Diesel Spray Flame Research Articles

Effects of Oxygenated Fuel on Soot Particle Formation in a Diesel Spray Flame

Effects of Oxygenated Fuel on Soot Particle Formation in a Diesel Spray Flame

Modeling Diesel Spray Flame Liftoff, Sooting Tendency, and NOx Emissions Using Detailed Chemistry With Phenomenological Soot Model

A detailed chemistry-based CFD model was developed to simulate the diesel spray combustion and emission process. A reaction mechanism of n-heptane is coupled with a reduced NOx mechanism to simulate diesel fuel oxidation and NOx formation. The soot emission process is simulated by a phenomenological soot model that uses a competing formation and oxidation rate formulation. The model is applied to predict the diesel spray lift-off length and its sooting tendency under high temperature and pressure conditions with good agreement with experiments of Sandia. Various nozzle diameters and chamber conditions were investigated. The model successfully predicts that the sooting tendency is reduced as the nozzle diameter is reduced and/or the initial chamber gas temperature is decreased, as observed by the experiments. The model is also applied to simulate diesel engine combustion under premixed charge compression ignition (PCCI) conditions. Trends of heat release rate, NOx, and soot emissions with respect to EGR levels and start-of-injection timings are also well predicted. Both experiments and models reveal that soot emissions peak when the start of injection (SOI) occurs close to TDC. The model indicates that low soot emission at early SOI is due to better oxidation while low soot emission at late SOI is due to less formation. Since NOx emissions decrease monotonically with injection retardation, a late injection scheme can be utilized for simultaneous soot and NOx reduction for the engine conditions investigated in this study.

Structure and temperature distribution of a stagnation-point Diesel spray premixed flame

We experimentally examine the flow and flame characteristics of a stagnation point premixed flame influenced by Diesel sprays. In the experiment, distributions of drop size, drop axial velocity and its fluctuation as well as the gas phase temperature are measured by using the phase-doppler particle analyzer and a thin thermocouple. As might be expected, similar to the gasoline spray flame, the partially prevaporized Diesel spray flame is composed of a weak blue flame zone, indicating the burning of methane fuel, and a strongly luminous zone containing many bright yellow lines showing the passages of burning Diesel drops. It is found that the axial temperature profiles at various radial positions consist of an upstream preheat region, a maximum temperature downstream of the blue flame and a downstream region with a declined temperature curve because of the heat loss to the quartz plate. The SMD of the drops increases from the upstream preheat region to a maximum near the blue flame and then decreases in the downstream burning zone. Along the axial position, the drops are decelerated in front of the flame but accelerated when passing through the blue flame. It is also interesting to note that the radial distributions of SMD and number density of drops in the upstream region are mainly influenced by small drops flowing outward, since the upstream vaporization of Diesel drops is very limited; while those in the downstream region should be influenced by both small drops flowing outward and Diesel drops burning. From the experimental observations, there are impinging and bouncing of Diesel drops downstream of the spray flame near the quartz plate, resulting in a small amount of soot and carbon deposits on the wall. These interesting phenomena will be reported in the near future.

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

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.

Laser Spectroscopy of Early Soot Formation Process in a Transient Spray Flame(Imaging and Spectral Measurements of Laser-induced Emission at Two Different Excitation Wavelengthes)

In order to investigate early soot formation process in a diesel spray flame, 2 D 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. 3rd harmonic (355 nm) and 4th harmonic (266 nm) Nd : YAG 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 2-D imaging covered an area between 30 mm and 55 mm downstream from the nozzle orifice. The results of 2-D 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 mil excited emitters in the spray flame. The spectral measurments 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 2-D imaging study. The spectra measured in upstream positions showed broad emission peaked around 400 to 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.

