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形態: カラー図版あり, Physical characteristics: Original contains color illustrations, Accepted: 2020-06-24, 資料番号: PA2110026000

To understand the soot formation characteristics in a pulverized coal flame with a swirling flow, simultaneous imaging of Mie scattering of coal particles and laser induced incandescence (LII) of soot were performed in this study. The pulverized coal flame was stabilized by a hydrogen diffusion pilot flame. The characteristic structures of soot formation in the pulverized coal flame with a swirling flow were analyzed based on a comparison of the experimental results and two-dimensional numerical simulation in the mixing region of coal particles and the oxidizer. The interactions between coal particle clouds and soot formation are discussed in detail. The results clearly show that the averaged radial dispersions of scattering signals from coal particles and of the LII signals from soot are overlapped. The overlapping region appeared nearby the nozzle exit due to the turbulent mixing and the high temperature region formed by swirl-induced recirculation flow. This overlapping region radially expands with increasing the height from the burner. Additionally, the characteristic areas of soot formation were observed in the results of simultaneous imaging of Mie scattering and LII. These areas are 1) streaky soot formation areas around the particle clouds, 2) soot formation areas inside of the particle clouds and 3) soot formation areas around the large coal particles.

A reduced kinetic model for the combustion of n-heptane, i-octane, n-cetane and heptamethylnonane was developed based on a prior model designed for use with a primary reference fuel consisting of n-heptane and i-octane. The present model, which can be easily employed in conjunction with a conventional computational fluid dynamics code, contains 59 chemical species and 96 reactions. Predicted ignition delay times under high pressure and temperature conditions were generated using this new kinetic model and compared with those obtained from full kinetic models. The results indicate that the general trends exhibited by the ignition delay times as temperature and pressure are varied are accurately predicted with this reduced model. The present model was also combined with a commercial computational fluid dynamics code and used to simulate the ignition of a diesel spray at high pressure and temperature. Finally, the effects of the cetane number of the fuel on the ignition process were investigated.

The main objective of this study is to evaluate the characteristics of combustion that combine premixed charge compression ignition (PCCI)-based combustion with conventional mixing controlled combustion. In this type of combustion, it is supposed that the combustion duration is shortened due to the synchronization of the timing of two types of combustions. In addition, the cooling loss caused by spray impingement is expected to decrease by the reduction of the proportion of mixing controlled combustion. In this study, the effect of injection pressure, injection timing, and split injection on thermal efficiency and emissions were investigated in order to determine the appropriate injection parameters for PCCI-based combustion to realize the proposed combustion concept., 2019 JSAE/SAE Powertrains, Fuels and Lubricants

Numerical calculations were carried out to investigate the formation of a fuel–air mixture as well as ignition and combustion processes associated with a diesel spray impinging on a wall. This was performed by modeling the spray formed by injecting n-heptane into a constant-volume vessel under high temperature and pressure, with the fuel droplets described by a discrete droplet model. The flow and turbulent diffusion processes were calculated based on the large eddy simulation method to simulate the formation of a local non-homogeneous mixture and the accompanying heat release. The flame structure and heat transfer to the wall during impingement were also assessed. The results show that heat transfer to the wall is increased in the peripheral region around the stagnation point, as a result of the high temperature and thin boundary layer. Conversely, in the outer region, the heat transfer decreases as the boundary layer becomes more developed.

In previous study, post injection has shown a significant benefit in the reduction of soot emissions. Among them, the close post injection which only has short dwell between the main injection is supposed to be able to increase the thermal efficiency. The reduction of the cooling loss due to the main flame impingement is considered as the reason to the increase of thermal efficiency. However, there is a lack of study related to the influence of post injection timing and quantity on the combustion characteristics. Therefore, in this study, the engine performance and emission characteristics of injection strategy with close post injection was investigated. The post injection timing was modified from near zero interval to around 4.5°CA from the end of main injection. In addition, the post injection quantity was modified from 3, 9, to 15 mm3/cycle and the intake pressure was modified from 170, 200, to 220 kPa while maintaining the total injection quantity at 39 mm3/cycle. In order to clarify the mechanism of cooling loss by applying post injection and the effect of injection dwell to the cooling loss and combustion characteristics, an optical engine was used to observe the combustion process. As a result, a certain dwell is needed to fully exploit the effect of cooling loss reduction by post injection.

