Browsing by Author "Zizin, Anton"

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    Development of a reduced chemical-kinetic combustion model for practical fuels
    (2014) Zizin, Anton; Riedel, Uwe (Prof. Dr. rer. nat.)
    This work deals with modeling of the surrogate fuels including their physical as well as chemical properties. The huge number of components of real fuels makes it impossible to model its properties with equations for all of the constituents. Even less feasible is to create a kinetic mechanism for such a fuel. As an alternative a mixture of a few hydrocarbons is used to replace the real fuel. Such mixture is called a surrogate fuel. The goal of the present work is to construct a complete method for the creation of a reduced combustion model for a Jet-A surrogate fuel, starting from the development of the fuel model itself to the algorithms for a kinetic mechanism reduction. Prior to the development of the numerical code a short overview of available equations of state is given and a comparison of their advantages, disadvantages and precision is made. The developed numerical tools allow fast calculation of the fuels phase diagram, distillation curve and critical point. The physical model and developed tools are successfully validated over a number of experiments for different mixtures of hydrocarbons. The elaborated surrogate model is obtained by optimization of the above mentioned characteristics as well as combustion enthalpy, formation enthalpy, molar weight and sooting tendency index. The Jet-A surrogate model consists of 11% propylcyclohexane, 14% iso-octane, 22% n-dodecane, 28% 1-methylnaphtalene and 25% n-hexadecane. Kinetic models are developed for the large hydrocarbons: n-decane, n-dodecane and n-hexadecane. These models are thoroughly tested over a wide range of shock-tube experiments for each of these pure components as well as mixtures of n-decane and 1-methylnaphthalene.The developed sub-mechanisms are added to the sub-mechanisms of other surrogate fuel components developed at the Institute of Combustion Technology at the German Aerospace Center (DLR) producing the full mechanism with 183 species and 1239 reactions. Numerical simulations made with the full mechanism for the surrogate blend are compared with shock-tube experiments for Jet-A kerosene. Of special interest is the low-temperature region and negative temperature coefficient. A rate of production analysis for each surrogate fuel component and sensitivity analysis will be given. Due to the lack of the thermodynamical data for the low-temperature species another computational tool is developed. It calculates thermodynamical properties using the Benson group additivity method. For extrapolation of the heat capacity to higher temperatures Willhoit polynomials are used due to good precision and stability. The verification is made by comparison of the results for a range of hydrocarbon molecules with database values and calculations in other works. For the reduction algorithm several common reduction methods are described and compared. Using some of these methods analysis and reduction of the full kinetic mechanism is made. Finally, for the reduction of the full mechanism two additional programs are developed. Using the KINALC package they accomplish two analyses in several time points for chosen numerical experiments: importance of reactions and species. Comparing the obtained lists they automatically choose unimportant species and reactions and remove them from the mechanism. Excellent results are obtained even at high degrees of reduction. Nevertheless, there exists a minimal number of species, after which further reduction causes very fast growing errors. At the same time the algorithm tends to remove some very basic species at high degrees of reduction. This limits the minimal size of the mechanism to 70-80 species. The influence of different reduction parameters on the performance of the resulting reduced mechanism is investigated and will be given.