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Ph.D. Thesis Proposal by
(Advisor: Prof. Tim Lieuwen)
Monday, November 6th, 10:30 – 11:15 a.m.
Student Center Room 343
Abstract:
Nitrogen oxides (NOx) are an undesirable byproduct of hydro-carbon combustion in air. In lean, premixed combustion, NOx levels are exponential functions of temperature and linear functions of residence time. Historically, lean premixed technologies have enabled local flame temperatures below the value where substantial NOx production occurs; consequently, most combustor technology development to date has focused on fuel/air mixing and other operational challenges of premixed combustion (e.g., blowoff, flashback, etc). However, improved cooling technologies and high temperature materials have enabled steadily increasing combustor exit temperatures (>1800K), to the extent that this strategy is no longer capable of low NOx levels. Axial staging of combustion has been identified as a means to combat these issues while maintaining range of operability of combustors.
This paradigm shift motivates research into the canonical problem of a reacting jet-in-crossflow (RJICF). The NOx emissions of RJICF are influenced by the degree of premixing of the fuel jets before injection and jet/crossflow mixing before combustion. In turn, jet/crossflow mixing is controlled by the hydrodynamic stability of the jet, as well as degree of flame lifting. Previous work in this field has focused primarily on the flame behavior of RJICF and has established temperature rise across the jet (ΔT) as the primary driver of NOx production, but little work has been done to establish the sensitivities of NOx production of jets at equal ΔT.
Preliminary work with rich premixed jets of low momentum flux ratio (J < 5) has been conducted. Results have established a NOx emissions benefit due to axial staging above a combustor firing temperature threshold. At constant ΔT, reduced NOx production was associated with reacting jets of lower momentum flux ratio and higher jet equivalence ratio. Significant lifting of the flame was observed for jet equivalence ratios above 3.5 with a correlated impact on NOx emissions. This relationship is, however, confounded due to the sensitivities of lift-off distance to jet parameters that are shown to directly impact the NOx production of the reacting jet.
The proposed work seeks to investigate the NOx emissions and flame stabilization characteristics of premixed methane/air jets injected into a high temperature vitiated crossflow of lean natural gas/air combustion products. The work is designed to increase the understanding of NO production sensitivity to governing RJICF parameters over a wide parameter space, to include jets with: rich and lean equivalence ratios, low J (J < 10) and high J (J > 10), and even enhanced mixing due to wall effects. In addition, the proposed work will attempt to independently control degree of flame lifting in order to generate more precise understanding of its causes and NOx impact with regards to a RJICF. This will include emissions sensitivity studies as well as detailed high speed diagnostics of the flow/flame interaction of the lifted flame. Finally, utilizing this increased understanding, a model for a characteristic equivalence ratio of combustion based on jet and crossflow parameters as well as lift-off distance will be formulated to improve prediction and mitigation of RJICF NOx production.
