Combustion and Flame最新文献

Smoldering is a flameless combustion mode occurring on the surface of charring fuels, such as wood and cigarettes. Although the smoldering process is slow and has a low temperature compared to flaming, it is easy to be initiated by a weak heat source and persists under poor oxygen conditions. Extensive work has been done for flame extinction to develop scaling models to predict the limiting oxygen concentration (LOC), but limited work is available for the smoldering extinction. This study develops a reduced analytical model to predict the extinction limits of smoldering. The model simultaneously solves smoldering propagation rate, surface temperature, and surface oxygen mass fraction as part of the solutions. The extinction limit is determined as the critical condition where solutions satisfying all governing equations cease to exist. The model provides a qualitative description and captures the essential characteristics of a previous experiment. The smoldering rate decreases with increasing fuel diameter, and a larger-diameter fuel is easier to extinguish. The mechanisms of the extinction process are investigated, showing the dominant role of radiative heat loss in the smothering limit at low airflow velocities and convective heat loss near the blowoff limit at high airflow velocities. Further analysis of the effect of oxygen concentration shows an increasing trend of LOC with fuel diameter, and the smothering branch cannot be predicted without considering the heat loss through radiation from the solid surface.

Novelty and Significance Statement

The novelty of this research is the prediction of smoldering propagation rates and extinction limits across various fuel diameters using a reduced analytical model. The model is modified to accommodate a thin fuel configuration and high airflow velocity condition, where convective heat and mass transfer play a more important role. The model's significance is that it not only provides essential insights into the mechanisms but also has the capability to simultaneously determine key parameters such as spread rate, reaction temperatures, and surface oxygen concentration as part of the solutions. Additionally, this study demonstrates the model's ability to reproduce the limit conditions, including smothering/blowoff limits and limiting oxygen concentration (LOC).

The mitigation of combustion noise is crucial for preventing thermoacoustic instabilities in rocket and gas turbine engines. Entropy noise, arising from entropy fluctuations within the engine, serves as a prominent noise source. Therefore, understanding the mechanisms governing entropy generation in flames becomes imperative for identifying the roots of indirect combustion noise. While previous studies have broadly examined various pathways of entropy generation under different canonical configurations of laminar flames, surprisingly, the effect of strain and pressure has been overlooked. This aspect is analyzed in the present work. Building upon previous analysis, the entropy source term of the reacting flow is decomposed into three sub-terms: the unsteady heat release term ($S_{e,1}$), the sensible enthalpy term ($S_{e,2}$), and the partial entropy term ($S_{e,3}$). The extent to which these three sub-terms contribute to entropy generation is thoroughly analyzed. Simultaneously, the scaling laws of the three sub-terms with respect to strain rate and pressure are investigated. The inclusion of detailed chemical kinetics as well as flame structure in the simulation dataset allows for in-depth investigation into the mechanism of the strain and pressure effects. Three canonical laminar flame configurations – the freely propagating premixed flame, the premixed counterflow flame, and the counterflow diffusion flame – are investigated. The following main conclusions are drawn: the heat release term is the leading constituent of the entropy source; the sensible enthalpy term is negligible; and the partial entropy term is secondary while non-negligible. Under strain and pressure variations, the contribution of the partial entropy term to total entropy production can greatly off-set the total entropy source or even qualitatively affect the strain/pressure response of the entropy generation, i.e., the strain response of the premixed flame and pressure response of the hydrogen non-premixed flame.

Novelty and significance statement

1. Conducted a systematic investigation of entropy production due to chemical reactions under the influence of strain and pressure variations.

2. Revealed that the partial entropy term, often overlooked in previous studies, can surpass the heat release term in certain configurations, emerging as the leading constituent of the total entropy generation source.

3. Identified dominating reactions contributing to entropy production for two different fuels of hydrogen and methane.

The paper presents an analytical theory quantitatively describing the heterogeneous combustion of nonvolatile (metal) particles in the diffusion-limited regime. It is assumed that the particle is suspended in an unconfined, isobaric, quiescent gaseous mixture and the chemisorption of the oxygen takes place evenly on the particle surface. The analytical solution of the particle burn time is derived from the conservation equations of the gas-phase described in a spherical coordinate system with the utilization of constant thermophysical properties, evaluated at a reference film layer. This solution inherently takes the Stefan flow into account. The approximate expression of the time-dependent particle temperature is solved from the conservation of the particle enthalpy by neglecting the higher order terms in the Taylor expansion of the product of the transient particle density and diameter squared. Coupling the solutions for the burn time and time-dependent particle temperature provides quantitative results when initial and boundary conditions are specified. The theory is employed to predict the burn time and temperature of 10–100 $\mu$m iron particles, which are then compared with measurements, as the first validation case. The theoretical burn time agrees with the experiments almost perfectly at both low and high oxygen levels. The calculated particle temperature matches the measurements fairly well at relatively low oxygen mole fractions, whereas the theory overpredict the particle peak temperature due to the neglect of evaporation and the possible transition of the combustion regime.

