conduction Heat Transfer

Convection Heat Transfer

Radiation Heat Transfer

Heat Transfer Concept

The heat energy is the form of energy called thermal energy, which can

be transferred from one system to another as a result of temperature

difference.

The science, which deals with the determination of the rates of heat

energy, is called Heat Transfer.

Chapter (1): Fundamentals of Heat Transfer (Lect. 1)

Introduction

Examples are everywhere

Heat flows in the body

Home heating/cooling systems

Refrigerators, air conditioning, ovens, furnaces, other appliances

Automobiles, power plants, the sun, etc.

During heat transfer, the thermal energy always moves in the same

direction:

Heat energy only flows when there is a temperature difference from a

warmer area to a cooler area.


Introduction

HOT COLD

Modes (Mechanisms) of Heat Transfer

There are three fundamental modes in which heat is transferred:

Conduction heat transfer,

Convection heat transfer,

Radiation heat transfer.


Introduction


Introduction


Introduction

Conduction Heat Transfer is the transfer of heat energy through

stagnant (stationary) medium as solids and non-movable fluids from

the more heated particles of the substance to the adjacent, less heated

ones as a result of contact of atoms.



The Physical Mechanism of Conduction Heat Transfer

In Solids: Conduction heat transfer is due to the combination of the

vibrations of the molecules in a lattice and the energy transport by

free electrons.

In Fluids (gases and liquids): Conduction heat transfer is due to the

collisions of the molecules during their random motion.

How are the particles arranged in a solid, a liquid and a gas?



Solid Liquid Gas

Metal

as

Cu, Al, Fe

Non-Metal

as

Graphite



How do non-metals conduct heat?

Taking the graphite as example of non-metals that is a good conductor

of heat.



As the graphite rod is heated, the carbon atoms near the heat source

begin to vibrate. These vibrations make the adjacent atoms vibrate, and

so on along the rode. This is how heat energy travels along a non-metal.



Heat

How do metals conduct heat?

Metals are good conductors of heat. The outer electrons of metal atoms

are not attached to any particular atom. They are free to move between

the atoms. So, heat transfer is due to the combination of the vibrations

of the molecules in a lattice and the energy transport by free electrons.

When a metal is heated, the free electrons

gain kinetic energy.

This means that the free electrons move faster

and transfer the energy through the metal.

This makes heat transfer in metals very

efficient.

Insulators do not have free electrons and so

they do not conduct heat as well as metals.



T1

L x

QA

T2

w

H

The parameters that affect the rate of

heat conduction:

Surface area of wall normal to heat

flow (Q A)

Temperature difference across the wall

(Q T)

Wall thickness (Q 1/L)

Wall material (Q k)

L

TTA Q 21

cond

Conduction Heat Transfer is expressed by Fourier’s Law of conduction as

L

TTA k Q 21

cond

L

TTA k 12

dx

dTA k For Plane Wall



Where:

Qcond is the conduction heat transfer rate (W)

k is the thermal conductivity (W/m.C or W/m.K)

A is the heat transfer area normal to the heat transfer (m2)

dT/dx is the temperature gradient across the wall (C/m or K/m)

T1, T2 are surfaces temperatures (C or K)

−e sign refers to that the heat moves in the direction of temperature

decreasing

dx

dTA k

L

TTA k Q 21

cond

For Plane Wall

The thermal conductivity is a physical property of the wall material. The

significance of the thermal conductivity of a material comes from that; it

is a measure of how fast heat will be conducted in that material.

Convection heat transfer is the transfer of heat energy between a

surface and the adjacent moving fluid. It involves the combined effect

of conduction and fluid motion.



Modes (Mechanisms) of Convection Heat Transfer

According to the source of fluid motion, there are two modes of

convection heat transfer;

(a) Natural (Free) convection heat transfer and

(b) Forced convection heat transfer.

Natural (Free) Convection

The fluid motion is due to buoyancy

force caused due to density difference

due to temperature difference .



T

Ts

Forced Convection:

The fluid motion is due to inertia force

caused by external means of motion

as a fan, pump, or the wind.....

