Markus Kraft

Measurement and numerical simulation of soot particle size distribution functions in a laminar premixed ethylene-oxygen-argon flame

Abstract Spatially resolved measurement of the soot particle size distribution function (PSDF) was made in a laminar premixed ethylene-argon-oxygen flame (φ = 2.07) using a scanning mobility particle sizer. The emphasis of the study was to follow the evolution of the PSDF from the onset of particle inception to particle mass growth. At the onset of soot inception, the PSDF was found to follow a power-law dependence on particle diameter. The PSDF becomes bimodal at larger height above the burner surface, and remains bimodal throughout the flame. Numerical simulation using a kinetic model proposed previously and a stochastic approach to solve aerosol dynamics equations again showed a bimodal PSDF. Further analysis revealed that bimodality is intrinsic to an aerosol process involving particle-particle coagulation and particle nucleation dominated by monomer dimerization.

A stochastic approach to calculate the particle size distribution function of soot particles in laminar premixed flames

We introduce an efficient stochastic approach to solve the population balance equation that describes the formation and oxidation of soot particles in a laminar premixed flame. The approach is based on a stochastic particle system representing the ensemble of soot particles. The processes contributing to the formation and oxidation of soot particles are treated in a probabilistic manner. The stochastic algorithm, which makes use of an efficient majorant kernel and the method of fictitious jumps, resolves the entire soot particle distribution (PSDF) without introducing additional closure assumptions. A fuel-rich laminar premixed acetylene flame is computed using a detailed kinetic soot model. Solutions are obtained for both, the stochastic approach and the method of moments combined with a modified version of the Premix, CHEMKIN code. In this manner, the accuracy of the method of moments in a laminar premixed flame simulation is investigated. It is found that the accuracy for the first moment is excellent (5% error), and mean error for rest of the moments is within 25%. Also the effect of the oxidation of the smallest particles (burnout) has been quantified but was found not to be important in the flame investigated. The time evolution of computed size distributions and their integral properties are compared to experimental measurements and the agreement was found to be satisfactory. Finally, the efficiency of the stochastic method is studied.

Nickel Nanoparticles Encapsulated in Few-Layer Nitrogen-Doped Graphene Derived from Metal–Organic Frameworks as Efficient Bifunctional Electrocatalysts for Overall Water Splitting

Nickel nanoparticles encapsulated in few-layer nitrogen-doped graphene (Ni@NC) are synthesized by using a Ni-based metal-organic framework as the precursor for high-temperature annealing treatment. The resulting Ni@NC materials exhibit highly efficient and ultrastable electrocatalytic activity toward the hydrogen evolution reaction and the oxygen evolution reaction as well as overall water splitting in alkaline environment.

Investigation of combustion emissions in a homogeneous charge compression injection engine: Measurements and a new computational model

The CO and hydrocarbon emissions of a homogeneous charge compression injection engine have been explained by inhomogeneities in temperature induced by the boundary layer and crevices according to a stochastic reactor model. The boundary layer is assumed to consist of a thin film (laminar sublayer) and a turbulent buffer layer. The heat loss through the cylinder wall leads to a significant temperature gradient in the boundary layer. The partially stirred plug flow reactor (PaSPFR) model, a stochastic reactor model (SRM), has been used to model turbulent mixing between the boundary layer, crevices, and the turbulent core and to account for the chemical reactions within the combustion chamber. The combustion of natural gas in the engine is described by a detailed chemical mechanism that is incorporated in the SRM. Molecular diffusion induced by turbulent mixing is described by the simple interaction by exchange with the mean (IEM) mixing model. The turbulent mixing intensity that describes the decay of the species and temperature fluctuations is estimated from measurements of the velocity fluctuations and the integral length scale of the turbulent flow in the engine. Pressure, CO emissions, and unburned hydrocarbons are also measured. Comparison between the mean quantities obtained from the SRM and these measurements show very good agreement. It is demonstrated that the SRM clearly outperforms a previous PFR-based one-zone model. The PaSPFR-IEM model captures the pressure rise that could not be described exactly using a simple one-zone model. The emissions of CO and hydrocarbons are also predicted well. Scatter plots of the marginal probability density function of CO 2 and temperature reveal that the emissions of hydrocarbons and CO can be explained by stochastic particles that undergo incomplete combustion because they are trapped in the colder boundary layer or in the crevices.

SURFACE CHEMISTRY AND PARTICLE SHAPE: PROCESSES FOR THE EVOLUTION OF AEROSOLS IN TITAN'S ATMOSPHERE

We use a stochastic approach in order to investigate the production and evolution of aerosols in Titan’s atmosphere. The simulation initiates from the benzene molecules observed in the thermosphere and follows their evolution to larger aromatic structures through reaction with gas-phase radical species. Aromatics are allowed to collide and provide the first primary particles, which further grow to aggregates through coagulation. We also consider for the first time the contribution of heterogenous processes at the surface of the particles, which are described by the deposition of the formed aromatic structures on the surface of the particles, and also through the chemical reaction with radical species. Our results demonstrate that the evolution of aerosols in terms of size, shape, and density is a result of competing processes between surface growth, coagulation, and sedimentation. Furthermore, our simulations clearly demonstrate the presence of a spherical growth region in the upper atmosphere followed by a transition to an aggregate growth region below. The transition altitude ranges between 500 and 600 km based on the parameters of the simulation.

