TITLE

FORCED CONVECTION

OBJECTIVE

Demonstrate the use of a fin (extended surface) to improve the heat transfer in forced

convection.

ABSTRACT

This experiment was conducted to perform and demonstrate force convection heat transfer

using different type of extended surface plate. This following experiment outlines the proper

procedure for determining these temperature distributions along an extended surface and

consequently helps the students to demonstrate them graphically by doing the analysis

based on the data and readings obtained. Heat transfer by convection between and

surrounding can be increased, by attaching thin strips of metal fins to the surface. When

heat transfer takes place by convection from both interior and exterior surfaces of a plate,

generally fins are used on the surfaces where heat transfer coefficients are low. Heat

transfer by convection, whether free or forced, forms the basis of most industrial heat

exchanger and related equipment. The measurement and prediction of heat transfer

coefficients for such circumstances is achieved in free and forced heat convection by

studying temperature profile and extended transfer surfaces.

INTRODUCTION

Convection is the transfer of heat from one place to another by the movement of fluids.

Convection is usually the dominant form of heat transfer in liquids and gases. Although often

discussed as a distinct method of heat transfer, convection describes the combined effects

of conduction and fluid flow or mass exchange.

Two types of convective heat transfer may be distinguished:

Free or natural convection: when fluid motion is caused by buoyancy forces that

result from the density variations due to variations of thermal temperature in the fluid.

In the absence of an external source, when the fluid is in contact with a hot surface,

its molecules separate and scatter, causing the fluid to be less dense. As a

consequence, the fluid is displaced while the cooler fluid gets denser and the fluid

sinks. Thus, the hotter volume transfers heat towards the cooler volume of that

fluid.Familiar examples are the upward flow of air due to a fire or hot object and the

circulation of water in a pot that is heated from below.

Forced convection: when a fluid is forced to flow over the surface by an external

source such as fans, by stirring, and pumps, creating an artificially induced

convection current.

Convection is also classified as external and internal, depending on whether the fluid is

forced to flow over a surface or in a pipe. Internal flow occurs when a fluid is enclosed by a

solid boundary such when flowing through a pipe. An external flow occurs when a fluid

extends indefinitely without encountering a solid surface. Both of these types of convection,

either natural or forced, can be internal or external because they are independent of each

other.

THEORY

Heat transfer from an object can be improved by increasing the surface area in contact with

the air by adding fins or pins normal to the surface. This can be seen in Newton’s Law of

Cooling, which defines the convection heat transfer rate:

S(TS - T )

The effect of the surfaces can be demonstrated by comparing finned and unfinned surfaces

with a flat plate under the same conditions of power flow.

where Q is the heat transferred per unit time, h is the heat transfer coefficient, TS is the

object's surface temperature and T  is the fluid temperature.

The convective heat transfer coefficient is dependent upon the physical properties of the

fluid and the physical situation. Values of h have been measured and tabulated for

commonly encountered fluids and flow situations.

APPARATUS

Air Sensor Thermometer

Finned Plate Pinned Plate

Digital Temperature Scale Convection Heat Transfer Machine

PROCEDURE

1. The input power supply plug was connected to a nearest single-phase electrical

supply of 240VAC/50Hz.

2. The heater power control knob and fan speed control knob was turned fully

anticlockwise. The fan supply lead at the base of the duct was connected to the

socket beneath the control panel.

3. The thermistor probe lead was connected to the socket beneath the control panel.

The flat plate heat exchanger was clamped into the duct using two toggle clamps and

the heater power supply lead was connected to the socket on the cover.

4. The plate temperature connector was connected to the socket on the heat exchanger

and the anemometer was switched on, the protective cover was slide carefully

rearward to expose the probe head and the probe is located into the bush fitted to the

duct.

5. The meter was placed into the bracket situated on the side of the duct. The sensor

hole was ensured aligned with the direction of the airflow when inserting the probe

through the wall of the duct and the switch was set off when the anemometer is not

being used.

6. The equipment was switched on by switching on the main power. The LED

temperature meter and wattmeter was checked and illuminated and the temperature

meter was checked indicates ambient temperature.

7. The heater power was increased in the exchanger by rotating the power control knob

clockwise and the power supplied to the exchanger were shown in watts on the

meter.

