Experimental Flame Speed Research Articles

Ultra-slow ammonia flame speeds — A microgravity study on radiation

Ammonia/air flames burn slowly and are consequently affected by buoyancy-, radiation-, and stretch-related uncertainties more than many conventional hydrocarbon fuels. In this work, unique buoyancy-free ammonia/air flame speed data were acquired and analyzed, specifically considering the impact of radiation. First, outwardly propagating ammonia/air flames near the propagation limits were accurately measured under microgravity using two simultaneous techniques. Flame radius evolution was captured by a high-speed Schlieren arrangement (optical method) in a near-isobaric regime. Accompanying, pressure rise was measured and subsequently used for flame speed extraction during near-isentropic gas compression. A profound nonlinear dependence of flame propagation speed on stretch was captured over the optical results in the observable range of the current setup, which illustrates the limitations of the optical method in comparison to the pressure-rise method. Radiation-corrected optical and pressure-rise results have shown good agreement along multiple isentropes, thereby cross-validating applied methodologies and highlighting very good consistency of the acquired data. Pressure-rise data analysis has shown sensitivity in the assessment of kinetic mechanisms to the radiation effect. In the absence of appropriate radiation correction for flame speed data, misinterpretations may arise regarding the accuracy of kinetic models, introducing challenges in the development of accurate kinetic models. The performance of the chemical kinetic models was quantified in a wide range of unburned gas temperatures (< 500K) and pressures (< 9.3bar) against distinctive experimental flame speed data with and without radiation correction acquired under microgravity indicating good performance of the Han et al. (2019) model.

Unstretched unburned flame speed and burned gas Markstein length of diluted hydrogen/air mixtures

Fundamental combustion characteristics of H2/air flames with the addition of actual H2/air combustion residuals (a mixture of 65% N2 + 35% H2O by mole) are examined experimentally and numerically at 1–2 bar, 373–473 K, equivalence ratio of 0.7, and dilution ratios of 0–40%. Spherically expanding flame measurements at constant pressure show that flame speed and adiabatic flame temperature drop almost linearly with increasing diluent level. Detailed numerical simulations and analyses of sensitivity coefficients reveal that this is because of the low chemical reactivity of the dilution mixture. On the other hand, the change in burned gas Markstein length with the dilution mixture addition is found more complex and cannot be represented with a linear trend. Experimental flame speed data are compared with results of chemical kinetic analyses obtained by several chemical mechanisms in order to assess the accuracy of these models.

Improvement of H2/O2 chemical kinetic mechanism for high pressure combustion

An updated H2/O2 kinetic mechanism was proposed by incorporating carefully selected reaction rate coefficient and great progress in radical chain mechanisms, in which the uncertainties of rate coefficient were discussed. The performance of the current mechanism was compared to other H2 mechanism and validated against a wide range kinetic targets, including oxidation, decomposition in shock waves, ignition, flame speed and flame structure. Results show that the current mechanism obtains an overall improvement of performance, especially for the flame speed. By using the updated binary diffusion coefficient from ab initio calculations and the chemically termolecular reactions, the current mechanism presents better agreement with the new experimental flame speed at atmospheric pressure and obtains the improved performance with respect to the negative pressure dependence of high-pressure H2 flame. Furthermore, the flame speed predictions are strongly sensitive to the H2O third body efficiency in the H2 mechanism, affecting the water-contained H2 flame. The modeling results of rapid compression machine ignition show that present mechanism can more accurately predicts the ignition delay under engine-like conditions. However, all three mechanisms cannot accurately reproduce the negative pressure dependence behavior of mass burning rate in high-pressure H2 flame, which may be attributed to the fact that the important reaction O + OH(+M) = HO2(+M) that significantly affects lean high-pressure H2 flame is not included in current mechanism. Consequently, continuous works should be emphasized on the reactions that are important but neglected in H2 mechanism. All these not only develop an improved H2 reaction mechanism for high-pressure combustion, but also point out the direction for refining the H2 mechanism.

