Investigation of Bubble Break-Up in Microchannel Orifices

Abstract

Two-phase gas-liquid reactions often occur in chemical processes such as hydrogenation or oxidation. The flow of microbubbles in millichannels offers large specific contact surface for enhanced mass transfer and intensified chemical reactions. For low pressure loss, a combination of micro nozzles and millistructured channels is an alternative equipment design. Continuous dispersion flow through micro orifices with high flow velocity and pressure gradient deforms the phase boundary of bubbles triggering their break-up. In the nozzle’s outlet larger eddies are generated close to the wall disintegrating into smaller vortices with high flow oscillation. This has a major impact since eddies equaling bubble sizes initiate their break-up. In this work bubble break-up and its location in micro nozzles is studied in a flexible microchannel reactor concept. The dependence of the break-up location is investigated related to hydraulic diameter of the orifice, gas content, flow rate, and energy dissipation rate and its related volume. Regions of backflow increase energy loss; hence, the nozzle’s outlet angle was optimized reducing recirculation zones. Bubble dispersion and bubbly flow are studied in different orifices and channel elements with widths up to 7 mm. The outlet angle was varied between 6 and 45° to investigate different backflow regions. The effect of orifice dimensions on bubble sizes is evaluated for hydraulic diameters of 0.25 to 0.5 mm. The channel elements are fixed under a view glass enabling optical investigation of bubble size, first break-up points, and recirculation zones via high-speed camera. Analysis of bubble diameters and tracking of suspended particles was carried out by GIMP and ImageJ software. Generated bubble diameters are in the range of less than 0.1 mm up to 0.7 mm with narrow size distribution depending on the total flow rate through the channel. First break-up points; hence, the maximum energy input location are shifted closer to the outlet of the orifice with increasing velocities and smaller hydraulic diameters. However, the entire break-up region stays nearly constant for each orifice indicating stronger velocity oscillations acting on the bubble surface. A relation between smaller bubble diameter and larger energy dissipation was identified. Orifice outlet angles above 6° resulted in flow detachments and recirculation zones around the effluent jet. Ongoing investigation is carried out to further understand the mechanism and the influence of various parameters.