Showing papers on "Laminar flame speed published in 2019"

Influence of the in-cylinder flow on cycle-to-cycle variations in lean combustion DISI engines measured by high-speed scanning-PIV

Abstract: In this study, the influence of the three-dimensional (3D) in-cylinder flow on engine's cycle-to-cycle variations (CCV) in a spray-guided direct-injection spark-ignition engine is investigated. The engine is operated at homogeneous lean air–fuel mixture which enhances the sensitivity to CCV due to reduced laminar flame speed. To compensate this, intake velocity is increased by a tumble-flap (TF) in the intake-port. To address the 3D-nature of the temporal evolution of the instantaneous in-cylinder flow for different TF-positions, time-resolved scanning particle image velocimetry (Scanning-PIV) is applied to the engine. The required scan-frequency is provided by an acousto-optical-deflector (AOD) to measure the flow field quasi-simultaneously in the central tumble-plane and both mid-valve-planes. The three planes are 18 mm displaced from each other to capture the variability of the large-scale tumble vortex. The in-cylinder flow measurements are combined with combustion analyses by the in-cylinder pressure-trace and the detection of the location of ignition through the evaluation of the luminous spark-plasma. A correlation-map analysis is conducted to identify coherent flow features responsible for CCV of the combustion parameters. This reveals a strong dependency of the spark position to variations of an upward directed flow pointing onto the spark plug. The variations of the upward flow are due to strong CCV of the bended tumble-axis position. An increased tumble motion caused by the TF results in favorable flow conditions by stabilizing the tumble-axis in the middle of the cylinder which decreases the CCV of the spark position significantly. Further correlation analysis including the combustion process exhibits that flow-structures moving the spark and early flame kernel towards the cylinder center reduces the crank angle of 5% heat release and the combustion duration considerably.

Laminar flame speeds of DEMP, DMMP, and TEP added to H2- and CH4-air mixtures

Abstract: Organophosphorus compounds (OPCs) have long been known to have significant fire suppression capabilities but were outclassed by Halon 1301 due to toxicity concerns. Recent interest in finding replacements for Halon 1301 has provided an impetus to reconsider OPCs. To better understand the mechanism by which OPCs suppress flames, more information about how they interact with fuel/air mixtures via chemical kinetics is needed. In this study, dimethyl methylphosphonate (DMMP), diethyl methylphosphonate (DEMP), and trimethyl phosphate (TEP) were added to hydrogen/air and methane/air mixtures to assess their suppression capabilities at 0.1% and 0.3% (DMMP only) of the total mixture volume. Laminar flame speed experiments were performed in an optically tracked, spherically expanding flame setup at 1 atm and 120 °C. The resulting laminar flame speed data are the first to be recorded using these compounds. Results show a 30% decrease in laminar flame speed for all OPCs at 0.1% on the methane/air parent mixture, and the laminar flame speed curves, as a function of equivalence ratio, tend to be broader than for un-doped mixtures. For the hydrogen/air mixtures, the OPCs differentiate themselves by having an increasing suppression effect corresponding with higher carbon moiety, i.e., TEP (15% overall reduction) > DEMP (13%) > DMMP (9%). The OPCs also have an increasing effect with increasing equivalence ratio on hydrogen/air, but with methane/air, they have a non-monotonic effect. The reduction of laminar flame speeds is comparable to twice the concentration of Halon 1301 and 10 times as much for previously investigated Halon 1301 replacements. These results are ideal for improving existing OPC chemical kinetics mechanisms, and possible applications include both fire suppression technologies and destruction of dangerous OPC compounds.

Reference natural gas flames at nominally autoignitive engine-relevant conditions

Abstract: Laminar natural gas flames are investigated at engine-relevant thermochemical conditions where the ignition delay time τ is short due to very high ambient temperatures and pressures. At these conditions, it is not possible to measure or calculate well-defined values for the laminar flame speed sl, laminar flame thickness δl, and laminar flame time scale τ l = δ l / s l due to the explosive thermochemical state. Here, the corresponding reference values, sR, δR, and τ R = δ R / s R , that account for the effects of autoignition, are numerically estimated to investigate the enhancement of flame propagation, and the competition with autoignition that arises under nominally autoignitive conditions (characterised here by the number τ/τR). Large values of τ/τR indicate that autoignition is unimportant, values near or below unity indicate that flame propagation is not possible, and intermediate values indicate that a combination of both flame propagation and autoignition may be important, depending upon factors such as device geometry, turbulence, stratification, et cetera. The reference quantities are presented for a wide range of temperatures, equivalence ratios, pressures, and hydrogen concentrations, which includes conditions relevant to stationary gas turbine reheat burners and boosted spark ignition engines. It is demonstrated that the transition from flame propagation to autoignition is only dependent on residence time, when the results are non-dimensionalised by the reference values. The temporal evolution of the reference values are also reported for a modelled boosted SI engine. It is shown that the nominally autoignitive conditions enhance flame propagation, which may be an ameliorating factor for the onset of engine knock. The calculations are performed using a recently-developed, detailed 177 species mechanism for C0–C3 chemistry that is derived from theoretical chemistry and is suitable for a wide range of thermochemical conditions as it is not tuned or optimised for a particular operating condition.

