Flame Stabilisation in the Transition to MILD Combustion

Abstract

Emissions reduction and energy management are current and future concerns for governments and industries alike. The primary source of energy worldwide for electricity, air transport and industrial processes is combustion. Moderate or intense low oxygen dilution (MILD) combustion offers improved thermal efficiency and a significant reduction of CO and NOx pollutants, soot and thermo-acoustic instabilities compared to conventional combustion. Whilst combustion in the MILD regime offers considerable advantages over conventional combustion, neither the structure of reacting jets under MILD conditions, nor the boundaries of the MILD regime are currently well understood. This work, therefore, serves to fill this gap in the understanding of flame structure near the boundaries of the MILD regime. The MILD combustion regime has been previously investigated experimentally and numerically in premixed reactors and non-premixed flames. In this study, definitions of MILD combustion are compared and contrasted, with the phenomenological premixed description of MILD combustion extended to describe non-premixed flames. A simple criterion is derived analytically which offers excellent agreement with observations of previously studied cases and new, non-premixed MILD and autoignitive flames presented in this work. This criterion facilitates a simple, predictive approach to distinguish MILD combustion, autoignitive flames, and the transition between the two regimes. The adequacy of simplified reactors as a tool for predicting non-premixed ignition behaviour in the transition between MILD combustion and autoignition has not previously been resolved, and is addressed in this work. The visual lift-off behaviour seen in the transition between MILD combustion and conventional autoignitive flames seen experimentally is successfully replicated using simplified reactors. The location of the visible flame base in a jet-in-hot-coflow burner is shown to be highly sensitive to the relative location of the most reactive mixture fraction and the high strain-rate shear layer due to the strong coupling of between ignition chemistry and the underlying flow-field. Previous studies have demonstrated a strong dependence of ignition delay times to significant concentrations of minor species. Simulations presented in this work demonstrate that small concentrations of the hydroxyl radical (OH), similar to those expected in practical environments, significantly affect ignition delay and intensity of non-premixed MILD combustion, however have little effect on autoignitive flames. Importantly, such concentrations of OH do not result in a change in flame structure for the cases investigated. Whilst these results stress the importance of minor species in modelling the transient ignition of non-premixed MILD combustion, steady-state simulations do not demonstrate the same sensitivity to concentrations of minor species expected in hot combustion products. These results suggest that the temperature and oxygen concentration in the oxidant stream are the most important factors governing the boundaries of, the MILD combustion regime. Investigations of reaction zone structure and ignition in, and near the boundaries of, the MILD combustion regime have demonstrated the relative importance of different aspects of ambient conditions and differences in structure between non-premixed MILD and autoignitive flames. These findings build upon the understanding of this regime and provide critical insight for future studies towards both fundamental research, and the practical implementation, of MILD combustion.