Unburned Mixture Research Articles

Thermal performance of solid walls in a mesoscale combustor with a plate flame holder and preheating channels

The flame is very hard to keep steadily symmetrical for a wide flammability limit under some extreme combustion conditions (such as large heat loss and fuel of low caloric value). Recently, we manufacture a mesoscale combustor with a plate flame holder and preheating channels, which could take the advantage of the flow recirculation and heat recirculation effects. The experimental results show that this special configuration performs excellent in flame-anchoring, and the flame can remain steadily symmetrical at very low equivalence ratio in the normal environment. In order to provide theoretical basis to optimize this combustor, the three-dimensional numerical simulation is used to study the thermal performances of solid walls on unburned fuel mixture quantitatively. The results indicate that the combustor wall does not always have preheating effects on the unburned mixture. Some walls or some parts of walls have the negative effect of heat loss. Furthermore, some interesting boundary shapes of the preheating areas or heat loss areas are found. It is deduced that the thermal performances of combustor walls mainly depend on the side with higher temperature. In addition, the preheating area decreases with an increasing flame height, so a lower flame height is probably beneficial for preheating.

Reduction of flame development time in nanosecond pulsed high frequency discharge ignition of flowing mixtures

The effects of discharge and flow parameters on ignition kernel development time are explored in flowing methane–air mixtures. A nanosecond pulsed high frequency discharge in a pin-to-pin configuration is used as the ignition source, providing 2.9 ± 0.23 mJ/pulse. The effects of pulse repetition frequency (PRF) in the range of 10–300 kHz, number of pulses in the range of 1–50 (≈2.9–145 mJ), equivalence ratio in the range of 0.55–0.65, gap distance in the range of 0.5–2.5 mm, and flow velocity in the range of 2.5–10 m/s are explored. For all conditions, the ignition events are in the “fully-coupled” regime, in which high ignition probability is achieved and locally extinguished ignition kernels are avoided. It is found that reducing the PRF reduces the kernel development time for fixed total energy deposition due to an increased volume of unburned mixture exposed to the discharge. Increasing the number of pulses at a given PRF also decreases the kernel development time, again by increasing the volume of gas exposed to the discharge. The equivalence ratio only has an effect on the kernel growth rate after the discharge, with identical kernel areas measured at all equivalence ratios while the discharge is active. Increasing gap distance decreases the kernel development time by providing a larger initial kernel volume as well as reducing heat and radical quenching at the electrode surfaces. Finally, the flow velocity has an effect on the kernel growth rate at velocities greater than 5 m/s, with larger flow velocities resulting in shorter kernel development time. This is due to the competing rates of self-propagating flame expansion and kernel growth due to convection past the discharge region. The combined effects of all of the above parameters on the kernel area after the discharge are summarized in a correlation equation, which predicts the trends in kernel growth rate based on an estimated plasma area defined by the discharge duration, flow velocity, and gap distance.

Pressure characteristics during vented explosion of ethylene-air mixtures in a square vessel

The vented explosion in a 2 m × 1.1 m × 0.5 m stainless cylindrical vessel with the ethylene-air mixture is investigated as the function of the ethylene concentration, vent area and vent burst pressure. The internal pressure histories measured by two piezoelectric transducers with the vented door of different thickness are used to analyze the effect of vent burst pressure on the whole explosion. It is shown that the peak overpressures are the highest just at the ethylene concentration is 8 vol%. The internal overpressure is grown with the concentration and in turn decreases after internal pressure reached the peak. As the vent burst pressure increases, the same trend that internal peak overpressure is increasing accordingly is found. The dominant peak overpressure will be change with different vent areas. Not only that, it is apparent that venting is more effective on the account of the low ethylene concentrations. The high-speed images demonstrate that the flame is strongly distorted and the unburned mixture is ignited by the external flame. The comparison between experimental results and calculation values has been discussed in consideration of the influences of vent area and vent burst pressure. The predicted overpressures of different theoretical models are relatively conservative than measured peak overpressures.

