Sooting Limit Research Articles

Near-extinction transient behaviors of counterflow nonpremixed flames in a porous spherical burner at very low stretch rate

Near extinction transient behaviors of CH4/N2 versus O2/N2 counterflow nonpremixed flames were investigated by adopting a porous spherical burner with a large radius of curvature to achieve a very low buoyant stretch rate. By varying the oxygen concentrations in the oxidizer stream, four regimes were identified by three limits of sooting, instability, and extinction. The sooting limit was sensitive to flame temperature, and was characterized by two indexes of fuel injection velocity uF and oxygen concentration XO2. When decreasing uF, the sooting limit was different from high stretch flames because of appreciable heat loss toward the burner surface. In the instability regime, periodic holes were observed whose frequency decreased with XO2. Periodic holes were also found to occur with N2 dilution in the fuel stream, which were more regular with larger frequencies compared with the periodic holes in the diluted oxidizer streams. The edge propagation behaviors for periodic holes in diluted fuel streams were analyzed. The statistical average of the propagation speed of advancing edge (hole closing) Se decreased with increasing fuel concentration gradient, and the ratio of Se with the stoichiometric laminar burning velocity SLst of Se/SLst was larger than unity. Also, the stretch rate significantly affected the propagation speed of advancing edge flames. Determination of the propagation speed of the retreating edge (hole opening) had intrinsic difficulty in estimating the transient radial flow velocity, however, the approximated |Se/SLst| was larger than unity and decreased with the fuel mole fraction. Nitrogen dilution in the oxidizer stream was proven to be more effective than in the fuel stream for flame suppression of methane. This study could enrich the understanding of very low-stretch nonpremixed flames in a counterflow burner and provide information regarding low-stretch flame suppression, which may be relevant to microgravity flame behaviors.

Structure and extinction of methane and propane nonpremixed counterflow flames at extremely low stretch rates in buoyant flowfields

This study experimentally investigated the structures and extinction behaviors of methane-air and propane-air counterflow nonpremixed flames influenced by individually controlled fuel injection velocity uF at extremely low stretch rates, which were induced by natural convection alone. A sooting limit fuel injection velocity (SLFIV) index was introduced, which increased with stretch rate in the low stretch rate region. Methane had higher SLFIV than propane. Quasi-one-dimensional blue flames appeared for both methane and propane within a certain uF range. The range for methane was higher than that for propane. The steady flame standoff distances yf had a relationship with uF and low stretch rate ab of yf∝uFabλ∞1/2, which differed from high-stretch flames. Different flame instabilities occurred for methane and propane by changing uF only without fuel or oxidizer dilutions. At low uF near extinction limit of methane, periodic flame holes appeared, which was due to large heat loss. This instability finally caused flame extinction by keeping retreating edges. At high uF, simultaneous cellular and wave flame instability appeared for propane, which differed from cellular flames occurred alone in highly diluted oxy-fuel flames. By modulating uF alone in this experiment, the cellular/wave flame was due to Rayleigh-Taylor and hydrodynamic instabilities. Flame extinction at the lowest uF for propane was through outer edge shrinking, where the displacement speed increased with stretch rate. Flammability maps of limit uF versus low ab were established for both methane and propane. Both maps were divided into four regions by sooting, instability and extinction limits. The three limits uF all increased with ab. This changing tendency was opposite with high-stretch flames reported previously. For propane, the instability limit uF approached infinity at higher ab. This work provided knowledge of extremely low stretch flame combustion, which can be applied to microgravity flames, because the combustion characteristics of microgravity forced low stretch flames were identical to those of low stretch flame induced by natural convection alone.

