Experimental and numerical study on moving hot particle ignition

Ignition thresholds for n-hexane-air were experimentally and numerically determined using a moving hot sphere of 6 mm in diameter. The novel experimental setup built for this purpose was described in detail. Two-color pyrometry was used for surface temperature measurements, and shearing interferometry flow field visualization was used to observe the onset of an ignition kernel, and subsequent flame formation and propagation. The probability of ignition was found to be 90% at a sphere surface temperature of 1224 K. Analysis of the interferograms at the ignition threshold indicated that ignition occurs near the region of flow separation. Numerical simulations of the transient development of the 2-D axisymmetric motion and ignition were performed. Four reduced chemical mechanisms, including high and low temperature chemistry, and two diffusion models were used to determine their impact on the numerical prediction of ignition thresholds. The simulation results were unaffected by the choice of diffusion model but were found to be sensitive to the chemical kinetic mechanism used. The predicted ignition threshold temperatures were within 6-12% of the experimentally determined values. The numerical fields of the energy source term and a wall heat flux analysis confirmed the experimental observation that ignition occurs near the region of flow separation at the ignition threshold. Detailed analysis of the species temporal evolution at the ignition location revealed that n-hexane is present in small amounts, demonstrating the importance of accounting for fuel decomposition within the thermal boundary layer when developing simple chemical reaction models.