Improving performance of high Mach number scramjets: fuelling strategies and combustor design

Abstract

With prospective flight speeds exceeding Mach 12, airbreathing scramjets, incorporated in hybrid satellite launch vehicles, offer efficiency and operability benefits when compared to rocket-based systems. By capturing atmospheric air, the scramjet second stage negates the requirement for onboard oxidisers. Addressing the small satellite niche market, these hybrid systems provide a cost-effective solution to launching such payloads to orbit. At hypersonic speeds, however, conventional wing-mounted engine nacelles are not viable; the scramjet must be integrated to the airframe. Hence, the flight-candidate, Mach 12 Rectangular-to-Elliptical Shape-Transitioning (M12REST) scramjet blends a rectangular capture area (permitting parallel mounting of engine modules to the planar vehicle underside), with a structurally efficient elliptical combustor. As access-to-space scramjets suffer immense heating loads, and engine airflow residence times approach air-fuel reaction timescales, hydrogen fuel is utilised for its exceptional cooling capacity and rapid ignition characteristics. However, efficient fuelling techniques are required to avoid crippling the engine's performance through conservative combustor lengths. This thesis numerically and experimentally investigated whether the performance of such an airframe-integrated scramjet can be sufficiently enhanced through customisation of fuel injection and combustor geometry to achieve net thrust at Mach 12. Three-dimensional, chemically reacting Reynolds-averaged Navier-Stokes solutions enabled numerical analysis, while experimental validation was performed within The University of Queensland's, T4 Stalker Tube.Preliminary work investigated cascaded fuel injectors, targeting mixing and penetration improvements. Two streamwise-aligned jets were employed, with the upstream injector half the diameter of the rear, while the distance between each was varied. The upstream jet induced a low dynamic pressure region in its wake, shielding the downstream jet from the hypersonic crossflow. The leeward jet benefits from an increased local jet-to-freestream momentum ratio, while its larger diameter increases absolute penetration. Examining performance at M12REST mean combustor entrance conditions, unique optimal injector spacings existed for each performance metric, displaying improved penetration, spread, and mixing across the range of flight Mach numbers examined (6 ≤ M ≤ 12). However, jet-to-jet spacings of 4-6 total jet diameters displayed universal performance enhancements over single fuel jets. With penetration improvements of 30-40%, and mixing improvements of 40-70%, the simple fixed geometry, passive technique is ideal for use within an accelerating access-to-space scramjet.While cascaded injectors improved performance within uniform flows, the flows ingested by airframe-integrated scramjets are far less homogeneous. Non-uniform compression fields combine with thick boundary layers developed over the vehicle forebody to deliver density stratified flow to the combustor. This thesis ...

Adjoint-based aerodynamic design optimisation in hypersonic flow

“…It is stressed, however, that despite the grid being made up of structured elements, the underlying flow and adjoint solvers operate on the grid using numerical routines developed for unstructured grids. This methodology is common in computational analyses of complex hypersonic flows [215,216]. This work focussed on an unstructured grid implementation because we are optimistic that unstructured methods will mature in the near future to the point where residual convergence is no longer an issue.…”

Free-jet testing of a Mach 12 scramjet in an expansion tube

“…More recently, Landsberg [87] carried out a mixed experimental and numerical study on the M12REST engine, investigating several techniques to increase combustion and mixing efficiency in the engine. He proposed a reduced-length combustor, using the injection scheme of Barth, although removing the backward facing step.…”

“…The interactions of the boundary layer developed by the forebody ingested by the inlet and combustor at the design Mach number have been not investigated before. Additionally, notwithstanding the fact the engines of Wise, Barth, Doherty and Landsberg [14,82,83,87] were scaled down version of the original ideal flight engine, no pressure-length scaling was applied, and similarity with flight was not achieved.…”

“…The engine adopted for the experiments was the one used by Wise [82], Barth [83], and Landsberg [87]. The configuration chosen was the one proposed by Barth [83], with the tailored injection scheme proposed by the same author, and the long combustor.…”

Investigation of supersonic turbulent combustion in a Mach 10 scramjet engine

“…This accounts for the variance between shots, by normalising by nozzle supply pressure, and makes comparison between shots easier since all the traces from different shots will start at the same normalised value p(t) = 1. This normalisation procedure has been used before in experiments performed on T4 [5,67].…”