B. Pritz

Prediction of the Resonance Characteristics of Combustion Chambers on the Basis of Large-Eddy Simulation

In the last few years intensive experimental investigations were performed at the University of Karlsruhe to develop an analytical model for the Helmholtz resonator-type combustion system. In the present work the resonance characteristics of a Helmholtz resonator-type combustion chamber were investigated using large-eddy simulations (LES), to understand better the flow effects in the chamber and to localize the dissipation. In this paper the results of the LES are presented, which show good agreement with the experiments. The comparison of the LES study with the experiments sheds light on the significant role of the wall roughness in the exhaust gas pipe.

Investigation of the Effect of Surface Roughness on the Pulsating Flow in Combustion Chambers with LES

Self-excited oscillations often occur in combustion systems due to the combustion instabilities. The high pressure oscillations can lead to higher emissions and structural damage of the chamber. In the last years intensive experimental investigations were performed at the University of Karlsruhe to develop an analytical model for the Helmholtz resonator-type combustion systems [1]. In order to better understand the flow effects in the chamber and to localize the dissipation, Large Eddy Simulations (LES) were carried out. Magagnato et al. [2] describe the investigation of a simplified combustion system where the LES were carried out exclusively with a hydraulic smooth wall. The comparison of the results with experimental data shows the important influence of the surface roughness in the resonator neck on the resonant characteristics of the system. In order to catch this effect with CFD as well, the modeling of surface roughness is needed. In this paper the Discrete Element Method has been implemented into our research code and extended for LES. The simulation of the combustion chamber with roughness agrees well with the experimental results.

Comparison of DES and LES on the Transitional Flow of Turbine Blades

The prediction of the laminar to turbulence transition is essential in the calculation of turbine blades, compressor blades or airfoils of airplanes since a non negligible part of the flow field is laminar or transitional. In this paper we compare the prediction capability of the Detached Eddy Simulation (DES) with the Large Eddy Simulation (LES) using the high-pass filtered (HPF) Smagorinsky model (Stolz et al., 2003) when applied to the calculation of transitional flows on turbine blades. Detailed measurements from (Canepa et al, 2003) of the well known VKI-turbine blade served to compare our results with the experiments. The calculations have been made on a fraction of the blade (10%) using non-reflective boundary conditions of Freund at the inlet and outlet plane extended to internal flows by (Magagnato et al., 2006) in combination with the Synthetic Eddy Method (SEM) proposed by (Jarrin et al., 2005). The SEM has also been extended by (Pritz et al., 2006) for compressible flows. It has been repeatedly shown that hybrid approaches can satisfactory predict flows of engineering relevance. In this work we wanted to investigate if they can also be used successfully in this difficult test case.

A fluid-structure-interaction tool by coupling of existing codes

Fluid-Structure-Interaction (FSI), as a sub-discipline of computational mechanics, has been gaining relevance since the growth in clusters capacity has made it possible to simulate high resolution models. Although some commercial tools already present certain capabilities for coupled simulations, the lack of efficiency of these general purpose programs is still an issue. Moreover, the high cost of commercial licenses - on a per processor basis - hampers the computation of high resolution models for academic research. Instead, a coupled software solution that resorts to well established existent programs proves a good alternative to preserve the value of decades-long development and associated know-how. However, since such codes were mostly conceived for standalone run, an elegant software implementation is not easily achieved. In this work our CFD code SPARC has been coupled with the open-source structural solver CalculiX by means of the in-house developed software packet FSiM. FSiM stands for Fluid-Structure-Interaction Simulation Manager and is in charge of the communication between the fluid and structure solver using the Dynamic Process Management of the MPI-2 standard. This approach facilitates that the parallelization strategies of both part-solvers be used with minimal modifications and no risk of mutual interference. Results of a simple two-dimensional test case of the panel flutter problem are presented to show the capabilities of the new coupling tool.

