[论文解读] A Novel Approach for Direct Measurement of the Stretch Factor in Laminar Premixed Hydrogen-Air Flames Affected by Thermodiffusive Instabilities

Marcel Marburger, Christoph Möller+6 more|arXiv (Cornell University)|Mar 19, 2026

Combustion and flame dynamics被引用 0

一句话总结

本论文提出一种以棒锚定的V形火焰实验装置，利用 OH-PLIF 直接通过比较稳定和热扩散不稳定火焰支折来测量伸展因子 I0，并辅以二维仿真。

ABSTRACT

This study introduces a novel experimental configuration using OH-PLIF imaging to directly determine the stretch factor ($I_0$) in laminar premixed hydrogen flames transitioning from a quasi-stable to a thermodiffusively unstable regime. A rod-anchored V-flame is stabilised in a laminar premixed reactant flow. Near the anchoring rod, the mildly strained flame remains quasi-stable, exhibiting a smooth surface and a well-defined inclination angle ($θ_{\mathrm{s}}$) to the main flow. This stable branch is associated with a burning rate $S_{\mathrm{s}}$. Farther downstream, the flame abruptly transitions to a regime dominated by thermodiffusive (TD) instabilities, characterised by cellular structures and a wrinkled surface. The distance between this transition and the anchor decreases with increasing equivalence ratio. This TD-unstable branch exhibits a larger mean flame-surface angle ($θ_{\mathrm{u}}$), enabling direct determination of the flame-speed increase, $S_{\mathrm{u}}/S_{\mathrm{s}}$. It is assumed that this ratio represents the normalised flame consumption speed, $S_{\mathrm{c}}/S_{\mathrm{L}}$. Determination of $I_0$ additionally requires the increase in flame-surface area caused by the thermodiffusive instabilities. Three complementary methods are therefore used to evaluate the surface area of the TD-unstable branch ($A$) relative to a smooth reference area ($A_0$), yielding consistent trends in $A/A_0$ over the investigated equivalence-ratio range. The resulting $I_0$ values, with the main uncertainty arising from $A$, decrease monotonically with increasing equivalence ratio, from about 1.1--1.3 at $ϕ=0.35$ to 0.8--0.9 at $ϕ=0.40$, consistent with theoretical predictions. Additional numerical simulations in a reduced two-dimensional representation reproduce the same transition behaviour and yield qualitatively consistent results.

研究动机与目标

• Motivate and quantify the influence of thermodiffusive instabilities on lean hydrogen–air premixed flames.
• Develop a direct experimental method to estimate the stretch factor I0 from observable flame geometry and surface area.
• Isolate and characterize the stable and TD-unstable flame branches to enable model validation.
• Provide numerical support via reduced 2-D simulations to corroborate the experimental methodology and trends.

提出的方法

• Use OH-PLIF imaging of a rod-anchored V-shaped lean H2/air flame to identify a stable upstream branch and a downstream TD-unstable branch.
• Determine the TD onset point and extract flame-front inclination angles (θs, θu) to compute S_u/S_s geometrically from sin relationships.
• Estimate flame-surface area increase A/A0 in the TD-unstable region using three contour-detection strategies (f-canny, f-otsu, f-otsuCanny).
• Define the stretch factor I0 via I0 = (S_u/S_s) (A0/A) as an approximate measure when S_s ≈ S_L.
• Perform 2-D numerical simulations (CONVERGE) with SAGE chemistry and mixture-averaged diffusion to reproduce the experiment and compute Yc-based flame surfaces for comparison.]
• research_questions: ["Can a rod-anchored V flame configuration enable direct observation of TD-instability onset under lean H2/air conditions?","Can the ratio of stable to unstable flame speeds and the associated surface-area increase be combined to yield a reliable estimate of the stretch factor I0?","Do 2-D simulations reproduce the experimentally observed TD transition and support the proposed I0 estimation approach?","How does equivalence ratio influence the TD onset, flame angles, surface area, and the inferred I0?","What are the uncertainties in estimating A/A0 from OH-PLIF data, and how do different surface-detection methods compare?"]
• key_findings: ["A stable upstream branch transitions abruptly to a TD-unstable, highly wrinkled branch downstream, with TD onset location decreasing as equivalence ratio increases.","The TD-unstable branch shows a larger mean flame angle θu, indicating a higher effective flame speed S_u relative to S_s, inferred from θu/θs via S_u/S_s = sin(θu)/sin(θs).","The mean area ratio A/A0 of the unstable branch remains approximately constant across φ, while S_u/S_s decreases with increasing equivalence ratio, yielding a decreasing I0 with φ.","Experimental I0 estimates lie roughly between 1.1–1.3 at φ=0.35 down to 0.8–0.9 at φ=0.4, consistent with theoretical expectations and model predictions.","Numerical simulations qualitatively reproduce the transition and trends, and enforcing unity-Lewis-number transport suppresses the TD instability, confirming TDIs arise from sub-Lewis-number effects in lean H2/air.","Three independent experimental area-detection methods yield consistent A/A0 trends, supporting the robustness of the I0 estimation approach."]
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Fig. 1: Cross section of the test rig with imaging FOV.

实验结果

研究问题

• RQ1 Can a rod-anchored V flame configuration enable direct observation of TD-instability onset under lean H2/air conditions?
• RQ2 Can the ratio of stable to unstable flame speeds and the associated surface-area increase be combined to yield a reliable estimate of the stretch factor I0?
• RQ3 Do 2-D simulations reproduce the experimentally observed TD transition and support the proposed I0 estimation approach?
• RQ4 How does equivalence ratio influence the TD onset, flame angles, surface area, and the inferred I0?
• RQ5 What are the uncertainties in estimating A/A0 from OH-PLIF data, and how do different surface-detection methods compare?

主要发现

• A stable upstream branch transitions abruptly to a TD-unstable, highly wrinkled branch downstream, with TD onset location decreasing as equivalence ratio increases.
• The TD-unstable branch shows a larger mean flame angle θu, indicating a higher effective flame speed S_u relative to S_s, inferred from θu/θs via S_u/S_s = sin(θu)/sin(θs).
• The mean area ratio A/A0 of the unstable branch remains approximately constant across φ, while S_u/S_s decreases with increasing equivalence ratio, yielding a decreasing I0 with φ.
• Experimental I0 estimates lie roughly between 1.1–1.3 at φ=0.35 down to 0.8–0.9 at φ=0.4, consistent with theoretical expectations and model predictions.
• Numerical simulations qualitatively reproduce the transition and trends, and enforcing unity-Lewis-number transport suppresses the TD instability, confirming TDIs arise from sub-Lewis-number effects in lean H2/air.
• Three independent experimental area-detection methods yield consistent A/A0 trends, supporting the robustness of the I0 estimation approach.

Fig. 2: Selected single shots (left panels) and averages (right panels) for all equivalence ratios. The horizontal dashed red lines indicate the mean onset location of TDIs. An example of the calculated fit segments is given for the equivalence ratio of $\phi=0.36$ . The fit of the stable section is