The influence of radiation on the flame propagation through micro organic dust particles with non-unity Lewis number

In this paper, the analytical study of effects of radiation and non-unity Lewis number on the laminar premixed flames of organic dust clouds has been done. The research is focused on a combustion model for premixed flames and the flame structure is composed of preheat-vaporization, narrow reaction and finally the post-flame zone. The normalized governing equations with help of boundary and matching conditions are solved by perturbation method. The results show that increasing equivalence ratio and decreasing Lewis number are resulted in the increase of flame temperature and burning velocity. For the sake of this model validation, fuel conversion is compared by published experimental data and shows an acceptable agreement.     

Transient flame propagation process and flame-speed oscillation phenomenon in a carbon dust cloud

A detailed numerical study was conducted to understand the transient flame propagation process and the flame-speed oscillation phenomenon in a carbon dust cloud. The modeling included the solution of a set of time-dependent conservation equations developed for the gas phase and the particle phase in a spherical coordinate. The gas-phase reactions used detailed chemistry, variable thermodynamic properties, and multicomponent transport properties. The particle-phase equations include the two-phase force interactions in the momentum equation by considering Stoke drag force and thermophoretic force resulting from the gas-phase temperature gradient. Mass and species transfer between the two phases were modeled as a result of both gas-phase and particle surface reactions. Energy transfer between the two phases, including convective, conductive, and radiative heat transfer, were included. Radiation absorption and emission by particles were both especially considered. The results show that because of the different inertia between particles and gas, a velocity slip occurs between the two phases in the region ahead of the flame front. The slip is more significant in the early flame propagation stage than in the later stage. The radiation heat losses of the hot gases and particles to the cold ambient and the radiation gain as a result of the absorption of unburned particles are both important in the present dust flame, because the characteristic time scale of the chemical reactions is longer than that of gaseous flames. Lastly, an analysis of the detailed numerical simulations shows that a slip between the gas and particle velocities is the cause of flame-speed oscillation. The slip leads to a periodic change in local particle number density in the reaction zone, which in turn changes the local fuel equivalence ratio periodically, causing the oscillation.     

Simulations of laminar flame propagation in droplet mists

In order to clarify the conditions conducive to propagation of premixed flames in quiescent sprays, a one-dimensional code with detailed chemistry and transport was used. n-Heptane and n-decane, distinguished by their volatility, were studied under atmospheric and low temperature, low pressure conditions. The effects of initial droplet diameter, overall equivalence ratio ?₀ and droplet residence time before reaching the flame front were examined. Increasing the residence time had an effect only for n-heptane, with virtually no evaporation occurring before the flame front for n-decane. The trends were only marginally correlated with the local gaseous equivalence ratio ?_eff at the location of maximum heat release rate. ?_eff could be as low as 0.4 (beyond the lean flammability limit), but the flame speed could still be 40% of the gaseous stoichiometric flame speed S_L,0. For n-heptane, ?_eff increased towards ?₀ with smaller droplets while high flame speeds occurred when ?_eff was near 1. This implied that the highest flame speed was achieved with small droplets for ?₀ ? 1 and with relatively large droplets for ?₀ > 1. In the latter case, the oxidiser was completely consumed in the reaction zone and droplets finished evaporating behind the flame where the fuel was pyrolysed. The resulting small species, mainly C₂H₂, C₂H₄ and H₂, diffused back to the oxidation zone and enhanced the reaction rate there. Ultimately, this could result in flame speeds higher than S_L,0 even with ?₀ = 4. For n-decane, the same trends were followed but smaller droplets were needed to reach the same ?_eff due to the slow evaporation rate. Under low pressure and low temperature, the effects of pressure and temperature on ?_eff and the flame speed were competitive and resulted in values close to the ones at atmospheric conditions.     

