Percolating Reaction-Diffusion Waves (PERWAVES) — Sounding Rocket Combustion Experiments

Percolating reaction–diffusion waves in disordered random media are encountered in many branches of modern science, ranging from physics and biology to material science and combustion. Most disordered reaction–diffusion systems, however, have complex morphologies and reaction kinetics that complicate the study of the dynamics. Flames in suspensions of heterogeneously reacting metal-fuel particles is a rare example of a reaction–diffusion wave with a simple structure formed by point-like heat sources having well-defined ignition temperature thresholds and combustion times. Particle sedimentation and natural convection can be suppressed in the free-fall conditions of sounding rocket experiments, enabling the properties of percolating flames in suspensions to be observed, studied, and compared with emerging theoretical models. The current paper describes the design of the European Space Agency PERWAVES microgravity combustion apparatus, built by the Airbus Defense and Space team from Bremen in collaboration with the scientific research teams from McGill University and the Technical University of Eindhoven, and discusses the results of two sounding-rocket flight experiments. The apparatus allows multiple flame experiments in quartz glass tubes filled with uniform suspensions of 25-micron iron particles in oxygen/xenon gas mixtures. The experiments performed during the MAXUS-9 (April 2017) and TEXUS-56 (November 2019) sounding rocket flights have confirmed flame propagation in the discrete mode, which is a pre-requisite for percolating-flame behavior, and have allowed observation of the flame structure in the vicinity of the propagation threshold

Characterising Iron Powder Combustion using an Inverted Bunsen Flame

Iron powder may be used as a modern energy carrier, with high energy density, safe and easy transportation and a carbon free combustion cycle [1]. However iron powder combustion is not yet well understood.Here we present an inverted Bunsen flame burner, built to characterise various parameters of iron powder combustion, focusing on the burning velocity. We also present the results of initial experiments performed on this burne

Flame Stabilization and Blow-Off of Ultra-Lean H2-Air Premixed Flames

The manner in which an ultra-lean hydrogen flame stabilizes and blows off is crucial for the understanding and design of safe and efficient combustion devices. In this study, we use experiments and numerical simulations for pure H2-air flames stabilized behind a cylindrical bluff body to reveal the underlying physics that make such flames stable and eventually blow-off. Results from CFD simulations are used to investigate the role of stretch and preferential diffusion after a qualitative validation with experiments. It is found that the flame displacement speed of flames stabilized beyond the lean flammability limit of a flat stretchless flame (ϕ=0.3) can be scaled with a relevant tubular flame displacement speed. This result is crucial as no scaling reference is available for such flames. We also confirm our previous hypothesis regarding lean limit blow-off for flames with a neck formation that such flames are quenched due to excessive local stretching. After extinction at the flame neck, flames with closed flame fronts are found to be stabilized inside a recirculation zone

Quantifying the impact of heat loss, stretch and preferential diffusion effects to the anchoring of bluff body stabilized premixed flames

The response of a premixed flame subjected to either flame stretch (and associated Lewis number effects) or heat loss has been well documented in the literature and has enabled a good understanding of canonical configurations such as flat burner-stabilized, counter flow and tubular flames. However, in practical burners, flames are simultaneously subjected to stretch, heat transfer with the flame holder and preferential diffusion effects. For such flames, usually the collective effect of underlying contributions is studied and individual effects are only treated in a qualitative manner. In this paper, our objective is to use flame stretch theory to separate and quantify the underlying contributions from flame stretch, preferential diffusion and heat transfer with the flame holder to the flame speed of bluff body stabilized flames. It is shown that the theory adequately predicts the flame displacement speed in comparison to the results from the numerical simulations. Using the quantification of contributions, an overall stabilization mechanism for H 2 enriched CH 4-air mixtures is discussed. The role of competing contributions from preferential diffusion and heat loss is highlighted especially near the flame base region where the flame speed is heavily impacted by all the effects. Insights are also given for low Lewis number flashback prone flames

An investigation into flashback and blow-off for premixed flames stabilized without a recirculation vortex

