Autoignition of ethanol in a rapid compression machine

Ethanol is a renewable source of energy and significant attention has been directed to the development of a validated chemical kinetic mechanism for this fuel. The experimental data for the autoignition of ethanol in the low temperature range at elevated pressures are meager. In order to provide experimental data sets for mechanism validation at such conditions, the autoignition of homogeneous ethanol/oxidizer mixtures has been investigated in a rapid compression machine. Experiments cover a range of pressures (10–50 bar), temperatures (825–985 K) and equivalence ratios of 0.3–1.0. Ignition delay data are deduced from the experimental pressure traces. Under current experimental conditions of elevated pressures and low temperatures, chemistry pertaining to hydroperoxyl radicals assumes importance. A chemical kinetic mechanism that can accurately predict the autoignition characteristics of ethanol at low temperatures and elevated pressures has been developed and this mechanism is compared with other models available in the literature.     

Ignition delay study of moist hydrogen/oxidizer mixtures using a rapid compression machine

Autoignition of moist hydrogen/oxidizer mixtures has been studied experimentally using a rapid compression machine (RCM). This work investigated the effect of water addition on ignition delays of stoichiometric hydrogen/oxidizer mixtures in the end of compression temperature range of T_C = 907–1048 K at three different end of compression pressures viz. P_C = 10 bar (1 MPa), 30 bar (3 MPa), and 70 bar (7 MPa). RCM experiments were conducted with 0%, 10%, and 40% molar percentages of water in the reactive mixture. At P_C = 30 bar and 70 bar, the presence of 10% and 40% water vapor was shown to promote autoignition. However, at P_C = 10 bar, water addition (10%) was seen to retard the reactivity, thereby increasing the ignition delay. Comparison with different reaction kinetic mechanisms reported in literature shows widely different results of simulated ignition delays for the temperature and pressure range studied, although most of the mechanism predictions demonstrate similar trend in ignition delay with water addition. A recent chemical kinetic mechanism, which shows good agreement with the present experiments at higher pressure but some discrepancy at lower pressure, was used for brute force sensitivity analysis in order to identify the important reactions for the dry mixtures in the temperature and pressure window investigated. An important reaction identified was further adjusted within the uncertainty limit as an attempt to improve the results from mechanism prediction for the ignition delay at low pressure (P_C = 10 bar) without water addition. In addition, the modification in the reaction rate leads to good agreement between the experiment data and the mechanism prediction for the moist mixtures at varying compressed pressures.     

Autoignition of n-butanol at elevated pressure and low-to-intermediate temperature

Autoignition experiments for n-butanol have been performed using a heated rapid compression machine at compressed pressures of 15 and 30 bar, in the compressed temperature range of 675–925 K, and for equivalence ratios of 0.5, 1.0, and 2.0. Over the conditions studied, the ignition delay decreases monotonically as temperature increases, and the autoignition response exhibits single-stage characteristics. A non-linear fit to the experimental data is performed and the reactivity, in terms of the inverse of ignition delay, shows nearly second order dependence on the initial oxygen mole fraction and slightly greater than first order dependence on initial fuel mole fraction and compressed pressure. Experimentally measured ignition delays are also compared to simulations using several reaction mechanisms available in the literature. Agreement between simulated and experimental ignition delay is found to be unsatisfactory. Sensitivity analysis is performed on one recent mechanism and indicates that uncertainties in the rate coefficients of parent fuel decomposition reactions play a major role in causing the poor agreement. Path analysis of the fuel decomposition reactions supports this conclusion and also highlights the particular importance of certain pathways. Further experimental investigations of the fuel decomposition, including speciation measurements, are required.     

