Effect of Heat Transfer Peculiarities on Ignition and Combustion Behavior of Al Nanoparticles

Abstract

Nanoenergetic materials have some advantages against micrometric and bulk materials. This is due to enhanced surface area and intimacy between reactive components that leads to increase in the reaction rate and decrease in the ignition delay. However, till now there is very limited understanding of fundamental physical processes that control reaction and combustion wave propagation. The heat transfer in the case of nanoparticles is characterized some specifi c features which determine the sometime unusual ignition and combustion behavior. The paper is focused on discussing the ignition and combustion of nano Al particles in conditions of a shock tube and in a plastic tube. It is shown that tiny metal particles at high temperatures and pressures become “thermally isolated” from ambient gas environment and experimentally observed ignition delays may be two order magnitudes longer of those calculated without accounting real energy accommodation and sticking coeffi cients. When going to conditions of reaction propagation in a plastic tube, some different ways for heat transfer have to be carefully analyzed. Actually, there are no evidences for unique dominant process which may provide propagation of combustion wave with observed speed through the loose Al/CuO particles mixture. It can be stated that the process comprises 2 stages with very fast ignition, releasing large amount of heat and propelling hot gas and condensed material in direction of unreacted mixture followed by more slow reaction of remaining metal with evolved in oxide decomposition oxygen. Common conclusion is that further detailed studying the fundamental properties of nanoenergetics materials and their reaction behavior may open ways for purposed control of the combustion behavior and for effective use of nanoenergetics in practical applications.