The increasing adoption of lean premixed (LP) combustion technologies is rising the need to study... more The increasing adoption of lean premixed (LP) combustion technologies is rising the need to study the dynamic behavior of flames in combustors [1]. Indeed combustion in lean conditions is much more prone to “instabilities” that manifest as oscillations of the flame able to drive detrimental pressure oscillations (thermo-acoustic instabilities). The proper characterization of these instabilities is the loss, after the change or perturbation of some system parameters, of a stable and steady regime to acquire a new stable but oscillating regime. Even though some of the most fundamental results date back more than 100 years ago [2], a full clear understanding of the mechanisms leading to the appearance of a dynamic combustion behavior is still not achieved. The full characterization of these instabilities is a difficult process. Most of the approaches adopted up to now belongs to the class of linear stability analysis. However the informations that can be obtained in the framework of li...
Limit combustion phenomena, such as ignition, are rather sensitive to chemical kinetics and these... more Limit combustion phenomena, such as ignition, are rather sensitive to chemical kinetics and these properties are therefore used to physically characterize the behaviour of different fuels. In the framework of bifurcation theory, the ignition and extinction correspond to saddle-node bifurcation points in a classical S-shaped steady-state curve [11, 18]. Then, the location of ignition and extinction conditions and their dependence on the main parameters (like pressure, equivalence ratio, residence time or inlet temperature in reactors) can be reformulated as a problem of bifurcation analysis. However, this approach represents a computationally difficult task when reaction mechanisms with several hundreds of species and thousands of chemical reactions are considered. In the past, a full bifurcation analysis has been successfully conducted adopting detailed schemes for some elementary fuels like hydrogen [7, 13] or methane with the simpler versions of the GRI mechanism [14, 15] by using...
Ultra Fine Water Mist (UFWM) fire extinction is a promising technology to achieve fire suppressio... more Ultra Fine Water Mist (UFWM) fire extinction is a promising technology to achieve fire suppression with reduced quantities of water [1]. Its use can also limit properties losses by limiting the damage to structures and objects consequent to large water release. However the mechanisms of extinguishment are very complex because of the strong interaction of the aerosol particles with the fluid dynamics and fire structures. These effect poses several issues for a proper dimensioning of the system. Indeed it is difficult to predict the quantity of water able to reach the flame, because the convective motion in the compartment, caused by the fire itself, strongly affects the path of the released droplets already at few tens of centimetre from the injection point. Particles so small as those produced by UFWM injectors, with initial size of 50 micron or less, rapidly dissipate their initially high inertia (high injection pressure of about 100 bar) and, even if injected close to the fire, th...
Combustion phenomena very often, especially in industrial applications, involve an unsteady behav... more Combustion phenomena very often, especially in industrial applications, involve an unsteady behavior, being intrinsically transient (f.i. ignition and explosions) or because affected by unsteady phenomena (like noise, turbulence, instabilities). The correct prediction of the unsteady behavior of a reactive mixture depends on a number of factors that change varying the operative conditions assumed. Thus, the unsteady combustion behaviors can depend upon transport mechanisms and their parameters[1, 2], but they are also affected by the time scales of the chemical kinetics[3]. Actually, in real configurations, these two class of effects share a wide overlapping of mutual interaction, being effectively separated only in limiting cases of very large or very low Damkohler numbers.
The use of detailed chemical mechanisms is becoming increasingly necessary during the actual tran... more The use of detailed chemical mechanisms is becoming increasingly necessary during the actual transition of energy production from fossil to renewable fuels. Indeed, the modern renewable fuels are characterized by a composition more complex than traditional fossil fuels due to the variability of the properties of the primary source, i.e. biomass. The parametric continuation can be a formidable tool to study the behavior of these new fuels allowing to promptly assess equilibrium conditions varying the main operative parameters. However, parametric continuation is a very computationally demanding procedure, both for the number of elementary operations needed and for the memory requirements. Actually, only very recently some approaches that allow affording this computation with chemical mechanisms composed of hundreds of chemical species and thousands of reactions have been proposed [1, 2, 37]. Starting from the procedure presented in [1], this paper illustrates further improvements of ...
The entropy production (second law-based) and exergy loss analysis, from historical point of view... more The entropy production (second law-based) and exergy loss analysis, from historical point of view, were developed to evaluate and optimize the performance of energy conversion systems [1]. For those systems involving a combustion process, it has been concluded that the major part of the exergy loss is due exactly to the combustion process [1]. Therefore, entropy production has been investigated in several combustion processes, for example: droplet combustion [2]; spray combustion [3]; coal/carbon combustion [4]; laminar premixed flames [5, 6]; diffusion flames [7].
