Spray combustion simulation study of waste cooking oil biodiesel and diesel under direct injection diesel engine conditions

Spray combustion characteristics of waste cooking oil biodiesel (WCO) and conventional diesel fuels were simulated using a RANS (Reynolds Averaged Navier Stokes) based model. Surrogates were used to represent WCO and diesel fuels in simulations. N-tetradecane (C₁₄H₃₀) and n-heptane (C₇H₁₆) were used as surrogates for diesel. Furthermore for WCO, surrogate mixtures of methyl decanoate, methyl-9-decenoate and n-heptane were used. Thermochemical and reaction kinetic data (115 species and 460 reactions) were implemented in the CFD code to simulate the spray and combustion processes of the two fuels. Validation of the spray liquid length, ignition delay, flame lift-off length and soot formation data were performed against previous published experimental results. The modeled data agreed with the trends obtained in the experimental data at all injection pressures. Further investigations, which were not achieved in previous experiments, showed that prior to main ignition, a first stage ignition (cool flame) characterized by the formation formaldehyde (CH₂O) species at low temperature heat release occurred. The main ignition process occurred at high temperature with the formation of OH radicals. Furthermore, it was observed that the cool flame played a greater role in stabilizing the downstream lifted flame of both fuels. Increase in injection pressure led to the cool flame location to be pushed further downstream. This led to flame stabilization further away from the injector nozzle. WCO had shorter lift-off length compared to diesel as a result of its cool flame which being closer to the injector. Soot formation followed similar trends obtained in the experiments

Droplet size distribution and evaporation characteristics of fuel spray by a swirl type atomizer

Spray atomization and evaporation play extremely important roles in mixture formation and combustion processes of direct injection (DI) gasoline engines. In this study, the fundamental characteristics of a swirl spray injected into a constant volume vessel are investigated by means of several laser diagnostic techniques including the laser diffraction-based method for droplet size distribution, the laser induced fluorescence-particle image velocimetry for velocity distributions of droplets and spray-induced ambient air flow, and the two-wavelength laser absorption-scattering technique for concentration distributions of liquid and vapor phases in the spray. The results show that the droplets at outer zone of the spray exhibit larger diameter than those at inner zone under both ambient pressures 0.1 and 0.4 MPa. While this can be partially attributed to the effect of spray-induced ambient air flow, the strength of ambient air flow become small when increasing the ambient pressure from 0.1 to 0.4 MPa, indicating the strong influence of spray dynamics on the droplet size distribution. In the evaporating spray, there are higher vapor concentrations near the spray axis than at peripheral zones. At 4.0 ms after start of injection, spray droplets almost completely evaporate under ambient temperature 500 K and pressure 1.0 MPa, but there are significantly amount of fuels with equivalence ratio below 0.5 in the spray. Reduction in ambient pressure promotes the air entrainment and droplet evaporation, but lowered ambient pressure results in more fuel vapor of equivalence ratio > 1.3 along the spray axis

燃焼により誘起されるボルテックス・ブレイクダウン現象の解明

研究期間:平成16-18年度 ; 研究種目:基盤研究B ; 課題番号:1636010

Macroscopic and microscopic characteristics of a single-hole spray under low ambient pressure induced conditions

Flash boiling spray is a promising method to improve atomization, which subsequently affects the combustion phenomenon in engines. Therefore, an investigation on both macroscopic and microscopic characteristics of the flash boiling spray should be done to understand this behavior and mechanism. A mini-sac injector with a single hole was used under different injection pressures among 10, 15, and 20 MPa. Experiments were performed in a constant volume chamber with low ambient pressures varying from 5 to 100 kPa. First, macroscopic behaviors including morphology, spray tip penetration, spray angle, and spray cone angle were compared by a Mie scattering method. Then, microscopic characteristics of flash boiling spray were obtained with the help of particle image analysis technology. In order to analyze the droplet behaviors along the spray axis completely, a spray "slicer" was introduced to filter the spray. The influences of measurement location, injection pressure, and ambient pressure were discussed in detail. Results showed that although the injection pressure and measurement location have influences on the spray and atomization in both macroscopic and microscopic cases, the ambient pressure plays a more critical effect on it, indicating the low ambient pressure-induced flash boiling spray could improve the atomization much more efficiently