Experimental Studies Of Fuel Spray/wall Impingement Process

As one of the most important power equipment in the society,the diesel engine is receiving widespread attention for its combustion efficiency and exhaust emissions.Strict emission regulations have been formulated at home and abroad.And there are also many new combustion methods that improve combustion efficiency and reduce emissions.However,whether it is the traditional combustion method or the new combustion method,the spray/wall impingement phenomenon of the fuel in the engine is always an intractable problem.The fuel film formed from spray/wall impingement will greatly increase the emission of HC and soot,and will cause insufficient combustion and reduce the combustion efficiency.So an in-depth understanding of the fuel spray/wall impingement phenomenon has important guiding significance for improving fuel economy and reducing emissions.The work of this paper mainly focused on the spray/wall impingement phenomenon of fuel and the movement and vaporization characteristics of the fuel film on the wall.The experiments were carried out on a high-pressure common rail injection test bench.The development characteristics of fuel spray/wall impingement and the movement and vaporization characteristics of the wall film were capture by using the high-speed camera technology.Firstly,the experiments were carried out to study the variation of the radial penetration distance and the height of the spray wall-impingement under different wall roughness and different wall temperature conditions.The fuels used in the experiments were butanol,n-heptane,and a 50% mixed solution of butanol and n-heptane.The results showed that for one-component fuel,the increase of wall roughness would cause the decrease of the radial penetration of the wall spray and the increase of the height of the spray wall-impingement.When the wall temperature was raised,the change in the characteristics of the fuel spray/wall impingement would vary depending on the fuel.The spray/wall impingement characteristics of the mixed component fuel did not show significant regularity with wall roughness or wall temperature.At the same time,there was a certain interaction between wall roughness and wall temperature on the impact of spray/wall impingement characteristics.Then,the formation and vaporization characteristics of the N-heptane film were investigated by injecting the fuel from a single-hole injector onto a high-temperature,rough wall.The influences of fuel injection duration,injection pressure,and wall temperature on the film movement and vaporization process were discussed.The film thickness was measured using the refractive index matching method(RIM).The results indicated that when the wall temperature reached a critical value,the nucleate boiling of the fuel film appeared.The boiling intensity dramatically increased and the boiling duration decreased with the increasing wall temperature.It was also found that the boiling phenomenon of the wall film was closely associated with the thickness of the residual fuel film on the wall and the film temperature,while it was nearly independent on the fuel injection conditions.Based on the theoretical analysis,it was revealed that there was a critical bubble radius for the bubble nuclei growing in a certain thickness of the fuel film within a conical pit.