Design of microchannel annular flow generator using IASE method

In order to make up for the deficiency of liquid-liquid annular flow generator in microfluidic devices, IASE method is used to innovate the annular flow generator in microfluidic devices, and a feasible scheme is proposed, which meets the functional requirements of the annular flow generator in microfluidic devices and generates stable annular flow; Fluent software is used to analyze and compare the VOF model of the two schemes; Through their pressure, flow velocity and flow state screening, the device structure that meets the requirements is selected. This paper summarizes a flow channel structure of L-type flow channel and coaxial flow channel in series. This structure can generate stable annular flow pattern and is easy to manufacture, which is suitable for the requirements of microfluidic equipment manufacturing at present.


Introduction
The study of two-phase flow patterns in microchannels shows that the liquid-liquid two-phase flow in microchannels can form a series of flow patterns, such as slug flow, drop flow, parallel flow and annular flow. Compared with other flow patterns, annular flow can form two-phase flow with a larger flow ratio range, so it has a wider application range [1] .
Most of the researches on annular flow in microchannels are in theoretical and experimental state, and most of them focus on flow patterns. Dreyfus et al [2] . studied the flow patterns of liquid-liquid two-phase flow, observed various flow patterns through experiments, monitored the surface velocity with the help of relevant equipment, and obtained the flow pattern diagram with dimensionless number. Different flow patterns have a great impact on the mass transfer in the microchannel. Akira Matsuoka et al [3] . experimentally studied the liquid-liquid two-phase flow pattern and mass transfer rate in microchannels. It was mentioned that the annular flow in microchannels was related to the high flow rate of the fluid, and the two-phase flow would change from plug flow to annular flow at high flow rate.
Researchers have also made some attempts to develop some devices in micro channel annular flow liquid-liquid two-phase. Jin Yang of Sichuan University and others have studied and invented a series of microchannel annular flow forming devices [4~6] , proposed microchannel devices for forming stable annular flow, simple microchannel devices for stabilizing annular flow, and three-way combined microchannel devices for stabilizing annular flow, and proposed a method for constructing stable annular flow in microchannels. To sum up, at present, there are few researches on the microchannel liquid-liquid annular flow; The related devices have the problems of complex structure and difficult manufacturing. Based on the innovative method of IASE, a microchannel annular flow generator with simple structure and easy to form annular flow is designed.

Scheme design
2.1 design method IASE method [7] (identification, analysis, solve, evaluation) is an innovative method which combines the advantages of extenics and TRIZ theory.

problem description
Due to the difficulty of fluid viscosity, interfacial tension and micro size design of microchannel, the overall design of microchannel annular flow generator becomes difficult. Solving the problem of annular flow generator in micro scale is the core of this paper.

path analysis
According to the innovation tool selection strategy of IASE innovation method. The above problems mainly focus on whether the structure and function of macro annular flow transmitter can achieve the same function on the same structure of micro channel annular flow generator. According to the IASE method and the actual situation, the following solution path is adopted.
Solution path: function analysis -Function Oriented Searchgeneral solution and standard solution.

solution steps 1) Problem identification function analysis
The function of microchannel annular flow generator is analyzed, and the functional components of the system are determined and the functional model is established.
(1) Identify functional components The microchannel annular flow generator is analyzed and divided into system components, super system components and action objects, as shown in Table 1 below:   2)Problem analysisfunction oriented search The function oriented search tool is selected for analysis, and the solving process is as follows: 1.Problem identification-Stable annular flow generated by microchannel annular flow generator; 2.Function generalization-Key function: the runner structure is designed to generate stable annular flow; 3.Identify leading areas-The maturity of macroscopical annular flow generator design technology; 4.Leading domain solutions-Referring to the design scheme of macro annular flow generator, the structure is studied and applied to the microchannel.
Aiming at the research field of macro annular flow generator, various cases of annular flow generation by optimizing the channel structure have been proposed in recent years. For example, Yao et al [8] . Studied the formation mode of annular flow in abrupt pipeline; Lu et al [9] . adjusted the angle structure and pipe material of the annular flow pipe to study the formation of annular flow. In addition, some patents of macro annular flow generator have also been invented through structural optimization design. For example, Jiang et al [10] . Invented an annular flow generator and a low viscosity annular heavy oil transportation and stabilization device.
The forming process of annular flow is simple and the forming time is short, which is also a design problem that annular flow must face 3)Problem solvingfinal ideal solution After the problem analysis stage, some existing methods to solve the structure of macro annular flow generator are searched. These methods are used for the design of micro channel annular flow generator, and then the final ideal solution is used for further concretization.

