Increase In Burning Velocity Research Articles

Air humidity influence on combustion of R-1234yf (CF3CFCH2), R-1234ze(E) (trans-CF3CHCHF) and R-134a (CH2FCF3) refrigerants

The influence of air humidity on flame propagation in mixtures of hydrofluorocarbons (HFCs) with air was studied through numerical simulations and comparison with measurements from the literature. Water vapor added to the air in mixtures of fluorine rich hydrofluorocarbons (F/H ≥ 1) can be considered as a fuel additive that increases the production of radicals (H, O, OH) and increases the overall reaction rate. The hydrofluorocarbon flame is typically a two-stage reaction proceeding with a relatively fast reaction in the first stage transitioning to a very slow reaction in the second stage which leads to the combustion equilibrium products. The transition to the second stage is determined by the consumption of hydrogen-containing species and formation of HF. Despite a relatively small effect of water on the adiabatic combustion temperature, its influence is significant on the reaction rate and on the temperature increase in the first stage of the combustion leading to the increase in burning velocity. The main reaction for converting H2O to hydrogen-containing radicals and promoting combustion is H2O + F = HF + OH, as demonstrated by reaction path analyses for the fluorine rich hydrofluorocarbons R-1234yf, R-1234ze(E), and R-134a (F/H = 2). The calculated burning velocity dependence on the equivalence ratio ϕ agrees reasonably well with available experimental measurements for R1234yf and R-1234ze(E) with and without the addition of water vapor. In agreement with experimental data, with water vapor, the maximum of burning velocity over ϕ is shifted to the lean mixtures (near ϕ = 0.8).

The effect of hydrogen addition on methane/air explosion characteristics in a 20-L spherical device

The addition of hydrogen to methane changes its deflagration characteristics and increases the combustion rate. However, studies on the effect of hydrogen on methane deflagration remain insufficient. Therefore, based on the CFD code GASFLOW-MPI, a four-step combustion-mechanism model was established for methane/hydrogen mixtures. The deflagration characteristics of a premixed combustible gas in a 20-L spherical device was numerically simulated using a methane/hydrogen/air equivalence ratio of 1 and hydrogen addition in the range of 0–50%; subsequently. The results were compared with experimental data. The four-step methane/hydrogen combustion-mechanism could effectively reproduce the methane/hydrogen deflagration process on considering the heat losses. With an increase in hydrogen addition, the laminar burning velocity increases, and the deflagration duration reduces. It decreases the explosion heat loss and increased the maximum deflagration pressure. Under adiabatic simulation, the maximum deflagration pressure decreased with an increase in hydrogen addition, in contrast with the experimental results. This indicates that the heat-loss effect of the methane/hydrogen/air-mixture deflagration process should not be ignored. Moreover, the heat loss during the methane/hydrogen/air-mixture deflagration was mainly caused by thermal radiation. Thus, the influence of the thermal-radiation and convective heat-transfer mechanisms should be considered in the numerical simulations of methane/hydrogen/air-mixture deflagration.

Experimental and numerical simulation of effects of CO2/N2 concentration and initial temperature on combustion characteristics of biomass syngas

Biomass syngas is a form of renewable energy with very broad application prospects, and it has different combustion characteristics according to the fuel composition and processing technology of biomass syngas. The influence of combustion composition, diluent and temperature variation on combustion characteristics were studied in this paper. The FFCM-1 mechanism was used to investigate the combustion characteristics of CO/CH4/H2 under varied diluents CO2/N2 and temperature by using spherical expansion flame method and ANSYS CHEMKIN-PRO. The experimental laminar burning velocity was compared with the simulation results of FFCM-1 mechanism. The results reveal that the experimental data are in good agreement with the simulation results, which are somewhat different under the condition of rich fuel. The laminar burning velocity decreases significantly with the increase of diluent CO2/N2, with the effect of diluent CO2 being more significant. The laminar burning velocity increase dramatically with the increase of initial temperature, and the adiabatic flame temperature also decreases with the increase of diluent. The reduction caused by diluent CO2 is much larger than that caused by diluent N2. The change of initial temperature also affects the adiabatic flame temperature, but the range of variation is not as pronounced as that of diluent. Not only was the interaction between the combustion characteristics of CO/CH4/H2 under different diluents and temperature changes explored in this paper, but the influence mechanism was also revealed in depth.

