Steady and oscillatory flame spread over liquid fuels

The flow induced in a layer of liquid fuel at sub-flash temperature by the thermocapillary forces associated with the spreading of a flame that heats and vaporizes the liquid is analysed numerically and asymptotically, for large values of the Marangoni number and of the Reynolds number based on the propagation speed. Upstream heat convection in a recirculating region moving with the flame front is described for a steady model problem and for uniform and pulsating flame spread. A possible mechanism triggering flow oscillations entirely dependent on the liquid phase is identified and discussed.     

Effects of pressure rise rate on laminar flame speed under normal and engine-relevant conditions

Laminar flame speed (LFS) is one of the most important physicochemical properties of a combustible mixture. At normal and elevated temperatures and pressures, LFS can be measured using propagating spherical flames in a closed chamber. LFS is also used in certain turbulent premixed flame modelling for combustion in spark ignition engines. Inside the closed chamber or engine, transient pressure rise occurs during the premixed flame propagation. The effects of pressure rise rate (PRR) on LFS are examined numerically in this study. One-dimensional simulations are conducted for spherical flame propagation in a closed chamber. Detailed chemistry and transport are considered. Different values of PRR at the same temperature and pressure are achieved through changing the spherical chamber size. It is found that the effect of PRR on LFS is negligible under the normal and engine-relevant conditions considered in this study. This observation is then explained through the comparison between the unsteady and convection terms in the energy equation for a premixed flame.     

On the determination of laminar flame speed from low-pressure and super-adiabatic propagating spherical flames

The outwardly propagating spherical flame (OPF) method is popularly used to measure the laminar flame speed (LFS). Recently, great efforts have been devoted to improving the accuracy of the LFS measurement from OPF. In the OPF method, several assumptions are made. For examples, the burned gas is assumed to be static and in chemical equilibrium. However, these assumptions may not be satisfied under certain conditions. Here we consider low-pressure and super-adiabatic propagating spherical flames, for which chemical non-equilibrium exists and the burned gas may not be static. The objective is to assess the chemical non-equilibrium effects on the accuracy of LFS measurement from the OPF method. Numerical simulations considering detailed chemistry and transport are conducted. Stoichiometric methane/air flames at sub-atmospheric pressures and methane/oxygen flames at different equivalence ratios are considered. At low pressures, broad heat release zone is observed and the burned gas cannot quickly reach the adiabatic flame temperature, indicating the existence of chemical non-equilibrium of burned gas. Positive flow in the burned gas is identified and it is shown to become stronger at lower initial pressure. Consequently, the LFS measurement from OPF at low pressures is not accurate if the burned gas is assumed to be static and at chemical equilibrium. For super-adiabatic spherical flames, the burned gas speed is found to be negative due to the local temperature overshoot at the flame front. Such negative speed of burned gas can also reduce the accuracy of LFS measurement. It is recommended that the direct method measuring both flame propagation speed and flow speed of unburned gas should be used to determine the LFS at low pressures or for mixtures with super-adiabatic flame temperature.     

Effect of non-zero relative velocity on the flame speed of two-phase laminar flames

A numerical study of one-dimensional n-heptane/air spray flames is presented. The objective is to evaluate the flame propagation speed in the case where droplets evaporate inside the reaction zone with possibly non-zero relative velocity. A Direct Numerical Simulation approach for the gaseous phase is coupled to a discrete particle Lagrangian formalism for the dispersed phase. A global two-step n-heptane/air chemical mechanism is used. The effects of initial droplet diameter, overall equivalence ratio, liquid loading and relative velocity between gaseous and liquid phases on the laminar spray flame speed and structure are studied. For lean premixed cases, it is found that the laminar flame speed decreases with increasing initial droplet diameter and relative velocity. On the contrary, rich premixed cases show a range of diameters for which the flame speed is enhanced compared to the corresponding purely gaseous flame. Finally, spray flames controlled by evaporation always have lower flame speeds. To highlight the controlling parameters of spray flame speed, approximate analytical expressions are proposed, which give the correct trends of the spray flame propagation speed behavior for both lean and rich mixtures.     

