The effect of increased throat size on the nozzle-supply flow in reflected shock tunnels

The operation of free-piston-driven reflected shock tunnels has largely observed the rule of thumb that the nozzle throat diameter should not exceed one third the shock-tube diameter. This is intended to ensure drainage does not prematurely terminate the test time before wave effects. For successful testing at lower Mach numbers, a higher nozzle throat-to-exit area ratio is required, either resulting in smaller test flows, and therefore experimental models, or necessitating a larger nozzle throat. This work experimentally examines the effect of enlarging the nozzle throat past the widely observed one-third limit in the T4 reflected shock tunnel over a range of operating conditions. This is performed within the context of testing the feasibility of designing a new Mach 6 nozzle with a throat-to-shock-tube diameter ratio of 0.421. It is demonstrated that the increased test-gas drainage caused by an enlarged nozzle throat decreases the speed of the reflected shock and the nozzle-supply pressure. There is also a reduction in constant-pressure test time of 2.3%, although this falls well within normal experimental variability. Quasi-one-dimensional simulations confirm these results and also show that the reduced strength of the reflected shock changes the tailoring of the condition. While this results in the downstream propagation of the contact surface, Pitot measurements at nozzle exit have shown that driver-gas contamination is not a concern at the low-enthalpy conditions of interest. These results demonstrate that it is possible to operate free-piston-driven reflected shock tunnels at throat diameters exceeding the one-third rule. When designing low Mach number nozzles for this class of facility, the effect on the desired test flow from the reduction in nozzle-supply pressure and altered tailoring must be considered.