Flame Dynamics inside Rectangular Meso scale Channels

The present work is focused on the experimental study of flame dynamics in preheated meso-scale straight channels of various aspect ratios (2, 5, 12 and 15) and inlet dimensions.Premixed methane-air mixture were used for the reported experiments. To maintain a positive wall temperature gradient inside the channel, the lower part of the rectangular channels were heated at a constant temperature using an external electric heater. Laminar premixed flames were stabilized inside these channels. Various flame propagation modes such as concave, planar, and convex flames with respect to unburned mixture. Concave flames lead to flashback whereas convex flames lead to blowout. Increase in aspect ratio and decrease of flow velocity leads to flame flashback.

Acceleration of laminar hydrogen/oxygen flames in a tube and the possible onset of detonation

The possibility is analysed of a laminar flame accelerating along a cylindrical tube, closed at one end, and inducing a deflagration to detonation transition in a stoichiometric H2/O2 mixture. The pressure and temperature ratios at the ensuing shock wave increase, as do laminar burning velocities, while autoignition delay times decrease. Combined with appreciable elongation of the flame, these enhance the strength of the shock. The conditions necessary for delay times of 0.05, 0.1, 1.0 and 5.0 ms, at an unburned mixture critical Reynolds number of 2300, are computed for different tube diameters. Probable consequences of the different delay times and hot spot reactivity gradients, including detonation, are all considered. The probability of a purely laminar propagation leading to a detonation is marginal. Only when the initial temperature is raised to 375 K, do purely laminar detonations become possible in tubes of between about 0.5 and 1.35 mm diameter.

Flame propagation and combustion modes in end-gas region of confined space

Flame propagation is investigated in a designed experimental apparatus equipped with a perforated plate in a constant volume chamber. The effect of the perforated plate is to generate a rapidly accelerating flame based on Bychkov work (Bychkov et al. 2008), in which the flame across the obstacle will becomes a strong jet flame. The experiment was conducted with a hydrogen–air mixture at different conditions. In this work, six different turbulent flame propagation and combustion modes were clearly observed at various conditions in our designed experiment. In the presence of perforated plate, the turbulent flame formed through the perforated plate may perform six types of turbulent propagations at the end gas regime. These types form through the interaction between the flame and the shock or acoustic wave and because of the limited effect of the wall in confined space. The six forms are as follows: (1) a normal flame propagation with a low flame front tip velocity and combustion rate; (2) a weak pulsation propagation with weak fluctuation due to the acoustic wave; (3) a pulsation propagation only with a visible reflected shock wave; (4) a strong pulsation propagation with a forward shock wave and shock reflection; (5) a continuously accelerating flame propagation due to auto-ignition of the unburned mixture between flame front and shock wave, which also leads to strong pressure oscillation; and (6) a violent pulsation propagation with a multi-shock wave leading to end gas auto-ignition with large pressure oscillation.

Laminar syngas–air premixed flames in a closed rectangular domain: DNS of flame propagation and flame/wall interactions

The propagation of a lean laminar premixed syngas–air flame is investigated numerically in a confined rectangular domain with isothermal walls at a temperature that is lower than that of the unburned mixture. Initiated by a hot kernel, the flame propagates towards the cold walls, setting in motion the initially quiescent mixture and compressing the gases, so that propagation proceeds under varying flow and thermodynamic conditions. The complex flow field changes structure and its effect on the local propagation characteristics is quantified by following the local displacement speed together with the local flow velocity and stretch rate. The flame first quenches head-on at the horizontal walls and then propagates towards the lateral walls affected by side-wall quenching. During the final stage, thermodiffusive instabilities are triggered along the front whose thickness has become half of the initial mainly because of the increased pressure. The temporal evolution of the quenching distances, the wall heat flux distribution and the heat transfer to the cold walls are quantified during the whole process. Except for the stages of the initial flame kernel growth and the final consumption of the fuel in the near-wall region, the fuel is consumed at an almost constant rate.

