Ignition and lean blowoff dynamics of turbulent flames in annular combustors

Abstract

This thesis aims to provide experimental insights on the transient phenomena of ignition light-around and lean blowoff dynamics of turbulent flames in annular combustors. Numerous ignition experiments have been performed in single flame setups but are unable to replicate the physics of flame spreading and propagation from one injector to the next in an annular geometry. Similarly, lean blowoff studies of a single flame fail to account for any flame interaction present in multi-injector configurations which is known to affect the eventual lean blowoff limit. The present work aims to contribute to the existing limited data concerning these transient flame dynamics through various experiments conducted on three different laboratory-scale annular combustors. The first of such designs is the Cambridge/NTNU annular combustor which has a greatly simplified geometry to facilitate fundamental studies. Ignition studies using hydrocarbons corroborated past findings that the ignition speed is positively correlated to a lumped parameter of laminar flame speed and thermal expansion (quantified using dilatation ratio). Mixture blends of hydrogen and ammonia were used to isolate effect of the dilatation ratio and the laminar flame speed. The use of these non-carbon fuel blends strongly suggests the dominant role of thermo-diffusivity in influencing flame propagation speed. Notably, a decrease in the volumetric expansion effect (lower dilatation ratio) leads to an unexpected increase in flame speed which is contrary to behaviour of hydrocarbon fuels and highlights that due consideration is needed when generalising existing findings to non-carbon fuels which often have drastically different flame properties. In collaboration with SAFRAN Helicopter Engines and CERFACS, two new laboratory-scale combustors were conceived to replicate the design principles of a SAFRAN spinning combustion technology (SCT) engine: (i) strong azimuthal swirl, (ii) Rich-Quench-Lean (RQL) combustion mode, and (iii) azimuthal fuel staging. These setups are named SCT V1 and SCT V2. SCT V1 was used to assess effect of strong azimuthal swirl on light-around and lean blowoff in premixed flames. SCT V2 explored these effects in a RQL configuration with and without azimuthal fuel staging. The presence of strong azimuthal swirl in SCT V1 and V2 was shown to promote higher injector-to-injector propagation speeds, and improve ignition and lean blowoff limits. Under a RQL combustion mode in SCT V2, non-premixed flame ignition behaviour was observed in that no clear correlation could be established between propagation speed and bulk flow velocity. Careful design of azimuthal fuel staging may lead to substantial improvements in the ignition limits, increasing the operating envelope of an engine. Lean blowoff limits of the three annular combustors and setups representative of their single flame equivalent were also mapped. It is found that in all three annular configurations, multiple interacting flames may extend the global extinction limits. Notably, in the case of SCT V1 and V2, there are indications that under the influence of strong azimuthal swirl, flames from an injector often have a piloting (and stabilising) effect on adjacent injectors. This finding is substantiated with Large Eddy Simulation work performed by CERFACS. Efforts are currently underway to further characterise and elucidate the various findings reported in this dissertation which have sought to (i) explore the ignition dynamics of ammonia and hydrogen in anticipation of the transition to a carbon-free future, and (ii) evaluate the ignition and lean blowoff physics of various combustor designs closely mimicking that of a Spinning Combustion Technology (SCT) engine.