Aerodynamic Losses in an Aero Engine Centrifugal Compressor with a Close-Coupled Pipe-Diffuser and a Radial-Axial Deswirler

In the work presented here, a detailed aerodynamic analysis of an aero engine centrifugal compressor is given. Steady and unsteady 3D-RANS simulations, as well as extensive experimental data have been used for the analysis. The compressor investigated contains a close-coupled pipe-diffuser. A radial-axial deswirler guides the air into the combustion chamber.The investigation presented gives a detailed insight in the loss mechanisms as well as their origin within the centrifugal compressor. In the past, the aerodynamic loss mechanisms within the centrifugal compressor were discussed on a phenomenological basis. In this work, the loss mechanisms are identified by analyzing the irreversible entropy production. The single loss mechanisms are quantified, enabling consolidated statements on the potential improvement, as well the approach on how to reduce the losses induced.An investigation of four different diffuser concepts is given, representing the time-wise evolution of the diffusion system towards a higher efficiency, increased stall margin and smaller outer diameter. Within the original diffusion system, a flow separation in the pipe-diffuser results in a thick shear layer with a high level of entropy production. By truncating the pipe diffuser, the large flow separation is prevented. The original downstream deswirler is not matched with the truncated pipe-diffuser's discharge flow. In order to increase the compressor’s efficiency, two 3D deswirler designs using a single row and tandem rows respectively, are compared. The lower efficiency found for the tandem blade design is not in agreement with the established design rules for two-dimensional compressor blades with a high diffusion factor. This outcome can be traced back to the high impact of the end-wall effects for the low-aspect-ratio deswirler designs investigated. The unsteady investigation demonstrates the change in loss production due to unsteadiness. The decrease in isentropic efficiency due to unsteadiness within the impeller is traced back mainly to the unsteady tip clearance flow, induced by the diffusor’s potential field. Within the diffuser, counteracting unsteady mechanisms are found and discussed in detail

Theoretische Grundlagen und Konzeptanalyse eines Laserzündsystems für kryogene Raketentriebwerke

In der vorliegenden Arbeit werden die theoretischen Grundlagen zur Auslegung eines Laserzündsystems bereitgestellt und eine Konzeptanalyse verschiedener Laserzündsysteme für die Anwendung in kryogenen Raketentriebwerken durchgeführt. In dieser Konzeptanalyse werden vier mögliche Laserzündsysteme für die Anwendung in kryogenen Raketentriebwerken betrachtet. Dabei werden sowohl unterschiedliche Zündstellen, als auch verschiedene Fokussierungsmethoden vorgeschlagen. Die Fokussierung der Leistung erfolgt durch eine Strahlfokussierung, die Bestrahlung eines Festkörpers (Ablationszündung) oder eine mehrfache Reflexion in einer entsprechend geformten Kavität (Kavitationszündung). Die entsprechenden Zündparameter werden anhand experimenteller Daten und analytischer Berechnungen für die Zündung unter atmosphärischen Bedingungen untersucht. Die daraus entstandenen Konzepte werden verglichen und deren Vor- und Nachteile herausgearbeitet

A combined genetic-morphometric analysis unravels the complex biogeographical history of Polyommatus icarus and Polyommatus celina Common Blue butterflies

Widespread species have the potential to reveal large-scale biogeographical patterns, as well as responses to environmental changes possibly unique to habitat generalists. This study presents a continental-scale phylogeographical analysis of Polyommatus icarus, one of the most common Palaearctic butterflies, and the morphologically and ecologically similar Polyommatus celina, a recently discovered cryptic species. By combining data from mitochondrial [cytochrome c oxidase subunit I (COI)] and nuclear [internal transcribed spacer (ITS2)] molecular markers with geometric morphometrics, we document a complex phylogeographical history for the two species. Despite morphological similarities, the genetic divergence between these two species is high (more than 5% at COI) and they are not sister species.For the first time, we show that P. celina occurs not only in North Africa but also in Europe, where it inhabits several west Mediterranean islands, as well as large parts of Iberia, where it occurs in parapatry with P. icarus. The two species appear to completely exclude each other on islands, but we provide morphological and molecular evidence that introgression occurred in the Iberian Peninsula. We discovered strongly diverged lineages that seem to represent relict populations produced by past range expansions and contractions: Crete and Iberian isolates for P. icarus, Balearics–Sardinia and Sicily–Lipari for P. celina. This study shows that a combined genetic-morphometric approach can shed light on cryptic diversity while providing the necessary resolution to reconstruct a fine-scale phylogeographical history of species at both spatial and temporal levels.Peer reviewe