Computational and Experimental Investigation of Supersonic Convection on a Laser Heated Target

Abstract

This research concerns the development and validation of simulation of the beam-target Interaction to determine the target temperature distribution as a function of time for a given target geometry, surface radiation intensity and free stream flow condition. The effect of a turbulent supersonic flow was investigated both numerically and experimentally. Experiments were In the Virginia Tech supersonic wind tunnel with a Mach 4 nozzle, ambient total temperature, total pressure of 1.1 x 10⁶ Pa and Reynolds number of 5 x 10⁷/m. The target consisted of a 6.35 mm stainless steel plate painted flat black. The target was irradiated with a 300 Watt continuous beam Ytterbium fiber laser generating a 4 mm Gaussian beam at 1.08 micron 10 cm from the leading edge where a 4 mm turbulent boundary layer prevailed. An absorbed laser power of 65, 81, 101, 120 Watts was used leading to a maximum heat flux between 1035 to 1910 W/cm². The target surface and backside temperatures were measured using a mid-wave infrared camera. The backside temperature was also measured using eight type-K thermocouples. Two tests are made, one with the flow-on and the other with the flow-off. For the flow-on case, the laser is turned on aft.er the tunnel starts and the flow reaches a steady state. For the flow-off case, the plate is heated at the same power but without the supersonic flow. The cooling effect is seen by subtracting the flow-on temperature from the flow-off temperature. This temperature subtraction is useful in canceling the bias errors such that the overall uncertainty is significantly reduced. The GASP conjugate heat transfer algorithm was used to simulate the experiments at 81 and 65 Watts. Most computations were performed using the Spalart-Allmaras turbulence model on a 280,320 cell grid. A grid convergence study was performed. Compared to the 65 Watt case, the 81 Watt case displays more asymmetry and a region of increased cooling is found upstream. The increased asymmetry was also seen on the backside by both the thermocouple and infrared temperature measurements. The computation underpredicts the surface temperature by 7% for the flow-off case. For both thef 65 and 81 Watt cases, cooling is underpredicted at the surface near the center. For all power settings, convective cooling significantly increases the time required to reach a given temperature.

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