In an axial flow compressor, the presence of separated flow near the hub-end of a stator would result in an overall loss in the performance. In the present paper, stator hub-stall is attempted to be eliminated for a high hub-tip ratio (0.8) axial flow compressor stage consisting of a highly loaded rotor and stator. Numerical and experimental studies on an untreated straight stator (straight-stacked, twisted) blade show a large vortex near its hub. The large vortex is attempted to be reduced by modifying the present blade by (i) providing an additional twist at the hub-end of the stator blade (ii) providing a hub-clearance (a cantilevered blade fixed from the casing). The straight (untreated) stator, hub-end-bend version and the hub-clearance version are studied for two different rotor-tip clearances. Detailed computational analysis of the variation of hub-clearance at a fixed rotor-tip clearance is also carried out. Throughout the study, experiments are carried out on the above discussed different stator (untreated & hub-treated) configurations, in combination with the same rotor, at a fixed rotor-tip clearance. The studies show that the flow conditions are improved near the hub of the highly loaded stator blade both by the hub-end-bend design and by the hub-clearance provided.

Superhydrophobic microchannels have evolved recently as an accepted strategy to mitigate the hydrodynamic resistance tendered in micro-constrictions. In this work, hydrodynamics of a hydrophobic microchannel realized by entrapping air in the cavities located between transversely oriented ribs is numerically investigated. An interface formed between the liquid and air/vapor in the confinement facilitates a resistance free slipping surface for the flowing water. The shape of the meniscus is determined by the pressure difference between air and liquid and is classified as convex, flat, and concave depending on the protrusion angle. Several applications require a long hydrophobic channel in which the liquid pressure decreases lengthwise; consequently the interface shape changes as well. In this regard, a mathematical model is proposed to predict the protrusion angle at a specific distance from the inlet of microchannel. This is incorporated in the computational fluid dynamics (CFD) simulations to define the static geometry of the interface which is varying throughout the length of the channel. Moreover, the boundary is treated as a combination of flat no-slip and curved shear-free regions to mimic the ribs and cavities. Further, the evolution of interface morphology is captured using the volume-of-fluid (VOF) scheme by considering a static contact angle at the solid surface to check the validity of the suggested model. Dynamically evolved protrusion angle is measured for various liquid-gas interface pressures and it is observed that the theoretical scaling proposed by Laplace and Young is well obeyed. Though CFD-VOF simulation scheme is an effective tool for predicting the pressure dependent liquid-gas meniscus and concurrent hydrodynamics of the ribbed microchannel, it is resource intensive. The present study demonstrates that the developed model for static boundary may be adopted alternatively to predict the hydrodynamics of a long hydrophobic microchannel by saving computational resources. Keywordshydrophobic microchannel ribbed channel volume-of-fluid simulation liquid-gas interface

Renewable sources of energy are on the rise and will continue to increase the coming decades [1]. A common problem with the renewable energy sources is that they rely on effects which cannot be controlled, for instance the strength of the wind or the intensity of the sunlight. The ALPHEUS Horizon 2020 EU project has the aim to develop a low-head hydraulic pump-turbine which can work as a grid stabilising unit. This work presents numerical results of an initial hub-driven counter-rotating pump-turbine design within ALPHEUS. Computational fluid dynamics simulations are carried out in both prototype and model scale, for pump and turbine modes, and under steady-state and unsteady conditions. The results indicate that the initial design have a hydraulic efficiency of roughly 90 % in both modes and for a wide range of operating conditions. The unsteady simulations reveal a complex flow pattern downstream the two runners and frequency analysis show that the dominating pressure pulsations originates from the rotor dynamics. Given the promising high efficiency, this initial design makes an ideal platform to continue the work to optimise efficiency and transient operations further.

The pan-European power grid is experiencing an increasing penetration of Variable Renewable Energy (VRE). The fluctuating and non-dispatchable nature of VRE hinders them in providing the Ancillary Service (AS) needed for the reliability and stability of the grid. Therefore, Energy Storage Systems (ESS) are needed along the VRE. Among the different ESS, a particularly viable and reliable option is Pumped Hydro Storage (PHS), given its cost-effective implementation and considerable lifespan, in comparison to other technologies. Traditional PHS plants with Francis turbines operate at a high head difference. However, not all regions have the necessary topology to make these plants cost-effective and efficient. Therefore, the ALPHEUS project will introduce low-head PHS for regions with a relatively flat topography. In this paper, a grid-forming controlled converter coupled with low-head PHS that can contribute to the grid stability is introduced, emphasising its ability to provide different AS, especially frequency control, through the provision of fast Frequency Containment Reserve (fFCR) as well as synthetic system inertia. This paper is an extended version of the paper "The Contribution of Low-head Pumped Hydro Storage to a successful Energy Transition", which was presented at the 19 th Wind Integration Workshop 2020.