Low global-warming-potential refrigerant CH2F2 (R-32): Integration of a radiation heat loss correction method to accurately determine experimental flame speed metrics

Due to their high global warming potential (GWP), hydrofluorocarbon refrigerants are systematically being phased out. Replacements with low GWP exist, but give rise to safety hazards as they are found to be mildly flammable. The assessment of the safety hazards of such fluids is typically based on their laminar flame speeds. Typical laminar flame speeds are below 10 cm/s, and hence are challenging to measure. Flames propagating at this speed are strongly affected by radiation heat loss and have to be corrected if considered for kinetic model validation. In the present study, the laminar flame speed of the representative refrigerant difluoromethane (CH2F2, R-32) is measured experimentally in an outwardly propagating flame configuration at elevated pressure and temperature. To assess radiation heat loss effects, detailed simulations using a recent kinetic model for CH2F2 are conducted. Flame speed reductions due to radiation are found to be in the order of 15%. An analytical radiation correction model, as discussed by Santner et al. (2014), is adopted and its suitability for refrigerant/air flames is demonstrated based on simulation. After subsequent correction of experimental results, the predictability of the model is evaluated, showing good agreement.

Large eddy simulation of premixed hydrogen/methane/air flame propagation in a closed duct

In this paper, large eddy simulation (LES) is performed to investigate the propagation characteristics of premixed hydrogen/methane/air flames in a closed duct. In LES, three stoichiometric hydrogen/methane/air mixtures with hydrogen fractions (volume fractions) of 0, 50% and 100% are used. The numerical results have been verified by comparison with experimental data. All stages of flame propagation that occurred in the experiment are reproduced qualitatively in LES. For fuel/air mixtures with hydrogen fractions of 0 and 50%, only four stages of “tulip” flame formation are observed, but when the hydrogen fraction is 100%, the distorted “tulip” flame appears after flame front inversion. In the acceleration stage, the LES and experimental flame speed and pressure dynamic coincide with each other, except for a hydrogen fraction of 0. After “tulip” flame formation, all LES and experimental flame propagation speeds and pressure dynamics exhibit the same trends for hydrogen fractions of 0 and 100%. However, when the hydrogen fraction is 50%, a slight periodic oscillation appears only in the experiment. In general, the different structures displayed in the flame front during flame propagation can be attributed to the interaction between the flame front, the vortex and the reverse flow formed in the unburned and burned zones.

Experimental and modeling study of chemical-kinetics mechanisms for H2–NH3–air mixtures in laminar premixed jet flames

A combined experimental and modeling study of laminar flame speeds for premixed H2–NH3–air jet flames is performed for 0–80% NH3 in H2 by energy and for equivalence ratios from 0.5 (fuel lean) to 1.1 (fuel rich). Experimental flame speeds in the jet flame configuration compare well with previous data from freely propagating spherical flames after corrections for heat losses. These data are then used to validate flame-speed predictions using CHEMKIN software over a wide range of conditions and to evaluate the performance of the GRI-Mech3.0, Tian and Konnov detailed chemical kinetic mechanisms. It is found that these mechanisms perform well for H2–air combustion but begin to deviate substantially (by ∼2×) from each other with the addition of NH3. Differences in flame speeds and associated radical species concentrations (H, O, and OH) are found to be largest for higher levels of NH3 (50% by energy and greater) and with increasing equivalence ratio. A sensitivity analysis reveals that OH is the key radical leading to NH3 decomposition and is a likely source of deviation between model predictions. The experimental data show that the Tian and Konnov mechanisms are the most capable of accurately predicting reductions in flame speed as much as 90–95% with NH3 addition, although the accuracy of each mechanism differs with the level of NH3 within the mixture.

