Specific Tasks at the University of Texas at Austin Task 8 Installation of a hot streak generator and baseline tests

Hot Streak Installation

The electric heater used to produce the hot streak was installed in a tunnel section immediately upstream of the plexiglass test section. As mentioned in the previous report, the electric heater is a nominally 20 x 20 cm square duct heater with a heating capacity of 7kW, which may be run at full and half power. The heating element is contained within the square duct section followed by a transition section leading to an 20 cm diameter round section, i.e. 0.44P, where P is the pitch between airfoils. The tunnel section constructed to contain the hot streak generator has the same dimensions as the entrance to the plexiglass test section with a height of 55 cm and width of 102 cm. The tunnel section is 117 cm long in the streamwise direction and is situated immediately downstream of the converging or nozzle section of the wind tunnel. The duct containing the heater coils is suspended on steel rods, which support the heater at the center of the tunnel section vertically (or spanwise) and allow continuous movement of the hot streak in the horizontal (or pitch) direction. As expected, preliminary measurements indicate a sizeable velocity defect due to the heater coils and supports within the hot streak generator. The velocity reaches a minimum of 0.5*U0 where U0 is the mainstream velocity. Future work involves adding resistance to the air flow in the area surrounding the hot streak generator to induce a uniform velocity across the tunnel in both the spanwise and pitch directions.

Measured Temperature Profiles

Temperature profiles were measured at several positions downstream from the heater exit. An initial profile at 0.77 chord lengths downstream from the heater exit indicates a maximum temperature ratio of T/T0 = 1.18 using only the upstream heater stage with 3.5 kW of heating. For this airfoil, the chord length is 59.4 cm. As seen in Figure 11, the profile is peaked with the maximum value at 0.56*S where S = 53.5 cm is the span. Since there is large velocity defect through the hot streak generator, the temperature rise is much greater than was predicted assuming uniform flow throughout the tunnel. Figure 11 also shows a temperature profile at 1.38 chord lengths downstream of the heater before the flow has reached the geometric leading edge of the vane. This profile was taken about 0.25 chord lengths (or 15 cm) upstream of the leading edge before the vane has affected the streamlines. This temperature profile reaches a maximum temperature ratio of T/T0 = 1.17 at 0.59*S. Both temperature profiles were taken at a position where the temperature has reached a maximum in the pitch direction and this coincides with the vertical centerline of the hot streak generator. The third profile was taken 18 cm behind the trailing edge of the vane or about 3.0 chord lengths downstream of the heater exit. This profile was taken with the hot streak generator positioned to pass the hot streak through the center of the passage between vanes. Again, the spanwise temperature profile was made where the temperature ratio reaches a maximum in the pitch (horizontal) direction, and reaches a maximum value of T/T0 = 1.07 at 0.66S. It is believed that buoyancy or secondary flows in the tunnel contribute the change in the position of the hot streak as it moves downstream. Lines indicating the top, centerline, and bottom of the hot streak generator are provided for comparison with the temperature profiles. For the profiles upstream of the vane, the hot streak is approximately the same height as the hot streak generator. Behind the trailing edge, the hot streak width in the vertical direction has shrunk significantly. The reduced centerline temperature of the hot streak as it passes through the passage is an indication of a relatively strong dispersion of the hot streak. At this point, it is probably an artifact of the velocity defect within the hot streak, which will cause high turbulence levels. After the facility is modified to produce a uniform velocity profile, we expect that the hot streak will remain much more coherent through the passage.

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