Uncertainties in interpretation of high pressure spherical flame propagation rates due to thermal radiation

It has been suggested that radiation heat loss may be a large source of experimental uncertainty in flame speed measurements using the outwardly propagating spherical flame method. Thermal radiation is usually not considered in interpretation of these experiments, yet it may contribute significantly to uncertainty especially for model-constraining conditions at low flame temperature and high pressure. In the present work, a conservative analytical estimate of the effects of radiation heat loss is derived and validated against detailed numerical simulations. A solver with a graphical interface is provided in the Supplemental material to allow implementation of these analytical results. The analytical estimate considers the radiation induced burned gas motion as well as the decreasing flame temperature due to conduction to the radiating burned gas and radiation loss from the flame zone. The results show that previous measurements of hydrogen flame speeds at low flame temperature by Burke et al. (2010) [3] are minimally affected by radiation, but flames with low flame speeds can be strongly inhibited by radiative loss. Future laminar spherical flame measurements and interpretation of existing determinations with low adiabatic flame speeds must include consideration of radiation effects to avoid large uncertainties.     

Characterization of hydrogen triple flame propagation in vitiated laminar coaxial flow

This paper numerically analyzes the propagation characteristics of a hydrogen flame in coaxial vitiated flow in a confined quartz tube. The transient propagation of the flame is calculated using Li's Mechanism of hydrogen oxidation, and the propagation characteristics are discussed based on these calculations. The formation of the reaction zone, the ignition of fuel, the transformation of the flame's base structure, and the propagation behavior of the hydrogen jet flame base are characterized in the present study. The flame characteristics is analyzed based on calculated results, the information provide several insight into the flame propagation and ignition layer at the leading edge in vitiated situations. The results show that the leading point of a hydrogen flame in coaxial fresh air propagates along the preferred equivalence ratio isoline as the flame has a triple flame structure. On the contrary, in vitiated coaxial flow, the propagation of the hydrogen flame fits triple flame theory more precisely. The flame's kinetic properties show that hydrogen flame propagation in coaxial vitiated flow is still dominated by the triple flame. The results also suggest that the transformation of the flame during propagation is affected by the pool of radicals as well as the chemical reactions.     

PMMA微粒子云中传播火焰的基本结构

以揭示可燃粉尘云中传播火焰的基本结构和燃烧反应区的反应特性为目的，用CCD和高速摄像仪拍摄了PMMA粉尘云中传播火焰的自发光和和粒子的激光散射图片，结合离子探针和微细热电偶测试技术，根据实测结果分析了有机可燃物PMMA粉尘云中传播火焰的基本结构；揭示了燃烧反应区的反应特性、反应强度分布规律和温度分布规律。     

Mathematical modeling of premixed counterflow combustion of organic dust cloud

In the present study, a mathematical approach is utilized so as to modeling the flame structure of organic dust particle and air through a two-phase mixture consisting in a counterflow configuration where heat loss is taken into account. Lycopodium is considered as the organic fuel in our research. In order to simulate combustion of organic dust particles, a three-zone flame structure has been considered; preheat-vaporization zone, reaction and post flame zones. The variations of the gaseous phase mass fraction and fuel particle mass fraction as a function of the distance from the stagnation plate are obtained. Subsequently, flame temperature and flame velocity in terms of strain rate are studied. Finally, the effect of heat loss on the non-dimensionalized temperature at different heat loss coefficients is investigated.     

Effects of CO addition on the propagation characteristics of laminar CH4 triple flame

This study proposes a numerical and experimental investigation of the propagation characteristics of CO-added CH₄ flames in a confined quartz tube. The transient propagation of a CH₄/CO–air flames were modeled using GRI-Mech 3.0, and the propagation characteristics are discussed based on the calculated results. This study characterizes the formation of the reaction zone, the transformation of the flame base structure, the ignition of fuel, and the propagation phenomena of the leading point of the flame base. The leading point of methane flame with a large amount of added CO is found to be difficult to define unambiguously. During flame propagation, a complex combination of chemical reactions coupled with the fluid dynamics between the stoichiometric line and the preferred equivalence ratio line occurs. The results suggest that the leading point of the propagating flame is still dominated by the redirection effect, while the effect of the intrinsic chemical properties of the fuel mixture on a propagating flame has finite thickness cannot be neglected.     

Effect of Lewis number on flame propagation through vortical flows

The premixed gas flame spreading through an array of large-scale vortices is studied numerically. It is found that the flame speed is a nonmonotonic function of the stirring intensity. At sufficiently low Lewis numbers (Le<1) the system becomes bistable with a hysteretic transition between possible propagation modes. In the presence of volumetric heat losses the stirring invariably promotes extinction (reduces the flammability limits), provided Le>1. At Le<1 this holds only for sufficiently strong stirring, whereas moderate stirring actually expands the flammability limits. At Le>1 the deficient reactant is fully consumed up to the very quenching point. At Le<1, prior to the total extinction, part of the deficient reactant escapes the reaction zone and remains unconsumed.     