From molecular to sub-μm scale: The interplay of precursor concentrations, primary particle size, and carbon nanostructure during soot formation in counter-flow diffusion flames

The focus of this work is on counter-flow diffusion flames of C2H4/air near the sooting limit (fV < 500 ppb) to shed light on the transition of precursor molecules to primary soot particles and their nanostructure. The applied experimental techniques cover non-intrusive methods based on laser-induced incandescence (LII) for determination of soot volume fractions, primary particle sizes, and ratios of refractive indices for absorption at different wavelengths. The methods are complemented by intrusive methods such as sampling with microprobes and analysis of gas composition by gas chromatography and mass spectrometry, particle sizing by differential mobility analysis, and determining carbon nanostructures of particles by high-resolution transmission electron microscopy (HRTEM). For numerical simulation, the well-known ABF-model is applied.The experimentally determined structures of the counter-flow flames could be well reproduced within the experimental uncertainties by the ABF-model for varying fuel mass fractions and strain rates. The numerical simulations identify the different processes during particle formation and the main variation can be attributed to variation of surface growth rates when applying strain rates and fuel mass fractions leading to higher soot volume fractions. Particle size distributions detected in the fuel flow near and below the stagnation plane appear bimodal with distinct maxima at particles sizes about 5 nm and 20 nm. The nanostructure of the primary particles exhibits basic structural units (BSUs), that increase up to the size of 7 Å to 10 Å at increasing soot volume fractions from about 100 ppb to 300 ppb. This coincides with the increase of the mole fractions of the detectable soot precursors with increasing soot volume fraction. The ratios of refractive index functions for absorption E(m,λ532)/E(m,λ1064) and E(m,λ266)/E(m,λ1064) decrease approximately linearly with increasing soot volume fractions. This is corroborated by the analyses of HRTEM-images that clearly bring about increasing sized BSUs with increasing soot volume fractions causing a shift of the absorption to larger wavelengths.

Modeling and Control of Diesel Engine Emissions using Multi-layer Neural Networks and Economic Model Predictive Control

This paper presents the results of developing a multi-layer Neural Network (NN) to represent diesel engine emissions and integrating this NN into control design. Firstly, a NN is trained and validated to simultaneously predict oxides of nitrogen (NOx) and Soot using both transient and steady-state data. Based on the input-output correlation analysis, inputs to NN with the highest influence on the emissions are selected while keeping the NN structure simple. Secondly, a co-simulation framework is implemented to integrate the NN emissions model with a model of a diesel engine airpath system built in GT-Power and used to identify a low-order linear parameter-varying (LPV) model for emissions prediction. Finally, an economic supervisory model predictive controller (MPC) is developed using the LPV emissions model to adjust setpoints to an inner-loop airpath tracking MPC. Simulation results are reported illustrating the capability of the resulting controller to reduce NOx, meet the target Soot limit, and track the adjusted intake manifold pressure and exhaust gas recirculation (EGR) rate targets.

Incipient sooting tendency of oxygenated fuels doped in ethylene counterflow diffusion flames

The incipient sooting tendencies of oxygenated fuels in counterflow diffusion flames were systematically assessed by doping selected oxygenate fuels into the baseline fuel of ethylene with various mixing ratios. Laser light scattering and planar laser-induced fluorescence (PLIF) techniques were adopted. The critical oxygen mole fractions at different mixing ratios were identified with the use of laser scattering. It is found that the critical oxygen mole fraction varies with oxygenates, suggesting that the effect of mixing ethylene with oxygenates is highly sensitive to the types of oxygenate. While the doping of acetone moderately promoted the incipient sooting tendency, the doping of methanol, acetic acid, formic acid, and diethyl carbonate suppressed the formation of incipient soot. On the other hand, the doping of ethanol, dimethyl carbonate, diethyl ether, and dimethoxymethane had weak influences on the incipient sooting tendency. The production of polycyclic aromatic hydrocarbons (PAHs) of different sizes were measured by PLIF. The results show there was a strong correlation between the incipient sooting tendency and the production of large PAHs. One important finding is that, at their sooting limits, the total oxygen-to-carbon ratio of a stoichiometric fuel/oxygen mixture is linearly correlated with the mixing ratio of the ethylene-oxygenate mixture. This slope of these two quantities can be defined as an incipient sooting index (ISI) of oxygenated fuels for ranking their tendency of incipient sooting formation. Having the slopes correlated well with the oxygenates’ hydrogen/oxygen ratio, a reasonably accurate prediction of critical oxygen mole fractions from the molecule information of oxygenated fuels can be achieved.

