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Editor’s Choice articles are based on recommendations by the scientific editors of MDPI journals from around the world. Editors select a small number of articles recently published in the journal that they believe will be particularly interesting to readers, or important in the respective research area. The aim is to provide a snapshot of some of the most exciting work published in the various research areas of the journal.

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31 pages, 10339 KB

Open AccessFeature PaperEditor’s ChoiceArticle

Forcing Pulsations by Means of a Siren for Gas Turbine Applications

by Fabrice Giuliani, Markus Stütz, Nina Paulitsch and Lukas Andracher

Int. J. Turbomach. Propuls. Power 2020, 5(2), 9; https://doi.org/10.3390/ijtpp5020009 - 13 May 2020

Cited by 7 | Viewed by 6311

Abstract

A siren is a robust fast-valve that generates effective flow pulsations and powerful noise levels under well-controlled conditions. It operates under the inlet flow conditions of a gas turbine combustor. Its principle is based on a sonic air jet periodically sheared by a cogged wheel rotating at a given speed. It is used as an alternative to loudspeakers in combustion laboratories when the use of these is made difficult by aggressive flow conditions, such as hot air under pressure, possibly containing impurities. It is also a serious candidate as an effective flow actuator to be deployed on power gas turbine fleets. The authors have gathered more than twenty years of knowledge on siren technology. This pulsator was originally developed for research on thermoacoustics. By scanning through a given frequency range, one detects the acoustic resonance of specific parts of the combustor assembly, or possibly triggers a combustion instability during a sensitivity analysis of a flame to small perturbations. In 2010, Giuliani et al. developed a novel siren model with the capacity to vary the amplitude of pulsation independently from the frequency. In this contribution, the physics, the metrics, and the resulting parameters of the pulsator are discussed. Technical solutions are unveiled about visiting large frequency ranges (currently 6 kHz) and achieving elevated pressure fluctuations (150 dB SPL proven, possibly up to 155 dB SPL) with a compact device. A multimodal excitation is available with this technology, one idea being to dissipate the acoustic energy on nearby peaks. The contribution ends with a summary of the applications performed so far and the perspective of an industrial application. Full article

(This article belongs to the Special Issue Selected Papers from the 9th EVI-GTI International Gas Turbine Instrumentation Conference)

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16 pages, 3071 KB

Open AccessFeature PaperEditor’s ChoiceArticle

Technology Development of Fast-Response Aerodynamic Pressure Probes

by Paolo Gaetani and Giacomo Persico

Int. J. Turbomach. Propuls. Power 2020, 5(2), 6; https://doi.org/10.3390/ijtpp5020006 - 12 Apr 2020

Cited by 19 | Viewed by 4567

Abstract

This paper presents and discusses the recent developments on the Fast-Response Aerodynamic Pressure Probe (FRAPP) technology at the Laboratorio di Fluidodinamica delle Macchine (LFM) of the Politecnico di Milano. First, the different geometries developed and tested at LFM are presented and critically discussed: the paper refers to single-sensor or two-sensor probes applied as virtual 2D or 3D probes for phase-resolved measurements. The static calibration of the sensors inserted inside the head of the probes is discussed, also taking into account for the temperature field of application: in this context, a novel calibration procedure is discussed and the new manufacturing process is presented. The dynamic calibration is reconsidered in view of the 15-years’ experience, including the extension to probes operating at different temperature and pressure levels with respect to calibration. As for the probe aerodynamics, the calibration coefficients are discussed and the most reliable set here is evidenced. A novel procedure for the quantification of the measurement uncertainty, recently developed and based on the Montecarlo methodology, is introduced and discussed in the paper. Full article

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13 pages, 1466 KB

Open AccessEditor’s ChoiceArticle

Evaporation Modeling of Water Droplets in a Transonic Compressor Cascade under Fogging Conditions

by Adrian Seck, Silvio Geist, Janneck Harbeck, Bernhard Weigand and Franz Joos

Int. J. Turbomach. Propuls. Power 2020, 5(1), 5; https://doi.org/10.3390/ijtpp5010005 - 19 Feb 2020

Cited by 4 | Viewed by 3396

Abstract

High-fogging is widely used to rapidly increase the power outputs of stationary gas turbines. Therefore, water droplets are injected into the inflow air, and a considerable number enter the compressor. Within this paper, the primary process of droplet evaporation is investigated closely. A short discussion about the influential parameters ascribes a major significance to the slip velocity between ambient gas flow and droplets. Hence, experimental results from a transonic compressor cascade are shown to evaluate the conditions in real high-fogging applications. The measured parameter range is used for direct numerical simulations to extract evaporation rates depending on inflow conditions and relative humidity of the air flow. Finally, an applicable correlation for the Sherwood number in the form of

