Final Design

Description of Final Design Solution

The MAE171A heat transfer experiment setup was redesigned to accurately model axisymmetric stagnation point flow and to have controls that allowed for accurate changes of parameters such as temperature and flow speed. The design solution consisted of a wind tunnel and a heated copper plate, as seen in figure #.

Design Solution

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 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. 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.

Final Design Wind Tunnel

Wind Tunnel Design

The wind tunnel, was determined to be an acrylic cylinder with an inner diameter of 8.255 cm, slightly greater than the copper plate diameter of 7.62 cm to ensure that the entire copper plate experienced perpendicular uniform flow. The wind tunnel was a mere 15 cm in length to minimize the boundary layer development. A section of honeycomb was placed immediately after the fan to reduce the non-uniform characteristics of the rotating blades’ airflow. Meshes were then placed in order of decreasing opening size at the exit of the wind tunnel to ensure uniform flow. An RTD was also positioned within the wind tunnel to measure the free stream temperature.

Final Design Heated Copper Plate and Insulation

Cross-Sectional View of Copper Plate and Insulation Configuration

The heated plate was made from a tellurium copper alloy due to its high thermal conductivity and machinability. An Omega heating pad was used to heat the copper plate. The heating pad has a watt density of 15500 W/m2, which provides more than sufficient power to heat a 0.635 cm-thick copper plate fast enough that students would be able to reach multiple temperature set points within the time frame designated for the experiment. The two insulation materials utilized were Promalight microporous material— wherever structural integrity was unnecessary— and Teflon PTFE— where material strength was necessary. In order for the copper plate to be held concentric to the wind tunnel to simulate stagnation point-flow, the copper plate-insulation setup was attached to a plate mount. Flathead screws clamped the Teflon PTFE and the aluminum base together such that the heating pad and copper plate were spring-loaded. An aluminum stand of 10.16cm was attached to hold the entire setup upright; its length was ample to ensure that the axisymmetric flow around the plate was undisturbed. Two pockets were machined into the sides of the copper plate for placement of the RTDs. Though the temperature distribution of the copper would essentially be uniform, having two RTDs for an average value allowed for identification of any error.

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

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.