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.

对于以低总焓运行自由活塞驱动的冲击隧道，建议使用氮气作为合适的驱动气体。当绝热压缩时，氮气的声速比常用驱动气体（例如氩气和氦气）低，因此具有在较低的焓下实现定制条件和较长测试时间的能力。本文介绍了使用氮气作为驱动气体来设计运行条件的方法，并提供了在无活塞驱动的冲击隧道中实现定制条件的实验和数值演示。在此演示中，基于恒定喷嘴供气标准，在总焓为1.6 MJ / kg的情况下，有用的测试流程持续时间从不到0.5毫秒扩展到了4毫秒。然后，使用相同的设计方法来开发不同的氮气-氩气驱动器条件，以在自由活塞驱动的冲击通道T4中量身定制操作，从而使焓范围从1.6到3.2 MJ / kg。使用氮气驱动气体，T4原本设计用于在25马赫飞行条件下运行，现在可以在低至5.5马赫当量飞行条件下运行。这是很重要的，因为在数值模拟的支持下，实验结果清楚地表明，在低总焓下进行测试时，氮气可以用作自由活塞驱动的冲击通道中的驱动气体，以最大程度地延长测试条件保持恒定的持续时间。使用氮气驱动气体，T4原本设计用于在25马赫飞行条件下运行，现在可以在低至5.5马赫当量飞行条件下运行。这是很重要的，因为在数值模拟的支持下，实验结果清楚地表明，在低总焓下进行测试时，氮气可以用作自由活塞驱动的冲击通道中的驱动气体，以最大程度地延长测试条件保持恒定的持续时间。T4原本是为氮气驱动气体设计的，可在高达25马赫的飞行条件下运行，现在可以在低至5.5马赫的等效飞行条件下运行。这是很重要的，因为在数值模拟的支持下，实验结果清楚地表明，在低总焓下进行测试时，氮气可以用作自由活塞驱动的冲击通道中的驱动气体，以最大程度地延长测试条件保持恒定的持续时间。