Characterization of a spark ignition system for flameholding cavities

This paper presents an experimental investigation of a capacitive-discharge spark ignition system designed to promote ignition in CH- and CH-fuelled supersonic combustors. The purpose of this study is the characterization of the ignition system and the plasma generated in the discharge. Schlieren and luminescence imaging are used to visualize the temporal evolution of the spark plasma. Transient voltages and currents across the primary-side of the ignition coil and input-side of the ignition unit are recorded using a high-speed data acquisition system. Three different ignition coils are tested with two different spark plug gaps in an attempt to increase the performance of the ignition system which is evaluated through spatially and temporally integrated luminescence recordings as well as temporally integrated photo diode signals. The data suggests that an increase in performance of a factor of 4-5 over the baseline setup can be achieved. A capacitive ignition lead is used to assess whether or not any capacitance on the coil secondary side can increase the performance of the ignition system. The experiments have also shown that the ignition system parameters can be set to cause sufficient heating of the electrodes to support ignition from a combined glow-spark plug setup

Laser ignition of hypersonic air-hydrogen flow

An experimental investigation of the behaviour of laser-induced ignition in a hypersonic air-hydrogen flow is presented. A compression-ramp model with port-hole injection, fuelled with hydrogen gas, is used in the study. The experiments were conducted in the T-ADFA shock tunnel using a flow condition with a specific total enthalpy of 2.5 MJ/kg and a freestream velocity of 2 km/s. This study is the first comprehensive laser spark study in a hypersonic flow and demonstrates that laser-induced ignition at the fuel-injection site can be effective in terms of hydroxyl production. A semi-empirical method to estimate the conditions in the laser-heated gas kernel is presented in the paper. This method uses blast-wave theory together with an expansion-wave model to estimate the laser-heated gas conditions. The spatially averaged conditions found with this approach are matched to enthalpy curves generated using a standard chemical equilibrium code (NASA CEA). This allows us to account for differences that are introduced due to the idealised description of the blast wave, the isentropic expansion wave as well as thermochemical effects

Development of a 2-phase flow nozzle for fine droplet generation

Fine sprays with droplet diameters (< 50mm), are widely used in engineering applications, ranging from liquid pre-cooling in natural draft cooling towers to fuel injection for scramjet engines. Traditionally fine droplets are generated by passing high pressure liquid through small diameter nozzles. The downsides of this method are high energy consumption arising from pumping work and limited flow rates arising from the small nozzle exit areas. A way to overcome these limitations is to use a 2-phase Flow Nozzle. Here, liquid is injected together with a small quantity of gas through the same nozzle. The interaction of the two fluid streams significantly enhances liquid jet break-up. This paper presents the design and commissioning of an experimental test set-up for 2-phase nozzle development and the results obtained from testing a prototype nozzle injecting hydrocarbon fuel and nitrogen. Results show that the nozzle can generate droplets below 100mm at moderate operating pressures (5-10 bar), while maintaining flow rates comparable to those of high pressure single phase nozzles. In addition to proving the feasibility of the 2-phase nozzle concept, the experimental campaign has generated an extensive set of high speed videos, providing an insight into the liquid jet break-up process. These will be used in future computational fluid dynamic investigations

Scramjet Performance For Ideal Combustion Processes

Models used to analyse scramjet engine cycles are typically either very simple or focused on specific combustor geometries. This paper uses a quasi-one-dimensional, inviscid chemical equilibrium solver to examine scramjet engines defined by predetermined combustion processes. This solver is capable of rapidly analysing combustion processes without requiring predefined geometries and is validated against the Hyshot II flight experiment and a constant area combustor ground test. Combustion occurs using constant area, constant pressure, constant Mach number or constant temperature processes. Constant area combustion typically produces the highest specific impulse for given combustor entrance conditions. Maximum pressure and temperature limitations were introduced, and multiple engines with combinations of combustion process were examined. It was found that engines with a combination of all four combustion processes can have lower maximum values of pressure and temperature whilst maintaining high performance compared with constant area combustor engines

Code Development to Determine the Temperature from the OH* Chemiluminescence Recordings in a Supersonic Combusting Flow

Experimental investigations of scramjet combustion are the focus of experimental studies performed in T4 facility at The University of Queensland. The objective of the present study is to develop code support to determine OH temperatures in such high speed flows. For this purpose available source codes, namely SPARTAN and Photaura, had to be extended in order to include OH species in their databases. Validation of the OH modelling in SPARTAN and Photaura was done by comparison with the results of LIFBASE simulations. The results of these codes were compared with the OH* chemiluminescence spectra obtained using a Mach 9 enthalpy-equivalent flight condition. A two-dimensional scramjet model with a constant-area supersonic combustor was used in the experimental investigation. The OH* chemiluminescence signal was recorded and the spectrally resolved measurements of the OH* emission spectra have been conducted in order to characterise the ignition and combustion processes. When compared with the spectrally resolved OH* emission spectra, the results of spectral simulations demonstrate a very good agreement. The comparison with the experimental data indicate that results of three synthetic spectral simulation can be considered as a consistent representation of the physical phenomena and can serve as a pre-experiment indication of the combustion process studied. The presented results show the possibility of using OH* chemiluminescence spectra to infer relative temperatures in supersonic combustion. Absolute temperatures can be inferred if the offset between the intensity-versus-temperature functions for OH* and OH is known

