Burn time measurements of micron sized Al-Mg alloys and Mg in a heterogeneous shock tube in different oxidizing environments

by Jose L Guadarrama

Institution: University of Illinois – Urbana-Champaign
Year: 2016
Keywords: Combustion; Shock Tube
Posted: 02/05/2017
Record ID: 2105435
Full text PDF: http://hdl.handle.net/2142/90678


Metal additives have been widely used in combustion applications such as solid rocket motors and high explosives. There have been a vast number of studies performed on aluminum, magnesium, boron, beryllium to determine their combustion mechanism and to attempt at extracting their high energy content. Aluminum has been an extensively used metal additive as it has a high enthalpy of combustion, it is widely abundant and has a low toxicity. However, it has been shown to have ignition delays and incomplete energy conversion yields. Much of the research in present time has been focused on finding an energetic additive that can serve as a trigger in order to unleash its internal energy properly. Magnesium has been a preferential candidate due to its high temperature upon ignition, its porous oxide layer and its relatively high energy content as well. Majority of the work performed in the past on Mg and Mg-Al alloys has been performed with coarse particles (>50 μm), which tend to have burn times in the 1-100 milliseconds. Work on single aluminum particles below 10 microns has shown a transitional regime. This marks the shift from diffusion limited combustion to the kinetically limited mode. This shifts from a particle size dependence to a pressure and temperature dependence. The purpose of this study is to determine the burnt time of magnesium and aluminum-magnesium alloys by measuring their luminosity or photon emission. The work uses the heterogeneous shock tube facility at the UIUC to generate high temperature and pressure environments behind a reflected shockwave for tailored gas mixtures of oxygen and carbon dioxide with nitrogen used as the inert gas. To target the transition region for aluminum combustion, particles sizes of 5 and 10 micron, obtained commercially, are used. Numerous photometric measures are taken to try to gain as much quantitative data as possible. These include using a Si biased detectors, photomultiplier tubes and a CMOS high speed imaging camera. A thermo-fluid model uses transport properties to determine the approximate location of the particles in the post reflected flow. Burning time measurements are used to determine if any size, pressure, temperature and oxidizer dependence exists. Results collected from all photosensors gave insight on the combustion mechanism of the free stream particles in the shock tube. Magnesium Burning times were shorter than previously recorded Burning times from different experimental setups. Residence time, isothermal conditions and low turbulence could be attributors, as most previous tests had been performed with pulsed lasers or acetylene flames. Another observation is that Magnesium particles burn in more quiescent fashion than Al-Mg alloys at higher pressures. This could be due to a combination of internal and external stresses leading to crack formation on the particle surface which eventually spall or might also agglomerate.