Farid Bakir

2D and 3D Unsteady Flow in Squirrel-Cage Centrifugal Fan and Aeroacoustic Behavior

The objective of this paper is the study and the analysis of the complex phenomena related to the internal flow in a centrifugal fan, using Computational Fluid Dynamics (CFD) tools, completed with experimental investigation in order to validate the used numerical models. The CFD analysis concerns 2D and 3D unsteady flow. The studied phenomena are the interactions and unsteadiness induced by the motion of the rotating blades relatively to the volute and their impact on the aeroacoustic behavior of the fan. Thus, 3D and 2D unsteady calculations using Unsteady Reynolds Averaged Navier Stokes (URANS) approach has been applied on a hybrid mesh grid whose refinement has been studied and adapted to the flow morphology. Turbulence has been modeled with the k-ω-Shear Stress Model (SST) model. The computational domain has been divided into two zones, a rotating zone including the impeller and stationary zone including the volute. A sliding mesh technique has been applied to the interfaces in order to allow the unsteady interactions between the two zones. The overall performances predicted by the computations have been validated at different flow rate. For each geometry modeling (2D and 3D), the unsteady part of the study is illustrated by analyzing the pressure fluctuations on different points from the lateral surface of the volute. The analysis of the wake generated by the rotation of the blower shows that the volute tongue is the main zone of unsteadiness and flow perturbations. In order to predict the acoustic pressures, the unsteady flow field variables provided by the CFD calculations have been used as inputs in the Ffowks Williams-Hawkings equations.Copyright

Experimental investigation of an actively controlled automotive cooling fan using steady air injection in the leakage gap

In an axial fan, a leakage flow driven by a pressure gradient between the pressure side and the suction side occurs in the gap between the shroud and the casing. This leakage flow is in the opposite direction to the main flow and is responsible for significant energy dissipation. Therefore, many authors have worked to understand this phenomenon in order to reduce these inherent energy losses. Up to now, most of the studies reported in the literature have been passive solutions. In this paper, an experimental controlling strategy is suggested to reduce the leakage flow rate. To this end, a fan with hollow blades and a specific drive system were designed and built for air injection. Air is injected in the leakage gap at the fan periphery. The experiment was performed for three rotation speeds, five injection rates and two configurations: 16 and 32 injection holes on the fan’s circumference. The experimental results of this investigation are presented in this article.

Towards Numerical Simulation of Snow Showers in Jet Engine Fuel Systems

Aircraft fuel systems are subject to icing at low temperatures. If the flow rate is increased, sudden releases of large quantities of ice may occur, called “snow showers”. They threaten the safety of flights and have been the subject of several investigations over past years. Jet engine fuel system components may be sensitive to clogging. When a snow shower happens, ice particles settle in seconds, forming a porous layer. Modelling such events involves transient hydraulics and solid dynamics. We propose to investigate numerically the dynamics of transient particle clogging. Equations of motion for the incompressible fluid phase are discretized in a high-order finite volume context and solved using a pressure-based algorithm. The discrete phase is modelled in a Lagrangian frame. Contacts between solids are handled by a dedicated algorithm. Solid volume fraction is calculated in regions occupied by particles. Finally, two-way coupling is achieved by source terms for momentum exchange, viscous and inertial loss. 2D simulation of the clogging of an ideal filter is performed.

Influence of Design Parameters on the Unsteady Flow in a Centrifugal Fan

The aim of this study is to evaluate the influence of design parameters on the unsteady flow in a forward-curved centrifugal fan and their impact on the aeroacoustic behavior. To do so, numerical and experimental study has been carried out on four centrifugal impellers designed with various geometrical parameters. The same volute casing has been used to study these fans. The effects on the unsteady flow behavior related to irregular blade spacing, blade number and radial distance between the impeller periphery and the volute tongue have been studied. The numerical simulations of the unsteady flow have been carried out using Computational Fluid Dynamics tools (CFD) based on Unsteady Reynolds Averaged Navier Stokes approach (URANS). The sliding mesh technique has been applied at the interfaces between the rotating and stationary zones in order to model the blades’ motion relative to the volute casing. The study is focused on the unsteadiness induced by the aerodynamic interaction between the volute and the rotating impeller blades. In order to predict the acoustic pressure at far field, the unsteady flow variables provided by the CFD calculations (pressure and velocity fluctuations acquired upon the surfaces of the rotating blades) have been used as inputs in the Ffowcs Williams-Hawkings equations (FW-H). Using this model, the acoustic pressure has been computed at the fan exit duct. The experimental part of this work concerns measurement of aerodynamic performance of the fans using a test bench built according to ISO 5801 [1] standard. In addition to this, pressure microphones have been flush-mounted on the volute tongue surface in order to measure the wall pressure fluctuations. The sound pressure level (SPL) measurements have been carried out in an anechoic room in order to remove undesired noise reflections. Finally, the numerical results have been compared with the experimental measurements and a correlation between the wall pressure fluctuations and the far field noise signals has been found.Copyright

Unsteady Flow in Multistage Centrifugal Fans

In order to improve the design of the multistage centrifugal fans, theoretical and experimental works were carried out in the optimization field of the unsteady 3D flows. Particular attention is given to the flows located at the rotor-stator interface. This zone is the seat of strong interactions between the moving part and the fixed part. This phenomenon has as consequences: strongly unsteady flow, fluctuating efforts on the stator bladings and an efficiency decrease. The analysis of the fluid behavior in this zone allowed the judicious choice of the fan geometry, the aim is to improve the flow organization and consequently the aeroacoustic performances. A numerical simulation tool was used in order to determine the kinematics and the dynamics of these flows. The measurements of the steady and unsteady flow characteristics allowed the comparison of the theoretical and experimental results.Copyright