Experimental investigation of the effect of hydrogen addition on the sooting limit and structure of methane/air laminar counterflow diffusion flames

The influence of hydrogen (H2) addition on the sooting limit and structure of methane/air laminar counterflow diffusion flames has been studied. Soot limits have been obtained using the laser light scattering technique for CH4/H2 mixtures with H2 concentration varying from 0–20% by volume in the fuel mixture for different strain rates. The resulting soot limit map shows a significant increase in the fuel concentration at the soot limit with increasing amounts of H2. To understand this trend, temperature and species concentration profiles have been measured and compared under the influence of H2 addition in (a) an incipiently sooting and (b) in a sooting CH4/air flame. Flame temperatures have been measured using a thin-wire S-type thermocouple and species profiles have been measured using the gas chromatography technique (GC). The results indicate a significant decrease in the amounts of higher hydrocarbons such as benzene and naphthalene (which are considered important soot precursors) with an increase in H2 concentration for the sooting flame. To explain this decrease in the concentration of higher hydrocarbons, pathway analysis has been performed to elucidate the influence of H2 on the formation and consumption of soot forming species.

Probing sooting limits in counterflow diffusion flames via multiple optical diagnostic techniques

Sooting limit representing the critical flame condition where soot starts to appear is a useful quantitative index to assess the sooting tendency of a particular fuel. Experimental determination of sooting limit is closely related to the establishment of a well-defined “nucleation” flame which facilitates further scrutinization of the soot formation process. The present study was devoted to probing the earliest soot appearance in typical counterflow diffusion flames. The sooting limit was defined as the critical fuel (or oxygen) mole fraction in the nitrogen-balanced fuel (or oxidizer) stream. Multiple optical diagnostic techniques including flame luminosity, light extinction and scattering, spectral soot emission (SSE) as well as laser-induced incandescence (LII) were used to determine the sooting limits. The results showed that the value of sooting limit can be quantitatively different among the used detection techniques. Flame luminosity and SSE were found to be most sensitive to the presence of nascent soot, followed by LII and laser light scattering, while light extinction is the least sensitive technique. The effects of the wavelength of probing light on sooting limits were also assessed for the laser extinction and scattering technique. The contribution of the present work can be summarized into the following two aspects: (1) a quantitative database of sooting limits as determined from different soot detection measurements is provided to facilitate the choice of nascent soot-probing technique; (2) the first appearance of soot particles in nucleation flames are fully characterized and interpreted through a cross-comparison among multiple diagnostics techniques. It is also the authors’ expectation that the present results may provide new insights into the understanding of soot nucleation process and the interaction between light and nascent soot.

Experimental Investigation on the Effect of Hydrogen Addition on the Sooting Limit and Structure of Methane/Air Laminar Counterflow Diffusion Flames

Experimental Investigation on the Effect of Hydrogen Addition on the Sooting Limit and Structure of Methane/Air Laminar Counterflow Diffusion Flames

A comparative study of the sooting tendencies of various C5–C8 alkanes, alkenes and cycloalkanes in counterflow diffusion flames

In this work, sooting tendencies of various vaporized C5-C8 alkanes, alkenes, cycloalkanes were investigated in a counterflow diffusion flame (CDF) configuration. Mie scattering and laser induced incandescence (LII) were respectively employed to measure the sooting limits and soot volume fraction, both of which were used as quantitative sooting tendency indices of the flames. Research foci were paid to analyze the impacts of fuel molecular structure on sooting tendencies, ranging from the length of the carbon chain, fuel unsaturation, presence and position of the branched chain as well as the cyclic ring structure. It was found that the present CDF-based sooting tendency data did not always agree with existing literature results that were obtained from either coflow or premixed flame experiments, indicating the importance of considering flame conditions when assessing fuel sooting tendencies. Several interesting observations were made; in particular, our results showed that the sooting limits of C5-C8n-alkane is comparable, regardless of the carbon-chain length, indicating an interesting fuel similarity pertinent to sooting tendency. In addition, we found that cyclopentane, with a five-membered ring, has even stronger sooting propensity than the six-membered cyclohexane; this trend occurs because cyclopentane prefers to decompose to more odd-carbon radicals of cyclopentadienyl and allyl, which are efficient aromatic precursors. Our results are expected to serve as necessary complements to existing sooting index data for a deeper understanding of the correlation between fuel molecular structures and their sooting tendencies.

