The importance of thermal radiation transfer in laminar diffusion flames at normal and microgravity

The importance of radiation heat loss in laminar and turbulent diffusion flames at normal gravity has been relatively well recognized in recent years. There is currently lack of quantitative understanding on the importance of radiation heat loss in relatively small scale laminar diffusion flames at microgravity. The effects of radiation heat transfer and radiation absorption on the structure and soot formation characteristics of a coflow laminar ethylene/air diffusion flame at normal- and microgravity were numerically investigated. Numerical calculations were conducted using GRI-Mech 3.0 combustion chemistry without the NO_x mechanism and complex thermal and transport properties, an acetylene based soot formation model, and a statistical narrow-band correlated-k non-grey gas radiation model. Radiation heat transfer and radiation absorption in the microgravity flame were found to be much more important than their counterparts at normal gravity. It is important to calculate thermal radiation transfer accurately in diffusion flame modelling under microgravity conditions.     

Influence of wall heat loss on the emission characteristics of premixed ammonia-air swirling flames interacting with the combustor wall

The influence of wall heat loss on the emission characteristics of ammonia-air swirling flames has been investigated employing Planar Laser-Induced Fluorescence imaging of OH radicals and Fourier Transform Infrared spectrometry of the exhaust gases in combustors with insulated and uninsulated walls over a range of equivalence ratios, ?, and pressures up to 0.5 MPa. Strong influence of wall heat loss on the flames led to quenching of the flame front near the combustor wall at 0.1 MPa, resulting in large unburned NH₃ emissions, and inhibited the stabilization of flames in the outer recirculating zone (ORZ). A decrease in heat loss effects with an increase in pressure promoted extension of the fuel-rich stabilization limit owing to increased recirculation of H₂ from NH₃ decomposition in the ORZ. The influence of wall heat loss resulted in emission trends that contradict already reported trends in literature. NO emissions were found to be substantially low while unburned NH₃ and N₂O emissions were high at fuel-lean conditions during single-stage combustion, with values such as 55 ppmv of NO, 580 ppmv of N₂O and 4457 ppmv of NH₃ at ? = 0.8. In addition, the response of the flame to wall heat loss as pressure increased was more important than the effects of pressure on fuel-NO emission, thereby leading to an increase in NO emission with pressure. It was found that a reduction in wall heat loss or a sufficiently long fluid residence time in the primary combustion zone is necessary for efficient control of NH₃ and N₂O emissions in two-stage rich-lean ammonia combustors, the latter being more effective for N₂O in addition to NO control. This study demonstrates that the influence of wall heat loss should not be ignored in emissions measurements in NH₃-air combustion, and also advances the understanding of previous studies on ammonia micro gas turbines.     

Insight into the inner structure of stretched premixed ammonia-air flames

Ammonia as a fuel has sparked significant interest in the combustion community. Although, using ammonia has a lot of advantages including no carbon emissions, ammonia-air flames are characterized as thick flames with low flame speeds. It is important to understand the flame structure to know the combustion process better. Flame thickness is an important property of the flame which characterizes the reactivity of the flame. Identifying the preheat zone is necessary to determine the fresh gas surface which is used to determine flame speed. Also, understanding the behavior of the important species emitted helps to demonstrate the reaction pathway which may be implemented in chemical kinetics schemes. Further, it is interesting to know the effect of curvature on the emission of excited species which gives direct knowledge on the influence of curvature on the flame reactivity. It was seen that the change in reactivity was manifested as a change in thickness of the species. The experiments presented here were performed on a Bunsen burner at atmospheric conditions. The laminar flame speeds have been evaluated over a range of equivalence ratios by choosing the isotherm as specified by the definition of the flame speed which are slightly higher than the values obtained from the literature. Chemiluminescence from NH* and NH₂* was studied for different equivalence ratios. A 1D simulation performed in Chemkin-Pro-was used to compare the behavior of the counterpart non-excited species. This comparison helps to correlate excited and non-excited species and also to define the structure of the ammonia-air flame. Both NH* and NH₂* have been determined as heat release rate markers.     

