CFD Analysis of the Acoustic Behavior of a Centrifugal Compressor for High Performance Engine Application

Abstract The paper reports an activity aiming at the characterization of the acoustic noise of a centrifugal compressor for a currently made high performance engine. All the analyses are carried out through the use of Detached Eddy Simulation. During high-load/low- engine speed operations of the engine, the compressor exhibits noise peaks above 150 dBA at relatively low frequencies, whose origin is relatively hard to rationalize. The use of three-dimensional CFD simulation appears to be very promising to gain a better understanding of the complex flow structures at the compressor inlet as well as to promote design optimizations aiming at limiting the acoustic emissivity of the component. In view of the dependency of the acoustic phenomena on the instantaneous pressure waves and flow structures, fully transient CFD simulations are highly recommended, together with the use of sophisticated numerical techniques such as Large Eddy and Detached Eddy simulation [1] , [2] , which are widely recognized to be able to better capture highly unstable features than the common RANS approach [3] , [4] . In order to limit the computational cost of the analyses, preliminary steady-state RANS simulations are carried out to both initialize the flow field and to evaluate the grid capability to properly match the desired frequency spectrum

A methodology for the reduction of numerical diffusion in sloshing analyses through automated mesh adaptation

The paper proposes a methodology to improve the accuracy of Volume of Fluid (VoF) multiphase problems involving liquid/gas sloshing in fuel/lubricant tanks without penalizing the computational cost of the simulations. In order to correctly track the complex trajectory of the liquid/gas interface and the presence of liquid droplets in the gas phase, the VoF method requires a fine mesh at each interface location to reduce modeling errors. The investigated case is a lubricant tank of a sport car subject to typical race track maneuvers. Due to the geometrical extent and the complexity of the computational domain and to the relevant accelerations, resulting in dispersed liquid structures within the gas phase, the use of a generalized fine mesh would result in computational costs far beyond the industrial practice. A methodology is then proposed to reduce the overall number of computational cells through a combination of local interface tracking and mesh refinement, which is combined with an active control of the time step to comply with Courant-Friedrichs- Lewy number limits. The methodology is at first validated against experimental measurements for a simplified test case, and then applied to the actual oil tank sloshing case, showing a relevant reduction of the numerical diffusion and a consequent higher accuracy

A Methodology to Improve Knock Tendency Prediction in High Performance Engines

AbstractThe paper presents a comprehensive numerical methodology for the estimation of knock tendency in SI engines, based on the synergic use of different frameworks [1]. 3D-CFD in-cylinder analyses are used to simulate the combustion and to estimate the point-wise heat flux acting on engine components. The resulting heat fluxes are used in a conjugate heat transfer model in order to reconstruct the actual point-wise wall temperature distribution. An iterative loop is established between the two simulation realms. In order to evaluate the effect of temperature on knock, in-cylinder analyses are integrated with an accurate chemical description of the actual fuel

Two-Stage Turbocharging for the Downsizing of SI V-Engines

AbstractOne of the most critical challenges for the specific power increase of turbocharged SI engines is the low end torque, limited by two aspects. First, the big size of the compressor necessary to deliver the maximum airflow does not allow high boost pressures at low speed, due to the surge line proximity. Second, the flame front velocity may become slower than the end gas auto-ignition rate, thus increasing the risk of knocking.This study is based on a current SI GDI V8 turbocharged engine, modeled by means of CFD tools, both 1d and 3d. The goal of the activity is to lower by 20% the displacement, without reducing brake torque, all over the engine speed range.It was decided to adopt a smaller bore, keeping stroke constant. Obviously, the combustion chamber, the valves and the intake-exhaust ports have been re-designed, as well as the whole intake and exhaust system. Instead of the two turbochargers, one for each bank of cylinders, a triple-turbocharger layout has been considered.The development of the engine has been carried out by means of 1D engine cycle simulations, using predictive knock models, calibrated with the support of both experiments and CFD-3d simulations. A few operating conditions for the final configuration have been also analyzed by means of a 3-d CFD tool.The paper presents the results of this activity, and describes in details the guidelines followed for the development of the engine

