Time domain modelling and stability analysis of complex thermoacoustic systems

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.A methodology allowing for a modular setup of complex acoustic systems is developed. The transfer behaviour of the individual subsystems is formulated in time domain. Subsystem descriptions can be obtained by analytical considerations, numerical methods, or experimental data. Once the complex subsystems have been characterized experimentally, changes in system geometry can be implemented easily by exchanging or adding subsystems. To validate the modelling approach, experiments are conducted in an acoustic test rig with a combustor-type geometry. Results are compared to predictions from the model, demonstrating accuracy in frequency and time domain. Application to thermoacoustic instabilities arising in lean-premixed combustion is given. The influence of a modified fuel distribution on an unstable operating point of a lean-premixed combustor is studied and validated with experimental data. Additionally, a study on the parameters governing the flame transfer function is performed to generate a stability map of a model combustor. An advantage of the state-space approach is that stability of a thermoacoustic system can be determined by simply solving a matrix eigenvalue problem. This is in strong contrast to the traditional approach, where the complete model is formulated in frequency domain with infinite-dimensional transfer functions. The time domain approach is based on the methodology presented by Schuermans et al. [1]. In contrast to their work, however, subsystems are not obtained from modal expansions but are characterized by using system identification techniques. Additionally, accuracy of the time domain model is verified by experiments

Eine Echtzeit-Online-Methode zur Beurteilung der künstlichen Grundwasseranreicherung und Grundwasserentnahme = Real-time online method for assessing artificial groundwater recharge and abstraction

Eine nachhaltige Wasserversorgung in urbanen Gebieten stellt aufgrund des Nutzungsdrucks sowie vielfältiger anthropogener Einträge eine große Herausforderung dar. Künstliche Grundwasseranreicherung hat das Potenzial, den Wasserbedarf zu decken und ggf. vorhandene Schadstoffkonzentrationen zu verdünnen. Durch hohe Infiltrationsraten kann zudem eine hydraulische Barriere gegenüber Schadstoffen aus anliegenden Nutzungsflächen entstehen. Grundvoraussetzung, um diese Barriere so effektiv wie möglich nutzen zu können, ist die genaue Beschreibung der räumlichen und zeitlichen Verteilung der Schutzfunktion. Im geschilderten Fallbeispiel für ein Trinkwassergewinnungsgebiet in der Nordschweiz präsentieren wir ein einfaches Online-Werkzeug. Es wird genutzt, um die große Menge der erhobenen Daten zu digitalisieren sowie um die Interpretation von Konturenkarten zu ergänzen, indem es die Auswertung erhobener Daten vereinfacht und Grundwasserfließrichtungen und -magnituden visualisiert. Die Ergebnisse können zur Beschreibung komplexerer Fließsysteme genutzt werden, was zu einer effizienteren Grundwasserbeobachtung führt

Prediction of Groundwater Flow and Transport Processes in an Urban Area: A Combined Approach of Field Testing and Numerical Modeling

Drinking water supply in urban areas is challenging due to different kinds of water use and potential groundwater contamination. We investigate an area where drinking water production is close to different contaminated sites. The study site is characterized by a high complexity of the tectonic and geological setting with a gravel and a karstic aquifer. The two aquifers are partly connected, partly disconnected by an aquitard. To avoid drinking water contamination, artificial groundwater recharge with surface water into the gravel aquifer is used to create a hydraulic barrier between the contaminated sites and the water abstraction wells. Trace compounds, that were found in former times in the surface water but not nowadays, are still detected in the extracted drinking water. Different studies have been performed such as numerical modeling, intensive groundwater monitoring and investigation of drilling cores to get a differentiated overview of the distribution of the contaminants. Back-diffusion from the matrix due to changing hydraulic boundary was stated to be the reason for the actual distribution of the contaminants. In a first approach due to the lack of experimental data or evidence from field measurements, the permeabilities of the karstic aquifer were assumed as homogeneous. In our study, we seek to identify the flow and transport processes within the system including the fracture network in a combined approach of field work and 3D modeling with FEFLOW. During a field campaign we acquired water samples for the analysis of stable water isotopes as well as organic and inorganic compounds. Furthermore, tritium and helium samples were taken to estimate water ages and to determine the flow through the fracture networks. A combination of existing and recently obtained data was used to build and validate a 3D flow and transport model. The simulation of different scenarios such as the water flow for varying injection and extraction rates as well as particle transport from different sources is carried out. To investigate the effect of subsurface heterogeneity, PEST, an independent parameter estimation and uncertainty analysis software, was used. With the calibrated model we will be able to optimize the operational conditions of artificial recharge and drinking water production especially at the boundaries of the production field. Hence, we can provide guidelines for an improved water resource management

Multi-Jet Event Rates in Deep Inelastic Scattering and Determination of the Strong Coupling Constant

Jet event rates in deep inelastic ep scattering at HERA are investigated applying the modified JADE jet algorithm. The analysis uses data taken with the H1 detector in 1994 and 1995. The data are corrected for detector and hadronization effects and then compared with perturbative QCD predictions using next-to-leading order calculations. The strong coupling constant alpha_S(M_Z^2) is determined evaluating the jet event rates. Values of alpha_S(Q^2) are extracted in four different bins of the negative squared momentum transfer~\qq in the range from 40 GeV2 to 4000 GeV2. A combined fit of the renormalization group equation to these several alpha_S(Q^2) values results in alpha_S(M_Z^2) = 0.117+-0.003(stat)+0.009-0.013(syst)+0.006(jet algorithm).Comment: 17 pages, 4 figures, 3 tables, this version to appear in Eur. Phys. J.; it replaces first posted hep-ex/9807019 which had incorrect figure 4

Measurement of Leading Proton and Neutron Production in Deep Inelastic Scattering at HERA

Deep--inelastic scattering events with a leading baryon have been detected by the H1 experiment at HERA using a forward proton spectrometer and a forward neutron calorimeter. Semi--inclusive cross sections have been measured in the kinematic region 2 <= Q^2 <= 50 GeV^2, 6.10^-5 <= x <= 6.10^-3 and baryon p_T <= MeV, for events with a final state proton with energy 580 <= E' <= 740 GeV, or a neutron with energy E' >= 160 GeV. The measurements are used to test production models and factorization hypotheses. A Regge model of leading baryon production which consists of pion, pomeron and secondary reggeon exchanges gives an acceptable description of both semi-inclusive cross sections in the region 0.7 <= E'/E_p <= 0.9, where E_p is the proton beam energy. The leading neutron data are used to estimate for the first time the structure function of the pion at small Bjorken--x.Comment: 30 pages, 9 figures, 2 tables, submitted to Eur. Phys.

The effects of model complexity and calibration period on groundwater recharge simulations

ISSN:1029-7006ISSN:1607-796