We have recently shown that premixed CH4/air flames anchored behind flame holders can stabilize in two flame stabilization regimes characterised by the presence or absence of a recirculation vortex [1]. Focus of the present work is on the underlying mechanisms governing flames anchored without the presence of a vortex for methane-air flames. We revisit the definition of the flame anchoring location and define a new anchoring location which results from flame stretch considerations rather than heat loss considerations. This location can be unambiguously defined for flame holders of different sizes. It is argued that such an anchoring location is more relevant for flames stabilized behind flame holders with sharp corners and do take into account the multi-dimensional nature of heat transfer with the flame holder as well. A quantitative assessment of heat transfer, stretch and preferential diffusion effects is then carried out at the anchoring location for elucidating their impact on the flame speed as a function of the flame holder size. New insights into flame blow-off, flashback and emergence of a recirculation vortex are obtained as a result of this investigation

Burn time and combustion regime of laser-ignited single iron particle

An improved particle generator based on electrodynamic powder fluidization is proposed and constructed for investigating single metal particle's combustion. The designed setup is able to generate a single metal particle moving upward with a well controlled velocity and trajectory and ignite it at near-uniform conditions by an infrared laser beam with flattened elliptical beam profile. Mechanically sieved narrow fractions of spherical iron particles with mean sizes in the range of around 26–54 μm were used in experiments. Particles burned in O 2/N 2 mixtures with oxygen content varying from 21% to 36%. Particle's trajectories, velocities, and arbitrary radiant intensities were measured by taking images with a high-speed camera and processing them with an in-house developed data processing program. Two characteristic times associated with particle combustion were measured: 1) total duration of high-temperature phase (t tot) and 2) time to the maximum brightness (t max). The results show that t tot and t max can be described by a d n-law with 1.57≲n≲1.72 and 1.46≲n≲1.60, respectively. The effect of oxygen concentration on t tot, t max, and t dec=t tot−t max was analyzed for selected particle sizes of 30, 40, and 50 μm. It was found that t max∝(1/X O2) n with 1.04≲n≲1.18 is almost linearly proportional to 1/X O2, while t dec shows a very weak dependency on the oxygen concentration at 26%–36%. This can be explained by the idea that the overall combustion process of iron is controlled by first external and then internal diffusion of oxygen owing to the saturation of oxygen on the particle surface

Temperature and phase transitions of laser-ignited single iron particle

The combustion behavior of single laser-ignited iron particles is investigated. Transient particle radiant intensities at 850 nm and 950 nm are measured by post-processing recorded high-speed camera images using an in-house developed particle tracking program. Then, the time-resolved particle temperature is obtained based on two-color pyrometry. A plateau-like stage shortly after the ignition is repeatedly observed, and identified as iron particle melting by the measured temperature and the estimated melting time. Besides, an abrupt brightness jump near the end of combustion is observed for most burning particles, while a small portion of the particles ( Fe 3O 4 (s) at 1869 K (congruent melting) and ii) L2 Fe 3O 4 (s) + O 2 (g) at 1855 K (eutectic reaction), where L2 represents a liquid iron oxide. Based on this, the presence of the brightness jump (spear point) is explained by a sudden solidification of supercooled iron oxide droplets with an atomic O/Fe ratio larger than or close to 4/3. Particles’ near-peak temperatures are also measured based on time-integrated spectra. The results indicate that the near-peak temperature increases first fast and then slowly with an increase of oxygen concentration. At higher oxygen concentrations, smaller particles have a slightly lower temperature. The effect of particle size on the near-peak temperature is negligible at lower oxygen concentrations due to weaker radiation. The morphology of combusted particles is examined by micrography. Some burned particles appear as hollow thin-shell spheres at all adopted oxygen concentrations. Additionally, nano oxides are found at 13–51% oxygen concentrations. Less traces of nano oxides were observed at reduced oxygen concentrations. The nano-oxide formation mechanisms are analyzed based on thermochemical equilibrium calculations