Forced Convection Heat Transfer Essay Example

and the voltage control was adjusted to a maximum of 4 amperes at least an hour in advance by the lab TA.

The experiment includes two parts: Axial profile and Radial profile measurements. For the Axial profile, values from Table 1 are read from the respective displays. This process is repeated for 3 additional sets, with a time interval of 5 minutes for each set. The temperatures listed in Table 1 (T1 to T7) represent either the surface or wall temperatures of the test section.

Radial profile

In the Radial profile, values from Table 2 are read once. The measured temperatures in Table 2 (Te,1 to Te,13) correspond to air temperatures. It is recommended to allow the apparatus to run for 30 minutes to ensure that the test conditions stabilize.

The calculation method used for the axial profile is the Formula 1. This formula determines the volumetric flow rate of air at the orifice plate, represented by Qo, in cubic meters per second. The formula uses variables such as the discharge coefficient, Cd, which is equal to 0.63, and the area of the orifice plate, represented by Ao, calculated using the formula do^2/4. The pressure drop at the orifice is denoted as Porifice and the air density is also taken into consideration.

Additionally, Formula 2 emphasizes the importance of the method or theory of problem solving. Laboratories provide valuable experimental knowledge that helps us understand the origin of certain values used in solving complex problems. This understanding highlights the significance of these values for students.

At this lab, an experiment was conducted to determine various properties of convective heat transfer. By analyzing the obtained results and performed calculations, a wide range

of information can be inferred. Table 1 displays the power input at each thermocouple location, which exhibits a ramp-like increment. This ramp pattern is a result of increased air exposure to the heating wires. Table 2 provides an assessment of the accuracy in raising the air flow temperature and confirms the expected outcomes from Table 1. According to expectations, with a constant heat flux, the temperature difference is the primary factor influencing the convective heat transfer coefficient. The examination corroborates this theory. Examining the results from Table 3 reveals a higher coefficient at the initial thermocouple location, owing to a greater temperature disparity between the cool air and hot wall. As the flow progresses axially, (h) decreases as the air temperature approximates the wall temperature. However, as the wall temperature increases, (h) starts to rise again.

The following table displays results obtained from the radial section of the lab. It presents data on Pressure in head(m), Pressure drop across diameter of pipe (m), Velocity of air(m/s), Exit temp (C), Bulk temp rise (C), and Radial Position (in). Examining the pressure section reveals that the highest pressure is located in the center of the pipe. As we move radially outward, the pressure decreases. This outcome contradicts a well-established principle of fluid dynamics known as boundary layers.

The theory of boundary layers states that the fluid touching a surface has zero velocity as it forms a layer on the surface. This should theoretically result in increased pressure. The other values in the table are what we would expect, with exit temperature and bulk temperature rising outward radially, and velocity and pressure dropping radially. Looking at Figure 2 (Exit velocity

and exit temperature versus radial position), we can observe that the temperature curve is roughly opposite to the velocity curve of the fluid being examined.

According to this experiment, a higher velocity of the fluid results in greater heat transfer. In other words, fluid with higher velocity is more effective at removing heat compared to fluid with lower velocity. However, it is worth noting that this experiment may have some bias as the heating element is placed around the pipe. To enhance accuracy, a better approach would be to position the heating element at the center of the pipe's radius. Figure 3 illustrates the relationship between the pipe's wall temperature and its length. Ideally, this graph should display a linear relationship due to the consistent heat input as a ramp function. However, there is an outlier present, which hinders a completely linear relationship.

The outlier in figure 3 is due to errors and uncertainties encountered in the laboratory. Like all experiments, this lab has sources of error including inaccuracies in the thermocouples and pitot tubes, as well as the digital displays' ability to convert measurements to a digital output. The human element is often the biggest source of error, with mistakes potentially occurring during calculations and when obtaining properties from different resources. Calculation errors and the reliability of references are likely the main sources of error for this lab. In the reporting section, some obtained or calculated values were compared, starting with the comparison between the integrated mass flow rate (as shown in fig 3) and the calculated value.

The obtained value from integration is 0.1440, while the calculated value is o.0339 (kg/s). This significant error can be

attributed to the low quality of the function generated from excel and the inherent inaccuracy in computer-based integral evaluation. The second comparison involves the integrated value of the bulk temperature rise and its calculated value.

277.9K is obtained from integration while 318.88K is derived from calculation. This substantial error can be attributed to the same sources that impacted the previous integration. Additionally, the calculated results indicate that the Nusselt number (N) equals F1=0, serving as another point of comparison.

The friction factors obtained, with a constant Reynolds number equal to 3.28, are 0.0159 while the correlations produce a friction factor of 0.0196 with a Reynolds number of 160.15. Although the difference between the friction factors is relatively small, there is a significant discrepancy in the Nusselt numbers. This discrepancy can be attributed to an unreliable reference used to determine various properties of air, including the specific heat capacity (Cp) value.

If we examine the moody diagram, we can observe that the flow is turbulent based on the friction factors. Both axial and radial temperature profiles play a crucial role in this laboratory. Due to its significance, let's briefly discuss some of the factors that impact these profiles. The amount of heat introduced into the fluid flow through the pipe determines the axial temperature. While the heat transfer to the fluid flow is a major determinant of the temperature profile, velocity also plays a role. Higher velocity efficiently removes heat from the heating element but also decreases fluid temperature since there is less time for heat exposure.

The radial temperature profile is influenced by the positioning of the heating element, the fluid velocity, and the roughness of the

pipe. It is logical that the temperature of the fluid will rise as the radial distance increases since the heating element is placed around it. Additionally, the boundary layer formed along the inner diameter of the pipe causes the fluid to remain stationary. This stationary fluid has a longer exposure time to the heating element, resulting in a higher temperature accumulation. The size of the boundary layer is determined by the roughness of the pipe, assuming a constant fluid flow.

Throughout this lab, our main focus has been on bulk temperature, which is similar to energy in terms of its difficult definition. In essence, temperature can be described as the characteristic that measures how hot or cold an object is. However, it is important to understand that temperature is not just a property; it is also a concept. The bulk temperature represents the average value obtained from all the temperatures observed across the fluid moving through the pipe's cross-section. It is crucial to avoid confusing temperature with heat - while temperature assesses levels of hotness or coldness, heat relates to a specific form of energy.

Conclusion

This laboratory was focused on forced heat convection heat transfer. The experiment involved using a fan to force air through a heated pipe. Various quantities such as bulk temperature raise, friction factor, and Reynolds number were calculated based on the obtained results. The main findings of the laboratory were the calculation of the convective heat transfer coefficient and understanding how it was influenced by different aspects of the experiment.

The laboratory was considered successful in both educating the student and achieving its objectives. Some recommendations include reducing the calculation portion

of the laboratory as it was too long, which hindered the focus on the laboratory itself. Additionally, it would be beneficial for all labs to closely align with the course content.

References

1. Heat Transfer Laboratory, Department of Mechanical Engineering, The University of New Brunswick.
2. ME3415: thermodynamics/ ME3435Heat Transfer Laboratory Format, Department of Mechanical Engineering, The University of New Brunswick.
3. Archimedes: A Gold Thief and Buoyancy/ Larry "Harris" Taylor, PhD
4. Engineering tool box/air properties, 156. html. VIII Appendices