Experimental studies of high pressure combustion using spherically expanding flames

2017-12-07Experiments involving laminar, premixed, spherically expanding flames were performed using two different experimental facilities. The first one involves experiments in a cylindrical constant volume chamber with optical access in which the flame is directly observed with high speed imaging using the shadowgraph technique. The second involves performing experiments in a totally spherical chamber with no optical access, for which the pressure evolution is the only observable and is used to derive flame speeds at much higher, engine relevant pressures and temperatures, taking advantage of the isentropic compression stage of the experiment. ❧ The first part of the study was aimed to define the uncertainty in data of laminar flame speed obtained from spherically expanding flames. A novel DNS (Direct Numerical Simulation) assisted extrapolation methodology was proposed in order to minimize systematic errors emerging from the use of theoretical linear and non-linear extrapolation equations to obtain the final value of the propagation speed at zero stretch rates. Moreover a complete, rigorous uncertainty quantification methodology was proposed, in order to calculate the errors from all the experimental aspects and then propagate and combine them to estimate the uncertainty in the final value of laminar flame speed. Such a study had never been performed in the past and is of high importance in order for the experimental targets to be meaningful for chemical scheme optimization of interest to the kinetic modeling community. ❧ Upon completion of the uncertainty quantification methodology, the second part of the study included a detailed investigation regarding our current knowledge of laminar flame speeds of C₂ hydrocarbons. Those fuels are of great significance as they comprise the foundational chemistry for the combustion of heavy hydrocarbons. Experiments of spherically expanding flames of CH₄, C₂H₆, C₂H₄, C₂H₂ mixtures were performed on an equal basis and the comparisons with kinetic model predictions revealed serious deficiency in predicting the propagation speed of rich acetylene flames. The importance of this evidence can also be supported by the fact that acetylene is one of main species involved in the reaction pathways that lead to soot production in combustion of hydrocarbon fuels under rich conditions. ❧ The third study in the current thesis revolves around flame acceleration due to instability formation on the flame surface during the compression stage of propagation. All studies currently existing in the literature have solely focused on describing the propagation characteristics of unstable flames during the initial, constant pressure stage of the flame propagation. In this study experiments were performed in both cylindrical and experimental configurations taking advantage of every capability. The onset of flame acceleration due to surface area growth caused from instabilities was promoted at various parts during propagation. The results obtained show an attenuating trend in the unstable flame acceleration, once the spherical flame enter the compression stage that is following the initial constant pressure and temperature propagation part