Effect of crevice mass transfer in a rapid compression machine

Rapid Compression Machines (RCMs) often employ creviced pistons to suppress the formation of the roll-up vortex. However, the use of a creviced piston promotes mass transfer into the crevice when heat release takes place in the main combustion chamber. This multi-dimensional effect is not accounted for in the prevalent volumetric expansion approach for modeling RCMs. The method of crevice containment avoids post-compression mass transfer into the crevice. In order to assess the effect of the crevice mass transfer on ignition in a RCM, experiments are conducted for autoignition of isooctane in a RCM with creviced piston in the temperature range of 680–940 K and at compressed pressures of ∼15.5 and 20.5 bar in two ways. In one situation, post-compression mass transfer to the crevice is avoided by crevice containment and in other it is allowed. Experiments show that the crevice mass transfer can lead to significantly longer ignition delays. Experimental data from both scenarios is modeled using adiabatic volumetric expansion approach and an available kinetic mechanism. The simulated results show less pronounced effect of crevice mass transfer on ignition delay and highlight the deficiency of the volumetric expansion method owing to its inability to describe coupled physico-chemical processes in the presence of heat release. Results indicate that it is important to include crevice mass transfer in the physical model for improved modeling of experimental data from RCMs without crevice containment for consistent interpretation of chemical kinetics. The use of crevice containment, however, avoids the issue of mass transfer altogether and offers an alternative and sound approach.     

CFD studies of a twin-piston rapid compression machine

A transient 2-dimensional moving mesh CFD computer model was created, validated against experimental data, and used to investigate the flow and resulting temperature fields in a rapid compression machine. The sensitivity of the horizontally opposed twin-piston RCM to nonsynchronized and non-uniform piston strokes was determined and the effect of non-uniform heating on resulting pressure profiles was investigated. Predictions of the ignition temperature in a rapid compression machine are made very difficult due to the existence of a highly non-uniform temperature field at the end of the compression stroke. An optimally designed piston head crevice, determined by a number of criteria, can largely overcome this problem by eliminating the mixing of the cool boundary layer gas with the hot compressed core gas. We used the CFD model to optimize the piston head crevices for our RCM and determined some new factors that are important when optimizing the piston head crevice design. Our best crevice design was then applied to a range of test gases and recommendations regarding the use of these as bath gases were made.     

A rapid compression machine with crevice containment

A rapid compression machine (RCM) incorporating ‘crevice containment’ is designed and fabricated. ‘Crevice containment’ maintains the advantage of suppression of piston-motion induced roll-up vortex while avoiding undesirable multi-dimensional effects of crevice. The geometry of the combustion chamber is optimized with computational fluid dynamic simulations. The designed RCM is demonstrated to provide highly reproducible experimental data at compressed gas pressures up to 100 bar. Pressure traces also reveal that ‘crevice containment’ leads to significant reduction in the post-compression pressure drop. Further, the importance of ensuring instrumentation calibration and avoiding thermal shock of pressure sensor is highlighted to avoid systematic errors in measurements. High fidelity experiments are conducted for autoignition of hydrogen at compressed pressure of 50 bar. The experimental data is properly modeled by the kinetic mechanism from O’Conaire et al. [M. O’Conaire, H.J. Curran, J.M. Simmie, W.J. Pitz, C.K. Westbrook, Int. J. Chem. Kinet. 36 (11) (2004) 603–622] and discrepancy is noted from a recent mechanism [Z. Hong, D.F. Davidson, R.K. Hanson, Combust. Flame 158 (2011) 633–644].     

Computational fluid dynamics modeling of hydrogen ignition in a rapid compression machine

In modeling a rapid compression machine (RCM) experiment, a zero-dimensional code is commonly used along with an associated heat loss model. However, the applicability of such a zero-dimensional modeling needs to be assessed over a range of accessible experimental conditions. It is expected that when there exists significant influence of the multidimensional effects, including boundary layer, vortex roll-up, and nonuniform heat release, the zero-dimensional modeling may not be adequate. In this work, we simulate ignition of hydrogen in an RCM by employing computational fluid dynamics (CFD) studies with detailed chemistry. Through the comparison of CFD simulations with zero-dimensional results, the validity of a zero-dimensional modeling for simulating RCM experiments is assessed. Results show that the zero-dimensional modeling based on the approach of “adiabatic volume expansion” generally performs very well in adequately predicting the ignition delay of hydrogen, especially when a well-defined homogeneous core is retained within an RCM. As expected, the performance of this zero-dimensional modeling deteriorates with increasing temperature nonuniformity within the reaction chamber. Implications for the species sampling experiments in an RCM are further discussed. Proper interpretation of the measured species concentrations is emphasized and the validity of simulating RCM species sampling results with a zero-dimensional model is assessed.     

