Flame propagation speed and Markstein length of spherically expanding flames: Assessment of extrapolation and measurement techniques

Laminar burning velocities are of great importance in many combustion models as well as for validation and improvement of chemical kinetic schemes. Determining laminar burning velocities with high accuracy is quite challenging and different approaches exist. Hence, a comparison of existing methods measuring and evaluating laminar burning velocities is of interest. Here, two optical diagnostics, high speed tomography and Schlieren cinematography, are simultaneously set up to investigate methods for evaluating laminar flame speed in a spherical flame configuration. The hypothesis to obtain the same flame propagation radii over time with the two different techniques is addressed. Another important aspect is the estimation of flame properties, such as the unstretched flame propagation speed and Markstein length in the burnt gas phase and if these are estimated satisfactorily by common experimental approaches. Thorough evaluation of the data with several extrapolation techniques is undertaken. A systematic extrapolation approach is presented to give more confidence into results generated experimentally. The significance of the linear extrapolation routine is highlighted in this context. Measurements of spherically expanding flames are carried out in two high-pressure, high-temperature, constant-volume vessels at RWTH in Aachen, Germany and at ICARE in Orleans, France. For the discussion of the systematic extrapolation approach, flame speed measurements of methane / air mixtures with mixture Lewis numbers moderately away from unity are used. Conditions were varied from lean to rich mixtures, at temperatures of 298–373 K, and pressures of 1 atm and 5 bar.     

几种二阶矩模型模拟湍流旋流燃烧的比较

本文对几种二阶矩湍流反应模型,包括统一二阶矩模型、只考虑温度脉动的二阶矩模型、只考虑浓度脉动关联的二阶矩模型、及同时考虑温度脉动和浓度脉动关联的二阶矩模型,进行了比较。将上述模型加入到FLUENT6.0软件平台上,模拟了不同旋流数下甲烷-空气的旋流燃烧。模拟结果和实验结果进行了比较,探讨了各关联量大小及其对时平均反应率的影响。结果表明,统一二阶矩湍流反应模型具有最好的模拟效果。其原因是,各种关联矩中,化学反应率系数与浓度间的脉动关联最重要。     

Premixed turbulent combustion modeling using tabulated detailed chemistry and PDF

Tabulated chemistry and presumed probability density function (PDF) approaches are combined to perform RANS modeling of premixed turbulent combustion. The chemistry is tabulated from premixed flamelets with three independent parameters: the equivalence ratio of the mixture, the progress of reaction, and the specific enthalpy, to account for heat losses at walls. Mean quantities are estimated from presumed PDFs. This approach is used to numerically predict a turbulent premixed flame diluted by hot burnt products at an equivalence ratio that differs from the main stream of reactants. The investigated flame, subjected to high velocity fluctuations, has a thickened-wrinkled structure. A recently proposed closure for scalar dissipation rate that includes an estimation of the coupling between flame wrinkling and micromixing is retained. Comparisons of simulations with experimental measurements of mean velocity, temperature, and reactants are performed.     

The effect of hydrogen addition on flammability limit and NOx emission in ultra-lean counterflow CH4/air premixed flames

The effect of hydrogen addition to ultra lean counterflow CH₄/air premixed flames on the extinction limits and the characteristics of NO_x emission was investigated by numerical simulation. Detailed chemistry and complex thermal and transport properties were employed. The results show that the addition of hydrogen can significantly enlarge the flammable region and extend the flammability limit to lower equivalence ratios. If the equivalence ratio is kept constant, the addition of hydrogen increases the emission of NO in a flame due to the enhancement in the rate of the NNH or N₂O intermediate NO formation routes. The addition of hydrogen causes a monotonic decrease in the formation of NO₂ and N₂O, except flames near the extinction limits, where the emission of NO₂ and N₂O first increases, and then decreases with the increase in the fraction of hydrogen. Overall, hydrogen enrichment technology allows stable combustion under ultra lean conditions, resulting in significant CO₂ and NO emission reduction.     

Theoretical and experimental studies on mesoscale flame propagation and extinction

Mesoscale flame propagation and extinction of premixed flames in channels are investigated theoretically and experimentally. Emphasis is placed on the effect of wall heat loss and the wall–flame interaction via heat recirculation. At first, an analytical solution of flame speed in mesoscale channels is obtained. The results showed that channel width, flow velocity, and wall thermal properties have dramatic effects on the flame propagation and lead to multiple flame regimes and extinction limits. With the decrease in channel width, there exist two distinct flame regimes, a fast burning regime and a slow burning regime. The existence of the new flame regime and its extended flammability limit render the classical quenching diameter inapplicable. Furthermore, the results showed that at optimum conditions of flow velocity and wall thermal properties, mesoscale flames can propagate faster than the adiabatic flame. Second, numerical simulation with detailed chemistry demonstrated the existence of multiple flame regimes. The results also showed that there is a non-linear dependence of the flame speed on equivalence ratio. Moreover, it is shown that the Nusselt number has a significant impact on this non-linear dependence. Finally, the non-linear dependence of flame speed on equivalence ratio for both flame regimes is measured using a C₃H₈–air mixture. The results are in good agreement with the theory and numerical simulation.     

