Giovanni Delibra

On the Role of Leading-Edge Bumps in the Control of Stall Onset in Axial Fan Blades

Taking a lead from the humpback whale flukes, characterized by a series of bumps that result in a sinusoidal-like leading edge, this paper reports on a three-dimensional numerical study of sinusoidal leading edges on cambered airfoil profiles. The turbulent flow around the cambered airfoil with the sinusoidal leading edge was computed at different angles of attack with the open source solver OpenFOAM, using two different eddy viscosity models integrated to the wall. The reported research focused on the effects of the modified leading edge in terms of lift-to-drag performance and the influence of camber on such parameters. For these reasons a comparison with a symmetric airfoil is provided. The research was primarily concerned with the elucidation of the fluid flow mechanisms induced by the bumps and the impact of those mechanisms on airfoil performance, on both symmetric and cambered profiles. The bumps on the leading edge influenced the aerodynamic performance of the airfoil, and the lift curves were found to feature an early recovery in post-stall for the symmetric profile with an additional gain in lift for the cambered profile. The bumps drove the fluid dynamic on the suction side of the airfoil, which in turn resulted in the capability to control the separation at the trailing edge in coincidence with the peak of the sinusoid at the leading edge.

The application of sinusoidal blade-leading edges in a fan-design methodology to improve stall resistance

Taking inspiration from previous biomimetic studies on the performance of humpback whale flippers, this paper reports a programme of work to design a ‘whale-fan’ that incorporates a sinusoidal leading-edge blade profile that mimics the tubercles on humpback whales flippers. Previous researchers have used two-dimensional cascades of aerofoils to study the effects of a sinusoidal profile on aerofoil lift and drag performance. The research was primarily concerned with elucidating the fluid-flow mechanisms induced by the sinusoidal profile and the impact of those mechanisms on aerofoil performance. The results indicate that a sinusoidal leading-edge profile has improved lift recovery post-stall and, thus, is inherently more aerodynamically resistant to the effect of stall. The reported research focuses on the application of previous research conducted with infinite span cascades of aerofoils to the design and optimisation of a finite span aerofoil. The paper presents the assumptions when developing a three-dimensional aerofoil-design methodology that correlates the sinusoidal profile of the blade-leading edge with the desired vorticity distribution at the trailing edge. The authors apply the developed methodology to the design of a fan blade’s tip region to control separation at the trailing edge. The paper presents numerically derived whale-fan performance characteristics and compares them with both numerically and experimentally derived performance characteristics of the baseline fan.

Large-eddy simulation of a tunnel ventilation fan

In this paper we discuss a computational method focused on the prediction of unsteady aerodynamics, adequate for industrial turbomachinery. Here we focus on a single rotor device selected from a new family of large tunnel ventilation axial flow fans. The flow field in the fan was simulated using the open source code OpenFOAM, with a large-eddy simulation (LES) approach. The sub-grid scale (SGS) closure relied on a one-equation model, that requires us to solve a differential transport equation for the modeled SGS turbulent kinetic energy. The use of such closure was here considered as a remedial strategy in LES of high-Reynolds industrial flows, being able to tackle the otherwise insufficient resolution of turbulence spectrum. The results show that LES of the fan allows to predict the pressure rise capability of the fan and to reproduce the most relevant flow features, such as three-dimensional separation and secondary flows.

Large-eddy simulations of tip leakage and secondary flows in an axial compressor cascade using a near-wall turbulence model:

Abstract This paper reports on the application of unsteady Reynolds averaged Navier—Stokes (U-RANS) and hybrid large-eddy simulation (LES)/Reynolds averaged Navier—Stokes (RANS) methods to predict flows in compressor cascades using an affordable computational mesh. Both approaches use the ζ—f elliptic relaxation eddy-viscosity model, which for U-RANS prevails throughout the flow, whereas for the hybrid the U-RANS is active only in the near-wall region, coupled with the dynamic LES in the rest of the flow. In this ‘seamless’ coupling the dissipation rate in the k-equation is multiplied by a grid-detection function in terms of the ratio of the RANS and LES length scales. The potential of both approaches was tested in several benchmark flows showing satisfactory agreement with the available experimental results. The flow pattern through the tip clearance in a low-speed linear cascade shows close similarity with experimental evidence, indicating that both approaches can reproduce qualitatively the tip leakage and tip separation vortices with a relatively coarse computational mesh. The hybrid method, however, showed to be superior in capturing the evolution of vortical structures and related unsteadiness in the hub and wake regions.

A Critical Review of Computational Methods and Their Application in Industrial Fan Design

Members of the aerospace fan community have systematically developed computational methods over the last five decades. The complexity of the developed methods and the difficulty associated with their practical application ensured that, although commercial computational codes date back to the 1980s, they were not fully exploited by industrial fan designers until the beginning of the 2000s. The application of commercial codes proved to be problematic as, unlike aerospace fans, industrial fans include electrical motors and other components from which the flow will invariably separate. Consequently, industrial fan designers found the application of commercial codes challenging. The decade from 2000 to 2010 was focused on developing techniques that would facilitate converged solutions that predicted the fans’ performance characteristics over the stable part of their operating range with reasonable accuracy, using a practical computational effort. In this paper, we focus on elucidating aspects of the flow physics that one cannot easily study in a laboratory environment, discussing the challenges involved and the relative merits of the available modelling techniques. The paper ends with a discussion of the practical problems associated with the use of commercial codes in a development environment and finally the legislation that is driving the need for aerospace style computation methods.