The understanding of combustion dynamics and of instability generation is a fundamental topic. Bo... more The understanding of combustion dynamics and of instability generation is a fundamental topic. Both theoretical and experimental studies highlighted the role of different mechanisms, like unmixedness of reactants [1], flame dynamics response to perturbations [2–4] and thermo-acoustic instabilities [5]. Nevertheless, quantitative predictions on the role of single mechanisms often lack a satisfactorily agreement. An incomplete knowledge of the effective behavior of the combustion kinetic under dynamic conditions is still present even if the possibility of a steadily oscillating combustion behaviour sustained by the sole kinetic is known from long time [6].
The chemical composition of renewable fuels can be very complex and chemical mechanisms composed ... more The chemical composition of renewable fuels can be very complex and chemical mechanisms composed by hundreds of species and several thousands of reactions can be required for an accurate prediction of their combustion behavior [1,2]. An important step for their computationally efficient use in simulations is, therefore, the possibility to obtain reduced mechanisms with an assessed range of validity.
The performance of Stirling engines is directly related to the amount of energy that, in the form... more The performance of Stirling engines is directly related to the amount of energy that, in the form of heat, participates in the thermodynamic cycle. A peculiar characteristic of this type of engine is the closed circuit of the working fluid, implying a periodic admission and extraction, ideally involving the whole working fluid, of the heat exchanged during the cycle without any exchange of material with the external environment. Several correlations have been proposed in the literature to predict the heat exchange in the hot side heat exchangers, but most of them are based on nondimensional numbers, usually expressed in terms of an oscillatory Reynolds number and a non-dimensional length, that even if they take into account the amplitude and frequency of the oscillating flow inside the tube with respect to its diameter, do not include any dependency upon the length of the tube. Nevertheless, the length of the tube can have a great impact on the performance of a Stirling engine. The ...
ABSTRACT Transient simulations of the methane-air mixture combustion in a Perfectly Stirred React... more ABSTRACT Transient simulations of the methane-air mixture combustion in a Perfectly Stirred Reactor (PSR) with global one-step and two-step, reduced (by directed relation graph method) and detailed chemical reaction mechanisms, have been conducted. The simulation conditions were carefully selected to observe the dynamic of a PSR with an oscillating residence time close to the extinction. The effect of the oscillation amplitude and frequency have been investigated over a wide range of values. The temperature response of the global and reduced mechanism has been compared to that of the detailed reaction mechanisms, the GRI Mech 3.0. The discussion of results points out limits and problems associated with the adoption of global reaction mechanisms. The results of the comparison indicate that the adoption of a global mechanism is not adequate in order to obtain an accurate description of methane-air combustion in a PSR with an oscillating residence time. This comparison underlines that only a little benefit is added by the adoption of the selected two-step mechanism. Instead, the reduced mechanism shows an excellent agreement with the detailed reaction mechanism.
The use of of Large Eddy Simulation for practical applications requires to adopt congruent inflow... more The use of of Large Eddy Simulation for practical applications requires to adopt congruent inflow boundary conditions. An approach proposed in most industrial codes, consists to use turbulence synthesis method. However while the construction of a fluctuations field, solution of Navier-Stokes equations, can be pursued, the correct phase, orientation and strain of the turbulent eddies cannot be easily recovered.. This point could influence the dynamic of non premixed combustion flames which is governed by molecular diffusion and affected by the effects of turbulence scales. Depending on the ratio between the flame thickness and the dimension of the turbulent scales, the rate of reaction and the flame so on, is increased or unaffected.. Precursor simulation methods, which extract and rescale data from auxiliary simulations to be used as inlet conditions, have shown to be a valid alternative, in recovering genuine turbulence, to synthesis turbulence methods. (See Fig.1) This works aims ...
The work concerns the numerical simulation of the direct coupling between the Fluidized Bed Combu... more The work concerns the numerical simulation of the direct coupling between the Fluidized Bed Combustor (FBC) and the heat exchanger of a Stirling engine. However, several issues need to be considered. Here the proper coupling of the Stirling engine with the fluidized bed is analyzed to assess the effective heat exchange enhancement attainable by the proposed configuration. In order to simplify the problem, a 2D simulation has been set. With this approximation the tube bank of the hot side exchanger of the Stirling engine are represented by a envelope surface shown in red in figure 2. This approximation appears justified by the very close spacing of the present heater tubes in the inner part of their cylindrical disposition. The main aim of this simulation is to investigate the level of fluidization attainable along the depth of lateral immersion in the sand. The influence of various parameters on the heat transfer between the FBC and the Stirling exchanger, such as particle diameter,...
