Purpose The aim of this paper is to assess the ability of a stress-blended eddy simulation (SBES) turbulence model to predict the performance of a three-straight-bladed vertical axis wind turbine (VAWT). The grid sensitivity study is conducted to evaluate the simulation accuracy. Design/methodology/approach The unsteady Reynolds-averaged Navier–Stokes equations are solved using the computational fluid dynamics (CFD) technique. Two types of grid topology around the blades, namely, O-grid (OG) and C-grid (CG) types, are considered for grid sensitivity studies. Findings With regard to the power coefficient (Cp), simulation results have shown significant improvements of predictions using compared to other turbulence models such as the k-e model. The Cp distributions predicted by applying the CG mesh are in good agreement with the experimental data than that by the OG mesh. Research limitations/implications The current study provides some new insights of the use of SBES turbulence model in VAWT CFD simulations. Practical implications The SBES turbulence model can significantly improve the numerical accuracy on predicting the VAWT performance at a lower tip speed ratio (TSR), which other turbulence models cannot achieve. Furthermore, it has less computational demand for the finer grid resolution used in the RANS-Large Eddy Simulation (LES) “transition” zone compared to other hybrid RANS-LES models. Originality/value To authors’ knowledge, this is the first attempt to apply SBES turbulence model to predict VAWT performance resulting for accurate CFD results. The better prediction can increase the credibility of computational evaluation of a new or an improved configuration of VAWT.
Aeroengine bearing chambers typically contain bearings, seals, shafts and static parts. Oil is introduced for lubrication and cooling and this creates a two phase flow environment that may contain droplets, mist, film, ligaments, froth or foam and liquid pools. Some regions of the chamber contain a highly rotating air flow such that there are zones where the flow is gravity dominated and zones where it is rotation dominated. The University of Nottingham Technology Centre in Gas Turbine Transmission Systems, is conducting an ongoing experimental program investigating liquid and gas flow behavior in a relevant highly rotating environment. Previously reported work by the UTC has investigated film thickness and residence volume within a simplified chamber consisting of outer cylindrical chamber, inner rotating shaft and cuboid off-take geometry (termed the generic deep sump). Recently, a more aeroengine relevant bearing chamber offtake geometry has been studied. This geometry is similar to one investigated at Purdue University and consists of a “sub-sump” region approached by curved surfaces linked to the bearing chamber. The test chamber consists of an outer, stationary cylinder with an inner rotating shaft. The rig runs at ambient pressure and the working fluid (water) is introduced either via a film generator on the chamber wall or through holes in the shaft. In addition to visual data (high speed and normal video), liquid residence volume within the chamber and film thickness were the two numerical comparators chosen. Data was obtained for a number of liquid supply rates, scavenge ratios and shaft rotation speeds. The data from the current model is compared to that from the earlier studies. The data shows that in contrast to the previously reported generic deep sump study, the residence volume of the curved wall deep sump (CWDS) designs is far less sensitive to shaft speed, liquid supply rate and scavenge ratio. The method of liquid supply only makes a significant difference at the lowest scavenge ratios. Residence volume data for the Nottingham CWDS is comparable, when appropriately scaled, to that for the Purdue design. The film thickness data shows that at the lower shaft speeds investigated the flow is gravity dominated whereas at higher shaft speeds shear dominates.
Aeroengines incorporate various bearing chambers that house the shaft bearings and the oil used to cool and lubricate these bearings must subsequently be recovered from these chambers. Effective oil removal (scavenge) is essential to avoid heat generation through unnecessary working of the oil which can lead to excessive heat generation and reduced overall efficiency. Therefore the design of the scavenge region (sump) in a bearing chamber, as well as the ability to assess its performance is very important. An ongoing research program into bearing chamber scavenge comprising experimental and computational components is being conducted at the University of Nottingham Technology Centre in Gas Turbine Transmission Systems. This program is enhancing understanding of sump performance and design. In this paper an experimental study into a simplified but representative scavenge is reported. This experimental work helps to further understanding of the complex two-phase flow physics in a bearing chamber, particularly in the scavenge region, by means of various measurements and flow visualization. For the study a bespoke test rig has been built. It consists of a simplified, generic bearing chamber with simple sump geometry constructed entirely of Perspex to allow visualization. A shaft in the centre of the chamber capable of rotating up to 15,000 rpm is employed to introduce a windage flow in the chamber. Water (the working fluid) is fed to the chamber via an inlet pump and an outlet pump removes liquid from the chamber, closing the circuit. Several pneumatic pinch valves are installed in the flow circuit to allow residence volume measurement. A completely air-tight reservoir with internal baffle functions as a simple liquid-gas separator, allowing measurement of gas volumetric flow rate in the off-take pipe; hence the scavenge ratio (ratio of total exit volume to liquid volume) can be obtained. Residence volume measurements highlight the importance of sump geometry as an ill-designed sump can lead to an undesirable increase in residence volume.
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