Studies On Burning Velocities And Flame Acceleration Dynamics Of Laminar/Turbulent Premixed Spherically Expanding Flames

The high pressure and lean premixed turbulent combustion modes are widely utilized in highspeed spark-ignition engines for transportation and in advanced gas turbines for power generation,which could quickly convert chemical energy of fuels into thermal energy in a clean,efficient,and stable manner.However,there still exists many fundamental problems in high pressure lean premixed combustion,especially when carbon-free and renewable fuels such as hydrogen and biofuel gradually replace traditional fossil fuels.Laminar burning velocity(SL)is the most important fundamental parameter of premixed flame.Lean premixed conditions cause a strong nonlinear effect of flame stretch on local burning rate,leading to a great challenge on accurate measurement of SL.Flame intrinsic instabilites are always easy invoked by high pressure conditions.The flame self-acceleration introduced by the flame instabilities is a strong nonlinear process,which is very difficult to accurately predict by the state-of-art theory.The intervention of wide-scale turbulence makes the fundamental research on premixed flame more challenging.The physical mechanisms of the coupling effects of the strong molecular diffusion and heat diffusion imbalance and the multi-scale turbulent eddies on lean premixed turbulent flames are still not clear.The interaction of multi-scale disturbances on flame front caused by both turbulence and flame instabilities under high pressure conditions is extremely complex,and experimental research in such medium and intense turbulence is very scarce.With the support of National Basic Research Program of China and National Natural Science Foundation of China1,a medium-scale,cylinder-type,spark-ignited,fan-stirred turbulent combustion chamber was designed and established for high temperature,high pressure,and intense turbulence conditions.Burning velocities and flame acceleration dynamics of both laminar and turbulent premixed spherically expanding flames were studied to clarify the physical mechanisms of laminar and turbulent combustion under high pressure and lean premixed conditions.The main findings and innovative achievements are summarized as follows.1.To address the issue of complex perturbations on laminar burning velocity measurement using spherically expanding flames,an algorithm was developed to efficiently and accurately select the proper flame radius range under different conditions,which could avoid inaccurate measurement results caused by using the traditional fixed flame radius range.Results show that the critical lower flame radius depends non-linearly on Lewis number(Le)and pressure,which is controlled by the strong coupling of non-linear stretch and spark ignition effects.The critical upper flame radius is 0.3-0.33 times of the inner chamber radius for stable flames,which is dominated by the chamber size but also affected by fuel type and pressure.Furthermore,a high accuracy non-linear extraction equation was deduced from theory to obtain accurate SL under wide Le conditions.The new expression could not only increase the extraction accuracy but also extend the ability to process strong nonlinear stretched flames.Moreover,the SL of oxygen enriched methane flames at high pressures were measured based on the developed methods,which provides reliable experimental data and measurement methods for the verification of the detailed chemical kinetic mechanism under high-pressure conditions.2.Parameter well controlled experiments were conducted to investigate the accelerative propagation dynamics of laminar hydrogen flames subjected to intrinsic cellular instabilities.It is found that a self-similar acceleration stage characterized by a slowly self-similar increasing acceleration exponent(?)following a short transition acceleration period,rather than the usually suggested self-similar propagation stage characterized by a constant ? from the traditional theory and early experimental work.The experimental critical onset(Pecr)and transition(Pect)Peclet numbers of flame instability increase non-linearly with the burned gas Markstein number(Ma).The linear stability theory could not quantitatively predict the experimental results,because the linear correction of the diffusional-thermal term is inaccurate,and the key fundamental parameters of flame and transport cannot be obtained accurately.A regime diagram was proposed for unstable hydrogen flames based on experimental Pecr and Pect,including smooth propagation,transition acceleration,and self-similar acceleration regimes,which was validated by a numerous number of experimental data from different research groups in literature.In addition,an empirical correlation for ? was obtained.This correlation could reproduce not only the data of hydrogen flames but also the results from syngas flames,which provides an experimental basis for the development of the flame acceleration dynamics theory with flame intrinsic instabilities.3.The propagation of two turbulent expanding methane/hydrogen flames were compared with different molecular diffusions but almost the same SL and flame thicknesses(lf)under diffenent turbulence intensities(urms).Results show that the molecular diffusion have a strong influence on self-similar propagation and burning velocity of turbulent flame even in intense turbulence,while traditional theory neglects the molecular diffusion effects on flame propagation in intense turbulence.Furthermore,the turbulent expanding flame is in self-similar nature regardless of Le,and the strength of accelerative propagation increases significantly with the decreasing Le.The Le effects on turbulent flame could be explained by the strong coupling of molecular diffusion and flame stretch on promoting the local burning velocity and increasing the flame surface area within the turbulent brush,and as such enhancing the flame self-similar propagation.In addition,a modified correlation for turbulent burning velocity(ST)was obtained with the consideration of molecular diffusion based on experimental data and theoretical model.This correlation is able to describe not only the present experimental data but also the ST in literature measured under different conditions and even using different flame geometries,which provides experimental support for the development of turbulent combustion theory under lean premixed conditions.4.With the coupling analysis on the growth rates of wrinkles on the flame front caused by multi-scale turbulence and flame intrinsic instability,the combustion process was found to be dominated by flame instability and turbulence in weak and intense turbulence,respectively.While in medium turbulence environment,both flame instability and turbulence are of equal importance.Parameter well controlled experiments were conducted on spherically expanding flames in medium and intense turbulence to investigate the flame propagation process,acceleration exponent,and burning velocity.Experimental results confirme that both flame instability and turbulence control the flame accelerative propagation by respectively dominating large-scale and small-scale disturbances on the flame front in medium turbulence.In intense turbulence,the flame accelerative propagation is dominated by turbulence,while flame instability still plays a significant role in the enhancement of ST,which provides an experimental basis for the optimization of the high-fidelity turbulent combustion model under high pressure conditions.