Free-Piston Driven Expansion Tubes

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

“…However, even in over-tailored operation, when the test gas is initially denser than the secondary driver gas, a reflected shock will form at the sd3/2 interface, and lead to an overall performance reduction. This negates the use of the secondary driver for scramjet flow conditions in terms of pure performance, however it theoretically retains its ability to provide an acoustic buffer at these lower enthalpy conditions [4]. Future publications from this study will examine these issues in more detail, and provide more rigorous justification for the ideas proposed in this paper.…”

“…p sd1 <400 kPa, the condition a sd2 > a sd3 arises, and the secondary driver is said to be 'over-tailored' with respect to the primary driver gas. This is the fundamental operating regime which allows the secondary driver gas to achieve a stronger shock than the primary driver gas alone [3,4], and corresponds to a performance increase relative to the basic expansion tube.…”

“…the case shown in Figure 1c), and the shock tube shock speed is seen to reduce compared to the basic expansion tube. It was an explicitly stated assumption of earlier studies [4] that the secondary gas must unsteadily expand into the test gas in order to utilise its higher sound speed to drive a stronger shock. Nevertheless, observing Figure 2b, it becomes evident that for an air test gas, even though the secondary driver is over-tailored (a sd2 > a sd3 ) over most of the sensible facility operating envelope, the requirement that the shock-processed secondary driver gas (region sd2) must undergo an unsteady expansion into the test gas -to achieve a performance increase compared to a basic expansion tube -becomes very restrictive.…”

“…The black curve shows the shock speeds for a Mach 10 scramjet condition without a secondary driver; coloured curves show shock speeds when secondary drivers of differing fill pressures are introduced between the primary diaphragm (x = 0) and x = 3.4 m. While shock speed obviously varies through the secondary driver, all curves are seen to collapse to the same speeds further downstream. Morgan [3,4] suggested the use of a secondary driver to act as an 'acoustic buffer' for these types of scramjet conditions, because it had earlier been shown that over-tailored operation can prevent the transmission of driver disturbances from the expanded primary driver gas to the test gas [5]. Figure 9 demonstrates this 'acoustic buffer' using L1d simulations of X2, incorporating full piston dynamics, for the same Mach 10 condition in Figure 8.…”

“…It is a modification normally applied to expansion tubes [3,4], which involves placing a volume of helium between the primary diaphragm and test gas, and is typically used either to increase shock strength through the test gas [3,4], or to prevent transmission of primary driver disturbances to the test gas [3,5]. The theoretical principles of secondary driver operation are well established [3,4], however its effective implementation is nontrivial for two reasons: firstly, the device increases the already large number of facility variables which must each be configured to achieve the desired test condition; secondly, for certain parts of the facility operating envelope, it can introduce complex secondary wave processes [6] which must be accounted for in order to make accurate predictions of the test flow properties.…”