Showing papers by "Ulrich Maas published in 1995"

Coupling of chemical reaction with flow and molecular transport

Abstract: During the last years the interest in the numerical simulation of reacting flows has grown considerably. Numerical methods are available, which allow to couple chemical kinetics with flow and molecular transport. However, the use of detailed physical and chemical models, involving more than 100 chemical species, and thus more than 100 species conservation equations, is restricted to very simple flow configurations like one-dimensional systems or two-dimensional systems with very simple geometries, and models are required, which simplify chemistry without sacrificing accuracy. In many chemically reacting flows chemical processes occur with time scales differing by many orders of magnitude (e.g., 10$^{-10}$ s to 1 s in combustion processes), whereas the time scales of flow, molecular transport, and turbulence usually cover a much smaller range of time scales. Based on local time scale analyses it is possible to decouple the fast (and thus not rate limiting) chemical processes. In this way the chemistry can be described in terms of a small number of governing reaction progress variables, and computations of complex reacting flow problems become possible. Examples for calculations with detailed and simplified chemistry are shown for various reacting flows, such as hypersonic reacting flows or combustion processes.

Simulation of the behavior of rich hydrogen-air flames near the flammability limit

Abstract: This paper presents an investigation of the behavior of rich hydrogen-air flames near the flammability limit. Ignition processes arc simulated using a computational model which involves the solution of the governing equations (for one-dimensional geometries) by implicit methods on an adaptive non-uniform grid, using detailed chemistry and a multi-species transport model. The reaction mechanism consists of 37 elementary reactions and 9 species. Calculations have been performed for different one-dimensional geometries. An investigation of the fundamental flammability limit {intrinsic to the combustion system itself), which is governed only by the physical and chemical processes in the gaseous mixture, is carried out by eliminating external factors such as heat loss, buoyancy, etc, in the calculations. Compulations have been performed for hydrogen-air mixtures of varying hydrogen content. Mixtures containing 75% or less hydrogen are found to be steadily propagating, whereas mixtures containing 82% o...

Simulation and sensitivity analysis of the heterogeneous oxidation of methane on a platinum foil

Abstract: The heterogeneous oxidation of methane–air mixtures in a stagnation point flow onto a platinum foil is investigated numerically and results are compared with experiments. The analysis includes detailed reaction mechanisms for the gas phase as well as the surface. Gas‐phase transport and its coupling to the surface is described using detailed multicomponent models. The heterogeneous ignition, extinction, and autothermal behavior are interpreted in terms of elementary steps at the gas–surface interface. A sensitivity analysis of the surface reactions is performed to identify crucial steps in the mechanism, thus pointing to those reactions for which better kinetic and thermodynamic data are needed urgently.

Experimental and Modeling Studies on the Ignition of CH3OH/O2-Mixtures with a CO2-Laser System

Abstract: Detailed experimental studies on the C02-laser-induced ignition of CH3OH/02-mixtures in a quartz reactor are performed to supply quantitative data for direct comparison with the numerical results of a mathematical model for ignition processes in one-dimensional geometries. The CH3OH/02-mixtures are ignited by a cw C02-laser operated in pulsed mode. Hydroxyl radicals produced during combustion are excited by a tunable KrFexcimer laser in the OH(A-X), (3-0) band at 248 nm, and subsequent 2D-fluorescence is detected with a gated image-intensified CCD-camera. Fluorescence from lower-lying, non-predissociative vibrational levels of OH are effectively suppressed by narrow spectral filtering of the (3-2)-band of OH. In order to simulate the ignition process, a one-dimensional model has been used, which is based on a detailed reaction mechanism consisting of 22 species and 173 reactions as well as a multicomponent transport model. The use of a one-dimensional model allows detailed parametrical studies, since the computational effort is within reasonable limits. Minimum ignition energies are determined and presented together with the simulated values. Temporally and spatially resolved measurements of flame position and OH concentration are presented for different conditions and compared directly to the computational results.

Simulation of reacting flows with a portable parallel code using dynamic load balancing

Abstract: The prediction of pollutant formation in turbulent flames requires a direct resolution of the Navier-Stokes equations using several hundred reactive species. The needed computer resources cannot be found any more on classical vector supercomputers. It is therefore necessary to write codes for parallel computers, which should offer in a near future a much higher computing power. But success can only be achieved if the difficult problems of portability and of load-balancing between the different nodes are taken into account. In this paper, we describe the development of such a parallel Navier-Stokes solver, and we show the first applications of this code concerning the investigation of flame structure.