LS4 - Forced Convection

Thermalfluids Lab-MEC 554/ LS 4/ THOMAS Rev. 01-2007

UNIVERSITI TEKNOLOGI MARA FACULTY OF MECHANICAL ENGINEERING ________________________________________________________________________ Program : Bachelor Of Engineering ( Hons ) Mechanical Course : Thermalfluids Lab Code : MEC 554 ________________________________________________________________________ LAB SHEET NO:

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HEAT TRANSFER LABORATORY SHEET TITLE : FORCED CONVECTION 1.

OBJECTIVE

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

2.

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:

Q  h As 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 and flow.

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Thermalfluids Lab-MEC 554/ LS 4/ THOMAS Rev. 01-2007 3.

EQUIPMENT

The surfaces are shown in the figure below. The finned surface consists of 9 fins that are each 0.1 m high and 0.068 m wide. The pinned surface consists of 17 pins that each have a diameter of 0.013 m and are 0.068 m long.

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Thermalfluids Lab-MEC 554/ LS 4/ THOMAS Rev. 01-2007 4.

PROCEDURES a. Place the heat exchanger into the test duct and record the ambient temperature (T). b. Set the heater power control to 75 W. Allow the temperature to rise to 80°C then adjust the heater power control to 20 W. c. Start stopwatch wait 5 minute and record the heated plate surface temperature (Ts). d. Repeat step (b). Then set the fan speed control to give a reading of 1.0 m/s on the thermal anemometer. Allow 5 min for the reading to stabilize and then again record Ts. e. Then adjust the fan speed to 2.0 m/s and then 2.5 m/s, recording the Ts for each speed. [Same Procedure at step (d) ].

5.

RESULTS 1.

Start with the unfinned flat plate. Using the data tables shown below, record the ambient temperature (T) and the power input ( Q )

Ambient air temperature (T) = __________ °C Power input ( Q )

Table A Air Velocity [m/s]

= __________ W

Flat Base Plate (Unfinned)

Heater Temperature (Ts) [°C]

Ts-T

h

[°C]

[W/(m·°C)]

0 1.0 2.0 2.5

2.

Calculate the temperature difference (TsT) and record on the data table.

3.

Calculate the surface area of the base plate (Abase).

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Thermalfluids Lab-MEC 554/ LS 4/ THOMAS Rev. 01-2007 4.

Calculate the convection heat transfer coefficient (h) from the equation:

h

5.

Q As Ts  T 

Replace the flat plate with the finned surface plate and repeat the experiment. Then replace the finned surface with the pinned surface plate and repeat the experiment. An equation for the surface areas (As) of the finned and pinned plates are shown below:

A finned  9  L w  Abase plate

Apinned  17 

 DL

plate

Table B Air Velocity [m/s]

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 Abase

Finned Plate

Heater Temperature (Ts) [°C]

Ts-T

h

[°C]

[W/(m·°C)]

0 1.0 2.0 2.5

Table C Air Velocity [m/s]

Pinned Plate

Heater Temperature (Ts) [°C]

Ts-T

h

[°C]

[W/(m·°C)]

0 1.0 2.0 2.5

6.

Plot graphs of air velocity versus surface temperature (T s-T) of each plate. Compare the ability of each plate surface to disseminate heat by convection.

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