Novelty and significance statement

For the first time, we present a comprehensive and quantitative analytical theory elucidating the heterogeneous combustion of nonvolatile (metal) particles in the diffusion-limited regime. This novel theoretical model exhibits a remarkable capacity for quantitative prediction, obviating the need for supplementary information from numerical simulations or experimental data. The derivation process of analytical solutions for burn time and time-dependent particle temperature from conservation equations is elaborated, offering transparency and insight into the model’s foundations. To demonstrate the practical utility of the theory, we apply it to analyze the combustion of iron particles, providing valuable mathematical perspectives on the underlying processes. The model’s predictions for burn time and temperature align closely with experimental results, offering a partial validation of the theory within the realm of its applicable assumptions. This pioneering work contributes a robust and versatile analytical framework, advancing our understanding of diffusion-limited combustion phenomena of nonvolatile particles.

In the last decade, ammonia has gained interest as an alternative fuel for the energy sector. The main advantages include its carbon neutrality and ease of storage at industrial scale. However, to consider ammonia as an alternative carbon-free fuel it is necessary to solve two main issues, namely flame stability and pollutant emission. In this work we compared two promising technologies used to enhance ammonia combustion, specifically partial cracking and plasma assisted combustion (PAC). The experiments were carried out for different blends of ammonia–hydrogen–nitrogen–air that reproduce the conditions of cracking as well as for pure ammonia assisted by nanosecond repetitive pulsed (NRP) discharges for the PAC. The cracking and PAC were compared for the same added power ratio at atmospheric conditions. Different added power ratios, from 0.1% to 4% of the flame’s total thermal power, were tested. The lean blow-off limits were measured for a range of bulk flow velocities between of 4 and 12 m/s. For the pollutant emissions, NOx, NH${}_3$, and N${}_2$O were considered. The effect of both strategies on the Laminar burning velocity (LBV) was explored by numerical simulations. For the same added power ratio, partial cracking extended the lean blow-off limits further than NRP discharges, even with a high pulse repetition frequency of 30 kHz. This result indicated that cracking was more effective in stabilizing the flame than PAC. Regarding NOx emissions, NRP discharges induced only a third of the increase observed with cracking. However, the reduction in N${}_2$O associated with cracking was twice as much as that for NRP discharges. Finally, cracking proved more efficient in reducing the flame height, which was supported by the simulations of LBV. These results suggest that PAC strategies optimized for hydrocarbon flames are less efficient on ammonia combustion, and that more work is needed to develop plasma actuators for NH${}_3$ combustion.

Novelty and Significance statement This work marks the first one-to-one quantitative comparison between plasma actuation and thermal cracking on ammonia swirl flames. It serves as a pivotal exploration to assess and understand the economic aspects of these strategies, providing valuable insights into their efficiency and potential for practical implementation.

To analyze the composition changes during the combustion process of iron droplets using the energy balance equation, iron wires with diameters of tens of micrometers were burned to form temperature-stable iron droplets with diameters of hundreds of micrometers. High-speed two-color imaging pyrometry was employed to measure the spatially resolved surface temperature and volume changes of the droplets under varying oxygen concentrations. Experimental results showed that the droplet's temperature stabilized between the melting points of wüstite and iron. Quantitative analysis revealed the presence of bubbles within the droplets, primarily sourced from the oxidation of carbon in the iron to carbon dioxide. Simulation results of the droplet combustion process revealed that at the experimental combustion temperatures, the transport of oxygen in molten iron oxides was the rate-limiting step. The gas responsible for micro-explosions originates from the release of dissolved excess oxygen before the solidification of molten iron oxides, with rough estimates indicating that the mass of the released gas is only 0.02% to 0.03% of the droplet mass. The impact of reaction kinetics parameters on the simulation results, as well as the role of the micro-explosion mechanism in the self-sustained combustion of iron wires, were also discussed.

The number of experimental studies on ammonia (NH₃) oxidation has rapidly increased. However, species data at elevated or high pressures are still limited compared to those at atmospheric pressures. The study aims to understand ammonia oxidation under intermediate-temperature and elevated-pressure conditions by conducting species measurements of NH₃, H₂, N₂O, and NO in a micro flow reactor with a controlled temperature profile (MFR). The maximum wall temperatures and pressures in the MFR were varied in a range of T_{w, max} = 1100–1400 K and P = 0.1–0.4 MPa, and three equivalence ratios (ϕ = 0.5, 1.0, and 1.5) for NH₃/O₂/Ar mixtures were tested. With increasing pressures from 0.1 MPa to 0.4 MPa, the measured NH₃ fractions decrease and the measured N₂O mole fractions below T_{w, max} = 1250 K increase, which were found for all ϕ cases considered and more significant at ϕ = 0.5. Rates of production analyses reveal that OH production through R130: NO + HO₂ = NO₂ + OH is significantly enhanced at elevated pressures by a radical sensitization through NO and NO₂ (i.e., net reactions of R130, NH₂ + NO₂ = H₂NO + NO, H₂NO + O₂ = HNO + HO₂, and HNO + O₂ = NO + HO₂). The sensitization is induced by NO originating from the fuel NH₃ itself but without NO addition (i.e., self-induced radical sensitization through NO and NO₂), which is more active at the temperature range of around 1200–1300 K. By decreasing ϕ, OH production through the self-induced radical sensitization is enhanced because HO₂ consumption increases via R130 and decreases via NH₂ + HO₂ = HNO + H₂O. Additionally, as pressure increases, the onset of N₂O production shifts to lower temperatures, at which N₂O decomposition is much slower, resulting in a larger N₂O mole fraction at a higher pressure.