Determination Of Flame Speeds Research Articles

Effect of radiation on laminar flame speed determination in spherically propagating NH3-air, NH3/CH4-air and NH3/H2-air flames at normal temperature and pressure

The use of spherically propagating flames is common for measuring the laminar flame speed in NH3-air, NH3/CH4-air and NH3/H2-air mixtures. However, the radiation-induced uncertainty in such mixtures has not been thoroughly investigated. Due to the low laminar flame speed of ammonia mixtures, it is anticipated that the radiation effect is considerable for such mixtures. This study aims to fill this gap by conducting numerical simulations using different chemical mechanisms and the adiabatic and optical thin radiation models to examine the effects of radiation on spherically propagating NH3-air, NH3/CH4-air and NH3/H2-air flames. The simulations are performed for mixtures at normal temperature and pressure (Tu=298 K and P = 1 atm) and wide range of equivalence ratios. The radiation-induced uncertainty in spherical flames is quantified and compared to planar flames. The importance of the radiation-induced flow and thermal effects in spherical flames is compared between different mixtures and a correlation is developed to determine the radiation-corrected flame speed for spherical NH3-air flames. Considering the radiation effect in NH3-air, it was found that using different mechanisms results in considerable discrepancies in laminar flame speed determination. Some mechanisms showed that the radiation-induced flame speed in spherical flames was underpredicted by more than two times compared to planar flames, and the radiation-induced uncertainty for lean and rich spherically propagating NH3-air flames exceeds 20%. However, the radiation-induced uncertainty at normal temperature and pressure in spherically propagating NH3/CH4-air and NH3/H2-air flames was less significant, not exceeding 11%. Finally, an updated correlation is proposed to determine the radiation-corrected flame speed for NH3-air flames that can be directly used in spherical flame experiments measuring the laminar flame speed.

Experimental and kinetic study on laminar burning velocities of NH3/CH4/H2S/air flames

Recently, ammonia has been favored by scientists and engineers as a new type of carbon-free fuel for future carbon–neutral society. Before large-scale application, some existing drawbacks including extremely low reactivity need to be overcomed by enhancing combustion technology. Blending ammonia with sour gas, a type of natural gas resource containing amounts of H2S, can be a solution to enhance the combustion of ammonia as well as efficiently utilize natural gas resource. In this study, the laminar burning velocities of NH3/CH4/H2S/air flames were investigated using the Heat Flux method over a wide range of equivalence ratios and and mixing ratios. A kinetic model for NH3/CH4/H2S/air flames simulation was developed by integrating ammonia sub-mechanism with the mechanism of Han et al. and validated based on the experimental data. The experimental results show that the H2S addition has a negative effect on the flame speed, which reaches 13.36 % decrease at ϕ = 1.25 when the H2S mole fraction is 12 %. Kinetic analysis was performed for exploring the effect of H2S addition on the laminar flame propagation of NH3/CH4/H2S mixtures. The results show that the elevated H2S content significantly impacts the radicals pool in flame, where the mole fraction of H, OH radicals decreases while the S-containing species content increases obviously. Sensitivity analysis shows that the reactions involving S-containing species become important for laminar flame speed determination at high H2S content. Besides, the increament of NH3 ratio in the fuel significantly improves the concentration level of NH2 radical in the flame, leading to the increased importance of NH2 chemistry.

