Hidenori Kosaka

Two-dimensional imaging of ignition and soot formation processes in a diesel flame

Abstract The processes of ignition and formation of soot precursor and soot particles in a diesel spray flame achieved in a rapid compression machine (RCM) were imaged two-dimensionally using the laser sheet techniques. For the two-dimensional imaging of time and of location where ignition first occurs in a diesel spray, planar laser-induced fluorescence (PLIF) of formaldehyde was applied to a diesel spray in an RCM. Formaldehyde has been hypothesized to be one of the stable intermediate species marking the start of oxidation reactions in a transient spray under compression ignition conditions. In this study, the laser-induced fluorescence (LIF) images of the formaldehyde formed in a diesel fuel spray during the ignition process have been obtained by exciting formaldehyde with the third harmonic of a neodymium-doped yttrium aluminium garnet (Nd:YAG) laser. The LIF images of formaldehyde in a spray revealed that the time when the first fluorescence is detected is almost identical with the time when the total heat release due to low-temperature oxidation reactions equals the heat absorption by fuel vaporization in the spray. The formaldehyde level rose steadily until the high-temperature reaction phase of diesel spray ignition. At the start of this ‘hot-ignition’ phase, the formaldehyde concentration fell rapidly, thus signalling the end of the low-temperature ignition phase. Increases in the initial ambient gas temperatures advanced the hot-ignition starting time. The first hot ignition occurred in the periphery of spray head at initial ambient gas temperatures between 580 and 660 K. When the ambient gas temperature was increased to 790 K, the position of the first ignition moved to the central region of the spray head. For the investigation of soot formation processes in a diesel spray flame, simultaneous imaging of the soot precursor and soot particles in a transient spray flame in an RCM was conducted by PLIF and by planar laser-induced incandescence (PLII) techniques. The third harmonic (355 nm) and the fundamental (1064 nm) laser pulses from an Nd:YAG laser, between which a delay of 44 ns was imposed by 13.3 m of optical path difference, were used to excite LIF from the soot precursor and laser-induced incandescence (LII) from soot particles in the spray flame. The LIF and the LII were separately imaged by two image-intensified charge-coupled device cameras with identical detection wavelengths of 400 nm and bandwidths of 80 nm. The LIF from the soot precursor was mainly located in the central region of the spray flame between 40 and 55 mm (between 270 and 370 times the nozzle orifice diameter d°) from the nozzle orifice. The LII from soot particles was observed to surround the soot precursor LIF region and to extend downstream. The first appearance of the LIF from the soot precursor in the spray flame preceded the appearance of the LII from soot particles. The intensity of the LIF from the soot precursor reached its maximum immediately after rich premixed combustion. In contrast, the intensity of the LII from soot particles increased gradually and reached its maximum after the end of injection. Measured LIF spectra, of the soot precursor in the spray flame, were very broad with the peak between 430 and 460 nm.

Ignition, Combustion and Emissions in a DI Diesel Engine Equipped with a Micro-Hole Nozzle

In an attempt to achieve lean combustion in Diesel engines which has a potential for simultaneous reduction in NO and soot, the authors developed a micro-hole nozzle which has orifices with a diameter as small as 0.06 mm. Combustion tests were carried out using a rapid compression-expansion machine which has a DI Diesel type combustion chamber equipped with the micro-hole nozzle. A comparison with the result of a conventional nozzle experiment revealed that the ignition delay was shortened by 30%, and in spite of that, both peaks of initial premixed combustion and diffusion combustion increased significantly. The combustion in the case of the micro-hole nozzle experiment was accompanied with a decrease in soot emission, whereas an increase in NO emission. The trends in the heat release pattern and exhaust emissions described above could possibly be attributed to the fact that locally stoichiometric mixtures which are responsible for NO formation still remain in the combusting region in the flame even if the mean equivalence ratio in the flame is lean.

Simultaneous 2-D Imaging of OH Radicals and Soot in a Diesel Flame by Laser Sheet Techniques

The OH and soot in an unsteady flame, which was achieved in a rapid compression machine, were visualized simultaneously by the laser-induced fluorescence and laser-induced scattering techniques. The fuel mixture consisting of 90% paraffin hydrocarbon (reference fuel) and 10% polypropylene-glycol was used to reduce the optical attenuation caused by dense soot cloud. The simultaneous images of the fluorescence from OH and scattering from soot show that the soot and OH exist separately from each other in the leading portion of the spray flame, and the OH is formed earlier than the soot in the near field region of spray flame.

