Research On Acoustic Characteristics Of Bubbly Liquid And Bubble Detecting Application

Owing to the significant differences in acoustic properties of gas and liquid, bubbly liquid has many special characteristics, which are highly valuable in both theoretical study and application. The existence of bubbles in propellant will affect the performance of the engine as well as shorten its life-span, which will lead to enlarging the uncertainty of spacecraft on-orbit refueling mission, thus effective detection of bubbles in the propellant pipeline flow is required. This thesis studies the effect of bubble volume and size on sound properties(velocity and attenuation) in a gas-liquid two-phase flow. Furthermore, a method of bubble detection based on acoustic attenuation is designed and experimental investigation verifies the validity.The main work is as follows:1. Single spherical bubble dynamics in unbounded stationary liquidIn this thesis, the Flynn’s theory of the bubble dynamics is modified, and then the nonlinear oscillation behavior of a single bubble in the presence of a sound field is studied to analyze the effect of fluid and acoustic characteristics on bubble oscillation. A linear model of bubble oscillation and radiation pressure is obtained to investigate the bubble oscillation damping, resonance frequency and maximum amplitude. Research shows that three different modes of bubble radial oscillation exist: quasi resonant oscillation under acoustic modulation, nonlinear divergence oscillation and forced oscillation driving by sound field. The changes of acoustic frequency and amplitude, liquid viscosity coefficient and temperature will affect the bubble oscillation, or even alter the nonlinear oscillation characteristics. The bubble radius and excitation frequency both affect bubble radiation pressure amplitude. The relationship between bubble resonance frequency and bubble radius provides a feasible solution for bubble detection.2. Single spherical bubble dynamics in bounded stationary liquidStarting from the Doinikov’s model, present thesis simplifies the dynamic model and the sound radiation model of single bubble in different situations. Linearization is proposed to demonstrate the influence of boundary layer on the bubble oscillation and acoustic radiation. Research shows that, Doinikov’s model has general universality and simplifies to the model in chapter 2 if the static fluid is unbounded. In the rigid boundary condition, the bubble oscillation damping coefficient and resonance frequency are less than those in unbounded static condition. When the distance between bubble and the rigid wall is more than 9.5 times of the bubble radius, the effect of rigid boundary on bubble vibration damping coefficient and resonance frequency is negligible(less than 5%). The parametric analysis of the effects of various factors on bubble sound radiation amplitude has also been carried out, meanwhile comparisons are given with the case of unbounded static fluid.3. Plane wave propagation in gas-liquid two-phase mediumFor multiple bubbles, this thesis develops a Lagrange bubble dynamics model by taking bubble interactions into account, and the linearization of the model is given. The relationship between macro Euler continuum model and micro Lagrange bubble dynamics model is established via gas volume fraction. Research shows that: when the gas volume fraction is small, acoustic interaction increases the bubble oscillation damping while the resonance frequency remains unchanged, but the resonance peaks tend to be gentle. Numerical results show that the effect of bubble radius on sound attenuation is very small, so the gas volume fraction can be directly obtained by measuring the attenuation coefficient. This conclusion lays a theoretical foundation for the coming experiments.4. Experimental study of bubble detection in two-phase pipe flowA two-phase flow bubble detection system is designed and experimental study of bubble detection is carried out. Analysis of mean value and variance of acoustic signal amplitude in different gas volume fraction has been carried out to validate the measurement model from the experimental level, and a corresponding criterion scheme is designed through the analysis of testing data. The experiment result shows that: based on the interpretation results of the prototype and the gas flow meter flow, the two-phase flow bubble detection method is feasible in practice.