Experimental and numerical study of product gas characteristics of ammonia/air premixed laminar flames stabilized in a stagnation flow

Ammonia has widely attracted interest as a potential candidate not only as a hydrogen energy carrier but also as a carbon free fuel for internal combustion engines, such as gas turbines. Because ammonia contains a nitrogen atom in its molecule, nitrogen oxides (NO_x) and other pollutants may be formed when it burns. Therefore, understanding the fundamental product gas characteristics of ammonia/air laminar flames is important for the design of ammonia-fueled combustors to meet stringent emission regulations. In this study, the product gas characteristics of ammonia/air premixed laminar flames for various equivalence ratios were experimentally and numerically investigated up to elevated pressure conditions. In the experiments, a stagnation flame configuration was employed because an ammonia flame can be stabilized by using such a configuration without a pilot flame. The experimental results showed that the maximum NO mole fraction was about 3,500 ppmv, at an equivalence ratio of 0.9 at 0.1 MPa. The NO mole fraction decreased as the equivalence ratio increased. In addition, the maximum value of the NO mole fraction decreased with an increase in mixture pressure. Furthermore, it was experimentally clarified that the simultaneous reduction of NO and unburnt ammonia can be achieved at an equivalence ratio of about 1.06, which is the target equivalence ratio for emission control in rich-lean two-stage ammonia combustors. Comparison of experimental and numerical results showed that even though the reaction mechanisms employed have been optimized for predicting the laminar burning velocity of ammonia/air flames, they failed to satisfactorily predict the measured species in this study. Sensitivity analysis was used to identify elementary reactions that control the species profiles but have negligible effects on the burning velocity. It is considered that these reaction models need to be updated for accurate prediction of product gas characteristics of ammonia/air flames.     

Experimental and numerical study of product gas and N2O emission characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow

In order to achieve carbon neutrality, the use of ammonia as a fuel for power generation is highly anticipated. The utilization of a binary fuel consisting of ammonia and hydrogen can address the weak flame characteristics of ammonia. In this study, the product gas characteristics of ammonia/hydrogen/air premixed laminar flames stabilized in a stagnation flow were experimentally and numerically investigated for various equivalence ratios for the first time. A trade-off relationship between NO and unburnt ammonia was observed at slightly rich conditions. At lean conditions, NO reached a maximum value of 8,700 ppm, which was larger than that of pure ammonia/air flames. The mole fraction of nitrous oxide (N₂O) which has large global warming potential rapidly increased around the equivalence ratio of 0.6, which was attributed to the effect of a decrease in flame temperature downstream of the reaction zone owing to heat loss to the stagnation wall. To understand this effect further, numerical simulations of ammonia/hydrogen/air flames were conducted using the stagnation flame model for various equivalence ratios and stagnation wall temperatures. The results show that the important reactions for N₂O production and reductions are NH +NO = N₂O + H, N₂O + H = N₂ + OH, and N₂O (+M) = N₂ + O (+M). A decrease in flame temperature in the post flame region inhibited N₂O reduction through N₂O (+M) = N₂ + O (+M) because this reaction has a large temperature dependence, and thus N₂O was detected as a product gas. N₂O is reduced through N₂O (+M) = N₂ + O (+M) in the post flame region if the stagnation wall temperature is sufficiently high. On the other hand, it was clarified that an increase in equivalence ratio enhances H radical production and promotes N₂O reduction by H radical through the reaction of N₂O + H = N₂ + OH.     

碳氢扩散火焰实验研究：燃烧气氛对火焰光谱与温度的影响

辐射是各种燃烧过程中热传递的主要方式。在不同的火焰中,辐射光谱分布十分复杂。在这项工作中,利用光谱仪测量了可见光(200～900 nm),近红外(900～1 700 nm)和中红外(2 500～5 000 nm)波段火焰的光谱强度,分析了空气和富氧气氛下扩散火焰的光谱特征。并基于光谱分析,定量得到了火焰中碳烟以及气体发射的辐射力,计算了火焰的温度分布。结果表明,空气燃烧中的火焰温度低于富氧燃烧中的火焰温度。在空气气氛下,火焰中的碳烟和气体均对中的热辐射起着重要作用。而在富氧气氛下,气体对于火焰热辐射更为重要。在可见光和近红外波段,由于在空气气氛下火焰中碳烟的大量形成,光谱曲线显示出了良好连续性。而富氧气氛下火焰的辐射光谱降低。在中红外波段,空气气氛下火焰的气体辐射明显弱于富氧气氛下火焰的气体辐射。     

