Research On Phase Splitting Characteristics Of Gas-liquid Two-phase Flow At Microchannel Junction

In recent years, with the rapid development of micromachining technology, there has been a dramatic increase in the number of microfluidic experiments with channel diameter of several tens of microns to hundreds of microns. Compared with conventional pipe, microchannel has some unique academic value and application prospects. For example, the high specific surface area can enhance heat and mass transfer; the narrow flow space is conductive to some high-risk exothermic reaction; the miniaturized and massively parallel experiments contribute to carrying on high-throughput drug screening; the plug flow reaction process can be precisely controlled. As a simple and easy processing microchannel structure, micro junctions exist widely in various kinds of compact heat exchangers and micro reactors. However, it is well known that when a two-phase flow passes through a junction, the qualities of the two branches are inevitably inconsistent with that of the inlet, a phenomenon known as phase maldistribution. This uneven distribution of phases will affect the heat and mass transfer downstream, and even bring security. On the other hand, the phase maldistribution feature can also be made use of as a partial phase separation. Up till now, extensive studies on phase maldistribution have been carried out, and a lot of methods for improving even distribution of phases have been proposed. But most of them were carried out at normal sized junctions with diameter larger than 1 mm. With the diameter decreasing from macroscale to microscale, since the flow behaviors and the dominant factors affecting phase split have changed substantially, there may be some differences in phase split features at microchannel junctions, and the phase splitting models constructed in normal sized junctions are not suitable for microscale junctions. Therefore, by using the high speed recording technique and the microfluidic controlling/measurement system, the present work experimentally investigates the phase split that occurs at micro branching junction and micro impacting junction with square cross sections of 0.5mm × 0.5mm. In the experiment, high purity nitrogen is chosen as the gas phase, and de-ionized water with different mass concentrations of sodium dodecylsulfate(SDS) are used as the liquid phase. The study includes the following aspects:(1) The phase splitting characteristics of gas-liquid two-phase flow at micro branching junctions have been studied, including the effect of inlet flow patterns(slug flow, slug-annular flow and annular flow) and the effect of branch angles(30°, 60°, 90°, 120°, 150°) on phase split. In addition, the experimental results are compared with data of previous studies at normal sized branching junctions to identify the changes that occur as a result of the reduction in channel diameter.(2) The phase splitting characteristics of gas-liquid two-phase flow at micro impacting junctions have been studied, including the effect of inlet flow patterns(slug flow, slug-annular flow and annular flow), the effect of surface tension, and the effect of impacting angles(30°/150°, 60°/120°, 90°/90°) on phase split. In addition, the experimental results are compared with data of previous studies at normal sized impacting junctions to identify the changes that occur as a result of the reduction in channel diameter.(3) "Separation efficiency" is introduced to evaluate the degree of phase maldistribution. It is used to compare the uniformity degree of phase distribution between micro branching junction and micro impacting junction under different inlet flow patterns, and also used to compare the effect of surface tension and branch angle on phase split of these two kinds of junctions, which can provide a reference for improving the uniformity distribution of two-phase flow and the selection of junction in practical applications.(4) By analyzing the phase splitting data and clarifying the predominant influential factors on phase split, the empirical formulae for predicting the phase split of slug flow and annular flow at micro branching T-junction and micro impacting T-junction are constructed.