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To further our understanding of iron particle combustion, especially in the liquid state, in situ optical measurements are performed for the size evolution and temperature of laser-ignited iron particles in a narrow size range around 45-55μm. The particle size evolution is monitored by a high-speed shadowgraphy system, and the particle temperature is probed using a two-color pyrometer synchronized with the shadowgraphy system. Before a particle reaches the peak temperature, its diameter grows linearly with elapsed time. Near the peak temperature, three types of particle size behavior are detected, namely, smooth transition, micro-explosion, and rapid inflation. For particles with smooth transition, the size and temperature reach the maximum values at the same time, and thereafter the size remains constant, while the temperature drops until rapid solidification of the iron oxide droplet. The micro-explosion and rapid inflation clearly indicate towards a quick gas release inside the particle. The ambient oxygen concentration has a strong effect on the growth rate of the particle size before the peak temperature but no influence on the final particle size. The measured relative particle diameter at the peak temperature suggests that iron has been fully oxidized and the degree of oxidation could be higher than FeO. At the onset of solidification, gas release occurs again, resulting in a second inflation or burst of already inflated particles. This suggests that the oxygen dissolved in the liquid iron oxide is at least more than needed to form solid Fe3O4, and the solidification process is accompanied with gaseous oxygen release via the phase transition: L2 → Fe3O4(s) + O2(g) at 1855 K, where L2 represents liquid iron oxide with dissolved excess oxygen.

In the present work, the discrete flame model [1] is augmented by introducing the thermal inertia of particles in the preheating zone. The effect of particle thermal inertia on flame speed, propagation limits, and near-limits dynamics of one-dimensional discrete combustion waves is studied using the new model. It is found that, with the increase of particle thermal inertia, the propagation velocity of the discrete flame decreases due to a smaller heating rate of the particles. Besides, particle thermal inertia extends the propagation limits compared to the prediction of the old model. Furthermore, it is mathematically proven that the nonphysical branch of the solutions for the discrete flame speeds, found using the old discrete model, is a set of solutions for the propagation limits of steady-state discrete flames with particle thermal inertia included. The flame speed predicted using the new model is also compared with that determined analytically using a continuum model considering the thermal inertia of the condensed phase [2]. We find that the discrete flame speeds predicted by the both models become closer to each other with increasing particle thermal inertia. Finally, the two models converge regardless of the discrete nature of the heat sources when particle thermal inertia is large enough so that can limit the flame propagation. The particle thermal inertia controlled flames could be regarded as a new kind of combustion regime.

Temporally, spatially, and thermally resolved measurements of in situ nanoparticle cloud formation during combustion of single micron-sized iron particles are carried out. The critical particle temperature for the cloud formation is identified using high-speed and high-magnification shadowgraphy, recording the evolution of a nanoparticle cloud, synchronized with two-color pyrometry probing the surface temperature of the burning micron particle. The finding could serve as an important criterion to minimize nanoparticle generation in the flame of iron-particle suspensions, making iron powders a cleaner and more recyclable fuel.

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 (< 10%) show a second plateau-like stage instead. The particle temperature right after the brightness jump (1880±70 K) is almost identical to that during the second plateau-like stage. This temperature corresponds to two phase-change temperatures in the Fe-O phase diagram: i) L2 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.

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 μ m 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 t o t ) and 2) time to the maximum brightness ( t m a x ). The results show that t t o t and t m a x 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 t o t , t m a x , and t d e c = t t o t − t m a x was analyzed for selected particle sizes of 30, 40, and 50 μ m. It was found that t m a x ∝ ( 1 / X O 2 ) n with 1.04 ≲ n ≲ 1.18 is almost linearly proportional to 1 / X O 2 , while t d e c 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.