Ts

Forced

flow Fluid

T

Convection heat transfer is expressed by Newton’s Law of Cooling as:



TTA h Q ssconv

Where:

Qconv is the convection heat transfer rate (W)

h is the convection heat transfer coefficient (W/m2.C or W/m2.K)

As is the surface area (m2)

Ts is the surface temperature (C or K)

T is the fluid temperature (C or K)

Example for free convection: When the kettle is turned on, the heating element worms

up and heats the water around it. The heated water becomes less dense than the cold

water above. This means that the heated water rises up the kettle. As the heated water

rises, it displaces the cold water. The colder, denser water falls to the bottom, where it is

then warmed by the heating element. This creates circular movements of water.



Why is the freezer compartment at the top of a fridge?



At the bottom of the fridge, it is

warmer. This warmer air rises and so a

convection current is set up inside the

fridge, which helps to keep the fridge

cool.

The freezer cools the air at the top and

this cold air cools the food on the way

down.

Report:

Why is it windy at seaside??



Radiation heat transfer is the transfer of heat energy in the form of

electromagnetic waves (or photons) as a result of the changes in the

electronic configurations of the atoms or molecules.

Any body have temperature larger than absolute zero (0 K = −273C),

emit heat by radiation.

Radiation heat transfer needs no medium to transmit.



Stefan-Boltzmann Law

“The emissive radiation of a body over all wave lengths is proportional

to fourth power of temperature”.

4

srad T Q





Radiation heat transfer is expressed by Stefan-Boltzmann Law as:

Where:

Qrad is the radiation heat transfer rate (W)

is the surface emissivity, 0 ≤ ≤ 1

As is the surface area (m2)

is Stefan-Boltzmann constant; = 5.67*10−8

(W/m2.K4)

Ts is the surface temperature (K)

Tsur is the surrounding temperature (K)

4

sur

4

ssrad TT A Q


Notes

q is the amount of heat energy (J)

Q is the heat transfer rate (W), Q = q/t

q is the heat transfer rate per unit length (W/m), q = Q/w

q is the heat transfer rate per unit area (Heat Flux) (W/m2), q = Q/A

q is the heat transfer rate per unit volume (Heat Generation) (W/m3),

q = Q/V

Awall = w*H Vwall = w*H*L

As, cylinder = DL = 2Rl Vcylinder = D2L/4 = r2L

Ac, cylinder = D2/4 = r2

Asphere = D2 = 4r2 Vsphere = D3/6 = 4r3/3

The heat energy

Heat Transfer

Modes (Mechanisms) of Heat Transfer





Summary

L

TTA k Q 21

cond

TTA h Q ssconv 4

sur

4

ssrad TT A Q

Thank You

QUESTIONS?



Example for free convection: During the day, the land is warmer than the sea. The heat

of the land warms the air above the land. This warm air rises because it is less dense

than the surrounding cooler air. The cold air then moves from the sea to take the place of

the warm air that has risen above the land. This movement of air sets up a convection

current and creates the seaside breeze.



Example for free convection: During the day, the land is warmer than the sea. The heat

of the land warms the air above the land. This warm air rises because it is less dense

than the surrounding cooler air. The cold air then moves from the sea to take the place of

the warm air that has risen above the land. This movement of air sets up a convection

current and creates the seaside breeze.



Example for free convection: at night, the land becomes cooler than the see. The warm

air above the sea rises and is replaced by cold air from the land. This means that

convection now occurs in the opposite direction.



Example for free convection: at night, the land becomes cooler than the see. The warm

air above the sea rises and is replaced by cold air from the land. This means that

convection now occurs in the opposite direction.



HEAT TRANSFER

CONDUCTION CONVECTION RADIATION

q12sT14 sT2

4

q1 q2

11

11

A

22

21

A

121

1

FA

Documents

Dr. Mohamed Reda Salem - Bu shoubra/mechanical... · Dr. Mohamed Reda Salem ... J.P. Holman “Heat Transfer”,10th Edition, 2010. ... So, heat transfer is due to the combination