Effect of local variations of the laminar flame speed on the global finger-flame acceleration scenario

Numerous formulations describing the dynamics and morphology of corrugated flames, including the scenarios of flame acceleration, are based on a “geometrical consideration”, where the wrinkled-to-planar flame velocities ratio, S_w /S_L , is evaluated as the scaled flame surface area, while the entire combustion chemistry is immersed into the planar flame speed S_L , which is assumed to be constant. However, S_L may experience noticeable spatial/temporal variations in practice, in particular, due to pressure/temperature variations as well as non-uniform distribution of the equivalence ratio and/or that of combustible or inert dust impurities. The present work initiates the systematic study of the impact of the local S_L -variations on the global flame evolution scenario. The variations are assumed to be imposed externally, in a manner being a free functional of the formulation. Specifically, the linear, parabolic and hyperbolic spatial S_L -distributions are incorporated into the formulations of finger flame acceleration in pipes, and they are compared to the case of constant S_L . Both two-dimensional channels and cylindrical tubes are considered. The conditions promoting or moderating flame acceleration are identified, and the revisited equations for the flame shape, velocity and acceleration rate are obtained for various S_L -distributions. The theoretical findings are validated by the computational simulations of the reacting flow equations, with agreement between the theory and modelling demonstrated.     

Effects of Soret diffusion on the laminar flame speed and Markstein length of syngas/air mixtures

The effects of Soret diffusion on premixed syngas/air flames at normal and elevated temperatures and pressures are investigated numerically including detailed chemistry and transport. The emphasis is placed on assessing and interpreting the influence of Soret diffusion on the unstretched and stretched laminar flame speed and Markstein length of syngas/air mixtures. The laminar flame speed and Markstein length are obtained by simulating the unstretched planar flame and positively-stretched spherical flame, respectively. The results indicate that at atmospheric pressure the laminar flame speed of syngas/air is mainly reduced by Soret diffusion of H radical while the influence of H₂ Soret diffusion is negligible. This is due to the facts that the main reaction zone and the Soret diffusion for H radical (H₂) are strongly (weakly) coupled, and that Soret diffusion reduces the H concentration in the reaction zone. Because of the enhancement in the Soret diffusion flux of H radical, the influence of Soret diffusion on the laminar burning flux increases with the initial temperature and pressure. Unlike the results at atmospheric pressure, at elevated pressures the laminar flame speed is shown to be affected by the Soret diffusion of H₂ as well as H radical. For stretched spherical flame, it is shown that the Soret diffusion of both H and H₂ should be included so that the stretched flame speed can be accurately predicted. Similar to the laminar flame speed, the Markstein length is also reduced by Soret diffusion. However, the reduction is found to be mainly caused by Soret diffusion of H₂ rather than that of H radical. Moreover, the influence of Soret diffusion on the Markstein length is demonstrated to decrease with the initial temperature and pressure.     

Experimental evaluation of flame observables for simplified scalar dissipation rate measurements in laminar diffusion flamelets

We present a comparative evaluation of the potential of several flame observables to yield a simplified measurement of the scalar dissipation rate (χ). The realization of the importance of this quantity for the structure of diffusion flamelets has led to brilliant experimental efforts targeted to its measurement, with a particular emphasis on χ_stoich, i.e., its value at the stoichiometric surface, which has been shown to control extinction. Such measurements require a significant amount of experimental resources, since they necessitate the simultaneous acquisition of multi-scalar data. The possibility of a simplified measurement stems from the realization that the related gradient of the mixture fraction scales as the inverse of an appropriately defined thickness of the mixing layer. In this paper, we investigate experimentally the utilization of several flame observables for the measurement of this thickness. In a flat, nitrogen diluted, counterflow, methane/oxygen diffusion flame, the scalar dissipation rate was first measured directly using line Raman imaging of major species and a N₂-molecule based definition of the mixture fraction. Additionally, LIF measurements of the hydroxyl radical (OH) and formaldehyde (HCHO) as well as Raman measurements of carbon monoxide (CO) were performed across the flamelet. The precision of χ_stoich estimates based on the thickness of the layers of these three observables as well as the layers corresponding to [HCHO] × [OH] and [CO] × [OH] “overlap” zones was evaluated in terms of following the theoretically expected inverse-square-root dependence on strain rate. Also, the absolute thickness of these layers was recorded, since it may restrict the application of simplified techniques in turbulent flow fields.     

