• Energy Research

Abstract The effect of selected additives on the ignition delay of ethanol (EtOH)/air and dimethylether (DME)/air mixture is investigated. Computational Singular Perturbation (CSP) tools are utilized in an effort to determine algorithmically which species to select as additives and it is established that CSP can identify species whose addition to the mixture can affect ignition delay. However, this is not a necessary condition for additives to be effective. Additives that are not identified by CSP can have a substantial effect on ignition delay, provided that they drastically alter the prevailing chemistry, by altering the instant in time when the thermal runaway regime develops. Some of the additives that were studied computationally are unstable radicals whose injection in practical mixtures is unrealistic. However, chemically stable, relatively light species were also determined that can drastically affect ignition delay, such as hydrogen peroxide, formaldehyde and acetaldehyde.

A numerical model for planar premixed flames of methane in ceramic porous media has been developed to improve the understanding of the structure of such flames. The model successfully reproduces experimental data for both single- and two-layer surface flames. The success is attributed to the detail given to the boundary conditions and the radiation modelling, which was done by solving the radiation transfer equation inside the porous medium without any simplifying models. Surface-stabilized flames yielded S L/S L0 1 and their energy balance was similar to that of a free flame, which implies that the burning velocity acceleration is due to the...

Considering temporally evolving processes, the search for optimal input selection in Machine Learning (ML) algorithms is extended here beyond (i) the readily available independent variables defining the process and (ii) the dependent variables suggested by feature extraction methods, by considering the time scale that characterizes the process. The analysis is based on the process of homogeneous autoignition, which is fully determined by the initial temperature T(0) and pressure p(0) of the mixture and the equivalence ratio ϕ that specifies the initial mixture composition. The aim is to seek the optimal input for the prediction of the time at which the mixture ignites. The Multilayer Perceptron (MLP) and Principal Component Analysis (PCA) algorithms are employed for prediction and feature extraction, respectively. It is demonstrated that the time scale that characterizes the initiation of the process τe(0), provides much better accuracy as input to MLP than any pair of the three independent parameters T(0), p(0) and ϕ or their two principal components. Indicatively, it is shown that using τe(0) as input results in a coefficient of determination R2 in the range of 0.953 to 0.982, while the maximum value of R2 when using the independent parameters or principal components is 0.660. The physical grounds, on which the success of τe(0) is based, are discussed. The results suggest the need for further research in order to develop selection methodologies of optimal inputs among those that characterize the process.

The dynamics of a homogeneous adiabatic autoignition of an ammonia/air mixture at constant volume was studied, using the algorithmic tools of Computational Singular Perturbation. Since ammonia combustion is characterized by both unrealistically long ignition delays and elevated NO x emissions, the time frame of action of the modes that are responsible for ignition was analyzed by calculating the developing time scales throughout the process and by studying their possible relation to NO x emissions. The reactions that support or oppose the explosive time scale were identified, along with the variables that are related the most to the dynamics that drive the system to an explosion. It is shown that reaction H 2 O 2 (+M) → OH + OH (+M) is the one contributing the most to the time scale that characterizes ignition and that its reactant H 2 O 2 is the species related the most to this time scale. These findings suggested that addition of H 2 O 2 in the initial mixture will influence strongly the evolution of the process. It was shown that ignition of pure ammonia advanced as a slow thermal explosion with very limited chemical runaway. The ignition delay could be reduced by more than two orders of magnitude through H 2 O 2 addition, which causes only a minor increase in NO x emissions.

Abstract When the fast dissipative time scales become exhausted, the evolution of reacting processes is characterized by slower time scales. Here the case where these slower time scales are of explosive character is considered. This feature allows for the acquisition of significant physical understanding; among others, the identification of intermediates in the reacting process that can be used as additives for the control of the ignition delay. The case of the homogeneous autoignition of CH 4 /air mixtures is analyzed here and the effects of adding the stable intermediates CH 2 O and H 2 O 2 to the fuel. These two species are identified as those relating the most to the explosive mode that causes autoignition, throughout the largest part of the ignition delay. Small quantities of these species in the initial mixture decrease considerably the ignition delay, by expediting the development of the thermal runaway.

Lowering emissions from power generating gas turbines, while retaining efficiency and power output, constitutes a formidable task, both at fundamental and technical levels. Combined gas turbine cycles involving air humidification are particularly attractive, since they provide additional power with improved efficiency. Water or steam addition promotes the reduction of nitrogen oxides emissions, for both the premixed and non-premixed modes of operation. Consequently, there is an urgent need for thorough understanding of the combustion chemistry and flow-chemistry interaction under high pressure and high humidity conditions as well as simulating the turbulent flow field with realistic chemistry. Both objectives require the development of reduced kinetic mechanisms. Reduced mechanisms for methane combustion valid for high pressure and high humidity are developed here, using the CSP (computational singular perturbation) method. The effects of humidity and pressure on the dynamics of NO formation pathways are discussed. A reaction progress variable model for the simulation of turbulent combustion is also developed, valid for adiabatic, non-adiabatic, premixed as well as partly or non-premixed combustion of various fuels, including natural gas, hydrogen and syngas. The model utilizes the CSP methodology for accurate mapping of the pertinent thermochemical data on a set of two reaction progress variables. Preliminary results are displayed.