NO and OH* emission characteristics of very-lean to stoichiometric ammonia–hydrogen–air swirl flames

One of the main concerns regarding ammonia combustion is its tendency to yield high nitric oxide (NO) emissions. Burning ammonia under slightly rich conditions reduces the NO mole fraction to a low level, but the penalties are poor combustion efficiency and unburnt ammonia. As an alternative solution, this paper reports the experimental investigation of premixed swirl flames fueled with ammonia-hydrogen mixtures under very-lean to stoichiometric conditions. A gas analyzer was used to measure the NO mole fraction in the flame and post flame regions, and it was found that low NO emissions (as low as 100 ppm) in the exhaust were achieved under very lean conditions (? ≈ 0.40). Low NO emission was also possible at higher equivalence ratios, e.g. ? = 0.65, for very large ammonia fuel fractions (X_NH3 > 0.90). 1-D flame simulations were performed to elaborate on experimental findings and clarify the observations of the chemical kinetics. In addition, images of OH* chemiluminescence intensity were captured to identify the flame structure. It was found that, for some conditions, the OH* chemiluminescence intensity can be used as a proxy for the NO mole fraction. A monotonic relationship was discovered between OH* chemiluminescence intensities and NO mole fraction for a wide range of ammonia-hydrogen blends (0.40 < ? < 0.90 and 0.25 < X_NH3 < 0.90), making it possible to use the low-cost OH* chemiluminescence technique to qualify NO emission of flames fueled with hydrogen-enriched ammonia blends.     

Concept and combustion characteristics of ultra-micro combustors with premixed flame

Under micro-scale combustion influenced by quenching distance, high heat loss, shortened diffusion characteristic time, and flow laminarization, we clarified the most important issues for the combustor of ultra-micro gas turbines (UMGT), such as high space heating rate, low pressure loss, and premixed combustion. The stability behavior of single flames stabilized on top of micro tubes was examined using premixtures of air with hydrogen, methane, and propane to understand the basic combustion behavior of micro premixed flames. When micro tube inner diameters were smaller than 0.4 mm, all of the fuels exhibited critical equivalence ratios in fuel-rich regions, below which no flame formed, and above which the two stability limits of blow-off and extinction appeared at a certain equivalence ratio. The extinction limit for very fuel-rich premixtures was due to heat loss to the surrounding air and the tube. The extinction limit for more diluted fuel-rich premixtures was due to leakage of unburned fuel under the flame base. This clarification and the results of micro flame analysis led to a flat-flame burning method. For hydrogen, a prototype of a flat-flame ultra-micro combustor with a volume of 0.067 cm³ was made and tested. The flame stability region satisfied the optimum operation region of the UMGT with a 16 W output. The temperatures in the combustion chamber were sufficiently high, and the combustion efficiency achieved was more than 99.2%. For methane, the effects on flame stability of an upper wall in the combustion chamber were examined. The results can be explained by the heat loss and flame stretch.     

Experimental study on co-firing characteristics of ammonia with pulverized coal in a staged combustion drop tube furnace

Utilizing ammonia as a co-firing fuel to replace amounts of fossil fuel seems a feasible solution to reduce carbon emissions in existing pulverized coal-fired power plants. However, there are some problems needed to be considered when treating ammonia as a fuel, such as low flame stability, low combustion efficiency, and high NO_x emission. In this study, the co-firing characteristics of ammonia with pulverized coal are studied in a drop tube furnace with staged combustion strategy. Results showed that staged combustion would play a key role in reducing NO_x emissions by reducing the production of char-NO_x and fuel(NH₃)-NO_x simultaneously. Furthermore, the effects of different ammonia co-firing methods on the flue gas properties and unburned carbon contents were compared to achieve both efficient combustion and low NO_x emission. It was found that when ammonia was injected into 300 mm downstream under the condition of 20% co-firing, lower NO_x emission and unburnt carbon content than those of pure coal combustion can be achieved. This is probably caused by a combined effect of a high local equivalence ratio of NH₃/air and the prominent denitration effect of NH₃ in the vicinity of the NH₃ downstream injection location. In addition, NO_x emissions can be kept at approximately the same level as coal combustion when the co-firing ratio is below 30%. And the influence of reaction temperature on NO_x emissions is closely associated with the denitration efficiency of the NH₃. Almost no ammonia slip has been detected for any injection methods and co-firing ratio in the studied conditions. Thus, it can be confirmed that ammonia can be used as an alternative fuel to realize CO₂ reduction without extensive retrofitting works. And the NO_x emission can be reduced by producing a locally NH₃ flame zone with a high equivalence ratio as well as ensuring adequate residence time.     