Effect of ignition energy on the uncertainty in the determination of laminar flame speed using outwardly propagating spherical flames

The initial propagation processes of expanding spherical flames of CH₄/N₂/O₂/He mixtures at different ignition energies were investigated experimentally and numerically to reduce the effect of ignition energy on the accurate determination of laminar flame speeds. The experiments were conducted in a constant-volume combustion bomb at initial pressures of 0.07???0.7?MPa, initial temperatures of 298???398?K, and equivalence ratios of 0.9???1.3 with various Lewis numbers. The A-SURF program was employed to simulate the corresponding flame propagation processes. The results show that elevating the ignition energy increases the initial flame propagation speed and expands the range of flame trajectory which is affected by ignition energy, but the increase rates of the speed and range decrease with the ignition energy. Based on the trend of the minimum flame propagation speed during the initial period with the ignition energy, the minimum reliable ignition energy (MRIE) is derived by considering the initial flame propagation speed and energy conservation. It is observed that MRIE first decreases and then increases with the increasing equivalence ratio and monotonously decreases with increasing initial pressure and temperature. As the Lewis number rises, MRIE increases. The results also suggest that during the data processing of the spherical flame experiment, the accuracy of determination of laminar flame speeds can be enhanced when taking the flame radius influenced by MRIE as the lower limit of the flame radius range. Then the flame radius influenced by MRIE was defined as RFR. It can also be found that there exist nonlinear relationships between RFR and the equivalence ratio and Lewis number, and the RFR decreases with increasing initial pressure and temperature.     

Combined isochoric and isobaric acquisition methodology for accurate flame speed measurements from ambient to high pressures and temperatures

The present paper addresses measurement techniques of the most important fundamental property of a combustible mixture, the laminar flame speed. The accurate determination of this parameter has been investigated carefully using two different approaches: the constant pressure method based on the flame front trajectory obtained by means of a Schlieren optical technique, and the constant volume method based only on the pressure-time history recorded during the combustion process. Methane/air flames are selected, because they are relevant for many technical applications and believed to be well understood. Measurements were performed using two high-pressure, high-temperature constant volume vessels at RWTH and ICARE. First, uncertainties stemming from experiments are quantified, and the different sources of errors are identified. Second, the assumption that the unburned gas is compressed isentropically due to the burned gas expansion is verified experimentally for the first time to the authors’ knowledge. More importantly, it was found that combining the two approaches can provide high-fidelity data w.r.t. stretch, radiative heat loss, and instabilities. The data will be very helpful for mechanism development and combustion modeling. Ideally, both approaches are applied simultaneously in the same vessel to obtain a high confidence level in the measured data. Finally, a methodology is proposed to provide flame speeds over a wide range of pressures and temperatures up to 14 bars and 620 K, close to engine-relevant conditions.     

An experimental and modeling study of ammonia with enriched oxygen content and ammonia/hydrogen laminar flame speed at elevated pressure and temperature

Laminar flame speeds of ammonia with oxygen-enriched air (oxygen content varying from 21 to 30 vol.%) and ammonia-hydrogen-air mixtures (fuel hydrogen content varying from 0 to 30 vol.%) at elevated pressure (1–10 bar) and temperature (298–473 K) were determined experimentally using a constant volume combustion chamber. Moreover, ammonia laminar flame speeds with helium as an inert were measured for the first time. Using these experimental data along with published ones, we have developed a newly compiled kinetic model for the prediction of the oxidation of ammonia and ammonia-hydrogen blends in freely propagating and burner stabilized premixed flames, as well as in shock tubes, rapid compression machines and a jet-stirred reactor. The reaction mechanism also considers the formation of nitrogen oxides, as well as the reduction of nitrogen oxides depending on the conditions of the surrounding gas phase. The experimental results from the present work and the literature are interpreted with the help of the kinetic model derived here. The experiments show that increasing the initial temperature, fuel hydrogen content, or oxidizer oxygen content causes the laminar flame speed to increase, while it decreases when increasing the initial pressure. The proposed kinetic model predicts the same trends than experiments and a good agreement is found with measurements for a wide range of conditions. The model suggests that under rich conditions the N₂H₂ formation path is favored compared to stoichiometric condition. The most important reactions under rich conditions are: NH₂+NH=N₂H₂+H, NH₂+NH₂=N₂H₂+H₂, N₂H₂+H=NNH+H₂ and N₂H₂+M=NNH+H+M. These reactions were also found to be among the most sensitive reactions for predicting the laminar flame speed for all the cases investigated.     

