1,721,027 research outputs found
Understanding and modeling unstarting phenomena in a supersonic inlet cascade
Full text linkThe renewed interest in supersonic turbomachinery research was driven by its potential applications in emerging fields. However, the design of supersonic inlet cascades faces significant challenges due to the inherent limitations of supersonic flows. While several studies have been published on the unstarting of supersonic intakes, there exists a major knowledge gap in the unstarting of supersonic blade rows. This paper presents the research on a novel unstarting mechanism for supersonic inlet cascades induced by the formation of a collective shock. Tailored simulations were carried out to study the coalescence of the leading-edge bow shock waves and to investigate the stability and the hysteresis of this phenomenon. Then, a reduced order model was developed and verified to estimate the limit induced by this additional unstarting mechanism. Since the accuracy of the unstarting condition relies heavily on the predicted bow shock shape, novel strategies were proposed to improve the estimate of the asymptotic slope of the bow shock and to account for large incidence angles. Furthermore, the well-known Kantrowitz criterion for the self-starting of a supersonic channel was reviewed and adapted to supersonic blade rows by considering both weak and strong oblique shock waves in the calculation of the maximum contract ratio. Then, it was demonstrated the importance of accounting for shock-induced boundary layer separation in the starting process of a supersonic machine. Finally, computational fluid dynamics simulations reveal the high sensitivity of the self-starting limit to the cascade solidity and profile shape
The Effect of Upstream Unsteadiness on the Unstarting of a Supersonic Inlet Turbine
Full text linkA 15% increase in thermal efficiency at medium pressure ratios is promised by rotating detonation engine technology over conventional Joule-Bryton cycles. A supersonic inlet turbine is a viable option to extract substantial work from the highly fluctuating and supersonic flow delivered by the detonating combustor. However, an additional unstarting mechanism based on the generation of a collective shock from the coalescence of leading-edge bow-shock waves further restricts the available design space. First, the effect of unsteady inlet conditions with variable frequency, amplitude, and mean value on collective shock generation is investigated. Then, a fast and cost-effective tool was developed and verified to predict bow-shock wave motion from a prescribed inlet Mach trend; the parameters of the transfer function were estimated with a classical identification method based on a step-response computational fluid dynamics (CFD) simulation. The model was employed to rapidly calculate the maximum amplitude of fluctuations accepted by the supersonic blade row for each frequency and average inlet Mach number. Finally, the influence of the inlet geometric angle, pitch to leading-edge thickness ratio, and static temperature on the model parameters is studied. The preliminary observations of the parametric analysis were confirmed by the rigorous quantitative approach of a global sensitivity analysis based on Sobol sensitivity indices
Design and Parametric Analysis of a Supersonic Turbine for Rotating Detonation Engine Applications
Full text linkPressure gain combustion is a promising alternative to conventional gas turbine technologies and within this class the Rotating Detonation Engine has the greatest potential. The Fickett–Jacobs cycle can theoretically increase the efficiency by 15% for medium pressure ratios, but the combustion chamber delivers a strongly non-uniform flow; in these conditions, conventionally designed turbines are inadequate with an efficiency below 30%. In this paper, an original mean-line code was developed to perform an advanced preliminary design of a supersonic turbine; self-starting capability of the supersonic channel has been verified through Kantrowitz and Donaldson theory; the design of the supersonic profile was carried out employing the Method of Characteristics; an accurate evaluation of the aerodynamic losses has been achieved by considering shock waves, profile, and mixing losses. Afterwards, an automated Computational Fluid Dynamics (CFD) based optimization process was developed to find the optimal loading condition that minimizes losses while delivering a sufficiently uniform flow at outlet. Finally, a novel parametric analysis was performed considering the effect of inlet angle, Mach number, reaction degree, peripheral velocity, and blade height ratio on the turbine stage performance. This analysis has revealed for the first time, in authors knowledge, that this type of machines can achieve efficiencies over 70%
THE ROLE OF ENDWALL SHAPE OPTIMIZATION IN THE DESIGN OF SUPERSONIC TURBINES FOR ROTATING DETONATION ENGINES
Full text linkRotating detonation engines (RDE) are characterized by a thermodynamic cycle with an efficiency gain up to 15% at medium pressure ratios with respect to systems based on the conventional Joule-Bryton cycle. Multiple turbine designs can be considered and this paper deals with the supersonic inlet configuration. After having reviewed the main design steps of an exemplary RDE supersonic turbine, the paper focuses on the considerable effects that endwall losses have on the performance of supersonic-inlet turbines and on the reasons why endwall contouring is strongly recommended for an efficient design. Parametric analyses, carried out by a novel in-house mean-line code validated against CFD, reveal that endwall friction losses contribute significantly to the overall stage loss. Endwall boundary layers also reduce the effective area, which can be critical for the self-starting capability of the supersonic channel. Therefore, a variable blade height geometry is necessary to extend the design space and guarantee a higher efficiency with respect to a constant-span configuration. The in-house CFD-based evolutionary shape optimization code was adapted to search for the optimal endwall shape for these unconventional machines. The optimal shape reduces shock losses and deviation angles and provides a significant gain in efficiency and work extraction. Finally, a novel technique is proposed to design the three-dimensional shape of the rotor based on the method of characteristics and tailored on the flow delivered by the stator
ADVANCED DESIGN OF A TRANSITION DUCT FOR SUPERSONIC INLET TURBINES IN ROTATING DETONATION ENGINES
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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