Joint Imaging of Rayleigh and LIF-OH at the Base of Lifted Flames Issuing in Vitiated Coflow

The stabilisation of a lifted flame issuing in a vitiated co-flow has been studied both experimentally and numerically with the objective of resolving whether the dominant mechanism is due to auto-ignition or to premixed flame propagation. A key parameter used in both studies is the lift-off height, LH defined at the first location of occurrence of heat release downstream of jet exit plane. Experimentally, LH is either observed visually or obtained from direct photographs of the flame. Numerically, LH is obtained from the profiles of temperature or reactive scalar such as the hydroxyl radical OH. This paper presents measurements of temperature and the hydroxyl radical, OH made at the base of lifted flames using joint Rayleigh-LIF-OH imaging. Temperature is deduced from Rayleigh imaging while the quenching correction for OH is applied using correlations generated, as functions of temperature, from opposed flow flame calculations. These measurements form a sound platform for comparison with calculations. Also, the images may highlight differences in the flame structure depending on whether stabilisation is due to auto-ignition or premixed flame propagation.