Propagation of sub-atmospheric methyl formate flames

Laminar flame speeds of methyl formate/air mixtures were measured at sub-atmospheric pressures for which limited data exist. The experiments were carried out in the counterflow configuration at an unburned mixture temperature of 333 K. The flow velocities were measured using particle image velocimetry. Particle phase slip correction was applied to low-pressure data sets for which the density disparity between the flow tracers and the gaseous phase is notable. The data were modeled using two recently developed kinetic models of methyl formate oxidation, and significant disagreements were realized at all pressures especially under fuel-rich conditions. Additionally, the computed species profiles of CO and CO2 in the burner-stabilized flame configuration using the two models were found to differ significantly. Reaction path analysis revealed that the kinetics of CH2OCHO that is produced directly from the fuel affects the overall reactivity, and the attendant rate constants differ between the two models. The variation of laminar flame speed with pressure revealed also a different behavior between experiments and simulations. Further insight into the sources causing the observed discrepancies were investigated and it was determined that reactions involving formyl radical, methanol, and formaldehyde could also be responsible for the reduction in reactivity specifically under fuel-rich conditions.

Actuation of a lean-premixed flame by diffuse non-equilibrium nanosecond-pulsed plasma at atmospheric pressure

This study investigates the effect of diffuse non-equilibrium nanosecond-pulsed plasma at atmospheric pressure on a lean-premixed CH4-air flame (ϕ = 0.65, P ∼ 0.3 kW). The domain of diffuse plasma existence is explored for both the case of the cold flow (no flame) and the case where a flame is stabilized downstream. The dynamics of plasma propagation and the flame displacement, following a high-voltage pulse, were measured using intensified charge-coupled device imaging. The energy of the plasma was measured using electrical probes and measurements of the second positive system of nitrogen were used to determine the rotational temperature and vibrational populations in the plasma. The effect of plasma on a flame was investigated by varying the pulse repetition frequency gradually from 1 to 7 kHz. Time-resolved imaging of the plasma emission shows that the primary streamer travels at higher velocities with increased pulsing frequency and with the presence of a flame ignited downstream of the discharge. Time-resolved imaging of the flame, following a high-voltage pulse, shows that the flame moves upstream into the unburned methane-air mixture with increased pulsing frequency. As the flame is displaced upstream, the nature of the discharge also changes, whereby less energy is coupled to the gas volume. Spectroscopic results reveal that the region in which the flame stabilizes is that of highest vibrational excitation and lowest rotational temperature. This actuation method is evidence of low-temperature chemical flame enhancement and potential control of a lean-premixed laminar flame at atmospheric pressure.

A DNS study of the physical mechanisms associated with density ratio influence on turbulent burning velocity in premixed flames

Data obtained in 3D direct numerical simulations of statistically planar, 1D weakly turbulent flames characterised by different density ratios σ are analysed to study the influence of thermal expansion on flame surface area and burning rate. Results show that, on the one hand, the pressure gradient induced within a flame brush owing to heat release in flamelets significantly accelerates the unburned gas that deeply intrudes into the combustion products in the form of an unburned mixture finger, thus causing large-scale oscillations of the burning rate and flame brush thickness. Under the conditions of the present simulations, the contribution of this mechanism to the creation of the flame surface area is substantial and is increased by σ, thus implying an increase in the burning rate by σ. On the other hand, the total flame surface areas simulated at σ = 7.53 and 2.5 are approximately equal. The apparent inconsistency between these results implies the existence of another thermal expansion effect that reduces the influence of σ on the flame surface area and burning rate. Investigation of the issue shows that the flow acceleration by the combustion-induced pressure gradient not only creates the flame surface area by pushing the finger tip into the products, but also mitigates wrinkling of the flame surface (the side surface of the finger) by turbulent eddies. The latter effect is attributed to the high-speed (at σ = 7.53) axial flow of the unburned gas, which is induced by the axial pressure gradient within the flame brush (and the finger). This axial flow acceleration reduces the residence time of a turbulent eddy in an unburned zone of the flame brush (e.g. within the finger). Therefore, the capability of the eddy for wrinkling the flamelet surface (e.g. the side finger surface) is weakened owing to a shorter residence time.

