Critical Parameters for Pulse Detonation Engine Pre-detonator Tubes

There has recently been considerable interest in the concept of pulse detonation engines (PDEs). From a practical perspective, it is highly desirable that PDEs use combustible fuels which have already been approved by the aviation industry (e.g., JP-10 or Jet-A). However, one drawback to the use of these fuels in PDEs is that the resulting fuelair mixtures are relatively difficult to detonate. One initiation scheme receiving considerable attention involves the use of a ‘pre-detonator’ or ‘driver’ tube [1,2]. In this concept, a detonation is first formed in a sensitive fuel-oxygen mixture by spark ignition followed by rapid deflagration-to-detonation transition. The established wave is then used to initiate the less sensitive fuel-air mixture contained in the main combustion chamber. It is desirable from both safety and performance points of view to keep the volume of the pre-detonator as small as possible. Therefore, the efficiency of transmission from the driver to the main chamber is an important issue. Very little published information is available about this topic despite its importance to the PDE community. In a previous report [3], the transmission of detonation from a fuel-oxygen driver to a larger, co-axially aligned receptor tube containing fuel-air mixture was investigated. This geometry is considered to be the simplest generic predetonator concept. The results showed that both the power of the driver tube mixture and the confinement provided by the receptor tube walls play an important role in determining the overall effectiveness of detonation transmission. For example, a driver tube containing stoichiometric C2H2-O2 was found to be capable of initiating lean C2H2-air in a receptor tube nearly three times larger in diameter. In the limit, the driver tube diameter was approximately 24 times smaller than the critical tube diameter for the C2H2-air mixture being initiated. The results were even more impressive for an equimolar C2H2-O2 driver because of its higher detonation velocity and correspondingly stronger transmitted shock wave. Using the same set-up, the driver tube was found to be nearly 40 times smaller than the critical tube diameter for the C2H2-air mixture initiated. In the present paper, additional results are presented for a smaller-scale apparatus which confirm that the previously proposed scaling relationship is applicable. The minimum driver tube length is also investigated in experiments employing a variable-length driver section.