The Effects of Droplets and Bubbles on On-orbit Propellant Volumetric Measurements Using Cavity Resonances

The accuracy of propellant volumetric measurements based on cavity resonances may be impeded by droplets and bubbles under on-orbit microgravity environment. Effects of three different configurations, namely droplet in the cavity, droplet in the neck and bubble in the propellant, on resonant mechanism are investigated in this paper. An acoustic resonant model is provided to analyze the size and position of droplets and bubbles on the resonant frequency of a propellant tank. Analytical results are validated through three dimensional numerical simulations implemented by COMSOL software. The present work shows that the position and size of droplets and bubbles have different influences on the resonant mechanisms. The effect of the axial position is less obvious than that of the radial position when the droplet is in the cavity, while the effect of the droplet in the neck is vice versa. Effects of the droplet with small size in the cavity, located far from the inlet of the tank, is negligible, but becomes significant when the cross section area of the drop is nearly identical to the cross section area of the cavity. Furthermore, bubbles in the propellant are shown to impose significant influence on the acoustic resonance of the system. Theoretical and numerical simulation show that droplets in the neck and bubbles surrounded by the propellant decrease the performance of volumetric measurement based on acoustic resonant method.