Hydrogen- and C 1–C 3 Hydrocarbon-Nitrous Oxide Kinetics in Freely Propagating and Burner-Stabilized Flames, Shock Tubes, and Flow Reactors

A detailed, chemical mechanism has been compiled for modeling the combustion of C 1–C 3 hydrocarbon-nitrous oxide mixtures. The compiled, chemical mechanism has been compared and validated against available data in the literature, including flame structure measurements of hydrogen-nitrous oxide and ammonia-nitrous oxide flames, flow reactor data on hydrogen-nitrous oxide and moist, nitrous oxide decomposition mixtures, shock tube ignition delay data on hydrogen- and methane-nitrous oxide mixtures, experimental flame speed data on carbon monoxide-nitrous oxide mixtures, and experimental, laminar flame speed data for hydrogen-, methane-, acetylene-, and propane-nitrous oxide mixtures. Feature sensitivity studies and reaction path analyses have been employed to elucidate important pathways to the overall reaction rate for these systems, and include the nitrous oxide decomposition step (N 2 O(+M) ⇌ N 2 + O(+M)) and nitrous oxide reactions with H atoms (N 2 O + H ⇌ N 2 + OH and N 2 O + H ⇌ NH + NO). Modifications to the compiled mechanism were considered on the basis of the most sensitive reactions with the highest apparent rate constant uncertainty, to closely predict the various experimental data considered.

Experimental flame speed in multi-layered nano-energetic materials

This paper deals with the reaction of dense Metastable Intermolecular Composite (MIC) materials, which have a higher density than conventional energetic materials. The reaction of a multilayer thin film of aluminum and copper oxide has been studied by varying the substrate material and thicknesses. The in-plane speed of propagation of the reaction was experimentally determined using a time of- flight technique. The experiment shows that the reaction is completely quenched for a silicon substrate having an intervening silica layer of less than 200 nm. The speed of reaction seems to be constant at 40 m/s for silica layers with a thickness greater than 1 μm. Different substrate materials such as glass and photoresist were also used.

Laminar Burning Velocities for Hydrogen-, Methane-, Acetylene-, and Propane-Nitrous Oxide Flames

Mixtures of four fuels (hydrogen, methane, acetylene and propane) with nitrous oxide were studied to experimentally and numerically determine the laminar flame speeds at near atmospheric pressure (0.8 atm). Using the flat flame method, laminar flame speeds for these nitrous oxide flames were determined for different levels of dilution with nitrogen. A comprehensive hydrocarbon oxidation mechanism in the literature was integrated with a hydrogen-nitrous oxide sub-mechanism, and computational flame speed results using this mechanism were compared to experimental data. For all four fuel systems (hydrogen-, methane-, acetylene-, and propane-nitrous oxide), the compiled chemical mechanism under-predicted the measured laminar flame speeds over the whole range of equivalence ratio investigated. Flame speed sensitivity analyses for the hydrogen-, acetylene-, and propane-nitrous oxide systems have shown that the most sensitive reactions are within the hydrogen-nitrous oxide sub-mechanism. A revision to the reaction rate constant expression for the N2O + H ⇌ N2 + OH pathway, one of the most sensitive reaction steps, within the reported uncertainty has improved computational predictions of experimental flame speed data, although further optimization and validation of the kinetic mechanism are required.

Experimental and numerical determination of laminar flame speeds: Mixtures of C 2-hydrocarbons with oxygen and nitrogen

Using the counterflow flame technique, laminar flame speeds of mixtures of ethane, ethylene, acetylene and propane with oxygen and nitrogen have been accurately determined over extensive lean-to-rich fuel concentration ranges and over the pressure range of 0.25 to 3 atm. These data are then compared with the numerically calculated values obtained by using the various kinetic schemes in the literature as well as one compiled in the present study. The present scheme yields close agrrement with all of the experimental flame speeds except for diluted, rich acetylene flames, for which the calculated values are higher. The relative importance and influence of the individual reactions on the flame speed and reaction mechanism are assessed and discussed with the aid of sensitivity analysis. The study also demonstrates that C2 schemes validated through comparisons based on methane flame speeds may not be accurate enough for flame speed predictions of the C2 fuels, and that the C2 schemes developed through comparisons with the flame speeds of the C2 fuels are rather insensitive to the details of the C3 sub-mechanism. The importance of having accurate values of the thermophysical properties of radicals for flame simulation is also emphasized.