Physical and chemical effects of phosphorus-containing compounds on laminar premixed flame

Phosphorus-containing compounds are the promising halon alternatives for flame inhibitions. However, some literatures suggested that the phosphorus-related inhibitors may behave as the unfavorable ones that will increase the burning velocity under lean-burn conditions, and this indeed posed potential threats to the fire prevention and fighting. To seek deeper insights into the reaction process, a numerical investigation was actualized to study the phosphorus-related effects on methane-air flames. By replacing a phosphorus-related inhibitor with the corresponding decomposed molecules, the detailed promoting and inhibiting effects of combustion were separated from the general chemical effect. A comparative study was carried out to identify the interaction between the two effects under different combustion conditions. It is observed that the promoting effect becomes the dominant factor during the reaction process when the equivalence ratio is smaller than 0.60. In this lean-burn condition, the exothermic reactions were faster than the others within the reaction chains due to the reduction of radical recombination in hydrocarbon oxidation. The results are believed to be useful for the further application and improvement of flame inhibitors.     

Influence of water mist on propagation and suppression of laminar premixed flame

The combustion of premixed gas mixtures containing micro droplets of water was studied using one-dimensional approximation. The dependencies of the burning velocity and flammability limits on the initial conditions and on the properties of liquid droplets were analyzed. Effects of droplet size and concentration of added liquid were studied. It was demonstrated that the droplets with smaller diameters are more effective in reducing the flame velocity. For droplets vaporizing in the reaction zone, the burning velocity is independent of droplet size, and it depends only on the concentration of added liquid. With further increase of the droplet diameter the droplets are passing through the reaction zone with completion of vaporization in the combustion products. It was demonstrated that for droplets above a certain size there are two stable stationary modes of flame propagation with transition of hysteresis type. The critical conditions of the transition are due to the appearance of the temperature maximum at the flame front and the temperature gradient with heat losses from the reaction zone to the products, as a result of droplet vaporization passing through the reaction zone. The critical conditions are similar to the critical conditions of the classical flammability limits of flame with the thermal mechanism of flame propagation. The maximum decrease in the burning velocity and decrease in the combustion temperature at the critical turning point corresponds to predictions of the classical theories of flammability limits of Zel'dovich and Spalding. The stability analysis of stationary modes of flame propagation in the presence of water mist showed the lack of oscillatory processes in the frames of the assumed model.     

甲烷/氧气层流反扩散火焰形态及滞后特性研究

对空气气氛中甲烷/氧气反扩散火焰的形态和推举滞后特性进行了实验研究. 实验中通过改变气体流量考察了气速变化对火焰形态演变及滞后特性的影响, 并利用紫外相机系统研究了气速对不同形态火焰中OH^*分布的影响. 研究结果表明: 甲烷气速、氧气气速和火焰的历史状态是决定火焰形态的三个重要参数, 并以此对实验范围内的火焰形态进行了分区; 氧气气速对不同形态反扩散火焰轴线上的OH^*分布有相似的影响, 当氧气缺乏时, 反扩散反应区较短, 当氧气富余时, 反扩散反应区在轴向分布较广; 同轴甲烷的气速对反扩散火焰的滞后特性影响显著, 随着甲烷气速的增加, 反扩散火焰的推举速度和再附着速度呈线性减小, 部分预混火焰向反扩散火焰转变的速度呈线性增加.     

Mid-infrared absorption measurements of liquid hydrocarbon fuels near

Quantitative absorption spectra for several hydrocarbon fuels in the liquid phase at are presented. Measurements of toluene, n-dodecane, n-decane, and three samples of gasoline were made over the spectral region 2700–3200 to support the development of mid-infrared laser-absorption diagnostics for measurements of fuel vapor in the presence of liquid films and aerosols. A procedure for quantitative Fourier transform infrared (FTIR) absorption measurements of strongly absorbing liquids is described and the resulting absorption spectra are compared with previously measured absorption spectra in the vapor phase. The measured absorption spectra for liquid gasoline are shown to scale with the volume percent of olefin, alkane, and aromatic hydrocarbons in each sample. Finally, the observed frequency shift of in the spectra of vapor and liquid hydrocarbons is discussed, including the potential for measurements of fuel vapor in the presence of liquid films.     

