Quantifying the early stage of bag-type breakup of droplet is an important way to study the mechanism of drop breakup, but remains a challenge due to the lack of spatial-temporal resolved diagnostic technique. High-speed digital in-line holography at 20 kHz is employed to characterize secondary droplets formed in bag rupture of an ethanol drop exposed in the gas stream. Droplets as small as 10 μm are resolved at the beginning of bag rupture at Weber number of 11. The velocities of secondary droplets can almost reach that of the gas stream. Then the thin wall shrinks to form wrinkles that will generate relatively larger secondary droplets with smaller velocities. Droplet diameters are statistically displayed and the relationship between velocity and diameter as well as time is analyzed. This will help the further understanding of fuel spray generation in gas turbine engines.

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