Int. J. Turbomach. Propuls. Power, Volume 3, Issue 1 (March 2018) – 8 articles

A highly loaded turbine exit guide vane with active boundary layer control was investigated experimentally in the High Speed Cascade Wind Tunnel at the University of the German Federal Armed Forces, Munich. The experiments include profile Mach number distributions, wake traverse measurements as well as boundary layer investigations with a flattened Pitot probe. Active boundary layer control by fluidic oscillators was applied to achieve improved performance in the low Reynolds number regime. Low solidity, which can be applied to reduce the number of blades, increases the risk of flow separation resulting in increased total pressure losses. Active boundary layer control is supposed to overcome these negative effects. The experiments show that active boundary layer control by fluidic oscillators is an appropriate way to suppress massive open separation bubbles in the low Reynolds number regime. Full article

This article reports on systematic numerical studies on an axial compressor stage with distorted inflow. Four operating points at two speedlines were simulated with an inflow distortion generated by a ${120}^{\circ }$ -sector segment with a wedge-type cross-section. With this setup, the interaction between the distorted inflow and the rotor flow was studied. The focus was put on the differences of the interaction between the distorted inflow and rotor flow as well as on the compressor behaviour at subsonic and transonic blade tip speeds, as the general mechanisms have been analysed in more detail in previous publications. The distorted flow itself is not influenced by the blade tip speed, but the interaction phenomena depend strongly on the spool speed and operating point. Additionally, the blade tip speed influences the circumferential sector of the compressor stage exit, which is affected by the distorted flow. The impact reaches from a small sector at 65% spool speed, with the peak efficiency operating point up to nearly the entire annulus at 100% spool speed, near the stall operating point. Full article

Flow measurement using a linear compressor or turbine cascade is a well-established technique to characterize the flow in turbomachines with a certain degree of abstraction. A common way to obtain a general characterization of the flow is to measure the flow downstream of the cascade with a five-hole probe, obtaining, e.g., total pressure losses and flow turning. Pneumatic five-hole probes are used to capture steady or time-averaged flow quantities, if not specified otherwise. In dependency of probe geometry, measurement set-up and flow properties, such measurements can be very time-consuming. Various techniques, in order to decrease the measurement time, are proposed in literature but for certain applications the efforts required to implement such techniques can outweigh the enhanced measurement speed. In this paper, methods proposed by other authors are combined and extended to allow for fast or transient five-hole probe measurements at strongly varying flow conditions. The effectiveness of this method is presented for flow measurements downstream of a compressor cascade with attached and stalled flow (by varying the Reynolds number) as well as with steady and periodically unsteady inflow. The new method allows to reduce the measurement time by up to 90 percent without compromising measurement accuracy. In fact, due to higher spatial resolution, the flow downstream of the cascade can be better resolved with the new method. Full article

Secondary flow and loss development in the T106Div-EIZ low-pressure turbine cascade are investigated utilizing (U)RANS simulations in cases with and without periodically incoming wakes at $M{a}_{2th}=0.59$ and $R{e}_{2th}=2\times {10}^5$ . The predictions are compared to experimental data presented by Kirik and Niehuis (2015). The axial mid-span and overall loss development in the T106Div-EIZ and the T106A-EIZ in the steady case are analyzed regarding the effects caused by the different loading distributions and by the divergent endwall geometry. Furthermore, the entropy generation is analyzed in the T106Div-EIZ with periodically incoming wakes in several axial positions of interest and compared to the undisturbed steady case. It is found that in the front-loaded T106Div-EIZ, the incoming wakes cause a premature endwall loss production in the front part of the passage, resulting in a lower intensity of the secondary flow downstream of the passage and a redistribution of the loss generation components. Full article

The continuing rise in turbine entry temperatures has necessitated the development of ever-more advanced cooling techniques. Effusion cooling is an example of such a system and is characterised by a high density of film cooling holes that operate at low blowing ratios, thereby achieving high overall cooling effectiveness. This paper presents both an experimental and computational investigation into the cooling performance of effusion systems. Two flat-plate geometries (with primary hole pitches of 3.0D and 5.75D) are experimentally investigated via a pressure sensitive paint technique yielding high resolution film effectiveness distributions via heat-mass transfer analogy. A computational fluid dynamics (CFD) scalar tracking method was used to model the setup computationally with the results comparing favourably to those obtained from the experiments. The CFD domain was modified to assess the cooling performance from a single film hole ejection. A superposition method was developed and applied to the resulting two-dimensional film effectiveness distribution that quickly yielded data for an array of closely-packed holes, allowing a rapid assessment of a multi-hole effusion type setup. The method produced satisfactory results at higher pitches, but at lower pitches, high levels of jet interactions reduced the performance of the superposition method. Full article

The decrease in the performance of centrifugal compressors operating at low Reynolds numbers (e.g., unmanned aerial vehicles at high altitudes or small turbomachines) can reach 10% due to increased friction. The purposes of this review are to represent the state-of-the-art of the active and passive flow control methods used to improve performance and/or widen the operating range in numerous engineering applications, and to investigate their applicability in low-Reynolds-number centrifugal compressors. The applicable method should increase performance by reducing drag, increasing blade loading, or reducing tip leakage. Based on the aerodynamic and structural demands, passive methods like riblets, squealers, winglets and grooves could be beneficial; however, the drawbacks of these approaches are that their performance depends on the operating conditions and the effect might be negative at higher Reynolds numbers. The flow control method, which would reduce the boundary layer thickness and reduce wake, could have a beneficial impact on the performance of a low-Reynolds-number compressor in the entire operating range, but none of the methods represented in this review fully fulfil this objective. Full article

The aim of this paper is to demonstrate how the harmonic balance method can be used to predict rotor–rotor and stator–stator interactions in turbomachinery. These interactions occur in the form of clocking and indexing. Whereas clocking refers to the dependency of the performance on the relative circumferential positioning of the rotors or stators, the term indexing is used when different blade (or vane) counts lead to an aperiodic time-averaged flow. The approach developed here is closely related to the one presented by He, Chen, Wells, Li, and Ning, who generalised the Nonlinear Harmonic method to zero-frequency disturbances. In particular, configurations with only one passage per blade row are used for the simulations. We validate the methods by means of the simulation of a fan stage configuration with rotationally asymmetric inlet conditions. It is demonstrated that the harmonic balance solver is able to accurately predict the inhomogeneity of the time-averaged flow field in the stator row. Moreover, the results show that the approach offers a considerable gain in computational efficiency. Full article