Correlation method for measuring the combustion time of micro-sized metal particles in a dust flame

The paper considers the possibilities of experimental determination of the combustion time of metal particles using the correlation analysis of flame luminosity. Experiments were carried out for dust flames of micro-sized spherical particles (d10< 5 µm) of Fe, Zr and Al in an axisymmetric laminar diffusion dust flame. The number density of particles in the gas suspension (in nitrogen) was about 1012 m-3. The width of the combustion zone at temperature T = (2000÷3000) K in the flame was 1÷2 mm. Under these conditions, the combustion zone is optically thin. This ensures the additive contribution of each particle to the combustion zone radiation. It was shown experimentally, as well as by simulation modelling methods, that the accuracy of combustion time measurements is affected by the stationarity of the object of study, the shape of the radiation trace from the burning particles, and the polydispersity of the initial fuel particles.It was found that the main cause of flame nonstationarity is low-frequency oscillations of different nature, which arise in the reacting two-phase flow at the moment of radiation registration.Studies have shown that processing of flame intensity time series by a high-pass filter (HPF) with a cut-off frequency of about 20 Hz significantly improves the appearance of the autocorrelation function (AF) and allows a more accurate determination of the effective correlation time (particle burning time). Limitations of the application of the HPF, which may lead to distortion of the AF and correlation time, are discussed.The interpretation of AF and correlation times is significantly complicated for polydisperse particle gas suspensions due to the dependence of the particle burning time and their radiative characteristics on the particle size. Simulation modelling methods show that in practice the range of monodispersity of fuel particles can be extended to values of the coefficient of variation of 20-25%.