Experimental study on structure and blow-off characteristics of NH3/CH4 co-firing flames in a swirl combustor

Experimental study on the unconfined lean premixed NH3/CH4/air flames with three CH4 fractions of 0, 50%, and 100% stabilized on a bluff-body and swirl burner was investigated. The flow fields and instantaneous OH distributions were captured by synchronous PIV/OH-PLIF measurement. The macrostructure and critical mechanism of blow-off for the compact NH3/air flame at conditions far away and near blow-off, and the effect of CH4 addition on the topology and flame stabilization was discussed. Results show that the NH3/air flame possesses a poor lean blow-off limit Φb, and the NH3/air mixture can not be ignited when Uin is higher than 5 m/s. After adding 50% CH4 fuel, the laminar flame speed SL and the extinction strain rate Kext are approximately 3 times and 7 times that of the NH3/air flame with equivalence ratio Φ = 0.8, resulting in a more stable flame. And the NH3/air flame Φb = 0.78 with inlet bulk velocity Uin = 4 m/s can be extended wider by 16.6% to Φb = 0.65. The NH3 flame front has been locally extinguished due to excessive stretching, which is the key factor of triggering the overall blow-off. Stretching is mainly caused by shear strain. CH4 addition makes the flame front more wrinkled. Furthermore, the flame is strengthened due to the enhanced resistance to stretching by blending CH4 with NH3. But the root of the NH3/CH4/air flames with methane fractions of 50% and 100% also have excessive straining near the blow-off conditions, which induces flame blow-off. The increase of burning velocity and temperature by adding CH4 is also conducive to improving flame stability. The Inner Shear Layer (ISL) vortexes promote the uniform mixing of combustion products in the Inner Recirculation Zone (IRZ) and downstream combustion gas, which is conducive to the stability of CH4/air flame. For NH3/air flame, the ISL vortexes accelerate the flame blow-off by entraining the cold reactants into the IRZ.

Impact of Syngas Addition to Methane on Laminar Burning Velocity

Abstract Exhaust gas recirculation (EGR) in spark-ignited (SI) engines is a key technique to reduce in-cylinder NOx production by decreasing the combustion temperature. The major species of the exhaust gas in rich combustion of natural gas are hydrogen and carbon monoxide, which can subsequently be recirculated to the cylinders using EGR. In this study, the effect of hydrogen and carbon monoxide addition to methane on laminar burning velocity and flame morphology is investigated. Due to the broad flammability limit and high burning velocity of hydrogen compared to methane, this addition to the gaseous mixture leads to an increase in burning velocity, less emissions production, and a boost to the thermal efficiency of internal combustion engines. Premixed CH4–H2–CO–air flames are experimentally investigated using an optically accessible constant volume combustion chamber (CVCC) accompanied with a high-speed Z-type Schlieren imaging system. Furthermore, a numerical code is applied to quantify the laminar burning velocity based on the pressure rise during flame propagation within the CVCC. According to the empirical and numerical results, the addition of hydrogen and carbon monoxide enhances laminar burning velocity while influencing the flame structure and development.

The Equivalent Effect of Initial Condition Coupling on the Laminar Burning Velocity of Natural Gas Diluted by CO2

