W. K. Bushe

An aerosol model to predict size and structure of soot particles

An aerosol model to simulate soot formation and growth was developed using moving- and fixed-sectional methods. The new model is composed of a set of subroutines that can be easily combined with the Chemkin package. Using the model, we have simulated soot formation and growth in plug flow reactors. Our model was compared with a previously published method of moments model for a simulation of the plasma pyrolysis of methane in a plug flow reactor. Inclusion of the transition correction factor for the condensation coefficient led to the prediction of a smaller condensation rate compared with the method of moments model. The average coagulation rate calculated by the sectional model was much higher than that by the method of moments model for a broad particle size distribution. The two models predicted significantly different soot precursor concentration and rates of aerosol processes, but substantially similar particle mass and number for the pyrolysis process. We have also simulated soot formation and growth in a jet-stirred/plug flow reactor (JSR/PFR) system for which soot size distribution measurements are available in the literature. It is shown that the adjusted-point fixed-sectional method can provide comparable accuracy to the moving-sectional model in a simulation of soot formation and growth. It is also shown that the measured surface growth rate could be much higher than the value used in this study. Soot mass concentrations and size distributions for particles larger than 10 nm were well predicted with a surface reaction enhancement. The primary particle size was underpredicted by only about 30% compared with the measurements, without any model adjustments. As the new model can predict both the particle size distribution and structure, and is suitable for application in complex flows, its application to diverse soot formation conditions will enhance our knowledge on the evolution of soot structures.

Simulation of transient turbulent methane jet ignition and combustion under engine-relevant conditions using conditional source-term estimation with detailed chemistry

The ignition and combustion processes of transient turbulent methane jets under high-pressure and moderate temperature conditions were simulated using a computationally efficient combustion model. Closure for the mean chemical source-terms was obtained with Conditional Source-term Estimation (CSE) using first conditional moment closure in conjunction with a detailed chemical kinetic mechanism, which was reduced to a Trajectory-Generated Low-Dimensional Manifold (TGLDM). The accuracy of the manifold was first validated against the direct integral method by comparing the predicted reactive scalar profiles in three methane–air reaction systems: a laminar premixed flame, a laminar flamelet and a perfectly stirred reactor. Detailed CFD simulations incorporating the CSE-TGLDM model were able to provide reasonably good predictions of the experimental ignition delay and initial ignition kernel locations of the methane jets reported in the literature with relatively low computational cost. Nitrogen oxides formed in the methane jet flame were found to be underpredicted by the model by as much as a factor of 2. The discrepancy may be attributable to the inability of the simulation to account for the effects of the rarefaction wave in the shock-tube experiments.

Presumed PDF modeling for RANS simulation of turbulent premixed flames

In this work, a turbulent premixed Bunsen flame is simulated using a RANS approach for turbulence and a flamelet model for turbulence–chemistry interactions. In this flamelet model, the mean reaction rates are approximated using a progress variable approach and a Flame Prolongation of ILDM (FPI) for chemistry reduction. This method requires a presumption for the shape of the probability density function of the reaction progress variable. Two shapes have been examined: a widely used β-function and a modified laminar flamelet PDF. Radial distributions of the calculated temperature field, axial velocity and chemical species mass fraction have been compared with experimental data. This comparison shows that using the modified laminar flamelet PDF leads to predictions that are similar, and often superior to those obtained using the β-PDF. Given that the new PDF is based on the actual chemistry – as opposed to the ad hoc nature of the β-PDF – these results suggest that it is a better choice for the statistical description of the reaction progress variable in a highly strained turbulent field.

Combustion in a heavy-duty direct-injection engine using hydrogen—methane blend fuels

Abstract Adding hydrogen to the fuel in a direct injection natural gas engine offers the potential significantly to reduce local and global air pollutant emissions. This work reports on the effects of fuelling a heavy-duty engine with late-cycle direct injection of blended hydrogen—methane fuels and diesel pilot ignition over a range of engine operating conditions. The effect of hydrogen on the combustion event varies with operating condition, providing insight into the fundamental factors limiting the combustion process. Combustion stability is enhanced at all conditions studied; this leads directly to a significant reduction in emissions of combustion byproducts, including carbon monoxide, particulate matter, and unburned fuel. Carbon dioxide emissions are also significantly reduced by the lower carbon—energy ratio of the fuel. The results suggest that this technique can significantly reduce both local and global pollutant emissions associated with heavy-duty transport applications while requiring minimal changes to the fuelling system.