CO &lt;sub&gt;2&lt;/sub&gt; Well-to-Wheel Abatement with Plug-In Hybrid Electric Vehicles Running under Low Temperature Combustion Mode with Green Fuels

&lt;div class="section abstract"&gt;&lt;div class="htmlview paragraph"&gt;Plug-in Hybrid Electric Vehicles (PHEVs) can be considered as the most promising technology to achieve the European CO&lt;sub&gt;2&lt;/sub&gt; targets together with a moderate infrastructure modification. However, the real benefits, in terms of CO&lt;sub&gt;2&lt;/sub&gt; emissions, depend on a great extent on the energy source (fuel and electricity mix), user responsibility, and vehicle design. Moreover, the electrification of the powertrain does not reduce other emissions as NOx and particulate matter (mainly soot). In the last years, low temperature combustion (LTC) modes as the reactivity controlled compression ignition (RCCI) have shown to achieve ultra-low NOx and soot emissions simultaneously due to the use of two fuels with different reactivity together with high exhaust gas recirculation (EGR) rates. Therefore, the aim of this work is to assess, through numerical simulations fed with experimental results, the effects of different energy sources on the performance and emissions of a series RCCI PHEV. The dual-fuel engine was fueled with diesel as high reactivity fuel and bioethanol as low reactivity fuel. The powertrains are optimized to meet the European homologation legislation (WLTP) for PHEVs. The tailpipe emissions and fuel consumption of the series RCCI PHEV is analyzed and compared to the OEM no-hybrid passenger vehicle running under conventional diesel combustion and RCCI. A life-cycle analysis (LCA) is performed to evaluate the potential of the different energy sources to reduce the overall CO&lt;sub&gt;2&lt;/sub&gt; emissions. Different electricity sources are investigated as well as different liquid fuels: e-diesel and bio-ethanol. The series RCCI PHEV technology achieved the tank-to-wheel 50 g/km CO&lt;sub&gt;2&lt;/sub&gt; emissions target with medium battery size (13 kWh). In addition, thanks to the RCCI technology, the vehicle is able to meet the Euro 6 NOx and soot limits. The LCA shows that, depending on the fuel source, the total CO&lt;sub&gt;2&lt;/sub&gt; reduction benefits variate. Also, the final use and the charging events of the series PHEV have a great impact in this sense.&lt;/div&gt;&lt;/div&gt;

Evaluation of semi-empirical soot models for nonpremixed flames with increased stoichiometric mixture fraction and strain

Due to the size and complexity of industrial-scale systems and computing limitations, semi-empirical soot models are often employed in CFD simulations rather than computationally-expensive detailed models. Many of these models were developed for specific applications with unique characteristic timescales and/or validated only under fuel-air combustion conditions. Hence, their use in different contexts, such as oxy-combustion, could lead to inaccurate predictions. In this study, twelve semi-empirical models (1-step or 2-step) are evaluated on their ability to respond to changes in stoichiometric mixture fraction (Zst) and strain in a series of ethylene counterflow flames which span the experimental sooting-to-non-sooting (yellow to blue) transition. A unique approach of plotting soot formation rate in normalized, local equivalence ratio space is introduced to aid in the analysis. Results show that no existing model is able to predict a blue flame when Zst is increased beyond the experimentally-measured sooting limit. Many models give the counter-intuitive result of increasing peak and/or integrated soot volume fraction (svf) as Zst increased, contrary to experimental observations. Two models are able to successfully predict a blue flame upon increase of strain to the sooting limit value. For two-step models, the strong dependence of growth rate on surface area results in high sensitivity of svf to changing flame boundary conditions, but with mixed accuracy. There remains a significant need for a robust semi-empirical model which can accurately predict soot fraction in systems with oxygen-enrichment or variable strain.

