Acoustic excitation and destruction of liquid sheets (Coaxial spray)

Flat sheets and swirl coaxial liquid sheets are investigated experimentally while under conditions similar to those present in an unstable liquid rocket engine combustor. Both spray geometries were found to respond to the imposed acoustic field.; Photographs show that the flat sheet can be driven to Support waves at acoustic frequencies between 1700 Hertz and 600 Hertz at both the pressure and velocity antinodes. For a fixed acoustic decibel level, the amplitude of the waves on the flat sheet increased as the acoustic driving frequency was decreased. The sheet showed a natural shedding frequency of 900 Hertz. A light scattering experiment was validated using the flat sheet as a test article.; Photographs of the swirl coaxial sheet showed no discernible structural change under the imposed acoustic field, but the light scattering experiments showed that the swirl coaxial sheets did respond to the acoustic energy. Fast Fourier transforms of the light scattering data showed that the swirl coaxial sheets were driven to shed ligaments at the frequency of the imposed acoustic field. As coaxial air velocity increased, the swirl coaxial sheet responded more readily to higher frequency acoustics and less readily to lower frequencies. This is in keeping with earlier analytical work found in literature. Contrary to expectation, no difference in the size of droplets was measured by a phase Doppler particle analyzer when the spray was acoustically driven.; This work supports the theory that liquid rocket engines are less susceptible to combustion instability if the injected spray is directly atomized by strong aerodynamic forces. In such a case, the liquid sheet is torn apart before surface waves can be formed. The spray resulting from liquid/liquid impinging injector elements forms a liquid sheet, and such injectors have historically been unstable.