Tomohiko Furuhata’s research while affiliated with Gunma University and other places

In recent years, diesel engines have been downsized in order to improve combustion efficiency by using high-pressure injection to atomize the fuel and to increase fuel economy. As a result, the diesel spray flame inevitably impinges on the combustion chamber wall, resulting in heat loss due to heat transfer between the flame and the wall. According to Newton's cooling law, the heat transferred from the flame to the wall is proportional to the heat transfer coefficient, which is closely related to the flow of the spray flame. However, there are few reports on the flow of a spray flame impinging on a wall. The flow characteristics of a non-vaporized spray impinging on a wall surface, assuming that the flow of a spray flame impinging on a wall surface is equivalent to that of a non-vaporized spray. The fuel film formed on the wall affected on the flow characteristics of the spray. When the spray flame impinges on the wall in the engine cylinder, no liquid film is formed on the wall. Still, during cold start, a liquid fuel film may form on the piston wall due to the low temperature in the engine cylinder, resulting in HC and soot emissions. As a result, the spray may flow over the formed film. Therefore, it is necessary to investigate the flow characteristics of the spray flowing over the fuel film. In this study, the experiments using an impingement gas jet were conducted on a wall surface with a fuel film, and the velocity field of the jet after impingement of the wall surface was measured by the time-series PIV. In addition, the thickness of the wavy liquid film formed by the gas jet impingement was measured by using the laser-induced fluorescence (LIF) method, discussing the relationship between the velocity of the jet after wall impingement and the characteristics of the wavy liquid film.

A laminar diffusion flame was measured by X-ray Compton scattering. The temperature distribution was measured from an analysis of Compton scattered X-ray intensity. The chemical state distribution was obtained from a Compton scattered X-ray spectrum analysis (s-parameter analysis). The analysis of intensity and s-parameter of Compton scattered X-ray spectra indicate that the propane molecule emitted from the cylindrical Bunsen burner collapse immediately coincides with soot generation. Furthermore, the temperature increases up to 1500 K and a large amount of CO2 was generated at the combustion field. Our results show that the Compton scattered X-ray analysis can be a novel nondestructive measurement for temperature and chemical states in a combustion reaction.

Recently, a combustion chamber of a high speed diesel engine has been downsized in order to improve fuel economy. Moreover, injection pressure of the diesel fuel tends to increase due to promotion of fuel atomization. Therefore, the diesel spray flame impinges on a cavity wall of a combustion chamber in recent diesel engines. It results that promotion of heat loss from the engine occurred due to the heat transfer between the impingement spray flame and the cavity wall. Therefore, it is necessary to understand the impingement phenomenon of the diesel spray flame for improvement of the thermal efficiency. In order to understand the impingement mechanism between the spray flame and the wall, velocity measurement of an impingement diesel spray flame is required. Moreover, the heat transfer strongly depends on the flow characteristics of the impingement spray flame. Velocity information of the impingement spray might provide heat transfer mechanism between the spray and the wall although the diesel spray was not a flame. In this study, velocity field of an impingement diesel spray was measured with time-resolved PIV. Effect of ambient gas density, injection pressure and impingement angle on flow characteristics of the impingement diesel spray was investigated. Moreover, velocity of the diesel spray just before impingement on the wall was measured in order to investigate dependence of the impinging spray velocity on the post-impingement spray velocity. As the result, averaged radial peak velocities of the post-impingement diesel spray decreased with an increase of ambient gas density. Moreover, dependence of the impinging spray velocity on the averaged peak velocities of the post-impingement spray was clarified.

In recent years, fuel injection pressure tends to be higher as compared with the conventional direct injection gasoline engine in order to promote the atomization of the spray. Injection pressure of the fuel is related to atomization process of the fuel, and the atomization of the fuel affects the mixture formation between the fuel and air. Therefore, in order to improve the efficiency of the engine, it is important to investigate the flow characteristics of the spray during compression stroke of the engines. In this study, effects of injection pressure and injection timing on flow characteristics of the gasoline spray near the nozzle exit injected in the chamber of rapid compression expansion machine (RCEM) during compression stroke were investigated. Velocity fields of the spray was measured by time-resolved PIV by using the ultra-high speed camera and the CW laser. In order to investigate the influence on the disturbance of the spray by the compression stroke, the turbulent energy was obtained. As results, normalized axial velocity distribution of the spray in radial direction showed good agreement with that of a single phase jet. However, there was a difference of normalized turbulent intensity between the spray and single phase jet.

