Study On The Formation And Control Of Soot In Flame-wall Interactions

Flame-wall interactions(FWI)refer to the complex physical and chemical changes occurring between the flame and walls when the flame directly contacts with walls.As a fundamental combustion phenomenon,flame-wall interactions are universally found in practical combustion devices such as internal combustion engines,gas turbines and industrial furnaces.It could significantly affect the energy conversion efficiency and pollutants emission of combustors.The formation of soot due to the incomplete combustion in FWI significantly reduces the combustion efficiency and causes the serious environmental and health issues.Facing to the more stringent emission regulations and the pursuit of the higher efficiency and the lower harmful emissions,the increased understanding of soot formation in FWI is necessary.To the best of our knowledge,there has been little work to analyze the soot formation in FWI.Thereby,it is necessary to systemically conduct the fundamental investigation of soot formation and evolution in FWI.It will contribute to the comprehensive understanding and mastery of soot formation processes and mechanism in FWI and the proposal of relevant strategies to reduce soot emissions.It will also offer the crucial experimental guideline and theoretical reference for the controlling of particulate emissions from practical combustion devices.On the basis of the ethylene jet diffusion impinging flame experimental platform,the combination of the local probe sampling and the overall quartz sampling methods were used to deeply analyze the effects of various nozzle to plate distances(H),Reynolds numbers(Re),collecting times(T)and oblique angles of burners(?)and the addition of different types of fuel(dimethyl ether(DME)and hydrogen(H₂))on soot characteristics from different flame zones in symmetrical and asymmetrical impinging flame structures.The utilization of transmission electron microscopy(TEM),high-resolution transmission electron microscopy(HRTEM),X-ray diffraction spectroscopy(XRD),Raman spectroscopy(Raman)and thermogravimetric analyzer(TGA)were adopted to analyze the variations of soot physicochemical properties such as morphology,nanostructure,graphitization degree and oxidation degree.The characteristics of soot formation and evolution from FWI in symmetrical and asymmetrical impinging flame structures were obtained and the relevant strategies to reduce soot formation were developed.Firstly,the characteristics of soot formation and evolution in FWI from symmetricalimpinging flame structures were explicitly analyzed.The combination of ethylene vertical jet diffusion impinging flames and the quartz sampling technique were adopted to investigate the influence of different nozzle to plate distances,Reynolds numbers and collecting times on soot physicochemical properties such as morphology,nanostructure and oxidation reactivity from different impinging flame zones.A series of soot rings which corresponded to various impinging flame zones were formed on the plate surface due to the FWI.Soot from the inner ring which located in the wall jet region was the irregularly shaped film-like material with the amorphous structure.The outer ring which surrounded the wall jet region contained soot escaped from the flame due to thermophoretic effects.The outer ring soot consisted of suborbicular primary particles with the classic core-shell structure.The increase of nozzle to plate distances promoted the penetration of ambient air into the stretched free jet region which accelerated soot oxidation.The soot disordered degree reduced,the fringe length increased and the fringe tortuosity decreased,leading to a reduction in oxidation reactivity.Meanwhile,the increase of H made the impinging flame layer shrunken.The air entrainment of related regions was reduced.The crystallization degree of outer ring soot was non-monotonic with the increase of the nozzle to plate distance.Soot at H=30 mm had the longest fringe length and the lowest fringe tortuosity.It also presented the lowest reactivity.When Re increased from 300 to 900,the ambient air was more easily participated in reactions due to the expanded impinging flame regions.It enhanced the oxidation process of soot.The crystallization degrees of soot from outer rings increased with Reynolds numbers.Inner ring soot at Re=600 had the most amorphous structure related with the shortest fringe length and the largest tortuosity because of the change of the free jet region.It presented the highest content of disordered carbons and reactivity.The reactivity of soot from both inner and outer rings decreased with the increase of collecting times at H=10 mm and Re=600,which might result from