Matthias Hettel

Evaluation of Advanced Two-Phase Flow and Combustion Models for Predicting Low Emission Combustors

The development of low emission aero engine combustors strongly depends on the availability of accurate and efficient numerical models. The prediction of the interaction between two-phase flow and chemical combustion is one of the major objectives of the simulation of combustor flows. In this paper, predictions of a swirl stabilized model combustor are compared to experimental data. The computational method is based on an Eulerian two-phase model in conjunction with an Eddy Dissipation (ED) and a presumed-shapePDF (JPDF) combustion model. The combination of an Eulerian two-phase model with a JPDF combustion model is a novelty. It was found to give good agreement to the experimental data.

Low-NOx Combustor Development pursued within the scope of the Engine 3E German national research program in a cooperative effort among engine Manufacturer MTU, University of Karlsruhe and DLR German Aerospace Research Center

Abstract MTU Aero Engines, the University of Karlsruhe and the DLR Aerospace Research Centre co-operated within the scope of the German national aeronautical research program Engine 3E. The program was focused on improving high-bypass turbofan engines. As a part of this program, a low-emission single-annular combustor was developed. The NOx emissions of this combustor are significantly reduced by using the rich-lean combustion concept. The basic idea of this concept is to avoid stoichiometric combustion conditions by splitting the combustion domain into a fuel-rich zone (low-oxygen zone) and a fuel-lean (low temperature zone). The NOx reduction capability of a combustor of this type scales with the homogeneity of the mixture in the rich zone and the time interval needed for the transition from the rich to the lean zone. Based on the insights gained from this cooperative research, an annular combustor was developed and tested at pressures up to 20 bar and inlet temperatures up to 800 K. The tested annular combustor was found to have NOx emissions of about 40% of the ICAO 96 standard. The carbon monoxide and unburned hydrocarbon emissions of the combustor are of about the same levels as present state of the art combustors.

Numerical verification of the similarity laws for the formation of laminar vortex rings

From analytical investigations it is well known that the roll-up of an inviscid plane vortex sheet which separates at the edge of a body is a self-similar process which can be described by scaling laws. Unlike plane vortices, ring vortices have a curved rotational axis. For this special vortex type experimental investigations as well as calculations in the literature suggest that the scaling laws are only partially valid. The main goal of this work is to clarify how far these similarity or scaling laws are also valid for the formation of viscid laminar vortex rings. Therefore, the formation process of laminar vortex rings was investigated numerically using a CFD (computational-fluid-dynamics) code. The calculations refer to an experimental setup for which detailed experimental data are available in the literature. In this setup, laminar ring vortices are generated by ejecting water from a circular tube into a quiescent environment by means of a piston. First, a case based on a constant piston velocity was investigated. Comparing calculated and measured data yields a very good agreement. Further calculations were made when forcing the velocity of the piston by three different time-dependent functions. The results of these calculations show that the formation laws for inviscid plane vortices are also valid for the formation process of viscid ring vortices. This applies to the normalized axial and radial position of the vortex centre as well as the normalized diameter of the vortex spiral. However, the similarity laws are valid only if the process is considered in a special frame of reference which moves in conjunction with the front of the jet and if the starting time of the formation process with respect to the starting time of the ejection is taken into account. Additionally, the formation of a ring vortex, which occurs during the start-up process of a free jet flow, was calculated. The results confirm a dependence for the motion of the jet front, which is known from analytical considerations and allows some interesting features to be identified.

Chapter Two - Spatial Resolution of Species and Temperature Profiles in Catalytic Reactors: In Situ Sampling Techniques and CFD Modeling

Spatial resolution of species and temperature profiles can provide valuable information for understanding, design, and optimization of catalytic reactors. The combination of experimental investigation and CFD modeling does not only improve our knowledge but also helps to discover uncertainties and limitations of novel scientific techniques for an adequate interpretation of the observations. Two lab-scale reactor configurations with in situ capillary techniques are investigated experimentally and numerically for the resolution of spatial species and temperature profiles: the stagnation flow on a catalytically coated disc and the flow through a catalytically coated honeycomb monolith, in which CO is totally and CH4 is partially oxidized, respectively, over Rh/Al2O3 catalysts. CFD simulations reveal two significant items for the interpretation of the measured profiles: internal mass transport inside the catalyst in the stagnation flow reactor and the impact of the capillary probe in the honeycomb monolith.

Numerical Modelling of Technical Combustion

This contribution gives a short overview over modern numerical combustion modelling. Numerical simulation of combustion is a multi-scale problem, because the specific issues of fluid mechanics and chemical reaction systems accumulate. There exist a large variety of combustion models for different flame types, which are more or less universal. For some turbulent reacting flows, existing methodologies are acceptably accurate, and have justifiable computational cost. Depending on the expected answers of numerical simulation, substantial advances are required and have to be worked out.

Two-Zone Fluidized Bed Reactors for Butadiene Production: A Multiphysical Approach with Solver Coupling for Supercomputing Application

The application of multiphysical modelling is steadily increasing in the last decade, which also leads to a corresponding increase of the complexity and of the diversity of software packages used. To deal with this complexity, users of supercomputing clusters are often challenged to couple two or more software systems of different software vendors together. However, the combined use of complex software systems usually raises additional limitations, thus reducing considerably the efficiency of the parallel simulations. In the present work, an example of such complex software utilization has been shown and the particular limitations are identified. The most severe limitation for the current supercomputing simulations has been the relatively high RAM requirement per computing core. At this stage of the numerical investigation, in order to overcome the limitations, the software packages have been ported to a different, more suitable hardware architecture with increased RAM per node. This way, the efficient use of the parallel computational resources has been guaranteed which was confirmed by means of strong scaling tests.

Numerical Investigations of the Noise Sources Generated in a Swirl Stabilized Flame

Compressible Large Eddy Simulations (LES) have been performed to study the reacting flow field of a typical industrial double concentric swirl burner. A turbulent flame speed closure (TFC) was implemented into the code to model the premixed flame. The results have been analysed with respect to the acoustic sources used in Lighthills acoustic analogy. It is shown, that the noise sources generated by the combustion process are significantly larger than the sources caused by the aerodynamics. A spectral analysis allowed to detect two dominant frequencies in the swirling flow field, which are suspected to contribute to the noise emissions. The frequencies are caused by the eigenfrequency of the burner and an oscillating coherent structure near the burner outlet.