Flame Kernel Radius Research Articles

Development and Validation of a CFD Combustion Model for Natural Gas Engines Operating with Different Piston Bowls

Nowadays, an accurate and precise description of the combustion phase is essential in spark-ignition (SI) engines to drastically reduce pollutant and greenhouse gas (GHG) emissions and increase thermal efficiency. To this end, computational fluid dynamics (CFD) can be used to study the different phenomena involved, such as the ignition of the charge, combustion development, and pollutant formation. In this work, a validation of a CFD methodology based on the flame area model (FAM) was carried out to model the combustion process in light-duty SI engines fueled with natural gas. A simplified spherical kernel approach was used to model the ignition phase, whereas turbulent flame propagation was described through two variables. A zero-dimensional evolution of the flame kernel radius was used in combination with the Herweg and Maly formulation to take the laminar-to-turbulent flame transition into account. To estimate the chemical composition of burnt gas, two different approaches were considered—one was based on tabulated kinetics, and the other was based on chemical equilibrium. Assessment of the combustion model was first performed by using different operating points of a light-duty SI engine fueled with natural gas and by using the original piston. The results were validated by using experimental data on the in-cylinder pressure, apparent heat release rate, and pollutant emissions. Afterward, two other different piston bowl geometries were investigated to study the main differences between one solution and the others. The results showed that no important improvements in terms of combustion efficiency were obtained by using the new piston bowl shapes, which was mainly due to the very low (+4%) or null increase in turbulent kinetic energy during the compression stroke and due to the higher heat losses (+20%) associated with the increased surface area of the new piston geometries.

Prediction of the flame kernel growth rate in spark ignition engine fueled with natural gas, hydrogen and mixtures

The knowledge of combustion duration is a key tool in the development of engines, specially nowadays for engines adapted to new fuels with low C/H ratio such as natural gas and hydrogen. This work is aimed to develop a correlation that predicts the duration of the first phase of combustion until the process becomes turbulent in a SI engine. The flame kernel radius when this transition occurs, Rtr, is the study variable.To determine this variable from the experimental pressure records, a flame kernel growth predictive model is used. The predictive model is adjusted to the experimental data, determining the most appropriate Rtr value.The pressure records of 500 consecutive cycles of 48 test points have been processed. The averaged values of Rtr of each test point have been correlated with the characteristic parameters of the process: turbulence and properties of the fuel–air mixtures. Finally, Rtr and integral length scale ratio is correlated with Damköhler number.A wide range of operating conditions have been studied, reaching the novel conclusion that it is possible to analyze the kernel growth phenomenon from a spatial point of view rather than from a temporal point of view, as had been studied in many previous works.The developed correlation can be used in combustion predictive modeling to support SI engine design. Other practical conclusion from the work, that can be used in SI engine development, is that decreasing the integral length scale reduces the time of the first phase of combustion.

Study of Flame Kernel Development at High EGR and High Flow Speed Using Conventional Spark Igniter and Non-Thermal Plasma Under Gasoline Engine Relevant Conditions

ABSTRACT As a novel ignition approach, nanosecond repetitively pulsed discharge (NRPD)-based non-thermal plasma offers significant benefits such as low energy consumption, and lean operating conditions. However, there is no investigation conducted on the early flame kernel formation and development induced by non-thermal plasma. Therefore, in this paper, experiments of conventional spark and non-thermal plasma ignition systems in constant volume combustion chamber (CVCC) are conducted under gasoline engine relevant conditions: wide initial ambient pressures (6.5, 8.3, 11.3 bar), a range of equivalence ratios (0.7–1.0), EGR rates (10−25%), and cross flow speeds (0–30 m/s). The discharge energy of non-thermal plasma is of around 210 mJ, while a single spark event can generate energy from 65 mJ to 83 mJ depending on ambient conditions. A consecutive spark strategy is adopted to guarantee comparable total input energy to non-thermal plasma. The ignition delay and combustion phase obtained from the chamber pressure history are calculated. In the meanwhile, flame kernel radius, flame propagation rate, and flame front length ratio via schlieren images are quantified and analyzed. Results showed that the flame initiated by non-thermal plasma can maintain a robust flame kernel and propagates fast. Under high-speed cross flow and high EGR rate conditions, non-thermal plasma can maintain a robust flame kernel at early stage supporting the initial flame kernel to survive and improve its ignition probability. Considering the effects of the EGR ratio and high-speed cross flow on flame kernel development, non-thermal plasma ignition can efficiently enhance the flame propagation and ignition probability. It is also concluded that non-thermal plasma can successfully ignite under lean (equivalence ratio between 0.7–1.0), high-diluted mixture (25% EGR), and high-speed cross flow of 30 m/s within the range of current study.