variations of soot oxidation rates due to the increase of the plate temperature.The results also confirmed the relationship between the soot structure and oxidation reactivity that soot with the higher degree of crystallization was more difficult to oxidize.Subsequently,soot characteristics from FWI with addition of DME(oxygenated fuel)and H₂(clean fuel)were respectively analyzed to develop relevant strategies controlling soot formation and evolution in FWI.The variations of soot morphology,nanostructure and oxidation reactivity from different impinging flame regions were investigated with different addition of DME(constant carbon flows)and H₂(constant fuel flows)in symmetrical ethylene jet diffusion impinging flames via the quartz sampling method.Soot from different impinging flame regions exhibited different characteristics with the addition of DME.The initial impingement region soot production was promoted and its oxidation reactivity was decreased due to the increase of PAHs(Polycyclic aromatic hydrocarbons)with 5%and 20%DME addition.When 5%DME was added,the surface growth of soot was enhanced due to the increase of C₂H₂ concentration.Soot from the post-impingement region had the larger production and presented the higher graphitization degree and the lower reactivity.The addition of 20%DME suppressed the production of soot from post-impingement regions and promoted the oxidation reactivity.The reduction of soot growth rates caused by the decrease of C₂H₂ led to the phenomenon.In conclusion,the large proportion of DME addition performed well in reducing the soot formation in FWI.It reduced the graphitization degree and enhanced the oxidation reactivity.It contributed to improving the energy efficiency and reducing soot emissions.When the fuel flow rate was constant,properties of soot from different impinging flame regions varied with the increasing addition of H₂.The inner ring soot contained more amorphous carbons with the addition of H₂.The fringe length tended to be shorter and the tortuosity seemed to be larger.It might be attributed to the lower soot nucleation rates and the less condensation of PAH on particle surfaces.When part of ethylene was substituted by H₂,the outer ring soot had the higher graphitization degree.Its fringe length and tortuosity exhibited the opposed variation trends.The higher flame temperature of post-impingement regions and the increase of OH radical enhanced the soot oxidation.The oxidation reactivity was increased for inner ring soot and decreased for outer ring soot with the increasing addition of hydrogen.To extend the relevant studies of soot formation in FWI,we investigated soot characteristics from asymmetrical impinging flame structures with various burner oblique angles,Reynolds numbers and nozzle to plate distances via the combination of probing sampling and quartz sampling methods on the basis of ethylene inclined jet diffusion impinging flames.The information of soot formation and evolution from FWI in asymmetrical impinging flame structures was obtained.The augment of the burner oblique angles suppressed the uphill combustion and promoted the downhill combustion.The crystallization degree reduced,the fringe length reduced,the fringe tortuosity increased and the oxidation reactivity improved for soot from uphill.Soot aggregates from downhill had longer carbon chains.The coagulation of soot was enhanced.Soot particles had the higher graphitization degree and the less oxidation reactivity.When Re increased from 300 to 900,the flow rates increased and the velocity of mixtures which arrived at the plate surface improved.The promotion of the plate on combustion processes was enhanced.The intensified soot formation and oxidation were observed at uphill and downhill.The primary particle diameter and the number of mature particles in soot aggregates of uphill and downhill were increased.The multi-core structures were found in particles.The carbon layers tended to be compact and the graphitization degree were higher.The soot oxidation reactivity of uphill and downhill decreased with the augment of Re.When H increased from 10 to 20 mm,the stretch of the free jet region made the vertical velocity of impinging flame decreased which impaired the plate promotion on combustion.Correspondingly,the particle boundary seemed to be obscure in uphill and downhill soot aggregates.The fullerene structures were found in several particles.More amorphous carbons were formed in soot.The fringe length decreased,the fringe tortuosity increased and the carbonized degree reduced.Soot particles became easier to be oxidized by oxygen.This study could offer the crucial experimental guideline for the research of soot characteristics in FWI from practical combustion systems.