Experimental study on instability of round nanofluid jets at low velocity

The effects of nano-alumina additives on the instability of round water jets were experimentally studied. The flow visualization inside and near the exit of two transparent nozzles was obtained by high-speed shadow imaging. A spectrum analysis method was applied to determine the surface wavelengths by performing fast Fourier transform on the image grayscale standard deviation data. According to the flow structure inside the sharp-edged nozzle, an integrated model for wavelength prediction was constructed based on the boundary layer instability theory. The effects of the nanoparticle additives on the jet instability were mainly caused by viscosity enhancement and cavitation promotion. The effects were divided into three categories according to the cavitation state in the nozzle. Firstly, for the no-cavitation flow, the nanoparticles exerted a frequency-reduction effect owing to the viscosity increase. Secondly, for the partial cavitation state, the nanoparticles had an increasing-frequency effect owing to the internal cavitation enhancement. Thirdly, for supercavitation or high-speed flow, the dominant frequency no longer changed, whereas the nanoparticles increased the jet instability owing to the enhancements in the external cavitation and aerodynamic force.