A numerical assessment of the novel concept of crevice containment in a rapid compression machine

Rapid compression machines (RCMs) typically incorporate creviced pistons to suppress the formation of the roll-up vortex. The use of a creviced piston, however, can enhance other multi-dimensional effects inside the RCM due to the crevice zone being at lower temperature than the main reaction chamber. In this work, such undesirable effects of a creviced piston are highlighted through computational fluid dynamics simulations of n-heptane ignition in RCM. Specifically, the results show that in an RCM with a creviced piston, additional flow of mass takes place from the main combustion chamber to the crevice zone during the first-stage of the two-stage ignition. This phenomenon is not captured by the zero-dimensional modeling approaches that are currently adopted. Consequently, a novel approach of ‘crevice containment’ is introduced and computationally evaluated in this paper. In order to avoid the undesirable effects of creviced piston, the crevice zone is separated from the main reaction chamber at the end of compression. The results with ‘crevice containment’ show significant improvement in the fidelity of zero-dimensional modeling in terms of predicting the overall ignition delay and pressure rise in the first-stage of ignition. Although the implementation of ‘crevice containment’ requires a modification in RCM design, in practice there are significant advantages to be gained through a reduction in the rate of pressure drop in the RCM combustion chamber and a quantitative improvement in the data obtained from the species sampling experiments.     

CFD modeling of two-stage ignition in a rapid compression machine: Assessment of zero-dimensional approach

In modeling rapid compression machine (RCM) experiments, zero-dimensional approach is commonly used along with an associated heat loss model. The adequacy of such approach has not been validated for hydrocarbon fuels. The existence of multi-dimensional effects inside an RCM due to the boundary layer, roll-up vortex, non-uniform heat release, and piston crevice could result in deviation from the zero-dimensional assumption, particularly for hydrocarbons exhibiting two-stage ignition and strong thermokinetic interactions. The objective of this investigation is to assess the adequacy of zero-dimensional approach in modeling RCM experiments under conditions of two-stage ignition and negative temperature coefficient (NTC) response. Computational fluid dynamics simulations are conducted for n-heptane ignition in an RCM and the validity of zero-dimensional approach is assessed through comparisons over the entire NTC region. Results show that the zero-dimensional model based on the approach of ‘adiabatic volume expansion’ performs very well in adequately predicting the first-stage ignition delays, although quantitative discrepancy for the prediction of the total ignition delays and pressure rise in the first-stage ignition is noted even when the roll-up vortex is suppressed and a well-defined homogeneous core is retained within an RCM. Furthermore, the discrepancy is pressure dependent and decreases as compressed pressure is increased. Also, as ignition response becomes single-stage at higher compressed temperatures, discrepancy from the zero-dimensional simulations reduces. Despite of some quantitative discrepancy, the zero-dimensional modeling approach is deemed satisfactory from the viewpoint of the ignition delay simulation.     

