14 Externe wissenschaftliche Einrichtungen

Permanent URI for this collectionhttps://elib.uni-stuttgart.de/handle/11682/15

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Boundary layer response to combustion instabilities and associated heat transfer
(2010) Panara, Daniele; Aigner, Manfred (Prof. Dr.-Ing.)
The development of efficient and environmental sustainable combustion systems is critical in the nowadays economy. The efficiency of an energy cycle is proportional to the highest cycle temperature but unfortunately, due to the major mechanism of nitric oxide formation, there is a temperature trade off between improved cycle efficiency, material constraints and low emissions. This considerations have pushed combustion chamber technology towards lean premixed flames where the tendency is to reduce local temperature peaks making use of a well studied air management. Unfortunately this kind of innovative systems have shown to be prone to combustion instabilities and higher wall heat load. In the present work, making use of numerical simulations, the wall boundary layer response to combustion instabilities has been studied in order to asses the effect of flow pulsations on heat transfer. In Chapter 2 the status of the art of combustor simulation has been presented. A full scale burner has been simulated with a commercial CFD (Computational Fluid Dynamic) code. The results have shown that in such complex simulations and in the presence of combustion instabilities, the correct predictions of wall heat load rely not only on the correct modeling of air-fuel turbulent mixing, chemical reactions and heat radiation, but also depend strongly on the near wall turbulence treatment and on the correct solution of the conjugate solid-fluid heat transfer problem at the wall (which details are discussed in Chapter 3). The main interest of the work is however the study of the near wall turbulence and the associated heat transfer in the presence of flow unsteadiness. In Chapter 4 the fundamental equations for the solution of the thermo-Fluid dynamic problem in turbulent unsteady flows have been introduced. Moreover, in Chapter 5 some unsteady analytical solutions in simple channel and pipe configurations as well as turbulent channel and pipe flow heat transfer and viscous loss correlations have been presented. In Chapter 6 and 7, making use of simplified but well defined academic test cases, the accuracy of different turbulence models for the prediction of the wall heat transfer response in presence of thermo-acoustic instabilities has been discussed. The results have shown the clear limitation of the use of wall functions both in URANS and LES applications and discrepancies with some experimental results. The most interesting results are however presented In Chapter 8 where the turbulent near wall structures and the associated heat transfer were in detail investigated by means of pulsating channel flow DNS (Direct Numerical Simulation) simulations. The response of the turbulent kinetic energy (k) and fluctuating temperature variance (k_theta) as well as their dissipation rates (epsilon and epsilon_theta) were reported at different flow pulsations and amplitudes. In order to highlight the most critical (for the turbulence models) flow conditions, the pulsation amplitudes and frequencies were chosen so to span different pulsating flow regimes. The DNS results have shown very complex turbulence unbalance phenomena depending on the flow pulsation and amplitude. Each term on the budget equation of k, k_theta and epsilon_theta have shown different answer to pulsation frequency and amplitude. The mean and phase locked averaged turbulent Prandtl number seems to be affected by flow pulsation as well as the unsteady heat transfer. The pulsation frequency increase determines an unsteady heat transfer amplitude decrease. The flow pulsation amplitude seems to affect the overall mean heat transfer value increasing up to two times its steady value with the increasing of pulsation amplitude. The present DNS database represents a highly valuable and unique work which can serve as a reference for the development of innovative unsteady turbulent heat transfer models. Finally, in Chapter 9, starting from the DNS data, a new turbulent heat transfer closure has been proposed. The new model was capable, in all the flow condition studied, to correctly predict the unsteady heat transfer mean value and pulsating amplitude showing the added value of the present DNS database for the understanding of the near wall turbulence behavior and the associated heat transfer in presence of flow unsteadiness.
Open Access
Modellierung der Turbulenz-Chemie-Interaktion in technischen Brennkammern
(2000) Theisen, Peter; Aigner, Manfred (Prof. Dr.-Ing.)
Die Modellierung von turbulenten Strömungen mit chemischer Reaktion in praxisnahen Brennkammern stellt nach wie vor hohe Ansprüche an heutige Rechnersysteme. Diese Arbeit liefert einen Beitrag zur sinnvollen Vereinfachung der bestehenden Modelle und zeigt Wege auf, adäquate Ergebnisse in vertretbaren Rechenzeiten zu erreichen. Hierfür werden Ergebnisse zeitlich aufgelöster Messungen an Hand numerischer Methoden analysiert. Die Messergebnisse liefern die Wahrscheinlichkeiten, mit der die verschiedenen Kombinationen der Einflussparameter auf die chemische Reaktion auftreten. Diese Größen, die normalerweise vom Strömungslöser bereitgestellt werden, sind hier die Grundlage für den Vergleich verschiedener Modellierungsmethoden für die Bestimmung der mittleren chemischen Quellterme. Insbesondere werden verschiedene Ansätze für die Modellierung der Wahrscheinlichkeitsdichtefunktion der Einflussparameter auf die chemische Reaktion mit den gemessenen Schwankungen verglichen. Aus dem Vergleich der Ergebnisse ergeben sich Kriterien, in welchen Fällen vereinfachte Modelle zur Verringerung der Rechenzeit Anwendung finden können, ohne Einbußen in der Qualität der Ergebnisse hinnehmen zu müssen. Um die Vielzahl von möglichen Berechnungsmethoden flexibel einsetzen zu können, wurde eine Programm-Struktur entwickelt, die das modulare Hinzufügen neuer Modelle zulässt. Durch die neue Programm-Struktur kann das entsprechende Chemiemodell während der Berechnung in Abhängigkeit der lokal herrschenden thermodynamischen Zustände frei gewählt werden. Zur weiteren Effizienzsteigerung wurde ein n-dimensionales Tabellensystem entwickelt, welches sich nach Angabe der geforderten Genauigkeit in Punkto Zugriffszeit und Speicherplatzbedarf automatisch optimiert. Das entstandene Programmsystem zur Bereitstellung der mittleren chemischen Quellen kommt an einem praxisnahen Methan-gefeuerten Drallbrenner zur Anwendung.