Study Of Instantaneous Flame Structure In Supersonic Flow By Using Laser-based Imaging Techniques

As one of air-breathing propulsion systems for hypersonic flight,scramjets need to organize combustion in supersonic flow.The supersonic combustion occurs under environments of high compressibility,extreme turbulence,strong flow discontinuity and intense flow fluctuations.For the supersonic combustion,its flame structure and flame stabilization mechanism are still not fully understood.Responding to the demands on diagnostics of instantaneous flame structures in scramjets,this work developed multi-species planar laser-induced fluorescence（PLIF）imaging for supersonic combustion,and realized simultaneous visualization of instantaneous preheating,heat-releasing（HR）and post-flame zones in a scramjet.The PLIF,the laser photofragmentation-induced fluorescence（LPIF）and the photofragmentation laser-induced fluorescence（PF-LIF）were utilized for capturing the relative distributions of CH₂O and CH₃species,and those of CH₃and CH species in premixed jet flames.In low-speed jet flames,the CH₃layers were located between the CH₂O and CH layers,and clung to them.With increased turbulence,the CH₂O layers were obviously broadened,the CH₃layers were slightly broadened,and the CH layers’thicknesses were nearly unaffected.The CH₃and CH layers overlapped with each other to a large extent,and then the CH₃distributions approximated the instantaneous HR zones.In high-speed jet flames,the CH₂O and CH₃species pervaded in the jet core of the flames;the CH layers appeared at the outer edges of CH₃distributions.In the CH₃regions,there existed holes formed by weak CH₃signals surrounded by strong CH₃signals.These holes usually corresponded to obvious CH₂O signals.The ridge lines,thicknesses and curvatures of species layers in low-speed jet flames were quantitatively extracted.The CH₃radicals were proposed as an indicator for the transitional zone between the preheating and HR zones in hydrocarbon flames.The one-dimensional laminar flame simulation revealed that both the position and temperature range of CH₃radicals lie between those of CH₂O and CH species,under diverse equivalence ratios.In premixed jet flames,the simultaneous imaging of two species’distributions showed the same relative positions of the three species.The sensitivity of CH₃layer thicknesses to turbulence was found to be weaker than that of CH₂O,but stronger than that of CH radicals.The CH₃LPIF and CH₃PF-LIF are able to visualize the transitional zones with high spatial and temporal resolutions,thus being significant for studies of highly turbulent flames.For the ethylene-air flames,LPIF produced CH₃images of high signal-to-noise ratios（SNRs）.For the methane-air flames,however,CH₃PF-LIF was required to obtain qualified CH₃images.This work systematically optimized the in-scramjet CH₂O PLIF scheme,in order to resolve the problem that the CH₂O PLIF imaging in scramjets suffered from serious interference.In the aspect of fluorescence excitation,the 353.06-nm laser was selected for CH₂O PLIF based on the measurement of CH₂O excitation spectrum within a wide wavelength range.With respect to the fluorescence detection,a laser trapper was used to repress the laser scattering,and the double imaging technique was applied for reducing the flame chemiluminescence interference.The optimized CH₂O PLIF scheme was implemented in a direct-connected scramjet facility.The SNRs of generated CH₂O PLIF images were 2 to 10 times those of images by conventional CH₂O PLIF schemes.The instantaneous preheating zones,HR zones and post-flames zones were captured in a scramjet,through simultaneous CH₂O and OH PLIF.The CH₂O PLIF imaged the preheating zones,the OH PLIF imaged the post-flame zones,and the pixel-by-pixel product of CH₂O and OH PLIF images represented the HR zones.The flame spreading angle and boundary were calculated from the high-speed CH*and schlieren images.The OH radicals were mainly distributed below the flame spreading boundary,and pervaded the cavity.CH₂O species chiefly appeared in the vicinity of cavity shear layer（CSL）and the jet wake,and were occasionally observed above the flame spreading boundary.The upper boundary of OH signals followed the lower boundary of CH₂O well,indicating that interaction of the jet wake and the CSL introduced a large amount of hot products into the jet wake.These hot products induced initial thermal decomposition of ethylene,thus yielding large blocks of CH₂O.In large scales,the instantaneous HR zones were in forms of ribbons and blocks,but in small scales the HR zones were shredded into small pieces.The shredded HR zones were possibly due to the local extinction resulted from intense strain rates in the flowfield.Based on the CH*images,the schlieren images and the flame structure images,it was deduced that there were two piloting processes during the flame stabilization:the cavity recirculation flow pilots the shear layer flame（SLF）,and the SLF pilots the jet wake flame.The hot products were recirculated into the CSL,which made the ethylene directly entrained into the CSL be burned quickly.This SLF accounted for the observation that the flame head was stably anchored in the CSL near the cavity leading edge.Hot products from the SLF were continuously transported into the jet wake,thus igniting the partially-premixed mixture there.In the middle and rear parts of the cavity section,there occasionally captured thin,curved and continuous OH layers with thicknesses of about 0.5 to 1 mm.This kind of OH layers demonstrated the existence of flame propagation in the jet wake,and the jet wake flame was not only consecutively auto-ignition and extinction.This work focused on the techniques of PLIF,LPIF and PF-LIF.CH₃LPIF and PF-LIF were proposed for visualizing instantaneous transitional zones.The CH₂O PLIF scheme for imaging the preheating zones in a scramjet was established,and then multiple flame zones in the scramjet were simultaneously captured through CH₂O and OH PLIF.This work revealed the actual flame structures of supersonic combustion,and provided valuable flame structure data and diagnostic tools for further studying the combustion in scramjets.