Study Of Laminar Premixed Flame Propagating In Confined Spaces And Chemical Kinetic Mechanisms For Typical Combustible Gases

Laminar premixed flame propagating in confined spaces is a very important combustion science and technology subject on safe utilization of combustible gases, internal combustion engine and detonation theory. Making proper measures to prevent the fire and explosion (detonation) accidents, suppress their spreads and mitigate the damage, quite depends on the scientific research. And the study of internal combustion engine also needs fundamental flame propagation data and detailed chemical mechanism to develop combustion model for design optimization and assessment of alternative fuels. Therefore, aiming to more comprehensively reveal the intrinsic rules and mechanisms of flame initiation, development, acceleration and mutation, deeply scrutinize the reaction zone structure and chemical process in the flame, build robust chemical kinetic model and collect more and useful fundamental flame data, we carefully investigate the laminar premixed flames of typical combustible gases in confined spaces, including flame structure and shape, flame acceleration and mutation, laminar flame speed and chemical kinetic mechanism, through experiment, theory and numerical simulation.Firstly, the laminar premixed hydrogen-air and propane-air flames in a closed duct are examined, including flame and pressure dynamics, interactions of flame with flame-induced flow and pressure wave, etc. The high speed schlieren photography captures and records the flame shape change and position. The high-precision pressure sensor detects the instant pressure change in the duct. Results show that, the premixed flame in the closed duct undergoes complicated shape changes with formation of classic Tulip and possible distorted Tulip structures. Due to the flame deceleration and acceleration, wall confinement, boundary effect, pressure wave and flame-induced flow, the movements of flame skirt, contact point of flame and side wall, Tulip cusp and flame tip, can be divided into stages. The prediction of movement parameters by Bychkov model does not agree well with the experiment, especially at later stages. We find the fluctuations of flame tip position (velocity) and pressure in all flames but with different frequencies and amplitudes. The distorted Tulip structure is not an exclusive behavior of premixed hydrogen-air flame, but is also found in the stoichiometric premixed propane-air flame in the closed duct. The Tulip distortion is always accompanied by the salient flame tip velocity fluctuation. The pressure wave does not trigger the flame vibration and Tulip distortion, but indeed enhances the flame deformation. The combination effect of wall and boundary effect, squish flow, hydrodynamic instability, and flame-induced flow might be the physical origin.Subsequently, the laminar flame speeds of C2hydrocarbons (mainly ethane, ethylene, and acetylene) are measured using the outwardly propagating spherical flame method at atmospheric and elevated pressures in a10cm diameter cylindrical chamber with a concentric pressure release chamber and two windows. The flame speed is very sensitive to equivalence ratio, initial pressure and molecular structure of fuel. Our atmospheric data are in good agreement with those reported in the literatures. The previous scattered and inadequate flame data are validated and supplemented. Evidently, the CO2dilution plays different roles in various fuel systems. Apparently, CO2dilution inhibits the propagation of ethylene flame and rich ethane flame, while it has little or no impact on acetylene flame and lean ethane flame as a manifestation of the offset of its inhibition effect. Generally, the agreement between experiment and model prediction from USC Mech Ⅱ is not good enough, and even worse at high pressures. The model is just able to qualitatively reproduce the trend of flame speed change.After a discussion about the deficiency of USC Mech Ⅱ, a new comprehensive model is developed and proposed using quantum chemistry and experimental measurement based on recent advances in chemical kinetics. The performance of new model prediction at atmospheric and elevated pressures is much better than USC Mech Ⅱ. New model gives us a detailed insight of dominant reaction path ways of acetylene, ethylene and ethane flames. Some similarities of flame structure and combustion behavior among hydrocarbon fuels are pointed out. The important elementary reactions in high pressure flame systems are distinguished and analyzed. The chemical and third body effects of CO2dilution on flame propagation appear in two contradictory forms: inhibition and enhancement. On one hand, CO2dilution increases the H atom production via equilibrium shift of CO+OH=CO2+H and enhancement of third body reaction H+O2(+M)=HO2(+M); on the other hand, the loss of H atom production is compensated via HCO(+M)=H+CO(+M). CO2dilution effect is an apparent manifestation of the competition and counteraction of these two mechanisms. In acetylene flame, CO+OHNCO2+H plays a minimal role in CO2formation and thus has less importance resulting in weakened CO2dilution effect. In ethane and ethylene flames, the sensitivity of flame speed to HCO decomposition rate coefficient decreases, especially at high pressures, which leads to inhibited flame propagation. In particular, the chemical and third body effects of CO2dilution on rich ethane flame are not dominant in flame deceleration; but the changes of thermal and mass transport, mixture concentration and density after dilution are the main cause. Different from ethane and ethylene flames,O atom production (not H atom production) is crucial in acetylene flame. The concentration change of O atom directly affects the flame behavior.