Stefan Brieschenk

Visualization of jet development in laser-induced plasmas

Laser-induced plasmas in gases are known to generate gaseous jets in the postplasma gas plume. The gaseous jet typically develops toward the laser source, and the experiments presented here show, for the first time to our knowledge, that, under certain conditions, these jets can develop in the opposite direction or may not form at all. The data suggest that this is related to the ratio between the energy absorbed in the plasma and the threshold breakdown energy, effectively leading to multiple plasma initiation sites in the focal waist.

The effect of blast wave re-focusing on a laser-induced plasma

The effect of laser-induced plasma confinement on lifetime and temperature is reported using cylindrical reflectors. This is determined experimentally in a gas cell, with hydrogen as test gas, and cylindrical shock reflectors of different diameters. The temperature evolution of confined and unconfined laser-induced plasma has been measured using plasma emission spectroscopy. Temperatures were determined through the plasma line-to-continuum thermometry technique in the hydrogen Balmer series using the Hα and Hβ transitions at λ = 656 nm and λ = 486 nm, respectively. The experiments found that re-focusing the blast wave can significantly increase temperatures during the exponential decay of the plasma. The experimental results also show that confinement increases peak plasma temperatures, and that plasma lifetimes are only marginally affected by the confinement.

A Detailed Investigation of Nominally 2-D Radical-Farming Scramjet Combustion

This paper reports on an investigation of the flow/chemistry coupling inside a nominally two-dimensional inlet-fuelled scramjet configuration. The experiments were conducted at a freestream Mach number of 7.3 and a total flow enthalpy of 4.3 MJ/kg corresponding to a Mach 9.7 flight condition. The phenomenon of radical-farming has been studied in detail using two-dimensional OH* chemiluminescence imaging and emission spectroscopy. High signal levels of excited OH (OH*) were detected behind the first shock reflections inside the combustion chamber upstream of any measurable pressure rise from combustion, which occurred towards the rear of the combustor. The production of OH in the first hot pocket initiates the ignition process and then accelerates the combustion process in the next downstream hot pocket. This was confirmed by numerical simulations of premixed hydrogen/air flow through the scramjet. Chemical kinetics analyses reveal that the ignition process is governed by the interaction between various reaction groups leading to a chainbranching explosion for low mean temperature and pressure combustion flowfields.

Laser-Induced Plasma Ignition Experiments in a Scramjet Inlet

An experimental investigation on the behavior of laser-induced plasma (LIP) ignition for scramjets has been conducted. Two different ignition methodologies are examined which are referred to as shear-layer-LIP-ignition and fuel-jet-LIP-ignition. A two-dimensional scramjet model with inlet injection, fueled 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 2.7 MJ/kg and a Mach 9 freestream. In the shear-layer-LIP-ignition experiment, LIP is formed in the shear-/mixing layers immediately downstream of four transversely orientated port hole injectors. In the fuel-jet-LIP-ignition experiment, LIP is formed inside the sonic throat of a single fuel injector. The influence of using fuel diluted with a plasma buffer gas (8% Ar, 92% H2) to extend plasma lifetimes is also investigated. The planar laser-induced fluorescence technique on the hydroxyl radical (OH-PLIF) is used to yield qualitative concentration images of the hydroxyl combustion species allowing us to compare the two different ignition methodologies in their effectiveness. Time-integrated schlieren images have been obtained and superimposed on the OH-PLIF intensity maps to determine the effect of the flow structure on the hydroxyl concentration. The broadband self-luminescence signal of the LIP in the early stages immediately after initiation is also recorded. Both ignition techniques are found to be effective in terms of post-LIP hydroxyl production. The fuel-jet-LIP-ignition technique allows lower laser energies to be used but requires a plasma buffer to compete with the shear-layer-LIP-ignition technique.

