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

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. When compressed adiabatically, nitrogen has a lower speed of sound than the commonly used driver gases, e.g., argon and helium, thus providing the ability to achieve tailored conditions and longer test durations at lower enthalpies. This paper describes the methodology used to design operating conditions using nitrogen as the driver gas and presents 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 less than 0.5 ms to 4 ms based on a constant-nozzle-supply-pressure criterion for tests at total enthalpies of 1.6 MJ/kg. The same design methodology was then used to develop different nitrogen–argon driver conditions for tailored operation in the free-piston-driven shock tunnel T4 for enthalpies spanning from 1.6 to 3.2 MJ/kg. With a nitrogen driver gas, T4, which was originally designed for operation up to Mach 25 flight conditions, can now operate at conditions as low as an equivalent flight Mach number of 5.5. This is significant because the experimental results, supported by numerical simulations, clearly demonstrate that nitrogen can be used as a driver gas in free-piston-driven shock tunnels to maximise the duration at which test conditions are held constant when testing at low total enthalpies.

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