Elucidating the challenges in extracting ultra-slow flame speeds in a closed vessel—A CH[formula omitted]F[formula omitted] microgravity case study using optical and pressure-rise data

Refrigerants with a low global warming potential (GWP) possess mild flammability. Hence, a fundamental understanding of their combustion characteristics is required to assess their fire-hazardous potential. The laminar burning velocity is one fundamental property to describe these under safety evaluation aspects. However, measuring laminar burning velocities for slow-burning components, such as low-GWP alternatives, is challenging. This is due to influencing effects such as buoyant flame deformation, stretch, radiation, and confinement, especially at the flammability limits. In the present study, we investigated near-limit flames of the representative refrigerant difluoromethane (CH2F2) with nitrogen-enriched oxidizer mixtures to elucidate the potentials and limitations at ultra-slow flame speeds of two widely used flame speed measurement methods in the closed vessel configuration: the optical flame speed measurement in the early quasi-isobaric regime and the flame speed determination from the pressure-rise during the later phase of the isochoric combustion process. Experiments were performed under terrestrial- and microgravity, conducted in a specially designed setup suitable for drop tower applications, using both methods in parallel. Recommendations for the extraction of flame speed data are derived from the non-buoyant reference investigations under microgravity and transferred to ultra-slow flames at terrestrial gravity. Near-limit flames under microgravity were found to be significantly affected by stretch effects for the optical method so that flame speed extrapolations to unstretched conditions are prone to errors. Stretch also affects the pressure method’s lower evaluation limit, and errors can be estimated by combining flame speed data from the pressure method and Markstein lengths from optical experiments.

Effect of wall heat transfer on the dynamics of premixed spherical expanding flames

Premixed laminar flame speed determination is a crucial point since its values are used for the sizing of every combustion systems. In order to measure this parameter, an isochoric combustion method can be used. It consists in measuring the pressure (and possibly other parameters) for a spherical expanding flame. This method allows to get flame speed data for a large scope of important pressures and temperatures supposing an isentropic compression. However, the entirety of the flame propagation process cannot be used to compute the flame speed as heat losses will start to appear as the flame come close the wall, making the isentropic compression assumption invalid. In order to precisely determine when significative heat losses occur, a criterion based on the evolution of the flame preheat zone thickness is described in this paper. The evaluation of this new method is performed using Direct Numerical Simulations for different mixtures (methane with various diluents) at different equivalence ratios and thermodynamic conditions. Finally, the criterion is compared to already existing methods, showing a relatively good accuracy to describe wall heat losses effect on the flame dynamics at high pressure.

Quantifying the role of Darrieus–Landau instability in turbulent premixed flame speed determination at various burner sizes

To probe the impact of Darrieus–Landau (DL) instability on turbulent premixed flame propagation at various burner sizes, methane–air premixed flames from five Bunsen-type burners with different nozzle diameters (4 mm, 6 mm, 8 mm, 10 mm, and 12 mm) were investigated at Reynolds numbers ranging from 1000 to 8500. The flame curvatures used to identify DL instability were determined using Mie scatter images captured by a particle image velocimetry system. The flame speed was further derived by applying an asymmetric hypothesis to the images. The energy-frequency spectrum of the inflow disturbance was determined using a hot-wire anemometry system, and specific wavelet transform analysis was performed to investigate the dependence of DL instability on the proportion of effective disturbances (Ped) and quantify the role of DL instability in determining the turbulent flame speed. The results showed that the burner diameter had an obvious effect on the presence of DL instability and its role in flame propagation. The ability of DL instability to enhance the flame curvature skewness and the turbulent flame speed was closely related to Ped. Ped increased when the burner diameter increased from 6 mm to 12 mm, thus enhancing the DL instability. Changing the burner diameter also affected the interplay between DL instability and turbulence. The above interactions and their effects on the flame speed during the change of inflow disturbances could be formulated by Ped. Finally, a Ped-based correlation was proposed to describe the dependence of the turbulent flame speed on the burner size.