A deep understanding of the ignition characteristics of syngas in O2/CO2 and O2/H2O atmospheres is essential for the application of oxy-syngas combustion. In the present work, ignition properties of a syngas with a typical H2-to-CO ratio (1:2) in O2/N2, O2/CO2 and O2/H2O atmospheres were investigated numerically. The ignition temperatures were determined by a 1-D model of a micro flow reactor with a controlled wall temperature profile, demonstrating that CO2 and H2O can lead to an increase in the ignition temperature compared to N2, and the increase is more pronounced for the O2/H2O atmosphere. The analysis manifests that CO2 and H2O can suppress OH production at the region with relatively lower wall temperature by promoting R10: H + O2(+M) = HO2+(M) to compete with R11: H + HO2 = 2OH for H radical. Moreover, the direct reaction effect (directly take part in reactions as reactants) and third-body effect of CO2 and H2O on ignition temperature were numerically isolated by adopting artificial species. The computation results reveal that the increase in ignition temperature mainly results from the enhanced reaction rate of R10 by the third-body effects of CO2 and H2O.

The radiation effect on flame temperature and NO emission of H2-lean (0.2H2 + 0.8CO) and H2-rich (0.8H2 + 0.2CO) syngas/air counterflow diffusion flames was numerically investigated using OPPDIF code incorporated with the optical thin model, statistical narrow band model and adiabatic condition. Firstly, the coupled effect of strain rate and radiation was studied. Disparate tendencies of NO emission with an increasing strain rate between H2-lean and H2-rich syngas flames were found at very small strain rate, and the effect of radiation reabsorption on NO formation can be neglected when the strain rate was greater than 100 s−1 for both H2-lean and H2-rich syngas flames. Because the radiation effect is vital to flames with small strain rate, its impact on flame temperature and NO emission was investigated in detail at a strain rate of 10 s−1. The results indicated that NO formation is more sensitive to radiation reabsorption than flame temperature, especially for the H2-rich syngas flame. The underlying mechanism was discovered by using reaction pathway analysis. Furthermore, the radiation effect under CO2 dilution of the syngas fuel was examined. It was demonstrated that the radiation effect on flame temperature became more prominent with the increase of CO2 concentration for both H2-lean and H2-rich syngas. The radiation effect on NO emission increased first and then decreased with an increasing CO2 content for H2-lean syngas, whereas for H2-rich syngas the radiation effect is monotonic.

Diffusion methane-air flames in Y-shaped meso-scale combustors of three different diameters (i.e., d = 4, 5 and 6 mm) were experimentally studied. The impact of adding fibrous porous media on mixture ignitability, flame stability and flammable range were systematically investigated. Without porous media, the mixture can only be ignited at positions farther than a certain distance, which was defined as ignition distance in this paper. The ignition distance increases with an increasing inlet velocity and decreases with a decreasing channel diameter. The flame cannot be stabilized and propagates downstream with an inclination angle. The propagation velocity increases with the increase of inlet velocity or with the decrease of channel diameter. With the addition of porous media, the mixture can be ignited near the splitter due to improved methane/air mixing. Moreover, the flames become more stable and stationary flames were observed under lower velocities in the combustors of d = 5 mm and 6 mm with fibrous porous media. Furthermore, the application of porous media can significantly expand the flammable range. In summary, the combustion performances can be notably improved by the application of fibrous porous media.

Flame stability of methane/air mixtures in mesoscale channels with different diameters (6 mm, 5 mm and 4 mm) filled with fibrous porous media was experimentally investigated. Standing combustion waves (namely, stationary flame) are observed under low inlet velocity and high equivalence ratio conditions. Moreover, the standing wave regime becomes narrower as the channel diameter is reduced from 6 mm to 5 mm and vanishes for the 4-mm channel. For a fixed equivalence ratio, the flame length becomes shorter at a smaller channel or a less inlet velocity. Regarding the downstream propagating wave, its propagation velocity increases with the decrease of channel diameter. Splitting flame appears at large inlet velocities. Besides, at low equivalence ratios, the downstream propagating flames grow into small flame balls and can survive until the channel exit. Numerical results demonstrate that for a smaller channel, although the total heat loss rate is reduced, its heat loss ratio is increased, which leads to a lower wall temperature level and the flame is quenched out near the wall. The combustion efficiency is decreased significantly for the 4-mm channel due to fuel leakage from the near-wall “dead space”.