Radiation effects in confined spherically expanding flames: Application to C5C10 flames at engine-relevant conditions

Radiation heat loss introduces one of the main uncertainties associated with the determination of laminar flame speeds from experiments using the spherically expanding flame constant volume method. In this study, a radiation model was developed to solve for the volumetric radiative power in spherical geometry using discrete transfer method accounting for spectrally dependent emission and absorption. The model was validated against the results obtained from the discrete ordinate method. Subsequently, the newly developed radiation model was integrated into a hybrid thermodynamic-radiation model used to derive laminar flame speeds from experimental pressure-time history data. Laminar flame speeds were measured for flames of C₅C₁₀ n-alkanes and isooctane for pressure and unburned mixture temperature ranges of 13–25 atm and 540 – 670 K respectively, and the data are reported with properly derived uncertainties. Additionally, the data are free from measurable instability-induced effects and to assure that a systematic analysis of cell formation was undertaken. Comparison of the experimental data with predicted laminar flame speeds showed consistency and reasonable agreement with two kinetic models. The development of new modeling tools over the last few years, allows for the derivation of accurate laminar flame speeds using the spherically expanding flame constant volume method for engine-relevant fuels and thermodynamic conditions, which other methods cannot accommodate.     

Experimental study of a confined premixed metal combustor: Metal flame stabilization dynamics and nitrogen oxides production

This work presents a study of a magnesium/air combustion process in the context of innovative zero carbon dioxide (CO₂) energy carriers for reducing global warming effects. In order to analyze more deeply the confined combustion of magnesium under fluctuating overpressure conditions (0 to 24 hPa) and the generated gaseous by-products, magnesium/air flames have been realized in a combustion chamber with a conical bluff-body as flame holder and different contraction ratios diaphragms at the exit duct. Sieved magnesium samples with two size-fractions were tested: 20–50?µm and 50–70?µm. The gaseous emissions of nitrogen oxides (NO_x) and dioxygen (O₂) were analyzed with on-line infrared, ultraviolet and paramagnetic analyzers. A flame pulsating behavior was clearly observed from light emission intensity (monitored by a photodiode) and pressure fluctuations (monitored by a pressure sensor); the frequencies obtained ranged between 3 and 10?Hz. The frequency of the pulsation exhibited strong dependence on the geometric configuration of the chamber: a contraction diaphragm divided by two the frequency level of the fluctuations in the studied range of maximum overpressure. Such fluctuations may probably be the consequence of periodic perturbations of the recirculation zone behind the bluff-body. These periodic perturbations are themselves caused by strong periodic overpressure variations due to stiff contraction downstream responding to gas velocity fluctuations. This feed-back-loop mechanism was considered in this study. NO_x emissions produced through the thermal pathway were analyzed for equivalence ratios ranging from 0.29 to 1. The representation of NO_x versus equivalence ratio exhibited a parabolic shape with a maximum for an equivalence ratio of 0.4. Moreover, NO_x emissions of this metal combustor have shown a similar order of magnitude than current internal combustion engines.     

