Pressure and Diffusion Driven Bubble Growth on a Static Surface

This thesis describes the growth of gas bubbles on a static surface. During the course of normal operation, flow within devices such as pumps, pipes, and reservoirs create low pressure regions. Though the pressure decrease may not be sufficient to cause vaporous cavitation, bubbles containing gases that were either dissolved in the fluid or trapped in crevices on solid surfaces will form. These bubbles have a significant effect on the performance of machinery and make it difficult to simulate the behavior of the flow in this range of conditions. Instead of being a simple single-phase incompressible liquid, the fluid is now a compressible two-phase gas-liquid mixture. The understanding of the process of formation and growth of cavitation bubbles is important in the design of any fluid system. The growth of a spherical bubble under static conditions and positioned far from any interfering surface is well understood and has an analytical solution. The problem of a bubble growing on a surface is more complex because the symmetry of the system is broken.The experiment described in this thesis has three components. As an initial verification of the experimental apparatus, the surface tension of three separate fluid (distilled water, dodecane, and Jet-A) is calculated using a force balance involving measured quantities at the instant of bubble departure. To examine pressure-driven growth, the pressure experienced by a bubble trapped on a surface is cycled to and from atmospheric pressure. By comparing the changes in volume to the magnitude of the pressure changes, pressure-driven bubble growth was found to be an isothermal polytropic process and independent of the fluid. To observe diffusion growth, an initial bubble is exposed to a step change in pressure that is maintained over the lifetime of the bubble. The change of the bubble volume over time due to diffusion is determined by two parameters, the diffusion coefficient (D) and Henry's Law constant (H). The experiment demonstrates that the Epstein-Plesset solution to the concentration gradient within the system is viable for limited applications when suitably modified to account for the non-spherical nature of the bubble. To use the Epstein-Plesset model as a predictive tool, both D and H, must be known a priori as the problem is indeterminant.