Experimental study of the effect of helium/nitrogen concentration and initial droplet diameter on nonane droplet combustion with minimal convection

In this paper, we study the influence of inert concentration and initial droplet diameter on nonane (C₉H₂₀) droplet combustion in an environment that promotes spherical droplet flames. The oxygen concentration is fixed while the inert is varied between nitrogen and helium. A range of initial droplet diameters (D_o) are examined in each ambient gas: 0.4 mm < D_o < 0.8 mm; and an oxidizing ambiance consisting of 30% oxygen (fixed) and 70% inert (fixed), with the inert in turn composed of mixtures of nitrogen and helium in concentrations of 0, 25, 50, 75, and 100% N₂. The experiments are carried out at normal atmospheric pressure in a cold ambiance (room temperature) under low gravity to minimize the influence of convection and promote spherical droplet flames. For burning within a helium inert (0% N₂), the droplet flames are entirely blue and there is no influence of initial droplet diameter on the local burning rate (K). With increasing dilution by nitrogen, droplet flames show significant yellow luminosity indicating the presence of soot and the individual burning histories show K reducing with increasing D_o. The evolution of droplet diameter D(t) is nonlinear for a given D_o in the presence of either helium or nitrogen inerts indicating that soot formation has little to do with nonlinear burning. A correlation is presented of the data in the form where the effective burning rate, K′, and ε are concentration-dependent. Correlations for these parameters are presented in the paper.