Giuseppe Cantore

Analysis of hydraulic components using computational fluid dynamics models

Abstract This paper presents some results obtained during the computational fluid dynamics (CFD) analysis of internal flows inside a hydraulic component, using a scaling technique applied to numerical pre- and post-processing. The main aim of the work is to demonstrate the reduction of computational work needed for a complete analysis of component behaviour over a wide range of operating conditions. This result is achieved through the adoption of a methodology aimed at giving the highest level of generality to a non-dimensional solution, thereby overcoming the two major limitations encountered in the use of CFD in fluid power design: computer resources and time. In the case study, the technique was applied to a hydraulic distributor and computations were performed with a commercial computational fluid dynamics code. The key factor of this technique is the evaluation, for a given distributor opening, of the Reynolds number of the flow in the metering region. Provided that this number is high enough to ensure that the discharge coefficient has reached its asymptotic value, the characterization of the flow by a single non-dimensional numerical run can be shown. The theoretical contents of the analysis of the re-scaling technique, which focuses on the engineering information necessary in component design, are described in detail. The bases for its subsequent application to actual cases are then outlined. Finally, a fairly close correlation between numerical results and experimental data is presented.

An Analysis on Time Scale Separation for Engine Simulations with Detailed Chemistry

The simulation of combustion chemistry in internal combustion engines is challenging due to the need to include detailed reaction mechanisms to describe the engine physics. Computational times needed for coupling full chemistry to CFD simulations are still too computationally demanding, even when distributed computer systems are exploited. For these reasons the present paper proposes a time scale separation approach for the integration of the chemistry differential equations and applies it in an engine CFD code. The time scale separation is achieved through the estimation of a characteristic time for each of the species and the introduction of a sampling timestep, wherein the chemistry is subcycled during the overall integration. This allows explicit integration of the system to be carried out, and the step size is governed by tolerance requirements. During the subcycles each of the species is only integrated up to its own characteristic timescale, thus reducing the computational effort needed by the solver. The present ODE solver was first validated using constant pressure batch reactor simulations with two different reaction mechanisms. Then the solver was coupled with the KIVA-4 code, and validated using HCCI and DI diesel combustion cases. Performance is compared with the commonly used DVODE chemistry solver and the results show that significant reductions in the total computational time with comparable accuracy are obtained with the new solution methodology.

Analysis of a 6 Cylinder Turbocharged HCCI Engine Using a Detailed Kinetic Mechanism

When analyzing HCCI combustion engine behavior, the integration of experimental tests and numerical simulations is crucial. Investigations of possible engine control strategies as a function of the different operating conditions have to take the behavior of the whole HCCI engine into account, including the effects both of the combustion process and of complex devices. Therefore the numerical simulation code must be able both to model accurately the gas-dynamic of the system and to evaluate the combustion chemical kinetics. This paper focuses on the coupling between the commercial one-dimensional fluid-dynamic GT-Power Code and our in-house detailed chemical kinetic Ignition Code. An interface has been developed in order to exchange information between the two codes: the Ignition Code considers as boundary conditions the GT-Power Code values provided for the gas composition at IVC and the pressure and temperature at every time step and passes back to GT-Power the burnt fuel fraction and stores in an external file the in cylinder gas composition. Thus the whole engine cycle can be accurately simulated, estimating the interactions between the gas-dynamics phenomena along the intake and exhaust pipes and through the valves, and the chemical processes occurring during the closed valves period. This tool makes it possible to analyze the engine behavior under duty cycle operating conditions, and therefore it represents a useful support to the experimental measurements, reducing the number of tests required to assess the proper engine control strategies.Copyright

Analysis of a HSDI Diesel Engine Intake System by Means of Multi-Dimensional Numerical Simulations: Influence of Non Uniform EGR Distribution

In order to comply with stringent pollutant emissions regulations a detailed analysis of the overall engine is required, assessing the mutual influence of its main operating parameters. The present study is focused on the investigation of the intake system under actual working conditions by means of 1D and 3D numerical simulations. Particularly, the effect of EGR distribution on engine performance and pollutants formation has been calculated for a production 6 cylinder HSDI Diesel engine in a EUDC operating point. Firstly a coupled 1D/3D simulation of the entire engine geometry has been carried out to estimate the EGR rate delivered to every cylinder; subsequently the in-cylinder flow field has been evaluated by simulating the intake and compression strokes. Finally the spray and combustion processes have been studied accounting for the real combustion chamber geometry and particularly the pollutants formation has been determined by using a detailed kinetic mechanism combustion model. The 1D/3D analysis highlighted a significant cylinder to cylinder EGR percentage variation affecting remarkably the pollutant emissions formation, as evaluated by the combustion process simulations. A combined use of commercial and in-house modified codes has been adopted.Copyright