Laminar flame speed determination at high pressure and temperature conditions for kinetic schemes assessment

Given the experimental difficulties, most of the available flame speed database is for relatively reduced thermodynamic conditions and for non-simultaneous variations of pressure and temperature. This limitation may be overpassed by using spherically expanding flames with the constant volume method. This methodology, introduced in the 30 s by Lewis and von Elbe, requires the knowledge of the pressure evolution in the combustion chamber. It has been penalized for a long time because of the underlying assumptions and problems in flame instability detection. This method has been greatly renewed recently by Egolfopoulos following a coupled experimental/numerical approach integrating the effects of radiation and dissociation while maintaining moderate computing costs. In parallel with this study, we have worked on an alternative method providing a maximum of information for each test minimizing uncertainties. The current study uses a new experimental device allowing simultaneous recording of pressure and flame radius inside the chamber during the full combustion process. The direct use of these data over the whole flame propagation allows testing kinetic schemes over large pressure and temperature domains with good accuracy. These new experimental targets allowed the identification of key reactions needing improvements.

Effects of data point number on laminar flame speed extrapolation

This study aims to investigate the effects of data points number on the accuracy of flame speed extrapolation using expanding spherical flame, which is a practical issue in high flame speed experimental measurement. Monte Carlo simulations were conducted based on the flame radius data obtained from one-dimensional spherical flame simulation. Given number of points were randomly picked from 70 points to obtain the unstretched flame speed by linear and nonlinear extrapolation under different Lewis number, fuel type, and initial pressures. The results were statistically analyzed and show that the least number for flame speed extrapolation is related to the Lewis number of mixtures. Mixtures with Lewis number smaller than unity need more data points to ensure accurate extrapolating results. In addition, the accuracy of flame speed determination is very sensitive to the error in flame radius when data points are sparse. When 0.1% of Gauss noise was added to flame radius data, additional dozens of points are required to keep the same accuracy. At least 30 data points were suggested to be adopted in the extrapolation of flame speed to reduce the uncertainty from data points number to less than 1%. The effects of the flame radius range on the determination of flame speed again validated by the current study, indicating a wide range of flame radius should be used to avoid uncertainty. It was also shown that the nonlinear extrapolation method needs fewer data points than the linear method to achieve the same accuracy for mixtures.

Effects of radiation on the uncertainty of flame speed determination using spherically propagating flames with CO/CO2/H2O dilutions at elevated pressures

This work investigates numerically the effects of spectral dependent radiation on laminar flame speed determination using spherically propagating CH4/air and H2/air flames with CO/CO2/H2O dilutions at elevated pressures. Three different models, adiabatic, optically thin radiation, and fitted statistically narrow band correlated k (FSNB-CK) models, are employed. The effects of radiation-induced negative burned gas velocity, increased density ratio, and chamber confinement induced flow compression are investigated. It is found that compared to the FSNB-CK model, the adiabatic flame model over-predicts the flame speed by 7% and the optically thin model makes more significant under-prediction. Moreover, this discrepancy increases with pressure. The results also show that a large negative velocity in the burned gas is induced by radiative heat loss and magnified further by the flow compression in a small combustion chamber. The radiation-induced negative burned gas velocity causes an under-estimation of flame speed. Moreover, radiation also increases the density ratio between the burned and the unburned gases. The use of the density ratio of adiabatic flame also causes under-prediction of flame speed. Two radiation corrections taking into account of the negative burned gas velocity and the increased density ratio are recommended for flame speed determination using propagating spherical flame for radiative mixtures. The corrections proposed in this study reduce the uncertainty of flame speed due to radiation.

Effects of compression and stretch on the determination of laminar flame speeds using propagating spherical flames

The effects of flow compression and flame stretch on the accurate determination of laminar flame speeds at normal and elevated pressures using propagating spherical flames at constant pressure or constant volume are studied theoretically and numerically. The results show that both the compression-induced flow motion and flame stretch have significant impacts on the accuracy of flame speed determination. For the constant pressure method, a new method to obtain a compression-corrected flame speed (CCFS) for nearly constant pressure spherical bomb experiments is presented. Likewise, for the constant volume method, a technique to obtain a stretch-corrected flame speed (SCFS) at elevated pressures and temperatures is developed. The validity of theoretical results for both constant pressure and constant volume methods is demonstrated by numerical simulations using detailed chemistry for hydrogen/air, methane/air, and propane/air mixtures. It is shown that the present CCFS and SCFS methods not only improve the accuracy of the flame speed measurements significantly but also extend the parameter range of experimental conditions. The results can be used directly in experimental measurements of laminar flame speeds.

