TY - JOUR
T1 - Flame structure and ignition limit of partially premixed cool flames in a counterflow burner
AU - Reuter, Christopher B.
AU - Won, Sang Hee
AU - Ju, Yiguang
N1 - Funding Information:
The authors recognize the support of NSF grant CBET-1507358, NASA ISS Post-Graduate award NNX15AB67G, and the DoD NDSEG Fellowship program.
Publisher Copyright:
© 2016 The Combustion Institute. Published by Elsevier Inc.
PY - 2017
Y1 - 2017
N2 - Due to their natural coupling of low-temperature chemistry and transport, cool flames are a valuable platform for drawing fundamental understandings of complicated phenomena relevant to real engines. In this study, self-sustaining partially premixed cool flames of dimethyl ether are investigated in detail through the use of an ozone-assisted counterflow burner. A double cool flame with distinct diffusion flame and premixed flames sides is visibly observed at increased fuel loading and equivalence ratio. The examination of double cool flames and double hot flames through planar laser-induced fluorescence measurements reveals the stark differences in the role of CH2O in each. The results show that while CH2O is one of the main product species of the cool flame via low-temperature chemistry, for the hot flame it is only a short-lived intermediate produced in the preheat zone. Comparisons of experimental results with numerical calculations based upon detailed chemical kinetic models show fair agreement on the double cool flame structure but a noticeable discrepancy in the prediction of the second-stage ignition limit, which triggers the transition from cool flames to hot flames. The critical strain rate for second-stage ignition is shown to be much more sensitive to fuel addition on the premixed side of the double flame than on the diffusion side. A mechanism for second-stage ignition in partially premixed cool flames is proposed based upon numerical modeling and experimental observations: H2O2 is primarily formed in the premixed cool flame, diffuses toward the stagnation plane, and then finally decomposes into OH radicals upon approaching the cool diffusion flame.
AB - Due to their natural coupling of low-temperature chemistry and transport, cool flames are a valuable platform for drawing fundamental understandings of complicated phenomena relevant to real engines. In this study, self-sustaining partially premixed cool flames of dimethyl ether are investigated in detail through the use of an ozone-assisted counterflow burner. A double cool flame with distinct diffusion flame and premixed flames sides is visibly observed at increased fuel loading and equivalence ratio. The examination of double cool flames and double hot flames through planar laser-induced fluorescence measurements reveals the stark differences in the role of CH2O in each. The results show that while CH2O is one of the main product species of the cool flame via low-temperature chemistry, for the hot flame it is only a short-lived intermediate produced in the preheat zone. Comparisons of experimental results with numerical calculations based upon detailed chemical kinetic models show fair agreement on the double cool flame structure but a noticeable discrepancy in the prediction of the second-stage ignition limit, which triggers the transition from cool flames to hot flames. The critical strain rate for second-stage ignition is shown to be much more sensitive to fuel addition on the premixed side of the double flame than on the diffusion side. A mechanism for second-stage ignition in partially premixed cool flames is proposed based upon numerical modeling and experimental observations: H2O2 is primarily formed in the premixed cool flame, diffuses toward the stagnation plane, and then finally decomposes into OH radicals upon approaching the cool diffusion flame.
KW - Cool flame
KW - Low-temperature chemistry
KW - Ozone
KW - Partially premixed flame
KW - Second-stage ignition
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U2 - 10.1016/j.proci.2016.06.067
DO - 10.1016/j.proci.2016.06.067
M3 - Article
AN - SCOPUS:85016251435
SN - 1540-7489
VL - 36
SP - 1513
EP - 1522
JO - Proceedings of the Combustion Institute
JF - Proceedings of the Combustion Institute
IS - 1
ER -