Previously, some efforts were made to investigate the effect of combining premixed charge compression ignition (PCCI) combustion with conventional mixing controlled combustion using two injectors for single cylinder. In this experiment, the PCCI combustion was introduced by a sub injection fixed at early stage and the conventional diesel combustion was introduced by main injection starts near top dead center (TDC). The self-ignitability of sub injection fuel was modified by blending n-heptane and isooctane. Therefore, the ignition delay of PCCI combustion was extended to overlap with conventional diesel combustion near TDC by increasing the isooctane volume percentage in sub fuel. By overlapping PCCI and conventional diesel combustion, the cooling loss could be reduced. Later, the combustion and emission characteristics of the combined combustion strategy was examined. The experimental results showed that with larger PCCI combustion ratio at the fixed load, the degree of constant volume (DCV) increased. At the same time, the cooling loss was decreased slightly.

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. Computational fluid dynamics calculations were performed to reveal the causes of the lower cooling loss; however, the results could not capture the experimental trend when using an ordinary spray cone angle. While using a wider spray angle for the second part of main injection, the calculated trend improved. The total cooling loss depends on the balance between the cooling losses by the first and second main sprays., International Powertrains, Fuels & Lubricants Meeting

The direct-injection-premixed charge compression ignition–based combustion process with a high exhaust gas recirculation ratio and early injection timing is simulated using a Reynolds-averaged Navier–Stokes–based commercial computational fluid dynamics code with a nonhomogeneous mixture auto-ignition combustion model. In addition, the formation processes of nitrogen oxide (NO), carbon monoxide (CO), and unburnt hydrocarbon are calculated. The calculation results are compared with the experimental data. According to the calculation results, the formation processes of NO, CO, and hydrocarbon are discussed in relation to the spray development and ignition point. Furthermore, the combustion process of two-stage injection is also calculated. The result shows that the combustion process is described well by this model except in the case where auto-ignition occurs in the squish area. Additionally, the relationship between the combustion process and the mixture distribution is clarified. The main origins of the unburnt hydrocarbon and CO emissions are located in the center region of the combustion chamber, where the mixture becomes excessively lean and low in temperature.

The factors affecting knock resistance of fuels, including hydrogen (H2), ethane (C2H6), propane (C3H8), normal butane (n-C4H10), and iso-butane (i-C4H10), were determined using modeling and engine operation tests with spark-ignition gas engines. The results of zero-dimensional detailed chemical kinetic computations indicated that H2 had the longest ignition delay time of these gaseous fuels. Thus, H2 possessed the lowest ignitability. Results of engine operation tests indicated that H2 was the fuel most likely to result in knocking. The use of H2 as the fuel produced a temperature profile of the unburned gas compressed by the piston and flame front that was higher than that of the other fuels due to the high specific heat ratio and burning velocity of H2.The relation between knock resistance and secondary fuel ratio in methane-based fuel blends also was investigated using methane (CH4) as the primary component, and H2, C2H6, C3H8, n-C4H10, or i-C4H10 as the secondary components. When the secondary fuel ratio was small, the CH4/H2 blend possessed the lowest knocking tendency. But as the secondary fuel ratio increased, the CH4/H2 mixture possessed a greater tendency to knock than did CH4/C2H6 due to the high specific heat ratio and burning velocity of H2. These results indicate that the knocking that can occur with gaseous fuels is not only dependent on the ignitability of the fuel, but it also the specific heat ratio and burning velocity.Copyright © 2015 by ASME

Numerical calculation based on large eddy simulation was carried out to investigate mixture formation process in a spray formed by injecting n-heptane into a constant volume vessel under high-temperature and high-pressure conditions. Liquid phase of fuel was described by discrete droplet model. The results show that the shape of spray is similar to that of visualized image and spray tip penetration corresponds well to experimental results. In addition, distributions of fuel concentration and local non-homogeneous distribution were discussed by comparing the calculation results with the mixture distributions measured using planer laser induced fluorescence technique. The local distributions were well described except around downstream spray boundary. Calculations were also carried out for lower injection pressure condition and smaller nozzle orifice diameter condition. Although fuel concentration in a spray does not quantitatively reproduce the experimental results, the effects of the injection conditions qualitatively agreed with the experiments. Furthermore, ignition and combustion processes were investigated using Schreiber's model for calculating the progress of oxidation reactions.