Measurement on minimum ignition energy of n-decane with pyrolysis

Measurement on the minimum ignition energy (MIE) of n-decane with pyrolysis is investigated experimentally. Certain decomposition rate n-decane vapor is simulated by blending certain proportion of ethylene into n-decane vapor. Two equivalence ratios (1 and 0.7) and five blending ratios (0,0.2 0.4,0.6,0.7) of ethylene are, respectively, designed to represent different decomposition rates and equivalence ratios. The effect of different decomposition rates and equivalence ratios on the MIE of n-decane is investigated in following experiments. Two experimental methods are used for measurement. Technique of high-speed schlieren system is adopted to obtain images of the flame kernel by which the relationship between flame kernel radius and flame speed is calculated. An ignition probability model based on the logic regression theory is established to obtain the ignition probability curve. Results indicate that MIE is sensitive to the blending ratio of ethylene when equivalence ratio is 0.7. With elevated blending ratio of ethylene, MIE decreases rapidly firstly and with the continuous increasing of the blending ratio, the decreasing of MIE is lagging which indicates that there is a limit to the effect of the blending ratio of ethylene on the MIE. It is also found that values of MIE are closed when ethylene blending ratio is big enough whatever the equivalence ratio is.

A model to determine the effects of low proportion of hydrogen and the flame kernel radius on combustion and emission performance of direct injection spark ignition engine

The only product of hydrogen combustion is water, so it is very clean. In this paper, the effects of low hydrogen content and FKR on the performance of the GDI engine are mainly studied. The simulation results show that adding less than 3% hydrogen in the GDI engine can improve the BP and BT of the engine under typical urban conditions such as 1500 rpm and 2000 rpm. As the hydrogen volume fraction and the FKR increase, the LFS of the mixture increases, resulting in a shorter ignition delay, combustion delay, and 10–90 % MFB. However, an increase in FKR results in a decrease in the BP and BT. In terms of fuel economy, the BSFC decreases as the hydrogen content increases, but as FKR increases, the BSFC increases. In terms of emissions, the proportion of hydrogen increases, HC and NOx emissions increase, and CO and CO2 emissions decrease. Therefore, adding hydrogen to the GDI engine while adjusting the FKR can improve the dynamic performance of the engine and reduce CO and CO2 exhaust emissions.

Effects of strain rate and Lewis number on forced ignition of laminar counterflow diffusion flames

Forced ignition is one of the most fundamental and important combustion problems. It is used in advanced engines and closely related to fire safety and accidental explosion. In this study, forced ignition of laminar counterflow diffusion flames with one-step chemistry is studied through two-dimensional simulations. The initial ignition kernel is simply modelled as a hot spot of unburned gas with a specified radius and uniform high temperature inside it. We focus on the subsequent ignition kernel development, during which an expanding axisymmetric triple flame appears under certain conditions. Non-monotonic change of the flame kernel radius with time is observed near the critical ignition conditions. The effects of strain rate and Lewis number on the ignition kernel development and critical ignition conditions are assessed. For the same initial hot spot, successful ignition can be achieved at relatively low strain rate and fuel Lewis number; while the ignition kernel quenches at relatively high strain rate or fuel Lewis number. The strain rate and Lewis number are shown to have great impact on the critical hot spot radius and on the minimum ignition energy. At relatively low strain rates, the minimum ignition energy increases linearly with the strain rate. This is consistent with previous theory on flame ball in a mixing layer. Besides, the optimum ignition position and the influence of the hot spot shape are investigated. It is demonstrated that the optimum ignition position is close to the stoichiometric surface and it depends on the Lewis number and global equivalence ratio.

