Hydrodynamics and Scaling Laws of Gas–Liquid Taylor Flow in Viscous Liquids in a Microchannel

Abstract

The hydrodynamics of gas–liquid Taylor flow in microchannels is vital to the microreactor’s application, but its characteristics in viscous liquids are rarely reported. Accordingly, this study concentrates on the characteristics of bubble behavior in viscous liquids (45.6–240.5 mPa·s). The results show that the viscosity effect could dominate the bubble morphology. Importantly, considering the liquid film distribution, a novel parameter as the equivalent liquid film thickness is proposed for the square microchannel and it is strongly related to the capillary number and gas–liquid flow rate ratio. Besides, the bubble velocity first decreases to a minimum value and then becomes faster with the flow rate of the liquid, and the ideality of the multiphase system decreases with the increase of liquid viscosity and pressure drop. Finally, the viscosity term is introduced to the pressure drop equation based on the Lockhart–Martinelli type. This work enriches the understanding of the viscosity effect on gas–liquid microflows.

Gas–Liquid Microfluidics: Transition Hysteresis Behavior between Parallel Flow and Taylor Flow

“…As shown by the blue dots in Figure 3(a), the liquid pressure increases with the liquid flow rate, which is mainly because the pressure drop is mainly determined by the liquid phase and is positively correlated with this value. 20 However, to the blue dots in Figure 3(b), the liquid pressure drop first decreases with the decrease in gas injection pressure but abruptly increases when the gas pressure is 37 kPa. The gas flow rate decreases with the decrease in gas pressure at a fixed liquid flow rate, 27 and the pressure drop is positively correlated with the gas flow rate; 37 thus, the liquid pressure gradually decreases.…”

“…From the parallel flow to the Taylor flow, within the time duration of 0–5.9 min, the flow state always remains at the parallel flow (see Figure (a)), and the gas–liquid system just needs 0.1 min (5.9–6.0 min) to self-regulate the bubble length from the maximal value to the minimal value to realize the stable Taylor flow. During this process, the gas volumetric flow rate gradually decreases as the liquid flow rate gradually transfers from 150 to 170 μL·min –1 under fixed gas pressure . The bubble length has a positive relationship with the gas volumetric flow rate ratio; ,, thus, the bubble size gradually decreases and reaches a stable state.…”

“…During this process, the gas volumetric flow rate gradually decreases as the liquid flow rate gradually transfers from 150 to 170 μL•min −1 under fixed gas pressure. 20 The bubble length has a positive relationship with the gas volumetric flow rate ratio; 2,11,15 thus, the bubble size gradually decreases and reaches a stable state. However, from the Taylor flow to the parallel flow, once the Q L value changes, the bubble length is very sensitive to the decrease in the Q L value, and therefore, the bubble size A flow pattern map can provide some useful information for the operation of gas−liquid microfluidics.…”

“…From the above discussion, we can see that different operating methods result in different gas−liquid flow patterns, and this phenomenon is the same as the hysteresis phenomenon of gas−liquid flow pattern transition under the gas constant-flowrate injection method. 32 Unfortunately, researchers did not report how their operating model was done in their works, 2,8,11,20,21,24,25,36 which may lead to different people obtaining different results and further forming a misleading conclusion when using the gas−liquid microfluidic technology.…”

“…For the gas–liquid Taylor flow in the microfluidic device, many investigations have put efforts to find the influencing factors on the Taylor bubble formation and develop the size prediction model over the past two decades. It mainly contains the channel size, liquid viscosity, gas feed methods, gas–liquid interfacial tension, gas–liquid squeezing models, etc. According to the flow pattern map for the gas–liquid system in the microdevice, the transformation between the parallel flow and Taylor flow can be precisely realized by adjusting the operating conditions of the gas–liquid phases .…”