8. The fan was switched and speed is increased by rotating the fan speed control knob

anticlockwise. The anemometer was set in switched on and the air velocity was

observed is indicated the meter scale.

9. The exchanger temperature lead was connected to the socket beneath control panel

and the temperature meter was checked to indicate the increasing temperature of the

heat exchanger metalwork.

10. The heater power control knob (anti-clockwise) and fan speed control knob

(clockwise) was set to minimum. The anemometer and the power control

knob(anticlockwise) was switched off and the fan speed control knob was set to the

maximum to cool down the hot plate exchanger

11. Lastly, the main power supply was shut down after the plate was cooled down to

room temperature.

RESULT

Ambient air temperature (T∞) = 26°C

Power input (  ) = 86.9 W

Finned Plate

Air Velocity (m/s) Heater Temperature

(Ts) [°C ]

Ts – h [W/m.°C]

0.0 63.70 37.70 31.93

1.0 53.20 27.20 44.25

1.5 45.50 19.50 61.72

SAMPLE CALCULATION

Sample calculation

=0.0722 m2

= 54.96 (W/m.oC)

Pinned Plate

Air Velocity (m/s) Heater Temperature

(Ts) [°C ]

Ts – h [W/m.°C]

0.0 65.70 39.70 63.26

1.0 54.60 28.60 87.82

1.5 47.90 21.90 114.68

SAMPLE CALCULATION

Sample calculation

=0.0346 m2

= 128.80 (W/m.oC)

MUHAMMAD ASRAF BIN AZMI 2014672104

DISCUSSION

From the experiment that been conducted, student need to demonstrate the use of

extended surface to improve the heat transfer in forced convection. In free convection the

heat transfer rate from the surface is limited by the small movements of air generated by this

heat. More heat is transferred if the air velocity is increased over the heated surface. This

process of assisting the movement of air over the heated surface is called Forced

Convection. Moreover, in Newton’s Law of Cooling stated that heat transfer from an object

can be improved by increasing the surface area in contact with the air. The surface that used

for this experiment are finned plate and pinned plate in order to study about the heat

transfer.

The comparing between two plates with a flat plate under the same conditions of power

input, ( = 83 W), same ambient or room temperature 26˚C and the same velocity of air. The

area of both plate is differing which is the finned plate has greater and more surface area,

0.07222 m2 than the pinned plate that is 0.03461 m2. Besides that, the velocity that been

used to run the experiment has different value which 0 m/s, 1.0 m/s and 1.5 m/s in order to

see the effect of heat transfer between this both plate.

Surface Air velocity

(m/s)

Heater

Temperature

(Ts) (oC)

( oC)

h

(W/m.oC)

Fin 1.0 53.60 27.60 43.61

Pin 1.0 53.20 27.20 92.34

The table above shows the different of the finned plate and pinned surface in heater

temperature, the difference of the temperature and the convection heat transfer coefficient.

In the same condition of the air velocity (m/s), the pinned plate has more heater temperature

compare to the finned plate. This is because the finned plate has fast or greater cooling rate

than the pinned plat and the finned surface has more ability to eliminate heat by convection

compare to pinned surface. Therefore, the finned plate is good heat discriminate than the

pinned plate. The pinned plate also has more store temperature in surface area which is the

heat is slow to get out than the finned plate. This can be referring in Newton’s Law of

Cooling which the rate of heat loss of a body is proportional to the difference in temperatures

between the body and its surroundings while under the effects of a breeze. The heat transfer

coefficient for finned pin is lower than the pinned surface because the finned plate has small

surface area than the pinned area. For the best surface area, use the finned plate to

improving the heat transfer of cooling rate and the air velocity plays the important role in heat

transfer, either in the form of change of enthalpy across the channel or by convection heat

transfer from a surface. For the best design in fast rate, use the pinned plate.

MUHAMMAD ASRAF BIN AZMI 2014672104

CONCLUSION

The conclusion for the experiment is successfully achieved and conducted. Student

are able to demonstrate the use of extended surface to improve the heat transfer in forced

condition which finned plate has good surface area to cool down faster than the pinned

plate. The higher the surface area of the surface, the slower the cooling rate to remove the

heat. Therefore, it is affect the heat transfer of the components. The best design for surface

area is finned plate than the pinned plate. Although it has some error and issue, it does not

affect the experiment and it can reduce in order to good result which need to some

improvement or calibrate the apparatus for experiment.

