Sofiane Khelladi

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.

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.

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: