Dieter Brüggemann

Scalar and joint scalar-velocity-frequency Monte Carlo PDF simulation of supersonic combustion

Approaches based on probability density functions (PDF) are a natural choice for the simulation of high-speed and supersonic turbulent reacting flows due to their ability to represent chemical sources in closed form. This paper aims in particular at the application of scalar and joint scalar-velocity-turbulent frequency PDF methods to supersonic combustion. Because transport equation PDF simulations of supersonic combustion are still rare, emphasis is placed on the peculiarities of this kind of flow. The paper reports detailed results for two supersonic hydrogen-air non-premixed flames and makes comparison with available experimental data. A reaction scheme consisting of seven elementary reactions is considered, which provides an adequate description of chemical kinetics. As mixing models still are the weakest point of transport equation PDF methods, the three most frequently applied formulations have been investigated. Only minor differences are observed for the supersonic test cases. Moreover, these simulations represent the first published application of joint scalar-velocity-turbulent frequency PDF calculations to supersonic flows with complex geometry and hydrogen chemistry. It is shown that significant improvements are achieved by the joint PDF approach for one of two high speed reacting flows investigated. Results are somewhat unconclusive in the other case, partly due to uncertainties in the experimental setup.

Comparison of Eulerian and Lagrangian Monte Carlo PDF methods for turbulent diffusion flames

Methods based on Monte Carlo solutions of the transport equation for the joint probability density function (pdf) of scalars are considered to be among the most promising approaches for modeling turbulent reactive flows. For the treatment of convection and diffusion two approaches are well established, either from Eulerian or Lagrangian point of view. Simply speaking, the former is computationally less expensive but also less accurate than the latter one. In this paper we provide an extension of a previous analysis of numerical diffusion of Eulerian schemes to two-dimensional flows in order to shed light on the shortcomings of this approach. For convergence acceleration, local time stepping is extended to Lagrangian pdf methods. Furthermore, the adaption of the Continuous Mixing Model to unequal particle weights is presented. The differences between the Eulerian and Lagrangian approaches are analyzed by application to simple test problems and by simulation of a supersonic hydrogen-air diffusion flame using a chemical scheme consisting of seven elementary reactions.

MULTIGRID CONVERGENCE ACCELERATION FOR TURBULENT SUPERSONIC FLOWS

A multigrid convergence acceleration technique has been developed for solving both the Navier-Stokes and turbulence transport equations. For turbulence closure a low-Reynolds-number q-ω turbulence model is employed. To enable convergence, the stiff non-linear turbulent source terms have to be treated in a special way. Further modifications to standard multigrid methods are necessary for the resolution of shock waves in supersonic flows. An implicit LU algorithm is used for numerical time integration. Several ramped duct test cases are presented to demonstrate the improvements in performance of the numerical scheme. Cases with strong shock waves and separation are included. It is shown to be very effective to treat fluid and turbulence equations with the multigrid method.

Conductometric soot sensor for automotive exhausts: initial studies.

In order to reduce the tailpipe particulate matter emissions of Diesel engines, Diesel particulate filters (DPFs) are commonly used. Initial studies using a conductometric soot sensor to monitor their filtering efficiency, i.e., to detect a malfunction of the DPF, are presented. The sensors consist of a planar substrate equipped with electrodes on one side and with a heater on the other. It is shown that at constant speed-load points, the time until soot percolation occurs or the resistance itself are reproducible means that can be well correlated with the filtering efficiency of a DPF. It is suggested to use such a sensor setup for the detection of a DPF malfunction.

In-Operation Monitoring of the Soot Load of Diesel Particulate Filters: Initial Tests

Diesel particulate filters (DPF) are indispensable parts of modern automotive exhaust gas aftertreatment systems due to the stringent emissions legislation. For a fuel-efficient control strategy, it would be beneficial to determine directly and in-operation their actual trapped soot mass. Two novel approaches—based on the electrical conductivity of trapped soot particles—emerged recently. By measuring the electrical resistance between different single walls inside the filter, the soot load is determined with local resolution. The microwave-based technique is a contactless approach that gives an integral value depending on the soot mass in the DPF. We present investigations on loading and regeneration of DPFs in a dynamometer test bench applying both methods. The results are compared with each other and correlated with the differential pressure and the soot mass. Especially the microwave-based technique has a potential for serial application.