Effect of ammonia/oxygen/nitrogen equivalence ratio on spherical turbulent flame propagation of pulverized coal/ammonia co-combustion

Because ammonia is one of the most promising candidates for energy carrier in the future, various applications of ammonia as a fuel are currently considered. One medium for utilizing ammonia is by introducing it to coal-fired boilers. To the best of our knowledge, this paper is the first to report the fundamental mechanism of the flame propagation phenomenon for pulverized coal/ammonia co-combustion. The effects of the equivalence ratio of the ammonia-oxidizer mixture on the flame propagation velocity of pulverized coal/ammonia co-combustion in turbulent fields were clarified by the experiments employing a unique fan-stirred constant volume chamber. The flame propagation velocities of pulverized coal/ammonia co-combustion, pure ammonia combustion, and pure pulverized coal combustion were compared. As expected, the flame propagation velocity of pulverized coal/ammonia was higher than that of the pure pulverized coal combustion for all conditions. However, the comparison of the flame propagation velocities of pulverized coal/ammonia co-combustion and that of the pure ammonia combustion, revealed that whether the flame propagation of the pulverized coal/ammonia was higher than that of the pure ammonia combustion was dependent on the equivalence ratio of the ammonia-oxidizer. This unique feature was explained by a mechanism including three competing effects proposed by the authors. In the ammonia lean condition, the positive effects, which are the strong radiation from the luminous flame and the increment of local equivalence ratio by the addition of volatile matter, are larger than the negative effect, which is the heat absorption by coal particles in preheat zone. In the ammonia rich condition, the effect of an increment of the local equivalence ratio by the addition of volatile matter turns into a negative effect. Consequently, the negative effects overcome the positive effect in the ammonia rich condition resulting in a lower flame propagation velocity of pulverized coal/ammonia co-combustion.     

Effects of OH concentration and temperature on NO emission characteristics of turbulent non-premixed CH4/NH3/air flames in a two-stage gas turbine like combustor at high pressure

This study examined the effects of OH concentration and temperature on the NO emission characteristics of turbulent, non-premixed methane (CH₄)/ammonia (NH₃)/air swirl flames in two-stage combustors at high pressure. Emission data were obtained using large-eddy simulations with a finite-rate chemistry method from model flames based on the energy fraction of NH₃ (E_NH3) in CH₄/NH₃ mixtures. Although NO emissions at the combustor exit were found to be significantly higher than those generated by CH₄/air and NH₃/air flames under both lean and stoichiometric primary zone conditions, these emissions could be lowered to approximately 300 ppm by employing far-rich equivalence ratios (?) of 1.3 to 1.4 in the primary zone. This effect was possibly due to the lower OH concentrations under far-rich conditions. An analysis of local flame characteristics using a newly developed mixture fraction equation for CH₄/NH₃/air flames indicated that the local temperature and NO and OH concentration distributions with local ? were qualitatively similar to those in NH₃/air flames. That is, the maximum local NO and OH concentrations appeared at local ? of 0.9, although the maximum temperature was observed at local ? of 1.0. Both the temperature and OH concentration were found to gradually decrease with the partial replacement of CH₄ with NH₃. Consequently, NO emissions from CH₄/NH₃ flames were maximized at E_NH3 in the range of 20% to 30%, after which the emissions decreased. Above 2100 K, the NO emissions from CH₄/NH₃ flames increased exponentially with temperature, which was not observed in NH₃/air flames because of the lower flame temperatures in the latter. But, the maximum NO concentration in CH₄/NH₃ flames was occurred at a temperature slightly below the maximum temperature, just as in NH₃/air flames. The apparent exponential increase in NO emissions from CH₄/NH₃ flames is attributed to a similar trend in the OH concentration at high temperatures.     

Effect of fuel ratio of coal on the turbulent flame speed of ammonia/coal particle cloud co-combustion at atmospheric pressure

