Direct determination of turbulent burning velocity during aluminum flame propagation: A comparison of three experimental methods

Burning velocity is a key parameter of main flame propagation models. However, its experimental determination while studying propagating dust flame is still challenging. In this work, aluminum flame propagation in a vertical tube is studied. Two aluminum powders with median diameters of 6.2 and 20.7 mu m are analyzed for different equivalence ratios with air. The main objective of this work is to compare the methods commonly used in the literature to determine the burning velocity in the case of propagating flames. One of these methods is based on the estimation of the thermal expansion coefficient. This article focuses first on the estimation of this coefficient and presents the limits of considering the adiabatic flame temperature for its estimation. As detailed in the paper, these methods have some limitations and are therefore compared with an innovative method based on a local direct determination of the burning velocity. This local method is based on the measurement of the unburned flow velocity just ahead of the propagating flame front by Time-Resolved Particle Image Velocimetry (TR-PIV). The methods commonly used in the literature mainly underestimate the burning velocity when compared with the local method. The local method is then used to study the influence of the particle size distribution and the equivalence ratio on the turbulent burning velocity. Firstly, we observe that the turbulent burning velocity increases while the flame is propagating in the vertical tube. Furthermore, the turbulent burning velocity with the 6-mu m powder is higher than with the 20-mu m powder.