Initial temperature has a promoting effect on laminar burning velocity, while initial pressure and dilution rate have an inhibitory effect on laminar burning velocity. Equal laminar burning velocities can be obtained by initial condition coupling with different temperatures, pressures and dilution rates. This paper analysed the equivalent distribution pattern of laminar burning velocity and the variation pattern of an equal weight curve using the coupling effect of the initial pressure (0.1–0.3 MPa), initial temperature (323–423 K) and dilution rate (0–16%). The results show that, as the initial temperature increases, the initial pressure decreases and the dilution rate decreases, the rate of change in laminar burning velocity increases. The equivalent effect of initial condition coupling can obtain equal laminar burning velocity with an dilution rate increase (or decrease) of 2% and an initial temperature increase (or decrease) of 29 K. Moreover, the increase in equivalence ratio leads to the rate of change in laminar burning velocity first increasing and then decreasing, while the increases in dilution rate and initial pressure make the rate of change in laminar burning velocity gradually decrease and the increase in initial temperature makes the rate of change in laminar burning velocity gradually increase. The area of the region, where the initial temperature influence weight is larger, gradually decreases as the dilution rate increases, and the rate of decrease gradually decreases.

Laminar burning velocity and flame structure of DME/methane + air mixtures at elevated temperatures

The experimental measurements and computational predictions of the laminar burning velocity of DME and methane blends with air are reported in the present work. The well documented and validated rectangular diverging channel was used for the measurements. Whereas, one dimensional freely propagating flame simulations were performed using PREMIX code detail reaction mechanisms. The measured experimental data and predictions were successfully validated against the data available at ambient conditions. Then, high temperature burning velocity measurement and predicted data is reported. Influence of mixture equivalence ratios, blend combinations, and initial temperatures are critically analysed and discussed. The detail flame structure variation is also reported with the help of heat release rates, major and minor species profiles, reaction pathways, sensitivity analysis, and fake/pseudo-specie analyses. The addition of DME into methane enhances the reactivity and burning velocity. However, the rate of increase in burning velocity with respect to an increase in initial temperature is higher for pure methane + air mixtures.

An improved CFD model for vented deflagration simulations – Analysis of a medium-scale hydrogen experiment

Hydrogen is a promising alternative fuel which is expected to gain a significant portion of the energy market in the near future. However, it is a flammable gas with significant safety concerns as it can easily cause deflagrations or even detonations. The aim of this work is twofold: firstly, to derive a new CFD model for vented deflagration simulations improving an existing one and secondly, to better understand the physics of the phenomenon. A recent experiment conducted in Karlsruhe Institute of Technology is used for that purpose. The new model improves substantially the predictions representing successfully the experiment. The analysis indicates that turbulence generated outside the vent is responsible for the violence of the external explosion which leads to the sudden pressure increase. Rayleigh-Taylor instability on the other hand does not seem to contribute to burning velocity increase. From the analysis that is made combustion models are formulated highlighting the important components for a successful CFD deflagration model.

Effect of hydrogen addition on the OH* and CH* chemiluminescence emissions of premixed combustion of methane-air mixtures

Chemiluminescence emissions measurements of OH* and CH* are used to characterize the combustion of blends of methane and hydrogen in air in a constant volume combustion bomb, with two sets of initial conditions. The combined results of combustion development cover from 0.1 to 2.5 MPa. Burning velocity, heat release and unburned/burned gas temperatures are obtained from the pressure by using a two-zone thermodynamic combustion diagnostic model. Intensity of OH* and CH* increases with the initial temperature and the percentage of hydrogen, in parallel with the usual increase in burning velocity. The timings of the peaks of OH* and CH* chemiluminescence emissions are found to correlate respectively with the maximum rate of heat release and flame temperature. These results show that both chemiluminescence signals can be used to monitorize the burning process in combustion devices operating in the pressure range studied.

Study on the combustion of micro organic dust particles in random media with considering effect of thermal resistance and temperature difference between gas and particles

This paper presents an analytical model to investigate the effect of randomly distributed micro-organic dust particles on the flame propagation through the combustible mixture by considering the thermal resistance and temperature difference between the gas and the particles. The effect of thermal resistance on flame characteristics is intended by the dimensionless Biot number. It is concluded that the increase in Biot number leads to decrease in the burning velocity as well as flame temperature. Conclusively, flame characteristics for different Lewis numbers and particles radii are reported. Results show that both flame temperature and burning velocity increase with enhancement in the Lewis number. This research leads us to a deep understanding of random combustion of lycopodium particles and more practical prediction of particle combustion compared to the published theoretical studies.

