Hybrid RANS-LES for turbomachinery

A range of popular hybrid Reynolds averaged Navier-Stokes- large eddy simulation (RANS-LES) methods are tested for cavity and labyrinth seal flows using an in-house high order computational fluid dynamics (CFD) code and a commercial CFD code. The models include the Spalart-Allmaras (SA) and Menter SST variants of delayed detached eddy simulation (DDES), the Menter scale adaptive simulation (SAS) model, and a new enhanced variant of SA DDES recently presented in the literature. The latter modifies the original definition of the subgrid length-scale used in DDES based on local vorticity and strain. For both geometries, the meshes are hybrid RANS-LES adequate. Very low levels of resolved turbulent content are observed for both the cavity and labyrinth seal flows for all models apart from the enhanced version of DDES. Similar findings are observed for both the commercial and in-house CFD codes. For both cases most models essentially produce a quasi-two-dimensional flow field with minimal resolved content. For the cavity simulations there is a significant under prediction of turbulent statistics. The enhanced version of SA-DDES shows a significant improvement and resolves turbulent content over a wide range of scales. Improved agreement with experimental measurements is also observed. It is recommended that extreme care should be taken where hybrid RANS-LES simulations are essentially steady but have lower than RANS levels of eddy viscosity

Differential Equation-Based Specification of Turbulence Integral Length Scales for Cavity Flows

A new modeling approach has been developed that explicitly accounts for expected turbulent eddy length scales in cavity zones. It uses a hybrid approach with Poisson and Hamilton–Jacobi differential equations. These are used to set turbulent length scales to sensible expected values. For complex rim-seal and shroud cavity designs, the method sets an expected length scale based on local cavity width which accurately accounts for the large-scale wakelike flow structures that have been observed in these zones. The method is used to generate length scale fields for three complex rim-seal geometries. Good convergence properties are found, and a smooth transition of length scale between zones is observed. The approach is integrated with the popular Menter shear stress transport (SST) Reynolds-averaged Navier–Stokes (RANS) turbulence model and reduces to the standard Menter model in the mainstream flow. For validation of the model, a transonic deep cavity simulation is performed. Overall, the Poisson–Hamilton–Jacobi model shows significant quantitative and qualitative improvement over the standard Menter and k–ε two-equation turbulence models. In some instances, it is comparable or more accurate than high-fidelity large eddy simulation (LES). In its current development, the approach has been extended through the use of an initial stage of length scale estimation using a Poisson equation. This essentially reduces the need for user objectivity. A key aspect of the approach is that the length scale is automatically set by the model. Notably, the current method is readily implementable in an unstructured, parallel processing computational framework

SAS – SST simulations of the flow and heat transfer inside a square ribbed duct with artificial forcing

Scale Resolving Simulations (SRS) are emerging as a promising compromise of cost and accuracy for industrial simulations of flows inside turbine blade cooling systems as they represent a necessary increase of accuracy with respect to Reynolds Averaged Navier Stokes (RANS) in the field. In this paper, several hybrid RANS-LES (Large Eddy Simulation) and SRS approaches are investigated. A Scale Adaptive Simulation (SAS) with spectrally calibrated artificial forcing is used to simulate flow inside a development section of a square duct with eight square equispaced ribs. Energy spectra, two-point correlations as well as other standard metrics are used to assess resolved content qualitatively as well as quantitatively. It is found that unmodified SST-SAS offers a marginal improvement over Unsteady RANS (URANS) for the present type of flow even on a LES-type grid and the solution is essentially steady. The artificial forcing used seems to trigger the resolving capability of the model and the solution is noticeably closer to experimental results while requiring minor extra computational demand. Effects of rotation are examined and it is found that the rotation appears to trigger the resolving mode of the unforced SAS model

Towards investigation of external oil flow from a journal bearing in an epicyclic gearbox

High loads and bearing life requirements make journal bearings the preferred choice for use in high power, planetary gearboxes in jet engines. With the planet gears rotating about their own axis and orbiting around the sun gear, centrifugal forces generated by both motions interact with each and generate complex kinematic conditions. This paper presents a literature and state-of-the-art knowledge review to identify existing work performed on cases similar to external journal bearing oil flow. In order to numerically investigate external journal bearing oil flow, an approach to decompose an actual journal bearing into simplified models is proposed. Preliminary modeling considerations are discussed. The findings and conclusions are used to create a three dimensional (3D), two-component computational fluid dynamic (CFD) sector model with rotationally periodic boundaries of the most simplistic approximation of an actual journal bearing: a non-orbiting representation, rotating about its own axis, with a circumferentially constant, i.e. concentric, lubricating gap. In order to track the phase interface between the oil and the air, the Volume of Fluid (VoF) method is used. External journal bearing oil flow is simulated with a number of different mesh densities. Two different operating temperatures, representing low and high viscosity oil, are used to assess the effect on the external flow field behaviour. In order to achieve the future objective of creating a design tool for routine use, key areas are identified in which further progress is required

A phase-change model for diffusion-driven mass transfer problems in incompressible two-phase flows

