TRANSMISSION OF SHOCK AND DETONATION WAVES INTO SEMICONFINED CHANNELS FILLED WITH LIQUID SATURATED BY GAS BUBBLES

Abstract

Instead of applying mechanical propellers for producing thrust in water vehicles, patent [1] proposed using a pulsed detonation hydrojet consisting of a pulsed detonation tube inserted into a water guide. It was implied that the pulsed detonation tube could periodically detonate a fuel oxidizer mixture and generate shock waves pushing water out of the guide and producing thrust. For e¨ective shock-to-water momentum transfer, it was proposed to increase water compressibility by saturating it with bubbles of a chemically inert or reactive gas. It was found in [2] that the optimal gas content required for the maximum shock-to-water momentum transfer was about 20 %(vol.). However, experiments and calculations in [2] were made for a single shock interacting with bubbly water, thus implying that this ¦nding was valid for relatively low frequencies of shock generation. The shock-to-water momentum transfer is obviously dependent of operation frequency as shock waves propagating in a compressible medium tend to merge with each other and each preceding shock wave changes the gas content ahead of the succeeding shock wave. The objective of this work was to study the e¨ect of shock generation frequency on the §ow pattern in the water guide and on the e©ciency of shock-to-water momentum transfer. The frequency of shock-wave pulses entering a column of bubbly water was about ∼ 7 kHz which is characteristic of continuous-detonation combustors rather than pulsed detonation tubes. Interaction of the wave package in the form of the high-frequency sequence of three shock waves with bubbly water (see the ¦gure) and the shock-to-water momentum transfer were studied experimentally. The wave package was generated by detonating the gaseous stoichiometric propane oxygen mixture in a detonation tube with three tube branches of di¨erent lengths submerged in a column of bubbly water with free surface. In the experiments, the initial gas content in water was varied from 2 to 16 %(vol.) at the average diameter of air bubbles 3 4 mm and shock wave velocity in bubbly water in the range of 40 to 180 m/s. Experiments showed that the use of high-frequency shock-wave pulses in a hydrojet is pointless because of the arising interference of pulses which worsens the momentum transfer: on the one hand, the waves penetrating water quickly merge, thus feeding each other and increasing the bubbly water velocity, but on the other hand, the initial gas content for each successive shock wave decreases and, accordingly, the e©ciency of the momentum transfer decreases. The maximum operation frequency of the pulsed detonation tube in the hydrojet was shown to be limited by 50 60 Hz.