This study aims to clarify the effect of fuel ratio of coal on the turbulent flame speed of ammonia/coal particle cloud co-combustion at atmospheric pressure under various turbulence intensities. High-fuel-ratio coals are not usually used in coal-fired thermal power plants because of their low flame stability. The expectation is that ammonia as a hydrogen-energy carrier would improve the ignition capability of coal particles in co-combustion. Experiments on spherical turbulent flame propagation of co-combustion were conducted for various coal types under various turbulence intensities, using the unique experimental apparatus developed for the co-combustion. Experimental results show that the flame speed of co-combustion with a low equivalence ratio of ammonia/oxidizer mixture for bituminous coal case was found to be three times faster than that of pure coal combustion and two times faster than that of pure ammonia combustion. On the other hand, the flame speed of co-combustion for the highest-fuel-ratio coal case is lower than that of the pure ammonia combustion case, although the flame propagation can be sustained due to the ammonia mixing. To explain the difference of tendencies depending on the fuel ratio of coal, a flame propagation mechanism of ammonia/coal particle cloud co-combustion was proposed. Two positive effects are the increases of local equivalence ratio and the increases of radiation heat flux, which increases the flame speed. In opposite, a negative effect is the heat sink effect that decreases the flame speed. The two positive effects on the flame speed of co-combustion overwhelm a negative effect for bituminous coal case, while the negative effect overcomes both positive effects for the highest-fuel-ratio coal case. The findings of the study can contribute to the reduction of solid fuel costs when the ammonia is introduced as CO₂ free energy carrier and can improve the energy security through the utilization of high-fuel-ratio coals.     

Extinction strain rates of premixed ammonia/hydrogen/nitrogen-air counterflow flames

Chemical energy vectors will play a crucial role in the transition of the global energy system, due to their essential advantages in storing energy in form of gaseous, liquid, or solid fuels. Ammonia (NH₃) has been identified as a highly promising candidate, as it is carbon-free, can be stored at moderate pressures, and already has a developed distribution infrastructure. As a fuel NH₃ has poor combustion properties that can be improved by the addition of hydrogen, which can be obtained energy-efficiently by partially cracking ammonia into hydrogen (H₂) and nitrogen (N₂) prior to the combustion process. The resulting NH₃/H₂/N₂ blend leads to significantly improved flame stability and resilience to strain-induced blow-out, despite similar laminar flame properties compared to equivalent methane/air flames. This study reports the first measurements of extinction strain rates, measured using the premixed twin-flame configuration in a laminar opposed jet burner, for two NH₃/H₂/N₂ blends over a range of equivalence ratios. Local strain rates are measured using particle tracking velocimetry (PTV) and are related to the inflow conditions, such that the local strain rate at the extinction point can be approximated. The results are compared with 1D-simulations using three recent kinetic mechanisms for ammonia oxidation. By relating the extinction strain rates to laminar flame properties of the unstretched flame, a comparison of the extinction behaviour of CH₄ and NH₃/H₂/N₂ blends can be made. For lean mixtures, NH₃/H₂/N₂-air flames show a significant higher extinction resistance in comparison to CH₄/air. In addition, a strong non-linear dependence between the resistance to extinction and equivalence ratio for NH₃/H₂/N₂ blends is observed.     

Towards the development of an efficient low-NOx ammonia combustor for a micro gas turbine

Recent studies have demonstrated stable generation of power from pure ammonia combustion in a micro gas turbine (MGT) with a high combustion efficiency, thus overcoming some of the challenges that discouraged such applications of ammonia in the past. However, achievement of low NOx emission from ammonia combustors remains an important challenge. In this study, combustion techniques and combustor design for efficient combustion and low NOx emission from an ammonia MGT swirl combustor are proposed. The effects of fuel injection angle, combustor inlet temperature, equivalence ratio, and ambient pressure on flame stabilization and emissions were investigated in a laboratory high pressure combustion chamber. An FTIR gas analyser was employed in analysing the exhaust gases. Numerical modeling using OpenFOAM was done to better understand the dependence of NO emissions on the equivalence ratio. The result show that inclined fuel injection as opposed to vertical injection along the combustor central axis resulted to improved flame stability, and lower NH₃ and NOx emissions. Numerical and experimental results showed that a control of the equivalence ratio upstream of the combustor is critical for low NOx emission in a rich-lean ammonia combustor. NO emission had a minimum value at an upstream equivalence ratio of 1.10 in the experiments. Furthermore, NO emission was found to decrease with ambient pressure, especially for premixed combustion. For the rich-lean combustion strategy employed in this study, lower NOx emission was recorded in premixed combustion than in non-premixed combustion indicating the importance of mixture uniformity for low NOx emission from ammonia combustion. A prototype liner developed to enhance the control and uniformity of the equivalence ratio upstream of the combustor further improved ammonia combustion. With the proposed liner design, NOx emission of 42?ppmv and ammonia combustion efficiency of 99.5% were achieved at 0.3?MPa for fuel input power of 31.44?kW.     