Investigation of early soot formation process in a diesel spray flame via excitation—emission matrix using a multi-wavelength laser source

Abstract In order to investigate the early soot formation processes in diesel combustion, spectral analysis of polycyclic aromatic hydrocarbons (PAHs) formed in the early soot formation region in a diesel spray flame was conducted via the excitation—emission matrix (EEM) technique using a multi-wavelength laser source. The experiments were conducted using an optically accessible constant volume combustion vessel under diesel-like conditions (ambient temperature, Ta = 750–1130 K and ambient pressure, Pa = 2.0–3.0 MPa) at different ambient oxygen concentrations (10–21 per cent). The PAHs formed in a diesel spray flame in the combustion vessel were excited by a coherent multi-wavelength ‘rainbow’ laser light (mainly 266, 299, 342, and 398 nm, total 20 mJ) generated by converting the fourth harmonic (266 nm, 60 mJ) of a pulsed neodymium-doped yttrium aluminium garnet (Nd:YAG) laser using a Raman cell frequency converter filled with hydrogen (500 kPa). The spectra of laser-induced fluorescence from the PAHs excited by the different laser wavelengths in the flame were simultaneously captured with a spectrometer and an intensified charge-coupled device (ICCD) camera as EEM images. The EEM measured in diesel spray flames at different ambient temperatures and oxygen concentrations revealed that the timing and the region for PAH growth and soot particle formation are delayed and extended downstream at lower ambient temperatures and oxygen concentrations. At an ambient temperature of 940 K and an ambient oxygen concentration of 21 per cent, a variety of PAHs are detected around the ignition timing in the central region of the diesel spray flame and the PAHs grow into larger PAHs and soot particles as the combustion process progresses. At a lower ambient temperature of 750 K or lower ambient oxygen concentrations down to 10 per cent, the fluorescence from PAHs is detected in the central region of the diesel spray flame before the ignition timing and the intensity and the spectral characteristics of the PAH fluorescence do not change for a longer period until they are finally converted to soot particles in the downstream regions in the spray flame.

Effects of Fischer—Tropsch diesel fuel on soot formation processes in a diesel spray flame

Abstract In order to examine the mechanism by which diesel soot emission is reduced when using Fischer—Tropsch diesel (FTD) fuel compared with conventional diesel fuel (JIS#2) under exhaust gas recirculation conditions, the soot formation processes in a diesel spray flame of the two different fuels (FTD and JIS#2) under different ambient oxygen concentrations (21 to 10%) were investigated via excitation—emission matrix analysis of polycyclic aromatic hydrocarbons (PAHs) and high-speed laser shadowgraphy of soot particles. The experiments were conducted using an optically accessible constant-volume combustion vessel under a diesel-like condition (Ta = 940 K and Pa = 2.5 MPa). In the FTD-fuelled diesel spray flame, the timing and region for the first appearance of PAH laser-induced fluorescence (LIF) and soot particles in the flame were delayed and shifted downstream compared with JIS#2. For JIS#2, the LIF appeared first in the shorter wavelength region (350 to 400 nm) and then shifted to the longer wavelength region (above 400 nm), while in the case of FTD, the LIF was observed not in the shorter wavelength but only in the longer wavelength region. The production of soot in the flame was increased by lowering the ambient oxygen concentration from 21 to 15% for both fuels, while the timing and region for the first appearance of soot and PAHs in the flame were delayed and shifted downstream. By lowering the oxygen concentration further down to 10%, the timing and region for the first appearance of PAHs and soot were further delayed and shifted downstream and the production of soot was decreased.

Investigation of the Early Soot Formation Process in a Transient Spray Flame Via Spectral Measurements of Laser-Induced Emissions

Abstract In order to investigate the early soot formation process in a diesel spray flame, two-dimensional imaging and spectral measurements of laser-induced emission from soot precursors and soot particles in a transient spray flame achieved in a rapid compression machine (2.8 MPa, 710 K) were conducted. The 3rd harmonic (355 nm) and 4th harmonic (266 nm) Nd: YAG (neodymium-doped yttrium aluminium garnet) laser pulses were used as the light source for laser-induced fluorescence (LIF) from soot precursors and laser-induced incandescence (LII) from soot particles in the spray flame. The two-dimensional imaging covered an area between 30 and 55 mm downstream from the nozzle orifice. The results of two-dimensional imaging showed that strong laser-induced emission excited at 266 nm appears only on the laser incident side of the spray flame, in contrast to an entire cross-sectional distribution of the emission excited at 355 nm, indicating that 266 nm-excited emitters are stronger absorbers and more abundant than 355 nm-excited emitters in the spray flame. The spectral measurements were conducted at three different positions, 35, 45, and 55 mm downstream from the nozzle orifice, along the central axis of the spray, where LIF from soot precursors was observed in a previous two-dimensional imaging study. The spectra measured in upstream positions showed that broad emission peaked at around 400–500 nm, which is attributable to LIF from polycyclic aromatic hydrocarbons (PAHs). The spectra measured in downstream positions appeared very much like grey-body emission from soot particles.