Hybrid LES/RANS of Internal Flows: A Case for More Advanced RANS

The Hybrid LES/RANS is emerging as the most viable modelling option for CFD of real-scale problems, at least in the aerospace design. Entrusting LES to resolve the intrinsic unsteadiness and three-dimensionality in the flow bulk reduces the modelling empiricism to a relatively small wall-adjacent RANS region, arguably justifying the use of very simple models. We argue, however, that for internal flows in complex passages, and involving heat and mass transfer, the role of the near-wall RANS should not be underestimated. The issue is discussed by two examples of flows in turbomachinery: a pinned internal-cooling passage in a turbine blade and tip leakage and wake in a compressor cascade with stagnant and moving casing. The examples illustrate the need for a topology-free wall-integration RANS model that accounts for versatile effects of multiple bounding walls. A HLR using an elliptic relaxation (\(\upsilon ^{2}/k-f\)) RANS model coupled with a dynamic LES showed to perform well in the cases considered.

An les insight into convective mechanism of heat transfer in a wall-bounded pin matrix

We report on an LES (large-eddy-simulations) study of flow and heat transfer in a longitudinal periodic segment of a matrix of cylindrical rods in a staggered arrangement bounded by two parallel heated walls. The configuration replicates the set-up investigated experimentally by Ames et al. (ASME Turbo Expo, GT2007-27432) and mimics the situation encountered in internal cooling of gas-turbine blades. LES have been performed using the in-house finite-volume computational code T-FlowS. Considered are two Reynolds numbers, 10000 and 30000, based on the rod diameter and maximum velocity in the matrix. The unstructured grid contained around 5 and 15 million cells for the two Re numbers respectively. After validating the simulations with respect to the available experimental data, the paper discusses the characteristic vortex and plume structures, streamline and heatline patterns and their evolution along the pin matrix, around individual pins and at the pin-endwall junctions. It is concluded that the convection by organized vertical structures originated from vortex shedding govern the thermal field and play the key role in endwall heat transfer, exceeding by far the stochastic turbulent transport.Copyright

LES and Hybrid LES/RANS Study of Flow and Heat Transfer around a Wall-Bounded Short Cylinder

The flow in plate-fin-and-tube heat exchangers is featured by interesting dynamics of vortical structures, which, due to close proximity of bounding walls that suppress instabilities, differs significantly from the better-known patterns around long cylinders. Typically, several distinct vortex systems can be identified both in front and behind the pin. Their signature on the pin and end-walls reflects directly in the local heat transfer. The Reynolds numbers is usually moderate and the incoming flow is non-turbulent, transiting to turbulence on or just behind the first or few subsequent pin/tube rows. Upstream from the first pin a sequence of several horseshoe vortices attached to the boundingwall is created, while the unsteady wake contains also multiple vortical systems which control the entrainment of fresh fluid and its mixing with the hot fluid that was in contact with the heated surfaces [1]. The conventional CFD using standard turbulence models, as practiced by heat exchangers industries, falls short in capturing the subtle details of the complex vortex systems. A fine-grid LES can provide accurate solutions, but for more complex configurations and higher Re numbers a hybrid RANS/LES using a coarser grid seems a more rational option, provided it can capture all important flow and vortical features.

Flow Analysis of a Wave-Energy Air Turbine with the SUPG/PSPG Method and DCDD

We present flow analysis of a wave-energy air turbine, specifically a Wells turbine. The analysis is based on the Streamline-Upwind/Petrov-Galerkin (SUPG) and Pressure-Stabilizing/Petrov-Galerkin (PSPG) methods and discontinuity-capturing directional dissipation (DCDD). The DCDD, first introduced to complement the SUPG/PSPG method in computation of incompressible flows in the presence of sharp solution gradients, was also shown to perform well in turbulent-flow test computations when compared to the Smagorinsky large eddy simulation model. Our computational analysis of the Wells turbine here, with results that compare favorably to the available experimental data, shows that the DCDD method performs well also in turbomachinery flows.

SIMULATION OF PARTICLE-LADEN FLOWS IN A LARGE CENTRIFUGAL FAN FOR EROSION PREDICTION

Regulations require that industrial fans utilised in power generation, cement and steel applications must operate as part of a process that produces erosive particles. Over time these erosive particles erode centrifugal fan impeller blades, changing the blade profile and consequently, degrading fan performance. To replace the eroded impellers, operators must shut down the process. If one must replace an impeller between scheduled maintenance intervals, the associated costs with lost production become significant. Consequently, the industrial fan community is interested in predicting the erosion, and ultimately, a fan impeller’s in-service life when operating in an erosive environment. Industrial fan designers face challenges when attempting to predict impeller erosion. Industrial centrifugal fan impeller blades are routinely constructed from cambered plate, usually with backward or forward sweeping, with the inevitable consequence of separated flow regions. This separated flow is within a highly three dimensional flow-field making difficult an accurate prediction of the flow-field though an impeller with cambered plate blades. Assuming that one can accurately predict this three dimensional flow-field one must then go on to simulate the erosive particles’ trajectory.This paper builds on the work of other scholars who have developed a computational approach that accurately predicts the flow-field though an impeller with cambered plate blades. The authors report an unsteady numerical analysis with the finite volume open-source code OpenFOAM. The analysis was undertaken using a moving mesh technique, based on Arbitrary Mesh Interface technology. Reynolds Averaged Navier-Stokes equations for incompressible flow were solved with a non-linear first order turbulence closure. They modelled particle transport and dispersion using a Lagrangian approach coupled with a Particle Cloud Tracking (PCT) model. Understanding the particle size effect facilitates identifying critical regions on the impeller blades most prone to erosion for each combination of particle sizes. Identifying the most critical regions thus provides a basis for modifying overall impeller and individual blade geometry in an effort to reduce susceptibility to erosion. This then increases in-service life, and consequently the time between maintenance intervals.© 2014 ASME