The effect of gravity and thermal expansion on the propagation of a triple flame in a horizontal channel

We investigate the effect of thermal expansion and gravity on the propagation of a triple flame in a horizontal channel with porous walls, where the fuel and oxidiser concentrations are prescribed. The triple flame therefore propagates in a direction perpendicular to the direction of gravity, a configuration that does not seem to have received any dedicated investigation in the literature. In particular, we examine the effect of the non-dimensional flame-front thickness ? on the propagation speed of the triple flame for different values of the thermal expansion coefficient α and the Rayleigh number Ra. When gravity is not accounted for (Ra = 0), and for small values of ?, the numerically calculated propagation speed is found to agree with predictions made in previous studies based on scaling laws [1]. We show that the well known monotonic relationship between U and ?, which exists in the constant density case when the Lewis numbers are of order unity or larger, persists for triple flames undergoing thermal expansion. Under strong enough gravitational effects (Ra ? 1), however, the relationship is no longer found to be monotonic. For a fixed value of ?, the relationship between the Rayleigh number and the propagation speed is shown to vary qualitatively depending on the value of ? chosen, exhibiting hysteresis if ? is small enough and displaying local maxima, local minima or monotonic behaviour for other values of ?. All of the steady solutions presented in the paper have been found to be stable, except for those on the middle branches of the hysteresis curves.     

The effect of pre-heating on flame propagation in nanocomposite thermites

Flame propagation in a confined tube configuration was evaluated for aluminum (Al) and molybdenum trioxide (MoO₃) thermites starting at room temperature and pre-heated up to 170 °C. Flame propagation was analyzed via high speed imaging diagnostics and temperatures were monitored with thermocouples. Experiments were performed in a semi-confined flame tube apparatus housed in a reaction chamber initially at standard atmospheric pressure. The flame propagation behavior for the nano-particle thermite was compared to micron particle thermite of the same composition. Results indicate that increasing the initial temperature of the reactants results in dramatically increased flame speeds for nanocomposite thermite (i.e., from 627 to 1002 m/s for ambient and 105 °C pre-heat temperature, respectively) and for micron composite thermite (i.e., from 205 to 347 m/s for ambient and 170 °C pre-heat temperature, respectively) samples. Experimental studies were extended giving a cooling time for the heated thermites prior to ignition and flame propagation. It is shown that when 105 °C and 170 °C pre-heated thermites are cooled at a rate of 0.06 K/s, almost the same flame speeds are obtained as thermites at ambient temperature. However, when the cooling rate is increased to 0.13 K/s, the measured flame speeds approach the flame speeds of pre-heated samples.     

Detailed numerical simulation of thermal radiation influence in Sandia flame D

In order to investigate the influence of thermal radiation in turbulent combustion processes, Sandia flame D is numerically simulated, with multiple-time scale (MTS) k–ε turbulence model for turbulence, the combination of probability density function (PDF) transportation method, Lagrangian flamelet model (LFM) and the detailed chemical reaction mechanism GRI 3.0 (consisting of 53 species and 325 elemental reactions) for combustion and finite volume/correlated-k (FV/CK) method for radiation heat transfer. To account for turbulence’s influence on radiation, the effects of turbulence–radiation interactions (TRI) are investigated in radiation calculations and it is recommended that for detailed numerical simulation TRI should be considered. Numerical results with and without radiation influence being taken into accounted are compared with experimental data. Different from reports by other researchers, our simulation results show that although the magnitude of thermal radiation is relatively small, its influence on combustion process is significant. It is suggested that turbulence and chemical reactions may magnify the influence of thermal radiation.     