Injection Parameter Effects on a Direct Injected, Pilot Ignited, Heavy Duty Natural Gas Engine with EGR

Pilot-ignited direct injection of natural gas fuelling of a heavy-duty compression ignition engine while using recirculated exhaust gas (EGR) has been shown to significantly reduce NO x emissions. To further investigate emissions reductions, the combustion timing, injection pressure, and relative delay between the pilot and main fuel injections were varied over a range of EGR fractions while engine speed, net charge mass, and oxygen equivalence ratio were held constant. PM emissions were reduced by higher injection pressures without significantly affecting NO x at all EGR conditions. By delaying the combustion, NO x was reduced at the expense of increased PM for a given EGR fraction. By reducing the delay between the pilot and main fuel injections at high EGR, PM emissions were substantially reduced at the expense of increased total hydrocarbon (tHC) emissions. In this research, no attempt was made to optimize the injector or combustion chamber for natural gas fuelling with EGR.

Application of the conditional source-term estimation model for turbulence–chemistry interactions in a premixed flame

Conditional Source-term Estimation (CSE) is a closure model for turbulence–chemistry interactions. This model uses the first-order CMC hypothesis to close the chemical reaction source terms. The conditional scalar field is estimated by solving an integral equation using inverse methods. It was originally developed and has been used extensively in non-premixed combustion. This work is the first application of this combustion model for a premixed flame. CSE is coupled with a Trajectory Generated Low-Dimensional Manifold (TGLDM) model for chemistry. The CSE-TGLDM combustion model is used in a RANS code to simulate a turbulent premixed Bunsen burner. Along with this combustion model, a similar model which relies on the flamelet assumption is also used for comparison. The results of these two approaches in the prediction of the velocity field, temperature and species mass fractions are compared together. Although the flamelet model is less computationally expensive, the CSE combustion model is more general and does not have the limiting assumption underlying the flamelet model.

Source Apportionment of Particulate Matter from a Diesel Pilot-Ignited Natural Gas Fuelled Heavy Duty DI Engine

In recent years there has been a growing awareness that particulate matter, especially fine diesel particulate, is a health concern. This has stimulated research to develop new technologies to reduce particulate emissions without increasing nitrogen oxide (NO x ) emissions or fuel consumption. Westport Innovations has developed a technology involving high pressure direct injection and combustion of natural gas for medium and heavy-duty engine platforms. At practical compression ratios, the natural gas will not auto-ignite, so a diesel pilot injection is used for ignition. Thus, the soot emissions can have contributions from the combustion of natural gas, diesel pilot, or lubricating oil. While the soot emissions with natural gas as the main fuel are significantly lower than in a conventional diesel engine, it remains important to determine where the soot is coming from to aid in emission reduction strategies. In this study, the contribution of the pilot fuel (a biodiesel blend with higher 14 C content than diesel fuel) was determined using accelerator mass spectrometry (AMS) measurements of 14 C in the exhaust particulate. Results indicate that the pilot fuel contribution to soot ranges from 4-40% over the tested operating conditions; correspondingly, the contribution by natural gas and lubricating oil combined ranges from 60-96%. The highest fraction of soot from the pilot source is at low load without exhaust gas recirculation. The lowest fraction of soot from the pilot source is at high load with exhaust gas recirculation, i.e. the conditions contributing most to mode-averaged emissions.