The increasing adoption of lean premixed (LP) combustion technologies is rising the need to study... more The increasing adoption of lean premixed (LP) combustion technologies is rising the need to study the dynamic behavior of flames in combustors [1]. Indeed combustion in lean conditions is much more prone to “instabilities” that manifest as oscillations of the flame able to drive detrimental pressure oscillations (thermo-acoustic instabilities). The proper characterization of these instabilities is the loss, after the change or perturbation of some system parameters, of a stable and steady regime to acquire a new stable but oscillating regime. Even though some of the most fundamental results date back more than 100 years ago [2], a full clear understanding of the mechanisms leading to the appearance of a dynamic combustion behavior is still not achieved. The full characterization of these instabilities is a difficult process. Most of the approaches adopted up to now belongs to the class of linear stability analysis. However the informations that can be obtained in the framework of li...
Limit combustion phenomena, such as ignition, are rather sensitive to chemical kinetics and these... more Limit combustion phenomena, such as ignition, are rather sensitive to chemical kinetics and these properties are therefore used to physically characterize the behaviour of different fuels. In the framework of bifurcation theory, the ignition and extinction correspond to saddle-node bifurcation points in a classical S-shaped steady-state curve [11, 18]. Then, the location of ignition and extinction conditions and their dependence on the main parameters (like pressure, equivalence ratio, residence time or inlet temperature in reactors) can be reformulated as a problem of bifurcation analysis. However, this approach represents a computationally difficult task when reaction mechanisms with several hundreds of species and thousands of chemical reactions are considered. In the past, a full bifurcation analysis has been successfully conducted adopting detailed schemes for some elementary fuels like hydrogen [7, 13] or methane with the simpler versions of the GRI mechanism [14, 15] by using...
Ultra Fine Water Mist (UFWM) fire extinction is a promising technology to achieve fire suppressio... more Ultra Fine Water Mist (UFWM) fire extinction is a promising technology to achieve fire suppression with reduced quantities of water [1]. Its use can also limit properties losses by limiting the damage to structures and objects consequent to large water release. However the mechanisms of extinguishment are very complex because of the strong interaction of the aerosol particles with the fluid dynamics and fire structures. These effect poses several issues for a proper dimensioning of the system. Indeed it is difficult to predict the quantity of water able to reach the flame, because the convective motion in the compartment, caused by the fire itself, strongly affects the path of the released droplets already at few tens of centimetre from the injection point. Particles so small as those produced by UFWM injectors, with initial size of 50 micron or less, rapidly dissipate their initially high inertia (high injection pressure of about 100 bar) and, even if injected close to the fire, th...
Combustion phenomena very often, especially in industrial applications, involve an unsteady behav... more Combustion phenomena very often, especially in industrial applications, involve an unsteady behavior, being intrinsically transient (f.i. ignition and explosions) or because affected by unsteady phenomena (like noise, turbulence, instabilities). The correct prediction of the unsteady behavior of a reactive mixture depends on a number of factors that change varying the operative conditions assumed. Thus, the unsteady combustion behaviors can depend upon transport mechanisms and their parameters[1, 2], but they are also affected by the time scales of the chemical kinetics[3]. Actually, in real configurations, these two class of effects share a wide overlapping of mutual interaction, being effectively separated only in limiting cases of very large or very low Damkohler numbers.
The use of detailed chemical mechanisms is becoming increasingly necessary during the actual tran... more The use of detailed chemical mechanisms is becoming increasingly necessary during the actual transition of energy production from fossil to renewable fuels. Indeed, the modern renewable fuels are characterized by a composition more complex than traditional fossil fuels due to the variability of the properties of the primary source, i.e. biomass. The parametric continuation can be a formidable tool to study the behavior of these new fuels allowing to promptly assess equilibrium conditions varying the main operative parameters. However, parametric continuation is a very computationally demanding procedure, both for the number of elementary operations needed and for the memory requirements. Actually, only very recently some approaches that allow affording this computation with chemical mechanisms composed of hundreds of chemical species and thousands of reactions have been proposed [1, 2, 37]. Starting from the procedure presented in [1], this paper illustrates further improvements of ...
The entropy production (second law-based) and exergy loss analysis, from historical point of view... more The entropy production (second law-based) and exergy loss analysis, from historical point of view, were developed to evaluate and optimize the performance of energy conversion systems [1]. For those systems involving a combustion process, it has been concluded that the major part of the exergy loss is due exactly to the combustion process [1]. Therefore, entropy production has been investigated in several combustion processes, for example: droplet combustion [2]; spray combustion [3]; coal/carbon combustion [4]; laminar premixed flames [5, 6]; diffusion flames [7].
