An experimental and numerical investigation of premixed combustion in a vortex in a laboratory dump combustor

Abstract

A one dimensional acoustic model was used to predict the resonant modes of the Caltech pulsed combustion facility. The model accurately predicted pressure FFTs found through experiments for the 2.5 and 7.6 cm duct height configurations. Heat addition locations were found to have only marginal effects on shifting the location of the facility's acoustic modes. A detailed experimental analysis of the reacting vortex structures shed from a rearward facing step was also performed using high speed shadowgraph and CCD cinematography. Premixed vortical combustion was found to have two ignition mechanisms depending on the prior status within the combustor. In the first, burning was initiated at the surface and proceeded toward the center while in the second ignition was initiated near the center and the flame propagated outward. Time delays measured from the start of vortex shedding to subsequent ignition or to the corresponding maximum burning intensity were found to vary inversely with combustor pressure during injection (shedding) and with combustor pressure during burning. Reducing the height of the combustor increased interactions between the burning vortex and the wall, inhibited vortex growth, and produced longer axial burning regions and higher overall straining throughout the structure's cycle. Vortex straining was defined in two ways: first, based on the growth rate of the core diameter of the structure and second, based on the effective length of the streamline separating hot combustion products and cold reactants. Straining provided a sufficient delay mechanism to shift vortex shedding from 237 to 188 Hz for the 5.1 cm case.