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.

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The objective of this study is to clarify the atomization behavior of a single droplet at high Weber number, which corresponds to catastrophic breakup. In this study, we compared the height and thickness of droplets in the initial stage of atomization by capturing single droplet atomization at high Weber number with high spatio-temporal resolution. The results show that the time variation of droplet height and thickness is independent of the Weber number. The wavelength of the droplet upstream interface from experiment roughly corresponds to that of the droplet upstream interface from the Rayleigh-Taylor instability theory.

The objective of this study is to obtain the particle size distribution of fine droplets formed by a breakup behind the shock wave. The algorithm for measuring the droplet diameter using the shadow sizing method was evaluated. The images of the liquid droplet breakup behavior were obtained using a high-speed camera attached to a long-range microscope. Calibration experiments were performed to obtain the relationship between the particle-camera distance, particle size, and particle image, and an algorithm was developed to derive a surface equation that identifies the particle-focus distance (-2 mm to 2 mm) within 0.3 mm error in depth of field and particle size (5-80 μm) within 6 μm error. The proposed particle size distribution measurement method was used to measure the particle size distribution of fine droplets by shock-induced atomization, and the volume probability density distribution of fine droplets in atomization was obtained in accordance with previous studies.