Performance of a conventional diesel aftertreatment system used in a medium-duty multi-cylinder dual-mode dual-fuel engine

Dual-mode dual-fuel combustion stands as one of the promising techniques to allow the dual-fuel operation along the whole engine map. This concept relies on using different combustion strategies as reactivity controlled compression ignition up to medium load, then migrating to diffusive dual-fuel combustion to reach full load. With this strategy, it is possible to obtain sensible reductions in NOx and soot while providing improvements in fuel consumption and CO2 emissions. However, the excessive quantities of HC and CO together with the low exhaust temperature can compromise the diesel oxidation catalyst (DOC) efficiency. In addition, the diffusive dual-fuel combustion applied at high engine load produces considerable soot amounts that should be reduced within the diesel particulate filter (DPF).Based on these facts, this work intends to evaluate the efficiency of a commercial aftertreatment system (DOC + DPF) while operating in dual-mode dual-fuel combustion. Additionally, fundamental studies where developed to understand the impact of the combination of fuels on the exhaust hydrocarbon species. First, the DOC performance was evaluated at steady-state and transient conditions under different operating conditions fulfilling the EUVI NOx and soot limits. In parallel, the different engine-out hydrocarbon species were measured by means of a Fourier-transform infrared spectroscopy (FTIR) gas analyzer. Finally, the passive and active regeneration processes were assessed by means of different methodologies aiming to evaluate the low NOx-low soot interaction and the capability of the active regeneration in dual-fuel dual-mode (DMDF) operating conditions. The DOC results showed an improper conversion efficiency at low load operating condition, where the exhaust temperature is low. By contrast, the thermal inertia at transient conditions allowed to improve the DOC behavior at low load, reaching DOC-out emissions one order of magnitude lower than those from the steady-state tests. Concerning the DPF, it was demonstrated that the low concentration of NOx and soot produced during the combustion does not lead to sensible changes in the NO2/NOx ratio before and after the DPF, indicating a low level/absence of passive regeneration. In the case of the active regeneration, both conventional diesel combustion (CDC) and DMDF operating conditions can obtain satisfactory reduction in the total soot trapped, being the increase in the exhaust temperature consequence of the HC and CO conversion the supporting mechanism for the active regeneration in the DMDF concept.

Microcombustion for micro-tubular flame-assisted fuel cell power and heat cogeneration

Flame-assisted fuel cell (FFC) studies have been limited to lower fuel-rich equivalence ratios (∼1–1.7, due to the upper flammability limit and sooting limit) where only small concentrations of H2 and CO can be generated in the exhaust. In this work, a non-catalytic microcombustion based FFC is proposed for direct use of hydrocarbons for power generation. The potential for high FFC performance (450 mW cm−2 power density and 50% fuel utilization) in propane/air microcombustion exhaust is demonstrated. The micro flow reactor is investigated as a fuel reformer for equivalence ratios from 1 to 5.5. One significant result is that soot formation in the micro flow reactor is not observed at equivalence ratios from 1 to 5.5 and maximum wall temperatures ranging from 750 to 900 °C. Soot formation is observed at higher wall temperatures of 950 °C and 1000 °C and equivalence ratios above 2.5. H2 and CO concentrations in the exhaust are found to have a strong temperature dependence that varies with the maximum wall temperature and the local flame temperature.

Micro-tubular flame-assisted fuel cells running methane, propane and butane: On soot, efficiency and power density

A two-stage combustor with 1st-stage premixed fuel-rich combustion, integrated micro-tubular flame-assisted fuel cell (FFC) and 2nd-stage, fuel-lean combustion is described. The current state of research in direct flame fuel cells (DFFCs) is assessed and the dilemma of obtaining high power density, high electrical efficiency and no soot is discussed. A method for deriving the maximum theoretical electrical efficiency from a FFC based system is developed. A method for comparing methane, propane and butane in the two-stage combustor with integrated FFC is developed. Methane, propane and butane are tested at different equivalence ratios and flow rates to assess the power density and electrical efficiency. High power density (>300 mW cm−2) and high electrical efficiency (>1.2%) are achieved for equivalence ratios below 1.6. Methane is found to achieve higher power density and electrical efficiency at lower equivalence ratios compared to propane and butane. Soot formation is avoided by operating below the critical sooting limit.