Homogeneous Charge Compression Ignition Engine: A Simulation Study on the Effects of Inhomogeneities

A stochastic model for the HCCI engine is presented The model is based on the PaSPFR-IEM model and accounts for inhomogeneities in the combustion chamber while including a detailed chemical model for natural gas combustion, consisting of 53 chemical species and 590 elementary chemical reactions. The model is able to take any type of inhomogeneities in the initial gas composition into account, such as inhomogeneities in the temperature field, in the air-fuel ratio or in the concentration of the recirculated exhaust gas. With this model the effect of temperature differences caused by the thermal boundary layer and crevices in the cylinder for a particular engine speed and fuel to air ratio is studied. The boundary layer is divided into a viscous sublayer and a turbulent buffer zone. There are also colder zones due to crevices. All zones are modeled by a characteristic temperature distribution. The simulation results are compared with experiments and a previous numerical study employing a PFR model. In all cases the PaSPFR-IEM model leads to a better agreement between simulations and experiment for temperature and pressure. In addition a sensitivity study on the effect of different intensities of turbulent mixing on the combustion is performed. This study reveals that the ignition delay is a Junction of turbulent mixing of the hot bulk and the colder boundary layer. (Less)

The Linear Process Deferment Algorithm: A new technique for solving population balance equations

In this paper a new stochastic algorithm for the solution of population balance equations is presented. The population balance equations have the form of extended Smoluchowski equations which include linear and source terms. The new algorithm, called the linear process deferment algorithm (LPDA), is used for solving a detailed model describing the formation of soot in premixed laminar flames. A measure theoretic formulation of a stochastic jump process is developed and the corresponding generator presented. The numerical properties of the algorithm are studied in detail and compared to the direct simulation algorithm and various splitting techniques. LPDA is designed for all kinds of population balance problems where nonlinear processes cannot be neglected but are dominated in rate by linear ones. In those cases the LPDA is seen to reduce run times for a population balance simulation by a factor of up to 1000 with a negligible loss of accuracy.

ANALYSIS OF A NATURAL GAS FUELLED HOMOGENEOUS CHARGE COMPRESSION IGNITION ENGINE WITH EXHAUST GAS RECIRCULATION USING A STOCHASTIC REACTOR MODEL

Abstract Combustion and emissions formation in a Volvo TD 100 series diesel engine running in a homogeneous charge compression ignition (HCCI) mode and fuelled with natural gas is simulated and compared with measurements for both with and without external exhaust gas recirculation (EGR). A new stochastic approach is introduced to model the convective heat transfer, which accounts for fluctuations and fluid-wall interaction effects. This model is included in a partially stirred plug flow reactor (PaSPFR) approach, a stochastic reactor model (SRM), and is applied to study the effect of EGR on pressure, autoignition timing and emissions of CO and unburned hydrocarbons (HCs). The model accounts for temperature inhomogeneities and includes a detailed chemical mechanism to simulate the chemical reactions within the combustion chamber. Turbulent mixing is described by the interaction by exchange with the mean (IEM) model. A Monte Carlo method with a second-order time-splitting technique is employed to obtain the numerical solution. The model is validated by comparing the simulated in-cylinder pressure history and emissions with measurements taken from Christensen and Johansson (SAE Paper 982454). Excellent agreement is obtained between the peak pressure, ignition timing and CO and HC emissions predicted by the model and those obtained from the measurements for the non-EGR, 38 per cent EGR and 47 per cent EGR cases. A comparison between the pressure profiles for the cases studied reveals that the ignition timing and the peak pressure are dependent on the EGR. With EGR, the peak pressure reduces and the autoignition is delayed. The trend observed in the measured emissions with varying EGR is also predicted correctly by the model.

Evaluating the EGR-AFR Operating Range of a HCCI Engine

We present a computational tool to develop an exhaust gas recirculation (EGR) - air-fuel ratio (AFR) operating range for homogeneous charge compression ignition (HCCI) engines. A single- cylinder Ricardo E-6 engine running in HCCI mode, with external EGR is simulated using an improved probability density function (PDF)-based engine cycle model. For a base case, the in-cylinder temperature and unburned hydrocarbon emissions predicted by the model show a satisfactory agreement with measurements. Furthermore, the model is applied to develop the operating range for various combustion parameters, emissions and engine parameters with respect to the air-fuel ratio and the amount of EGR used. The model predictions agree reasonably well with the experimental results for various parameters over the entire EGR-AFR operating range thus proving the robustness of the PDF based model. The boundaries of the operating range namely, knocking, partial burn, and misfire are reliably predicted by the model. In particular, the model provides a useful insight into the misfire phenomenon by depicting the cyclic variation in the ignition timing and the in-cylinder temperature profiles. Finally, we investigate two control options, namely heating intake charge and trapping residual burned fraction by negative valve overlap. The effect of these two methods on HCCI combustion and CO, HC and NOdx emissions is studied. (Less)

Detailed Modeling of soot formation in a partially stirred plug flow reactor

The purpose of this work is to propose a detailed model for the formation of soot in turbulent reacting flow and to use this model to study a carbon black furnace. The model is based on a combination of a detailed reaction mechanism to calculate the gas phase chemistry, a detailed kinetic soot model based on the method of moments, and the joint composition probability density function (PDF) of these scalar quantities. Two problems, which arise when modeling the formation of soot in turbulent flows using a PDF approach, are studied. A consistency study of the combined scalar-soot moment approach reveals that the molecular diffusion term in the PDF-equation can be closed by the IEM and Curl-type mixing models. An investigation of different kernels for the collision frequency of soot particles shows that the influence of turbulence on particle coagulation is negligible for typical flame conditions and the particle size range considered. The model is used as a simple toot to simulate a furnace black process, which is the most important industrial process for the production of carbon blacks. Despite the simplifications in the modeling of the turbulent flow reasonable agreement between the calculated soot yield and data measured in an industrial furnace black reactor is achieved although no adjustments were made to the kinetic parameters of the soot model. The effect of the mixing intensity on soot yield and different soot formation rates is investigated. In addition the influence of different operating conditions such as temperature and equivalence ratio in the primary zone of the reactor is studied. (Less)