Vinicio Magi

Streamtube model for analysis of vertical axis variable pitch turbine for marine currents energy conversion

Abstract Marine currents may represent a renewable energy source characterized by a limited environmental impact. In Italy, the Strait of Messina seems to be suited for exploitation of this energy source. A vertical axis turbine, with blades oscillating about the pivotal axis according to the Voith–Schneider system, has been considered. This paper presents a preliminary theoretical investigation of the performance of this kind of turbine that may be employed to tap marine currents energy sources. The investigation is conducted by means of a simple momentum model based on the “single-disk single-streamtube” approach. The theoretical results are compared with experimental measurements. The adequate agreement between experimental and theoretical results shows that such a simple model may be able to predict the power coefficient and the operating range of the turbine.

Exploring injected droplet size effects on steady liquid penetration in a Diesel spray with a two-fluid model

An approach to include droplet size effects in a two-fluid model for Diesel sprays when the locally-homogeneous flow (LHF) assumption is employed is developed. The model is then employed to study the effect of droplet sizes on the steady liquid penetration in vaporizing Diesel sprays when several injection and chamber parameters are changed. These parameters include the orifice diameter, injection pressure, ambient temperature and the ambient density. The computed steady liquid penetration is compared with constant volume measurements made under Diesel conditions at the Sandia National Laboratories. It appears that under typical Diesel conditions, the steady liquid penetration is controlled by entrainment and mixing alone. However, at lower ambient densities, droplet sizes may also be important.

APPLICATION OF THE DISCRETE ORDINATES METHOD TO COMPUTE RADIANT HEAT LOSS IN A DIESEL ENGINE

Abstract A three-dimensional model for computing flows, sprays, and combustion in internal combustion engines is modified to include radiant heat loss. Radiant heat loss is computed by solving the radiative transport equation using a discrete ordinates approximation method. Such a method solves the radiative transport equation for a set of discrete directions spanning the range of 4 π solid angle. Angular integrals of intensity are discretized by numerical quadrature. The resulting discrete ordinates equations are numerically solved by using a finite volume approach in contravariant formulation. Computations are made with and without radiant heat loss in a diesel engine, and the effects of the radiant heat loss on the computed temperature and NO and soot concentrations are discussed. Inclusion of radiant heat loss reduces the peak temperature by about 10%. As a result, the predicted frozen NO concentrations are found to be lowered. However, the soot concentrations are not significantly altered.

THE k-ε MODEL AND COMPUTED SPREADING RATES IN ROUND AND PLANE JETS

Several prior computational works that employ the steady, incompressible boundary layer (BL) equation have reported their inadequacy in predicting measured values of spreading rates in jets. This work employs a model that solves the complete set of transient conservation equations for multidimensional reacting flows and the k- k model without making the assumptions that lead to the steady, incompressible BL form of the equations. This work discusses the dependence of the computed spreading rates on the numerical resolution and on the additional terms that appear in the multidimensional equations, including the k- k model, relative to the BL equations.Several prior computational works that employ the steady, incompressible boundary layer (BL) equation have reported their inadequacy in predicting measured values of spreading rates in jets. This work employs a model that solves the complete set of transient conservation equations for multidimensional reacting flows and the k- k model without making the assumptions that lead to the steady, incompressible BL form of the equations. This work discusses the dependence of the computed spreading rates on the numerical resolution and on the additional terms that appear in the multidimensional equations, including the k- k model, relative to the BL equations.

Transient deformation and drag of decelerating drops in axisymmetric flows

Transient deformation and drag coefficients of decelerating drops in axisymmetric flows are numerically computed. The drag coefficients are compared with those of solid spheres. In the case of drops, the behavior of the drag coefficient is dependent on the deformation and internal circulation of the drops in addition to the factors which are important for solid spheres. These, in turn, are dependent on the gas-based Weber number (Weg) and the Ohnesorge number (Ohl). At the relatively low Weg of 1, when the deformation is small, the drag coefficients are about the same for the solid sphere and drop. When Weg is increased, the deformation increases and the difference increases. At the highest Weg of 100, the drop reaches a point of secondary breakup. In general, oblate shapes result in greater drag and prolate shapes in lower drag relative to the solid sphere. Increasing Ohl, which implies increasing viscous forces in the liquid relative to surface tension forces, leads to less deformation and hence lesser ...

