A Shock Tube Study Of Methane Combustion Chemical Kinetics Mechanism

As the main access to energy, combustion is much concerned with the environment-friendly and safe use of energy which becomes more and more important in today's social and economic development. Since shock tube is a primary apparatus to investigate the chemical kinetics of combustion, a shock tube experiment platform was designed and established and CFD simulations were carried out to better understand the unsteady flow fields in it. Further more, the fundamental researches on combustion mechanism of methane, especially ultra lean methane, were conducted.Firstly, a shock tube device as the fundamental equipment together with the gas delivery, pressure test and spectral measurement system was established. In order to control the arrival time of shock wave, we developed a low- voltage/high-current electro-heating diaphragm bursting device. It is demonstrated that this method have little influence on the chemical reaction process and EMI impact on the instruments, and the shock wave arrival time tolerance is less than±1ms. All synchronizations of instruments of Nd:YAG Laser pumped PLIF measurement were harmonized by a self-developed Atmega-MCU-based central controller. The coordinating process included laser preheating, shock wave creating and arriving, reaction process monitoring, diagnostic laser pulse firing and ICCD photography. The uniformity of incident shock wave was also validated by acetone PLIF measurement. Semi-quantitative analysis of the results agrees well with the empirical results. It gives a demonstration to the visualization of flow filed in shock tube experiments and confirms the validity of the high spatial and temporal resolution PLIF measurement technique as a new method for the mechanism research of the interactions between shock wave and gaseous matter, such as combustion, detonation and nonstationary wave.Secondly, Shock-induced ignition of stoichiometric methane/air mixture was experimentally studied by digital chemiluminescence imaging and OH-PLIF visualization technique. Shock waves of different strengths lead to different post shock temperatures of methane/air mixture, and then result in different energy release powers per unit mass. So there are two typical characteristics of reaction zones in strong and weak ignition models. The OH-PLIF results agreed well with the measurements of spontaneous luminescence and pressure profiles in this paper, and were in accordance with the former conclusion from experiments and numerical simulations. In weak ignition, because ignition delay was relatively long compared with the time scale of the shock wave propagation, the role of fluctuations caused by turbulence, pressure perturbations, etc., became more obvious. That means more time was provided to non-linear chemical reaction and the reaction zone showed distinctively irregular structure. When inducing shock wave was strengthened, the nonuniform characteristics decreased. Regular detonation wave structure could be seen in reaction zone of strong ignition. Lots of hot spots, in which the OH radical distribution or chemiluminescence is much more intensive than the surrounding area, exist in different stages of weak ignition process. At the initial stage of ignition, the hot spots are small kernels, then interact with each other to plaques and form the flame; at the subsequent flame propagating stage, the hot spots will appear at the interface between the unburned mixture and the flame front, speeding up the propagation of the flame front.Thirdly, we developed a computing platform for parallel Fluent. The third order MUSCL scheme for spatial discretization, second order implicit time integration, Roe Flux-Difference splitting scheme for convective fluxes, RNG k-εturbulence model, preconditioning and dual-time formulation were employed to solve 2D, unsteady, compressible, time-averaged N-S equations. The simulation results of unsteady fields exhibit the processes of shock forming, propagating and reflecting and the interactions between reflected shock and unsteady boundary layer and between bifurcated shock and contact surface in shock tube operation and show a good qualitative agreement with the previous research. Also the key impact factors of region 5 experimental condition are pointed out.Finally, the ignition delays of ultra lean methane (ULM) were measured by OH* and CH* spectral method in shock tube platform. The experimental results were compared with predicting results from detailed mechanism, the typical empirical expression and some reduced mechanisms and verified the applicability of GRI-Mech 3.0 mechanism. Through analyzing the results, we found the power law relationship between the effective active energy and methane fraction. An improved empirical expression for predicting ignition delay of methane oxidation at temperature of 1100～1900K is obtained. Also it provides a potential correction to overall reaction mechanism parameter. The reaction characteristics of ULM were studied by local and overall sensitivity analysis. There are several competing reactions to radical between methane and oxygen, like H+O₂<=>OH+O VS. H+CH₄<=>CH₃+H₂. Low methane concentration would certainly activate the most important branch reaction of H+O₂<=>OH+O. So the ignition delay time of ULM is decreased. Meanwhile, low methane concentration case weakens the influence of complex reaction of methyl (CH₃+O₂<=>O+CH₃O). And the oxidation of methyl (CH₃+O₂<=>O+CH₃O) would dominate ULM combustion.