MOHAMAD FAREEZ BIN HASLIM 2014684346

DISCUSSION

In this experiment, we have conducted the heat convection processes or heat

transfer. We use 2 different plate or surface to get the result of our experiment. The plate

that we used is pinned plate and finned plate.

PINNED PLATE

FINNED PLATE

Air velocity (m/s) Heater temperature (TS)(OC)

TS – T0

(OC)h ( W/m.OC)

0.0 65.70 39.70 63.261.0 54.60 28.60 87.821.5 47.90 21.90 114.68

TABLE 1 PINNED PLATE

Air velocity (m/s) Heater temperature (TS)(OC)

TS – T0

(OC)h ( W/m.OC)

0.0 63.70 37.70 31.931.0 53.20 27.20 44.251.5 45.50 19.50 61.72

TABLE 2 FINNED PLATES

The result has been obtained. From the result we can say the heater temperature was

decreasing as the velocity of the air is increasing in both pinned and finned plate. From the

differences of temperature in both plates, we can see the decreasing of temperature. But the

pinned plate showed the faster rate of decreasing in temperature.

The value of h that is heat convection coefficient, are depends on the surface area of the

plate. As recorded the value of h are increasing as the temperature differences are

decreasing for both plate. But the pinned plate gives the faster increasing rate of coefficient.

This is because differences surface area gives differences of result. As we observed, the

pinned plate has larger surface area than finned plate. So we can conclude that, the larger

the surface area the faster the heat convection coefficient increase.

There are some errors that have been identified in these experiments that is the voltage are

not constant as we control the voltage by hand, other than that the flow of air is not constant

because of human setting and the surrounding obstacles due to humidity, air flow and room

temperature are not well calibrated constant. So to overcome this problem, the several

actions must be done such as taken the result twice or wait for a few minutes as the reading

is constant.

MOHAMAD FAREEZ BIN HASLIM 2014684346

CONCLUSION

From the experiment of force convection that we have conduct, we have

demonstrated the use of the pinned plate and finned plate to improve the heat transfer in

force convection. So, the objective in this experiment is achieved as it shows that the heat

transfer convection coefficient for the pinned plate is higher than at the finned plate. The

importance variable is the air velocity as it will affect the temperature difference.

MOHAMAD IKHWAN BIN SAMER 2014630126

DISCUSSION

Based on the objective of the experiment which is to determine the use of the fin

plate and pin plate to improve the heat transfer in forced convection. Convection is a method

of heat transfer that uses fluid as a medium to transfer heat. In this experiment the medium

we use is air with three different air velocity 0 m/s, 1.0m/s and 1.5 m/s. When the experiment

was conducted with finned plate for air velocity of 0m/s the temperature different is 37.70°C

and the convection heat transfer coefficient is 31.93W/m°C. When the fan speed was turned

on for 1.0m/s the temperature different is 27.20°C and the convection heat transfer

coefficient is 44.25W/m°C. For the fan speed of 1.5m/s the temperature different is 19.50°C

and the convection heat transfer coefficient is 61.72W/m°C. It shows that in convection heat

transfer the fan speed act as a medium during heat transfer because when the fan speed

increase the temperature different decrease which mean heat loss in the plate thus the heat

transfer coefficient increase.

As for the pinned plate when the air velocity is 0m/s the temperature different is

39.70°C and the convection heat transfer coefficient is 63.26W/m°C. When the fan speed

was turned on for 1.0m/s the temperature different is 28.60°C and the convection heat

transfer coefficient is 87.82W/m°C. For the fan speed of 1.5m/s the temperature different is

21.90°C and the convection heat transfer coefficient is 114.68W/m°C. The graph below

show the relationship between air velocity and the temperature different:

From the graph it shows that finned plate suffers more heat loss than pinned plate

because the finned plate has lower temperature different than pinned plate when the air

velocity was done on both plate at constant value. Thus the convection heat transfer

coefficient value shows that the pinned plate was higher than the finned plate which leads us

to the surface area on both plates. The surface area of pinned plate is 0.0346m2 while finned

plate surface area is 0.0722m2. If the surface area value is higher the convection heat

transfer is considered the better one so the lower the convection heat transfer coefficient the

better the heat transfer.

