永岡 真

For the measurements of flow rate, pressure and/or temperature in an engine exhaust pipe, probes are often inserted into the exhaust pipe depending on the application. These measurement probes differ a lot in terms of their size and shape. The flow around the probes become further complicated due to the pulsation of engine exhaust flow. In this study, computational fluid dynamics (CFD) simulations were carried out and a zero-dimensional (0D) model was constructed to analyze the flow field around the probe and flow rate of a pulsating flow. The simulations and the measurements of the flow rate and pressure were performed on flows around a hexagonal prism inserted in a circular pipe which is intended to be a differential pressure flow meter. The velocity field was also measured using the particle image velocimetry (PIV) technique. The CFD simulation results were validated with the experiments for both steady and pulsating flows. In the 0D model for pulsating flow, the flow acceleration as well as pipe friction and prism drag losses were taken into account. The flow rates calculated using the model agreed well with the CFD simulation results. The relationship between the flow rate and the pressure was analyzed using the CFD and the 0D model. In the low flow rate and low pressure difference period, the relationship between the flow rate and the square root of pressure difference deviated from linear and exhibited hysteresis due to the flow acceleration. The cycle-averaged flow rates calculated using the 0D model were closer to those by the CFD simulations than those of a conventional steady flow correlation.

This report describes the implementation of the spark channel short circuit and blow-out submodels, which were described in the previous report, into a spark ignition model. The spark channel which is modeled by a particle series is elongated by moving individual spark particles along local gas flows. The equation of the spark channel resistance developed by Kim et al. is modified in order to describe the behavior of the current and the voltage in high flow velocity conditions and implemented into the electrical circuit model of the electrical inductive system of the spark plug. Input parameters of the circuit model are the following: initial discharge energy, inductance, internal resistance and capacitance of the spark plug, and the spark channel length obtained by the spark channel model. The instantaneous discharge current and the voltage are obtained as outputs of the circuit model. When two arbitrary spark particles of the spark channel get close, the short circuit occurs if the electric potential differences between the two locations exceed a certain threshold voltage, which is raised with increasing distance between the two particles and decreasing discharge current. When the current falls below a lower limit current for maintenance of discharge, the spark blow-out occurs. A new spark channel is formed if the secondary circuit has the remaining energy which can break the electrical insulation between electrodes. Each line element of the spark channel particles heats and ignites the surrounding mixture gas. The turbulent flame speed and extinction are considered in the flame kernel behavior. The behavior of the spark channel, the current and voltage of the secondary circuit, and the ignition limit due to in-creases in the EGR rate were consistent with data measured from the spark ignition process in a combustion chamber.