The flow of a high-speed unsteady jet is analyzed using computational fluid dynamics for an incompressible flow with the k–e turbulence model. The pseudo-nozzle concept is applied to the inlet condition with a large pressure gradient. The results show that the time history of the jet development agrees with the 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 effects of inlet conditions of the turbulence intensity and scale on the flow are investigated. © 2006 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(1): 1– 12, 2007; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.20141

Effect of hydrogen addition to hydrocarbon fuels on combustion characteristics were investigated using chemical full kinetics. The changes of ignition delay time and laminar flame velocity when hydrogen is added to methane or propane fuels were calculated. The result shows that for ambient pressure condition, ignition delay time decreases corresponding to the amount of hydrogen addition. However for high pressure condition, when 10% hydrogen is added to hydrocarbon fuel, ignition delay time decreases as short as that of hydrogen itself. Laminar flame velocity increases gradually with adding hydrogen, and when more than 70% hydrogen is added, laminar flame velocity rapidly increases. Furthermore, the production rate of OH is an important factor for the change of ignition delay time and laminar flame velocity, when hydrogen is added.

The combustion process of natural gas in a premixed charged compression ignition (PCCI) engine is analyzed using computational fluid dynamics included in heterogeneity of mixture concentration and temperature with stochastic approach. The non-uniform states of turbulence mixing, ignition process is statistically described using the probability density functions (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.

The velocity field and the temperature distribution are measured simultaneously in a nonpremixed jet flame by applying particle image velocimetry (PIV) and planar laser induced fluorescence (PLIF) of nitric oxide with known concentration added as a tracer. The fuel flow of low temperature is surrounded by the high temperature region. Also, turbulence kinetics in a jet flame was analyzed based on the statistical characteristics of the temperature and velocity, including mean velocity, turbulence intensity and Reynolds stress. It was shown that u' is quickly decayed in the high temperature region. And the maximum point of density weighted average of Reynolds stress is located at fuel stream side of shear layer. As a result, production of the turbulence is mainly occurred in the fuel stream.

To evaluate the promotion of turbulent mixing, the time-average and fluctuation of fuel concentration were measured in a mixing channel with triangular plates using Rayleigh scattering method. Based on the obtained results, the deviations of fuel distribution were calculated at some crosssections, and also probability density functions were estimated by comparing with measured deviation and predicted ones from turbulence-mixing model. Furthermore, the effect of mixture heterogeneity on NO concentration in a burnt gas was demonstrated, and the trade-off relation between a mixing evolution and an increase in pressure drop was exhibited.

Time changes of a two-dimensional distribution of velocities in a nitrogen jet and a methane jet flame are measured by cross-correlation particle-image-velocimetry (PIV). From the measured results in a jet and a jetting flame, it is shown that the velocity gradient at shear layer in the reacting zone is increased due to the local acceleration by buoyancy, resulting in higher turbulence intensities compared with those in a non-reacting jet. Also, from the change of the distribution of velocity vectors with time, it is observed that the turbulence eddies are carried downstream along the gas motion with a little transformation. The time scale of turbulence at every location in the flow is obtained from the autocorrelation function of the velocity fluctuations. Furthermore, this can afford to estimate the turbulence length-scale. It is shown that the characteristic length-scales of a flaming jet are about 1.5 times greater than those of a non-flaming jet.

A fully three dimensional numerical model for the combustion process in a spark-ignition engine was established using the combustion submodel described of the flame area evolution. Computations carried out the process of premixed combustion and they were compared with the results using the combustion submodel of the eddy break-up. The combustion in a disc chamber of constant volume was simulated. The results show that the course of flame propagation is well predicted. The calculations for different conditions in a natural-gas engine are performed. It is shown that the effects of fuel-air equivalence ratio and swirl intensity on flame propagation are described. The cylinder pressure histories are agreed with measured one, and the distributions of velocity vectors and temperature in a cylinder are revealed.

The transition from laminar to turbulent mode of an ethylene jet flame was investigated using the two-dimensional instantaneous photography of turbulent eddies by a laser-light sheeting method. Observations were made of the eddies around the break point in the fuel flow and the flame. The results show that in the laminar region the fuel flow is curved due to instability in the shear layer, whereas the outer soot layer has little curvature because of the high viscosity in the hot layer. In the transient region, eddies generated in the fuel flow deform the outer soot layer. Numerical calculations were performed to predict fluid motions due to interaction between density and pressure gradients in the flame boundary. The results show that the pressure gradient in a medium of varying density generates the vorticity along the flame. Deformation and stretching of the flame boundary take place once the vorticity becomes stronger than the dissipation due to viscosity.

An auto-ignition process of a non-homogeneous mixture is investigated using a numerical calculation based on chemical kinetics and the stochastic approach. This type of autoignition phenomenon is considered as a fundamental process of the initial stage of diesel combustion (Ishiyama, et al., 2003) or homogeneous charged compression ignition (Shimasaki, et al., 2003). In order to investigate these combustion processes, many numerical calculations have been performed and for many of those it has been assumed that the ignition process is dominated by the turbulent mixing (Kong and Reitz, 2000).