Investigation of Nanoparticle-Induced Differences in Premixed Blended Hydrogen Flames ��Rahat Mollick and Albert Ratner

University of Iowa

Iowa City, Iowa, USA

July 26^th, 2026

2026 Premixed Turbulent Flames Workshop

Presentation Outline

• Motivation
• Experimental Setup
• Results & Discussions
• Methane-Hydrogen Flames
• Ammonia-Hydrogen Flames
• Conclusions
• Acknowledgements

​

​

Ratner Research Group

Motivation- Hydrogen Enrichment of Methane

Ratner Research Group

[4] Koroll et. al (1993)

Hydrogen has a laminar flame speed of around 288 cm/s [4]; approximately 7 times that of Methane

Motivation- Hydrogen Enrichment of Methane

Ratner Research Group

[4] Koroll et. al (1993)

Hydrogen has a laminar flame speed of around 288 cm/s [4]; approximately 7 times that of Methane

100% Methane

80% Methane 20% Hydrogen

Motivation

Strategies to Reduce Hydrogen Usage

Optimized Reactor Design

Advanced designs can improve mixing and combustion.

Fuel Additives

Small amounts of additives can enhance flame stability.

Flame Stabilization

Techniques like swirl and recirculation can help.

Ratner Research Group

Acetylene Black

Ratner Research Group

Schematic of the Experimental Setup

Ratner Research Group

Visuals of Different Components of the Study

Fig. Seeder enclosed (left), opened (middle), Zoomed in (right)

Low Swirl Burner (top view)

Premixer (Front view)

Flame on top of the Premixer- Low Swirl Burner Assembly

Ratner Research Group

Results-Blowoff Limit

​

• Seeding effect is most prominent when there is no Hydrogen addition

​

• In terms of blowoff limits, while seeding provides slightly additional benefits for the 20% hydrogen blend, it shows no measurable effect on the 40% hydrogen blend’s blowoff limits

​

• This can be attributed to the predominant effects of hydrogen's elevated mass diffusivity and high reactivity, which tend to overshadow the impact of nanoparticles

​

Ratner Research Group

Results-High Speed Camera Images

Figure : High speed color images of (a) 100% Methane at Φ = 1; no seed (b) 100% Methane at Φ = 1; seeded (c) 80% Methane 20% Hydrogen at Φ = 1; No Seed (d) 80% Methane 20% Hydrogen at Φ = 1; Seeded

• For pure Methane flame at Φ=1, nanoparticle seeding led to a 64% increase in cumulative pixel intensity
• For 80% Methane 20% Hydrogen blend at the same equivalence ratio, seeding resulted in approximately 73% increase in cumulative pixel intensity

Ratner Research Group

Non-Dimensional Analysis for Non-seeded blends

• The non-dimensional analysis shows that 100% Methane operates in a weaker stabilization regime, with relatively low Damkohler number and higher Karlovitz number
• With 20% H₂ addition, the flame shifts toward higher Da and lower Ka, especially near richer conditions, meaning hydrogen already improves chemical reactivity and reduces turbulence–chemistry mismatch

Ratner Research Group

Results-Colormap of Detected Edge

• Non-seeded flames [(a), (b)] show intense, spatially extended OH zones with high pixel intensity near the shear layers, indicating stronger local heat release and greater small-scale wrinkling

​

• Seeded flames [(c), (d)] exhibit noticeably more compact and symmetric flame structures, with reduced intensity gradients and a narrower OH region. This suggests enhanced flame stabilization and suppressed turbulent wrinkling due to nanoparticle-induced radiative preheat and altered diffusion behavior

Ratner Research Group

Results- Seeding effects on Flame Front Curvature Statistics

• Curvature PDFs shift toward a sharper peak with seeding, indicating that seeded flames have a more concentrated curvature distribution and fewer extreme flame-front distortions.

Ratner Research Group

Results- Seeding effects on Flame Front Curvature Statistics

• Nanoparticle seeding consistently decreases curvature variance (up to ~31%) and the mean radius of curvature (up to ~17%) across equivalence ratios

Figure: Seeding effects on (a)variance of curvature (b) average absolute radius of curvature for all the tested fuel compositions across different equivalence ratio

(a)

(b)

Ratner Research Group

Results- Seeding Effect on Flame Brush

• A reduction in maximum FSD is indicative of a more coherent, less distorted flame front and improved combustion stability

• Seeding yields a more uniform colormap, fewer high-intensity regions

Ratner Research Group

Ammonia Flames

Results-High Speed Camera Images

Figure: High speed color images of (a) 100% Ammonia at Φ = 1; no seed (b) 100% Methane at Φ = 1; seeded (c) 80%Ammonia 20% Hydrogen at Φ = 1; No Seed (d) 80% Ammonia 20% Hydrogen at Φ = 1; Seeded

• Less localized flame extinction is observed in seeded Ammonia Flame
• For pure Ammonia flame at Φ=1, nanoparticle seeding led to a ~43% increase in cumulative pixel intensity
• For 80% Ammonia 20% Hydrogen blend at the same equivalence ratio, seeding resulted in approximately ~5% increase in cumulative pixel intensity

