The partial fuel stratification, by means of direct fuel injection, is one of the most suitable c... more 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.
The aim of this work is the study of the fluid dynamic structure of underexpanded hydrogen jets b... more 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.
Numerical simulations were carried out to study the influence of ozone on the Ignition Delay Time... more Numerical simulations were carried out to study the influence of ozone on the Ignition Delay Time (IDT) of iso-octane/air mixtures under typical operating conditions of HCCI engines. 0-D and 2-D CFD simulations were carried out to compute IDT and to characterize compression, combustion, and expansion in an HCCI engine, respectively. A kinetic model was developed by merging a mechanism for iso-octane, a sub-mechanism for nitrogen oxides, and a sub-mechanism for ozone. The model was used to investigate iso-octane/air/ozone mixtures under typical operating conditions of HCCI engines running with very lean mixtures (equivalence ratio equal to 0.3). Parametric analyses were carried out considering different values of temperature (500 to 1200 K), pressure (15 to 40 bar) and ozone concentration (0 to 50 ppm). The results show that as ozone concentration increases IDT decreases, with a greater impact at low temperatures, and the NTC (Negative Temperature Coefficient) effect decreases. However, the reduction of IDT with ozone addition is less as ozone concentration increases, especially at low temperatures. An increased pressure, on the other hand, generally promotes faster ozone decomposition and enhances ozone effect on IDT reduction for all temperatures except in the range 760-840 K, where the opposite effect occurs due to NTC phenomenon. Finally, when temperature is very high, i.e. 1200 K, both ozone and pressure have little influence on the percentage reduction of IDT.
This work explores the coupling of advanced combustion strategies for engines with bio-based fuel... more This work explores the coupling of advanced combustion strategies for engines with bio-based fuels. The characteristics of ethanol combustion in HCCI mode are investigated by using a multidimensional CFD model coupled with an accurate combustion model. In such a model, the chemical source terms are computed by a detailed kinetic mechanism and are corrected in order to take into account the influence of turbulence. The predictive capability of the model is proven by comparing the results with experimental measurements. The sensitivity analysis to initial and boundary conditions gives suggestions in order to increase engine efficiency and reduce pollutant emissions.
Ethanol is a very interesting alternative fuel for the transportation sector, for both its chemic... more Ethanol is a very interesting alternative fuel for the transportation sector, for both its chemical-physical properties and reduced environmental impact. As a matter of fact, ethanol has a relatively high octane number, about 107, and a high latent heat of vaporization. Therefore, it can be used in Spark Ignition (SI) engines in order to increase the compression ratio, especially in those engines working in direct injection mode. In Homogeneous Charge Compression Ignition (HCCI) engines, ethanol can be used in pure form or blended to conventional fuel to prevent knock occurrence and increase engine efficiency. Besides, as an oxygenated fuel, ethanol oxidation is characterized by lower CO emissions, if compared with combustion of conventional fuels. Finally, ethanol is produced from biomass, thus fitting the requirements of a sustainable energy system, concerning both greenhouse gas emissions and petroleum dependence. In this work, a three-dimensional CFD model is coupled with an accurate kinetic mechanism for ethanol combustion, in order to investigate the performance of engines fuelled by ethanol. The experimental results of a test engine are considered for comparisons. This test engine is a single cylinder engine working in HCCI mode. The numerical results are shown in terms of average in-cylinder pressure, heat release rate, CO and CO2 emissions for different equivalence ratios. They are compared with measurements and very good agreement is obtained. Sensitivity analyses of various initial and boundary conditions, such as initial temperature of the mixture and wall temperature, are performed. Finally, the influence of wall heat loss on CO emissions is considered.
The Direct Numerical Simulation (DNS) is one of the most suitable tools for a comprehensive analy... more The Direct Numerical Simulation (DNS) is one of the most suitable tools for a comprehensive analysis of fluid dynamic problems of engineering interest. Nowadays, direct simulation is used to analyse flow configuration confined in small domains, due to the need of a large amount of computational resources. Hence, from a practical point of view, DNS is a computational method for testing the validity and accuracy of turbulent models and for tuning the values of the model-constants of such models. In this paper, DNS is used to study the mixing of two gaseous streams at different velocities. Such a problem is encountered in Diesel engines during the injection process. The validity of such an approach has been proven by comparing the numerical results with experimental measurements available in literature.
The partial fuel stratification, by means of direct fuel injection, is one of the most suitable c... more 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.
The aim of this work is the study of the fluid dynamic structure of underexpanded hydrogen jets b... more 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.