Laser-induced incandescence and Raman measurements in sooting methane and ethylene flames

Laser-induced incandescence (LII) of soot particles is modeled using two non-linear coupled differential equations deduced from the energy- and mass-balance of the process. Experimental data of laminar sooting diffusion flames are taken and the model function is fitted to the individual data curves of every pixel in the two-dimensional flame image. The results of the least squares fit are maps of the soot particle radii and number densities in the flame. Methane and ethylene flames are examined and the results are compared with former measurements. The quality of this fitting algorithm is compared to the reproduction of the data curves by a conventional data evaluation method. The fitting routine for the methane flame is improved by providing local temperature data obtained by Raman measurements. Results for this flame are compared with the local gas concentrations.

Experimental Studies on the Influence of Diesel Engine Operating Parameters on Properties of Emitted Soot Particles

Physical and chemical properties of emitted soot particles are influenced by diesel engine operating parameters. In order to find correlations between the in-cylinder processes of combustion and the engine-out properties of soot particles, parameter studies were carried out on a modern light duty production diesel engine (turbo direct injection, TDI) and an optically accessed single cylinder diesel engine (single cylinder). Several techniques have been combined to analyze the combustion process as well as the emitted particles. The size of emitted soot particles of the TDI engine has been determined by a scanning mobility particle sizer (SMPS), and soot samples have been analyzed by a high-resolution transmission electron microscope (HR-TEM) and by thermogravimetry (TG). In addition, the internal combustion in an optically accessed single cylinder engine has been observed by time resolved spectroscopy as well as OH*- and C2-imaging. It turns out that in both cases, increasing injection pressure leads to a decreasing soot particle agglomerate and primary particle size with decreasing oxidation temperature.

Test of an optical parametric oscillator (OPO) as a compact and fast tunable stokes source in coherent anti-stokes raman spectroscopy (CARS)

An optical parametric oscillator (OPO), as a novel kind of broadband Stokes source, is employed for coherent anti-Stokes Raman spectroscopy (CARS). Compared to the conventional dye laser configuration OPO-CARS offers practical advantages. The tunable OPO allows a fast and comfortable frequency tuning. The excitation bandwidth of about 35 cm−1 (FWHM) limits the spectral range of effective and stable single pulse CARS generation but can be used to enhance selected spectral structures.

Matrix dissipation for central difference schemes with combustion

The effect of artificial viscosity is investigated for problems related to supersonic combustion. An implicit lower-upper symmetric Gauss-Seidel finite volume method is employed for solving the full, compressible, two-dimensional averaged Navier-Stokes and species transport equations. For the right-hand side discretization central differences are used. Therefore some kind of artificial viscosity is necessary to reduce oscillations near shock waves and to enable convergence to machine accuracy. In comparison to the standard second- and fourth-order scalar dissipation a matrix dissipation reduces the amount of artificial viscosity by scaling each equation individually. For the necessary absolute flux Jacobian matrix a new decomposition is presented keeping the additional cost moderate. With an appropriate sensor the scheme also gets total variation diminishing properties. Calculations using different dissipation models are presented and advantages using a matrix dissipation are shown.

Fuel vapor measurements by linear Raman spectroscopy using spectral discrimination from droplet interferences

Vapor-phase measurements by linear Raman spectroscopy are performed in the vicinity of methanol droplets. Several types of interference by these droplets are identified and removed by appropriate filtering. This procedure, together with the phase-dependent spectral shift of the OH stretching vibration frequency, is proved to permit single-pulse linear Raman measurements of methanol vapor and nitrogen on a line with coexisting droplets. Laser-induced droplet breakdown is found to limit the applicable laser irradiance to approximately 2 GW/cm(2) and is avoided by use of a flash-lamp-pumped dye laser with high energy (1-7 J) and long pulses (1.5 micross).