Showing papers by "Ulrich Maas published in 2015"

Ignition delay times of diethyl ether measured in a high-pressure shock tube and a rapid compression machine

Abstract: Ignition delay times of diethyl ether (DEE)/air/argon mixtures were studied in a shock tube in the temperature range from 900 to 1300 K at pressures of 10, 20, and 40 bar and in a rapid compression machine (RCM) at various equivalence ratios between 500 and 1060 K at pressures between 25 and 13 bar Between 25 and 55 bar, the RCM results show that ignition delay times of DEE exhibit a region (between 590 and 800 K) where ignition delay times are weakly temperature dependent only, while above 833 K and below 590 K, the ignition delay times are strongly temperature dependent Two-stage ignition was observed in the temperature range from 500 to 665 K in the RCM measurements At the conditions of the shock tube, a strong pressure and temperature dependence of the ignition delay times was observed, but no non-thermal (NTC) behavior was found in the investigated temperature range Simulations based on detailed chemistry using the mechanism of Yasunaga et al (2010) [15] indicate that at high pressures ignition delay times show a high sensitivity towards the two H-atom abstraction reactions by HO 2 from diethyl ether By increasing the rate coefficients of these two reactions relative to the original values by a factor of five, the mechanism well describes our measurements and still well reproduces the original data of Yasunaga et al (2010) [15]

Ignition by transient hot turbulent jets: An investigation of ignition mechanisms by means of a PDF/REDIM method

Abstract: Understanding the ignition of combustible mixtures by hot jets of burnt gases plays an important role in explosion protection. In this work a PDF method in conjunction with a reaction–diffusion manifold (REDIM) is used to investigate the ignition of a hydrogen/air mixture by a hot turbulent jet. In accordance with experimental results it is observed in numerical investigations that after an ignition delay time, the ignition is typically initiated at the jet head vortex. The scope of the current work is to investigate the mechanisms leading to ignition and explain the processes governing the ignition delay time as well as the ignition location. It is shown that macro- as well as micromixing and the chemical kinetics have a profound influence on the ignition process and that a realistic model for the ignition process has to account for all these processes in combination with a transient description of the jet penetration.

Validation of a novel numerical model for the electric currents in burner-stabilized methane–air flames

Abstract: This study presents measurements of electric currents in flat flames, induced by externally applied electric potentials. In addition to these measurements, a theoretical and numerical model for ionized methane–air flames was developed to predict the electric currents based on the charged particle distribution in the flame. Our model comprises Poisson’s equation and a multi-component diffusion model in order to incorporate an electric field in the existing CHEM1D combustion software. A comparison of the numerical simulations and experimental data showed a good agreement in the observed current–voltage characteristic for different electrode distances. The model also predicts the dependence of the saturation current on the equivalence ratio well for lean mixtures. Deviations were found in the rich regime, which are largely attributed to shortcomings in the chemical mechanism. For strong applied electric fields the electric current is independent of the applied field strength. This saturation effect is caused by the depletion of electrons from the flame plasma and a domination of the electric forces over Fick diffusion for the cations. According to the simulations, the diodic effect is mostly defined by the distance that the heavier and less mobile ions have to travel to reach the negatively charged electrode.

Prediction of NOx in premixed high-pressure lean methane flames with a MMC-partially stirred reactor

Abstract: The multiple mapping conditioning (MMC) method is used to extend the stochastic PDF approach to the flamelet regimes of premixed combustion. MMC permits for a desired degree of flamelet-like conditions while reflecting the fluctuating nature of turbulent flames and preserving the integrity and universality of the chosen mixing model. The model is implemented in the context of the partially stirred reactor (PaSR), which is therefore generalised to have a wider range of applicability. A stochastic formulation of original MMC is deployed, where mixing of particle scalar values is conditioned on a Markovian reference variable which emulates an implied particle position relative to a flame. The model interactions with the reference variable are controlled through the flamelet localness parameter, Λ, which is also related to the ratio of diffusive to convective time scales. The model is implemented in a Monte Carlo numerical scheme using detailed GRI3.0 chemical kinetics without adjustments of kinetic coefficients. Predictions of NOx emissions are validated against experimental data for a lean premixed high-pressure combustor in which reactions fall between the flamelet and distributed regimes. There is good agreement between model predictions and experimental data.

Hierarchical Structure of Slow Manifolds of Reacting Flows

Abstract: Nowadays the mathematical description of chemically reacting flows uses very often reaction mechanisms with far above hundred or even thousand chemical species (and, therefore, a large number of partial differential equations must be solved), which possibly react within more than a thousand of elementary reactions. These chemical kinetic processes cover time scales from nanoseconds to seconds. An analogous scaling problem arises for the length scales. Due to these scaling problems the detailed simulation of three-dimensional turbulent flows in practical systems is beyond the capacity of even today's super-computers. Using simplified sub-models is a way out of this problem. The question arising in mathematical modeling of reacting flows is then: How detailed, or down to which scale has each process to be resolved (chemical reaction, chemistry-turbulence-interaction, molecular transport processes) in order to allow a reliable description of the entire process. Both the chemical source term and the transport term have one important property, namely, they cause the existence of low-dimensional attractors in composition space. When these manifolds can be constructed (described) and parametrized by a small number of variables, it can be used to reformulate and reduce the mathematical description for modeling reacting flows. In this work the hierarchical nature of these low-dimensional manifolds of slow motions is discussed. It is demonstrated how this important feature of reacting flows is accounted for by the standard model reduction methods (like e.g. PEA and QSSA methods) as well as by recently developed concepts of model reduction. The use of the hierarchical nature for identification of the low-dimensional manifolds to devise hierarchical modeling concepts (e.g. for turbulent reacting flows) is additionally discussed.

A PDF projection method: A pressure algorithm for stand-alone transported PDFs

Abstract: In this paper, a new formulation of the projection approach is introduced for stand-alone probability density function (PDF) methods. The method is suitable for applications in low-Mach number transient turbulent reacting flows. The method is based on a fractional step method in which first the advection–diffusion–reaction equations are modelled and solved within a particle-based PDF method to predict an intermediate velocity field. Then the mean velocity field is projected onto a space where the continuity for the mean velocity is satisfied. In this approach, a Poisson equation is solved on the Eulerian grid to obtain the mean pressure field. Then the mean pressure is interpolated at the location of each stochastic Lagrangian particle. The formulation of the Poisson equation avoids the time derivatives of the density (due to convection) as well as second-order spatial derivatives. This in turn eliminates the major sources of instability in the presence of stochastic noise that are inherent in particle-ba...

A numerical approach to investigate the maximum permissible nozzle diameter in explosion by hot turbulent jets

Abstract: The ignition of a combustible environment by hot jets is a safety concern in many industries. In explosion protection concepts, for a protection of the type “flameproof enclosures” a maximum permissible gap is of major importance. In this work a numerical framework is described to investigate the ignition processes by a hot turbulent jet which flows out from such gaps. A Probability Density Function (PDF) method in conjunction with a reaction-diffusion manifold (REDIM) technique is used to model the turbulent reactive flow. In this paper the ignition of a stoichiometric mixture of hydrogen/air gas by a hot exhaust turbulent jet is examined. The impact of the nozzle diameter on the ignition delay time is investigated, too. The method is used to explore the maximum nozzle diameter for specific boundary conditions for which there is no ignition.