Influence of injection pressure on droplet atomization and auto-ignition of dimethyl ether (DME) at elevated pressure

Abstract

This work presents the coupled measurements of droplet size distribution, droplet velocity distribution and droplet shape in a dimethyl ether (DME) spray with 6MPa and 8MPa injection pressures into a high pressure atmosphere with 2MPa. To investigate the influence of the spray characteristics on the auto-ignition characteristics of the spray, the auto-ignition delay time was measured. The comparison of the delay with the delay of gaseous DME free jets reveals the influence of atomization and evaporation on the auto-ignition delay time.The spray is analyzed by a shadow technique and digital image analysis with temporal and spatial resolution. The images are used to measure the spray characteristics, such as droplet velocity, droplet size and droplet shape. To study the auto-ignition of the DME spray, a high-speed video camera is applied.It was found that an increased injection pressure increases the spray angle in the high pressure atmosphere. Since the small droplets have a smaller momentum, only droplets with larger diameter are detected in greater distance from the nozzle outlet. With increased injection pressure, the droplet diameter distribution becomes more homogeneous. The correlations of droplet diameter and droplet velocity are presented as Joint Probability Density Functions (JPDF).Finally, the ignition delay times for different injection pressures are measured. They do not reveal influence of the injection pressure on the ignition delay time. Furthermore, the ignition delay time of the DME spray reveals no difference to the ignition delay time of gaseous DME free jets under identical conditions.