The objective of this thesis was to use the X2 expansion tube to produce the high Mach number and high total pressure scramjet flow conditions associated with access to space. Initial experimental attempts to produce a Mach 13 condition indicated that the existing free-piston driver arrangement, based on a 35 kg piston and 100% helium driver gas, did not produce high pressure driver gas for sufficient duration. Following expansion of the driver gas, the expansion wave processing the driver gas reflected off the piston face, interfering with the test gas prior to its arrival in the test section. The result was significant attenuation of the primary shock prior to its arrival in the test section. It was determined that a tuned driver condition could provide a significantly longer duration of high pressure driver gas; achieving this operating condition subsequently became the first major task of the investigation.
Tuned operation involves configuring the driver so that the piston is moving sufficiently fast following primary diaphragm rupture that the piston displacement compensates for driver gas loss to the driven tube. This can result in approximately constant driver pressures for a relatively long duration of time. An analysis of X2's free-piston driver indicated that for X2's relatively short (4.5 m) compression tube, tuned operation requires a very lightweight piston (approximately 10 kg). The tuned piston must be light so that it can be first accelerated to a high speed (>200 m/s), and then brought to rest, over the short compression tube length. A new 10.5 kg lightweight piston for X2 was developed, and three new tuned driver conditions were developed.
The theoretical performance envelope of X2 with the new driver was then investigated, and a set of new scramjet flow conditions was proposed based on analytical relations which were later refined using the 1-D CFD code L1d2. The final task in this study was to assess the new flow conditions both experimentally in X2, and numerically using a hybrid 1-D L1d2/2-D axisymmetric Eilmer3 CFD model. Four flow conditions were considered: Mach 10, 12.5, and 15 conditions in X2 without a nozzle, and a Mach 10 condition with a nozzle.
The experimental and numerical results indicated that the predicted primary wave processes were achieved. The detailed CFD analysis further predicted that the target test flow Mach number, velocity, temperature, and static pressure, were all approximately achieved at each condition. It is estimated that the maximum test flow total pressures were 3.75, 8.79, and 10.4 GPa, at Mach 10, 12.5, and 15 respectively. At these relatively low enthalpies (4.05, 6.68, and 10.4 MJ/kg respectively), these are the highest total pressure scramjet flows that have been reported in the literature to date.
Several challenges remain to be addressed following this experimental campaign. Satisfactory experimental Pitot pressure measurements could not be achieved in these harsh, short duration test flows, and therefore CFD Pitot calculations could not be experimentally validated. Partial impact pressure measurements with 15 deg half angle cone probes, specially developed for this experimental campaign, did demonstrate reasonable correlation with an equivalent pressure calculation from the CFD simulation results. Hence, there are reasons to be confident that better measurement techniques will demonstrate that good agreement exists with the experiment. This is based on a) matched wave processes, b) matched and steady tube wall static pressure measurements, c) correlation with cone probe pressure measurements, and d) the high fidelity of the CFD simulations.
Two other obvious limiting features of these test flows are the short test times and small core flow diameters (40-80 mm). X2 is a medium sized facility, and test time and core flow size are directly dependent on tube length and diameter. The purpose of this investigation was to demonstrate proof of concept, and this has been achieved. UQ's X3 facility is much larger than X2, and when these conditions are scaled upwards it is expected that test flow duration and core flow diameter will correspondingly increase to meet the requirements for actual scramjet testing.
In summary, this study has shown, for the first time, that an expansion tube can be configured to achieve the high Mach number, GPa total pressure, flow conditions associated with scramjet access to space. The CFD predicts some unsteadiness in these test flows; in the worst case, future testing may simply need to adapt to these imperfect test flows, since no ground testing technique, other than the expansion tube, is currently conceived which can produce flows even close to these total pressures. One of ground testing's most important functions is validation of CFD models, and these test flows can provide experimental data which permit validation of CFD models very close to the true flight conditions.