Image-based characterization of the bubbly shock wave generation and evolution in aviation fuel cavitation

Unsteady shock wave propagation and generation mechanisms in aviation fuel cavitation are experimentally characterized in a planar converging-diverging nozzle geometry via high-speed digital imaging and a signal processing technique that we denote as enhanced gradient shadowgraphy. Two fuels, JP-5, and JP-8, in two different flow cavitation regimes were studied: (1) aerated cavitating flow, where air microbubbles are injected into the flow at the nozzle inlet, and (2) nonaerated cavitating flow, where the flow evolved into the two-phase cavitating flow without bubble injection. For a sufficiently low range of imposed nozzle pressure ratios, gaseous cavitation led to choked flow for each working fluid. Two independent sustained mechanisms responsible for shock wave generation in the diverging section of the nozzle have been observed and characterized in the choked flow regime. The first mechanism exists only in the aerated case due to the injected bubbles. The second mechanism exists in both aerated and nonaerated cases and resulted in weaker shock waves than in the first case. This is related to the shed cavity clouds from a sheet cavity attached to nozzle sidewalls. More violent collapse events and stronger shock wave emissions were observed for aerated JP-5 than JP-8 due to the first mechanism. Detailed quantitative data are obtained to characterize shock wave intensity and velocity in aerated flow regimes under different bubble injection rates, nozzle back pressures, and void fractions. A strong similarity is obtained between shock speeds from our experimental data and the nonlinear solutions of the governing equations for nonbarotropic homogeneous flow. Results from the study shed some light on the complex physics of unsteady fuel cavitation phenomena and may lead to improved design of fuel system components, which suffer from system performance degradation due to the damaging impact of these shock waves.