I think I actually know what’s going on in this photo, so let’s write it down and see what I learn.
The mortar was loaded like this: powder in the depression in the bottom, fuse coming in lying over that, then bowling ball. Then two ziploc bags of gasoline were placed on top of the bowling ball.
Here’s what we’re seeing in the photo:
B is the bowling ball itself.
A1 and A2 are the two ziploc bags, ruptured and just about done spewing a mist of gasoline (the enlargement at left makes this a bit clearer).
C is the mist of gasoline recently released (see also E and F).
D is the flame front, as it works its way up the cloud of mist.
E shows actual droplets of gasoline, not yet vaporized (may not be clear at screen resolution, but is quite clear at full res).
F shows actual droplets at the side, as well; the flame front has not yet consumed them.
Note that the projectile B has already passed the bags A1 and A2. This is not that surprising, given their relative density and aerodynamic coefficients.