Flow Characterization of a Hypersonic Expansion Tube Facility for Supersonic Combustion Studies

Abstract

A hypersonic expansion tube facility was designed and constructed to primarily study supersonic combustion phenomena. This paper provides an overview of the range of conditions the facility is capable of generating along with a discussion on how well a number of parameters required to accurately replicate conditions for supersonic combustion are achievable. A series of experiments using three low-to-mid enthalpy conditions were used to characterize and assess the extent of impact that flow-nonidealities have on the final test gas properties. Aspects such as shot-to-shot repeatability, temporal and spatial test gas uniformity, boundary-layer properties, and the presence of flow disturbances are analyzed. The shot-to-shot variation in the measured shock speeds corresponded to acceptable bulk variations of the aerothermodynamic properties of the test gas flow. Temporally, both the static and total pressures increased over the test gas slug, offsetting each other in a manner that kept the inferred test gas Mach number constant. Spatially, it was found that the core flow size increases with increasing test gas pressure and to a first order can be approximated to span $2/3$ of the pipe diameter for the range of conditions considered. The presence of test gas flow disturbances was found to be negligible as the conditions studied exhibited a weak frequency focusing effect. Lastly, an improved method of acquiring high signal-to-noise ratio static pressure measurements in such impulse facilities was demonstrated to reduce measurement noise by as much as 70%.