Abstract
Moderate or Intense Low-oxygen Dilution (MILD) combustion is an emerging combustion technology which can simultaneously improve the efficiency and reduce the emission levels of the combustion systems. In this study, Direct Numerical Simulation (DNS) is used to study the role of gaseous additives on combustion characteristics and reaction pathways of MILD combustion. The heat loss effects on the ignition behaviour of MILD flames and the effects of exhaust gas addition on the combustion characteristics of syngas enriched methane flames are also investigated. Syngas and carbon monoxide are used as the gaseous additives, and methane is used as the base fuel.Syngas addition affects the ignition delay, flame structure, NO formation and reaction pathways of MILD methane flames. Syngas addition reduces the CH4 content of MILD methane flames which decreases their NO mass fraction levels (prompt NO) at least by an order of 10−1. The preferential diffusion effects become
important in MILD methane flames with syngas addition. Furthermore, in case of MILD methane flames, the ignition characteristics are controlled by the exhaust gas content of the oxidiser stream. Upon the addition of syngas, both exhaust gas content of the oxidiser stream and hydrogen content of the fuel control the ignition characteristics of MILD methane flames. For instance, syngas addition to MILD methane flames shortens their
ignition delay by up to 10−2 s. The syngas addition also increases the flux rate between the major intermediate species formed during the combustion process, and affects the major intermediate species formed during the ignition stage. Exhaust gas addition to syngas enriched methane flames reduces their flame temperature, flame structure, NO formation, and ignition delay. The changes observed in these parameters and properties depend on the amount of exhaust added to the flames.
The carbon monoxide addition to adiabatic MILD methane flames shortens its ignition delay up to 1.28 ms (at oxidiser temperature 1400 K). This shortened ignition delay is found to be significant especially at oxidiser temperatures lower than 1600 K. Additionally, the carbon monoxide addition affects the concentration of the major intermediate species, increases the scalar dissipation rate, and makes preferential diffusion effects
important especially in the flame development stage post the ignition stage.
The heat loss effects lengthen the ignition delay of MILD methane flames by 1.11 ms (from 1.01 ms to 2.12 ms) and the ignition delay of carbon monoxide enriched MILD methane flames by 1.01 ms (from 0.91 ms to 1.92 ms) at oxidiser temperature 1700 K. In case of adiabatic MILD methane flames, the ignition characteristics are governed by the molecular diffusion process. However, in MILD flames with the inclusion of heat loss effects, both molecular diffusion and turbulence govern the ignition characteristics. The investigation of carbon monoxide addition to turbulent MILD methane flames with the inclusion of heat loss reveals that such trends agree well with those observed in the adiabatic turbulent MILD methane flames.
The reaction pathway analysis shows that the carbon monoxide addition to MILD methane flames increases the flux rate between the intermediate species. In addition, in MILD methane flames, only the ethane to vinyl pathway seems to be important in the methane to carbon dioxide oxidation pathway. In carbon monoxide enriched MILD methane flames both ethane to vinyl and formaldehyde pathways become important in the methane to carbon dioxide oxidation pathway. However, in both the flames, the top intermediate species involved in methane to water vapour oxidation pathways are identical, and their oxidation pathways are similar.
Date of Award | 2022 |
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Original language | English |
Awarding Institution |
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Supervisor | Mansour Qubeissi (Supervisor), Ebrahim Abtahizadeh (Supervisor) & Nwabueze Emekwuru (Supervisor) |