Effect of dimethyl ether (DME) addition on sooting limits in counterflow diffusion flames of ethylene at elevated pressures

The effects of dimethyl ether (DME) addition to ethylene fuel on sooting tendencies with varying pressure were investigated in counterflow diffusion flames by using a laser scattering technique. Sooting limit maps were determined in the fuel (XF) and oxygen (XO) mole fraction plane, separating sooting and non-sooting regions. The results showed that when DME is mixed to ethylene, the sooting region was appreciably shrank, especially in the cases of soot formation/oxidation (SFO) flames as compared with the cases of soot formation (SF) flames. This indicated an inhibiting role of DME on sooting. An interesting observation was that the critical XO required for sooting initially decreased and then increased with the DME mixing ratio to ethylene β for the cases of SF flames, exhibiting a non-monotonic behavior. This implied a promoting role of DME on sooting when small amount of DME is mixed to ethylene. As the pressure increased, the sooting region generally expanded. Specifically, the range of β in promoting soot formation extended with pressure. This implies that a strategy in reducing soot by adding DME to ethylene at high pressures required a large amount of DME addition. To interpret the observed phenomena, kinetic simulations including reaction pathway and sensitivity analyses were conducted with the opposed-flow flames model using the KAUST-Aramco PAH Mech. The results showed that the thermal effect of DME addition on sooting tendency monotonically decreases with β. The chemical effect was found to be the main contributor to the DME addition effect on sooting tendency, resulting in the non-monotonic sooting limt behavior. The pathway analysis showed the role of methyl radicals generated from DME promoted incipient benzene ring formtion when small amount of DME was added, which can be attributed to the soot promoting role of DME addition for small β.

Experimental and soot modeling studies of ethylene counterflow diffusion flames: Non-monotonic influence of the oxidizer composition on soot formation

Previous soot studies in counterflow diffusion flames revealed that the sooting limit curve in regions with large oxygen mole fractions (XO) exhibited marked bending behaviors that indicated a non-monotonic variation of sooting tendency with oxygen concentration. The underlying mechanisms of this bending behavior remained unclear. In this regard, the present study systematically investigated the effect of oxygen mole fraction in the oxidizer stream on the sooting characteristics of ethylene counterflow diffusion flames. We used the near-infrared light extinction technique to measure soot volume fractions of two types of flames that significantly differed in sooting structures: soot formation oxidation (SFO) flames with a fixed fuel mole fraction (XF) of 0.28 and varying XO between 0.5 and 1.0 and soot formation (SF) flames with pure ethylene (XF = 1.0) in the fuel stream and varied XO from 0.25 to 0.3. We also conducted detailed soot modeling studies by combining the gas-phase chemistry with a sectional soot model that accounted for soot inception, surface growth, particle coagulation as well as soot oxidation. Our experimental and modeling results demonstrated the non-monotonic relationship between soot volume fractions and XO in SFO flames. A detailed analysis of the evolutionary process of soot formation revealed that the suppression of soot inception and the enhancement of the soot oxidation process with increasing XO led to a reduction of soot volume fraction. On the contrary, the surface growth rates increased with XO, resulting in an increase in soot mass concentration. These competing effects led to the non-monotonic variation of soot volume fractions with XO in SFO flames. On the other hand, in SF flames both the inception and surface growth of soot increased as XO increased, resulting in the observed monotonic relationship between soot volume fraction and XO. We also analyzed the soot zone structures and made comparisons between SFO and SF flames.

Measurements of sooting limits in laminar premixed burner-stabilized stagnation ethylene, propane, and ethylene/toluene flames

The systematic investigation of the effect of two major experimental parameters (equivalence ratio and temperature) as well as fuel structure on the sooting limits of ethylene, propane, and ethylene/toluene flames was carried out in premixed burner-stabilized stagnation flames through qualitatively visual observations of flame yellow luminosity and quantitatively direct measurements of incipient soot particle size distributions by a scan mobility particle sizer. A strong temperature dependence of incipient soot formation was observed in the three flames. Detailed chemical kinetic modeling of the flame structure and the soot precursor chemistry was performed to explain the experimental observations. The results show that the incipient soot formation is prohibited in either low or high temperature flames. While the former is due to slow reactions induced by both low temperatures and small concentrations of soot precursors, the latter is attributed to the thermodynamic reversibility of polycyclic aromatic hydrocarbons (PAHs) at high temperatures. Sooting limits have been found to be not a function of concentrations of PAHs alone at a fixed flame temperature; sooting propensity generally increases with increasing fuel carbon/hydrogen atom ratio, presenting the following order: propane < ethylene < ethylene/toluene.

