On the measurement of laser-induced plasma breakdown thresholds

The breakdown threshold of a gas exposed to intense laser-radiation is a function of gas and laser properties. Breakdown thresholds reported in the literature often vary greatly and these differences can partially be traced back to the method that is typically used to determine breakdown thresholds. This paper discusses the traditional method used to determine breakdown thresholds and the potential errors that can arise using this approach, and presents an alternative method which can yield more accurate data especially when determining breakdown thresholds as functions of gas pressure

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 H α and H β 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

Numerical modelling of porous fuel injection in a radical farming scramjet

A numerical investigation of the behaviour of fuel injection through a porous surface in an inlet-fuelled, radial-farming scramjet is presented. The performance of porous fuel injection is compared to discrete port hole injection at an equivalence ratio of φ ≈ 0.4 for both cases. The comparison is performed at a Mach 6.5 flow condition with a total specific enthalpy of 4.3 MJ/kg. The numerical results are compared to experiments performed in the T4 shock tunnel where available. The presented results demonstrate for the first time, that porous fuel injection has the potential to outperform port hole injectors in scramjet engines in terms of fuel-air mixing, ignition delays and achievable combustion efficiencies despite reduced fuel penetration heights

Ignition characteristics of laser-ionized fuel injected into a hypersonic crossflow

This paper presents an experimental investigation on the ignition characteristics of laser-ionized fuel injected into a hypersonic air-crossflow. A Q-switched laser causes breakdown in the sonic H injector stream before interaction with the air-crossflow traveling at 2km/s with a total specific enthalpy of 2.5MJ/kg. The flowfield is visualized using schlieren imaging and planar laser-induced fluorescence (PLIF) on the NO molecule. The temporal evolution of the ignition process is visualized using PLIF on the OH molecule. We compare the ignition effectiveness, in terms of the OH PLIF signal, between a series of test cases with different injector pressures and laser energies. These results are also compared to a previous study, where the laser-spark was generated in the jet interaction region, outside the fuel injector. The influence of using H fuel diluted with Ar, which serves as a plasma buffer gas, to extend plasma lifetimes is also investigated. The ignition technique is found to be effective in terms of post-plasma hydroxyl production, with a significant increase in performance observed when Ar is used as a plasma buffer gas

Experimental investigation of inlet-injection radical-farming scramjet combustion

The flow/chemistry coupling inside a nominally twodimensional inlet-fuelled scramjet configuration has been studied using a combination of experimental surface pressure measurements, optical diagnostics and 2-D premixed CFD simulations. The experiments were conducted at a flow total enthalpy of 4.6MJ/kg which corresponds to a Mach 10 equivalent flight condition, and a flight dynamic pressure of 58 kPa, corresponding to an altitude of 32 km. Radical-farming behaviour was investigated employing two-dimensional chemiluminescence imaging of excited OH (OH*) within the scramjet combustor. The streamwise location of the onset of combustion inferred from the OH* measurements correlates well with the start of the measured combustion-induced pressure rise along the combustor. 2-D premixed CFD results confirm the experimental observations where the ignition process is seen to be initiated downstream of the first shock reflection causing the production of highly reactive radicals, which then travel with the flow and enhance the combustion process further downstream at subsequent shock reflections. The intensity of the OH* signal was found to increase with increasing fuel equivalence ratio indicating mixing limited combustion kinetics

OH imaging in a non-uniform, hydrogen-fueled scramjet engine

The hydroxyl radical, OH, acts as a convenient indicator of combustion within hypersonic engines. This paper presents results from OH imaging within a scramjet combustor in the T4 hypersonic shock tunnel. Three methods are presented, being time-integrated emission, time-resolved emission, and planar laser-induced fluorescence. All methods capture emission in ultraviolet wavelengths, around λ = 310 nm. The resulting 2D images are representative of OH density and hence combustion. These images show the dependence of combustion on the shock structure within the scramjet combustor

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

Laser-induced plasma ignition studies in a model scramjet engine

An experimental investigation of the behavior of laser-induced plasma (LIP) ignition for scramjet inlet injection is presented. The presented results demonstrate for the first time, that LIP can be used to promote the formation of hydroxyl in a hypersonic flow. The temporal evolution of the LIP-ignited region is monitored using the planar laser-induced fluorescence technique on the hydroxyl radical. This study is the first laser spark study in a hypersonic flow, shown to generate combustion products where they would not otherwise occur

Simulation of laser-induced-plasma ignition in a hypersonic crossflow

A numerical study of a laser-induced-plasma ignition system in a scramjet-like geometry is conducted. Simulations are based on experiments in which hydrogen is injected into hypersonic air-crossflow, compressed by a 9º ramp from a freestream Mach number of 8 to below hydrogen autoignition conditions. Ignition of the jet is conducted using a laser pulse, modelled in the simulations as a small region of high temperature and pressure corresponding to the energy input measured in the experiments. To capture the highly unsteady nature of the fuel-air interface and its coupling with the dynamics of the expanding laser spark, Wall-Modelled Large Eddy Simulations are used to resolve the majority of turbulent motion. Detailed non-equilibrium thermochemistry models are included in the calculations. Simulations have revealed that the cloud of hydroxyl radicals detected in the experiments is created by the blast wave which forms from the expanding plasma kernel merging with the jet’s bow shock. This event raises the temperature and pressure in the upstream mixing layer and spontaneous ignition of the fuel is observed. Shortly after this ignition, combustion ceases due to hostile conditions for ame anchoring. Downstream of the jet, the simulations suggest that the plasma kernel remains can initiate some radical formation in the wake region, but no stable ame forms due to nominally low pressures in the experimental condition studied