lab 2

“Heat Conduction”

Introduction

Thermal conduction is a mode of heat or energy transfer that occurs in a solid or fluid due to the presence of temperature gradient. It is the transfer of energy from the more energetic particles to the adjacent less energetic particles in the form of interactions between the particles. Simply put, it is defined as heat transferred by molecules that travel a very short distance before colliding with another molecule and exchanging energy.

Theory

There are two main studies that the thermal conduction is based on:

1. Linear conduction heat transfer
Heat is applied to one end of a bar which results to an elevated temperature at that end and causes the heat to flow towards the cooler end. This will make a gradual increase of temperature at the cooler end until an equilibrium temperature is established. Usually, the changes that occur within the bar is not instantaneous and will be different at both end.

2. Radial conduction heat transfer
Fluid motion that resulted by force over the surface by an external source, such as fans or pumps. Small movement of air is generated which limits the heat transfer rate from the hotter surface to the surrounding air. Therefore, more heat is transferred when the fluid velocity is increased over the heated surface.

Objectives:

- To study the Fourier’s Law on linear and radial conduction heat transfer.
- To illustrate the transfer of heat by conduction in solid materials.

Equipments Required:

- PHYWE Heat Conduction Study Bench Unit (Model FF105)
- Thermocouples

Working Procedures:

Part A – Linear Conduction
1. The power cable of the linear test unit is connected to the display unit.
2. The brass specimen is inserted to the test unit.
3. Thermocouples are inserted to their respective slots (by following the labels 1 to 8).
4. The equipment is turned on by turning the main power knob at clockwise direction.
5. Water flow is set to 1.4 L/min.
6. From the display, press the ESC button (top right) and then the F1 button to choose the cylindrical test unit.
7. Heater switch is pressed to switch in the heater and the power knob is set for 10 W by checking the display unit.
8. Wait until the steady state is achieved (by checking the temperature readings).
9. The temperature, thermal conductivity and other informations are to be recorded.

Part B – Radial Conduction along circular metal plate
10. The power cable of the radial test unit is connected to the display unit.
11. Thermocouples are inserted to their respective slots (by following the labels 1 to 6).
12. The equipment is turned on by turning the main power knob at clockwise direction.
13. Water flow is set to 1.4 L/min.
14. From the display, press the ESC button (top right) and then the F2 button to choose the radial test unit.
15. Heater switch is pressed to switch in the heater and the power knob is set for 10 W by checking the display unit.
16. Wait until the steady state is achieved (by checking the temperature readings).
17. The temperature, thermal conductivity and other informations are to be recorded. Data Collection

Linear Conduction
I =
V =
Q =
Inner radius =
Outer radius =
Thermal conductivity, K =

Power (W) 10
Specimen 25mm diameter brass T1 (°C)
T2 (°C)
T3 (°C)
T4 (°C)
T5 (°C)
T6 (°C)
T7 (°C)
T8 (°C)

Radial Conduction
I =
V =
Q =
Inner radius =
Outer radius =
Thermal conductivity, K =

Power (W) 10
R1 (°C)
R2 (°C)
R3 (°C)
R4 (°C)
R5 (°C)
R6 (°C)

Graphs:

Calculations:

Analysis and Discussion:

Theoretically, changing the radius means that the area is changed. Based on the Fourier’s equation, the heat transfer is proportional to the area of the material. Since the power input was set equal for both cases of aluminum radius of 2 cm and 2.5cm, the only parameter affected was the thermal conductivity calculated. Since there are variation in the temperature reading collected, the thermal conductivity for the material was found to be different which in this case should be the same.

The graphs show the same characteristic, which can be described in three sections. The first section of the graph is in the heater region ranging 1-3. The temperature gradient along these points is decreasing. The second section is in the material or specimen region ranging from 3-6. This region is the vital part of all because it shows the behaviour of the temperature across the surface of the material. The temperature gradient in this region is decreasing. The third section is in the cooling section ranging from 6-8 and as expected the temperature along these points also decreasing.
From the calculated thermal conductivity, k, it was found that the error was so great that the Fourier’s Law was not found to be valid in this experiment. This may be due to some error which could not be avoided such as a malfunction in the probe that stops us from getting the correct temperature of the cylinder. As a conclusion, it was found that the Fourier’s Law explained the behavior of the temperature in the linear and radial heat conduction. The temperature gradient in the linear heat transfer was found decreasing along the material from the hot surface end to the cold surface end. The heat transfer also depended on the area of the cylinder where the higher the area, the more the heat can be transferred. The radial heat transfer also exhibited the same results as the linear conduction. The temperature gradient of the brass was also found to decrease away from the heat source to the outer most radial displacement.

Conclusion:

Through this experiment, we had concluded the practical use of extended surfaces that is used to improve heat transfer from a surface. We also had managed to experiment on different type of extended surfaces that had different effects on the flow velocity as seen from our discussed findings. Since these are based on the convective heat transfer entirely, it is safe to say that they can be improved by the use of these extended surfaces, often in conjunction with a fan.

Recommendations:

This experiment has really benefited me a lot in understanding the convective heat transfer that will definitely be needed in the future reference.
However, since this was a replacement experiment, we were limited by some time constraint that we could have adhere during the standard experimental procedure.

Despite of this, the demonstrator has done his best at giving us all the information we needed to know for this experiment and for future usage.

References:

Moran, Shapiro, Munson, DeWitt. (2003). Introduction to Thermal Systems Engineering: Thermodynamics, Fluid Mechanics, and Heat Transfer. John Wiley & Sons.

Manufacturing Engineering Lab II: Laboratory Manual. Department of Manufacturing and Materials Engineering. (2013).

Various internet resources. Retrieved December 10th 2013.

References: Moran, Shapiro, Munson, DeWitt. (2003). Introduction to Thermal Systems Engineering: Thermodynamics, Fluid Mechanics, and Heat Transfer. John Wiley & Sons. Manufacturing Engineering Lab II: Laboratory Manual. Department of Manufacturing and Materials Engineering. (2013). Various internet resources. Retrieved December 10th 2013.