Internal pressure,ultrasonic velocity and viscosity of multi-component liquid systems

Ultrasonic velocity, density and viscosity were measured in two ternary liquid systems namely,n-pentane +n-hexane + benzene(I) andn-hexane + cyclohexane + benzene(II) and one quaternary liquid system,n-pentane +n-hexane + benzene + toluene (III). The experimental as well as literature values of thermal expansion coefficient and iso-thermal compressibility of pure liquid components were utilized to deduce the ideal value of internal pressure and excess internal pressure for the above liquid systems at 298·15K using two different approaches. In the conventional approach one needs the experimental values ofα andβ _T of mixtures for computing internal pressure, which was not possible. The second method which is proposed here utilizes only the density, ultrasonic velocity and viscosity data of the mixture. This method is used in computing internal pressure and its excess value for multicomponent liquid systems. A satisfactory agreement has been observed.     

High-pressure laminar flame speeds and kinetic modeling of carbon monoxide/hydrogen combustion

Laminar flame speeds were accurately measured for CO/H₂/air and CO/H₂/O₂/helium mixtures at different equivalence ratios and mixing ratios by the constant-pressure spherical flame technique for pressures up to 40 atmospheres. A kinetic mechanism based on recently published reaction rate constants is presented to model these measured laminar flame speeds as well as a limited set of other experimental data. The reaction rate constant of CO + HO₂ → CO₂ + OH was determined to be k = 1.15 × 10⁵T^2.278 exp(−17.55 kcal/RT) cm³ mol⁻¹ s⁻¹ at 300-2500 K by ab initio calculations. The kinetic model accurately predicts our measured flame speeds and the non-premixed counterflow ignition temperatures determined in our previous study, as well as homogeneous system data from literature, such as concentration profiles from flow reactor and ignition delay time from shock tube experiments.     

Ultrasound-assisted oxidative desulfurization of liquid fuels and its industrial application

Latest environmental regulations require a very deep desulfurization to meet the ultra-low sulfur diesel (ULSD, 15 ppm sulfur) specifications. Due to the disadvantages of hydrotreating technology on the slashing production conditions, costs and safety as well as environmental protection, the ultrasound-assisted oxidative desulfurization (UAOD) as an alternative technology has been developed. UAOD process selectively oxidizes sulfur in common thiophenes in diesel to sulfoxides and sulfones which can be removed via selective adsorption or extractant. SulphCo has successfully used a 5000 barrel/day mobile “Sonocracking” unit to duplicate on a commercial scale its proprietary process that applies ultrasonics at relatively low temperatures and pressures. The UAOD technology estimate capital costs less than half the cost of a new high-pressure hydrotreater. The physical and chemical mechanisms of UAOD process are illustrated, and the effective factors, such as ultrasonic frequency and power, oxidants, catalysts, phase-transfer agent, extractant and adsorbent, on reaction kinetics and product recovery are discussed in this review.     

Laser wavelength measurements and the speed of light

Early determinations of the speed of light, based on astronomical observations or terrestrial time-of-flight experiments, were largely superseded in the 1940's by measurements of the frequency and wavelength of microwave radiation. The results were limited by the uncertainty in the wavelength measurements, and it was apparent that greater accuracy could be achieved by using radiation of shorter wavelength. It was, however, not until the development of lasers, and the nonlinear optical techniques made possible by their high output intensities, that frequency measurements could be extended towards the visible region of the spectrum.Stimulated by a demand for a more precise knowledge of the speed of light for application in space research, geodesy, and metrology, a new series of determinations has recently taken place. The measurements were made upon the radiations from a number of different stabilized-laser systems operating in the near infrared and visible spectral regions. Several different interferometric techniques were developed for the wavelength measurements. The results have led to a new recommended value for the speed of light, 299 792 458±1.2 m s^–1, and to the possibility of re-defining the unit of length.