3213 感熱燐光体を用いたディーゼル機関燃焼室壁面熱流束分布の画像計測(S44-1 エンジンの新しい計測技術とモデリング(1),S44 エンジンの新しい計測技術とモデリング)

The potential of laser-induced phosphorescence technique for imaging temperature and heat flux distribution on diesel combustion chamber wall was explored with a view towards improving our understanding of heat transfer phenomena in diesel engine. Two-dimensional distribution of temperature on a phosphor-coated quartz chamber wall, onto which a diesel spray flame impinges, was measured under diesel like conditions using a rapid compression and expansion machine (RCEM). The phosphor coated wall was irradiated by a 355nm ND:YAG laser pulse and a phosphorescence image was captured by a CCD camera. Two-dimensional temperature distribution was obtained from pixel-by-pixel ratio of phosphorescence intensities at reference and measured temperatures.

Laser Diagnostics of Early Soot Formation Processes in a Diesel Spray Flame(Measurement PM in Flames)

Laser Diagnostics of Early Soot Formation Processes in a Diesel Spray Flame(Measurement PM in Flames)

A stagnation-point gasoline-spray premixed flame

In order to understand interactions of droplets, flamelets, and flows in the process of spray premixed burning, we have experimentally investigated the flow and flame characteristics and concentration field of a stagnation-point gasoline-spray premixed flame. As might be expected, similar to the diesel-spray flame, the gasoline-spray flame was found to be composed of a weak blue flame zone, indicating the burning of gaseous fuel, and a strong luminous zone, containing many distinct lines of bright yellow color showing the trajectories of burning gasoline drops. Distributions of drop size, drop axial velocity, and the gas-phase temperature for the flow field of the gasoline-spray flame were similar to the previous findings for the diesel-spray flame. Concentration measurements further show that, in the center core of the flow, the O 2 concentrations are nearly 20% in the upstream region and decrease slowly in the preheat zone of the blue flame; while passing through the thin blue flame, the O 2 concentrations decrease more pronouncedly in the droplet-burning zone and almost remain constant in the downstream region. It was also shown that the increase of the CO 2 concentrations follow the decrease of the O 2 concentrations, and the CH 4 variation is similar to the O 2 variation. For the spray flow examined in this study, the increase of CO concentration simply corresponded with the droplets burning after the blue flame.

1930 不燃性気体先立ち噴射による非定常噴流火炎の NOx 低減効果

Similarities in the structure of spray flames suggest that higher fuel injection speeds would reduce NOx emission as the fuel residence time in the reaction zone would shorter. However, in diesel combustion it is commonly known that NOx emissions increase when the fuel injection velocity is increased. The authors have suggested from experimental and numerical analysis that the mixing time scale is significantly large at the spray tip region and existence of this unsteady structure is the major difference of the two flames. The purpose of this paper is to examine the effect of pre-injection of inert gas and air for the reduction of NOx : the aim was to eliminate the weak mixing zone at the tip and to create a fuel flow similar to a well developed jet structure. Experimental results showed NOx emissions decreased with his method, and the results provide clues to ways of reducing NOx in diesel spray flames.

2-D Imaging of Soot Formation Process in a Transient Spray Flame by Laser-induced Fluorescence and Incandescence Techniques.

In order to inventigate the soot formation process in a diesel spray flame, simultaneous imaging of soot precursor and soot particles in a transient spray flame achieved in a rapid compression machine was conducted by laser-induced fluorescence (LIF) and incandescence (LII) techniques. 3rd harmonic (355nm) and fundamental (1064nm) laser pulses from single Nd : YAG laser, between which a delay of 44ns was imposed by 13.3m of optical path difference, were used to excite LIF due to soot precursor and LII due to soot particles separately in the spray flame. The LIF and the LII were separately imaged by two imageintensified CCD cameras with identical detection wavelength of 400nm and bandwidth of 80nm. The LIF due to soot precursor was mainly located in the central region of the spray flame between 40 and 55mm (270 to 370 times nozzle orifice diameter d0) downstream from the nozzle orifice. The LII due to soot particles was observed to surround the soot precursor LIF region and to extend downstream. First appearance of the LIF due to soot precursor in the spray flame preceded the appearance of the LII due to soot particles. The intensity of the LIF due to soot precursor reached its maximum immediately after rich premixed combustion. In contrast, the intensity of the LII due to soot particles increased gradually and reached its maximum after the end of injection. Measured LIF spectrum of the soot precursor in the spray flame was very broad and showed no apparent spectral structure.

