Nitrogen driver for low-enthalpy testing in free-piston driven shock tunnels

Nitrogen is proposed as a suitable driver gas candidate for the operation of free-piston driven shock tunnels at low total enthalpies. Considering current operating conditions of the T4 reflected shock tunnel with a fixed compression ratio of 60, when nitrogen is compressed adiabatically to the same final pressure its sound speed is 63% that of argon (which is currently used together with helium for testing at total enthalpies between 3.2 MJ/kg and 15 MJ/kg). This lower sound speed under compression helps in achieving tailored conditions at lower enthalpies, providing long test times. This paper describes the methodology used to design operating conditions using nitrogen as the driver gas, with an experimental and numerical demonstration of its use to achieve tailored conditions in a free-piston driven shock tunnel. In this demonstration, the useful test flow duration was extended from almost zero to four milliseconds based on a constant pressure criterion for tests at total enthalpies of 1.6 MJ/kg