On the Air-Entrainment Characteristics of Diesel Sprays and Flames in a Quiescent Atmosphere

Air-entrainment characteristics of non-evaporating sprays and flames are measured by means of high speed photography including ordinary shadowgraphy of sprays, backscattered light illumination photography and laser shadow photography of flames. Effects of injection pressure and nozzle orifice diameter on air entrainment characteristics are investigated. Using a two-zone thermo-dynamic model, the amounts of air entrained into flames are calculated from the data obtained from the photographs and the pressure measurement in the combustion chamber. The air-entrainment characteristics of flames are compared with those of the corresponding sprays. It has shown that immediately after the start of ignition, the air entrainment into a flame increases more rapidly as compared with the corresponding spray and then, with the development of diffusion combustion, the air entrainment gradually approaches that of the spray.

A Numerical Simulation of Turbulent Mixing in Transient Spray by LES (Comparison between Numerical and Experimental Results of Transient Particle Laden Jets)

The purpose of this study is to investigate the turbulent mixing in a diesel spray by large eddy simulation (LES). As the first step for the numerical simulation of diesel spray by LES the LES of transient circular gas jets and particle laden jets were conducted. The simulation of transient circular jets in cylindrical coordinates has numerical instability near the central axis. To reduce the instability of calculation, azimuthal velocity around the central axis is calculated by the linear interpolation and filter width around the axis is modified to the radial or axial grid scale level. A transient circular gas jet was calculated by the modified code and the computational results were compared with experimental results with a Reynolds number of about 13000. The computational results of mean velocity and turbulent intensity agreed with experimental results for z/D>10. Predicted tip penetration of the jet also agreed to experimental data. Next, the simulations of transient circular particle laden jets were performed at the mass loading ratios of particles of 0.13 and 0.8. The particles were glass beads and the Sauter mean diameter of beads was 0.035 mm. Prediction underestimated the turbulent intensity of particles and overestimated the length of potential core. When the mass loading of particles increased, the discrepancy between predicted and experimental results increased.

Micro particle image velocimetry investigation of near-wall behaviors of tumble enhanced flow in an internal combustion engine

A micro particle image velocimetry has been performed to investigate tumble enhanced flow characteristics near piston top surface of a motored internal combustion engine for three inlet valve open timing (−30, −15, 0 crank angle degrees). Particle image velocimetry was conducted at 340, 350 and 360 crank angle degrees of the end of the compression stroke at the constant motored speed of 2000 r/min. The measurement region was 3.2 mm × 1.5 mm on the piston top including central axis of the cylinder. The spatial resolution of particle image velocimetry in the wall-normal direction was 75 µm and the vector spacing was 37.5 µm. The first velocity vector is located about 60 µm from the piston top surface. The micro particle image velocimetry measurements revealed that the ensemble-averaged flow near the piston top is not close to the turbulent boundary layer and rather has tendency of the Blasius theorem, whereas fluctuation root-mean-square velocity near the wall is not low. This result shows that revision of a wall heat transfer model based on an assumption of the proper characteristics of flow field near the piston top is required for more accurate prediction of heat flux in gasoline engines.

A Study on Effect of Heterogeneity of Oxygen Concentration and Temperature Distributions in a Combustion Chamber on Diesel Combustion

In order to clarify the effect of heterogeneity of distribution of oxygen concentration in a combustion chamber induced by EGR on the soot and NO emissions from the diesel engine, the combustion characteristics of diesel spray flame achieved in a rapid compression and expansion machine (RCEM) at various patterns of oxygen distribution in the chamber are investigated. The distribution of oxygen in a chamber is controlled by changing the location, phase, and duration of EGR gas injection with multi-port intake system. The multi-port intake system can make the circler distributions of oxygen with different gradient of oxygen concentration between-260 and 260 mol/m4. NO and soot emissions are measured at different oxygen concentration distributions. Results indicate that the amount of oxygen entrained into the spray upstream the luminous flame region affects the NO and soot emissions from diesel flame strongly.In order to clarify the effect of heterogeneity of distribution of oxygen concentration in a combustion chamber induced by EGR on the soot and NO emissions from the diesel engine, the combustion characteristics of diesel spray flame achieved in a rapid compression and expansion machine (RCEM) at various patterns of oxygen distribution in the chamber are investigated. The distribution of oxygen in a chamber is controlled by changing the location, phase, and duration of EGR gas injection with multi-port intake system. The multi-port intake system can make the circler distributions of oxygen with different gradient of oxygen concentration between-260 and 260 mol/m4. NO and soot emissions are measured at different oxygen concentration distributions. Results indicate that the amount of oxygen entrained into the spray upstream the luminous flame region affects the NO and soot emissions from diesel flame strongly.