Numerical simulations were carried out to study the influence of ozone on the Ignition Delay Time... more Numerical simulations were carried out to study the influence of ozone on the Ignition Delay Time (IDT) of iso-octane/air mixtures under typical operating conditions of HCCI engines. 0-D and 2-D CFD simulations were carried out to compute IDT and to characterize compression, combustion, and expansion in an HCCI engine, respectively. A kinetic model was developed by merging a mechanism for iso-octane, a sub-mechanism for nitrogen oxides, and a sub-mechanism for ozone. The model was used to investigate iso-octane/air/ozone mixtures under typical operating conditions of HCCI engines running with very lean mixtures (equivalence ratio equal to 0.3). Parametric analyses were carried out considering different values of temperature (500 to 1200 K), pressure (15 to 40 bar) and ozone concentration (0 to 50 ppm). The results show that as ozone concentration increases IDT decreases, with a greater impact at low temperatures, and the NTC (Negative Temperature Coefficient) effect decreases. However, the reduction of IDT with ozone addition is less as ozone concentration increases, especially at low temperatures. An increased pressure, on the other hand, generally promotes faster ozone decomposition and enhances ozone effect on IDT reduction for all temperatures except in the range 760-840 K, where the opposite effect occurs due to NTC phenomenon. Finally, when temperature is very high, i.e. 1200 K, both ozone and pressure have little influence on the percentage reduction of IDT.
This work explores the coupling of advanced combustion strategies for engines with bio-based fuel... more This work explores the coupling of advanced combustion strategies for engines with bio-based fuels. The characteristics of ethanol combustion in HCCI mode are investigated by using a multidimensional CFD model coupled with an accurate combustion model. In such a model, the chemical source terms are computed by a detailed kinetic mechanism and are corrected in order to take into account the influence of turbulence. The predictive capability of the model is proven by comparing the results with experimental measurements. The sensitivity analysis to initial and boundary conditions gives suggestions in order to increase engine efficiency and reduce pollutant emissions.
Ethanol is a very interesting alternative fuel for the transportation sector, for both its chemic... more Ethanol is a very interesting alternative fuel for the transportation sector, for both its chemical-physical properties and reduced environmental impact. As a matter of fact, ethanol has a relatively high octane number, about 107, and a high latent heat of vaporization. Therefore, it can be used in Spark Ignition (SI) engines in order to increase the compression ratio, especially in those engines working in direct injection mode. In Homogeneous Charge Compression Ignition (HCCI) engines, ethanol can be used in pure form or blended to conventional fuel to prevent knock occurrence and increase engine efficiency. Besides, as an oxygenated fuel, ethanol oxidation is characterized by lower CO emissions, if compared with combustion of conventional fuels. Finally, ethanol is produced from biomass, thus fitting the requirements of a sustainable energy system, concerning both greenhouse gas emissions and petroleum dependence. In this work, a three-dimensional CFD model is coupled with an accurate kinetic mechanism for ethanol combustion, in order to investigate the performance of engines fuelled by ethanol. The experimental results of a test engine are considered for comparisons. This test engine is a single cylinder engine working in HCCI mode. The numerical results are shown in terms of average in-cylinder pressure, heat release rate, CO and CO2 emissions for different equivalence ratios. They are compared with measurements and very good agreement is obtained. Sensitivity analyses of various initial and boundary conditions, such as initial temperature of the mixture and wall temperature, are performed. Finally, the influence of wall heat loss on CO emissions is considered.
The Direct Numerical Simulation (DNS) is one of the most suitable tools for a comprehensive analy... more The Direct Numerical Simulation (DNS) is one of the most suitable tools for a comprehensive analysis of fluid dynamic problems of engineering interest. Nowadays, direct simulation is used to analyse flow configuration confined in small domains, due to the need of a large amount of computational resources. Hence, from a practical point of view, DNS is a computational method for testing the validity and accuracy of turbulent models and for tuning the values of the model-constants of such models. In this paper, DNS is used to study the mixing of two gaseous streams at different velocities. Such a problem is encountered in Diesel engines during the injection process. The validity of such an approach has been proven by comparing the numerical results with experimental measurements available in literature.
In this paper, two advanced combustion models for premixed charge spark ignition (SI) engines hav... more In this paper, two advanced combustion models for premixed charge spark ignition (SI) engines have been analysed, implemented and validated. This study arises from the need to have a more reliable and accurate computational tool for analysing the physical and chemical phenomena during the combustion process in SI engines. The Local Equilibrium Characteristic Time (LECT) model and the Coherent Flame Model (CFM) have been considered. These sub-models have been implemented in a full 3-D model that can simulate reciprocating and rotary engines. Simulations of the combustion process have been carried out in a premixed charge spark ignition engine in a relatively simple geometry. The combustion chamber has an axisymmetric geometry with the spark plug located on the engine head along the axis. A parametric study of the two combustion models has been carried out and, by changing the initial values of some physical quantities, it is shown that correct trends are obtained. The sensitivity of the results to model parameters shows a strong dependence when CFM is used.