Effect of operating condition on particulate matter and nitrogen oxides emissions from a heavy-duty direct injection natural gas engine using cooled exhaust gas recirculation

Abstract Two methods for reducing nitrogen oxides (NOX) emissions from direct injection, compression ignition, heavy-duty engines are exhaust gas recirculation (EGR) and the high-pressure direct injection of natural gas. Tests combining these two techniques were carried out on a single-cylinder research engine (SCRE) based on a modified heavy-duty automotive engine. No attempt was made to optimize the engines combustion chamber or the injector geometry for EGR operation. The SCREs independent charge-air system allowed for more controlled testing over a wider range of test variables than can be carried out by a standard engine. These tests investigated the effects of cooled EGR on particulate matter (PM) and NOX emissions while varying the injection timing, engine speed, equivalence ratio and intake manifold pressure. The results suggested that, with EGR, higher equivalence ratios reduced power-specific NOX but increased PM emissions. Increasing the charge mass at a constant EGR fraction resulted in significant reductions in PM, at the cost of slightly increased NOX By advancing the injection timing at high EGR fractions, PM emissions and fuel efficiency were improved, with only a slight increase in NOX emissions compared to the more retarded injection timings. The engine speed influenced the amount of EGR that could be recirculated, with lower speeds resulting in higher achievable EGR fractions. These results suggest that EGR fractions in excess of 20 per cent can achieve NOX reductions beyond 75 per cent, without causing unacceptable increases in PM emissions or significant reductions in fuel efficiency.

Hydrogen-methane blend fuelling of a heavy-duty, direct-injection engine

Combining hydrogen with natural gas as a fuel for internal combustion engines provides an early opportunity to introduce hydrogen into transportation applications. This study investigates the effects of fuelling a heavy-duty engine with a mixture of hydrogen and natural gas injected directly into the combustion chamber. The combustion system is unmodified from that developed for natural gas fuelling. The results demonstrate that hydrogen can have a significant beneficial effect in reducing emissions without affecting efficiency or requiring significant engine modifications. Combustion stability is enhanced through the higher reactivity of the hydrogen, resulting in reduced emissions of unburned methane. The fuel’s lower carbon-energy ratio also reduces CO2 emissions. These results combine to significantly reduce tailpipe greenhouse gas (GHG) emissions. However, the effect on net GHG’s, including both tailpipe and fuel-production emissions, depends on the source of the hydrogen. Cleaner sources, such as electrolysis based on renewables and hydro-electric power, generate a significant net reduction in GHG emissions. Hydrogen generated by steam-methane reforming is essentially GHG neutral, while electrolysis using electricity from fossil-fuel power plants significantly increases net GHG emissions compared to conventional natural gas fuelling.Copyright

Stochastic simulation of variations in the autoignition delay time of premixed methane and air

A mesoscopic stochastic particle model for homogeneous combustion is introduced. The model can be used to investigate the physical fluctuations in a system of coupled chemical reactions with energy (heat) release/consumption. In the mesoscopic model, the size of the homogeneous gas volume is an additional variable, which is eliminated in macroscopic continuum models by the thermodynamic limit N→∞. Thus, continuous homogeneous models are macroscopic models wherein fluctuations are excluded by definition. Fluctuations are known to be of particular importance for systems close to the autoignition limits. The new model is used to investigate the stochastic properties of the autoignition delay time in a homogeneous system with stoichiometric premixed methane and air. Temperature and species concentrations during autoignition of sub-macroscopic volumes, including physically meaningful fluctuations, are presented. It is found that different realizations mainly differ in the time when ignition occurs; besides this the development is similar. The mesoscopic range and the macroscopic limit are identified. Which range a specific system is assigned to is not only a question of the length scale or particle number, but also depends on the complete thermodynamic state. The stochastic algorithm yields the correct results for the macroscopic limit compared to the continuous balance equations. The sensitivity of the results to two different detailed reaction mechanisms (for the same system) is studied and found to be low. We show that when approaching the autoignition limit by decreasing the temperature, the fluctuations in the autoignition delay time increase and an increasing number of realizations will have exceedingly long ignition delay times, meaning they are in practice not autoignitable. With this result the mesoscopic simulations offer an explanation of the transition between autoignitable and non-autoignitable conditions. The calculated distributions were compared with ten repetitions of the same experiment. A mesoscopic distribution that matches the experimental results was found.