Tetsuya Aizawa

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.

Diode-Laser Wavelength-Modulation Absorption Spectroscopy for Quantitative in situ Measurements of Temperature and OH Radical Concentration in Combustion Gases.

The in situ quantitative profiles of temperature and OH radical concentration in a postflame region of methane-air premixed counterflow flames were measured by wavelength modulation spectroscopy with a 1.5-mum external cavity diode laser. The second harmonic (2f) signal was generated from absorption by overtone vibrational-rotational transitions of OH: the ?(3/2) (v?, v?) = (2, 0) P11.5e (nu(0) = 6421.35 cm(-1)) or the ?(3/2) (v?, v?) = (3, 1) P5.5f (nu(0) = 6434.61 cm(-1)) transitions. The absorption occurred in the postflame region between methane-air premixed twin flames stabilized in a two-dimensional laminar counterflow burner (Tsuji burner) with a 60-mm line-of-sight path length. The temperature and OH concentration profiles at an equivalence ratio of phi = 0.85 were determined by least-squares fitting of theoretical 2f line shapes to the experimental counterparts and by calculation of the ratio of the line intensities of the two different OH transitions (two-line thermometry). The measured temperature and OH concentration profiles were cross checked by Rayleigh scattering thermometry, thermocouple measurements, and two-dimensional numerical prediction of premixed combustion by use of a detailed chemical kinetic mechanism. The measurements and the prediction showed reasonable agreement.

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.

Diesel flame imaging and quantitative analysis of in-cylinder soot oxidation

This study concerns a quantitative analysis of late-cycle soot oxidation in diesel engines that focuses on two-dimensional KL factor images obtained by the two-color method. The spatially integrated KL factor was converted into the in-cylinder soot mass using a new formula of diesel soot emissivity. This methodology was applied to two combustion systems: a heavy-duty optical engine which was tuned for a higher fuel–air mixing capability and a rapid compression and expansion machine which had a lower mixing performance. The in-cylinder soot mass history during the last stage of soot oxidation phase was converted into a normalized soot mass history and was used for comparison with simulated soot mass history. A model calculation of in-cylinder soot mass history which was based on oxidation of a primary soot particle was performed with the surface-specific soot oxidation rate as a parameter. A value of the surface-specific soot oxidation rate was specified from the curve fitting approach between the experimental and simulated in-cylinder soot mass traces. The resultant soot oxidation rates plotted on the Arrhenius diagram were found to lie in domains with different oxidation mechanisms. The reason for the scattered plots was discussed referring to model predictions of soot oxidation in the literature, and it was concluded that the higher oxidation rates could be attributed to well-mixed soot oxidizer structure.

Thermocouple temperature measurements in diesel spray flame for validation of in-flame soot formation dynamics

It is well known that the soot formation is governed by equivalence ratio and temperature. However, there are only few examples of temperature measurements in the soot formation region of diesel spray flame, which has been impeding better understanding and model validation. In this study, the time histories of temperature at different axial locations in a single-shot diesel spray flame were measured in a constant volume vessel using a 50-µm-thin wire type R thermocouple and used to demonstrate their usefulness for the model validation of in-flame soot formation dynamics. The measured temperature was (1) compared and cross-checked with two-color temperatures of diesel flame periphery and core, (2) compared with predicted temperature from large eddy simulation of the diesel flame for validation and (3) contrasted with previous laser spectroscopic measurements of soot precursors (polycyclic aromatic hydrocarbons) in diesel spray flame and predicted soot processes from the large eddy simulation of diesel spray flame employing detailed chemical kinetics. The measured steady-state temperature increased from upstream to downstream in diesel spray flame corresponding to the progress of mixing and combustion. The measured temporal histories of temperature exhibited notable increase after the end of injection duration considered due to entrainment of ambient gases and resulting heat release in the wake of the injection pulse. The thermocouple-measured “core” temperatures and two-color “peripheral” temperatures of diesel flame were significantly different as up to 700 K in the upstream and gradually converged to around 2000 K in the downstream. The thermocouple-measured and large eddy simulation–predicted temperature histories showed similar general trends; however, 500 K as significant discrepancy was observed between the measured and predicted temperatures of the polycyclic aromatic hydrocarbon onset locations in diesel spray flame, suspected partially due to deficiency in the current polycyclic aromatic hydrocarbon formation model employed in the simulation.

