Air gun with water bullet

The gun is a 12 m long inclined pipe of 0.1 m diameter which is connected to a charge of compressed air contained in a 0.5 m3 vessel. The bullet is a slug of water sitting in the upstream lower end of the pipe. The trigger is a hand-operated valve. The target is an elbow at the upstream higher end of the pipe. The smoking gun effect is created by a mist of water coming out of the pipe after each shot. The apparatus is not a toy but meant for serious research. When steam lines are out of operation and/or lack thermal insulation, liquid water collects in the lower parts of the system. System restart may accelerate the water slugs to velocities as high as 50 m/s, and subsequent slug impacts on elbows and orifices may cause pressure peaks with magnitudes only encountered in water-hammer events. The experimental programme consists of water slugs fired towards an elbow with an open end, a closed end, and an orifice end. The varied parameters are air pressure, water mass, outlet condition (open, closed, orifice). Upstream driving pressure and downstream impact pressure are measured in each experimental run. Pressure peaks up to 50 bar have been observed. Experimental results are compared with preliminary predictions from basic one-dimensional models.