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Heat Transfer Wind Tunnel

Spring 2016  MAE 156B Sponsored Project

University of California, San Diego

Elizabeth Huang - Austin Lee - Tian Shi - Anthony Tran

Sponsored by 

Nicholas Busan

Mechanical and Aerospace Engineering Department

Advised by 

Kalyanasundaram Seshadri

Background

The project was for the Mechanical and Aerospace Engineering (MAE) Department at UC San Diego, specifically for MAE171A. MAE171A is a mechanical engineering laboratory course where undergraduate students perform experiments tailored towards specific engineering disciplines, such as solid mechanics, vibrations, and linear circuits. Nicholas Busan, Associate Dev Engineering-Supervisor of all the MAE laboratories, and Dr. Kalyanasundaram Seshadri, a professor specializing in combustion, recognized the need for redesign of the experiment pertaining to fluid mechanics and heat transfer due to lack of conformity with theory. The former heat transfer wind tunnel used for the experiment consisted of a very small testing chamber and a flat heating plate. The goal was to change the orientation of the heated copper plate, so that stagnation-point flow was modeled instead of external flow over a flat plate.

Objectives

The goal was to complete a working experiment setup for mechanical engineering students in the MAE171A class in the upcoming quarter. The following requirements of the final design were set with input from the sponsor Nicholas Busan and the advisor Kalyanasundaram Seshadri.

Final Design

The redesigned MAE171A heat transfer experiment setup needed to accurately model axisymmetric stagnation point flow and have controls that allow for accurate changes of parameters such as temperature and flow speed. Therefore the final design solution was a vertically 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. Within the wind tunnel is a layer of honeycomb and several layers of 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 flush against the plate. Surrounding the copper plate is Promalight microporous 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. The fan is PWM controlled using LabVIEW while the heating pad is attached to a LabVIEW temperature controller to produce accurate temperature set points. 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. They are wired to LabVIEW which displays the temperature and enables students to easily record temperature measurements at free stream and at the copper plate.

 

Performance Results

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 4.1 m/s.

Theoretical Heat Transfer Coefficient Values

The team first verified the flow velocity at maximum fan speed. Then, using a hot wire thermo-anemometer, the velocity at three points across the flow duct exit was recorded. The average flow velocity was then used to calculate the Reynolds number.

Flow Velocity Measurements

When the displayed temperature plot leveled out, meaning that steady-state temperature was reached, the displayed power-to-heater value was recorded. The power measurements and plate temperature readings were then used to calculate the convective heat transfer coefficient for the varying flow velocities. The results can be seen as follows:

Experimental Heat Transfer Coefficient Values

The flow velocity measurements at maximum fan speed revealed a few significant things. Firstly, a maximum airflow velocity of 5.4 m/s was achieved, which was greater than the desired 3 m/s. This gives room for future expansion of the MAE 171A lab scope. Even at maximum temperature, the Reynolds number was less than 5x105, meaning the flow was laminar, as the theory calls for. Secondly, the velocity profile was fairly uniform, with variation of at most 17%. Since the maximum velocity was greater than desired, more meshes can be added to further increase the flow uniformity.

There was some discrepancy between the experimental and theoretical heat transfer coefficient hc values, particularly for forced convection at the lowest temperature set point (about 64 oC). The heat transfer coefficient hc is a function of the Reynolds number and therefore airflow velocity, and should not be affected by changes in temperature. However, as seen in the Experimental Heat Transfer Coefficient Values table, there was some variation in the hc values at each average airflow velocity. In fact, the power values, and thereby the hc values, did not make sense at the lowest temperature set point. For the lowest temperature set point, as the average airflow velocity increased, hc decreased from the free convection value, when it should be greater at higher speeds. This resulted in a percent error of up to 64%. Excluding the lowest temperature set point data, however, the largest percent error was 1-24%, so the experimental matched the theoretical for other temperature set points.

More data needs to be collected in order to pinpoint the cause and solution to the incorrect hc values at the lowest temperature set point. Despite this issue, there was reassurance in that the rest of the hardware and the controllers were working as intended and that the experimental setup conformed to theory.

Click to view Executive Summary