S h (R e^{1 / 2} S c^{1 / 3})

is suggested. Full article

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11 pages, 6050 KB

Open AccessEditor’s ChoiceArticle

Operating Range Extension of an Open Impeller Centrifugal Compressor Stage Utilising 3D Diffuser End Wall Contouring

by Daniel Hermann, Manfred Wirsum, Douglas Robinson and Philipp Jenny

Int. J. Turbomach. Propuls. Power 2020, 5(1), 4; https://doi.org/10.3390/ijtpp5010004 - 7 Feb 2020

Cited by 9 | Viewed by 4088

Abstract

State-of-the-art centrifugal compressor units utilised in pipeline and in energy storage applications face the challenge of flexible and highly efficient operation. Geometric contouring on the hub side near a vaned diffuser affects the flow in a way which increases operational flexibility by delaying the incipience of instability and thereby increases compressor operating range. In the present paper, a hub-side wall contouring is applied within the vaneless space and the vaned diffuser of an open impeller centrifugal compressor stage. The performance characteristic of the novel hub contouring is evaluated in a scale-model test rig and compared against a baseline design. A stable operating range increase of 8% is achieved for the contoured design at

{Mu}_{2} =

1.16. 5-hole probe measurements covering a complete diffuser blade-to-blade passage are performed upstream the diffuser and compared both against CFD simulations and against the measurements of the baseline design for an operating point near the stability limit. Full article

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12 pages, 6898 KB

Open AccessEditor’s ChoiceArticle

Detection and Analysis of an Alternate Flow Pattern in a Radial Vaned Diffuser

by Victor Moënne-Loccoz, Isabelle Trébinjac, Nicolas Poujol and Pierre Duquesne

Int. J. Turbomach. Propuls. Power 2020, 5(1), 2; https://doi.org/10.3390/ijtpp5010002 - 19 Jan 2020

Cited by 11 | Viewed by 3488

Abstract

The flow in the vaned diffuser of an aeronautical centrifugal compressor designed by Safran Helicopter Engines is analyzed through steady and unsteady pressure measurements at different rotation speeds. The analysis leads to the identification of different operating zones thanks to a new variable, [...] Read more.

A

. It allows the characterization of a specific behavior of the vaned diffuser consisting of an alternate stall pattern in two adjacent channels of the diffuser. While it is close to zero at low speed, the alternate rate reaches a maximum value at a higher speed before collapsing with a further increase in the rotation speed. Depending on the value reached by the alternate rate, three distinct regimes of the flow within the diffuser can be distinguished. For low

A

values, the regime is the most common one with an equivalent flow pattern in each channel of the diffuser. For moderate

A

values, a mild difference of the flow fields which develop in two adjacent channels can be observed but it remains time independent. Finally, for high values of

A

, the alternate pattern is amplified and becomes time dependent, pulsating together with the mild surge of the entire compressor. Full article

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15 pages, 9889 KB

Open AccessEditor’s ChoiceArticle

Focusing Schlieren Visualization of Transonic Turbine Tip-Leakage Flows

by Maximilian Passmann, Stefan aus der Wiesche and Franz Joos

Int. J. Turbomach. Propuls. Power 2020, 5(1), 1; https://doi.org/10.3390/ijtpp5010001 - 2 Jan 2020

Cited by 19 | Viewed by 6835

Abstract

This paper presents a focusing schlieren system designed for the investigation of transonic turbine tip-leakage flows. In the first part, the functional principle and the design of the system are presented. Major design considerations and necessary trade-offs are discussed. The key optical properties, e.g., depth of focus, are verified by means of a simple bench test. In the second part, results of an idealized tip-clearance model as well as linear cascade tests at engine representative Reynolds and Mach numbers are presented and discussed. The focusing schlieren system, designed for minimum depth of focus, has been found to be well suited for the investigation of three-dimensional transonic flow fields in turbomachinery applications. The schlieren images show the origin and growth of the tip-leakage vortex on the blade suction side. A complex shock system was observed in the tip region, and the tip-leakage vortex was found to interact with the suction side part of the trailing edge shock system. The results indicate that transonic vortex shedding is suppressed in the tip region at an exit Mach number around

M_{2, i s}

= 0.8. Full article

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