Transition between free, mixed and forced convection

In this contribution, numerical methods are discussed to predict the heat transfer to liquid metal flowing in rectangular flow channels. A correct representation of the thermo-hydraulic behaviour is necessary, because these numerical methods are used to perform design and safety studies of components with rectangular channels. Hence, it must be proven that simulation results are an adequate representation of the real conditions. Up to now, the majority of simulations are related to forced convection of liquid metals flowing in circular pipes or rod bundle, because these geometries represent most of the components in process engineering (e.g. piping, heat exchanger). Open questions related to liquid metal heat transfer, among others, is the behaviour during the transition of the heat transfer regimes. Therefore, this contribution aims to provide useful information related to the transition from forced to mixed and free convection, with the focus on a rectangular flow channel. The assessment of the thermo-hydraulic behaviour under transitional heat transfer regimes is pursued by means of system code simulations, RANS CFD simulations, LES and DNS, and experimental investigations. Thereby, each of the results will compared to the others. The comparison of external experimental data, DNS data, RANS data and system code simulation results shows that the global heat transfer can be consistently represented for forced convection in rectangular flow channels by these means. Furthermore, LES data is in agreement with RANS CFD results for different Richardson numbers with respect to temperature and velocity distribution. The agreement of the simulation results among each other and the hopefully successful validation by means of experimental data will fosters the confidence in the predicting capabilities of numerical methods, which can be applied to engineering application.

Prediction of Stability Limits of Combustion Chambers with LES

Lean Premixed combustion, which allows for reducing the production of thermal NOx, is prone to combustion instabilities. There is an extensive research to develop a reduced physical model, which allows—without time-consuming measurements—to calculate the resonance characteristics of a combustion system consisting of Helmholtz resonator type components (burner plenum, combustion chamber). For the formulation of this model numerical investigations by means of compressible Large Eddy Simulation (LES) were carried out. In these investigations the flow in the combustion chamber is isotherm, non-reacting and excited with a sinusoidal mass flow rate. Firstly a combustion chamber as a single resonator subsequently a coupled system of a burner plenum and a combustion chamber were investigated.

Stability Investigation of Combustion Chambers with LES

Our primary energy consumption is supported in 81% by the combustion of fossil energy commodities (IEA, 2010). The demand on energy will grow by about 60% in the near future (Shell, 2008). The efficiency of the combustion processes is crucial for the environment and for the use of the remaining resources. At the Karlsruhe Institute of Technology the longterm project Collaborative Research Centre (CRC) 606: “Non-stationary Combustion: Transport Phenomena, Chemical Reactions, Technical Systems” was founded to investigate the basics of combustion and for the implementations relevant processes coupled to combustion (Bockhorn et al., 2003; SFB 606, 2002). Modern combustion concepts comprise lean premixed (LP) combustion, which allows for the reduction of the pollutant emissions, in particular oxides of nitrogen (NOx) (Lefebvre, 1995). Lean premixed combustors are, however, prone to combustion instabilities with both low and high frequencies. These instabilities result in higher emission, acoustical load of the environment and even in structural damage of the system. A subproject in CRC 606 was dedicated to investigate low frequency instabilities in combustion systems. The main goal of this subproject was to validate an analytical model, which was developed to describe the resonant characteristics of combustion systems consisting of Helmholtz resonator type components (burner plenum, combustion chamber) (Buchner, 2001). The subproject included experimental and numerical investigations as well. The goal of the numerical part was to find a reliable tool in order to predict the damping ratio of the system. The damping ratio is a very important input of the analytical model. The combination of the numerical prediction of the damping ratio and the analytical model enables the stability investigation of a system during the design phase. In the numerical part Large Eddy Simulation (LES) was used to predict the damping ratio as previous investigations with unsteady Reynolds-averaged Navier-Stokes simulation (URANS) failed to predict the damping ratio satisfactorily (Rommel, 1995). The results of LES showed a very good agreement with the experimentally measured damping ratio. The focus of this chapter is to show results of further numerical investigations, which sheds light on a very important source of self-excited combustion instabilities, and to show how can provide LES the eigenfrequencies of a system. In this chapter firstly a short description to combustion instabilities is given. After it the experimental and the numerical investigations of the resonant characteristics of the combustion systems will be shown briefly. In these investigations the system was excited

Stability Analysis of a Coupled Helmholtz Resonator with Large Eddy Simulation

Lean Premixed combustion, which allows for reducing the production of thermal NOx, is prone to combustion instabilities. There is an extensive research to develop a reduced physical model, which allows - without time-consuming measurements - to calculate the resonance characteristics of a combustion system consisting of Helmholtz-resonator-type components (burner plenum, combustion chamber). For the formulation of this model numerical investigations by means of compressible Large Eddy Simulation (LES) are carried out. In these investigations the flow in the combustion chamber is isotherm, non-reacting and excited with a sinusoidal mass flow rate. The foregoing investigations concentrated on the single combustion chamber as a single resonator.