A detailed parameter study is made of chemically active spherical bubbles. The calculations apply an up-to-date chemical mechanism for pure oxygen initial content, taking into account pressure dependency, duplication of chemical reactions, and proper third-body efficiency coefficients. The chemical yield is defined as the amount of substance at the maximum bubble radius, and the dissipated power is approached in a relatively new method. The parameter study focuses on finding the parameter combinations where maximum yield and maximum energy efficiency arise for various chemical species (O3, OH radical, H2 and H2O2). Results show that the locations of maximum yield and efficiency points differ significantly, depending on the chemical species. Usually, neither chemical yield nor efficiency values arise at maximum pressure amplitude and minimum driving frequency (as one would presumably expect).

This chapter introduces state-of-the-art modelling techniques of chemical kinetics inside a single spherical oscillating bubble placed in an infinite domain of liquid water. The initial content of the bubble is pure oxygen and water vapor. The reaction mechanism that governs chemical kinetics inside the bubble takes into account many aspects that are usually neglected in previous sonochemical investigations. First, at the collapse state of a bubble, the pressure inside can reach several hundreds of atmospheres; thus, the incorporation of the pressure dependence of reactions in which a third body plays a role is mandatory. Second, third body efficiencies are also taken into account. Third, the backward reactions are computed via thermodynamic equilibrium conditions. Fourth, reactions that have non-Arrhenius temperature dependence can be described by two sets of Arrhenius constants. These reactions are identified and modelled properly. As more experimental data have been accumulated over the decades, the Arrhenius constants of certain reactions have been changed even by orders of magnitude. Therefore, it is also important to employ up-to-date values of the Arrhenius constants. The behavior of the proposed model is demonstrated with reaction condition sets (pressure amplitude, frequency and bubble size) typically used during the experiments. The production of important chemical species (e.g., hydrogen or free radicals) are investigated from energy efficiency points of view (yield in mole per unit dissipated power of the bubble).

A state-of-the-art chemical mechanism is introduced to properly describe chemical processes inside a harmonically excited spherical bubble placed in water and saturated with oxygen. The model uses up-to-date Arrhenius-constants, collision efficiency factors and takes into account the pressure-dependency of the reactions. Duplicated reactions are also applied, and the backward reactions rates are calculated via suitable thermodynamic equilibrium conditions. Our proposed reaction mechanism is compared to three other chemical models that are widely applied in sonochemistry and lack most of the aforementioned modelling issues. In the governing equations, only the reaction mechanisms are compared, all other parts of the models are identical. The chemical yields obtained by the different modelling techniques are taken at the maximum expansion of the bubble. A brief parameter study is made with different pressure amplitudes and driving frequencies at two equilibrium bubble sizes. The results show that due to the deficiencies of the former reaction mechanisms employed in the sonochemical literature, several orders of magnitude differences of the chemical yields can be observed. In addition, the trends along a control parameter can also have dissimilar characteristics that might lead to false optimal operating conditions. Consequently, an up-to-date and accurate chemical model is crucial to make qualitatively and quantitatively correct conclusions in sonochemistry.

The sonochemical activity and the radial dynamics of a harmonically excited spherical bubble are investigated numerically. A detailed model is employed capable to calculate the chemical production inside the bubble placed in water that is saturated with oxygen. Parameter studies are performed with the control parameters of the pressure amplitude, the forcing frequency and the bubble size. Three different definitions of collapse strengths (extracted from the radius vs.time curves) are examined and compared with the chemical output of various species. A mathematical formula is established to estimate the chemical output as a function of the collapse strength; thus, the chemical activity can be predicted without taking into account the chemical kinetics into the bubble model. The calculations are carried out by an in-house code exploiting the high processing power of professional graphics cards (GPUs). The results shown that chemical activity can be approximated qualitatively from the values of relative expansion. This could be helpful in order to optimise chemical output of sonochemical reactors either from measurement data or simulations as well.

The purpose of the present study is the investigation of condition of centrifugal pumps via pressure signals. Instead of vibration measurement on the housings that is widely used in industry, our method is based on pressure signal measurement on the pressure side of the pump. Fourier transforming such a signal can get us to make conclusions about the behavior of the pump. By changing the operating point along a characteristic curve, we can create waterfall diagrams that provide useful information about the pump at constant rotational speed. For example, it is possible to differentiate the mechanical and the hydrodynamical effects predicting the occurrence of many constructional failures (such as unbalance, angular misalignment, bearing misalignment, motor instability, etc.); thus, preventing heavy damage of the equipment.

The present study focuses on the possibilities of dynamic stabilization of a gas–vapour bubble below Blake’s critical threshold (Blake in the onset of cavitation in liquids, 1949) by harmonic forcing. In bubble dynamics, in terms of the ambient pressure, this threshold is known as a special limit where bubbles tend to grow infinity due to the non-strictly dissipative nature of the governing equations. The employed model is the harmonically excited Rayleigh–Plesset equation that is a nonlinear, second-order ordinary differential equation. Partial results have already been published in the literature (Hegedűs in Ultrasonics 54(4):1113, 2014), Hegedűs in Phys Lett A 380(9–10):1012, 2016). Throughout this paper, however, the investigated parameter space is significantly extended: excitation properties (pressure amplitude and frequency), ambient pressure, bubble size and liquid viscosity (amount of dissipation). The numerical results have indicated that domains where stable oscillations exist can always be found below Blake’s threshold. However, from application point of view, it is mandatory to raise the dissipation rate of the system to significantly increase the extent of these domains making the process of dynamic stabilization robust.