Modeling thermal behavior and work flux in finite-rate systems with radiation

We apply thermodynamic analysis in modeling, simulation and optimization of radiation engines as non-linear energy converters. We also perform critical analysis of available data for photon flux and photon density that leads to exact numerical value of photon flux constant. Basic thermodynamic principles lead to expressions for converter’s efficiency and generated work in terms of driving energy flux in the system. Steady and dynamical processes are investigated. In the latter, associated with an exhaust of radiation resource measured in terms of its temperature decrease, real work is a cumulative effect obtained in a system composed of a radiation fluid, sequence of engines, and an infinite bath. Variational calculus is applied in trajectory optimization of relaxing radiation described by a pseudo-Newtonian model. The principal performance function that expresses optimal work depends on thermal coordinates and a dissipation index, h, in fact a Hamiltonian of the optimization problem for extremum power or minimum entropy production. As an example of work limit in the radiation system under pseudo-Newtonian approximation the generalized exergy of radiation fluid is estimated in terms of finite rates quantified by Hamiltonian h. The primary results are dynamical equations of state for radiation temperature and work output in terms of process control variables. In the second part of this paper these equations and their discrete counterparts will serve to derive efficient algorithms for work optimization in the form of Hamilton–Jacobi–Bellman equations and dynamic programming equations. Significance of non-linear analyses in dynamic optimization of radiation systems is underlined.     

Concentration profile of particles across a flame propagating through an iron particle cloud

The number density profile of particles across a flame propagating through an iron particle cloud has been examined experimentally. The iron particles were suspended in air and ignited by an electric spark. Measurements were performed using high-speed photomicrography combined with laser light scattering technique. It is shown that for relatively large (agglomerated) particles the number density of iron particles changes in the range of x smaller than 11.0 mm, where x is the distance from the leading edge of the combustion zone. The number density increases with the decrease of x in the range 0.6 ≤ x ≤ 11.0 mm, reaches a maximum at x ≈ 0.6 mm, and then decreases. The maximum value of the number density is about 2.6 times larger than that at the region far ahead of the flame (x >11.0 mm). This increase in the number density of particles must cause a change of the lower flammability limit. By assuming that the increase in the number density is caused by the velocity difference of particles from surrounding gas flow, the profile of the number density of particles has been estimated on the basis of measured velocities of particles. The estimated number density profile of particles agrees well with that of the measured profile. The increase in the number density of particles just ahead of the flame will appear not only in iron particle cloud but also in any two-phase combustion systems, such as combustible particle cloud, combustible spray and so on.     

Influence of thermal radiation during aluminum thermal evaporation on organic solar cells

Organic solar cells (OSCs) based on the blends of poly(3-hexylthiophene) and fullerene derivative [6, 6]-phenyl-C₆₁ butyric acid methyl ester have been fabricated with p-type chromium oxide film as hole-transporting layer. The distribution of temperature and heat flow in vacuum chamber was investigated by temperature probes and finite element method. It is found that the two factors have significant effect on the quality of the thermally evaporated Al cathode. The bigger momentum impinging Al particles facilitates the formation of C-Al or C-O-Al bonds at the interface between photoactive layer and Al electrode, which is propitious to the electron transfer, and the corresponding devices have higher electron mobility. The higher substrate temperature that induced by thermal radiation during Al evaporation can act as a function of thermal annealing treatment, which can eliminate some electron traps and lead to better ohmic contacts for electrons to transport at the interface. The combination of the bigger momentum impinging Al particles and the higher substrate temperature can induce the improvement of electron mobility and OSC performance.     

Modeling of thermal radiation in a participating media with conjugate heat transfer

Modeling a combination of thermal radiation and conjugate heat transfer in a three-dimensional rectangular domain which has a participating media CO₂ flowing through is done numerically in OpenFOAM. The rectangular duct has a vertical step (facing forward to the inlet) which is located at a distance from the inlet (the distance is same as the height of the inlet section). The domain is divided into two regions (namely solid and fluid). Carbon dioxide, a highly absorbing fluid with extinction, is used here as the participating medium. The ability of the code is verified to analyze the thermal radiation in a participating media with conjugate heat transfer. The study was carried out for a constant Reynolds number 250 and a contraction ratio of 0.5. The study focused primarily on the importance of adding thermal radiation on to thermal analysis and the reason behind the Nusselt number variation on different regions of solid–fluid interface. It also discussed the effect of radiative properties, such as optical thickness and linear scattering albedo, on the average convective Nusselt Number.