Reynolds-averaged Navier–Stokes and wall–modelled large–eddy simulations of sonic hydrogen injection into hypersonic crossflow

The combustion efficiency in supersonic combustion ramjets (scramjets) is strongly dependent on the fuel injection process. This paper investigates the transverse injection of hydrogen into a hypersonic air crossflow at Mach 6 . The flow physics are investigated using both Reynolds-averaged Navier-Stokes (RANS) simulations and wall-modelled large-eddy simulations (WMLES). We focus on the comparison of the results of these two methods and their agreement with experimental temperature measurements. Assessing the performance of RANS and its shortcomings in this context is of particular interest due to its significantly reduced computational costs and its widespread use in the hypersonics community compared to WMLES

Chemiluminescence imaging in supersonic combustors operating in radical-farming mode

Emission spectroscopy was used to investigate ignition and combustion characteristics of supersonic combustion ramjet engines. Two-dimensional scramjet models with inlet injection, fuelled with hydrogen gas, were used in the study. The scramjet engines were configured to operate in radical farming mode, where combustion radicals are formed behind shock waves reflected at the walls. The chemiluminescence emission signals were recorded in a two-dimensional, time-integrated fashion to give information on the location and distribution of the radical farms in the combustors. High signal levels were detected in localised regions immediately downstream of shock reflections, an indication of localised hydroxyl formation supporting the concept of radical farming. Results are presented for a symmetric as well as an asymmetric scramjet geometry. These data represent the first successful visualisation of radical farms in the hot pockets of a supersonic combustor. Spectrally resolved measurements have been obtained in the ultraviolet wavelength range between 300 and 400 nm. This data shows that the OH! chemiluminescence signal around 306nm is not the most dominant source of radiation observed in the radical farms

Experimental design of a cavity flameholder in a Mach 8 Shape-Transitioning Scramjet

The significant volume constraints placed on the design of airframe-integrated scramjet engines calls for fuels with high energy per unit volume. Therefore, low-order hydrocarbons, such as ethylene and methane are candidates for use in engines such as the Mach 8 Rectangular-to-Elliptical Shape Transition (REST) engine, a flow-path which has been extensively tested for hydrogen. Cavity ame- holders are being investigated as a means of ensuring the robust combustion of these fuels. A modular cavity has been designed for a Mach 8 REST engine model suitable for shock tunnel testing. The depth, L/D ratio, and aft wall angle of the cavity ameholder have been determined using previous experimental studies, coupled with axisymmetric simulations. The L/D ratio and aft wall angle have been chosen to minimise stagnation pressure losses and promote a stable flowfield for ameholding. Unsteady axisymmetric simulations reveal that for all cavity depths investigated, the recirculating cavity flow is established within a small fraction of the test time available in the University of Queens- land's T4 Hypersonic Shock Tunnel. The flow temperatures achieved within the simulated cavities are used in conjunction with correlations for cavity residence time and fuel ignition delay to estimate the cavity depths required to auto-ignite low-order hydrocarbon fuels. Consideration is also given to the level of disturbance to the flow through the core of the combustor caused by cavities of various depths. The initial cavity dimensions selected to be experimentally tested are L/D ratio, depth and aft wall angle of 4:0, 4:4 mm, and 22:5°, respectively. Analysis indicates conditions in this cavity should cause the auto-ignition of ethylene. An external ignition source, using conventional spark plugs, will be installed in the cavity to promote ignition of the low-order hydrocarbon fuels, should auto-ignition not occur

Supersonic combustion of hydrogen in Mach 2 flows over an axisymmetric model

An external axisymmetric model featuring a cavity flame-holder fuelled by hydrogen was adopted for the study of the supersonic combustion in Mach 2 flows. The self-sustained hydrogen flames ignited by a spark gap igniter were detected using two optical techniques: an intensified camera attachment coupled with a high speed camera with a narrow-band-pass filter centred at 310 nm for detection of OH* chemiluminescence; and high-speed schlieren imaging for flow visualization. The spark ignition was identified on the schlieren images which also enabled visualisation of changes in the flow structure associated with the combustion. Local concentrations of the radiating radical OH* were determined using the inverse Abel transformation based on the ICCD images and calibration against a light source of known radiance. The combustion flow was reconstructed numerically using a CFD Solver, Eilmer4 in an axisymmetric configuration coupled with a detailed H2/O2 oxidation chemistry mechanism and a OH* chemiluminescence sub-scheme. The maximum magnitude of the results for the OH* chemiluminescence simulations demonstrated reasonable agreement with measurements