Martin Gabi

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.

Large Eddy Simulation (LES) with Moving Meshes on a Rapid Compression Machine: Part 2: Numerical Investigations Using Euler–Lagrange-Technique

The flow inside a simplified one-stroke engine with squared cross section has been calculated with compressible Large Eddy Simulation (LES) using our code SPARC and compared with the measurements on the same geometry. The one-stroke engine has a turbulence generator, which can ether generate a tumble or homogenous turbulence depending on the configuration. By waiting different amount of time after the turbulence generation process a variable turbulence level can be achieved. During the up going motion of the piston the turbulent fuel mixture is compressed and ignited by a row of spark plugs. The simulation has been using more then 8 million points for the space discretization. A space conservation law was used to calculate the grid motion with Euler-Lagrange technique. The mesh was refined in the shear layers and close to the wall so that y+ < 1 results almost everywhere. A comparison between Miles (monotonically integrated large eddy simulation) approach and conventional subgrid scale modelling (dynamic Smagorinsky) showed very similar solutions. Mean and fluctuating velocities at TDC are compared with available experimental findings.

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.

Numerical Investigation of the VKI Turbine Blade by Large Eddy Simulation

In the frame of this work, the numerical investigations of the flow through the VKI turbine cascade were performed by means of Large Eddy Simulation.

The Buffer Layer Technique Applied to Transonic Flow Calculations

The main goal of numerical simulations is to predict a flow field as close to the real one as possible. All disturbances which travel down to an inlet/outlet boundary of the domain should pass through the boundary. Unfortunately, the standard numerical boundary conditions produce unphysical reflections of the disturbances. This is especially undesirable for unsteady calculations. To avoid this effect, a non-reflecting boundary condition must be applied. According to recent papers, one can find some different approaches to avoid the unphysical reflections, called non-reflecting boundary conditions. One of those is the buffer layer technique. The buffer layer non-reflecting boundary, proposed by Freund3, has been implemented into our code10 and tested for various test cases.

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.

Investigation of the performance of short diffusers configurations for different inflow profiles

Diffusers are used to connect the outlet of axial fans to the following piping. In thermal power plants the diffuser design is often restricted by limited installation space. Two different diffuser configurations for limited diffuser length and fixed inlet and outlet diameter are studied at a scaled diffuser test rig. Configuration 1 combines an annular diffuser and a Carnot diffuser to achieve the required diffuser outlet diameter. Configuration 2 consists of an annular diffuser (diffuser 1) and a diffuser 2 with installed guiding bodies. These bodies are arranged in a star-like configuration and reduce the effective opening angle of the diffuser. The tests have been carried out with different inlet velocity profiles. Configuration 1 was more sensitive on inlet profile variation regarding flow separation. The pressure recovery for both configurations is compared for different positions downstream the diffuser. There is a significant static pressure increase due to a reduction of over-speed areas.

A Coupled Blade Adjustment and Response Surface Method for the Optimization of Radial Fans Without Housing

The optimization process of a fan based on 3D viscous CFD calculations is time consuming, especially if many variables are taken into account. With the focus on computational cost efficiency and reliable CFD-results a specific optimization algorithm for radial fans based on CFD calculations is presented. The algorithm is derived from the classical knowledge of flow phenomena occurring in radial fans. The leading edge is adjusted in reference to the stagnation point caused by the incoming flow. The trailing edge is adjusted to achieve the required pressure rise. The 5 blades of the investigated fan are constructed as 3D free surface blades; each blade is separated into 5 profile sections. The optimization process regarding the blade includes 10 independent parameters of the leading and trailing edges.An additional potential to increase efficiency is obtained by changing the meridional shape of the impeller. To investigate the meridional shape, the blade adjustment algorithm is coupled with a response surface method using the Kriging approximation to find a highly efficient meridional shape.Copyright