Experimental investigation of flame propagation and stabilization in a meso-combustor with sudden expansion

Combustion of gaseous fuels in a meso-scale combustor duct with a sudden expansion is studied experimentally. Premixed mixtures of methane-air and propane-air are considered. Different flame states are observed, namely, travelling flames within the duct, steady/oscillating flame within or at the exit of the duct, and no ignition. The travelling flames suffer quenching either head-on at the sudden expansion or along the walls of the duct. Regime transitions across these flame states are mapped comprehensively over a wide range of operating conditions. The time scale of propagation of the travelling flames corresponds to transient heat conduction in the duct wall material, regardless of the manner of quenching experienced by the flame. These flame speeds relative to the unburned mixture are below the adiabatic flame speed due to the flame-wall interaction, but follow the same trend with equivalence ratio as the latter. Across the regime transition from the wall-quenched travelling flame to the steady/oscillating flame in the duct, the flame propagation slows down abruptly with slight increase in the duct Reynolds number to allow adequate time for significant heating of the wall and upstream reactants that enables the flame stabilization. The location of flame stabilization, however, follows the same trend as the location of wall-quenching, as the Reynolds number is increased. The final locations of the wall-quenched and the stable/oscillatory flames are further downstream of the flow recirculation zone at the sudden expansion.

Numerical investigation of the effects of hydrogen enrichment on combustion and emissions formation processes in a gasoline rotary engine

The study aims to numerically investigate the combustion and emissions formation processes in a spark-ignition rotary engine fueled with hydrogen-gasoline blends. The Renormalization Group k-ε turbulence coupled with a skeletal primary reference fuel mechanism were adopted to simulate the engine working process under 0%, 2% and 4% hydrogen volume fractions in CONVERGE software. The flow field variation and detailed combustion processes were analyzed and discussed. Results showed that a mainstream flow field along with the rotor movement was formed during the compression stroke and sustained in the combustion chamber until the exhaust valve opening. The center of the burned zone would move in same direction with the mainstream flow. Meanwhile, due to the effect of the mainstream flow, the flame propagation in direction of the mainstream flow was expedited. While the flame propagation in contrary direction was retarded, consequently, the unburned mixtures at rear region of combustion chamber suffered incomplete combustion. The distributions of nitric oxide and carbon monoxide emissions were also dominantly affected by the flow field in the combustion chamber. After hydrogen addition, the increased OH, H and O radicals concentrations combined with the intense flow flied accelerated the combustion process, which resulted in the improvement and advancement of in-cylinder pressure and temperature. Compared with original gasoline case, the peak in-cylinder pressure was increased by 9.1% and 13.7% with 2% and 4% hydrogen blends, respectively. With hydrogen enrichment, the increased in-cylinder temperature promoted the formation of nitric oxide emission. Besides, Carbon monoxide emission was decreased with the increase of hydrogen addition fraction.

Semiempirical Correlation for Predicting Laminar Flame Speed of H2/CO/Air Flames with CO2 and N2 Dilution

A semiempirical correlation was developed for predicting the laminar flame speed of H2/CO/air flames with N2 and CO2 dilution without solving the detailed governing equations, detailed chemical kinetics, and molecular mass transport. The correlation, derived through an asymptotic analysis, comprised a theoretically based expression with a series of experimentally fitted parameters. New experimental measurements were conducted using the Bunsen flame method over broad ranges of unburned mixture temperature (300–600 K), H2 ratio in the H2/CO blends (0.25–0.75), CO2 or N2 dilution ratio (0–0.67), and equivalence ratio (0.8–4.0) to comprehensively validate the performance of the proposed correlation. Detailed numerical simulation and sensitivity analysis using CHEMKIN-Pro were also carried out to gain an insightful understanding of the experimental observations. Results showed that the proposed correlation was able to satisfactorily predict the laminar flame speed, with error < 15%, of a wide range of H2/CO/N2...