Laser-induced incandescence study of diesel soot formation in a rapid compression machine at elevated pressures

Simultaneous laser-induced incandescence (LII) and laser-induced scattering (LIS) were applied to investigate soot formation and distribution in a single cylinder rapid compression machine. The fuel used was a low sulfur reference diesel fuel with 0.04% volume 2-ethylhexyl nitrate. LII images were acquired at time intervals of 1 CA throughout the soot formation period, for a range of injection pressures up to 160 MPa, and in-cylinder pressures (ICP) up to 9 MPa. The data collected shows that although cycle-to-cycle variations in soot production were observed, the LII signal intensities converged to a constant value when sufficient cycles were averaged. The amount of soot produced was not significantly affected by changes in in-cylinder pressure. Soot was observed distributed in definite clusters, which were linked to slugs of fuel caused by oscillations in the injector needle. The highest injection pressure exhibited lower soot productions and more homogeneous soot distributions within the flame. Despite diffusion flames lasting longer with lower injection pressure, it appeared that the extended oxidation time was insufficient to oxidize the excess production of soot. In addition, soot particles were detected closer to the nozzle tip with higher injection pressures. The recording of LII sequences at high temporal resolutions has shown that three distinct phases in soot formation can be observed. First, high soot formation rates are observed before the establishment of the diffusion flame. Second, a reduced soot formation rate is apparent from the start of diffusion flame until the end of injection. Finally, high soot oxidation rates occur after the end of injection and for the duration of the flame.     

Early flame propagation of hydrogen enriched methane-air mixtures at quasi laminar conditions in a rapid compression expansion machine

The temporal evolution of the kinematic properties of hydrogen enriched, lean premixed methane-air flames was studied experimentally in a rapid compression expansion machine (RCEM) during transient operation. Schlieren imaging was used to capture the flame front propagation, while the temporally evolving flow field was simultaneously acquired by particle image velocimetry (PIV). A statistical analysis based on probability density functions (PDFs) of non-dimensional formulations of the flame curvature, stretch rate and displacement speed was performed to study the effect of the onset of flame instabilities on the aerodynamic characteristics of the propagating flame kernel. It was found that initial perturbations stemming from the presence of the spark plug electrodes and from the gas expansion led to the formation of large scale cusps related to the hydrodynamic instability, which is manifested by a notable negative skewness of the curvature PDF, validating recent numerical findings. Under the influence of thermal-diffusive effects, small scale cellular structures develop along the flame front beyond a certain radius, with their appearance shifting to earlier times with increasing hydrogen content or for leaner mixtures. The increasing width of the normalized curvature and stretch rate PDFs is proposed as an indicator for the onset of this type of instability. The adopted methodology allows to track the self-acceleration of the propagating flame front effecting from hydrogen enrichment of methane. The findings of this study provide valuable physical insight and aid in the development of more accurate models, capable of capturing these complex phenomena.     

Experiments and modeling of the autoignition of methylcyclohexane at high pressure

New experimental data are collected for methyl-cyclohexane (MCH) autoignition in a heated rapid compression machine (RCM). Three mixtures of MCH/O₂/N₂/Ar at equivalence ratios of ? = 0.5, 1.0, and 1.5 are studied and the ignition delays are measured at compressed pressure of 50 bar and for compressed temperatures in the range of 690–900 K. By keeping the fuel mole fraction in the mixture constant, the order of reactivity, in terms of inverse ignition delay, is measured to be ? = 0.5 > ? = 1.0 > ? = 1.5, demonstrating the dependence of the ignition delay on oxygen concentration. In addition, an existing model for the combustion of MCH is updated with new reaction rates and pathways, including substantial updates to the low-temperature chemistry. The new model shows good agreement with the overall ignition delays measured in this study, as well as the ignition delays measured previously in the literature using RCMs and shock tubes. This model therefore represents a strong improvement compared to the previous version, which uniformly over-predicted the ignition delays. Chemical kinetic analyses of the updated mechanism are also conducted to help understand the fuel decomposition pathways and the reactions controlling the ignition. Combined, these results and analyses suggest that further investigation of several of the low-temperature fuel decomposition pathways is required.     

