(54c) Integration of Computational Fluid Dynamics and Advanced Combustion Models Using An On-the-Fly Kinetic Reduction Approach

Fossil fuels contribute approximately 85% of the energy consumed in the United States each year, with transportation being an essential part. Of particular importance are aviation fuels such as jet A-1, JP08, JP-10, etc. As future global energy and environment regulations becoming more stringent, the importance of research oriented on saving energy along with reducing pollutants emissions and green house gases has continuously grown over last few decades. In recent studies detailed chemical kinetic mechanisms of aviation fuel combustion have been developed to address issues such as ignition, combustion efficiency, and pollutant formation. Due to the complexity of jet fuels which may contain in excess of 1000 compounds [1], simple multicomponent surrogate models have been developed to reproduce the combustion behavior of complex fuels. A comprehensive review of experimental and kinetic modeling studies is given in [2].

In addition to fuel oxidation mechanisms, NOx, soot, and polycyclic aromatic hydrocarbons (PAH) formation mechanisms have been developed and supplemented to the fuel mechanisms to predict pollutants emission in aviation engine combustion. The emission of soot from combustion causes significant health problems associated with respiratory system. Soot formation is considered to contain three partially parallel processes [3]: (a) Formation of molecular precursors of soot. PAH of increasing size are mainly formed from reactions between smaller PAH, PAH radicals, and acetylene. (b) Surface growth. Reactions at the surface of growing PAH particles facilitate the accumulation of carbon mass. (c) Particle coagulation. Particle sizes increase further by collision of growing soot particles. To predict soot formation, detailed PAH formation kinetics have been developed and extended to particle formation. A detailed review of modeling approaches is available in [4]. Nitrogen oxides formation in combustion systems is another significant pollutant source. Nitrogen oxides consist of nitric oxide (NO), nitrogen dioxide (NO2), and nitrous oxide (N2O). NO and NO2 are referred as NOx. Advances in experimental study of NOx reactions and increasing computer power have facilitated comprehensive modeling of NOx formation and destruction. Modeling approaches of NOx formation has been reviewed in [5].

Detailed kinetic mechanisms can provide comprehensive description of fuel chemistry. However, they usually consist of hundreds of species and thousands of reactions. The incorporation of these detailed mechanisms in CFD calculations is computationally expensive. Thus two categories of approximation approaches have been proposed: (a) implementing comprehensive CFD codes by including simplified kinetic models, and (b) employing detailed kinetic mechanisms by using simplified description of the flow field. Approximation approaches focusing on kinetic reduction include global lumping techniques, skeletal reduction, and adaptive reduction. Kuo and Wei [6] proposed a lumping approach, which lumps concentrations of chemical species into a reduced species set and lumps elementary reactions into a few global steps. Approaches aiming at developing skeletal kinetic mechanisms include sensitivity analysis [7, 8], quasi-steady-state-assumption (QSSA) and partial equilibrium approximations [9, 10], and optimization-based approaches [11-13]. Adaptive kinetic reduction strategy has been explored to use different mechanisms for different conditions. Several adaptive schemes have been proposed such as In situ Adaptive Tabulations (ISAT) [14], the ?store and retrieve? approach [15], the mathematical programming approach [16], and a graph-based reduction approach [17]. On the other hand, various approximation approaches have been oriented towards simplified flow calculation. This usually evolves from a zone-method approach. The earliest example of this type of model was single-zone models [18] which considered the entire engine chamber to be single cell. Later, multi-zone models have been developed which divide the combustion chamber into multiple zones based on physical conditions [19].

In this paper, an on-the-fly kinetic reduction methodology which takes advantage of both accurate CFD models and detailed kinetic mechanisms is described. The on-the-fly reduction scheme analyzes local reactive conditions and develops reduced mechanisms dynamically during the flow calculation. The reduction technique used in the scheme is based on element flux analysis, which was first introduced by Revel et al. [20] and implemented in mechanism reduction by Androulakis et al. [21]. The atomic fluxes for each atom (C, H, O, N, etc.) between sources and sinks are calculated at each time step and sorted in descending order. A user-selected cutoff is then applied on the descending-sorted flux list. Source-sink pairs above the cutoff are included in the reduced mechanism while those under the cutoff are considered to be dormant at current time step. As the system evolves, flux is re-calculated and the reduced mechanism is updated. The on-the-fly scheme enables efficient integration of detailed fuel kinetics with CFD calculation, which is especially useful to the simulation study of complex aviation fuel combustion systems. However, the on-the-fly scheme was not able to capture soot formation due to the fact that soot species usually have much smaller flux values. A tree-building searching algorithm has been proposed to complement the on-the-fly scheme in order to incorporate soot formation kinetics in mechanism reduction. The proposed methodology was tested in plug flow reactor (PFR) model to predict fuel ignition delay and engine simulation code KIVA [22] to demonstrate its application in multi-dimensional CFD. Detailed soot formation mechanism [3] and JP-10/NOx mechanism [23] were employed in the demonstration.

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