A numerical investigation on methane combustion and emissions from a natural gas-diesel dual fuel engine using CFD model

Natural gas (NG)-diesel dual fuel engines have been criticized for their high emissions of unburned methane. The past research on methane emissions from dual fuel engines has focused on the measurement of methane concentration in exhaust gases. The development of approaches capable of minimizing methane emissions requests the detailed spatial distribution of methane in-cylinder during the combustion and post combustion processes. However, it is difficult to experimentally measure the spatial distribution of methane in-cylinder.This research presents a numerical study on the combustion process of a NG-diesel dual fuel engine using the computational fluids dynamics (CFD) model CONVERGE coupled with a reduced primary reference fuel (PRF) mechanism. The model was validated against the heat release process and the emissions of nitrogen oxide, methane and carbon monoxide measured in a single cylinder dual fuel engine. The validated CFD model was applied to investigate the combustion of methane and n-heptane and the spatial distribution of methane in the dual fuel engine. This is most likely the first attempt to visualize the spatial distribution of methane in dual fuel engines using CFD. The objective of this study is to numerically simulate the methane combustion process, especially the methane present outside the pilot spray, quantify the methane combustion in each combustion stage, and visualize the spatial methane distribution in cylinder. The results showed that the momentum produced by the pilot fuel injection and combustion pushed the combustion products of pilot fuel and methane within the pilot spray plume toward the unburned methane-air mixture. Such a movement enhanced the mixing of the hot combustion products and the relatively cold unburned methane-air mixture during the main combustion process and dominated the combustion of methane presented outside the pilot fuel spray plume. Based on the simulation results at a low load condition (4.05bar), the main combustion process consumed 43–53% of the methane fumigated into the intake mixture. The post-combustion oxidation process consumed 17–29% of the intake methane, which was 36.2–51.8% methane that survived the main combustion process. In comparison, 27–35% methane emitted the engine without participating the combustion process. The unburned methane at exhaust valve opening was mainly observed at the center of the cylinder. In comparison, the contribution of the crevice and boundary layer around the cylinder liner to methane emissions was relatively small. The slip of methane through the dual fuel engines was due to the fact that the premixed mixture was too lean to support the propagation of the turbulent flame initiated by the pilot fuel and the lack of pilot fuel vapor reaching the center of the combustion chamber because of the geometric limitations of the fuel injection system and the reduced mass of pilot fuel injected into the cylinder. The approaches aiming to enhance the combustion of methane and minimize methane emissions from dual fuel engines should focus on those capable of increasing the volume of pilot fuel vapor formed after injected into the cylinder.

Flammability limit of moderate- and low-stretched premixed flames stabilized in planar channel

The experimental study of flammability limits of counterflow premixed methane-air flames in a planar micro channel is described in the paper. Combustion occurs in the gap between quartz plates which are connected with two countercurrent slot burners. The proposed counterflow burner allows investigation of the flame structure at low flow rates which are outside the applicability range of conventional counterflow burners at the normal gravity conditions. It was found that the heat return from combustion products to unburned mixture through the heat conducting plates is an essential effect at low gas flow rates.

Scalar transport and the validity of Damköhler’s hypotheses for flame propagation in intense turbulence

The turbulent burning velocity of premixed flames is sensitive to the turbulence intensity of the unburned mixture. Premixed flame propagation models that incorporate these effects of turbulence rest on either of the two hypotheses proposed by Damköhler. The first hypothesis applies to low-intensity turbulence that acts mainly to increase the turbulent burning velocity by increasing the flame surface area. The second hypothesis states that, at sufficiently high intensities of turbulence, the turbulent burning velocity is governed mainly by enhanced diffusivity. Most studies to date have examined the validity of the first hypothesis under increasingly high intensities of turbulence. In the present study, the validity of Damköhler’s second hypothesis is investigated. A range of turbulence intensities is addressed by means of direct numerical simulations spanning the “flamelet” and “broken reaction zones” regimes. The validity of Damköhler’s second hypothesis is found to be strongly linked to the behaviour of turbulent transport within the flame.