Experimental study of microwave assisted spark ignition on expanding C2H2-Air spherical flames

Microwave Assisted Spark Ignition (MAI) is a potential technology to improve lean combustion characteristics in spark ignition engines. This paper investigates the MAI performance with C2H2-Air spherical expanding flames in a Constant Volume Combustion Chamber (CVCC) system under ambient pressures from 0.1 to 0.6 MPa and equivalence ratios (φ) from 0.5 to 1.9. Results show that MAI greatly increases early flame kernel radius compared with SI mode particularly in the cases close to lean or rich limit under 0.1 MPa. The enhancement decreases obviously as equivalence ratio approaches stoichiometric ratio. With ambient pressure increasing, the MAI enhancement gets weakened. Normalized Reduced Electric Field (REF) is found to correlate well with the decreasing trend of normalized flame radius enhancement with increasing ambient pressure, indicating that REF is probably one of the key factors affecting MAI performance. The influence of microwave Pulse Repetition Frequency (PRF) on MAI under PRF from 1 to 80 kHz are studied and discussed for both non-combustion discharge and ignition tests. Increasing PRF, the enhancement of MAI is strengthened as more microwave energy is injected in high-absorption-efficiency period which is dominated by electron density. At last, the formation mechanism for “fluorescence spot” and its dependence on pressure are offered to explain the observed increasing light intensity in the flame kernel with ambient pressure.

Effects of the initial flame kernel radius and EGR rate on the performance, combustion and emission of high-compression spark-ignition methanol engine

As clean fuel, methanol is considered to be one of the most promising alternative to conventional fuels for internal combustion engines. In this paper, a four-cylinder ignited methanol engine simulation model was built by using a one-dimensional simulation software. The methanol engine was converted from a diesel engine with a compression ratio of 17.5, and the methanol injector was installed in the inlet. Setting the engine speed as 2400 r/min, the effects of the initial flame kernel radius and EGR rate on the performance, combustion and emissions of methanol engine were studied when the throttle opening was 10%, 30%, 50%, 70% and 90%, respectively. The results showed that at any throttle opening, with the increase of EGR rate, the performance of the engine decreased, the in-cylinder combustion deteriorated, the NOx emission decreased significantly, but the HC and CO emission increased. At any throttle opening, the engine performance decreased slightly with the increase of the initial flame kernel radius, and the combustion was optimized. In terms of emission, when the throttle opening was between 10% and 70%, the NOx and CO emission decreased with the increase of the initial flame kernel radius. When the throttle opening was 90%, the NOx and CO emission increased with the increase of the initial flame kernel radius.

Comparative study on combustion and emission characteristics of methanol, ethanol and butanol fuel in TISI engine

At present, methanol, ethanol and butanol are recognized clean fuels and have been extensively studied. This paper mainly studies the combustion and emission characteristics of methanol, ethanol and butanol blended with gasoline under different engine speed, load, flame kernel radius and ignition time. The results show that adding alcohol fuels to gasoline can increase the brake torque (BT) and heat release rate (HRR) of the engine. Moreover, the ability of methanol to increase BT and HRR is higher than that of ethanol and butanol, and the increase effect of butanol is the worst. In terms of fuel economy, the addition of alcohol fuels to gasoline can increase the brake thermal efficiency (BTE) of the engine, but brake specific fuel consumption (BSFC) is also increased. Compared with ethanol and butanol, methanol can better increase BTE of the engine, but it also increases BSFC even more. In terms of emissions, the addition of alcohol fuels to gasoline can reduce CO and CO2 emissions, but HC emissions are increased. Under certain conditions, M20 combined with ignition time can better reduce NOx emissions. In general, the addition of alcohol fuels to gasoline while adjusting the ignition time and flame kernel radius can effectively improve engine torque and reduce fuel consumption and exhaust emissions.

Characteristics of flame kernal radius in a spark ignition engine according to the electric spark ignition energy

To study flame propagation in a spark ignition engine, it must concentrate on the initial combustion stage, i.e., the formation and development of flame. Therefore, we must study the theoretical calculation of minimum flame kernel radius which affects the formation and development of flame kernel. To calculate the minimum flame kernel radius, we must know the thermal conductivity, flame temperature, laminar burning velocity, etc. To evaluate the accuracy of the minimum flame kernel radius based on the theory, authors compared with those from calculation and those from tests. According to the increase of number of sparks and spark intervals, the flame kernel radii become to increase and the expected lean limits are extended. It is considered that is due to the stability of combustion as increase of flame kernel size according as high ignition energy supplies in initial period and discharge energy period lengthens.

