Research On Droplet Impacting On To Wall And Evaporation Characteristics Under Conditions Of Convection

In the rocket and the cars, the engine combustion process is divided into two stages, namely spray and combustion. When droplet inject into the combustion chamber, the evaporation characteristics and the mixture forming control the process of combustion. In the spray process, droplet always hitting on wall, and it may form frustrating oil film that can affect fuel evaporation and result incomplete combustion. In order to improve the combustion efficiency and control pollutants effectively, the study of factors that affect impingement and evaporation is necessary.In this article, the commercial software FLUENT combination with VOF method is used to simulate droplets impacting on wall and a droplet evaporation. For the process of impingement, the simulation about two droplets hitting on wall vertically and a droplet impacting on membrane. Through the analysis of non-dimensional parameters Reynolds number, Webber number, the ratio of droplet initial diameter and wall size (do/H), the ratio of droplet initial diameter and the center distance(d0/s) to investigate droplet morphological change. In order to simulate droplet evaporation more accurately, the method of FLUENT software combine with user defined function(UDF) is used to simulate one droplet evaporation on the condition of convection. For evaporation process, qualitative analysis of the influence of inlet velocity on temperature, velocity field and mass transfer rate.The numerical results show that the double droplets impingement process is similar with single droplet impacting on liquid membrane process. The phenomenon of spread, rebound, splash, breakup and so on will be also happened. To a certain extent, the factors of Webber number, Reynolds number, droplet center distance and wall size all have effect on droplet shape and number of secondary droplet. Especially, Webber number has greater influence on the height of crown. Increasing the factors of Reynolds number, Webber number and the ratio of droplet initial diameter and wall size are conductive to secondary droplet generation, but increasing the ratio of droplet initial diameter and the center distance is adverse to secondary droplet generation. For a single droplet evaporation process adding convection, the heat exchanger of droplet and ambient gas is no longer diffusion-based, but mainly to convection heat transfer, and heat transfer rate is significantly enhanced. Numerical simulation shows that secondary droplets number can be more, and its diameter will be smaller by changing these factors above. Airflow rate on the entrance have an affect on temperature distribution, velocity distribution and mass transfer rate. Increasing the inlet velocity can obviously enhance the heat transfer and speed up droplet evaporation.