Int. J. Turbomach. Propuls. Power, Volume 4, Issue 2 (June 2019) – 10 articles

The aim of this study was to assess the capabilities of different simulation approaches to predict the flutter stability of a steam turbine rotor. The focus here was on linear and nonlinear frequency domain solvers in combination with the energy method, which is widely used for the prediction of flutter onset. Whereas a GMRES solver was used for the linear problem, the nonlinear methods employed a time-marching procedure. The solvers were applied to the flutter analysis of the first rotor bending mode of the open Durham Steam Turbine test case. This test case is representative of the last stage of modern industrial steam turbines. We compared our results to those published by other researchers in terms of aerodynamic damping and local work per cycle coefficients. Time-domain, harmonic balance, and time-linearised methods were compared to each other in terms of CPU efficiency and accuracy. Full article

Shaped holes are considered as an effective solution to enhance gas turbine film-cooling performance, as they allow to increase the coolant mass-flux, while limiting the detrimental lift-off phenomena. A great amount of work has been carried out in past years on basic flat plate configurations while a reduced number of experimental works deals with a quantitative assessment of the influence of curvature and vane pressure gradient. In the present work PSP (Pressure Sensitive Paint) technique is used to detail the adiabatic effectiveness generated by axial shaped holes with high value of Area Ratio close to 7, in three different configurations with the same 1:1 scale: first of all, a flat plate configuration is examined; after that, the film-cooled pressure and suction sides of a turbine vane model are investigated. Tests were performed varying the blowing ratio and imposing a density ratio of $2.5$ . The experimental results are finally compared to the predictions of two different correlations, developed for flat plate configurations. Full article

Experimental activity has been performed to study different impingement cooling schemes in static and rotating conditions. Geometry replicates a leading-edge cold bridge system, including a radial supply channel and five rows of film-cooling and showerhead holes. Two impingement geometries have been studied, with different numbers of holes and diameters but with equal overall passage area. Reynolds numbers up to 13,800 and rotation numbers up to 0.002 have been investigated (based on an equivalent slot width). Tests have been performed using a novel implementation of transient heat transfer technique, which allows correct replication of the sign of buoyancy forces by flowing ambient temperature air into a preheated test article. Results show that complex interactions occur between the different features of the system, with a particularly strong effect of jet supply condition. Rotation further interacts with these phenomena, generally leading to a slight decrease in heat transfer. Full article

The present research focused on the analysis of the leakage flows developing from advanced blade tip geometries. The aerodynamic field of a contoured blade tip and of a high-performance rimmed blade were investigated against a baseline squealer rotor. Time-resolved numerical predictions were combined with high-frequency pressure measurements to characterize the tip leakage flow of each tip design. High spatial and temporal resolution measurements provided a detailed representation of the unsteady flow in the near-tip region and at the stage outlet. Numerical computations, based on the nonlinear harmonic method, were employed to assess the unsteady blade row interactions and identify the loss generation mechanisms depending on the tip design. The space- and time-resolved flow field was analysed by modal decomposition to identify the main periodicities of the near-tip and outlet flow and classify the most relevant sources of aerodynamic unsteadiness and entropy generation across the stage. Full article

Since the 1960s, turbomachinery design has mainly been based on similarity theory and empirical correlations derived from experimental data and manufacturing experience. Over the years, this knowledge was consolidated and summarized by parameters such as specific speed and diameters that represent the flow features on the meridional plane, hiding however the direct correlations between all the actual design parameters (e.g., blade number or hub-to-tip ratio). Today a series of statistical tools developed for big data analysis sheds new light on correlations among turbomachinery design and performance parameters. In the following article we explore a dataset of over 10,000 axial fans by means of principal component analysis and projection to latent structures. The aim is to find correlations between design and performance features and comment on the capabilities of this approach to give new insights on the design space of axial fans. Full article

Erosive wear of hydraulic machines is a common issue, which results in efficiency degradation, the enhancement of cavitation, and the need for expensive maintenance. Although numerical simulations of the erosion process could be very useful, both for understanding and predicting the process, its multiscale nature renders it very difficult to simulate. A previously validated multiscale model of erosion is presented. It consists of two coupled sub-models: On the microscale, the sediment impacts are simulated by means of comprehensive physical models; on the macroscale, the turbulent sediment transport and erosion accumulation are calculated. A multiscale simulation of the erosion of a prototype-scale Pelton bucket impacted by a sediment-laden water jet is presented. The simulation results, namely the erodent flux and the distributions of average impact angle and velocity on the bucket surface, bring insight into the erosion process. Furthermore, the results explain the obtained erosion distribution, which is in very good agreement with the experimental erosion measurements available in the literature for the same test case. Full article

