Effect of flow conditions on burn rates of metal particles

Micron-sized aluminum and titanium powder particles were carried by an air flow into a focal point of a CO2 laser beam, where they were ignited. The ignited particles continued to burn in room temperature air. The air flow pattern was varied in different experiments to produce both laminar and turbulent flow conditions. The flow pattern was described using a computational fluid dynamics model; characteristics of the turbulent flow were established and correlated with the particle combustion characteristics. Particle burn times and temperatures were measured optically. Effect of turbulent flow conditions was observed on combustion characteristics for both aluminum and titanium powders. For both powders, luminous combustion streaks became shorter. For aluminum, the brightness of the streak and the optically measured temperature were also substantially reduced in the turbulent flow conditions. For titanium, reduction in both streak brightness and temperature were minor for different flow conditions. The burn rates of aluminum were observed to increase more than 4 times in the turbulent flows compared to the burn rates measured for the same powder in laminar flow. For titanium, the burn rates in turbulent flows were approximately twice as fast as in the laminar flow. Simple empirical correlations are proposed for both metals enabling one to predict the increase in the metal particle mass burn rate for turbulent flow conditions taking into account the particle size and such flow characteristics as Kolmogorov length scale and average kinetic energy.