E​RCOFTAC PC Germany

Ignition of Iron Particles in a Turbulent Jet with Hot Co-Flow

A​uthors:  Bich-Diep Nguyen, Arne Scholtissek, Christian Hasse
(Technical University of Darmstadt, Simulation of Reactive Thermo-Fluid Systems (STFS), Germany)

Christopher Geschwindner, Janik Hebel, Benjamin Böhm, Andreas Dreizler,
(Technical University of Darmstadt, Reactive Flows and Diagnostics (RSM), Germany)

Iron particle ignition in a particle-laden turbulent jet injected into a hot co-flow. The upper half displays high-speed experiments by Hebel et al. at 50 kHz, showing particle dispersion via Mie scattering and ignition through combustion luminosity. The lower half shows complementary large-eddy simulations of Nguyen et al. resolving entrainment, particle heating to ignition and oxidation progress from metallic iron to iron oxide. The combined visualization displays the interacting mechanisms governing ignition and flame development.

Scientific Abstract

Combustion of iron particles attracts growing interest as a pathway to carbon-free energy conversion, in which iron serves as a recyclable energy carrier for transport and storage of renewable energy. As a metal fuel, iron enables the retrofit of existing coal-fired power stations for dispatchable power generation without CO2 emissions. To translate this conceptual idea into robust technology, iron particle combustion must be understood under conditions relevant to practical combustors, where laboratory-scale studies have revealed strong interactions between turbulent mixing, particle clustering, and ignition behavior [1]. A particle-laden turbulent jet injected into a hot co-flow provides a canonical yet application-relevant configuration to investigate these effects in a controlled setting. In this contribution, a visualization of a combined experimental and numerical investigation of such a jet configuration is presented, with the aim of revealing the mechanisms governing particle ignition and subsequent oxidation.

The experimental recordings are obtained using high-speed optical diagnostics operating at 50 kHz and are shown in the upper half of the animation [2]. A dual-camera arrangement enables the simultaneous recording of iron particles illuminated by a fiber laser via Mie scattering (green color map) and particle ignition through combustion luminosity (yellow–red color map). The shown field of view extends 45 mm downstream from the jet nozzle, capturing the near-exit region where ignition first occurs. The Mie-scattering images reveal a pronounced edgepeaked particle distribution at the jet exit, originating from turbophoretic transport and near-wall accumulation of inertial particles inside the injection lance. These preferentially edge-aligned particles are subsequently drawn into the jet by entrainment of the hot co-flow, forming characteristic thread-like particle structures. Downstream of the nozzle, localized ignition events occur preferentially within this shear layer, where enhanced mixing and entrainment promote rapid particle heating.

To further interrogate the mechanisms underlying the experimentally observed ignition behavior, large-eddy simulations employing an Euler–Lagrangian framework are performed under matching conditions. The numerical results are shown in the lower half of the animation [3]. The initial visualization is chosen to closely mimic the appearance of the experimental recordings, displaying particle trajectories and ignition in a manner that allows direct qualitative comparison and demonstrates very good agreement in terms of particle dispersion, shear-layer ignition, and flame development.

Subsequently, the visualization blends into the mixture fraction field between the turbulent jet and hot co-flow, highlighting the entrainment and mixing processes that govern particle heating. In this view, individual particles are colored according to their oxidation state, with initially metallic iron particles shown in black and progressively changing to blue and white as oxidation proceeds toward iron oxides. This combined representation makes it possible to directly associate local mixing conditions with particle ignition and oxidation history. By correlating ignition events with local mixture fraction and gas temperature, the simulations reveal that particle ignition is initially dominated by convective heating due to entrainment of the hot co-flow into the turbulent shear layer. As the number of reacting particles increases downstream, heat released to the surrounding gas promotes additional ignition through inter-particle thermal coupling, leading to a cascade-like amplification of the reaction zone.

Associated Publications

[1] P. Steffens, J. Hebel, D. Braig, A. Vahl, L. L. Berkel, S. Schary, H. Nicolai, A. Scholtissek, A. Dreizler, B. Böhm, C. Hasse: Exploring turbulent methane-assisted iron dust combustion: A combined experimental and numerical study of a 47 kWth lab-scale combustor, Fuel 392 (2025). DOI: 10.1016/j.fuel.2025.135205.
[2] J. Hebel, C. Geschwindner, K. Westrup, A. Dreizler, B. Böhm: Ignition of iron particles in a turbulent round jet with hot co-flow, Fuel, under review (2026).
[3] B.-D. Nguyen, A. Scholtissek, J. Hebel, C. Geschwindner, B. Böhm, A. Dreizler, C. Hasse: Flame characteristics of a turbulent iron particle-laden jet issued into a hot co-flow, Proceedings of the Combustion Institute 41, under review (2026).