Dynamics of premixed CH4/air flames in a micro combustor with a plate flame holder and preheating channels

It is challenging to achieve a wide flammable range in micro combustors due to the increased heat loss. In the present work, the flammable range based on equivalence ratio for a novel micro combustor is investigated, and some interesting dynamic flame behaviors are observed, which include the flame blow-off, pulsating flame and the flame with repetitive flashback and pulsating (FRFP). Our analysis indicates that the above flame behaviors mainly depend on the competition of the burning velocity and flow speed. For the lean fuel, a higher equivalence ratio leads to a faster burning velocity and a higher preheating temperature. A higher preheating temperature further elevates the burning velocity as well as the flow speed of incoming gas mixture. The faster burning velocity makes the flame shift upstream, but the bigger flow speed pushes the flame downstream. In such a way, the increase of equivalence ratio gives rise to the increase of burning velocity and flow speed, and the burning velocity increases faster than the flow speed. In addition, the flame dynamics distributions are almost symmetrical with respect to the stoichiometric ratio, i.e., the flame behaviors under lean and rich fuel are the same basically.

Modes of chocked flame instability defined by the peculiarities of combustion kinetics at rising pressure

The aim of the paper was to analyze the structure and the stability of the chocked flames to understand the origins of different possible combustion modes, including quasi-stable supersonic flames and deflagration-to-detonation transition. By means of numerical analysis, it is shown that the chocked flame structure and its stability are defined by two basic mechanisms: compression of the fresh mixture ahead of the flame front and compression of the reacting mixture inside it. The first mechanism provides burning velocity increase; the second one can either accelerate or decelerate reaction depending on the pressure-dependent reaction behavior in the observed pressure range and depending on the rate of compression. In case of reaction intensification with rising pressure, a detonation forms on the leading edge of the flame front. Otherwise, the flame propagates in a quasi-stable supersonic regime consisting of consequential stages of deceleration and re-acceleration of the flame. On the deceleration stage, the compressed fresh mixture priorly chocked by the supersonic flow near the flame tip flows downstream generating the compression wave ahead. The new contact surface between this packet of compressed mixture and the fresh mixture ahead of the flame front can become the kernel of the exothermal reaction, evolving in a new deflagration wave or even detonation.

Freely-propagating flames in aluminum dust clouds

The free propagation of isobaric flames through aluminum dust clouds is investigated in an extensive series of experiments using two facilities with different scales. In small-scale laboratory experiments, spherical flame propagation occurs in aluminum dust clouds contained within 30-cm-diameter latex balloons, whereas in large-scale tests, flames propagate vertically through unconfined aluminum dust clouds with a vertical scale of about 4 m. The balloon experiments are performed with suspensions of aluminum powder in oxygen mixed with nitrogen, argon, or helium with various concentrations of oxygen and aluminum. It is found that stable flame propagation is only observed for aluminum concentrations near stoichiometric to rich conditions. Pulsating and spiral-like flames are discovered in fuel-lean mixtures, and flames with cellular patterns occur in very-fuel-rich suspensions. The burning velocities in the stable propagation regime derived from the balloon experiments correlate well with data previously obtained with stabilized Bunsen-type flames. The flame speed of stable flames is found to be a strong function of the heat conductivity of the gas mixture. In addition, the oxygen concentration has a strong influence on the flame speed for fuel-rich mixtures but dependence is reduced for fuel-lean mixtures. In the large-scale experiments, the burning velocity is estimated to be about two times larger than for the small-scale experiments. The increase in burning velocity is attributed to preheating of the unburned mixture by radiation from the condensed-phase combustion products. The degree of preheating, determined with an array of fine thermocouples, is found to be in the range of 150–200 K. The propagation of stable flames is discussed in light of existing qualitative dust flame models, whereas the pulsating flame propagation regime observed is interpreted in terms of the thermo-diffusive instability theory developed for high Lewis number flames in gases and solid reactive powder mixtures.

