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Lookup NU author(s): Edward Hunt, Dr Andrew AspdenORCiD
This work is licensed under a Creative Commons Attribution 4.0 International License (CC BY 4.0).
This paper presents direct numerical simulations (DNS) of thermodiffusively-unstable lean premixed hydrogen flames in the canonical turbulent flame-in-a-box configuration. A range of reactant (pressure, temperature, and equivalence ratio) and turbulent (Karlovitz and Damk\"ohler number) conditions are used to explore the effects of the small and large turbulent scales on local and global flame response. Turbulence-flame interactions are confirmed to be independent from integral length scale (or equivalently, from Damk\"ohler number) for a fixed Karlovitz number. Furthermore, a recent model that predicts mean local flame speed as a function of an instability parameter and Karlovitz number is also demonstrated to be independent from integral length scale. This model thereby reduces turbulent flame speed modelling for thermodiffusively-unstable cases to predicting surface area enhancement. Flame surface area wrinkling is found to have good agreement with Damk\"ohler's small-scale limit. There is some scatter in the data, although this is comparable with similar experimental data, and the freely-propagating flame properties have a greater impact on the turbulent flame speed than the flame surface area. It is demonstrated that domain size can have an effect on flame surface area even if the integral length scale remains unchanged; the larger volume into which flame surface area can develop results in a higher turbulent flame speed. This is not accounted for in conventional algebraic models for turbulent flame speed. To investigate the influence of the fuel Lewis number $\LeF$, an additional study is presented where $\LeF$ (alone) is artificially modified to span a range from 0.35 to 2. The results demonstrate that more flame surface area is generated for smaller $\LeF$, but the difference for $\LeF$\,$\lesssim$\,1 is much smaller than that observed for $\LeF$\,$>$\,1. A volume-filling-surface concept is used to argue that there is a limit to how much flame surface can develop in a given volume, and so there is only so much more flame surface can be induced by the thermodiffusive response; whereas the thermodiffusive response at high $\LeF$ is to reduce flame surface area. The agreement of the present data (and previous work) with Damk\"ohler's small-scale limit (even for low-to-moderate Karlovitz numbers) suggests that a distinction should be made between the small-scale limit and the distributed burning regime. Furthermore, it is argued that the distinction between large- and small-scale limits should be made based on Damk\"ohler number. Consequently, the flamelet, thin reaction and distributed regimes should be distinguished by Karlovitz number (as usual), but the two latter regimes both have separate large- and small-scale regimes. Finally, implications for the turbulent premixed regime diagram are discussed, and a modified regime diagram is proposed.
Author(s): Hunt EF, Aspden AJ
Publication type: Article
Publication status: Published
Journal: Combustion and Flame
Year: 2024
Volume: 272
Print publication date: 01/02/2025
Online publication date: 30/11/2024
Acceptance date: 13/11/2024
Date deposited: 07/11/2024
ISSN (print): 0010-2180
ISSN (electronic): 1556-2921
Publisher: Elsevier
URL: https://doi.org/10.1016/j.combustflame.2024.113855
DOI: 10.1016/j.combustflame.2024.113855
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