Experimental study on the effect of injector nozzle K factor on the spray characteristics in a constant volume chamber: Near nozzle spray initiation, the macroscopic and the droplet statistics

摘要

In this work, the effects of the injector K factor on the spray characteristics have been investigated by using a high pressure common rail injection system and a constant volume chamber. The near nozzle region spray tip behavior, the spray macroscopic characteristics, and the droplet statistics in a fixed spray location is sequentially examined. Specifically, in the near nozzle region, laser pulse illumination and microscopic imaging technique was used to capture the very initial spray evolution morphology and it is found to take on the form of a mushroom like structure which quickly breaks up. Increasing K factor advances the spray tip evolution and favors the formation of the shockwave which might be caused the accelerated nozzle inner flow. For different K factor nozzles, the spray tip penetration (STP) evolution before the primary breakup is all found to include two sub-stages: in the very initial stage when the needle valve open is relatively small, the breakup of the mushroom like tip results in a slightly decelerated STP evolution. Subsequently, when the needle valve is fully opened, the STP evolution is gradually accelerated by the increased mass flux. The classic Hiroyasu and Arai [1] empirical model, which shows constant STP evolution speed before the primary breakup, is then refined to account this unsteady STP evolution behavior. Secondly, the spray macroscopic behavior data such as the spray tip penetration and the spray cone angle from varying K factor nozzles were collected by high speed visualization. It is found that as the K factor is increased, the STP evolution is accelerated and spray cone angle decreases. The classic model for the STP by Hiroyasu and Arai [1] which is applied K = 0 nozzle was extended to accommodate the effect of K factor on the STP evolution after the primary breakup and the revised empirical model show perfect performance in predicting the STP behavior after the primary breakup for all different K factor nozzles. Finally, by using the particle/droplet image analysis (PDIA) system, the droplet statistics and the Sauter Mean Diameter (SMD) that characterize the atomization behavior at a certain spray location were obtained. It is seen that high K factor nozzle yields a slightly smaller SMD and it's believed to enhance the atomization process. However, our experimental results show that this K nozzle effect is relatively weak and it vanishes for nozzles with larger outlet diameter. (C) 2017 Elsevier Ltd. All rights reserved.