MOHAMAD IKHWAN BIN SAMER 2014630126

CONCLUSION

Based on the objective of the experiment which is to demonstrate the use of a fin and

pin to improve the heat transfer in forced convection. It was concluded that the finned is

better in the heat transfer because the finned plate has lower temperature different than the

pinned plate which mean finned plate heat loss was much more than the pinned plate. This

is because the finned plate has higher surface area which it has more tendency to suffer

heat loss during convection. The lower value of heat transfer coefficient in the finned plate

also show that finned plate is better than pinned plate in convection heat transfer.

MOHD KHAIRUDDIN B CHE LONG 2013834608

DISCUSSION

For this experiment, we used two different type of surfaces which are finned and pinned

surfaces. We studied about two different surfaces and which one is better to improve the

heat transfer in forced convection.

Theoretically, by increasing the surface area that contact with the air, the convection heat

transfer from an object can be improved. In this experiment, the effects of extended surface

can be demonstrated by using finned and pinned surface with same the power inputs

(86.9W), ambient air temperature 26ºC and the air velocity flows. The value for pinned

surface is always higher than finned surfaces for every air velocity. It is because the pinned

have more heat stored compared to the finned surface. For an example, in 1.0(m/s) air

velocity the heater temperature for finned surface is 53.2ºC while pinned plate is 54.6ºC. For

pinned plate, it is square in shape. The heat release from the plate is high, and thus the

temperature stored is low. On the other hand, for pinned plate, it is cylinder in shape. The

heat release from the plate is low compared to the finned plate. Thus, the temperature

stored in the pinned plate is high. By calculating, we got that the pinned plate has higher

value of the heat transfer coefficient (h) in each air velocity. By referring the Newton’s Law of

Cooling:

Q=h As (Ts-T∞)

We can say that, the area of a plate and temperature difference will affect the heat transfer

coefficient of a certain surface. In this experiment, we can conclude that the pinned plate is

better heat transfer than the finned plate. However, there are some errors may occur during

the experiment that may affect our result. According to thermodynamic law of conduction of

heat transfer, total amount of energy will not be 100% efficiency because they will be having

some minor loses during the heat conduction. Technically when the heat is transfer from one

medium to another, there will be some friction that will cause some significant heat losses.

One of the factor is ambient temperature may be not fix due the room not too close and

some time the door of room is opened. We need to setup a close room and make sure have

fix ambient temperature. Next is about the sensor temperature that is very sensitive since

there are many person and group using these machines. Generally, we know that sensor is

very sensitive thing. So to get best result we need to make sure the sensor performs in good

condition by services regularly. Lastly about the air velocity that supply is very difficult to set

up according the procedure. May be we can get some automatic air velocity with more

accurate according to the procedure.

MOHD KHAIRUDDIN B CHE LONG 2013834608

CONCLUSION

From this experiment, we can see that the use of extended surfaces enhance heat transfer

from a surface. It is sound that finned extended surface release heat faster than pinned

extended surface due to higher surface area. In theory, higher flow velocity would encourage

convection heat transfer. This theory is clearly illustrated based on the result obtained in this

experiment. The higher the air velocity, the faster convection heat transfer occur. In

conclusion, this experiment can be considering as successful since the objective of this

experiment have been covered.

REFERENCES

1. Incropera DeWitt VBergham Lavine 2007, Introduction to Heat Transfer, 5th ed., pg.

6 ISBN 978-0-471-45727-5.

2. Retrieved from http://biocab.org/Heat_Transfer.html Biology Cabinet organization,

April 2006, “Heat Transfer”, (Accessed on 3 November 2015).

3. Retrieved from http://www.engineersedge.com/heat_transfer/convection.htm

Engineers Edge, 2009, “Convection Heat Transfer”, (Accessed on 3 November

2015).

4. Cengel, Ghajar, “Heat and Mass Transfer: Fundamentals and Applications, Fourth

Edition in SI Units, Mc Graw Hill,2015, Singapore, pg 303 ISBN 978-1-121-90576-4.