Asymmetric Bubble Formation at Rectangular Orifices

Abstract

Bubble formation in liquids is frequently observed in nature and applied in various industrial processes. These include pool and flow boiling for thermal management systems, where bubbles may form asymmetrically at narrow slits and in convective flows. While previous studies have focused on symmetric bubble formation at circular orifices, the dynamics of asymmetric bubble formation remains poorly understood. Here, we experimentally investigate bubble formation at rectangular orifices and examine the effects of the orifice size and aspect ratio and the gas flow rate on the bubble size. The asymmetric bubble shape evolution at the rectangular orifice is analyzed, and we find that the size of the bubble neck is controlled either by the orifice size or by the capillary length. Based on these findings, we develop a static force balance model to predict the bubble size in the quasi-static regime, where the roles of Bond number and aspect ratio are identified. The bubble size evolution in the dynamic regime is further understood by introducing a Weber number that evaluates the effect of the virtual mass force induced by gas flow. Our study provides physical understanding of the dynamics of asymmetric bubble formation and guidance to predict the bubble size at asymmetric orifices.

Shape Oscillation-Induced Early Detachment of Bubble from a Submerged Microcapillary Nozzle

“…It is noted that V d,i and V d,c are equal in single bubbling, as no bubble coalescence occurs. The gas flow rate almost has no effect on the bubble initially detachment diameter for the nozzles with diameters of 0.15, 0.2, and 0.25 mm in the first three regimes, which is consistent with the experimental results of Xie et al, 6 Qu et al, 24 and Mohseni et al 23 With increasing the gas flow rate, the detachment volume of bubbles generated from nozzles of 0.27 and 0.4 mm in diameter is almost constant at low Q but increases at high relatively Q. This is due to the transition of bubble formation from quasi-static regime to dynamic regime.…”

“…After the detachment of the former bubble, the new bubble is not formed immediately since the pressure difference between the gas and liquid is lower than the threshold capillary pressure. 24 Increasing the gas flow rate decreases the waiting time and distance between the detached bubble and growing bubble. As the gas flow rate is higher than a critical value, a unique bubble coalescence phenomenon is observed for the capillary nozzle with a diameter smaller than 0.3 mm, as shown in Figure 2b.…”

“…At a low gas flow rate, only one bubble experiences growth, necking, detachment, and rising without coalescence in a single period, as shown in Figure a, which is termed single bubbling. After the detachment of the former bubble, the new bubble is not formed immediately since the pressure difference between the gas and liquid is lower than the threshold capillary pressure . Increasing the gas flow rate decreases the waiting time and distance between the detached bubble and growing bubble.…”

“…This regime called bubbling with coalescence has been reported by many researchers. 15,[17][18][19][20][21][22]24 In the present study, the bubble after the first detachment and final detachment is termed as an initially detached bubble and completely detached bubble, respectively. The bubble merged into the detached bubble is referred to as trailing bubble, as shown in Figure 2b.…”

“…Mohseni et al developed a mechanistic model for calculating bubble volume from orifices ranging from 0.03 to 0.193 mm in a quiescent liquid, considering the effects of gas momentum, liquid inertia forces, and detached bubble-induced liquid flow. Recently, Zhou et al investigated the bubble formation on rectangular orifices with widths ranging from 0.24 to 0.95 mm and lengths from 0.24 to10.21 mm and found that the size of the bubble neck was determined by either the orifice size or capillary length. Xia et al investigated the impact of surface wettability on bubble formation on superhydrophilic and superhydrophobic orifices.…”