100% Ammonia

80% Ammonia 20% Hydrogen

Ratner Research Group

Non-Dimensional Analysis for Non-seeded Ammonia Blends

• The non-dimensional analysis shows that 100% Ammonia operates in a weaker stabilization regime, with relatively low Damkohler number and higher Karlovitz number
• With 20% H₂ addition, the flame shifts toward higher Da and lower Ka, especially near richer conditions, meaning hydrogen already improves chemical reactivity and reduces turbulence–chemistry mismatch

Ratner Research Group

Results-Colormap of Detected Edge

• The seeded–non-seeded contrast is more apparent in the averaged OH-PLIF fields, where random single-shot fluctuations are reduced, and the mean flame brush becomes easier to compare.
• The reduced spatial spread in the seeded case indicates a narrower flame brush, suggesting that acetylene-black seeding limits large-amplitude flame-front motion and supports more repeatable flame anchoring.
• This qualitative PLIF trend supports the blowoff data, where seeding provides measurable stabilization for weak NH₃ flames operating under low-Da/high-Ka conditions.

Ratner Research Group

Results- Seeding effects on Flame Front Curvature Statistics

• Curvature PDFs shift toward a sharper peak with seeding, indicating that seeded flames have a more concentrated curvature distribution and fewer extreme flame-front distortions.

Ratner Research Group

Results- Seeding effects on Flame Front Curvature Statistics

Seeding consistently decreases both curvature variance and average absolute radius of curvature relative to the non-seeded case, indicating reduced flame-front distortion and a more stable reaction zone; the effect is stronger for the NH₃/H₂ blend

Ratner Research Group

Results- Seeding Effect on Flame Brush

• Seeding expands and sustains the flame-brush region, increasing axial persistence in pure NH₃ and broadening the active branches in NH₃/H₂ flames, which indicates improved reaction-zone stability and greater resistance to local extinction.

​

• Mechanistically, this is consistent with nanoparticle-assisted radiative preheating and modified local transport, which help the weak ammonia flame remain ignited and spatially sustained.

​

• The upward shift indicates greater downstream persistence of the flame surface, meaning the seeded flame remains active over a longer axial distance and is less confined to the immediate stabilization region.

Ratner Research Group

Results- Seeding Effect on Flame Surface Density

• For 100% NH₃, seeding increases the conditional mean FSD over most of the progress-variable range, showing that acetylene-black particles help sustain flame surface density in the weak ammonia flame.

• For the NH₃/H₂ blend, non-seeded cases already show high FSD at intermediate-to-high progress variable, indicating that hydrogen strongly enhances the reaction-zone structure even without seeding.

• The OH-based progress variable organizes the flame from weak-OH to strong-OH regions, while the conditional mean FSD shows how much flame surface exists at each stage of that reaction progress.
• Overall, the progress-variable-conditioned FSD confirms that seeding has the strongest stabilizing influence in pure NH₃, while its effect becomes more selective when H₂ chemistry already strengthens the flame.

Ratner Research Group

Conclusions about Methane-Hydrogen Flames

. Nanoparticle Seeding (Acetylene Black, 0.01 mg/s)

• Shifts lean blowoff by ~6.7% for pure CH₄ (better anchoring/quench resistance).
• Sharper PDF peaks & narrower widths → reduced flame-front variability.
• Curvature variance ↓ up to ~31%; near ϕ ≈ 0.8–1.1.
• Maximum FSD ↓ up to ~23%, indicating less wrinkling and a more coherent flame brush.
• Benefits persist at 20% H₂; diminish at 40% H₂ but do not reverse.
• Hydrogen Addition:
1. Adding hydrogen (e.g., 20% or 40%) reduces absolute mean curvature, improving overall flame stability due to hydrogen's higher diffusivity and reactivity.
2. However, hydrogen addition increases the variance of curvature, indicating a broader distribution of curvatures and a higher probability of localized wrinkling.
3. Increasing hydrogen content beyond 20% (e.g., 40%) yields limited additional smoothness/stability, suggesting a plateau in benefits due to diminishing returns.
• Practical Implications:
• Optimizing fuel composition (e.g., hydrogen blending), equivalence ratio can significantly improve flame properties and, apply trace nanoparticle seeding to reduce wrinkling and enhance stability.
• Potential applications in enhancing combustion efficiency and reducing emissions in industrial and energy systems.

Ratner Research Group

Conclusions about Mixed Ammonia Flames

• Acetylene-black seeding measurably improves ammonia flame stability, with pure NH₃ showing a clear blowoff-limit improvement of about 5.3%

​

• Imaging show that seeding produces a more compact, symmetric, and anchored reaction zone, supporting the observed improvement in flame stability

​

• Curvature statistics confirm reduced flame-front distortion with seeding, indicating suppression of turbulent wrinkling and more stable flame-front behavior

​

• Flame-brush and conditional FSD analyses show that seeding redistributes and sustains flame surface density, especially in weak low-Da/high-Ka ammonia flames.

Ratner Research Group

Acknowledgements

KBIH Foundation

Ratner Research Group

Thank You!!

​

Happy to take your questions!

​

Feel Free to reach out with questions to albert-ratner@uiowa.edu

​

​

​

Ratner Research Group