Laminar flame propagation in supercritical hydrogen/air and methane/air mixtures

Abstract: The propagation of laminar hydrogen/air and methane/air flames in supercritical conditions was computationally simulated for the planar flame configurations, incorporating descriptions of supercritical thermodynamics and transport as well as high-pressure chemical kinetics. The inaccuracies associated with the use of ideal gas assumptions for various components of the supercritical description were systematically assessed with progressively more complete formulation. Results show that, for hydrogen/air flames, the laminar flame speeds at high pressures increase due to the non-ideal equation of state (EoS), and is mainly due to the density modification of the initial mixture. Including the thermodynamic properties of heat capacity reduces the flame speed because of the correspondingly reduced adiabatic flame temperature. Transport properties were found to have small effect because of the inherent insensitivity of the laminar burning rate to variations in the transport properties. For methane/air flames, the use of recently reported high-pressure chemical kinetics considerably affects the laminar flame speed, even for the same flame temperature.

Combustion properties of H2/N2/O2/steam mixtures

Abstract: The present work reports new experimental and numerical results of the combustion properties of hydrogen based mixtures diluted by nitrogen and steam. Spherical expanding flames have been studied in a spherical bomb over a large domain of equivalence ratios, initial temperatures and dilutions at an initial pressure of 100 kPa (Tini = 296, 363, 413 K; N2/O2 = 3.76, 5.67, 9; %Steam = 0, 20, 30). From these experiments, the laminar flame speed S L 0 , the Markstein length L’, the activation energy Ea and the Zel'dovich β number have been determined. These parameters were also simulated using COSILAB® in order to verify the validity of the Mevel et al. [1] detailed kinetic mechanism. Other parameters as the laminar flame thickness δ and the effective Lewis number Leeff were also simulated. These new results aim at providing an extended database that will be very useful in the hydrogen combustion hazard assessment for nuclear reactor power plant new design.

Experimental and kinetic study of 2,4,4-trimethyl-1-pentene and iso-octane in laminar flames

Abstract: Laminar flame speeds of 2,4,4-trimethyl-1-pentene are investigated at equivalence ratios of 0.7–1.6, initial temperatures of 298–453 K and initial pressures of 0.1–0.5 MPa. The comparison between 2,4,4-trimethyl-1-pentene and iso-octane is also performed. Results show that 2,4,4-trimethyl-1-pentene has faster laminar flame speed than iso-octane. Chemical kinetic models (Metcalfe model, Modified model I) were tested against the present experimental data. The laminar flame speeds are apparently over-estimated by the Metcalfe model and under-predicted by the Modified model I. Therefore, high-level quantum mechanical calculations were used to revise the Modified model I to obtain Modified model II and it can give fairly good prediction at various conditions on laminar flame speeds. In addition, the chemical kinetic analysis was conducted. The analysis indicates both thermal and kinetic effects result in the discrepancy of laminar flame speeds between 2,4,4-trimethyl-1-pentene and iso-octane. Furthermore, IC4H8 plays a dominant role in laminar flame speeds of 2,4,4-trimethyl-1-pentene and iso-octane.

Ignition delay times, laminar flame speeds, and species time-histories in the H2S/CH4 system at atmospheric pressure

Abstract: Hydrogen sulfide (H 2 S) composes up to 30% of certain natural-gas resources (“sour gas”) and can considerably alter combustion properties of methane (CH 4 ), but few data on H 2 S/CH 4 are available in the literature. In this work, new shock-tube and laminar flame speed data were obtained to facilitate future model validation. For the shock-tube experiments, a fuel-lean (φ = 0.5) 30/70 H 2 S/CH 4 blend in 99% argon by volume was shock-heated to temperatures between 1538 and 2144 K and pressures near 1 atm. Laser absorption diagnostics at 4.5 and 1.4 µm were employed to measure CO and H 2 O time-histories, respectively. OH* chemiluminescence profiles were measured using an emission diagnostic at 307 nm. For the laminar flame speed experiments, measurements were carried out in a constant-volume vessel at 295 K and 1 atm for CH 4 /argon and H 2 S/CH 4 /argon (8.25% H 2 S) mixtures from φ = 0.7 to φ = 1.4. The predictions of several recent chemical kinetics mechanisms were compared to the data, leading to the conclusion that species containing both carbon and sulfur are unimportant for shock-tube conditions but can be quite influential for laminar flames. By combining the modeling efforts of two recent works, a tentative new model is proposed that shows marked improvement over the older models in terms of shock-tube ignition delay times. Flame speed predictions show a discrepancy with the new model but follow general experimental trends. To the best of the authors’ knowledge, this study provides the first shock-tube data and laminar flame speeds measured in the H 2 S/CH 4 system.

