Performance considerations for expansion tube operation with a shock-heated secondary driver

Abstract

A shock-heated secondary driver is a modification typically applied to an expansion tube which involves placing a volume of helium between the primary diaphragm and the test gas. This modification is normally used to either increase the driven shock strength through the test gas for high-enthalpy conditions, or to prevent transmission of primary driver flow disturbances to the test gas for low-enthalpy conditions. In comparison to the basic expansion tube, a secondary driver provides an additional configuration parameter, adds mechanical and operational complexity, and its effect on downstream flow processes is not trivial. This paper reports on a study examining operation of a shock-heated secondary driver across the entire operating envelope of a free-piston-driven expansion tube, using air as the test gas. For high-enthalpy conditions it is confirmed that the secondary driver can provide a performance increase, and it is further shown how this device can be used to fine tune the flow condition even when the free-piston driver configuration is held constant. For low-enthalpy flow conditions, wave processes through the driven tube are too closely coupled, and the secondary driver no longer significantly influences the magnitude of the final test gas flow properties. It is found that these secondary driver operating characteristics depend principally on the initial density ratio between the secondary driver helium gas and the downstream test gas.