Use of degree of disequilibrium analysis to select kinetic constraints for the rate-controlled constrained-equilibrium (RCCE) method

The Rate-Controlled Constrained-Equilibrium (RCCE) method provides a general framework that enables, with the same ease, reduced order kinetic modelling at three different levels of approximation: shifting equilibrium, frozen equilibrium, as well as non-equilibrium chemical kinetics. The method in general requires a significantly smaller number of differential equations than the dimension of the underlying Detailed Kinetic Model (DKM) for acceptable accuracies. To provide accurate approximations, however, the method requires accurate identification of the bottleneck kinetic mechanisms responsible for slowing down the relaxation of the state of the system towards local chemical equilibrium. In other words, the method requires that such bottleneck mechanisms be characterized by means of a set of representative constraints. So far, a drawback of the RCCE method has been the absence of a systematic algorithm that would allow a fully automatable identification of the best constraints for a given range of thermodynamic conditions and a required level of approximation. In this paper, we provide the first of two steps towards such algorithm based on the analysis of the degrees of disequilibrium (DoD) of chemical reactions in the underlying DKM. In any given DKM the number of rate-limiting kinetic bottlenecks is generally much smaller than the number of species in the model. As a result, the DoDs of all the chemical reactions effectively assemble into a small number of groups that bear the information of the rate-controlling constraints. The DoDs of all reactions in each group exhibit almost identical behaviour (time evolution, spatial dependence). Upon identification of these groups, the proposed kernel analysis of N matrices that are obtained from the stoichiometric coefficients yields the N constraints that effectively control the dynamics of the system. The method is demonstrated within the framework of modeling the expansion of products of the oxy-combustion of hydrogen through a quasi one-dimensional supersonic nozzle. The analysis predicts and RCCE simulations confirm that, under the geometrical and boundary conditions considered, the underlying DKM is accurately represented by only two bottleneck kinetic mechanisms, instead of the three constraints identified for the same problem in a recently published work also based, in part, on DoD analysis

Laminar burning velocity of mixtures of air with indolene, isooctane, methanol and propane

Thesis (Sc.D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1980.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.Includes bibliographical references.by Mohamad Metghalchi.Sc.D

Laminar burning velocity of isooctane-air, methane-air, and methanol-air mixtures at high temperature and pressure

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1977.MICROFICHE COPY AVAILABLE IN ARCHIVES AND ENGINEERING.Includes bibliographical references.by Mohamad Metghalchi.M.S

Development and validation of A quasi-dimensional model for (M)Ethanol-Fuelled SI engines

RESEARCH OBJECTIVE - The use of methanol and ethanol in spark-ignition engines forms an interesting approach to decarbonizing transport and securing domestic energy supply. Experimental work has produced promising results, however, the full potential of light alcohols in modern engine technology remains to be explored. Today, this can be addressed at low cost using system simulations of the whole engine, provided that the employed models account for the effect of the fuel on engine operation. The goal of current work is to develop an engine cycle model that can accurately predict performance, efficiency, pollutant emissions and knock onset in state-of-the-art neat alcohol engines. METHODOLOGY - Two-zone thermodynamic engine modeling, in combination with 1D gas dynamics, is put forward as a useful tool for cheap and fast optimization of engines. Typically, this model class derives the mass burning rate of fuel from turbulent combustion models. A fundamental building block of turbulent combustion models is an expression for the laminar burning velocity of the fuel-air-residuals mixture at instantaneous cylinder pressure and temperature. This physicochemical property basically groups the contribution of the chemical reactions (of the fuel) to combustion. Consequently, an important part of our study consisted of calculating (using chemical kinetics) and measuring the laminar burning velocity of methanol and ethanol at engine-like conditions. In order to validate the developed engine model, its predictions were compared against a database of experimental results obtained on three different flex-fuel and dedicated alcohol engines. RESULTS - Comparison of the experimental and simulated cylinder, intake and exhaust pressure traces confirmed the predictive power of our engine model for methanol-fuelled engines. A wide variety of engine operating points were accurately reproduced thanks to a new laminar burning velocity correlation, which correctly accounts for changes in pressure, temperature, mixture richness and residual ratio. The Flame Closure Model of Zimont-Lipatnikov emerged as the most widely applicable model from a comparison of several turbulent combustion models. With regard to the gas dynamics it proved necessary to include a fuel puddling submodel to take the cooling effect due to alcohol injection into consideration. LIMITATIONS - The developed model was successfully validated for normal combustion in port-injected neat methanol engines. The validation of the routines for ethanol combustion and engines with direct injection is part of ongoing work. Now that normal combustion can be accurately simulated, further work will look at the prediction of pollutant emissions and knock onset in these engines. NOVELTY - This paper presents the first recent attempt to model the application of neat alcohols in modern and anticipated future engine technologies. Compared to previous work the effects of in-cylinder and mixture conditions on the combustion are more accurately predicted thanks to the inclusion of a new and widely validated laminar burning velocity correlation. In contrast to other studies, the current experimental database also includes measurements on turbocharged, high compression ratio engines with elevated amounts of EGR, which is representative of future dedicated alcohol engines. CONCLUSIONS - The current work focused on adapting the various submodels of quasi-dimensional engine codes to the properties of light alcohols. The developed simulation tools can be used with confidence to optimize current and future engines running on neat methanol and ethanol. This work also forms the starting point for an extension of the modelling concepts to alcohol-gasoline blends, which hold more industrial relevance

Entropy transport equation in large eddy simulation for exergy analysis of turbulent combustion systems

The transport equation of entropy is introduced in large eddy simulation to perform exergy analysis of turbulent combustion systems. The sources of exergy destruction can be evaluated by analyzing entropy generation terms, which appear in unclosed forms in this equation. The closure is based on the filtered density function (FDF) methodology. The primary advantage of FDF is that chemical reaction and its entropy generation effects appear in closed forms. This methodology involves a stochastic model, which is being developed to account for the subgrid scale transport of entropy