COMBINED-CYCLE POWER PLANTS FROM GAS TURBINES AND GEOTHERMAL SOURCE INTEGRATION

Geothermal power plants have difficulties due to the low conversion efficiencies achievable.Geothermal integrated combined cycle proposed and analyzed in this paper is a way to achieve high efficiency.In the proposed cycle the geothermal fluid energy is added, through suitable heat ecxhangers, to that of exhaust gases for generating a steam cycle.The proposed cycle maintains the geothermal fluid segregated from ambient and this can be positive on the environmental point of view.Many systems configurations, based on this possibility, can be taken into account to get the best thermodynamic result.The perfomed analysis examines different possible sharings between the heat coming from geothermal and exhaust gases, and gives the resulting system efficiencies.Various pressures of the geothermal steam and water dominated sources are also taken into account.As a result the analysis shows that the integrated plant power output is largely greater than the total power obtained by summing the gas turbine and the traditional geothermal plant power output, considered separately.Copyright

Comparison Between Steady and Unsteady CFD Simulations of Two Different Port Designs in a Four Valve HSDI Diesel Engine: Swirl Intensity and Engine Permeability

Swirl control strategies are useful methods for controlling mixture formation in HSDI Diesel engines. Test rigs allows only steady state measurements of the Swirl number, and give only a rough estimation of the charge motion during the actual compression stroke within the engine. On the contrary, CFD simulations are powerful tools to characterize the air flow drawn into the cylinder, since they allow not only steady state operations, but also full dynamic modeling of the intake and compression strokes. This paper studies an application of computational fluid dynamics for predicting intake swirl intensity in an automotive 4 valve per cylinder C.I. Diesel engine. Two different intake ports are compared and the best trade off between engine permeability and swirl intensity is assessed. Both steady state and dynamic simulations of the induction process are carried out, and results demonstrate that steady state analysis is a reliable tool for predicting the port permeability, while the same capability is not proved in investigating the organized charge motion within the chamber.Copyright

Advances in The Design of Two-Stroke, High Speed, Compression Ignition Engines

An interesting concept in order to meet the conflicting requirements mentioned above is the 2-Stroke cycle combined to Compression Ignition. Such a concept is widely applied to large bore engines, on steady or naval power-plants, where the advantages versus the 4-Stroke cycle in terms of power density and fuel conversion efficiency (in some cases higher than 50% [1]) are well known. In fact, the double cycle frequency allows the de‐ signer to either downsize (i.e. reduce the displacement, for a given power target) or “down-speed” (i.e. reduce engine speed, for a given power target) the 2-stroke engine. Furthermore, mechanical efficiency can be strongly improved, for 2 reasons: i) the gas ex‐ change process can be completed with piston controlled ports, without the losses associ‐ ated to a valve-train; ii) the mechanical power lost in one cycle is about halved, in comparison to a 4-Stroke engine of same design and size, while the indicated power can be the same: as a result, the weight of mechanical losses is lower.

Off-Design Evaluation of a Multiple Expansion Reheating Gas Turbine in Cogeneration Applications

The multiple expansion reheating gas turbine proves to have a potential of good operational flexibility for the intrinsic capability of responding to variations in electric and thermal power demands without appreciable impact on efficiency.The present study deals with evaluation of the performance attainable in off-design operation, with power control obtained through changes in the first and second combustor firing temperatures and in the compressor intake air flow achieved by means of variable inlet guide vanes.Because of the important impact of the hot part cooling air flows on performance, the study includes also a hypothesis of controlling such flows in off-design operation through external means.The predicted off-design performance results superior in the hypothesis of external cooling air flow control, thus making such a system worthy of consideration for possible future developments of machines in this category.To evaluate the suitability of the multiple expansion reheating gas turbine in cogeneration applications, the electric efficiency and the electrical index have been taken into consideration.The capability of varying the reheating temperature represents an effective way of controlling the electrical index with good efficiencies in industrial cogeneration with strongly varying electric power and process heat requirements.With regard to the cooling air control through external means, implementation of such a more complex system seems to be avoidable at least when the gas turbine is intended specifically for application in cogeneration, because of its smaller impact on the overall efficiency of the system.Copyright

Performance Levels Obtainable From Steam-Gas Turbine Combined Cycles

This study aims at the evaluation of the best performance obtainable from steam-gas turbine combined plants both in a new plant design and in improving existing steam plants by adding a topping gas turbine system. A method of comparison is presented based on the choice of a steam-gas reference cycle which as shown to be be particularly suitable for a general study. A thermodynamic analysis has been carried out showing the influence on the combined plant overall efficiency of the parameters characterizing both the gas and steam cycles. The reference cycle as well as those derivable from it by modifying the gas portion cycle only has been studied. The analysis was also extended to evaluation of the gas to steam units output power ratio and of the efficiency increase when repowering a steam unit. It has been shown that the combined cycle plants maximum overall-efficiency is achieved, whatever the steam cycle, when the gas turbine cycle operates at maximum specific work.

Experimental investigations into vortex flow in a straight, annular section channel

Abstract Results of experimental velocity measurements in air at several cross-sections along a straight annular section channel comparable to a hydraulic machine admission duct are presented. The flow analysis focuses on the axial and tangential velocity components and their modification with changes in the intensity of swirl along the channel. Particular prominence is given to an anomalous flow, the ‘dead water core’, occurring at the greatest swirl intensities. An earlier method of evaluating the core size of this anomalous flow on the basis of a vortex-type schematization is tested; Strscheletzkys method is also analysed. Finally a definition of the dead water core dimensions based on the axial velocity component distribution is proposed