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Fundamental investigation is conducted on flow and the spark-ignited combustion process of a high-speed, unsteady hydrogen jet, by experimental and theoretical approaches. Jet development and flame propagation in a constant-volume vessel were visualized by means of the shadowgraph technique. The effects of ignition timing and ignition location on the combustion process were investigated. Furthermore, a numerical simulation was performed by using incompressible-flow type computational fluid dynamics with the k-ε turbulence model and the flamelet concept. The pseudo-nozzle concept is applied to the inlet condition with a large pressure gradient. The flame propagation process is described by reference to the flame area evolution model. The results show that the pressure-history in a vessel and the flame propagation process are successfully described for experimental data. Furthermore, the flame propagation process in a jet is investigated.

Diesel combustion model for CFD simulation is established taking account of an auto-ignition process of non-homogeneous mixture. Authors revealed in their previous paper that the non-homogeneity of fuel-air mixture affected more on auto-ignition process such as its ignition delay or combustion duration than the turbulent mixing rate. Based on these results, novel diesel combustion model is proposed in this study. The transport calculation for local variation of fuel-air PDF is introduced and the chemical reaction rate is provided by the local non-homogeneity. Furthermore, this model is applied the RANS based CFD simulation of the spray combustion in a Diesel engine condition. The results show that the combustion process is well described for several engine operations.

A reduced kinetic model for n-heptane, i-octane, n-cetane and heptamethylnonane is developed based on that for the primary reference fuel (n-heptane and i-octane). The present model, which can be easily applied to a conventional CFD code, is constructed simply from 59 chemical species and 96 reactions. The ignition delay times are calculated by this kinetic model and compared with those by full kinetic models under high pressure and temperature conditions. The results indicate that the general trend of the ignition delay times for various temperatures and pressures is well described with this reduced model. Furthermore, the present model is combined with a commercial CFD code and used to simulate the ignition process of a diesel spray under a high pressure and temperature condition. The effect of the cetane number of the fuel on the ignition process is investigated.

The flow and mixing process of a high-speed unsteady jet are analyzed by using computational fluid dynamics for incompressible flow with the k-ε turbulence model and a stochastic approach. The pseudo-nozzle concept is applied to the inlet condition with a large pressure gradient. The non-uniform states of turbulent mixing are statistically described using probability density functions (PDFs). The results show that the time history of the jet development agrees with experimental data for methane and hydrogen fuels. In addition, the effect of the injection condition on the development of the jet tip is well described with this model. Furthermore, the micro-mixing process is successfully described with this PDF model.

The combustion process of natural gas in a premixed charged compression ignition (PCCI) engine is analyzed using computational fluid dynamics via stochastic approach. The nonuniform states of turbulent mixing and the ignition process are statistically described using probability density function (PDF). The results show that the course of in-cylinder pressure is good agreement with experimental data, and the effect of mixture heterogeneity on the ignition delay and the rate of heat release is revealed.

This study aims to utilize high-pressure split-main injection for improving the thermal efficiency of diesel engines. A series of experiments was conducted using a single-cylinder diesel engine under conditions of an engine speed of 2,250 rpm and a gross indicated mean effective pressure of 1.43 MPa. The injection pressure was varied in the range of 160–270 MPa. Split-main injection was applied to reduce cooling loss under the condition of high injection pressure, and the split ratio and the number of injection stages were varied. The dwell of the split main injection was set to near-zero in order to minimize the elongation of the total injection duration. As a result, thermal efficiency was improved owing to the combined increase in injection pressure, advanced injection timing, and split-main injection. According to the analysis of heat balance, a larger amount of the second part of the main injection decreased the cooling loss and increased the exhaust loss.

A series of experiments was conducted using a single-cylinder small-bore (85 mm) diesel engine to investigate the smoke-reduction effect of post injection by varying the number of injection nozzle orifices and the injection pressure. The experiments were performed under a constant injection quantity condition and under a fixed NOx emission condition. The results indicated that the smoke emission of six-hole, seven-hole, and eight-hole nozzles decreased for advanced post injection, except that the smoke emission of the 10-hole nozzle increased as the post injection was advanced from a moderately late timing around 17° ATDC. However, the smoke emission of the 10-hole nozzle with a higher injection pressure decreased for advanced post injection. These trends were explained considering the influence of the main-spray flames on post sprays based on CFD simulation results.

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.

The effects of jet-jet angle on the combustion process were investigated in an optical accessible rapid compression and expansion machine (RCEM) under various injection conditions and intake oxygen concentrations. The RCEM was equipped with an asymmetric six-hole nozzle having jet-jet angles of 30° and 45°. High-speed OH* chemiluminescence imaging and direct photo imaging using the Mie scattering method captured the transient evolution of the spray flame, characterized by lift-off length and liquid length. The RCEM operated at 1200 rpm. The injection timing was -5°ATDC, and the in-cylinder pressure and temperature were 6.1 MPa and 780 K at the injection timing, respectively, which achieved a short ignition delay. The effects of injection pressure, nozzle hole diameter, and oxygen concentration were investigated.

To clarify the relationship between the local heat release and the velocity distribution inside the diesel spray flame, simultaneous optical diagnostics of OH* chemiluminescence and particle image velocimetry (PIV) have been applied to the diesel spray flame under the elevated in-cylinder pressure and temperature conditions formed in a rapid compression expansion machine (RCEM). The cranking speed of the RCEM was 900 rpm, and the in-cylinder pressure and temperature were 8 MPa and 800 K at the start of injection, respectively. The amount of fuel was 10.2 mg. The injection pressure was 120, 90, and 60 MPa. To minimize the disturbance of luminous flame on optical diagnostics, a solvent, with comparable combustion characteristics to diesel fuel was used as fuel. The oxygen concentration was set to 15%. Results clearly show that PIV can successfully analyze the velocity distribution in diesel spray flames.

A diesel combustion model for CFD simulation is established taking into account the auto-ignition process of a non-homogeneous mixture. In a previous paper, the authors revealed that the non-homogeneity of a fuel-air mixture has a more significant effect on the auto-ignition process with respect to, for example, ignition delay or combustion duration, as compared to the turbulent mixing rate. Based on these results, a novel diesel combustion model is proposed in the present study. The transport calculation for the local variation of the fuel-air PDF is introduced, and the chemical reaction rate is obtained based on the local non-homogeneity. Furthermore, this model incorporates RANS-based CFD simulation of the spray combustion in a constant-volume vessel under a high-temperature, high-pressure condition. The results show that the combustion process is well described for a wide range of temperature and pressure conditions.