A reduced chemical kinetic model for HCCI combustion of primary reference fuels in a rapid compression machine

A model for the Homogeneous Charge Compression Ignition (HCCI) of Primary Reference Fuels (PRFs) in a Rapid Compression Machine (RCM) has been developed. A reduced chemical kinetic model that included 32 species and 55 reactions was used and the affect of wall heat transfer on the temperature of the adiabatic core gas was taken into account by adding the displacement volume of the laminar boundary layer to the cylinder volume. A simple interaction between n-heptane and iso-octane was also included. The results showed the well-known two-stage ignition characteristics of heavy hydrocarbons, which involve low and high temperature cycles followed by a branched chain explosion. The first stage energy release decreases and the ignition delay increases nonlinearly with increasing octane number and decreasing the initial pressure. The energy release rate and total energy released were determined primarily by the rate of CO oxidation during the explosive phase following the ignition delay. The model reproduced the pressure curves obtained in the RCM experiments over a wide range of conditions remarkably well and was very sensitive to the fuel structure, the mixture composition and the initial temperature and pressure. Thus, the model can be easily adapted for predicting “knock” in spark-ignition engines and ignition-delays and burning rates in HCCI engines.     

Ignition properties of methane/hydrogen mixtures in a rapid compression machine

We investigate changes in the combustion behavior of methane, the primary component of natural gas, upon hydrogen addition by characterizing the autoignition behavior of methane/hydrogen mixtures in a rapid compression machine (RCM). Ignition delay times were measured under stoichiometric conditions at pressures between 15 and 70 bar, and temperatures between 950 and 1060 K; the hydrogen fraction in the fuel varied between 0 and 1. The ignition delay times in methane/hydrogen mixtures are well correlated with the ignition delay times of the pure fuels by using a simple mixing relation reported in the literature. Simulations of the ignition delay times using various chemical mechanism are also reported. The mechanism given by Petersen et al. shows excellent agreement with the measurements for all mixtures studied. Initial results on fuel–lean mixtures show a modest effect of equivalence ratio on the delay times.     

Interpretation of experimental data from rapid compression machines without creviced pistons

Over the last two decades, experimental data of the nature of species evolution profiles and ignition delays from rapid compression machines (RCMs) has been used to develop and validate chemical kinetic mechanisms at low-to-intermediate temperatures and elevated pressures. A significant portion of this overall dataset is from RCMs that had not employed a creviced piston to contain the roll-up vortex. The detrimental influence of the roll-up vortex and the thermokinetic interactions due to the resulting temperature non-homogeneity during the negative temperature coefficient (ntc) regime have been documented in the literature. However, the adequacy of the homogeneous modeling of RCMs without creviced pistons during reactive conditions has not been investigated. In this work, computational fluid dynamics simulations of an RCM without a creviced piston are conducted for autoignition of n-heptane over the entire ntc regime over a range compressed pressures from 5 to 18 bar. The results from the CFD simulations highlight the non-homogeneity of autoignition and reveal significant quantitative discrepancy in comparison to homogeneous modeling, particularly for the hot ignition delay in the ntc regime. Specifically, the roll-up vortex induced temperature non-homogeneity leads to diminution of the ntc behavior. The experimental data from RCMs without creviced piston needs to be taken with caution for quantitative validation and refinement of kinetic mechanism, particularly at conditions when ntc behavior is highly pronounced.     

Autoignition of n-decane under elevated pressure and low-to-intermediate temperature conditions

An experimental investigation of the autoignition for various n-decane/oxidizer mixtures is conducted using a rapid compression machine, in the range of equivalence ratios of ?=0.5-2.2, dilution molar ratios of N₂/(O₂ + N₂) = 0.79-0.95, compressed gas pressures of PC=7-30 bar, and compressed gas temperatures of TC=635-770 K. The current experiments span a temperature range not fully investigated in previous autoignition studies on n-decane. Two-stage ignition, characteristic of large hydrocarbons, is observed over the entire range of conditions investigated, as demonstrated in the plots of raw experimental pressure traces. In addition, experimental results reveal the sensitivity of the first-stage and total ignition delays to variations in fuel and oxygen mole fractions, pressure, and temperature. Predictability of two kinetic mechanisms is compared against the present data. Discrepancies are noted and discussed, which are of direct relevance for further improvement of kinetic models of n-decane at conditions of elevated pressures and low-to-intermediate temperatures.     

