Moderate or Intense Low oxygen Dilution (MILD) combustion has been established as a combustion regime with improved thermal efficiency and decreased pollutant emissions, including NO x and soot. MILD combustion has been the subject of numerous experimental studies, and presents a challenge for computational modelling due to the strong turbulence-

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The present paper is focused on prevaporised ethanol flames in the transition from conventional combustion to the MILD combustion regime. Photographs and imaging of OH * chemiluminescence reveal a distinctive flame structure when ethanol carried by air burns in a 3% O 2 coflow (typical of MILD combustion), differing from that at higher oxygen levels. In comparison to flames carried by air in a 9% O 2 coflow, the spatial gradient and the peak in the OH * signal profile are significantly reduced in the 3% O 2 coflow, indicating a more uniform distribution of heat release and temperature. The use of N 2 as a carrier gas renders the OH * profile for the 6% O 2 coflow case similar to that of flames carried by air in the 3% O 2 coflow. The experimental results indicate a transition from conventional combustion to MILD combustion with the decrease of coflow O 2 level and/or the use of N 2 as a carrier gas. Calculations reveal that a substantial drop in the peak heat release rate and/or overall net heat release rate might contribute to the lack of luminosity of flames in the 3% O 2 coflow, suggesting a need for threshold values of these two in defining MILD combustion. A series of laminar flame calculations are performed to identify the MILD combustion regime based on the absence of a negative heat release region. The absence of a negative heat release region is found to be strain rate dependent at a given temperature and O 2 level of the oxidant stream. This is mainly a result of the enhanced transportation of O 2 across the reaction zone at a higher strain rate. At low strain rates, the negative heat release region is more likely to disappear in a 3% O 2 oxidant flow due to a combination of low flame temperature and high availability of O 2 .

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