Experimental and numerical determination of laminar flame speeds of methane/(Ar, N2, CO2)-air mixtures as function of stoichiometry, pressure, and flame temperature

By using the counterflow method, the laminar flame speeds of methane/(Ar, N 2 , CO 2 )-air mixtures have been accurately and extensively determined over the stoichiometric range from very lean to very rich, the pressure range from 0.25 to 2 atm, and flame temperature range from 1,550 to 2,250 K; independent variation in the flame temperature is achieved by substituting nitrogen in the air by an equal amount of argon or carbon dioxide. These data are expected to be useful for the partial validation of proposed kinetic mechanisms. In the present study numerical simulation of the experimental flame speeds has been conducted by using a C 1 mechanism and a full C 2 mechanism. The calculated results agree well with the experimental data, for both the C 1 and C 2 mechanisms, except for very rich mixtures for which there is substantial overprediction by the C 2 mechanism. Sensitivity analyses have also been performed where appropriate for enhanced insight into the controlling elementary reactions.

Turbulence Effects on Flame Speed and Flame Structure

Turbulence effects on methane-air flames stabilized in grid turbulence were investigated through measurements of flame speed and mean and fluctuating flame temperature profiles. Published turbulent flame speed correlations were able to correlate the experimental flame speed data but were contradictory in indicating flame structure and combustion mechanisms. A simple, one-dimensional, wrinkled laminar flame model was used to predict characteristic flame temperature fluctuation levels. Comparison of these predictions with measured temperature fluctuations indicated that the majority of the flames studied were wrinkled laminar flames. However, a wrinkled laminar flame structure was inappropriate for the most intensely turbulent flames examined.

The hydrazine flame

Experimental data on the hydrazine flame speed are pressure-independent at low pressures. Thus, a second-order reaction mechanism is assumed to be controlling. Kinetics are given for a chain mechanism in which the NH 2 radical is formed by a pseudo-unimolecular process, tending to first order at high pressures, but second order at low pressures. The consequent set of flame equations can be solved using the chemical steady-state hypothesis. In this instance, however, the solution to the flame equation leaves both the flame speed and the overall chemical rate constant undetermined for the hydrazine decomposition. By inserting the experimental flame speed, a value for this rate constant is obtained. It is also possible to calculate the second-order rate constant for the formation of activated N 2H 4 radicals. The rate k a⋍3 × 10 18 exp(−60 000/RT) cm 3/mole sec. The unusually high value for this constant suggests energy transfer to the NN bond from many vibrational modes of N 2H 4 . Re-examination of earlier data of Szwarc on the formation of NH 2 radicals treated as a first-order process indicates that a second-order process may be implied from the data and, also, a value for k a⋍10 19 exp(−60 000/RT) cm 3/mole sec may be deduced. Investigation of the validity of the chemical steady state for free radicals in the hydrazine flame is made. It is determined that the flame may only be roughly approximated by the chemical steady state.

A study of burning velocity

Using the counterflow flame technique, laminar flame speeds of mixtures of ethane, ethylene, acetylene and propane with oxygen and nitrogen have been accurately determined over extensive lean-to-rich fuel concentration ranges and over the pressure range of 0.25 to 3 atm. These data are then compared with the numerically calculated values obtained by using the various kinetic schemes in the literature as well as one compiled in the present study. The present scheme yields close agrrement with all of the experimental flame speeds except for diluted, rich acetylene flames, for which the calculated values are higher. The relative importance and influence of the individual reactions on the flame speed and reaction mechanism are assessed and discussed with the aid of sensitivity analysis. The study also demonstrates that C2 schemes validated through comparisons based on methane flame speeds may not be accurate enough for flame speed predictions of the C2 fuels, and that the C2 schemes developed through comparisons with the flame speeds of the C2 fuels are rather insensitive to the details of the C3 sub-mechanism. The importance of having accurate values of the thermophysical properties of radicals for flame simulation is also emphasized.