The transition of ethanol flames from conventional to MILD combustion

Abstract

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 .

Understanding and Interpreting Laser Diagnostics in Flames: A Review of Experimental Measurement Techniques

“…Signal may be effectively increased through the use of on-chip binning where a square number of pixels (usually four) act as a "superpixel" at a cost of in-plane resolution. This in-plane resolution is often less than the light sheet thickness which is typically ∼0.1-0.2 mm (Gordon et al, 2008;Medwell et al, 2008;Kaiser and Frank, 2011;Ye et al, 2016) and limits the actual resolution of the imaging system. Smoothing is a means of improving SNR in raw data prior to post-processing.…”

“…Concentrations and images of major, intermediary and radical combustion species in jet flames in hot and diluted coflows have been obtained through single-point Rayleigh-Raman-LIF (Cabra et al, 2002(Cabra et al, , 2005Dally et al, 2002;Dunn et al, 2009;Sepman et al, 2013) and planar LIF (Medwell et al, 2007(Medwell et al, , 2008(Medwell et al, , 2009bGordon et al, 2008Gordon et al, , 2009Dunn et al, 2009;Oldenhof et al, 2011;Oldenhof et al, 2012;Masri, 2011, 2012;Arndt et al, 2012Arndt et al, , 2013Sidey and Mastorakos, 2015;Ye et al, 2016Ye et al, , 2017Ye et al, , 2018Kruse et al, 2019) measurements.…”

“…The hot and diluted coflows of JHC flames suppress the formation of soot (Medwell et al, 2008;de Joannon et al, 2012;Ye et al, 2016Ye et al, , 2017Evans et al, 2017bEvans et al, , 2019b. Soot may be suppressed for typically sooty fuels such as ethylene under MILD combustion conditions (Medwell et al, 2008;Evans et al, 2017b).…”

“…Fundamental studies of these flames have not only provided significant insight into autoignition processes, but have been used to generate extensive datasets in simplified configurations for validating turbulence-chemistry interaction models for numerical modeling of combustion systems. Fundamental studies of laminar and turbulent jet flames issuing into high temperature, low oxygen environments have been undertaken in jet in hot coflow (JHC) burners (Dally et al, 2002;Medwell et al, 2007Medwell et al, , 2008Oldenhof et al, 2010Oldenhof et al, , 2011Oldenhof et al, 2012;Medwell and Dally, 2012a;Sepman et al, 2013;Ye et al, 2016Ye et al, , 2017Ye et al, , 2018Evans et al, 2017bEvans et al, , 2019aKruse et al, 2019), hot cross-flow burners (Sidey and Mastorakos, 2017), vitiated coflow burners (VCBs) (Cabra et al, 2002(Cabra et al, , 2005Gordon et al, 2008Gordon et al, , 2009Macfarlane et al, 2018Macfarlane et al, , 2019Ramachandran et al, 2019), and partially premixed jet burners (PPJBs) (Dunn et al, 2007a;Dunn et al, 2009), as have spray flames in hot coflow burners (Correia Rodrigues et al, 2015a,b;Wang et al, 2019b). In each case, fresh fuel issues from a jet into a stream of hot gas generated by lean premixed flames, resulting in 15% O 2 (by vol.).…”

“…Such improved understanding these transitions-and the structure of the upstream flames leading to their formation-may be achieved through targeted laser-diagnostics studies and subsequently validated, complementary numerical modeling. The high fidelity data that laser diagnostics can provide allows for the detailed study of reactive structures across the broad range of spatial and temporal scales in different optically-accessible JHC burners and reactors (Plessing et al, 1998;Cabra et al, 2002Cabra et al, , 2005Dally et al, 2002;Medwell et al, 2007Medwell et al, , 2008Gordon et al, 2008Gordon et al, , 2009Oldenhof et al, 2010Oldenhof et al, , 2011Oldenhof et al, 2012;Medwell and Dally, 2012a;Sepman et al, 2013;Sorrentino et al, 2015Sorrentino et al, , 2016Ye et al, 2016Ye et al, , 2017Ye et al, , 2018Evans et al, 2017bEvans et al, , 2019aSidey and Mastorakos, 2017;Macfarlane et al, 2018Macfarlane et al, , 2019Kruse et al, 2019;Ramachandran et al, 2019). Laser diagnostics a provide means to investigate the small-scale and distributed reaction zones in macroscopically-near-homogeneous MILD combustion conditions.…”