Laminar Burning Velocity of Methane/Air Mixtures and Flame Propagation Speed Close to the Chamber Wall

Experimental studies on the laminar burning velocity (LBV) and the flame propagation speed close to the wall of premixed methane-air flames at a pressure of 1 bar and ambient temperature, for poor and rich mixtures, have been conducted and analyzed. The methane-air mixture LBV and flame propagation speed are studied in a constant volume combustion chamber with acrylic windows using the shadowgraph technique and a high-speed camera. The recorded images are analyzed using a Matlab script and both the system and the Matlab code have been validated against literature data. The results are in quite good agreement with available literature for LBV that shows its decreasing with pressure increase. For the flame propagation speed little literature has been found for validation but using a different set-up. The results show that the flame propagation speed decreases almost linearly except close to the wall where some oscillations are present, due to a combination of multiple phenomena such as compression and heating of the unburned mixture, a combination of radiative heat loss to the wall, flame curvature and flame stretch.

Laminar flame speed correlations for methane, ethane, propane and their mixtures, and natural gas and gasoline for spark-ignition engine simulations

The laminar flame speed plays an important role in spark-ignition engines, as well as in many other combustion applications, such as in designing burners and predicting explosions. For this reason, it has been object of extensive research. Analytical correlations that allow it to be calculated have been developed and are used in engine simulations. They are usually preferred to detailed chemical kinetic models for saving computational time. Therefore, an accurate as possible formulation for such expressions is needed for successful simulations. However, many previous empirical correlations have been based on a limited set of experimental measurements, which have been often carried out over a limited range of operating conditions. Thus, it can result in low accuracy and usability. In this study, measurements of laminar flame speeds obtained by several workers are collected, compared and critically analyzed with the aim to develop more accurate empirical correlations for laminar flame speeds as a function of equivalence ratio and unburned mixture temperature and pressure over a wide range of operating conditions, namely [Formula: see text], [Formula: see text] and [Formula: see text]. The purpose is to provide simple and workable expressions for modeling the laminar flame speed of practical fuels used in spark-ignition engines. Pure compounds, such as methane and propane and binary mixtures of methane/ethane and methane/propane, as well as more complex fuels including natural gas and gasoline, are considered. A comparison with available empirical correlations in the literature is also provided.

EFFECT OF HYDROGEN ADDITION ON FLAME PROPAGATION CHARACTERISTICS THROUGH TUBE OF METHANE-AIR MIXTURES USING OPTICAL TECHNIQUE

The effect of Hydrogen addition on laminar flame speed (Uf) of Methane – Air premixed mixtures using optical technique has been experimentally investigated inside a tube. the flame front location had been positioned by a photocell. The (Uf) measured at a laboratory conditions for an extensive range of equivalence ratios (F). In order to use density ratio method for the calculation of laminar burning velocity (UL), all experimental work was carried out at constant pressure (Pre-pressure period). The flame temperature (Tb) has been calculated theoretically. Mixture strength (F) and hydrogen content (RH) dependence of (UL) is represented by empirical correlation over the ranges F = (0.6-1.4), RH = (0, 0.1, 0.2, 0.3, 0.4), all at initial unburned mixture temperature Tu = 298 K and a pressure of (1 atm). In overlapping ranges, the results were found in satisfactory agreement with those previously published.

Effect of natural gas components on its flame temperature, equilibrium combustion products and thermodynamic properties

It is a known fact that the composition of natural gas varies widely from source to source and from time to time even in the same source. Such variations in gas composition cause variations in flame temperature, its combustion products and thermodynamic properties which can affect gas-fueled engine performance and its emissions. The purpose of this study is to investigate effects of varying amounts of natural gas diluent components such as ethane (C2H6), isobutane (C4H10), propane (C3H8), carbon dioxide (CO2) and nitrogen (N2) on methane–air combustion under different pressures, unburned mixture temperatures and equivalence ratios. Results show that adiabatic flame temperature is mostly increased by isobutane followed by propane and ethane however it is mostly influenced by CO2 content in the gas which decreases flame temperature 80 K in %70 CH4 - %30 CO2 mixture at stoichiometric conditions. As for specific heat, the highest increase is again for isobutane followed by propane and ethane content but it is decreased mostly by N2. Effect of increasing secondary fuel content on product species is greatest at equivalence ratio near unity except for CO and H2. Analysis results including equilibrium compositions are validated and expected to be a reference guide for scientists, engineers and NG consumers to evaluate and compare natural gas samples of different compositions.