watershed model and physical parameters
In order to determine the advantages and disadvantages of the two schemes, the two schemes are

governing equations
The annular flow forming process of microchannel studied in this paper is a two-phase Newtonian fluid system. The fluids selected in this paper can be regarded as immiscible and incompressible viscous fluids. These two fluids in microchannel are controlled by continuity equation and momentum equation: The interface tracking between two phases in the VOF model is completed by solving the continuous equation of the volume fraction of one or more phases. For the second phase , there are the following equations: Where, is the velocity vector of the fluid, is the source term, and are phase to phase mass transport and phase to phase mass transport respectively. In this study, there is no mass transport between the two phases, so the right term in the above formula has only the source term.

boundary conditions
Both the external phase inlet and the internal phase inlet are velocity inlet boundaries, in which the internal and external phase velocity is 0.25m/s; The outlet is the pressure outlet boundary, and the pressure is set to atmospheric pressure. gap. The pressure gradient at the annular flow gap is approximately symmetrically distributed.As shown in the figure4 The flow velocity vector diagram of two schemes of microchannel annular flow generator is shown in. It can be seen from figure (c) that when the two phases meet, due to the influence of interfacial tension, the two-phase velocity forms a good uniform direction velocity in the intersection area. In figure (d), the two-phase velocity forms the velocity vector region of turbulent flow in the intersection region, and there is a strong interference between the two-phase velocities.

3.4.2Flow pattern waveform results
As shown in the figure5 Shown are the flow pattern fluctuation diagrams of two design schemes of microchannel annular flow generator. It can be seen from the two figures that both design schemes can form micro channel annular flow. In the observation figure (a), it can be seen that a stable interference layer is formed between the two-phase flow without strong fluctuation. It can be seen from the observation figure (b) that although a micro channel annular flow is formed between the two-phase flow, the interference layer between the two phases is wavy, and the fluctuation range is relatively large, and there are signs of instability near the outlet.

Scheme determination
Through numerical analysis, after comparing the pressure, velocity and flow pattern stability of the two schemes, it is found that the flow pattern fluctuation caused by the first L-shaped channel in series with the coaxial channel is small. The second type can be clearly seen in the pressure cloud diagram and velocity vector diagram Ψ. There are local reflux and eddy current phenomena in the channel structure of series connection between type channel and coaxial channel. In conclusion, taking the micro channel annular flow pattern waveform and flow stability as the main evaluation criteria, the first channel structure with L-shaped channel in series with coaxial channel is determined as the final design scheme of micro channel annular flow generator.

Conclusion
Microfluidic equipment has a good development and application prospect in the fields of preparation, chemical reaction and micro chemical technology. The microchannel liquid-liquid annular flow generator device has the advantages of stable wrapping function and long-distance transportation in micron scale. It provides a device for multiphase flow transportation, droplet preparation, mass transfer between phases and chemical reaction between phases. Therefore, this paper uses IASE method to imitate and learn from the original macro annular flow generator, introduces it into the micro channel liquid-liquid annular flow generator, innovates the micro channel liquid-liquid annular flow generator, and gives two kinds of micro channel structures. Finally, the numerical simulation method based on VOF model of FLUENT software is used to verify the two schemes, and finally the