Flow structure/chemistry coupling in the ignition process in shock-induced-combustion scramjets

Lorrain, Philippe (2014). Flow structure/chemistry coupling in the ignition process in shock-induced-combustion scramjets PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland. doi:10.14264/uql.2014.357

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Author Lorrain, Philippe
Thesis Title Flow structure/chemistry coupling in the ignition process in shock-induced-combustion scramjets
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
DOI 10.14264/uql.2014.357
Publication date 2014-01-01
Thesis type PhD Thesis
Supervisor Russell Boyce
Richard Morgan
Timothy McIntyre
Total pages 270
Total colour pages 69
Total black and white pages 201
Language eng
Subjects 0901 Aerospace Engineering
Formatted abstract
A coupled experimental and numerical study has been conducted to probe the flow structure/chemistry interactions in the ignition process inside a nominally two-dimensional scramjet configuration designed to operate in radical-farming mode. The flow conditions employed for the experiments are at the lower limit of where ignition can be achieved for pure scramjet operation. The scramjet model employed a two-ramp intake, constant area combustor and single ramp expansion nozzle. Hydrogen fuel was injected at an angle of 45° to the local direction of the flow in the intake from portholes on both the bottom and top first intake ramps. Two fuelling condition were investigated corresponding to global fuel-air equivalence ratios (Φ) of 0.5 and 0.8. The majority of tests were conducted for an intake geometry with sharp leading edges (LE), however, additional tests were carried out with blunt LE’s to investigate the influence of increased boundary layer temperatures on the ignition behaviour.

The combustion experiments were performed in the T4 shock tunnel facility at the University of Queensland for a Mach number of ∼ 7.6 and flow stagnation enthalpy of ∼ 3.7 MJ/kg (flight Mach 9 at a dynamic pressure of 73 kPa), which provided more realistic scramjet inflow conditions than in most previous studies of radical-farming combustion where nominal Mach 6 nozzles were used for combustion tests at similar flow stagnation enthalpies. In particular, the average combustor entrance temperature for the test flow condition in this study is ∼ 660 K, which is well below the auto-ignition temperature of ∼ 850 K for hydrogen-air mixtures. Despite the low mean combustor entrance temperature level, sustained supersonic combustion was achieved in the experiments, which enabled an investigation of the ignition characteristics at the highly temperature sensitive, lower limit of pure scramjet operation where ignition relies entirely on the interaction between the flow structures and the combustion chemistry.

A comprehensive suite of sophisticated optical diagnostic techniques was applied to obtain experimental evidence of the enhancement of chemical activity by the flow structures inside the scramjet combustor. More specifically, high-speed schlieren was used to visualise the flowfield in the intake, including the fuel plume penetration from the wall injection process, and in the first third of the combustor. Chemical activity throughout the combustor was visualised through two-dimensional imaging of the chemiluminescence from excited OH (OH*) using an intensified CCD (ICCD) camera. OH* is a very short-lived species which is formed as a reaction intermediate in the production of OH. This means that the recorded signal intensities are directly indicative of elevated local temperatures and increased chemical activity. These measurements were supported by visualisations of the flow self-luminosity in the first third of the combustor. From these measurements, chemical activity was observed to be confined to the near wall regions where temperatures are higher compared to the central region of the flow due to viscous dissipation from boundary layers. A significant finding is that OH* signals were detected in the expansion regions between consecutive shock reflections where temperatures and pressures were expected to be too low for chemical activity to occur, indicating that chemical reactions and heat release, initiated by shock reflections, continued throughout the expansion region rather than being quenched due to the drop in temperature. A similar behaviour has been observed in previous numerical studies and has now been shown experimentally for the first time. A strong correlation (in terms of peaks and troughs) was found to exist between the surface pressure measurements and spatially integrated OH* intensity distributions along the combustor which indicates a strong coupling between the flow structures and chemical reactions. The streamwise location of the onset of combustion inferred from the OH* intensity distributions was found to correlate with the onset of combustion induced pressure rise along the combustor surface. When blunt LE’s were used, significantly higher OH* intensities were detected along the combustor indicating increased chemical activity due to the higher boundary layer temperatures.

To support the analysis of the experimental data, three-dimensional Reynolds Averaged Navier Stokes (RANS) simulations were carried out for the non-reacting and reacting experimental flows to develop further insight into the flow physics and ignition characteristics. Initial simulations provided strong evidence that the semi-empirical method for determining the experimental test conditions greatly overpredicts the flow stagnation temperature, hence freestream static temperature, as the combusting flow simulations significantly overpredicted the measured combustion induced pressure rise. Simulations performed for a lower stagnation temperature test condition corresponding to the lower uncertainty bound on the nominal value, resulted in very good agreement between numerical and experimental surface pressures distributions. This is an important finding which emphasises firstly the strong temperature sensitivity of the ignition process at the present flow conditions, and secondly the importance of obtaining accurate estimates of the test flow and test thermodynamic conditions for shock tunnel combustion experiments, as this has significant implications for the conclusions that may be drawn from the experimental combustion data. In addition to the temperature sensitivity, vitiation effects due to the presence of atomic oxygen (O) in the shock tunnel freestream were found to enhance chemical activity in high temperature regions around the fuel injectors in the intake providing a source of combustion radicals to aid the ignition processes inside the combustor.

Finally, the numerical study revealed that ignition and combustion processes do not occur uniformly across the flow due to the limited mixing between fuel and air. Ignition and combustion emerges from isolated reaction zones with approximately stoichiometric or fuel-lean mixtures in the corner regions near the side walls of the rectangular flowpath and in between fuel plumes.
Keyword Scramjet
Air-breathing propulsion
Supersonic combustion

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Created: Thu, 18 Sep 2014, 02:52:36 EST by Philippe Lorrain on behalf of Scholarly Communication and Digitisation Service