Executive Summary

This project was sponsored by Nicholas Busan, the supervisor of undergraduate Mechanical and Aerospace engineering laboratories at University of California, San Diego. The objective of the project was to improve the heat transfer wind tunnel experiment that was part of the curriculum for the Mechanical Engineering Laboratory course, MAE171A.

The requirements of the improved experiment setup included a more accurate-to-theory wind tunnel and temperature controller. The redesigned experiment modeled stagnation point flow— the flow was perpendicular to a heated plate. The wind tunnel setup needed to be simple for students to use and to allow them to gather accurate power, temperature, and airflow velocity measurements for their experimental calculations. This required that the wind tunnel display uniform flow that was not disrupted by corner boundary layers and that the copper plate was heated quickly with a uniform temperature distribution.

Figure 4: Full assembly of heat transfer experiment setup

The final design solution was a vertical oriented wind tunnel equipped with a fan, heated plate, temperature sensors, and temperature and fan speed controller. The wind tunnel is an acrylic tube with an inner diameter of 8.255 cm. Air is pushed through the wind tunnel using a Sunan Fan PF80381BX which is PWM controlled using LabView. Within the wind tunnel is a layer of honeycomb and mesh screens with varying fineness to create uniform flow. At the base of the wind tunnel is a copper plate, which is heated using an Omega heating pad. The heating pad is attached to a LabView temperature controller to produce accurate temperature set points. Surrounding the copper plate is promalight insulation and teflon PTFE which ensures the heat from the Omega heating pad is transferred to the surface of the copper plate and there is no other significant heat loss. Finally, the temperature of both the airflow within the wind tunnel and at the copper plate will be measured using RTDs. RTDs are located within the copper plate and on the inner side of the wind tunnel duct, and are wired to LabView which displays the temperature enabling students to easily record temperature measurements at free stream and at the copper plate.

When testing the heat transfer wind tunnel, the team first calculated the theoretical Nusselt number and convective heat transfer coefficient for flow speeds of 2.5 and 5.4 m/s.

Table 12: Theoretical heat transfer coefficient values

The flow velocity was then measured using an anemometer at several locations to get the flow velocity distribution. The average flow velocity was then used to calculate the Reynolds number.

Table 13: Flow Velocity Measurements

In conclusion, there was a large discrepancy between the experimental and theoretical heat transfer coefficient values. Additionally, the power values did not logically make sense. For example, at 2.5 m/s, the power consumed was much greater than that at a higher velocity of 5.4 m/s. This resulted in heat transfer coefficient values that did not make logical sense: at the same Tw, the hc at 2.5 m/s was 375.8 W/m2K compared to 83.51 W/m2K at 5.4 m/s. These discrepancies were due to the power-sensing circuit not working as intended since it could not account for the rapidly fluctuating power consumed—from the temperature controller turning the heating pad on and off. An approximated average was calculated by reading the fluctuating power values, thereby creating large random error.

Despite the unreliable data gathered from the test, there was reassurance in that the rest of the hardware and the controllers were working as intended. Also until the power-sensing circuit is fixed, the discrepancy caused by the non-uniform flow cannot be pinpointed. However, Dr. Seshadri assured that it could be accounted for through calculations. Additionally, for future use, the air source could be replaced with one that supplies greater air pressure—rendering the stagnation point pressure drop negligible—since the mechanical setup is easily adaptable.