High-speed ultraviolet chemiluminescence imaging of late combustion in diesel spray flame

For modern diesel engines employing relatively small injector nozzle holes, reduction of late combustion in high load operation is an attractive potential strategy to shorten the combustion period and thus thermal efficiency. However, the governing mechanism of the late combustion has not intensively been studied. This study aims to experimentally investigate where in the diesel spray flame the late combustion heat release is occurring and what governs the phenomenon. As a practical and qualitative marker of local heat release location and existence, which is applicable not only to idealized vessel experiments but also to future production engine experiments, ultraviolet emission from diesel spray flame during the late combustion was examined. First, the ultraviolet emission spectra of diesel spray flame during the late combustion were measured and found to mainly consist of OH* chemiluminescence with some background due to broadband CO2* chemiluminescence and soot luminosity ultraviolet components. Second, simultaneous high-speed imaging of 310 nm ultraviolet emission and visible soot luminosity from flame were performed to distinguish the OH* chemiluminescence from the soot luminosity. Third, simultaneous high-speed imaging of 310 nm ultraviolet emission, visible soot luminosity, and 266 nm ultraviolet absorption in diesel spray flame were performed, showing similar spatial and temporal distributions of heat release locations and combustible mixtures during the late combustion. The observation results suggest that the late combustion heat release occurs in the mixtures losing momentum and accumulated at the spray tip region.

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.

Spectroscopic Analysis of Early Soot Formation Region in a Transient Spray Flame via Pyrene-LIF Technique

In order to investigate early soot formation process in diesel combustion, spectral analysis and optical thermometry of early soot formation region in a transient spray flame under diesel-like conditions (Pg=2.8 MPa, Tg=620-820 K) was attempted via laser-induced fluorescence (LIF) from pyrene (C16H10) doped in fuel. Pyrene is known to exhibit a temperature dependent variation of LIF spectrum, the ratio of S 2/S 1, fluorescence yields from the lowest excited singlet state S 1 and the second excited singlet state S 2, depends strongly on temperature. In the present study, pyrene was doped (1% wt) in a model diesel fuel (0-solvent) and the variation of LIF spectra from the pyrene in the spray flame in a rapid compression machine were examined at different ambient temperatures, oxygen concentrations, measurement positions and timings after start of fuel injection. Nd : YAG-pumped dye laser pulse at 309 nm (5 mJ) was used as the excitation source and the LIF spectra were measured by a spectrometer. The pyrene LIF spectra measured in non-combusting evaporating spray in nitrogen atmosphere showed increase in the S 2/S 1 ratio of LIF intensity and shift of the peak wavelength of S 1 band towards red with increasing ambient temperature. The pyrene LIF spectra measured in spray flame between start of injection and ignition showed increase in the S2/ S1 ratio of LIF intensity and shift of the peak wavelength of S 1 band towards red as the time progresses after start of injection, showing that the temperature variation of the gas mixture up to ignition can be captured by this technique. The LIF spectra measured in spray flame after ignition showed increase in emission intensity in longer wavelength region due to LIF from combustion products and the S 2/S 1 ratio of LIF intensity decreased. The pyrene LIF spectra measured in spray flame at 1.0 ms after start of injection at different ambient oxygen concentrations (5 to 21%) showed increase in the S 2/S 1 ratio of LIF intensity and shift of the peak wavelength of S 1 band towards red with increasing oxygen concentration, showing that the temperature difference of the gas mixture caused by progress of reaction at different oxygen concentrations can be captured by this technique.