Annarita Viggiano

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%.

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.

Dataset of working conditions and thermo-economic performances for hybrid organic Rankine plants fed by solar and low-grade energy sources.

This article provides the dataset of operating conditions of a hybrid organic Rankine plant generated by the optimization procedure employed in the research article “A genetic optimization of a hybrid organic Rankine plant for solar and low-grade energy sources” (Scardigno et al., 2015) [1]. The methodology used to obtain the data is described. The operating conditions are subdivided into two separate groups: feasible and unfeasible solutions. In both groups, the values of the design variables are given. Besides, the subset of feasible solutions is described in details, by providing the thermodynamic and economic performances, the temperatures at some characteristic sections of the thermodynamic cycle, the net power, the absorbed powers and the area of the heat exchange surfaces.

Large Eddy Simulation of High-Density Ratio Hydrogen Jets

The aim of this work is the analysis of a subsonic high-density ratio hydrogen jet by using a Large Eddy Simulation (LES) model in order to gain new insights into the fluid dynamic process of turbulent mixing of compressible jets. FLEDS (Flow - Large Eddy and Direct Simulation) code has been employed, where a sixth-order compact finite difference scheme is used to discretize the governing equations. The simulation of compressible free jets is challenging in the presence of large density gradients. The injection of hydrogen in air implies high velocities, large diffusion and even higher density gradients, that can cause non-physical spurious oscillations. Therefore, a Localized Artificial Diffusivity (LAD) scheme has been implemented in order to remove the numerical instabilities, that are not dumped by the high-order non-dissipative numerical scheme employed in this work. A hydrogen jet with Mach number equal to 0.8 issuing into still air has been considered. The LES results show that the normalized cente...

Numerical Simulations of an Ethanol Partially Stratified Charge CI Engine With Adaptively Reduced Kinetic Mechanisms

This work deals with the analysis of the performance and emissions of ethanol HCCI/PSCCI engines by means of a Dynamic Adaptive Chemistry (DAC) technique. The implementation of such a technique provides a reduction of the computational cost of the simulations without compromising the reliability of the results. Very accurate results, in terms of pressure and heat release rate profiles and CO, CO2 and UHC emissions, are obtained with ethanol as the only species for the DRGEP graph search both with the charge uniformly distributed in the combustion chamber and by directly injecting liquid fuel in the same chamber. The global speed up of DAC simulations is twice with respect to a full mechanism which consists of 57 species and 288 reactions and a further increase is expected when DAC is compared to very detailed kinetics.Copyright

A Numerical Analysis of Hydrogen Underexpanded Jets

The aim of this work is the study of the fluid dynamic structure of underexpanded hydrogen jets by using a High Performance Computing (HPC) methodology. An axial symmetric two-dimensional turbulent flow model, which solves the Favre-averaged Navier-Stokes equations for a multicomponent gas mixture, has been implemented and validated. In order to predict the decrease in spreading rate with increasing Mach number, a compressibility correction has been added to the turbulence closure model.The flow model has been assessed by comparing spreading and centerline property decay rates of subsonic jets at different Mach numbers with those obtained both by theoretical considerations and experimental measurements. Besides, the Mach disk structure of an underexpanded jet has been analysed, thus confirming the suitability 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 computations performed under ICEs conditions show that the values of Mach number and pressure just behind the Mach disk are affected by the use of real gas equations.Copyright

Advanced Numerical Methods for Non-Premixed Flames

Engine designers are under increasing pressure to reduce emissions and pollutants. Multidimensional models, as well as advanced experimental techniques, provide fundamental knowledge to meet regulations in terms of efficiency and emissions. Recently, considerable efforts have been addressed to develop advanced numerical techniques and comprehensive theoretical models, in order to study the dynamic of flames under the operating conditions typical of internal combustion engines, aircraft engines, gas turbine combustors, etc. (Bilger et al., 2005; Hilbert et al., 2004). Dealing with high Reynolds number reactive flows, invaluable knowledge can be achieved by using accurate numerical methodologies, such as Large-Eddy Simulation (LES) (Pitsch, 2006) and Direct Numerical Simulation (DNS) (Moin & Mahesh, 1998). The frontier research in this field concerns the coupling of such techniques with proper combustion models, in order to study engineering combustion devices fueled by conventional hydrocarbons, hydrogen and renewable bio-based fuels. In this work, a DNS methodology, coupled with detailed kinetic mechanisms for fuel oxidation, is described. This technique is implemented in an in-house developed CFD software package for the analysis of multicomponent free shear flows (Magi, 2004). Such a tool solves the Navier-Stokes equations for reacting flows by using a detailed description of thermal and transport properties and an accurate modelling of chemical source terms. A high order compact finite difference scheme is adopted for the solution of the partial differential equations (Lele, 1989; 1992). Although the computational code is able to perform both LES and DNS analysis, the latter is used in this work. MPI libraries are employed to fully parallelize the code, thus allowing to execute the computations on high performance parallel machines. In this contribution, the software package is used to simulate the reacting mixing layer between two streams of air and fuel with different velocity, even though the same methodology has been used to simulate other interesting phenomena for the study of combustion systems, such as the autoignition of fuel in starting transient jets (Viggiano & Magi, 2004; Viggiano, 2009). The role of some physical parameters, such as the mixture fraction, the scalar dissipation rate and the initial conditions, in terms of temperature and velocity, has been explored. The localization of the ignition spots and the ignition delay time have been investigated and the results have been compared with those of several experimental and numerical works in the literature (Mastorakos et al., 1997; Mastorakos, 2009; Sreedhara & Lakshmisha, 2000). Besides,