Laminar burning speed measurement, autoignition and flame structure study of spherically expanding flames.

摘要

Laminar burning speed is a thermophysical property of a combustible mixture. It is a measure of the rate of energy released during combustion in quiescent gas mixtures and incorporates the effects of overall reaction rates, energy (heat) of combustion and energy and mass transport rates. There are several experimental techniques to measure laminar burning speed and they can be broadly categorized into two general categories of stationary flames methods and those that are based on propagating flames. Investigation of spherical flame propagation in constant volume vessels is recognized to be one of the most accurate approaches for laminar burning speed measurement and flame structure study.;In this thesis flame structure, laminar burning speed and onset of autoignition are studied for different premixed combustible mixtures including n-decane, jet-fuels, and Hydrofluorocarbon (HFC) refrigerants in air at high temperatures and pressures over a wide range of fuel-air equivalence ratios. The experimental facilities consist of two spherical and cylindrical vessels. The spherical vessel is used to collect pressure data to measure the burning speed and cylindrical vessel is used to take pictures of flame propagation with a high speed CMOS camera located in a shadowgraph system.;A thermodynamic model is employed that assumes unburned gases compress isentropically and that burned gases are in local thermodynamic equilibrium. Burning speed is derived from the time rate change of mass fraction of burned gases. The major advantages of this method are that it circumvents the need for any extrapolation due to having low stretch rates and that many data points can be collected along an isentrope in a single experiment.;Flame structures are studied to determine the cell formation conditions. Critical pressures at which the flame becomes cellular are identified and the effects of important parameters on cell formation are studied. Autoignition experiments are carried out for JP-8 fuels with high initial pressures in the spherical chamber. Autoignition occurs at specific temperature and pressure during the compression of unburned gas due to flame propagation.