Investigation into the flow physics of hypersonic variable geometry inlet starting

A coupled numerical-experimental investigation studying the applicability of a Sliding Doors variable geometry inlet starting mechanism for a 2D SCRAMjet inlet at Mach 6 has been undertaken. Door opening speeds ranging from 3 to 20ms repeatedly produced started inlet conditions when tested within the TUSQ Low-Enthalpy Wind Tunnel. Transient flow during door retraction was captured both quantitatively and qualitatively via surface pressure measurements and Schlieren visualization techniques. A blunt wedge protruding upstream into the combustor from the rear of the model was also installed, simulating a large pressure rise within the combustor at the centerline. Inlet unstart was encountered for a wedge position protruding 7mm upstream into the combustor, with door opening speeds as low as approximately 1ms were seen to have no visible effect on inlet startability. A viscous, transient, time accurate numerical investigation was conducted, recreating the experimental conditions by solving the Reynolds-Averaged Navier-Stokes equations. These simulations provide insight into pockets of the flowfield for which optical access was not available, as well as also allowing faster door opening speeds to be employed which were not mechanically possible in the tunnel. It was found that even extreme door opening speeds of 0.1ms were not able to start the more aggressive forward wedge positions. Interactions between shock structures around the leading edge of the wedge and the boundary layer at the rear of the combustor were seen to supersede the positive effects of employing unsteady effects.

H2-O2 porous fuel injection in a radical farming scramjet

This paper reports on the experimental testing of oxygen compatible ceramic matrix composite porous injectors in a nominally two-dimensional hydrogen fuelled and oxygen enriched radical farming scramjet in the T4 shock tunnel facility. All experiments were performed at a dynamic pressure of 146 kPa, an equivalent flight Mach number of 9.7, a stagnation pressure and enthalpy of 40 MPa and 4.3 MJ/kg respectively and at a fuelling condition that resulted in an average equivalence ratio of 0.472. Oxygen was pre-mixed with the fuel prior to injection to achieve enrichment percentages of approximately 13%, 15% and 17%. These levels ensured that the hydrogen-oxidiser mix injected into the engine always remained too fuel rich to sustain a flame without any additional mixing with the captured air. Addition of pre-mixed oxygen with the fuel was found to significantly alter the performance of the engine; enhancing both combustion and ignition and converting a previously observed limited combustion condition into one with sustained and noticeable combustion induced pressure rise. Increases in the enrichment percentage lead to further increases in combustion levels and acted to reduce ignition lengths within the engine. Suppressed combustion runs, where a nitrogen test gas was used, confirmed that the pressure rise observed in these experiments as attributed to the oxygen enrichment and not associated with the increased mass injected.

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 306 nm 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- lands 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.

An experimental investigation of a thermal compression scramjet with OH imaging

Scramjet engines must be designed to operate over a range of Mach numbers. In order to maintain robust starting characteristics at low freestream Mach numbers, the inlet contraction ratio must be minimized, reducing performance at high Mach numbers. The technique of thermal compression can be used to maintain robust combustion at low mean pressure and temperature, improving the performance of low inlet contraction ratio engines over a range of Mach numbers. This paper examines a scramjet engine with a threedimensional inlet that induces spanwise gradients of pressure and temperature, producing high inlet compression on one side and low inlet compression on the other. The effect of combustion in each half on pressure throughout the entire flowfield is examined by suppressing combustion on either half of the combustor through injecting either a combusting fuel, hydrogen, or a non-combusting replacement, helium. Two optical techniques are used to examine how the concentration of the OH radical varies throughout the combustor. Images of OH* Chemiluminescence and OH Planar Laser-Induced Fluorescence (PLIF) map the concentration and production of OH radicals indicating chemical activity throughout the combustion process. The thermal compression effect is isolated, and is shown to increase combustion-induced pressure rise in a scramjet engine.

Quasi-one-dimensional investigation of combustion processes on scramjet performance

Scramjet engines can decrease the cost of access-to-space systems. One-dimensional solvers can be used to rapidly analyse many scramjet engine configurations to maximise performance. This paper uses an inviscid, quasi-one-dimensional, chemical equilibrium solver to examine the effects of combusting 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. When a maximum engine pressure constraint is imposed the constant pressure process becomes more effective. This work shows that if combustion occurs at constant area until the maximum pressure is reached, then continued using a constant pressure process, the required intake compression and contraction ratios can be decreased by 53% and 35% respectively from the constant pressure case. This decrease results in improved starting characteristics for a negligible cost to performance. The effect of maximum engine pressure on performance is examined and it is shown that there is little benefit from increasing intake compression ratios above 60, or maximum engine pressure over a factor of 150 above the freestream pressure.