Reducing Diesel Engine Drive Cycle Fuel Consumption through Use of Cylinder Deactivation to Maintain Aftertreatment Component Temperature during Idle and Low Load Operating Conditions

Modern on-road diesel engine systems incorporate flexible fuel injection, variable geometry turbocharging, high pressure exhaust gas recirculation, oxidation catalysts, particulate filters and selective catalytic reduction systems in order to comply with strict tailpipe-out NOx and soot limits. Fuel consuming strategies, including late injections and turbine-based engine exhaust throttling are typically used to increase turbine outlet temperature and flow rate in order to reach the aftertreatment component temperatures required for efficient reduction of NOx and soot. The same strategies are used at low load operating conditions to maintain aftertreatment temperatures. This paper demonstrates that cylinder deactivation (CDA) can be used to maintain aftertreatment temperatures in a more fuel efficient manner through reductions in airflow and pumping work. The incorporation of CDA to maintain desired aftertreatment temperatures during idle conditions is experimentally demonstrated to result in fuel savings of 3.0% over the HD-FTP drive cycle. Implementation of CDA at non-idle portions of the HD-FTP where BMEP is below 3 bar is demonstrated to reduce fuel consumption further by an additional 0.4%, thereby resulting in 3.4% fuel savings over the drive cycle.

Experimental and numerical investigation of methane thermal partial oxidation in a small-scale porous media reformer

This study deals with the topic of synthesis gas (syngas) production from preheated, rich methane/air mixtures. The examined process is based on non-catalytic partial oxidation within a small-scale porous media based reformer, intended for application in Solid Oxide Fuel Cell (SOFC) based systems. For this purpose, process characteristics like temperature profiles within the porous material and exhaust syngas compositions were experimentally and numerically investigated under conditions that can be encountered in such systems. The soot content of the generated syngas was also measured using the technique of Scanning Mobility Particle Sizing. An important feature of the reformer, which was demonstrated during the experiments for a wide range of thermal loads (380–1895 kW/m2) and equivalence ratios (1.9–2.6), is the ability to operate based on stationary flames. This is achieved using a two-section design. The sections show a conical and a cylindrical geometry, whereas the same porous medium is installed in both of them. For this study, the solid matrix was created as packed bed of Al2O3-Raschig rings (62% open porosity). The process was simulated with a quasi-1D numerical model, which uses a volume-averaged approach. The model solves both the gas- and solid-phase energy balances explicitly and accounts for the radiative heat transport in the solid-phase. Peak temperatures measured within the porous zone provide evidence of superadiabatic combustion, which is also confirmed by the numerically predicted temperature profiles with the model. Syngas compositions reveal a maximum reforming efficiency of 65% based on H2 and CO, while the soot limit of the process was found to lie at φ = 2.2, regardless of thermal load and preheat temperature of the fresh mixture.

Formation of Soot in Counterflow Diffusion Flames with Carbon Dioxide Dilution

ABSTRACTExperimental and numerical modeling studies have been performed to investigate the effect of CO2 dilution on soot formation in ethylene counterflow diffusion flames. Thermal and chemical effects of CO2 addition on soot growth was numerically identified by using a fictitious CO2 species, which was treated as inert in terms of chemical reactions. The results showed that CO2 addition reduces soot formation both thermodynamically and chemically. In terms of chemical effect, the addition of CO2 decreases soot formation through various pathways, including: (1) reduced soot precursor (PAH) formation leading to lower inception rates and soot number density, which in turn results in lower surface area for soot mass addition; (2) reduced H, CH3, and C3H3 concentrations causing lower H abstraction rate and therefore less active site per surface area for soot growth; and (3) reduced C2H2 mole fraction and thus a slower C2H2 mass addition rate. In addition, the sooting limits were also measured for ethylene counterflow flames in both N2 and CO2 atmosphere and the results showed that sooting region was significantly reduced in the CO2 case compared to the N2 case.