Investigation Of The Flame Stabilization Of Supersonic Combustion Based On High Repetition Rate Multi-point Laser Induced Plasma

One of the difficulties of hypersonic is to achieve stable and reliable combustion.The conventional flame stabilizer represented by cavity and strut has some limitations,such as thermal protection and great total pressure loss.As a new ignition method,laser induced plasma(LIP)has the advantages of non-invasive components,so it can be considered to apply the high repetition rate multi-point LIP to the chamber of hypersonic.The characteristics of high repetition rate can be simulated by using the adjustable pulse interval.The multi-point breakdown can be realized by the combination of beam splitter and convex lens.In this paper,the evolution of high repetition rate multi-point LIP were studied by the photography and schlieren shooting were taken simultaneously.The effects of laser incident energy,pulser interval and lattice layout(distance between focal points)on LIP envolution are investigated.Firstly,the experiment was carried out in static air to study the spatio-temporal evolution process of high-repetition multi-point LIP and compare it with dual-pulse single-point LIP and single-pulse single-point LIP.According to the experimental results,although the overall evolution law of multi-point LIP is similar to that of single-point LIP,the energy absorption rate of multi-point LIP is higher at low energy(less than 200 m J).The energy absorption rate is also related to the lattice layout.Among the several configurations adopted in the experiment,The energy absorption rate is the highest when the distance between focal points is 8.32 mm.Considering the shock energy loss,it can be considered that multi-point LIP can achieve more efficient energy utilization in a larger space range.Taking all these factors together,a reliable ignition and combustion stabilization scheme should require a laser pulse interval of no more than 200 ns.The high repetition rate breakdown of LIP is studied in a flush wall supersonic combustor.The plasma has a short lifetime under the existing laser energy and frequency.Breakdown results show that the strength of the plasma can be improved by increasing laser energy or choosing the appropriate breakdown location(such as the place far away from the fuel nozzle,where turbulence dissipation is weak).In addition,with a proper pulse interval(less than 200 ns),the energy of the second laser pulse will be deposited in the plasma formed by the previous laser pulse,thus increasing the intensity and survival time of the plasma and contributing to the survival of the fire core.The ignition and flame stabilization tests in a combustor model with a cavity as flame stabilizer show that the multi-point LIP does not shorten the ignition time under monopulse conditions.For dual-pulse multi-point LIP ignition,when the laser energy absorption rate is high and the pulse interval is less than a few microseconds,the initial fire nuclei generated by multiple plasmas tend to fuse.The process of ignition and the flame reaching stable combustion will be accelerated.In addition,the lattice layout also has an impact on the ignition process.The ignition completion time will be prolonged with a excessive distance between focal points.