In this study, formation behaviors of soot in laminar diffusion flames of diesel fuel with fatty acid or fatty acid methyl ester (FAME) were investigated. Oleic acid and oleic acid methyl ester were selected as fatty acid and fatty acid methyl ester. Combustion gas emitted from the laminar diffusion flame was sampled, and PM composition in the gas was analyzed. Laser induced incandescence (LII) and laser induced fluorescence (LIF) techniques were applied to measure soot and polycyclic aromatic hydrocarbon (PAH) distributions in the laminar diffusion flames. As the results, soot emission and soot incandescence distributions were decreased by the addition of fatty acid or fatty acid methyl ester. Moreover, PAH concentration in the closed flame became high by addition of fatty acid or fatty acid methyl ester.

The effect of non-thermal plasma technology for particulate matter removal and nitrogen oxide emission reduction from diesel exhaust has been investigated. A sample of exhaust was cooled to the ambient temperature and passed through a dielectric barrier discharge reactor. This reactor was employed for producing plasma inside the diesel exhaust. A range of discharge powers by varying the applied voltage from 7.5 to 13.5 kV (peak–peak) at a frequency of 50 Hz has been evaluated during the experiments. Regarding the NOx emission concentration, the maximum removal efficiency has been achieved at energy density of 27 J/L. Soot, soluble organic fraction and sulphate components of diesel particulate matter have been analysed separately, and the consequence of plasma exposure on particle size distribution on both the nucleation and accumulation modes has been studied. Plasma was found to be very effective for soot removal, and it could approach complete removal efficiency for accumulation mode particles. However, when applied voltage approached 12 kV, the total number of nucleation mode particles increased by a factor of 50 times higher than the total particle numbers at the reactor inlet. This increase in nucleation mode particles increased even more when applied voltage was set at 13.5 kV.

Recently, in order to improve the thermal efficiency of diesel engines, there has been a tendency to increase injection pressure of diesel fuel to promote atomization of the fuel. Moreover, the combustion chamber of the engine has been downsized to reduce fuel consumption. These strategies for the improvement of thermal efficiency result in impingement of the diesel spray on a cavity wall of the combustion chamber. Therefore, it is necessary to understand the impingement process of the diesel spray in order to consider reduction strategy of heat loss by heat transfer between the impingement spray and the cavity wall. Moreover, ambient gas density in a combustion chamber of a modern engine tends to increase by a high boost as compared with a conventional engine in order to apply an exhaust gas recirculation (EGR) system. In this study, the effects of ambient gas density and injection pressure on flow characteristics of the impingement diesel spray were investigated. The velocity field of the impingement diesel spray was measured with time-resolved PIV, and the mean velocity field and turbulent kinetic energy inside the impingement spray were evaluated. As a result, there was not much difference in normalized velocity distributions in the wall spray flow region, even though ambient gas density and injection pressure were changed. The normalized velocity distributions of the spray were compared with those of impingement air jets obtained from the existing literature. The velocity distribution of the spray was similar to that of the impingement air jet. It means that the analogy between the spray and the jet might be maintained, even though there was a difference between single-phase flow and two-phase flow. Moreover, turbulent kinetic energy of the impingement spray near the wall decreased with a raise of ambient gas density and with a decrease in injection pressure.

Exhaust Gas Recirculation (EGR) system with EGR coolers is one of the promising ways for NOx reduction in a diesel engine. Since PM and condensable hydrocarbons are contained in the exhaust gas of a diesel engine, they cause PM deposition on the wall in an EGR cooler. To improve performance of EGR cooler, the PM deposits in it should be reduced because the heat transfer performance is deteriorated by the PM deposition. In this study, effect of wall temperature on PM deposition in an experimental EGR cooler was investigated. The wall temperature of the EGR cooler could be controlled by the temperature of cooling water. The exhaust gas from a diesel engine was passed through the EGR cooler. The thickness of PM deposit was measured by a laser displacement sensor. As a result, the thickness of PM deposit increased with an increase of the exhaust gas passed through it, and the thickness did not so depend on the temperature of cooling water. The PM deposit layer in the upstream region of EGR cooler was thicker than that in the downstream region. It was found that PM deposit layers were separated from the wall in the case of lower cooling water temperature. The separation was probably caused by condensation of water in the exhaust gas.