A Numerical Analysis of Hydrogen Underexpanded Jets Under Real Gas Assumption

This work examines the fluid dynamic structure of underexpanded gas jets by using a high-performance computing (HPC) methodology in order to untangle the question of whether it is necessary to include the real gas assumption dealing with hydrogen jets. The answer to this question is needed in order to guarantee accurate numerical simulations of such jets in practical engineering applications, such as direct-injection hydrogen engines. An axial symmetric turbulent flow model, which solves the Favre-averaged Navier–Stokes equations for a multicomponent gas mixture, has been implemented and validated. The flow model has been assessed by comparing spreading and centerline property decay rates of subsonic jets at different Mach numbers with those obtained by both theoretical considerations and experimental measurements. Besides, the Mach disk structure of underexpanded jets has been recovered, thus confirming the suitability and reliability of the computational model. To take into account the effects of real gases, both van der Waals and Redlich–Kwong equations of state have been implemented. The analysis of a highly underexpanded hydrogen jet with total pressure equal to 75 MPa, issuing into nitrogen at 5 MPa, shows that the use of real gas equations of state affects significantly the jet structure in terms of temperature, pressure, and Mach number profiles along the jet centerline and also in terms of jet exit conditions, with differences up to 38%.

A Computational Investigation of the Interaction of Pulses in Two-Pulse Jets

Large-eddy simulations (LES) and Reynolds-averaged Navier-Stokes (RANS) simulations of the interaction of the second pulse with the first pulse in a double-injection event are presented in this study. The opposing effects of the turbulence generated by the first pulse to accelerate mixing of the second pulse, and the residual bulk velocity from the first pulse to accelerate penetration of the second pulse are analyzed. The LES results show greater influence of the turbulence whereas the RANS results show greater influence of the bulk velocity.

An Investigation on the Performance of Partially Stratified Charge CI Ethanol Engines

The partial fuel stratification, by means of direct fuel injection, is one of the most suitable combustion strategies in order to overcome the limits of ignition control and operating range of HCCI engines. In this work, a multidimensional model, coupled with a detailed kinetic mechanism for ethanol oxidation, is used to investigate the performance of a partially stratified charge CI engine fueled by ethanol. The model, which accounts for turbulence effects on combustion, has been validated in a previous work, against experimental results in terms of both HCCI engine performance and emissions. In this work, computations have been carried out by varying the fraction of the fuel stratified charge and the injection timing and by considering different flow structures within the cylinder. By increasing the amount of stratified fuel, the rate of the pressure rise and the heat release rate reduce, while the peak of the heat release rate delays, since the zones of the chamber, where the liquid fuel is located, are relatively cold and rich to ignite, thus the combustion process slows down. However, the ignition timing remains nearly constant, since the remaining zones of the combustion chamber are characterized by nearly uniform conditions, in terms of temperature and mixture composition, typical of an HCCI combustion. On the other hand, by increasing the fraction of the directly injected fuel, higher values of the maximum temperature are reached, thus producing an increase of NOx emissions. In order to avoid high values of temperature during combustion, the fuel stratification can be coupled with both swirl and early injection timing since, in both cases, a more uniform distribution of the injected fuel is obtained before ignition. Finally, simulations have been performed by increasing the fuel load up to 30%, thus showing the suitability of direct injection strategy in order to extend the operation range of HCCI engine. In: SAE Paper 2011-01-0837, SAE 2011 World Congress, Detroit, Michigan, USA, April 12-14, 2011, doi:10.4271/2011-01-0837, http://papers.sae.org/2011-01-0837 Copyright

Preliminary Design of a Hypersonic Air-breathing Vehicle

*, † ‡ § ¶ This paper describes the capabilities of a new in-house code, named SPREAD 2.0, to provide real time guidance to select the optimal parameters for preliminary design of hypersonic propulsion systems. Such a new solver drastically reduces the time and costs associated with excessive use of Computational Fluid Dynamics (CFD) and/or experimental tests. The accuracy of the model has been assessed by comparing the results with a 2-D CFD simulation performed with the C3NS-CIRA code. Finally, SPREAD 2.0 has been used to address the influence of air/fuel equivalence ratios and of craft angles of attack on the thermodynamic variables, which in turn affect the design, and on the pollutant emissions.

Enhancing the Performance of a Parallel Solver for Turbulent Reacting Flow Simulations

This article describes results of an effort to improve the parallel efficiency of a solver for turbulent reacting flows on two computer architectures. The compact finite-difference scheme employed for the solution of the differential equations involves the inversion of multiple tridiagonal matrices at each time step. Detailed performance evaluation of the standard LU, parallel partition LU, and parallel diagonal dominant algorithms are presented. The speed-up and efficiencies of these parallel strategies are critically compared and evaluated based on both computation and communication complexities, on the CRAY XT4 and IBM Blue Gene/P architectures.