Michael Deligant

Experimental Study of Turbocharger’s Performances at Low Speeds

One of the most efficient ways to reduce the pollution and fuel consumption of an automotive engine is to downsize the engine, whilst maintaining a high level of power and torque. This is achieved by using turbochargers. In urban, and often in suburban, traffic conditions the engine power demand is weak in relation to the maximum power available, so the turbocharger runs at low speed. To appreciate and improve engine performance, it is necessary to know the characteristics of the turbomachinery in this functioning area, characteristics which are not given by turbocharger manufacturer. The reason for this lack of information will be explained and the experiments we are currently conducting at low turbocharger speed are presented. Experimentally, it has been demonstrated that the measured performances of the compressor are dependent on heat exchange (convection and conduction) and are also linked to the pressure and temperature of the lubricating oil. At the CNAM laboratory, the turbocharger test rig has been equipped with a special torquemeter, allowing rotation speeds of up to 120000 rpm, set up between the turbine and the compressor. The turbine is thus separated from the compressor and could be considered as a drive which provides mechanical power to the turbocharger (torquemeter + compressor + bearing unit). Temperature and pressure of the lubricating oil can be adjusted to an experiment’s requirements. The test bench lay out is described. To achieve accurate measurements and evaluate the influence of heat exchanges, tests have been carried out with the whole compressor thermally isolated and with preheated inlet air. The compressor can be assumed to be adiabatic, and the power given to the air flow can be calculated using the first law of thermodynamics. Mechanical bearing losses can be deduced from this calculation and torquemeter power, but also from measurements of lubricating oil flow, and oil temperature at inlet and outlet. The results of experiments for different lubricating oil temperatures and pressures and turbocharger speeds are presented. Real compressor characteristics curves are set up and a comparison of experimental mechanical power losses with a journal bearing CFD model is presented.Copyright

Smoothed Particle Hydrodynamics: A consistent model for interfacial multiphase fluid flow simulations

Abstract In this work, a consistent Smoothed Particle Hydrodynamics (SPH) model to deal with interfacial multiphase fluid flows simulation is proposed. A modification to the Continuum Stress Surface formulation (CSS) [1] to enhance the stability near the fluid interface is developed in the framework of the SPH method. A non-conservative first-order consistency operator is used to compute the divergence of stress surface tensor. This formulation benefits of all the advantages of the one proposed by Adami et al. [2] and, in addition, it can be applied to more than two phases fluid flow simulations. Moreover, the generalized wall boundary conditions [3] are modified in order to be well adapted to multiphase fluid flows with different density and viscosity. In order to allow the application of this technique to wall-bounded multiphase flows, a modification of generalized wall boundary conditions is presented here for using the SPH method. In this work we also present a particle redistribution strategy as an extension of the damping technique presented in [3] to smooth the initial transient phase of gravitational multiphase fluid flow simulations. Several computational tests are investigated to show the accuracy, convergence and applicability of the proposed SPH interfacial multiphase model.

Computational Fluid Dynamics Calculations of Turbocharger's Bearing Losses

Fuel consumption in internal combustion engines and their associated CO2 emissions have become one of the major issues facing car manufacturers ev eryday for various reasons: the Kyoto protocol, the upcoming European regulation concerning CO2 emi ssions requiring emissions of less than 130g CO2/km before 2012, and customer demand. One of the most efficient solutions to reduce fuel consumption is to downsize the engine and increase its specific power and torque by using turbochargers. The engine and the turbocharger have to b chosen carefully and be finely tuned. It is essential to understand and characterise the turboc harger’s behaviour precisely and on its whole operating range, especially at low engine speeds. T he characteristics at low speed are not provided by manufacturers of turbochargers because compresso r maps cannot be achieve on usual test bench. Experiments conducted in our laboratory on a specia l test rig equipped with a high-precision torquemeter, demonstrate that compressor performanc es in this area cannot be deduced from adiabatic assumption. Nevertheless, our study sugge sts that as long as torque at the shaft end is measured and mechanical power losses are known, the effective power provided to the air flow can be calculated. Tests and calculations reveal that t hese mechanical power losses cannot be evaluated by general physical laws. A better knowledge of the se losses is required. In this paper, a CFD model of a turbocharger journal bearing system is propose d. The real behaviour of what occurs in the bearing system (such as leakage flow, heat transfer from the inner film to the outer through the brass bearing material) has been computed with the en rgy equation. The bearing system performance is presented against the rotational spe ed at various oil inlet temperatures and pressures. The impact of those parameters has been studied in detail and presented in this paper. It is demonstrated that the oil temperature rise decrease s the friction torque along the rotational speed by making the viscosity drop. Moreover, an increase of the oil inlet pressure results into a higher friction torque. This paper provides an analysis of this trend showing the link between oil inlet pressure, oil mass flow and thermal exchange inside the bearing. Results also present the variation of oil mass flow along the entire speed range and i ts distribution between the inner and outer clearances.

