A Quasi-DNS Of Diesel Spray And The Investigation Of Its Atomization Mechanism

One of the key factors which affect the quality of the combustion of I.C.Engine is fuel spray which has been investigated for a long time. Due to the limit of the computer performance and testing technology, most of the present researches in this field at present are focus on the geometrical and physical characters but not the mechanism of atomization which is considered as the key to control the atomization process and quality. Researches about atomization mechanism are basically based on the experience and presume.In order to reduce the cost of calculation, DDM is used in most of the calculation of spray simulation at present. Although it is effective in engineering calculation, some important details of the spray field are lost due to four basic premise of DDM.In this thesis, LES theory and VOF method are combined to make a Quasi-DNS simulation of the spray field in the near nozzle region. The mechanism of the initial breakup and secondary atomization is investigated by the results of the simulation and Hiroshi Hattori's experiment, some concludes are obtained as below: the initial breakup process is effected mostly by the resistance and sheering force of the ambient air; the breakup is mainly occurred at the tip of the liquid pillar; the initial liquid pillar can not be broken by the surface wave, so that, a liquid pillar as long as 30d is observed.Some conclusion from the investigation of the secondary atomization process is carried out: primary and secondary atomization are not two processes separated but interactional; coalescence process occurred along with the secondary atomization, including the amalgamation between those thin liquid fog and droplets; secondary atomization process should be occurred under the effect from the ambient air outside and the turbulence movement inside, and the former is the main factor.Differences between the spray configurations of different inject speed, environment pressure and nozzle structure is investigated: increasing the inject speed and environment pressure is benefit to the atomization; increasing the length-diameter rate is not propitious to the atomization definitely, a best length-diameter rate of a given spray must be existent.The nozzle diameter of modern diesel is much more smaller, so that, the spray configurations under no-slip wall and slip wall boundary condition are compared in order to investigate the effect of boundary layer, because the premise that the flow speed fell to zero in near wall region is not correct any more in the nozzle with such a small diameter. The investigation indicated that the resistance of the wall is propitious to the atomization, but the penetration length is longer under no-slip wall boundary condition, which might leads to the decrease of spray angle.