We present a VOF-based numerical method for incompressible Direct Navier–Stokes (DNS) equations for diffusion-driven phase-change flows. A special emphasis is placed on the treatment of velocity discontinuities across the interface. A novel algorithm is presented to smoothly extend the liquid velocity field across the interface in a way that the interface can be transported by a divergence-free velocity field. The transport of species is treated with a two-scalar approach and special attention is paid to the advection and diffusion steps in order to prevent artificial mass transfer. The methodology is implemented in the open-source code Basilisk and is validated against analytical and semi-analytical models. The relative errors on the relevant quantities are generally below 1% for the finest grids. The method is finally applied to study the growth of electrochemically generated bubbles on planar electrodes and the effect of contact angles and number of nucleation sites is investigated

Windage Torque Reduction in Low-Pressure Turbine Cavities Part 2:Experimental and Numerical Results

Minimizing the losses within a low-pressure turbine (LPT) system is critical for the design of next-generation ultra-high bypass ratio aero-engines. The stator-well cavity windage torque can be a significant source of loss within the system, influenced by the ingestion of mainstream annulus air with a tangential velocity opposite to that of the rotor. This paper presents experimental and numerical results of three carefully designed Flow Control Concepts (FCCs) – additional geometric features on the stator surfaces, which were optimized to minimize the windage torque within a scaled, engine-representative stator-well cavity. FCC1 and FCC2 featured rows of guide vanes at the inlet to the downstream and upstream wheel-spaces, respectively. FCC3 combined FCC1 and FCC2. Superposed flows were introduced to the upstream section of the cavity, which modelled the low radius coolant and higher radius leakage between the rotor blades. In addition to torque measurements, total and static pressures were collected, from which the cavity swirl ratio was derived. Additional swirl measurements were collected using a five-hole aerodynamic probe, which traversed radially at the entrance and exit of the cavity. A cavity windage torque reduction of 55% on the baseline (which has no flow control) was measured for FCC3, at the design condition with superposed flow. For this concept, an increase in the cavity swirl in both the upstream and downstream wheel-spaces was demonstrated experimentally and numerically. With increasing superposed flow, the contribution of FCC1 surpassed FCC2, due to more mass flow enterin

Windage Torque Reduction in Low-Pressure Turbine Cavities Part 1:Concept Design and Numerical Investigations

The windage torque on rotational walls has negative effect on the performance of the low pressure turbine. In this paper, three novel flow control concepts (FCCs) were proposed to reduce the windage torque within a turbine stator well, with upstream and downstream cavities connected by an interstage labyrinth seal. The swirl and flow pattern inside a reference turbine cavity was first investigated and the potential locations for the FCCs were identified using numerical simulations. FCC1 was a circumferential row of leaned deflectors downstream of the labyrinth seal. FCC2 was a set of deflector vanes and platform to optimize the ingress swirl at high radius in the upstream cavity. FCC3 combined the two flow concepts and the superposition resulted in a stator well windage torque reduction of 70% when compared to the baseline design. The FCCs also showed performance benefits at off-design conditions and over a range of secondary flow rates to the cavity. In Part 2 [1], the numerical analysis and performance of the FCCs are validated in an experimental rig, using additively-manufactured components

Scalable Continuous Vortex Reactor for Gram to Kilo Scale for UV and Visible Photochemistry

© 2020 American Chemical Society. We report the development of a scalable continuous Taylor vortex reactor for both UV and visible photochemistry. This builds on our recent report (Org. Process Res. Dev. 2017, 21, 1042) detailing a new approach to continuous visible photochemistry. Here, we expand this by showing that our approach can also be applied to UV photochemistry and that either UV or visible photochemistry can be scaled-up using our design. We have achieved scale-up in productivity of over 300× with a visible light photo-oxidation that requires oxygen gas and 10× with a UV-induced [2 + 2] cycloaddition obtaining scales of up to 7.45 kg day-1 for the latter. Furthermore, we demonstrate that oxygen is efficiently taken up in the reactions of singlet O2, and for the examples examined, that near-stoichiometric quantities of oxygen can be used with little loss of reactor productivity. Furthermore, our design should be scalable to a substantially larger size and have the potential for scaling-out with reactors in parallel

High-Productivity Single-Pass Electrochemical Birch Reduction of Naphthalenes in a Continuous Flow Electrochemical Taylor Vortex Reactor

We report the development of a single-pass electrochemical Birch reduction carried out in a small footprint electrochemical Taylor vortex reactor with projected productivities of >80 g day-1 (based on 32.2 mmol h-1), using a modified version of our previously reported reactor [Org. Process Res. Dev. 2021, 25, 7, 1619-1627], consisting of a static outer electrode and a rapidly rotating cylindrical inner electrode. In this study, we used an aluminum tube as the sacrificial outer electrode and stainless steel as the rotating inner electrode. We have established the viability of using a sacrificial aluminum anode for the electrochemical reduction of naphthalene, and by varying the current, we can switch between high selectivity (>90%) for either the single ring reduction or double ring reduction with >80 g day-1 projected productivity for either product. The concentration of LiBr in solution changes the fluid dynamics of the reaction mixture investigated by computational fluid dynamics, and this affects equilibration time, monitored using Fourier transform infrared spectroscopy. We show that the concentrations of electrolyte (LiBr) and proton source (dimethylurea) can be reduced while maintaining high reaction efficiency. We also report the reduction of 1-aminonaphthalene, which has been used as a precursor to the API Ropinirole. We find that our methodology produces the corresponding dihydronaphthalene with excellent selectivity and 88% isolated yield in an uninterrupted run of >8 h with a projected productivity of >100 g day-1