Heqt Transfer١Lebanese university – Faculty of Engineering – Branch I

Eng. Wael Zmerly – 2007-2008General information on Heqt Transfer

Chapter IGENERAL INFORMATION ON HEAT

TRANSFER

Lebanese universityFaculty of Engineering

Branch I

By Eng. Wael Zmerly – 2007-2008

Heat Transfer

Heqt Transfer٢Lebanese university – Faculty of Engineering – Branch I

Eng. Wael Zmerly – 2007-2008General information on Heqt Transfer

1. MODES OF HEAT TRANSFER :

HEAT

T T

Conduction

Convection

Radiation

Two bodies A and B at different temperatures exchange energy in the form of heat until complete disappearance of their difference in temperature. This exchange can be done according to three fundamental modes of transmission: conduction, radiation, convection.Actually, any heat exchange is carried out simultaneously under the three modes of transfers which are competed with. The exchange is then the sum of the three effects. Example: cooling with the air of a metal plate at very high temperature.When one of the modes is prevalent, one will neglect obviously the two others.

Heqt Transfer٣Lebanese university – Faculty of Engineering – Branch I

Eng. Wael Zmerly – 2007-2008General information on Heqt Transfer

1. MODES OF HEAT TRANSFER :A-CONDUCTION

The phenomenon of pure conduction meets primarily in the solids. The propagation of heat by conduction inside a body is done according to two quite distinct mechanisms: a transmission by the vibrations of the atoms or molecules and a transmission by the free electrons. The relative importance of each of the two mechanisms depends obviously on nature on the body.Example: propagation of heat along an iron bar whose end is subjected to the action of a flame

Heqt Transfer٤Lebanese university – Faculty of Engineering – Branch I

Eng. Wael Zmerly – 2007-2008General information on Heqt Transfer

B-CONVECTION

1. MODES OF HEAT TRANSFER :

The heat transfers by convection which bring into play the displacement of matter, take place in the fluids primarily and find their origin in the movement of those.When the movement of the fluids is caused by an external mechanical action (action of a pump for the circulation of a fluid in a drain - agitator mechanical for a fluid heated in an engine…), the convection known as is forced.One will speak about natural convection when the movement of the fluids is due to the variations of density within the fluids which appear under the effect of heat exchange between fluids and walls in consequence of the differences in temperature. Example: the convector (the word radiator is unsuitable) of central heating raises the temperature of the air in contact with its wings.

Heqt Transfer٥Lebanese university – Faculty of Engineering – Branch I

Eng. Wael Zmerly – 2007-2008General information on Heqt Transfer

1. MODES OF HEAT TRANSFER :C-RADIATION

All the bodies emit by their surface of energy in the form of electromagnetic radiations. If this radiated energy meets a body absorbing for its wavelengths, it is transformed into heat. The heat transfer per radiation occurs even at the low temperatures (it is enough that the temperature is higher than 0 K) as soon as two bodies at different temperatures are separated by a permeable medium. This transfer of heat which does not require any material support produces even in the vacuum.The energy received by the surface of the coldest body is divided into three parts: a fraction of received energy is considered; another fraction can pass through the body; a third fraction finally is absorbed by the body and of this fact its temperature raises.Example: the sun heats the ground by radiation.

Heqt Transfer٦Lebanese university – Faculty of Engineering – Branch I

Eng. Wael Zmerly – 2007-2008General information on Heqt Transfer

1. MODES OF HEAT TRANSFER :

2-conduction3-convection

1-radiationP

D ANALOGY OF THE THREE METHODS

One can make an analogy between heat and water to extinguish the fire. The men represent the average to transmit heat.1 – the pipe transmits the water directly From Well P to the Building industry B, independent of any average of transmission. It is like the transfer of heat in the vacuum or some gas, the radiation.2 – In the bridge of the buckets, water passes from P to B by an average, It is likeConduction.3 – a man carries the water of P to B, It is like the Convection.

Heqt Transfer٧Lebanese university – Faculty of Engineering – Branch I

Eng. Wael Zmerly – 2007-2008General information on Heqt Transfer

y

Z

X

2. TEMPERATURE PATTERNS:

= 0dTdt

Steady operation

Transient state

Conservation ofenergy

≠ 0dTdt

=e sdQ dQ

Energy enteringdQe

dx

Energy leavingdQs

When the temperatures at the various points of the system are constant, i.e.independent of time, it is said that the temperature pattern is a steady operation. On the other hand, if the local temperatures vary according to time, that means that thermal balance is not reached yet. It is the transient state. It is the current case of the transfers of heat through the walls of a building where the outside temperature of the air varies (to evoke only it) during the year… and of hour in hour.

Limit of the study:The variable mode involves complex relations. Also will limit us to it study of the

transfers by conduction to steady operation, where one considers the temperature of a constant medium in time.

Note:The study in steady operation has as a consequence:- A simplification of the study of the transfers of heat (increase of the transfers)- No accumulation of whole or part of heat at the time of the passage of this one

through material. What results in:

= 0dTdt

=e sdQ dQ

Heqt Transfer٨Lebanese university – Faculty of Engineering – Branch I

Eng. Wael Zmerly – 2007-2008General information on Heqt Transfer

3. DENSITY AND HEAT FLOW FLUX:

ϕ =dQ

S . dt

ϕn

S

Density of Flow

Heat flow Φ =dQ dt

ϕΦ = . S

Φ = Heat flow [W]ϕ = Density flow [W/m ²]Q = Quantity of exchanged heat [J]

Whatever the mode of transmission, the study of the transfers is based on the concept of flow of heat. The flow of heat or Heat flow Φ (W) is the thermal output per unit of time. Flow of heat which crosses a surface whose surface is equal to the unit is called the Density Flux ϕ (W/m2).That is to say a homogeneous element of surface “S” (standard wall) with through which exchanged heat follows lines perpendicular to surfaces having a constant temperature (isothermal surface). That is to say “dQ” quantity of energy exchanged during time “dt”.One can write:From where, the exchanged total power will be written, in this case:Φ = Heat flow [W]ϕ = Density flux [W/m ²]Q = Quantity of exchanged heat [J]

ϕ =dQ

S . dt

ϕΦ = Φ =dQ et . Sdt

Heqt Transfer٩Lebanese university – Faculty of Engineering – Branch I

Eng. Wael Zmerly – 2007-2008General information on Heqt Transfer

3. DENSITY AND HEAT FLOW FLUX:

IsothermalSurface α

dS

ϕn

M

ϕ ϕ αΦ = • = n dS cos dSd

ϕ αΦ = . S . cos

In a more general way, for an element of surface “dS” directed by its unit normal “”The elementary flow of heat can be written:That is to say:Foot-note: If α = then Φ = 0 (and =): Circulating flow perpendicular to isothermal surface is null.

Note:- One calls Tubes of Flow, tangent surfaces with maximum flow. In the plan one will define Lines of flow.- Energy circulating perpendicular to the line of flow is null.- Energy is preserved in a tube of flowExample:Determine the flow and the quantity of heat crossing a wall of 100 m ², subjected to the solar radiation of density 1000 W/m ² arriving under an angle of 30 ° compared to the normal and this during 10 minutes.

ϕ ϕ αΦ = • = n dS cos dSdϕ αΦ = . S . cos

π2 ϕ 0