Homogeneous charge compression ignition of binary fuel blends

The present experimental investigation aims to understand the homogeneous combustion chemistry associated with binary blends of three surrogate components for practical fuels, including toluene, isooctane, and diisobutylene-1 (DIB-1). Specifically, high-pressure autoignition characteristics of the three neat fuel components as well as the fuel blends of toluene + isooctane and toluene + DIB-1 are studied herein. Experiments are conducted in a rapid compression machine at compressed pressures varying from 15 to 45 bar and under low to intermediate temperatures. To obtain insights into interactions among fuels, the relative proportion of the two neat fuels in the reactive mixtures is systematically varied, while the total fuel mole fraction and equivalence ratio are kept constant. Experimental results demonstrate that ignition delays for neat toluene are more than an order of magnitude longer than those for neat isooctane. Whereas DIB-1 has ignition delays shorter than those for isooctane at higher temperatures, at temperatures lower than 820 K DIB-1 shows a longer ignition delay. Although the ignition delays of binary blends lie in between the two extremes of neat components, the variation of ignition delay with the relative fuel proportion is seen to be highly nonlinear. Especially, a small addition of isooctane or DIB-1 to toluene can result in greatly enhanced reactivity. In addition, the effect of DIB-1 addition to toluene is more significant than the effect of isooctane addition. Furthermore, in the compressed temperature range from 820 to 880 K, ignition delay of the toluene + isooctane blend shows greater sensitivity to temperature than that of isooctane.     

Using rapid compression machines for chemical kinetics studies

Rapid compression machines (RCMs) are used to simulate a single compression stroke of an internal combustion engine without some of the complicated swirl bowl geometry, cycle-to-cycle variation, residual gas, and other complications associated with engine operating conditions. RCMs are primarily used to measure ignition delay times as a function of temperature, pressure, and fuel/oxygen/diluent ratio; further they can be equipped with diagnostics to determine the temperature and flow fields inside the reaction chamber and to measure the concentrations of reactant, intermediate, and product species produced during combustion.This paper first discusses the operational principles and design features of RCMs, including the use of creviced pistons, which is an important feature in order to suppress the boundary layer, preventing it from becoming entrained into the reaction chamber via a roll-up vortex. The paper then discusses methods by which experiments performed in RCMs are interpreted and simulated. Furthermore, differences in measured ignition delays from RCMs and shock tube facilities are discussed, with the apparent initial gross disagreement being explained by facility effects in both types of experiments. Finally, future directions for using RCMs in chemical kinetics studies are also discussed.     

Experimental study of the kinetic interactions in the low-temperature autoignition of hydrocarbon binary mixtures and a surrogate fuel

The oxidation and autoignition of five undiluted stoichiometric mixtures, n-heptane/toluene, isooctane/toluene, isooctane/1-hexene, 1-hexene/toluene, and isooctane/1-hexene/toluene, has been studied in a rapid compression machine below 900 K. Ignition delay times of two- and one-stage autoignition have been measured and compared to those for pure hydrocarbons. The largest influence of mixing is in the region of the negative temperature coefficient. Intermediate products have been analyzed. The main reaction paths of low-temperature co-oxidation are discussed according to current knowledge of the oxidation paths of pure hydrocarbons. The influence of toluene on the temperature coefficient of the first stage of ignition of isooctane cannot be accounted for by the current theories of low-temperature autoignition. Each hydrocarbon generates a pool of radicals whose reactivity and selectivity toward further attack changes with temperature and with the family of hydrocarbons. The overall behavior of mixtures may result from changing competition for HO₂ and OH as temperature increases during the delay time. Termination reactions between stable radicals seem to have a minor impact at low temperature.