The Study On The Interaction Between Premixed Flame Propagation And Wall Film In Closed Space

The wet wall formed by spray impinging on the piston top and cylinder wall is difficult to avoid,which becomes an important factor affecting the engine combustion and emission performance.The over rich concentration zone formed by film evaporation near the wall will lead to incomplete combustion,resulting in the generation of carbon monoxide（CO）,unburned hydrocarbons（UHC）and other pollutants.Meanwhile,the pool fire of film will significantly increase the soot emission.In addition,relevant research showed that the participation of lubricating oil film in combustion could also deteriorate the engine emission performance.In order to further study the interaction mechanism between wall film and flame propagation,optimize engine spray combustion system and reduce harmful emissions,the present study carried out experimental and simulation studies on the interaction between premixed flame propagation and wall film under different flow conditions in the closed space.The influence of wet wall conditions on flame propagation and quenching,heat release law,component distribution and pollutant emissions,and the occurrence mechanism of film evaporation and pyrolysis reactions in the combustion process under laminar flame propagation,local turbulent combustion and jet turbulent flame propagation combustion modes were studied.The main research contents and conclusions include:First,aiming at the interaction between wall film and flame propagation under laminar combustion,optical experimental test and CFD（Computational Fluid Dynamics）simulation were implemented.In terms of experiments,a constant volume visual combustion system which can realize the uniform thin film on the wall was built.Using the synchronous system of high speed schlieren photography and transient pressure measurement,the variation laws of flame propagation,pressure and heat release rate,film thickness and quality in the process of methane air premixed combustion under the conditions of dry wall and diesel film wall were compared.In terms of simulation,the direct numerical simulation of laminar premixed flame propagation under the condition of side wall film was carried out.The flame quenching process,the regular of film consumption and the distribution of film pyrolysis products under different conditions were analyzed.The results showed that the participation of film vapor in combustion changed the flame propagation characteristics and pressure rise.The film mass consumption rate increased significantly under the condition of disturbed laminar flame.Under the condition of side wall film,the flame quenching distance were larger than that of dry wall conditions and changed continuously with the flame propagation.The film evaporation and cracking mainly occurred in the burnt area after side wall quenching.The film vapor components and small molecule pyrolysis products gathered within the quenching layer.Secondly,the laminar flame was disturbed by setting necking baffles in combustion space,and a local turbulent flow was gradually formed.Then,the influence of wall fuel film on the disturbed flame propagation process was studied.In terms of experiments,the changes of flame morphology,propagation,and pressure parameters under different baffle structures were compared.The effects of different initial conditions on the development of turbulent spreading region were analyzed as well.It was found that the effect of initial pressure on turbulent spreading was relatively large.The results also indicated that the film evaporation promoted by turbulent disturbance and the increase of combustion heat release jointly led to the appearance of pool fire.The RANS（Reynolds-Averaged Navier-Stokes）model was applied for three-dimensional CFD simulation of several test conditions.The temperature field,flow variation,species distribution and film consumption were quantitatively analyzed.Meanwhile,the reaction steps of the main small molecule hydrocarbon products of film pyrolysis were calculated and analyzed by using the chemical reaction mechanism.The results showed that the turbulent disturbance promoted the evaporation and pyrolysis of fuel film in the end stage of flame propagation,and the main pyrolysis products were acetylene（C₂H₂）and ethylene（C₂H₄）formed by H-abstraction and decomposition reactions,which also led to the increase of unburned hydrocarbon and soot emissions.Finally,the influence of wall film on high speed turbulent flame propagation was studied.In terms of experiment,by setting a pre-combustion chamber in the vessel to form high speed jet turbulent flame propagation,the effects of different types of film on jet flame propagation,rebound turbulent combustion and film pool fire were studied.The pool fire spreading process,reaction paths of different film pyrolysis and PAH（Polycyclic Aromatic Hydrocarbons）formation,and pollutant emission results under different types of film conditions were analyzed.The results showed that the participation of film vapor in the combustion increased the peak value of jet flame propagation speed under the lean and stoichiometric premixed conditions.In the turbulent convergence area near the wet wall,the phenomenon of film pool fire appeared and spread rapidly within 2 ms,and the fire spreading velocity was related to the volatility of film.In addition,the lubricating oil film also had a low degree of pool fire after increasing the initial pressures.Through the analyses of flame propagation,film evaporation and pyrolysis,and the development of pool fire,the present study revealed the influence mechanism of wall film on combustion heat release and pollutant formation under turbulent combustion conditions,and provided experimental and theoretical support for improving the combustion and emission performance of the engine.