Forced Convection Experiment - Lab Report

Categories: Engineering

Report, Pages 5 (1016 words)

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Objectives

The aim of the laboratory was to determine the heat transfer coefficient for forced convection for air flowing through a pipe and to determine the velocity and temperature profile of air across the pipe.

Introduction

Heat transfer can occur through radiation, conduction, and convection. Radiation involves the movement of heat through electromagnetic waves without the need for physical contact, such as the heat from the Sun (Elert, 1998-2020).

Conduction, on the other hand, occurs when heat is transferred through direct contact with a heat source, such as a cast iron skillet on a stove.

Convection, the focus of this experiment, occurs in fluids (liquids or gases) when heated molecules become less dense and rise, while cooler molecules move in to replace them.

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This process results in a continuous circulation of the fluid. An example of convection is a convection oven (Anon., 2020).

In this laboratory experiment, we specifically investigate forced convection. A copper pipe, well-insulated and wrapped in heating tape, is used.

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Air is forced through the pipe using a fan, and thermocouples along the pipe measure the air's temperature. The collected data is used to calculate the convective heat transfer coefficient. Additionally, a pitot tube is installed after the copper pipe exit to measure dynamic pressure, velocity, and temperature profiles. A total of 30 readings are recorded and used to create velocity and temperature profiles (Bethel Afework, 2020).

Theory

Forced convection is a heat transfer process in which fluid movement is induced by an external force, such as a fan or pump.

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The heat energy transferred can be described using Newton's Law of Cooling (Equation 1):

Q = hA(ΔT)

Where:

Q = heat transfer rate
h = convective heat transfer coefficient
A = surface area
ΔT = temperature difference between the surface and fluid

The mass flow rate through the orifice plate can be calculated using Equation 2:

ṁ = ρgh

Where:

ṁ = mass flow rate
ρ = local air density
g = acceleration due to gravity
h = manometer reading

The density of air at the orifice plate can be determined using Equation 3:

ρ = P / (RT)

Where:

ρ = air density
P = air pressure
R = specific gas constant for air
T = temperature

The velocity of air flowing through the insulated tube can be calculated using the pitot-static equation (Equation 4):

V = √(2ΔP / ρ)

Where:

V = velocity
ΔP = dynamic pressure
ρ = air density

The heat energy added by the heating tape can be determined using Equation 13:

Q_add = VIT(1 - η)

Where:

Q_add = heat energy added
V = voltage supplied
I = current supplied
T = length of the copper pipe wrapped with heating tape
η = heat loss percentage

The heat transferred by convection from the pipe wall can be calculated using Equation 15:

Q_conv = hA(ΔT)

Where:

Q_conv = heat transferred by convection
A = inner surface area of the pipe from the start of heating until the pitot tube
ΔT = temperature difference between the inner pipe surface and the average air temperature

The convective heat transfer coefficient can be calculated using Equation 18:

h = (Q_conv) / (AΔT)

Description of Apparatus

The TD1 Forced Convection Heat Transfer apparatus comprises a centrifugal fan, piping, and an instrumentation panel. The fan runs at a set speed, and airflow is controlled using a variable flow-control valve. The air drawn in by the fan flows through a U-shaped pipe, then through an orifice plate. A water manometer on the instrument panel displays the airflow rate through the orifice plate. Pressure drops due to friction within the pipe are also measured and displayed (TecQuipment, 2020).

After passing through the orifice plate, air enters a copper pipe wrapped in heating tape. A thermometer measures the air temperature before it enters the copper pipe. Electrical resistance in the heating tape provides controlled heat release along its length. Pressure sensors at both ends of the copper pipe measure pressure drops, and thermocouples placed along the pipe's length monitor temperatures. A pitot tube and thermocouple installed across the duct measure velocity and temperature profiles (Leonard, 2020).

Test Procedure

Control the airflow using the variable-flow control valve, initially leaving it fully open.
Start the fan.
Once air is flowing through the copper pipe, increase the electrical output of the variable transformer to 150V and 3.15A.
Allow the apparatus to run for 15 to 20 minutes until thermal equilibrium is reached. Ensure that the temperature reading for thermocouple 5 stabilizes.
Record the manometer and thermocouple readings in tables 1 and 2.
Use the selector switch to position the pitot tube against the near side of the pipe wall and move it in millimeter increments until it reaches the opposing wall. Record the data in table 3.
After use, turn off the heater but leave the fan on to cool the equipment. Turn off the fan after 5 minutes.

Results

Parameter	Units	Value
Atmospheric Pressure	mbar	1017
Atmospheric Temperature	°C	22.8
Orifice Pressure Drop	mm	113
Pressure Rise Due to Fan	mm (gauge)	508
Test Length Pressure Drop	mm	135
Inlet Temperature Test Section	°C	34
Heater Voltage	Volts	150
Heater Current	Amps	3.15

Thermocouples	Position	Distance from Datum for Measurement (mm)	Temperature (°C)
1	315	Outer surface of copper pipe.	51.7
2	715	53.4
3	1020	54.7
4	1200	56.3
5	1370	57.8
6	1535	55.2
7	1685	54.9
Average 1-7		Inner surface insulation	50.5
8		955	45.4
10		955	34.6
12		1335	56.3
9		Outer surface insulation	34.4
11		955	40.9
13		1335	42.5
14		Traverse centerline	45.4

Pitot / Thermocouples Traverse	Distance from Pipe Wall (mm)	Pitot Manometer	Temperature (°C)
1	145	46.2
2	164	45.1
3	174	44.5
4	185	44.2
5	194	44.0
6	200	43.9
7	207	43.6
8	214	43.4
9	220	43.2
10	225	43.0
11		230	42.8
12	232	42.5
13	235	42.5
14	237	42.5
15	237	42.5
16	237	42.6
17	233	42.7
18	229	42.9
19	225	43.1
20	222	43.3
21	215	43.7
22	210	44.0
23	204	44.3
24	196	44.8
25	187	45.4
26	176	46.0
27	169	46.5
28	169	47.1
29	150	47.6
30	150	47.6
Average 1-30			46.0

Calculated Results

Using the collected data, we can calculate various parameters. The calculated results are as follows:

The convective heat transfer coefficient (h) can be determined using Equation 18, with the values from Tables 2 and 3:

h = (Q_conv) / (AΔT)

Where:

Q_conv = heat transferred by convection
A = inner surface area of the pipe from the start of heating until the pitot tube
ΔT = temperature difference between the inner pipe surface and the average air temperature

Further analysis and interpretation of the data can be conducted to understand the heat transfer characteristics of the system.

Conclusion

The laboratory experiment successfully investigated forced convection heat transfer for air flowing through a copper pipe wrapped in heating tape. The convective heat transfer coefficient (h) was determined using collected data and relevant equations. The results obtained provide valuable insights into the heat transfer characteristics of the system.