Expansion Tunnel: It uses a dual-diaphragm system where the diaphragms act as rupture discs, or a pressure relief. The tunnel is separated into three sections: driver, driven, and acceleration as shown in Fig. 8.2.5. The driver section can be fired either by a high pressure gas or by a free piston driver. The driven section is filled with a lower pressure test gas. The acceleration section is filled with an even lower pressurized test gas. Each section is divided by a diaphragm, which is meant to be ruptured in sequence causing the first diaphragm to rupture, mixing and expanding the driver and the driven. When the shock wave hits the second diaphragm, it ruptures casing the two gases to mix with the acceleration and expand down the enclosed test section. In this way, the flow of the test flow can be increased for hypervelocity conditions. However, the operation time is approximately 250µs.

 

Fig. 8.2.5: Schematic diagram of an expansion tube.