Numerical study of the effects of gravity on soot formation in laminar coflow methane/air diffusion flames under different air stream velocities

Numerical simulations of laminar coflow methane/air diffusion flames at atmospheric pressure and different gravity levels were conducted to gain a better understanding of the effects of gravity on soot formation by using relatively detailed gas-phase chemistry and complex thermal and transport properties coupled with a semi-empirical two-equation soot model. Thermal radiation was calculated using the discrete-ordinates method coupled with a non-grey model for the radiative properties of CO, CO₂, H₂O, and soot. Calculations were conducted for three coflow air velocities of 77.6, 30, and 5 cm/s to investigate how the coflowing air velocity affects the flame structure and soot formation at different levels of gravity. The coflow air velocity has a rather significant effect on the streamwise velocity and the fluid parcel residence time, especially at reduced gravity levels. The flame height and the visible flame height in general increase with decreasing the gravity level. The peak flame temperature decreases with decreasing either the coflow air stream velocity or the gravity level. The peak soot volume fraction of the flame at microgravity can either be greater or less than that of its normal gravity counterpart, depending on the coflow air velocity. At sufficiently high coflow air velocity, the peak soot volume fraction increases with decreasing the gravity level. When the coflow air velocity is low enough, soot formation is greatly suppressed at microgravity and extinguishment occurs in the upper portion of the flame with soot emission from the tip of the flame owing to incomplete oxidation. The numerical results provide further insights into the intimate coupling between flame size, residence time, thermal radiation, and soot formation at reduced gravity level. The importance of thermal radiation heat transfer and coflow air velocity to the flame structure and soot formation at microgravity is demonstrated for the first time.     

辐射传热对微重力环境下预混火焰传播特性的影响

1引言在火焰中，辐射过程是一种重要的传热方式。对该过程尽可能精确的计算，对于改进燃烧设备的设计、改善设备的运行性能十分有益。在正常重力环境下，与其它的释热现象相比，预混火焰中的辐射热损失十分微弱，因而，过去对预混火焰的分析中，往往忽略了辐射热损失的影响。近年来，对微重力（ug）环境下的预混火焰的研究结果表明，可燃极限与＃s最小点火能无关，自媳灭火焰（SEFs）发生时；其释放的能量比通常观察到的点火极限时的能量大几个数量级山，因此火焰伸张并不能解释“g环境下观察到的实验结果，辐射热损失可能是影响＃g火焰可…     

Turbulent flame propagation limits of polymethylmethacrylate particle cloud–ammonia–air co-combustion

Ammonia is a highly promising energy carrier for achieving a carbon-neutral society. The co-combustion of solid particle clouds–ammonia, in particular, is considered an efficient and feasible method of reducing carbon dioxide emissions. Understanding turbulent flame stabilization and extinguishment processes during the two-phase hybrid-mixture co-combustion of solid particle clouds–ammonia is essential for the co-combustion technology to be used in combustors. To the best of our knowledge, this is the first study to describe the turbulent flame propagation limits and associated mechanism on the co-combustion of solid particle clouds–ammonia–air. Turbulent flame propagation experiments on silica particle clouds–ammonia–air mixing combustion and polymethylmethacrylate (PMMA) particle cloud–ammonia–air co-combustion were conducted in this work using a novel fan-stirred constant-volume vessel to clarify the turbulent flame propagation limits and associate mechanism of solid particle cloud–ammonia–air co-combustion. Results showed that adding inert silica particles contracted the turbulent flame propagation limits of premixed ammonia–air mixtures. However, adding PMMA particles expanded and then contracted the turbulent flame propagation limits of a premixed ammonia–air mixture as the ammonia equivalence ratio increased from lean to rich. In the solid particle cloud–ammonia–air co-combustion, reactive particles induce two types of effects on the turbulent flame propagation limits of premixed ammonia–air mixtures: The local equivalence ratio increment effect is caused by adding volatile matter from preheated particles in the preheat zone of the flame front, and the heat sink negative effect is induced by the unburned particles.