Experimental study of turbulent flame kernel propagation

Flame kernels in spark ignited combustion systems dominate the flame propagation and combustion stability and performance. They are likely controlled by the spark energy, flow field and mixing field. The aim of the present work is to experimentally investigate the structure and propagation of the flame kernel in turbulent premixed methane flow using advanced laser-based techniques. The spark is generated using pulsed Nd:YAG laser with 20 mJ pulse energy in order to avoid the effect of the electrodes on the flame kernel structure and the variation of spark energy from shot-to-shot. Four flames have been investigated at equivalence ratios, φ j, of 0.8 and 1.0 and jet velocities, U j, of 6 and 12 m/s. A combined two-dimensional Rayleigh and LIPF-OH technique has been applied. The flame kernel structure has been collected at several time intervals from the laser ignition between 10 μs and 2 ms. The data show that the flame kernel structure starts with spherical shape and changes gradually to peanut-like, then to mushroom-like and finally disturbed by the turbulence. The mushroom-like structure lasts longer in the stoichiometric and slower jet velocity. The growth rate of the average flame kernel radius is divided into two linear relations; the first one during the first 100 μs is almost three times faster than that at the later stage between 100 and 2000 μs. The flame propagation is slightly faster in leaner flames. The trends of the flame propagation, flame radius, flame cross-sectional area and mean flame temperature are related to the jet velocity and equivalence ratio. The relations obtained in the present work allow the prediction of any of these parameters at different conditions.

The relative effects of fuel concentration, residual-gas fraction, gas motion, spark energy and heat losses to the electrodes on flame-kernel development in a lean-burn spark ignition engine

The potential of lean combustion for the reduction in exhaust emissions and fuel consumption in spark ignition engines has long been established. However, the operating range of lean-burn spark ignition engines is limited by the level of cyclic variability in the early-flame development stage that typically corresponds to the 0-5 per cent mass fraction burned duration. In the current study, the cyclic variations in early flame development were investigated in an optical stratified-charge spark ignition engine at conditions close to stoichiometry [air-to-fuel ratio (A = 15] and to the lean limit of stable operation (A/F = 22). Flame images were acquired through either a pentroof window (‘tumble plane’ of view) or the piston crown (‘swirl plane’ of view) and these were processed to calculate the intra-cycle flame-kernel radius evolution. In order to quantify the relative effects of local fuel concentration, gas motion, spark-energy release and heat losses to the electrodes on the flame-kernel growth rate, a zero-dimensional flame-kernel growth model, in conjunction with a one-dimensional spark ignition model, was employed. Comparison of the calculated flame-radius evolutions with the experimental data suggested that a variation in A/F around the spark plug of Φ(A/F) ≈ 4 or, in terms of equivalence ratio Φ, a variation in Φ ≈ 0.15 at most was large enough to account for 100 per cent of the observed cyclic variability in flame-kernel radius. A variation in the residual-gas fraction of about 20 per cent around the mean was found to account for up to 30 per cent of the variability in flame-kernel radius at the timing of 5 per cent mass fraction burned. The individual effect of 20 per cent variations in the ‘mean’ in-cylinder velocity at the spark plug at ignition timing was found to account for no more than 20 per cent of the measured cyclic variability in flame kernel radius. An individual effect of similar level was attributed to the heat losses to the electrodes, and a much lower effect than that, namely 5-10 per cent at most, could be attributed to cyclic variations in spark-energy release traces. Even when the variations in mean velocity were coupled to both the effects of heat losses to the electrodes and spark-energy release variations, the combined effect was found to account for no more than 40-50 per cent of the observed cyclic variability in flame-kernel radius at the 5 per cent mass-fraction burned timing of 40 ° crank angle after ignition timing.

Initiation of laminar flames in near-limit H 2/O 2/H 2O mixtures

A one-dimensional spherically symmetrical flame model including a detailed chemical mechanism for hydrogen oxidation, multispecies transport, and thermal radiation is used to examine the ignition and flame propagation properties for homogeneous hydrogen/oxygen/steam mixtures at elevated temperatures up to about 500 K and pressures ranging from 0.15 to 3 MPa. The model takes into account 26 elementary reactions and absorption of thermal radiation by water molecules. Time-dependent conservation equations of mass, species, and energy are integrated numerically to investigate the details of flame structure and calculate the flame speed. Kinetic sensitivity analysis of the burning velocity is used to elucidate the importance of individual elementary reactions at various pressures. Laminar flame propagation in stoichiometric mixtures heavily diluted with steam is investigated near the flammability limit through numerical simulation of the evolution of the spherical flame kernel initiated by a spatially localized ignition source. Critical conditions for flame propagation are analyzed in terms of ignition energy and flame kernel radius. The effect of pressure on these critical conditions is revealed and investigated. The predicted ignition nergy and limiting hydrogen concentration nonmonotonically depend on pressure. The computed results are in reasonable agreement with available experimental data on flammability limits.