Study on characteristics of fragment size distribution generated via droplet breakup by high-speed gas flow

Abstract

In a design of scramjet engine using liquid hydrocarbon fuels, predictions of the fragment size distribution generated by liquid droplet breakup in high-speed gas flow are useful. However, the characteristics of fragment size distribution may be unclear, especially in high-velocity flows. In this study, the diameter of fragments formed by the disintegration of water droplets in a high-speed gas flow behind the shock wave was measured. The fragment diameters of several μm to several tens of μm moving at high speeds were clearly captured via high resolution visualization using a microscope and pulse laser with a flash time of 20 ns as a backlight. The parameters used to measure the fragment diameter from the captured images were determined from calibration experiments using a device that can change the working distance; thus, highly reliable particle size measurements were conducted. From the experimental results, the time variation in the volume probability density distribution of fragments size, Sauter mean diameter (SMD), and mass median diameter (MMD) were calculated. As a result, it was clarified that the volume probability density was successfully described by a root-normal distribution for high Weber number, for which catastrophic breakup should occur according to conventional classification. It was also observed that SMD and MMD increase with time, and the ratio of MMD to SMD was found to be 1.2, except at the initial stage. In this study, the characteristics of size distribution of the fragments generated by liquid droplet breakup in high-velocity flows, which was unclear, has been clarified.