The understanding of combustion dynamics and of instability generation is a fundamental topic. Bo... more The understanding of combustion dynamics and of instability generation is a fundamental topic. Both theoretical and experimental studies highlighted the role of different mechanisms, like unmixedness of reactants [1], flame dynamics response to perturbations [2–4] and thermo-acoustic instabilities [5]. Nevertheless, quantitative predictions on the role of single mechanisms often lack a satisfactorily agreement. An incomplete knowledge of the effective behavior of the combustion kinetic under dynamic conditions is still present even if the possibility of a steadily oscillating combustion behaviour sustained by the sole kinetic is known from long time [6].
The chemical composition of renewable fuels can be very complex and chemical mechanisms composed ... more The chemical composition of renewable fuels can be very complex and chemical mechanisms composed by hundreds of species and several thousands of reactions can be required for an accurate prediction of their combustion behavior [1,2]. An important step for their computationally efficient use in simulations is, therefore, the possibility to obtain reduced mechanisms with an assessed range of validity.
The performance of Stirling engines is directly related to the amount of energy that, in the form... more The performance of Stirling engines is directly related to the amount of energy that, in the form of heat, participates in the thermodynamic cycle. A peculiar characteristic of this type of engine is the closed circuit of the working fluid, implying a periodic admission and extraction, ideally involving the whole working fluid, of the heat exchanged during the cycle without any exchange of material with the external environment. Several correlations have been proposed in the literature to predict the heat exchange in the hot side heat exchangers, but most of them are based on nondimensional numbers, usually expressed in terms of an oscillatory Reynolds number and a non-dimensional length, that even if they take into account the amplitude and frequency of the oscillating flow inside the tube with respect to its diameter, do not include any dependency upon the length of the tube. Nevertheless, the length of the tube can have a great impact on the performance of a Stirling engine. The ...
ABSTRACT Transient simulations of the methane-air mixture combustion in a Perfectly Stirred React... more ABSTRACT Transient simulations of the methane-air mixture combustion in a Perfectly Stirred Reactor (PSR) with global one-step and two-step, reduced (by directed relation graph method) and detailed chemical reaction mechanisms, have been conducted. The simulation conditions were carefully selected to observe the dynamic of a PSR with an oscillating residence time close to the extinction. The effect of the oscillation amplitude and frequency have been investigated over a wide range of values. The temperature response of the global and reduced mechanism has been compared to that of the detailed reaction mechanisms, the GRI Mech 3.0. The discussion of results points out limits and problems associated with the adoption of global reaction mechanisms. The results of the comparison indicate that the adoption of a global mechanism is not adequate in order to obtain an accurate description of methane-air combustion in a PSR with an oscillating residence time. This comparison underlines that only a little benefit is added by the adoption of the selected two-step mechanism. Instead, the reduced mechanism shows an excellent agreement with the detailed reaction mechanism.
The use of of Large Eddy Simulation for practical applications requires to adopt congruent inflow... more The use of of Large Eddy Simulation for practical applications requires to adopt congruent inflow boundary conditions. An approach proposed in most industrial codes, consists to use turbulence synthesis method. However while the construction of a fluctuations field, solution of Navier-Stokes equations, can be pursued, the correct phase, orientation and strain of the turbulent eddies cannot be easily recovered.. This point could influence the dynamic of non premixed combustion flames which is governed by molecular diffusion and affected by the effects of turbulence scales. Depending on the ratio between the flame thickness and the dimension of the turbulent scales, the rate of reaction and the flame so on, is increased or unaffected.. Precursor simulation methods, which extract and rescale data from auxiliary simulations to be used as inlet conditions, have shown to be a valid alternative, in recovering genuine turbulence, to synthesis turbulence methods. (See Fig.1) This works aims ...
The work concerns the numerical simulation of the direct coupling between the Fluidized Bed Combu... more The work concerns the numerical simulation of the direct coupling between the Fluidized Bed Combustor (FBC) and the heat exchanger of a Stirling engine. However, several issues need to be considered. Here the proper coupling of the Stirling engine with the fluidized bed is analyzed to assess the effective heat exchange enhancement attainable by the proposed configuration. In order to simplify the problem, a 2D simulation has been set. With this approximation the tube bank of the hot side exchanger of the Stirling engine are represented by a envelope surface shown in red in figure 2. This approximation appears justified by the very close spacing of the present heater tubes in the inner part of their cylindrical disposition. The main aim of this simulation is to investigate the level of fluidization attainable along the depth of lateral immersion in the sand. The influence of various parameters on the heat transfer between the FBC and the Stirling exchanger, such as particle diameter,...
Uploads