Parameters influencing the burning rate of laminar flames propagating into a reacting mixture

Abstract: The laminar flame speed is an important property of a reacting mixture and it is used extensively for the characterization of the combustion process in practical devices. However, under engine-relevant conditions, considerable reactivity may be present in the unburned mixture, introducing thus challenges due to couplings of auto-ignition and flame propagation phenomena. In this study, the propagation of transient, one-dimensional laminar flames into a reacting unburned mixture was investigated numerically in order to identify the parameters influencing the flame burning rate in the conduction-reaction controlled regime at constant pressure. It was found that the fuel chemical classification significantly influences the burning rate. More specifically, for hydrogen flames, the “evolution” of the burning rate does not depend on the initial unburned mixture temperature. On the other hand, for n-heptane flames that exhibit low temperature chemistry, the burning rate depends on the instantaneous temperature and composition of the unburned mixture in a coupled way. A new approach was developed allowing for the decoupling the flame chemistry from the ignition dynamics as well as for the decoupling of parameters influencing the burning rate, so that meaningful sensitivity analysis could be performed. It was determined that the burning rate is not directly affected by fuel specific reactions even in the presence of low temperature chemistry whose effect is indirect through the modification of the reactants composition entering the flame. The controlling parameters include but not limited to mixture conductivity, enthalpy, and the species composition evolution in the unburned mixture.

Effect of ignition energy on the uncertainty in the determination of laminar flame speed using outwardly propagating spherical flames

Abstract: The initial propagation processes of expanding spherical flames of CH4/N2/O2/He mixtures at different ignition energies were investigated experimentally and numerically to reduce the effect of ignition energy on the accurate determination of laminar flame speeds. The experiments were conducted in a constant-volume combustion bomb at initial pressures of 0.07 − 0.7 MPa, initial temperatures of 298 − 398 K, and equivalence ratios of 0.9 − 1.3 with various Lewis numbers. The A-SURF program was employed to simulate the corresponding flame propagation processes. The results show that elevating the ignition energy increases the initial flame propagation speed and expands the range of flame trajectory which is affected by ignition energy, but the increase rates of the speed and range decrease with the ignition energy. Based on the trend of the minimum flame propagation speed during the initial period with the ignition energy, the minimum reliable ignition energy (MRIE) is derived by considering the initial flame propagation speed and energy conservation. It is observed that MRIE first decreases and then increases with the increasing equivalence ratio and monotonously decreases with increasing initial pressure and temperature. As the Lewis number rises, MRIE increases. The results also suggest that during the data processing of the spherical flame experiment, the accuracy of determination of laminar flame speeds can be enhanced when taking the flame radius influenced by MRIE as the lower limit of the flame radius range. Then the flame radius influenced by MRIE was defined as RFR. It can also be found that there exist nonlinear relationships between RFR and the equivalence ratio and Lewis number, and the RFR decreases with increasing initial pressure and temperature.

A self-contained progress variable space solution method for thermochemical variables and flame speed in freely-propagating premixed flamelets

Abstract: Flamelet models for premixed combustion, which are based on equations formulated and solved in progress variable space, have been proposed in the past, but have not been adopted for chemistry reduction methods. This is due to one limitation of these models: they need a closure for both the magnitude and the shape of the gradient (or scalar dissipation rate) of the progress variable, which is essential for an accurate prediction of the flame displacement speed. So far, solution methods for the aforementioned models require gradient information as an input, which is either modelled and non-generic, or extracted from a previous physical space flame solution for the analogous problem. The objective of this work is to provide a self-contained solution method for freely-propagating premixed flamelets in progress variable space, by solving an additional flamelet equation for the gradient of the progress variable. With this, the novel method provides both magnitude and shape of the gradient. Studying hydrogen-air and methane-air configurations, it is demonstrated that an accurate prediction of the laminar flame speed without the necessity for further input parameters can be obtained.