On the role of excited species in hydrogen combustion

Recently updated hydrogen combustion mechanism was combined with ozone decomposition reactions and extended by reactions of excited species: O(1D), OH(2Σ+), and O2(a¹Δg). The reliability and the accuracy of the rate constants pertinent to these excited species were evaluated. Many reactions proposed in the literature and implemented in other kinetic schemes were found irrelevant or insignificant. The new mechanism for hydrogen combustion was then validated against commonly accepted sets of laboratory experiments. It was expected that new reactions incorporated into the model should not affect its predicting ability for “thermal” combustion of H2, i.e. in the absence of excited species in the initial mixtures. The model validation showed that predictions of ignition, oxidation, flame burning velocities and flame structure of hydrogen–oxygen–inert mixtures are indistinguishable or very close to those of the basic mechanism at all condition, except for hydrogen oxidation in a flow reactor close to explosion limit. It was further demonstrated that singlet oxygen formed in reaction H2 + O2(1Δ) = H + HO2 at ppm levels may notably accelerate the process. Kinetic role of O(1D) and OH(2Σ+) in the “thermal” combustion of H2 was found negligible. In addition, hydrogen + air flame enhancement by singlet oxygen was modeled. It was demonstrated that the burning velocity increase with 1% of O2(a¹Δg) seeded into the air is rather modest. Moreover, purely thermal effect due to additional enthalpy brought to the mixture exceeds chemical flame enhancement by the singlet oxygen.

Effect of initial turbulence on vented explosion overpressures from lean hydrogen–air deflagrations

To examine the effect of initial turbulence on vented explosions, experiments were performed for lean hydrogen–air mixtures, with hydrogen concentrations ranging from 12 to 15% vol., at elevated initial turbulence. As expected, it was found that an increase in initial turbulence increased the overall flame propagation speed and this increased flame propagation speed translated into higher peak overpressures during the external explosion. The peak pressures generated by flame–acoustic interactions, however, did not vary significantly with initial turbulence. When flame speeds measurements were examined, it was found that the burning velocity increased with flame radius as a power function of radius with a relatively constant exponent over the range of weak initial turbulence studied and did not vary systematically with initial turbulence. Instead, the elevated initial turbulence increased the initial flame propagation velocities of the various mixtures. The initial turbulence thus appears to act primarily by generating higher initial flame wrinkling while having a minimal effect on the growth rate of the wrinkles. For practical purposes of modeling flame propagation and pressure generation in vented explosions, the increase in burning velocity due to turbulence is suggested to be approximated by a single constant factor that increases the effective burning velocity of the mixture. When this approach is applied to a previously developed vent sizing correlation, the correlation performs well for almost all of the peaks. It was found, however, that in certain situations, this approach significantly under predicts the flame–acoustic peak. This suggests that further research may be necessary to better understand the influence of initial turbulence on the development of flame–acoustic peaks in vented explosions.

An Analytical Model for Flame Propagation through Moist Lycopodium Particles with Non-unity Lewis Number

In this investigation, the structure of one-dimensional flame propagation in uniform cloud of volatile organic particles has been analyzed in which the structure of flame is divided into three zones. The first zone is preheat zone which is divided into three subzones itself. In the first subzone (heating), particle cloud is heated until the moist particles reach to vaporization temperature (water vapor). In the next subzone (drying), particle moisture comes out, and in the final subzone the pyrolysis phenomenon takes place. The second zone is the reaction zone, and the last one is the post-flame zone. In this research, an analytical method is used in order to solve the governing equations of particle cloud combustion in aforementione d zones. The overall investigation of this study leads to a non-linear burning velocity correlation. Consequently, the results show that a decrease in particle moisture content or an increase in equivalence ratio ( u ϕ ) or Lewis number causes to increase in moisture evaporation and devolatization rates, and consequently both flame temperature and burning velocity increase