(3-22) Two-Dimensional Imaging of Reaction Zone Structure in Diesel Spray Flame by LIF technique((D-3)Diagnostics 3-Applications and Advamced Technolog)

(3-22) Two-Dimensional Imaging of Reaction Zone Structure in Diesel Spray Flame by LIF technique((D-3)Diagnostics 3-Applications and Advamced Technolog)

Effects of Operating Parameters on the Time Resolved Soot Concentration in Diesel Spray Flame.

An experimental study has been made of the instantaneous two dimensional visualization of soot distribution profile in a diesel spray flame. The spray flame was formed by a single shot of fuel injection in high pressure and high temperature gaseous environments inside a constant volume combustion chamber. A cross section fo the spray flame was illuminated by a sheet of incident laser light. The images of visible flame and scattered light were recorded simultaneously by using a CCD camera with a gated image intensifier at an arbitrary instant of time after the start of fuel injection. The results showed that the high concentration region of soot existed along the central axis of the spray flame, and that the soot distribution profile was highly heterogeneous. The soot in the spray flame would increase with the lapse of time and would reach a maximum after which it would decrease to a very low value approaching zero. The effects of ambient oxygen concentration, injection pressure and fuel properties on the soot formation were revealed.

Instantaneous and 2-Dimensional Measurement of Nitric Oxide Concentration in a Diesel Spray Flame.

Although the time-space mechanism of NOx formation in diesel injection combustion at high temperature and pressure has conventionally been pursued by the sampling method, and so forth[1], the cause-effect relationship of its formation factors remains to be more accurately clarified. In our present study, concentrations of NO, OH and soot in diesel spray flames from a single injection nozzle at high temperature and pressure were measured by use of a motoring compressed injection test system with application of laser-induced fluorescence[2], [3] and laser-induced incandescence[4], both of which can provide unperturbed measurements necessary for clarifying formation characteristics of harmful exhaust substances in diesel fuel flames at high pressure.

Flame Temperature and Ambient Gas Composition Effects on Soot Formation and Oxidation in Flame. Observation of Diesel Spray Combustion Using a Rapid Compression Machine.

A single-action rapid compression machine was developed to observe soot formation and oxidation processes in a diesel spray flame. The two-color method was applied to analyze the flame temperature and KL factor from the flame image taken by a high-speed camera. Variation in the surrounding gas-oxygen concentration was achieved by adding differing quantities of pure oxygen, nitrogen, carbon dioxide and argon gases when charging air, ranging from 17 to 25 vo1% of oxygen concentration, to examine the effect of flame temperature. The initial gas temperature has a great effect not only on ignition delay but also on soot formation speed. Higher oxygen concentration resulted in higher flame temperature and faster soot oxidation speed in the flame. Carbon dioxide has a soot-reduction effect in spite of its lower flame temperature.

Surrounding Gas Effects on Soot Formation and Oxidation Processes-Observation. Diesel Spray Combustion Using a Rapid Compression Machine.

A Single-action rapid compression machine was developed to observe soot formation and oxidation processes in a diesel spray flame. The two-color method was applied to analyze the flame temperature and KL factor from the flame image taken by a high-speed camera. Variation in the surrounding gas-oxygen concentration was achieved by adding differing quantities of pure oxygen, nitrogen, carbon dioxide and argon gases in charging air ranging from 17 to 25 vol% of oxygen concentration to examine the effect of flame temperature. The initial gas temperature has a great effect not only on the ignition delay but also on soot formation speed. Higher oxygen concentration resulted in higher flame temperature and faster soot oxidation speed in the flame. Carbon dioxides have a soot-reduction effect in spite of its lower flame temperature.