# Laboratory report

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GROS Kevin
Mechanical Engineering 096591462

Laboratory Report Forced Convection

Heriot Watt University

GROS Kevin
Mechanical Engineering 096591462

Summary :
1) 2) 3) 4) 5) 6) 7) 8)

Introduction Apparatus Theory Procedure Results Discussion Conclusion References

1) Introduction :
The understanding of heat transfer characteristics of materials is important to the engineeringworld because this process is useful to engineers for understanding fluid flow analysis to be performed. And they can determine temperature distribution of components which is useful in applications such as heat exchangers, heating systems or cooling and insulation materials. In this experiment, we are going to determinate the heat transfer characteristics of a copper rod which is placed in theheat exchanger. This is simply to blow air over the surface of the cylinder, by monitoring the rate of cooling of the copper rod. As the air flows across the rod it will be cooled by convection. The relevant readings are monitored using a thermocouple and of a computer software. From the experimental data and theory we can so determine the heat transfer characteristics.

Heriot Watt University GROS Kevin
Mechanical Engineering 096591462

2) Apparatus :
• • • • • • To realize this experiment we can use the following equipment : The air outlet orifice (for controling the percentage of air speed through the wind tunnel). The electric heater (for heating the copper rod until the desired temperature). The copper rod is placed in the test section. The wind tunnel (to produce a forcedconvection via an electric motor). The petro tube (for measuring the air speed). The PC (with a software for saving all the experimental data obtained).

Heriot Watt University

GROS Kevin
Mechanical Engineering 096591462

3) Theory :
Considering the heat lost by forced convection form the test rod. The amount of heat transferred is given by :

˙ ˙ Q=×A×T −T a  (1) where Q  A T Ta
So,in any period time,

: Rate of heat transfer (W) : Film heat transfer coefficient (W/m²K) : Area for heat transfer (m²) : Temperature pf the copper rod (°C or K) : Temperature of air (°C or K)

dt , then the fall in temperature, dT , will be given as :

˙ −Q dt =m×c p×dT (2) where m : Mass of copper rod (kg) c p : Specific heat of the copper rod (J/kgK)
Eliminating Q from (1) and (2) then:

−dT × A = ×dt T −T a m×c p

Since

T a is constant, dT =d T −T a 

Integrating gives :

ln T −T a =
at

−×A ×t C 1 m×c p

t=0 , T =T 0 , hence C 1=ln T −T 0  , hence : ln T −T a = −×A ×tln T 0−T a  m×C p

or

ln 

T −T a −× A = ×t T 0 −T a m×C p ln  T −T a −× A  against t should give a straight line of gradient m×C p T max−T a
from

Therefore a plotof

which the heat transfer coefficient,

 , can be found.

To find the velocity of air passing the rod, first the velocity upstream must be found. From basic fluid flow theory :

 . 2  P= 2

in the air stream.

Heriot Watt University

GROS Kevin
Mechanical Engineering 096591462 And Therefore

 P= . g . h in the measuring manometer.

a . 2 = w . g . h 2
a w  h(3)

Where

: Density of air : Density of fluid in manometer : Mean velocity of air : Head in manometer

Therefore measuring the air temperature and air pressure the density can be found,

a =

Pa R×T a
R=289 J /kg K.

Where

 2=

2 w . g . h a =140.1

If

 w is taken as 1000 kg /m3 and h is measured in m, then

h a

However the velocity, u, used in heattransfer calculations is normally based on the minimum flow area. Therefore with the single rod :

u=

10  , since, 9

Practical forced convection heat transfer relationships are often expressed in the dimensionless form

Nu=C . R e . Pr

n

m

However for gases, Pr is virtually constant, therefore

Nu=K . R e
Nu = Nusselt Number,

n

d k a . . d  C p . k
Where d : Diameter...