The propagation of a laminar reaction front during end-gas auto-ignition

A transient, one dimensional premixed laminar reaction front is used as a model problem to further understand the physical processes influencing reaction front propagation during the various stages of spark-assisted compression ignition (SACI) combustion for both constant and variable domain pressures. This approach is consistent with the wrinkled laminar flame representation of turbulent, spark ignited engine combustion. With the proper choice of timescales and pressure rise rate, it applies to the interaction of the flame with auto-igniting end-gas in a typical automotive engine. Under the conditions simulated by a transient flame code, the reaction front begins as a deflagration, propagating into an end-gas with an initially negligible level of reaction progress. The diffusive–reactive nature of the front is maintained until significant levels of end-gas reaction progress, where the burning velocity depends upon the degree of pre-reaction. At the time of the end-gas maximum chemical power, the maximum temperature gradient and peak rate of heat conduction within the front diminish to the point where combustion becomes chemically controlled. Although significant increases in burning velocity are observed at the onset of chemically controlled combustion within the front, the end-gas is within one front time from the completion of combustion. As a result, no more than one front thickness is consumed by the apparent propagation of the spontaneous ignition front.

NONLINEARITY EFFECT OF TRANSPORT PROPERTIES ON COMBUSTION OF VOLATILE ORGANIC PARTICLES

This paper presents the structure of laminar, one-dimensional, and steady-state flame propagation in a uniform cloud of volatile organic particles. In this model, it is assumed that particles vaporize to yield a gaseous fuel. In order to study the effects of thermal conductivity and thermal diffusion on combustion of organic particles, an asymptotic analysis, based on the large Zeldovich number or high rate of reaction, is used. In the preheat zone, the rate of chemical reaction is small, and transfer phenomena play a significant role in temperature and mass distributions. Therefore, according to the kinetic theory of gases, thermal conductivity (λ) and diffusion coefficient (D) are considered to be proportional to T1/2 and T3/2, respectively. Finally, flame characteristics for different equivalence ratios of solid mixture for different Lewis numbers are reported. Results show that both flame temperature and burning velocity increase with rise in the Lewis number.

Investigation of combustion enhancement by ozone additive in CH 4/air flames using direct laminar burning velocity measurements and kinetic simulations

The effect of ozone additive on the enhancement of the burning velocity for premixed methane–air flames is investigated by both experimental measurements and kinetic simulations. Laminar burning velocities with and without O 3 were directly measured using the Heat Flux method. The O 3 molecules were introduced into the system by a dielectric-barrier-discharge ozone generator installed in the O 2 gas line, which provided prompt control of on/off of the O 3 feed into the system, enabling a precise comparison of the measured burning velocity with and without ozone additives. Noticeable burning velocity enhancement was observed at off-stoichiometric conditions rather than stoichiometric conditions. With 3730 ppm O 3 additive in the oxidizer, experimental data shows ∼8% burning velocity increase in fuel-rich mixtures and ∼3.5% burning velocity increase for the stoichiometric mixture. With 7000 ppm ozone additive in the oxidizer, maximum ∼16% burning velocity increase was observed at fuel-lean conditions while ∼9.0% was found at fuel-rich conditions. An O 3 kinetic mechanism involving 16 elementary reactions together with the GRI-Mech 3.0 was composed and validated through CHEMKIN calculations, which gives good predictions of the burning velocities with and without O 3 additives. Extra O radicals contributed by O 3 molecules in the pre-heat zone initiate and accelerate the chain-branching reactions and consequently increase the burning velocity.

E131 高温予混合火炎の固有不安定性に及ぼすルイス数の効果(OS-12:次世代の燃焼技術(III))

This paper treated high-temperature premixed flames to study the effects of the Lewis number on intrinsic instability under the constant-enthalpy conditions. The growth rate increased as the unburned-gas temperature became higher, which was due to the increase in burning velocity of a planar flame. In addition, the unstable range widened and the maximum growth rate increased as the Lewis number became lower, which was due to the diffusive-thermal effects. To elucidate the characteristics of cellular flames induced by intrinsic instability, a disturbance with the critical wave number was superimposed. The superimposed disturbance evolved, and then a cellular flame front formed. When the unburned-gas temperature was high, the behavior of cellular flames was mild. This was because that the difference in temperature between burned and unburned gases decreased.