Vladimir A. Alekseev

Experimental Uncertainties of the Heat Flux Method for Measuring Burning Velocities

ABSTRACT The laminar burning velocity is a fundamental property of combustible mixtures important for kinetic model validation as well as for practical applications. Many efforts are directed towards its accurate determination. The heat flux method is one of the commonly recognized methods for measuring laminar burning velocity, however, the information on the accuracy of the method is scattered in the literature. In the present work, an attempt was made to summarize and extend the available information on the different factors contributing to the experimental uncertainty of the heat flux method. Experimental setup of the Lund University group, typical for the heat flux community, and the procedures used to determine the burning velocity are described. Furthermore, the influence of different uncertainty factors, originating from each part of the setup, is analyzed. Asymmetric heat fluxes and the method for determining flame surface area were found to give an important contribution to the total error. As a result of this, some of the previously published data have been re-evaluated. Finally, recommendations are presented on how to control or reduce the uncertainties in the heat flux measurements, and possible directions for future research, aimed at improvement of the accuracy and understanding of the method, are outlined.

Strategy for improved NH2 detection in combustion environments using an Alexandrite laser

A new scheme for NH2 detection by means of laser-induced fluorescence (LIF) with excitation around wavelength 385nm, accessible using the second harmonic of a solid-state Alexandrite laser, is presented. Detection of NH2 was confirmed by identification of corresponding lines in fluorescence excitation spectra measured in premixed NH3-air flames and on NH2 radicals generated through NH3 photolysis in a nonreactive flow at ambient conditions. Moreover, spectral simulations allow for tentative NH2 line identification. Dispersed fluorescence emission spectra measured in flames and photolysis experiments showed lines attributed to vibrational bands of the NH2 A2A1←X2B1 transition but also a continuous structure, which in flame was observed to be dependent on nitrogen added to the fuel, apparently also generated by NH2. A general conclusion was that fluorescence interferences need to be carefully considered for NH2 diagnostics in this spectral region. Excitation for laser irradiances up to 0.2GW/cm2 did not result in NH2 fluorescence saturation and allowed for efficient utilization of the available laser power without indication of laser-induced photochemistry. Compared with a previously employed excitation/detection scheme for NH2 at around 630nm, excitation at 385.7nm showed a factor of ~15 higher NH2 signal. The improved signal allowed for single-shot NH2 LIF imaging on centimeter scale in flame with signal-to-noise ratio of 3 for concentrations around 1000ppm, suggesting a detection limit around 700ppm. Thus, the presented approach for NH2 detection provides enhanced possibilities for characterization of fuel-nitrogen combustion chemistry.