Contour plots and image sequences pertaining to smalls-scale free-piston homogeneous charge compression ignition engines
Description
The contour plots and image sequences are presented for illustrating the ignition and combustion processes in smalls-scale free-piston homogeneous charge compression ignition engines. In contrast to transport-limited engine combustion modes, homogeneous charge compression ignition depends primarily upon the compression process and fuel oxidation kinetics. Therefore, matching engine operating conditions and homogeneous charge compression ignition combustion is essentially a reaction engineering problem. The governing equations for a non-isothermal and non-adiabatic system are mass conservation. To model heat transfer in a smalls-scale engine, several approaches with varying levels of complexity may be taken. However, the heat transfer model should be relatively simple but capture the surface-area-to-volume ratio dependence. Moreover, conduction from the charge to an isothermal wall represents a worst-case scenario in terms of heat loss. Hence, conduction is assumed to be the dominant heat transfer mode. The non-dimensional heat transfer rate is proportional to the non-dimensional surface-area-to-volume ratio and depends exclusively upon the compression ratio and the aspect ratio. Evidently, large aspect ratios tend to minimize heat transfer. Diffusion fluxes also increase with surface-area-to-volume ratio. Therefore, assuming that radical recombination reactions on the combustion chamber walls are mass transfer-limited, it is hypothesized that the frequency of these events will increase with the surface-area-to-volume ratio. Thus, this effect is taken into account by assuming the walls to be perfect radical sinks and that diffusion dominates. Wall species are assumed to have thermodynamic properties identical to their authentic counterparts. Temperature-dependent terms of the reaction rates are included in the kinetic mechanism while pressure and geometry-dependent parts are incorporated in the source code. Transport properties of the charge are assumed identical to air. To achieve homogeneous charge compression ignition, the charge must be brought by compression to a thermodynamic state such that the ignition delay time is short relative to the residence time. Therefore, in contrast to typical reaction engineering problems, the bulk temperature, chemical time, and residence time are variables. Additionally, homogeneous charge compression ignition depends strongly upon the initial conditions and the compression ratio. Hence, the relationships between the various parameters and homogeneous charge compression ignition are seldom obvious. Contributor: Junjie Chen, E-mail address: koncjj@gmail.com, ORCID: 0000-0002-5022-6863, Department of Energy and Power Engineering, School of Mechanical and Power Engineering, Henan Polytechnic University, 2000 Century Avenue, Jiaozuo, Henan, 454000, P.R. China
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To facilitate computational modeling of transport phenomena and chemical kinetics in the flowing system of complex chemical reactions involving gas-phase and surface species, steady-state analyses are performed and computational fluid dynamics is used. ANSYS FLUENT is applied to the problem involving surface chemistry. Physical properties depend on temperature and composition.