The Dynamic Adaptive Chemistry (DAC) technique is extended in this work to multidimensional simul... more The Dynamic Adaptive Chemistry (DAC) technique is extended in this work to multidimensional simulations of ethanol HCCI/PSCCI engines. Several DAC computations have been performed by using two kinetic reaction mechanisms of ethanol with different levels of detail, that include 57 species and 135 species, respectively. The specific choice of the DAC parameters, i.e. the set of search-initiating species and the tolerance value, has been carefully analyzed. The simulations show that 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 graph search both with the fuel uniformly distributed and by directly injecting liquid fuel in the combustion chamber. As regards NOx, specific attention has been addressed to the analysis of the NOx formation in order to correctly reproduce the paths that lead to NOx emissions for the different cases. The choice of ethanol–N2O as search-initiating set has given the best results with negligible errors with respect to the full mechanism. For the single-zone computations, the use of DAC provides a speed-up of the 135-species full mechanism more than 9, whereas, with respect to the 57-species mechanism, about 50% of the computational time is saved in the multidimensional simulations.
In the last decade, the interest towards non-petroleum based fuels is strongly increased due to t... more In the last decade, the interest towards non-petroleum based fuels is strongly increased due to the current concerns about the impact of the transport system on atmospheric pollutions, mainly due to CO2 emissions. Ethanol, either blended with conventional fuels or in its pure form, is one of such fuels already in use in the automotive industries. As an example, the flex-fuel engines are designed to employ gasoline from 10% up to 85% of ethanol (E10, E85). In this work, the oxidation kinetics of ethanol with air has been considered and analyzed in an ambient at high pressure, ranging from 4 to 6 MPa, by means of a Computational Singular Perturbation methodology. This study starts from a detailed kinetic reaction mechanism, which is extensively used in the literature and is made up by 235 reversible reactions among 46 chemical species. This mechanism has been used to get several families of simplified (skeletal) mechanisms by considering equivalence ratios in the range 0.2–2.0 and two sets of initial temperatures (900–1200 K and 1200–1700 K). The skeletal mechanisms have been validated by comparing the temperature and major chemical species profiles and the ignition delay time with those obtained by employing the detailed mechanism. Advantages and limitations of these mechanisms are highlighted. The accuracy of the skeletal mechanisms is very good in the range of equivalence ratio and temperature considered in the simplification procedure. Moreover, it is shown that a further simplification of the reaction mechanism is obtained by narrowing the range of equivalence ratio instead of the range of temperature. The most simplified skeletal mechanisms show an error in the prediction of temperature and fuel profiles lower than 3%, except for the case of low temperature and lean mixtures, where the maximum error increases up to 14%. Finally, the main reaction pathways are analyzed to show how the most important intermediate chemical species and products characterize the skeletal mechanisms for different values of equivalence ratio and temperature. The skeletal mechanisms are given as Supplementary data. The reader can refer to such data for the entire set of the retained reactions and species for each skeletal mechanism.
The stringent regulations on the reduction of both pollutant emissions and dependence from crude ... more The stringent regulations on the reduction of both pollutant emissions and dependence from crude oil have increased the interest toward alternative energy resources. Transport and energy production sectors are strongly involved in the pollutions net environmental balance and, at the same time, their primary energy requirements are significant. In this scenario, several efforts are carried out to identify new solutions. In the last two decades, in the automotive industry, the use of several alternative fuels for internal combustion engine applications has been investigated. Specifically, this work focuses on syngas and its use in spark ignition (SI) engines. First, a comprehensive analysis of the syngas combustion process has been carried out and accurate laminar flame speed correlations are proposed to characterize the fuel oxidation. Then, these correlations have been implemented in a CFD model to simulate a CFR engine combustion process. Different H2/CO–air mixtures (fuel molar ratio ranging from 50:50 to 100:0 of H2:CO) at different engine operating conditions (compression ratio from 6:1 to 10:1 and fuel equivalence ratio from 0.6 to 0.8) have been considered and the results have been compared with available experimental data. A good agreement has been observed in all conditions, in terms of pressure trace, heat release and other parameters that are useful to characterize the combustion process in SI engines, i.e. burn duration, ignition lag and rapid burn angle.
The aim of this work is the analysis, under dynamic conditions, of the energy performance of buil... more The aim of this work is the analysis, under dynamic conditions, of the energy performance of buildings based on different climatic conditions. Two school buildings, Liceo Classico “E. Duni” and Liceo Scientifico “D. Alighieri”, located in Matera, Italy, are considered. Furthermore, a strategy to improve the energy performance of the two school buildings is proposed by the installation of a co-trigeneration plant integrated with a solar plant. Such a plant is equipped with an absorption chiller to produce chilled fluid. The analysis under dynamic conditions has been performed by using a well-known simulation software, TRNSYS 17, and the results have been compared with those obtained under stationary conditions by employing a numerical solver, MC-11300, which is certified by the Italian Thermotechnical Committee. At first, the results obtained by considering the dynamic and stationary states and the experimental data measured in situ are compared by considering the actual buildings plants. Then, the energy performance of the two buildings is computed by considering three different climatic zones of Italy. Finally, a discussion of the advantages of the proposed requalification solution, which employs the trigeneration plant, is given.
The aim of this work is the study of the fluid dynamic structure
of underexpanded hydrogen jets ... more 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.
This paper describes the capabilities of a new in-house code, named SPREAD 2.0, to provide real t... more 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.
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