Further considerations on the determination of laminar flame speeds with the counterflow twin-flame technique

The accuracy of the laminar flame speed determination by using the counterflow twin-flame techniquehas been computationally and experimentally examined in light of the recent understanding that linear extrapolation of the reference upstream velocity to zero strain rate would yield a value higher than that of the laminar flame speed, and that such an overestimate can be reduced by using either lower strain rates and/or larger nozzle separation distances. A systematic evaluation of the above concepts has been conducted and verified for the ultralean hydrogen/air flames, which have relatively large Karlovitz numbers, even for small strain rates, because of their very small laminar flame speeds. Consequently, the significantly higher values of the previous experimentally measured flame speeds, as compared with the independently calculated laminar flame speeds, can now be attributed to the use of nozzle separation distances that were not sufficiently large and/or strain rates that were not sufficiently small. Thus, by using lower strain rates and larger nozzle separation distances, the experimentally and computationally redetermined values of these ultralean hydrogen/air flames agree well with the calculated laminar flame speeds. The laminar flame speeds of methane/air and propane/air mixtures have also been experimentally redetermined over extensive ranges of the equivalence ratio and are found to be slightly lower than the previously reported experimental values.

On the determination of laminar flame speeds from stretched flames

The effects of stretch on the determination of the laminar flame speed are experimentally studied by using the positively-stretched stagnation flame and negatively-stretched bunsen flame, and by using lean and rich mixtures of methane, propane, butane, and hydrogen with air whose effective Lewis numbers are either greater or less than unity. Results demonstrate that flame speed determination can be influenced by stretch through two factors: (1) Preferential diffusion which tends to increase or decrease the flame temperature and burning rate depending on the effective Lewis number, and (2) Flow divergence which causes the flame speed to assume higher values when evaluated at the upstream boundary of the preheat zone instead of the reaction zone. Recent data on flame speed including the present ones are then examined from the unified viewpoint of flame stretch, leading to satisfactory resolution of the discrepancies between them. The present study also proposes a methodology of determining the laminar flame speeds by using the stagnation flame and linearly extrapolating the data to zero stretch rate.

Theory of laminar flame propagation with non-normal diffusion

A study has been made of the problem of steady, one-dimensional, laminar flame propagation in premixed gases, with the Lewis number differing from (and equal to) unity. Analytical solutions, using the method of matched asymptotic expansions, have been obtained for large activation energies. Numerical solutions have been obtained for a wide range of the reduced activation temperature parameter ( n ≑ E/RT b), and the Lewis number δ. The studies reveal that the flame speed eigenvalue is linear in Lewis number for first order and quadratic in Lewis number for second order reactions. For a quick determination of flame speeds, with reasonable accuracy, a simple rule, expressing the flame speed eigenvalue as a function of the Lewis number and the centroid of the reaction rate function, is proposed. Comparisons have been made with some of the earlier works, for both first and second order reactions.

Theory of Linear Flame Propagation. Part II. Structure of the Steady State

A quantitative approximate theory of flame structure in the steady state, including formulas for the determination of flame speeds, is set forth. The theory predicts that in a first approximation, the sensitivity of a steady deflagration wave to external perturbations depends upon the shape of the temperature profile, but not upon the speed or width of the wave, or the pressure. It is shown that the sensitivity increases with increasing dilution of the explosive mixture, in agreement with the explanation of inflammability limits suggested in Part I.

Reactions between nitrogen dioxide and formaldehyde. Part III.—The determination of flame speeds

The first page of this article is displayed as the abstract.

The determination of flame speeds in gaseous mixtures

The determination of flame speeds in gaseous mixtures