Efficiency of bio- and socio-inspired optimization algorithms for axial turbomachinery design

Abstract Turbomachinery design is a complex problem which requires a lot of experience. The procedure may be speed up by the development of new numerical tools and optimization techniques. The latter rely on the parameterization of the geometry, a model to assess the performance of a given geometry and the definition of an objective functions and constraints to compare solutions. In order to improve the reference machine performance, two formulations including the off-design have been developed. The first one is the maximization of the total nominal efficiency. The second one consists to maximize the operation area under the efficiency curve. In this paper five optimization methods have been assessed for axial pump design: Genetic Algorithm (GA), Particle Swarm Optimization (PSO), Cuckoo Search (CS), Teaching Learning Based Optimization (TLBO) and Sequential Linear Programming (SLP). Four non-intrusive methods and the latter intrusive. Given an identical design point and set of constraints, each method proposed an optimized geometry. Their computing time, the optimized geometry and its performances (flow rate, head (H), efficiency (η), net pressure suction head (NPSH) and power) are compared. Although all methods would converge to similar results and geometry, it is not the case when increasing the range and number of constraints. The discrepancy in geometries and the variety of results are presented and discussed. The computational fluid dynamics (CFD) is used to validate the reference and optimized machines performances in two main formulations. The most adapted approach is compared with some existing approaches in literature.

Surge detection on an automotive turbocharger during transient phases

The surge limit on automotive turbocharger needs to be avoided to prevent operations with pressure and mass flow oscillations. Mild surge is accompanied by noise which is disturbing. Deep surge can cause significant loss of engine power and severe drivability issues. It is necessary to know the stationary limit in order to match a turbocharger with an engine, ensuring enough surge margin. However, this choice does not guarantee surge free operation during transient functioning. In this paper, the surge onset of a compressor while closing a downstream valve is studied. Various tests have been carried out varying the closing time, the position of the initial operating point and the volume of the circuit. The inlet and outlet signals of physical parameters are analyzed with spectral and temporal methods in order to define the instant of the surge occurrence.

Toward a near-field CAA-CFD coupling approach: Application to a centrifugal blower

Centrifugal blowers are widely used in heating, ventilation and air-conditioning to gen-erate high mass flow in compact geometries. Confined configurations imply important interactions between the impeller and the volute casing, which causes acoustic tonal noise. This study is focused on the simulation of the noise propagation into a subsonic centrifu-gal blower. In this con?ned domain, integral methods in acoustic reach their limits since they do not take into account reflection and diffraction. Instead, this near-field propaga-tion issue is tackled here by the use of a wave propagation operator such as Linearized Perturbed Compressible Equations (LPCE). First, a simple acoustic source model is pro-posed. Then, sources are extracted from CFD computations. This two kind of sources are experimented as inputs into the propagation operator. Spatial resolution is enforced by a high order finite volume CAA solver (FV-MLS). The moving parts are taken into account through an innovative sliding-mesh approach.

Surge Limit Prediction of Centrifugal Compressor Using Semi Classical Signal Analysis

This paper presents the use of the recently developed semi classical signal analysis (SCSA) method for surge limit detection of a centrifugal compressor. The SCSA is based on the fact that the studied temporal signal is considered as a trap-potential for a semi-classical particle and is represented by discrete energy levels given by the discrete spectrum of a Schrodinger operator. This approach provides new spectral parameters whom variability might be interesting for signals analysis. Inlet and outlet pressure signals recorded on a turbocharger test bench while reducing the compressor mass flow rate are analysed. The spectral parameters provided by the SCSA show interesting behaviour especially around the transition zone between stable operation and surge. Following these observations, a new criterion for surge limit detection is proposed. The sensitivity of the detection may be adjusted by the threshold value. The implementation of this new method combined with active regulation may extend the usable zone of the compressor map.Copyright