Free-piston driver performance characterisation using experimental shock speeds through helium

Abstract

Tuned free-piston driver operation involves configuring the driver to produce a relatively steady blast of driver gas over the critical time scales of the experiment. For the purposes of flow condition development and parametric studies, it is useful to establish some average working values of the driver pressure and temperature for a given driver operating condition. However, in practise, these averaged values need to produce sufficiently accurate estimates of performance. In this study, two tuned driver conditions in the X2 expansion tube have been used to generate shock waves through a helium test gas. The measured shock speeds have then been used to calculate the effective driver gas pressure and temperature after diaphragm rupture. Since the driver gas is typically helium, or a mixture of helium and argon, and the test gas is also helium, ideal gas assumptions can be made without significant loss of accuracy. The technique is applicable to tuned free-piston drivers with a simple area change, as well as those using orifice plates. It is shown that this technique can be quickly used to establish average working driver gas properties which produce very good estimates of actual driven shock speed, across a wide range of operating conditions. The use of orifice plates to control piston dynamics at high driver gas sound speeds is also discussed in the paper, and a simple technique for calculating the restriction required to modify an established safe condition for use with lighter gases, such as pure helium, is presented.

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

“…Helium flow processes were modeled using ideal gas assumptions, which provide good accuracy across the range of temperatures and pressures considered. [10].…”

Generating High Speed Earth Re-entry Test Conditions in an Expansion Tube for Interplanetary Return Missions

“…While both were originally designed to operate with an 80%He/20%Ar (by volume) driver gas and a choked throat condition, 36 they will be used here with a more powerful pure helium driver gas with a 65 mm diameter orifice plate to maintain the piston dynamics. 37 All test conditions discussed in this paper use a simulated Uranus entry test gas composition of 85%H 2 /15%He (by volume) based on values from the work of Conrath et al 38 which were found from Voyager fly by measurements. They found a helium mole fraction in the upper troposphere of Uranus (where methane is insignificant) of 0.152 ± 0.033.…”

“…In later work, where attempts were made to run the driver with pure helium, orifice plates were required to match the mass loss from the driver while maintaining the piston dynamics. 37 Even though some total pressure gains are lost by having a steady expansion to a supersonic Mach number into the first driven tube of the facility, up to a point the gains from using a lighter driver gas outweigh that, and a stronger shock can still be driven through the first driven tube. X2's driven section diameter is 85 mm, and in Gildfind et al, 37 a 65 mm diameter orifice plate was required to match the piston dynamics using a pure helium driver gas instead of an 80%He/20%Ar (by volume) driver gas, giving a supersonic throat Mach number of 2.15.…”

X3 expansion tube driver gas spectroscopy and temperature measurements

“…Here, the piston is slightly over-driven such that its speed at rupture is moderately greater than that required to compensate for driver gas flow through the ruptured diaphragm to the driven tube. The driver gas pressure is designed to rise to about 10% more than the rupture pressure and as a result tends to reduce minimally during the useful supply time [5]. There are, however, fluctuations in the driver gas pressure which occur at much shorter timescales (e.g.…”

“…Gildfind et al [5] derived an analytical technique to calculate an 'effective' driver temperature at the time of primary diaphragm rupture, appropriate for tuned drivers (see section II for a description of tuned driver operation), using experimentally measured shock speeds through a helium test gas. The technique assumes that along with pressure, temperature remains constant for a short duration after rupture, under tuned operation of the driver.…”