Polydispersity Effects in Low-order Ignition Modeling of Jet Fuel Sprays

Abstract

Low-order ignition models are important tools in the design of aviation gas turbines. In this paper, a stochastic model that predicts the ignition probability in a combustor based on a time-averaged coldflow solution is extended to include local fuel concentration fluctuations due to the polydisperse nature of the spray. For this, a stochastic approach to modeling such fluctuations is considered, and the effects of the flow and mixture parameters on the resulting equivalence ratio pdfs are investigated. The concentration of fuel in large droplets results in a high variation of the local equivalence ratio, hence affecting the local flammability factor at the model's cell scale. The extinction criterion of the ignition model based on a critical Karlovitz number is calibrated based on ignition probability data from canonical experiments using jet fuel, suggesting critical Karlovitz values of spray flames between 0.2 and 0.6, which is to be contrasted with values of 1.5 for gaseous fuels. ARTICLE HISTORY

Low-order modeling of high-altitude relight of jet engine combustors

“…The present work demonstrates the application of recent experimental findings in a canonical geometry 21 to the practical case of an aeroengine re-igniting at high-altitude conditions. Attention is given to flow conditions where atomization of the liquid fuel within the combustor is far from normal operation conditions, that is, characterised by high Sauter mean diameter and increased polydispersity.…”

“…The critical Karlovitz number Ka crit was set to 0.5 in the present studies, a value that was obtained from a backwards calibration of the present model using ignition probability measurements taken in canonical experiments of spherically propagating flames in uniform droplet dispersions with aviation fuel. 21 In the evaluation of Ka p , the laminar burning velocity of the spray flame,…”

“…The liquid-phase equivalence ratio / p;l is evaluated according to the stochastic procedure described previously. 21 Since liquid droplets are heavy compared to the gas and generally move with a velocity different from the gaseous carrier phase, the liquid equivalence ratio is not considered to be transported with the flame particles. Instead / p;l is evaluated for the cell volume at the instantaneous location of flame particle.…”

“…14 This approach has been a product of extensive validation on non-premixed flames, 14,19 premixed flames, 20 and more recently on kerosene spray flames. 21 Due to its the main assumptions related to flame transport, it is applied with good results to spray and gaseous nonpremixed conditions and high turbulence. Furthermore, the critical value for extinction was verified to be approximately 1.5 in experiments with gaseous premixed flames in isotropic turbulence, 18 and has been used in the code for non-premixed and spray flames 14,19,22 and premixed flames.…”

“…1,[28][29][30] This phenomenon was observed by de Oliveira and Mastorakos 1 mainly in the interdroplet and gaseous-like flame propagation modes, while, in contrast, continued propagation was attributed to the presence of large droplets in the vicinity of the reaction zone in the droplet-mode propagation regime. 1 As first approach to incorporate such complex combination of droplet-induced effects as fluctuations of flame speed, increased flame wrinkling at the droplet scale, and the change in the local flammability of the mixture due to the random position of the droplets in the spray, de Oliveira et al 21 have used a stochastic approach to evaluate the effect of local equivalence ratio fluctuations caused by the spray's polydisperse character was evaluated. In their work, measurements of ignition probability in kerosene spray flames 31 were used to directly evaluate the Karlovitz values at which kerosene spray flames extinguish or ignite.…”