Improving the Performance of Surface Flow Generated by Bubble Plumes

Gas–liquid two-phase flow is widely used in many engineering fields, and bubble dynamics is of vital importance in optimizing the engineering design and operating parameters of various adsorptive bubble systems. The characteristics of gas–liquid two-phase (e.g., bubble size, shape, velocity, and trajectory) remain of interest because they give insight into the dynamics of the system. Bubble plumes are a transport phenomenon caused by the buoyancy of bubbles and are capable of generating large-scale convection. The surface flow generated by bubble plumes has been proposed to collect surface-floating substances (in particular, oil layers formed during large oil spills) to protect marine systems, rivers, and lakes. Furthermore, the surface flows generated by bubble plumes are important in various types of reactors, engineering processes, and industrial processes involving a free surface. The bubble parameters play an important role in generating the surface flow and eventually improving the flow performance. This paper studies the effects of temperature on bubble parameters and bubble motion to better understand the relationship between the various bubble parameters that control bubble motion and how they impact the formation of surface flow, with the ultimate goal of improving the efficiency of the generation of surface flow (i.e., rapidly generate a strong, high, and wide surface flow over the bubble-generation system), and to control the parameters of the surface flow, such as thickness, width, and velocity. Such flow depends on the gas flow rate, bubble size (mean bubble diameter), void fraction, bubble velocity, the distance between bubble generator and free surface (i.e., water height), and water temperature. The experiments were carried out to measure bubble parameters in a water column using the image visualization technique to determine their inter-relationships and improve the characteristics of surface flow. The data were obtained by processing visualized images of bubble flow structure for the different sections of the bubble regions, and the results confirm that temperature, bubble size, and gas flow rate significantly affect the flow structure and bubble parameters.

Influence of pre-stage cavitation on performance of double-nozzle flapper pressure servo valves

Cavitation is common in electrohydraulic servo valves and often adversely affects their performance. This study explores the influence of pre-stage cavitation on the performance of double-nozzle flapper pressure servo valves. The mechanism of cavitation is theoretically studied, a dynamic model of cavitation bubble motion is deduced, and the main factors affecting the development and variation of cavitation are determined. The software ADINA is used to establish a servo valve pre-stage fluid–structure interaction (FSI) model using the finite volume method. Cavitation is introduced to the model to conduct an FSI simulation of the influence of pre-stage cavitation on the performance of double-nozzle flapper pressure servo valves with nozzle diameters of 0.5, 0.55, 0.6, and 0.7 mm. An experiment on the servo valve is carried out to verify the simulation results. The simulation and experiment show that a nozzle diameter that is too small or large weakens the performance of a servo valve, and 0.6 mm is an ideal diameter; asymmetric double nozzles may reduce or improve the performance of a servo valve, and a combination of 0.6 and 0.55 mm nozzles is optimal; pre-stage cavitation influences the performance of a servo valve by changing the size and pressure gain of the dead zone; and excessive cavitation strength decreases or increases the dead zone and increases the pressure gain.