Hierarchical 1D/3D Approach for the Development of a Turbulent Combustion Model Applied to a VVA Turbocharged Engine. Part II: Combustion Model

As discussed in the part I of this paper, 3D models represent a useful tool for a detailed description of the mean and turbulent flow fields inside the engine cylinder. 3D results are utilized to develop and validate a 0D phenomenological turbulence model, sensitive to the variation of operative parameters such as valve phasing, valve lift, engine speed, etc. In part II of this paper, a 0D phenomenological combustion model is presented, as well. It is based on a fractal description of the flame front and is able to sense each of the fuel properties, the operating conditions (air-to-fuel ratio, spark advance, boost level) and the combustion chamber geometry. In addition, it is capable to properly handle different turbulence levels predicted by means of the turbulence model presented in the part I. The turbulence and combustion models are included, through user routines, in the commercial software GT-Power". With reference to a small twin-cylinder VVA turbocharged engine, the turbulence/combustion model, once properly tuned, is finally used to calculate in-cylinder pressure traces, rate of heat release and overall engine performance at full load operations and brake specific fuel consumption at part load, as well. An excellent agreement between numerical forecasts and experimental evidence is obtained

Hierarchical 1D/3D Approach for the Development of a Turbulent Combustion Model Applied to a VVA Turbocharged Engine. Part I: Turbulence Model☆

Abstract It is widely recognized that air-fuel mixing, combustion and pollutant formation inside internal combustion engines are strongly influenced by the spatial and temporal evolution of both marco- and micro- turbulent scales. Particularly, in spark ignited engines, the generation of a proper level of turbulence intensity for the correct development of the flame front is traditionally based on the onset, during the intake and compression strokes, of a tumbling macro-structure. Recently, in order to both reduce pumping losses due to throttling and develop advanced and flexible engine control strategies, fully variable valve actuation systems have been introduced, capable of simultaneously governing both valve phasing and lift. Despite the relevant advantages in terms of intake system efficiency, this technology introduces uncertainties on the capability of the intake port/valve assembly to generate, at low loads, sufficiently coherent and stable structures, able therefore to promote adequate turbulence levels towards the end of the compression, with relevant effects on the flame front development. It is a common knowledge that 3D-CFD codes are able to describe the evolution of the in-cylinder flow field and of the subsequent combustion process with good accuracy; however, they require too high computational time to analyze the engine performance for the whole operating domain. On the contrary, this task is easily accomplished by 1D codes, where, however, the combustion process is usually derived from experimental measurements of the in-cylinder pressure trace (Wiebe correlation). This approach is poorly predictive for the simulation of operating conditions relevantly different from the experimental ones. To overcome the above described issues, enhanced physical models for the description of in-cylinder turbulence evolution and combustion to be included in a 1D modeling environment are mandatory. In the present paper (part I), a 0D (i.e. homogeneous and isotropic) phenomenological (i.e. sensitive to the variation of operative parameters such as valve phasing, valve lift, intake and exhaust pressure levels, etc.) turbulence model belonging to the K-k model family is presented in detail. The model is validated against in-cylinder results provided by 3D-CFD analyses carried ou

Pressure Losses in Multiple-Elbow Paths and in V-Bends of Hydraulic Manifolds

Hydraulic manifolds are used to realize compact circuit layouts, but may introduce high pressure losses in the system because their design is usually oriented to achieving minimum size and weight more than reducing the pressure losses. The purpose of this work is to obtain the pressure losses when the internal connections within the manifold are creating complex paths for the fluid and the total loss cannot be calculated simply as the sum of the single losses. To perform the analysis both Computational Fluid Dynamic (CFD) analysis and experimental tests have been executed. After the comparison between numerical and experimental results, it was possible to assess that the numerical analysis developed in this work is able to depict the correct trends of the pressure losses also when complex fluid path are realized in the manifold. Successively, the numerical analysis was used to calculate the pressure loss for inclined connections of channels (or V-bends), a solution that is sometimes adopted in manifolds to meet the design requirements aimed towards the minimum room-minimum weight objective