A mach 1.95 free-jet facility for experimental investigation of injectant flow patterns

Abstract

Inspired by the need to study injectant flow patterns near the test surface, a supersonic free-jet facility was designed and constructed. This facility provides a Mach 1.95 flow over a test section area of 6.35 cm by 5.08 cm. The facility was thoroughly tested and proved that it is durable, versatile, and capable of providing repeatable test conditions. When compared to the main supersonic tunnel at VPI & SU, the free-jet facility has many advantages, including greater optical and spatial access, longer available test durations, and less time needed between tests for the storage tanks to be refilled. As a part of the project, several diagnostic techniques were evaluated in the facility. Normal, sonic injection of helium through a circular injector was studied as a way to evaluate nanoshadowgraph photography, oil flow photography, and infrared imaging as qualitative flow visualization methods. Quantitative measurements of the local helium concentration within the boundary layer were taken to evaluate the effectiveness of an existing hot-film concentration probe. The tests showed that oil flow visualization is a very effective technique in the free-jet facility, producing clear photographs that could be directly scaled. Nanoshadowgraph photography also produced clear photographs of the flow field, although this method was more difficult to implement in the free-jet facility than in the main supersonic tunnel. Finally, infrared imaging, which was not possible in the main supersonic tunnel without major hardware reconfigurations, showed great promise as a method for studying normal injection. Although the tests revealed no conclusive information about injectant flow patterns in the boundary layer, a heat transfer analysis showed that it may be possible in future studies to use this technique as a way to quantitatively measure local helium concentration at the surface. Helium concentration was measured for test cases with both air and helium injection. The measurements were taken at two axial locations with the probe positioned at a distance of 0.1 injector diameters above the test surface. The air injection tests showed helium concentration levels up to 30 percent mole fraction, which is erroneous since no helium was present in the flow. Based on these results it was concluded that the existing probe was ineffective in the free-jet facility, with an uncertainty level of no less than 25 percent mole fraction helium. The cause of the high uncertainty and overall ineffectiveness was most likely the probe calibration and data reduction methods.