Makoto Nagaoka

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.

Direct numerical simulations (DNSs) of interactions between turbulent premixed flame and isothermal wall have been conducted to investigate heat losses and quenching mechanism of turbulent premixed flames near the wall. Near-wall behaviors of hydrogen/air and methane/air premixed flames were investigated by considering detailed kinetic mechanism. For hydrogen/air flame, heat release rate in near-wall region is higher than that of freely-propagating flame. For methane/air flame, however, heat release rate near the wall becomes lower than that of freely-propagating flame. This difference is caused by different contributions of certain low-temperature reactions that are enhanced near the wall to the total heat release rate. The wall heat flux of hydrogen/air turbulent flame is higher than that of cor-responding laminar flame, because the turbulent burning velocity increases significantly and pressure rises near the flame region. For methane flame, however, the wall heat flux nearly coincides with that of laminar flame since the turbulent burning velocity is not enhanced due to local extinction.

The CFD for automotive engines has been applied as a CAE tool for production engine design and development since around 2000. The development process has been changed. However, there are still many research problems on the mathematical modeling of IC engine and the numerical method. The applicable site of engine will be enlarged to the concurrent development process with establishment of design evaluation index.

We present a coupled interface capturing and multi-fluid model (CIM) method for computing fuel injection behavior from a nozzle internal flow to a liquid jet seamlessly. This method is a kind of CFD algorithm for multi-phase and multi-scale flow simulation. In a computational cell, a relatively large-scale gas/liquid interface captured by the computational mesh is handled by the PLIC-VOF method, and small-scale dispersed gas bubbles and liquid droplets are handled by the four-fluid model. The MF-ICE method is employed for coupling of pressure and velocity, where the compressibility of the gas phase is taken into account. A cavitation model and the CSF model are implemented into the four-fluid model and the VOF method, respectively. The present method demonstrated an ability to handle the multi-phase and multi-scale flows in which different size scales of the gas/liquid interfaces are coexisting. 2-D axisymmetrical calculations of a liquid jet from a cylindrical nozzle were performed. The transition from a cavitating flow regime to a hydraulic flip within a nozzle orifice was simulated well. Moreover the influence of the cavitation bubbles at the nozzle orifice exit and of the hydraulic flip on the subsequent liquid jet was also simulated by the uninterrupted calculation from the nozzle internal flow to the liquid jet. The predicted discharge coefficient of the nozzle and transition of the flow regime by the increase of the pressure difference were corresponding to measurement data.

A series calculation methodology from the injector nozzle internal flow to the in-cylinder fuel spray and mixture formation in a diesel engine was developed. The present method was applied to a valve covered orifice (VCO) nozzle with the recent common rail injector system. The nozzle internal flow calculation using an Eulerian three-fluid model and a cavitation model was performed. The needle valve movement during the injection period was taken into account in this calculation. Inside the nozzle hole, cavitation appears at the nozzle hole inlet edge, and the cavitation region separates into two regions due to a secondary flow in the cross section, and it is distributed to the nozzle exit. Unsteady change of the secondary flow caused by needle movement affects the cavitation distribution in the nozzle hole, and the spread angle of the velocity vector at the nozzle exit. The transient data of spatial distributions of velocity, turbulent kinetic energy, dissipation rate, void fraction rate, etc. at the nozzle exit were extracted. These output data were transferred to the spray calculation, in which a primary break-up model was applied to the Discrete Droplet Model (DDM). The calculation results were compared with the results of the measurement data of spray in a constant volume vessel, and the engine in-cylinder visualization results of high speed and full load operating condition. Predicted spray shape, Sauter mean diameter, penetration, and fuel mixture shape showed good agreement with the experimental data. Copyright © 2005 SAE International.

In direct-injection (DI) gasoline engines, spray characteristics greatly affect engine combustion. For the rapid development of new gasoline direct-injectors, it is necessary to predict the spray characteristics accurately by numerical analysis based on the injector nozzle geometry. In this study, two-phase flow inside slit nozzle injectors is calculated using the volume of fluid method in a three-dimensional CFD. The calculation results are directly applied to the boundary conditions of spray calculations, of which the submodels are recently developed to predict spray formation process in direct injection gasoline engines. The calculation results are compared with the experiments. Good agreements are obtained for typical spray characteristics such as spray shape, penetration and Sauter mean diameter at both low and high ambient pressures. Two slit nozzle injectors of which the slit thickness is different are compared. It is shown that the slit thickness controls the liquid sheet thickness and velocity distributions at the nozzle exit, and influences the spray tip shape and the Sauter mean diameter. Copyright © 2002 Society of Automotive Engineers, Inc.