Experimental determination of counterflow ignition temperatures and laminar flame speeds of C2–C3 hydrocarbons at atmospheric and elevated pressures

Experimental data were acquired for: (1) the ignition temperatures of nitrogen–diluted ethylene and propylene by counterflowing heated air for various strain rates and system pressures up to 7 atm; (2) the laminar flame speeds of mixtures of air with acetylene, ethylene, ethane, propylene, and propane, deduced from an outwardly propagating spherical flame in a constant-pressure chamber, for extensive ranges of lean-to-rich equivalence ratio and system pressure up to 5 atm. These data, respectively, relevant for low- to intermediate-temperature ignition chemistry and high-temperature flame chemistry, were subsequently compared with calculated results using a literature C₁–C₃ mechanism and an ethylene mechanism. Noticeable differences were observed in the comparison for both mechanisms, and sensitivity analyses were conducted to identify the reactions of importance.     

An experimental and kinetic modeling study of a premixed nitromethane flame at low pressure

A premixed nitromethane/oxygen/argon flame at low pressure (4.67 kPa) has been investigated using tunable vacuum ultraviolet (VUV) photoionization and molecular-beam mass spectrometry. About 30 flame species including hydrocarbons, oxygenated and nitrogenous intermediates have been identified by measurements of photoionization efficiency spectra. Mole fraction profiles of the flame species have been determined by scanning burner position at some selected photon energies. The results indicate that N₂ and NO are the major nitrogenous products in the nitromethane flame. Compared with previous studies on nitromethane combustion, a number of unreported intermediates, including C₃H₄, C₄H₆, C₄H₈, C₂H₂O, C₂H₄O, CH₃CN, H₂CNHO, C₃H₃N and C₃H₇N, are observed in this work. Based on our experimental results and previous modeling studies, a detailed oxidation mechanism including 69 species and 314 reactions has been developed to simulate the flame structure. Despite some small discrepancies, the predictions by the modeling study are in reasonable agreement with the experimental results.     

H2/air旋流预混燃烧的流场特性和火焰结构分析

本文采用完全可压缩的N-S方程,对当量比为1.0的H₂/air旋流预混火焰进行了直接数值模拟研究。氢气和空气的化学反应采用9种组分19步的详细机理。模拟结果表明,强旋流流场中存在回流区,碗形旋流火焰稳定在回流区的外围。在火焰面上沿火焰法向提取了局部火焰结构,将局部湍流火焰结构与层流预混火焰的火焰结构进行了比较,发现局部湍流火焰比层流预混火焰更薄,燃烧强度更高。     

Fire retarded polyurethane foam composites based on steel slag/ammonium polyphosphate system: A novel strategy for utilization of metallurgical solid waste

A series of FR-RPUF composites were prepared by a one-step water foaming process with ammonium polyphosphate (APP) and steel slag (SS) as flame retardants. Thermogravimetric analysis (TG), limiting oxygen index (LOI), UL-94 vertical combustion test, microscale combustion calorimetry (MCC), TG-Fourier transform infrared spectrometry (TG-FTIR), scanning electron microscopy (SEM), Raman spectra and FTIR were used to investigate the thermal stability, flame retardancy, combustion performance, gas phase products, and char residue morphology of FR-RPUF composites. TG test results showed that the initial decomposition temperature (T_-5wt%) and char residue rate at 700°C of RPUF/APP/SS composites were significantly enhanced by the addition of APP and SS, and the thermal stability of the composites was improved. Flame retardant test results confirmed the significantly increased LOI values of RPUF/APP/SS composites with V-0 rating. TG-FTIR also confirmed the obviously decreased release of toxic gases and flammable gases in the combustion of RPUF/APP/SS composites. SEM and Raman spectra of char residues for the composites suggested that APP/SS system improved the compactness and graphitization degree of char layer for RPUF/APP/SS composite. The above researches provide a new strategy for the utilization of SS in fire safety engineering.     

On the Fundamental Polymer Chemistry of Inverse Vulcanization for Statistical and Segmented Copolymers from Elemental Sulfur

In this concept review, the fundamental and polymerization chemistry of inverse vulcanization for the preparation of statistical and segmented sulfur copolymers, which have been actively developed and advanced in various applications over the past decade is discussed. This concept review delves into a discussion of step-growth polymerization constructs to describe the inverse vulcanization process and discuss prepolymer approaches for the synthesis of segmented sulfur polyurethanes. Furthermore, this concept review discusses the advantages of inverse vulcanization in conjunction with dynamic covalent polymerization and post-polymerization modifications to prepare segmented block copolymers with enhanced thermomechanical and flame retardant properties of these materials.