Dynamics of cavitation bubbles in compressible two-phase fluid flow
Final Report Abstract
Bubbles in liquids arise in various natural situations and technical systems when liquids are stressed beyond their tensile strength or energy is deposited locally, for example. Depending on their size, stability and a host of parameters, such cavitation bubbles show rich two-phase flow dynamics, e.g., shape deformations, oscillations, collapse with shock wave emission and liquid jet formation, or break-up. In particular, spherically symmetric bubbles can collapse strongly and heat up to high temperatures. All these mechanical and thermal phenomena make bubbles an interesting laboratory for fundamental research. They are equally important for practical applications, e. g., in engineering systems which use liquids, where many of the bubbles' useful as well as detrimental effects can be attributed to their interactions with solid structures, a wellknown example being cavitation erosion of pipes or turbine blades. The project had as its main goal the experimental investigation of thermal effects which are associated with bubble dynamics. Two complementary aspects were considered: bubble dynamics in a heated liquid, and temperature changes in the liquid close to the phase boundary during the oscillation cycle of a spherical bubble. In the first part, spherical bubbles were generated by laser-induced cavitation in a cuvette filled with water at controlled temperature in the range from 10° C to about 80°C. The radial dynamics of the bubbles was measured from series of short-exposure images and compared with results from numerical simulations of different ODE models. It could be shown that at elevated temperatures the spherical bubble collapse is less violent and that the rebound amplitude increases markedly with higher temperature. The numerical simulations gave results consistent with observation. The models which include the vapor dynamics and the effect of non-equilibrium condensation tend to agree better with the observations, indicating that the effect of vapor trapping may be relevant at higher liquid temperatures. In the second part, thermometry of the liquid was implemented by the method of laser-induced fluorescence, and applied to a laser-generated bubble. Despite considerable experimental efforts and improvements of the method (two-color ratiometric LIF) thermal variations in the liquid during a transient oscillation cycle could not be detected. The LIF method was also employed in the case of an acoustically driven, stably trapped bubble. With sustained oscillation a region of diminished fluorescence intensity could be observed, being convected away from the bubble. Unexpectedly, this finding cannot be explained as a temperature effect, but is probably caused by chemical inactivation of the dyes. The observation suggests that fluorescence methods could be employed to study the sonochemical activity of single bubbles. The LIF part of this project can be seen as a first step towards the direct microscopic investigation of processes taking place at the bubble surface. A detailed knowledge of surface phenomena will be important for future, novel applications of bubbles, for example, in chemical processing, cleaning, energy conversion, or biomedical applications.
Publications
- Numerical simulation of shock bubble interaction using a laser-induced cavitation bubble, Proceedings of the 8th International Symposium on Cavitation CAV 2012 - August 14-16, 2012, Singapore, C.-D. Ohl, E. Klaseboer, S. W. Ohl, S. W. Gong, B. C. Khoo (eds.), Abstract No. 162, 37- 41 (2012)
Bachmann, M., Müller, S., Alizadeh, M., Kurz, T., and Söhnholz, H.
(See online at https://dx.doi.org/10.3850/978-981-07-2826-7_162) - LIF temperature measurements on cavitation bubble collapse, Fortschritte der Akustik – DAGA 2014, p. 708 (Berlin: DEGA, 2014)
Söhnholz, H., and Kurz, T.
