Compression ignition driven shock tubes

Abstract

Severe pressure losses are experienced in free piston driven shock tunnels when operated at high driver gas compression ratios. Drivers relying solely on compression to produce high temperature driver gas must use high compression ratios to attain high tailored interface shock speeds. An investigation of a compression ignition driver (CID) which uses a combination of compression and combustion to increase gas temperature at low compression ratios is conducted. Analytical predictions of plateau pressure, Pp , increase obtainable by the combustion of hydrogen, without increasing piston work (Wp) show a high sensitivity to the speed of sound at diaphragm rupture (aR) with maximum gains reducing from 150% to 10% as aR is increased from 2.5 to 3.1 kms-1 (PR = 40 MPa). However, if ignition occurs early in the piston stroke then rupture pressures (PR) are greatly reduced from those required with helium at equivalent aR and Wp. This permits Wp to be increased and subsequently PP can be increased by substantial factors. For instance, with aR = 3.4 kms-1 a 570% increase in PP can be obtained by igniting the mixture before compression. Calculation of PP at fixed PR is much less sensitive to aR than when Pp is calculated at fixed Wp. Measured plateau pressure and shock speeds were both higher than calculated at high aR but the expected gains at low aR were not realized. The Ignition of Hydrogen at high pressure is studied by rapid compression in the driver and analytical predictions using a chemical kinetic analysis are found to be in good agreement with pressure measurements. An explanation of why the ignition temperature is independent of pressure is offered.