Numerical Modeling of Aerated Cavitation Using Compressible Homogeneous Equilibrium Model

Cavitation is a well-known physical phenomenon occurring in various technical applications such as hydraulic turbo-machines, pipe flows, and venturis. Coupling aeration in a cavitating flow is a recent technique to control the overall effect of the cavitation over the zone of interest. The aeration process is done by injecting spherical air bubbles into the fluid flow without having at the same time an interaction with it. The contact handling algorithm is based on the projection of the velocity field of the injected particles over the velocity field of the fluid flow, in such a manner that, at each time step the gradient of the distance between every two bubbles is kept non-negative as a guarantee of the non-overlapping. The collisions between the air bubbles are considered as inelastic. The differential equation system is composed of the Navier-Stokes equations, implemented with the Homogeneous Mixture Model. The latter accounts the three phases (liquid, vapor, and mixture) separately. In the mixture phase, the gas and liquid phases are considered in local thermodynamic equilibrium. A high-order Finite Volume solver based on Moving Least Squares approximations is used for this analysis. For the sake of the numerical simulations, structured and unstructured grids have been used. The code makes use of a SLAU-type Riemann solver for low Mach numbers in order to accurately calculate the numerical fluxes. To avoid any numerical oscillations in the zones of strong gradients, a slope limiter algorithm is coupled with a Moving Least Squares sensor detecting any discontinuities.

Estimation of the Turbulence Model of Cavitation Effect on the Reynolds Stress Equation with Rotor Vibrations

The objectives of the study are to find a computational method to realize the cavitation flows under a variable rotational motion and develop a turbulence model to describe this phenomenon. We propose an integral model (closure) in the momentum equation and obtain information about the intensity of correlations in the rotational turbulence model. The computational models are as follows. We took the integral of the pressure and moment on the blade balance to the moment of inertia and replaced the variable rotational term with the integral in the momentum equation. We set up the geometry of a pseudo twodimensional centrifugal blade system, and performed four cases of the computations: neither cavitations nor integral form, cavitations and no-integral form, no cavitations with integral form, and cavitations with the integral form. We used the phase field model (PFM) for cavitations and the compressible Navier Stokes equation (MUSCL, third order). We tested the instability of the integration model with the moment of inertia and found the computations were unstable if the no-dimensional moment of inertia was less than 0.3 in (1000rpm, 2000rpm, and 5000rpm). We also found the pressure on the blade relaxed somewhat if we added the integral terms. In cavitation problems without the integral terms, we set the edge of the bubble was located in the middle of the blade passage and the interface was always oscillating. If we added the integral term, the edge of the separation bubble drifted to the trailing edge. The lift was decreased for cavitation conditions, even with a volume ratio of air less than 0.1. We obtained the unsteady rotational terms in the Reynolds stress equation with the integration model. The normalized correlation of the variable rotational term was 0.2 for cavitation flows and 0.25 for no-cavitaion flows. For both cases the variable integral terms should be included in any turbulence modeling. The results infer that the blade made by lighter materials has less moment of inertia and tends to vibrate more easily. The results show that the present computational model can realize the cavitation flows in variable rotation frames.

Numerical and Experimental Study of Mass Transfer Through Cavitation in Turbomachinery

The vapour generation in a liquid can be caused by two different mechanisms: following a heat input, thus an increase in temperature at constant pressure, which is well known as the boiling phenomenon, or, at constant temperature, a decrease of pressure, which corresponds to the cavitation phenomenon. When the liquid pressure decreases below the saturation pressure, some liquid undergoes a phase change, from liquid to vapour. The saturation pressure, pv, is a fluid property which depends strongly on the fluid temperature. The cavitation phenomenon is manifested, in the fluid flow, by the formation of bubbles, regions of vapour or vapour eddies. The cavitation phenomenon frequently occurs in hydraulic machines operating under low pressure conditions. The cavitation phenomenon causes several undesirable effects on this type of machines, for example: the noise generated by the mass transfer between the phases, the efficiency loss of the hydraulic machines, and the erosion of certain elements caused by the vapour bubbles collapses near walls. Additionally, it should be mentioned the flow instabilities caused by the vapour appearance, such as alternate blade cavitation and rotating blade cavitation (Campos-Amezcua et al., 2009). The formation of cavitating structures in the hydraulic machines, their geometry and more generally, their static and dynamic properties, depend on several parameters (Bakir et al., 2003), such as: • Geometrical conditions: profile, camber, thickness, incidence, and leading edge shape of the blades, as well as the walls roughness. • Local flow conditions: pressure, velocities, turbulence, the existence of gas microbubbles dissolved in the flow. • Fluid properties: saturation pressure, density, dynamic viscosity and surface tension. This chapter presents an analysis of the cavitating flows on three axial inducers. These studies include numerical analyses at a range of flow rates and cavitation numbers, which were validated with experimental tests (Campos-Amezcua et al., 2009; Mejri et al., 2006). The obtained results can be summarized of the following way:

Modeling of the Broadband Noise Radiated by an Airfoil

The mechanisms of noise generation due to the turbulent flow around an airfoil are complex phenomena. In this paper, a prediction method of the broadband noise is presented then validated. The aim of this work is to develop tools of noise prediction in turbomachinery engaged by the laboratory since several years. The modeling presented is based on the unsteady aerodynamic linearized theory of an airfoil. An acoustic calculation based on the formulation of Amiet determines the far field acoustic power spectral density produced by an airfoil. It shows the importance of the noise due to the interaction of incident turbulence flow with the leading edge. The results obtained are compared with the experimental results available in the literature.Copyright