Wang, Xujiang; (2019) Direct Numerical Simulation of Lean Premixed Turbulent Flames at High Karlovitz Numbers under Elevated Pressures. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Lean premixed combustion is a promising strategy for the next generation of gas turbines, which is characterised by low pollutant emissions and high combustion efficiency. However, flame quenching and combustion instability arising from this technique could increase operating cost and decrease operating efficiency. The fundamentals behind these problems are not yet clarified. Therefore, it is essential to study and understand fundamental combustion phenomena of lean premixed flames in conditions relevant to practical combustion devices, which will promote the development of turbulent flame models and help to get full advantages of this technique. With the availability of increasingly powerful supercomputers, direct numerical simulation (DNS) of turbulent reacting flow has become feasible and affordable. This thesis investigates lean premixed turbulent H2/air flames at high Karlovitz (Ka) numbers under elevated pressures by using DNS with multi-step chemistry. The effects of the Karlovitz number, pressure, equivalence ratio and integral length scale on flame structures and chemical pathways are examined qualitatively and quantitatively. It is found that the relative probability of positive curvature to negative curvature is insensitive to Ka but sensitive to pressure and integral length scale (lt). On flame fronts, the local heat release rates in regions with high-positive curvatures are higher than those in regions with high-negative curvatures when conditioned on the same H2 consumption rate, whereas this phenomenon is getting weaker with decreasing Ka and increasing pressure. As pressure increases, the flame speed and thickness (δL) decreases, and the reaction zone moves to regions with higher values of progress variable. Moreover, the thickness of the inner layer conditioned on the laminar flame thickness becomes smaller under elevated pressures, which results in a lower probability of finding high curvatures in the high-pressure flames with a fixed Ka. Under conditions relevant to gas turbines, the heat release rate and scaled reaction zone thickness (δf /δL) increase with increasing equivalence ratio. However, flames demonstrate similar topological structures of flame fronts when Ka is fixed. Trenches of local equivalence ratio (φL) with small gradients are observed in concave structures outside the reaction zone, while φL plateaus with large gradients are observed in convex structures inside the reaction zone. When the integral length scale is smaller than the thickness of the corresponding laminar flame, turbulence is unable to stretch and interrupt the reaction zone and the flame presents laminar flamelet characteristics. However, the distributions of curvature and tangential strain rate are comparable with those in the same Ka flames with lt/δL ≥ 1.0. It is also found that keeping constant lt/δL ratio and Ka could isolate the effects of pressure on flame front structures. The turbulent flame with unity lt/δL ratio could capture the main features of heat release as those in flames with higher lt/δL ratios. Considering the chemical process, pressure could significantly modify the chemical pathways in both laminar and turbulent flames, and the effects are more significant than those of the Karlovitz number and integral length scale. Due to the combined effects of radical fractions and reaction rate constants, the local heat release is changed in different temperature windows when the mixture equivalence ratio varies.

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