Flame sensing in pulse detonation combustors using diodes, ion probes, and high speed video

“Pressure-rise” combustion in pulse detonation engines has long been considered an efficient alternative to constant pressure combustion utilized in traditional turbojet and turbofan aircraft engines. With skyrocketing fuel costs and global warming concerns placing additional pressure on the aviation industry to reduce fuel consumption, this technology is being revisited in earnest. A key parameter to the success of pulse detonation engines is the ability to detect the presence of fully formed detonation waves inside the engine during operation. Traditionally, pressure transducers have been used to provide shock wave detection, but they are not well suited for continuous operation in high temperature environments. Ionization probes are more durable and can provide an indication of the location of the detonation wave flame front in an engine. Though these ion sensors have been used to detect the presence of a combustion wave, very few fundamental studies of an ion probe signal response have been conducted in a detonation tube. The present study seeks correlate the signal response of ionization probes to visual indications provided by photodiodes and high speed video images that have been filtered to observe the chemiluminescence from the radical electronic ion species CH* and C2* during detonation. It is shown that the initial response of ion probes is closely coupled to the initial response of photodiodes and occurs after an induction delay. The initial response is measured at 5% to a peak in both signals and directly correlates to visual indications of the flame front recorded with high speed video; peak ionization is more closely coupled to reflected wave characteristics passing inside the engine after the initial detonation wave is exhausted from the tube