Prediction of nano/micro aluminum particles ignition in oxygen atmosphere

•Propose a validated model to predict ignition of multiscale aluminum particles in oxygen.•Heat transfer in transition regime is modeled and validated to yield a model covering whole Knudsen number.•Develop accurate prediction equations for ignition temperature and ignition delay time.•Key factors on the ignition of multiscale aluminum particles are examined to prompt ignition. Ignition prediction of aluminum particle is of great significance for a variety of propulsion and power systems to achieve optimal energy release within a limited residence time. In this study a heat transfer model employing temperature dependent coefficients was developed and validated to describe the heat exchange between quiescent/flow gas and aluminum particles from nano- to micro-size, covering the free-molecular to continuum regimes. By coupling heat transfer and aluminum oxidation, a theoretical model has been proposed to accurately capture ignition properties of both aluminum nanoparticle and microparticle (ANP and AMP) burning in hot oxygen atmosphere. Two formulas were obtained to predict the ignition temperature and ignition delay time for nano/micro particles, which show good agreements with experimental results, providing a convenient and accurate method for practical application. A parametric study illustrates that AMP ignition is affected by bulk flow velocity, radiation and oxygen concentration, particularly for AMP over 100 μm in diameter; in contrast, ANP is more sensitive to alumina thickness which generally raises both ignition temperature and ignition delay time. The present study not only deepens the fundamental understanding of aluminum combustion but also provides a guideline for prompting ignition.