This paper presents a gradient-based design optimization of a turbocharger radial turbine for automotive applications. The aim is to improve both the total-to-static efficiency and the moment of inertia of the turbine wheel. The search for the optimal designs is accomplished by a high-fidelity adjoint-based optimization framework using a fast sequential quadratic programming algorithm. The proposed method is able to produce improved Pareto-optimal designs, which are trade-offs between the two competing objectives, in only a few iterations. This is realized by redesigning the blade shape and the meridional flow channel for the respective target while satisfying imposed aerodynamic constraints. Furthermore, a comparative study with an evolutionary algorithm suggests that the gradient-based method has found the global Pareto front at a computational cost which is about one order of magnitude lower. Full article

The paper provides an evaluation of the errors embodied in the Axial Momentum Theory (AMT) as applied to a uniformly loaded actuator disk model without wake rotation. Although this model exhibits some unphysical features, such as the tip singularity and the violation of the angular momentum equation, it is still considered a touchstone in the theoretical aerodynamics of propellers. To simplify the model, a purely mathematical assumption is commonly used in the differential form of the axial momentum equation, i.e., the contribution of the pressure forces on the lateral surface of the infinitesimal streamtubes swallowed by the disk is neglected. In this paper, the errors introduced by this simplifying assumption are evaluated by comparing the results of the AMT with those of a nonlinear method modelling the free wake as the superposition of ring vortices distributed along the wake boundary. Firstly, the validity of this method is verified in terms of global performance coefficients. Then, using a CFD approach, it is also verified in terms of local flow quantities. The comparison between the ring-vortices method and the AMT shows that, for a highly loaded propeller, significant errors exist in the axial velocity at the disk, especially near the tip. Moreover, despite the uniform load, the axial velocity at the disk varies in the radial direction. Instead, the velocity magnitude remains almost uniform only for values of the thrust coefficient lower than 1. Full article

The present paper reports the results of an experimental and computational investigation of flat plate film cooling jets discharged from three fan-shaped holes. Measurements have been carried out at near unity density ratio in a low-speed wind tunnel, at low inlet turbulence intensity, with blowing ratios (BR) of 1 and 2. Aerodynamic results have shown that the jet stays attached to the flat plate. Thermal measurements have revealed that film cooling effectiveness decreases downstream of the holes, and BR equal to 1 provides the best trade-off between cooling air consumption and thermal protection. Consequently, BR = 1 was selected for assessing the performance of different turbulence models, implemented in STAR-CCM+, according with the steady Reynolds-averaged Navier–Stokes (RANS) approach. Predictions from realizable k-ε (RKE), shear stress transport k-ω (SST KW) and Reynolds stress model (RSM) were compared against measurements of laterally averaged and centerline adiabatic effectiveness, as well as off-the-wall velocity maps and profiles of stress components. RSM provided the most accurate predictions. Full article

Side channel pumps provide high pressure at relatively low flow rates. This comes along with a quite low specific speed and thus with the known disadvantage of a quite poor maximum efficiency. This paper describes the detailed analysis and optimisation of a typical 1-stage side channel pump with an additional radial suction impeller by means of computational fluid dynamics (CFD) simulations. In a first step, the model was successively generated and it was obvious that it has to contain all details including suction impeller and main stage (both 360° models) as well as the pressure housing and all narrow gaps to provide useful simulation results. Numerical simulations were carried out in a stationary and transient way with scale resolving turbulence models to analyse the components in detail. Finally the CFD-simulations were validated with model tests. For the optimisation process it was necessary to generate a reduced numerical model to analyse the effects of more than 300 geometry variations. The findings were then combined to establish the desired objectives. Finally the best combinations were validated again with the full numerical model. Those simulations predict a relative efficiency increase at best efficiency point (BEP) and part load >30% with respect to all given limitations like identical head curve, suction behavior, and dimensions. Full article