Multiphase Gas-Liquid Flow Behavior CFD Simulation in a T-junction Microchannel with Variation in Aspect Ratio and Cross-Sectional Shape with Constant Hydrodynamic Diameter

Authored by

Bintang Farhan Muhammad, Department of Mechanical Engineering, University of Indonesia

Abstract

N/A

Introduction

Multiphase flow provides an enhancement in performance parameters compared to single phase flow. Single phase flow has long diffusion time and Taylor dispersion and this problem can be reduced by adding a second immiscible fluid into the flow regime. This enhancement in mixing is measured by the reduction in the residence time distribution (RTD) due to the segmented flow that enhances mixing across phase boundaries and reduces dispersion. However, larger multiphase flow device usually cannot handle the flow parameters with high precision control. Thus, a new approach is required to deal with the pointed out limitation.

Figure 1 Types of flow regime in a vertical and horizontal micro-channels (Rouhani & Sohal, 1983)

Micro-channels is one type of micro-structured device that flows two phase of fluids into a micro scale channel. Micro-structured device have a higher precision in the control of dispersion using high precision ability to control the generation of bubbles/droplets in the desired size. Heat and mass transfer coefficients are two to three orders of magnitude higher than the conventional counterparts. In micro-channels, many previous research has shown different flow behavior due to significant difference with an order-of-magnitude in the hydraulic diameter.

Although the flow should be laminar and well predicted due to low Reynolds number, the interfacial forces add complexity in micro-channel flow regime. Based on the review done by Gunther & Jensen (2006), flow patterns take several different forms, including channel spanning, slugs, suspended droplets, and wall wetting films, which depends on interaction among gravitational, interfacial, inertia, and viscous forces. Flow patterns in micro-channels is shown in figure 1 for horizontal and vertical micro-channels. This forces varies with operational condition and the geometry of the micro-channel itself. Kawahara et al. (2002) did an experimental research on gas-liquid micro-channel and found that there is a gradual shift in flow patterns with increasing gas and liquid flow rates. Based on the review of Sattari-Nafabajadi et al. (2018), will produce different mass transfer coefficient with different inlet angles, with variation in 60, 90,120,and 150 degrees, and the highest mass transfer coefficient was found to be at 90 degrees, which is a T-junction microchannel.

Although micro-fluidic flow research has been widely done, several topics has not yet been covered. Sattari-Nafabajadi et al. (2018) reviewed that researches on the effect of microchannel size are not sufficient to decide whether it is worthy enough to decrease the cross-sectional area to increase mixing performance in compromise of the pressure drop. Other than that, the aspect ratio, which regarded as the effective parameter on the manufacturing expenses, was not investigated in depth in the past research. Detailed assessment of influence of the channel cross section on dispersion also has not yet been established (Gunther & Jensen, 2006). Therefore, it is necessary to find the influence of aspect ratio and other geometrical parameters to the flow regime and characteristics.

With computational fluid dynamics (CFD) we are able to conduct research with flexible variation in geometry with low-cost. Previous micro-channel research has been done utilizing CFD as a means to study the flow behavior with variations in geometry (Aubin et al., 2009; Naher et al., 2011). This work aims to simulate using CFD to analyze the effect of geometrical parameters to its flow pattern and characteristics. In more detail, the research objectives are as follows:

1) To investigate the effect of variation on hydrodynamic diameter of circular micro-channel with constant aspect ratio to its flow patterns, pressure drop, axial dispersion, and RTD;

2) To investigate the effect of variation on aspect ratio in circular cross-section micro-channel to the flow pattern, pressure drop, axial dispersion, and RTD;

3) To give suggestion on the possible future experimental research based on the numerical simulation results;

The following chapters after this introduction will be structured as follows: 1) methodology, explains the fluid model and parameters, software and hardware utilized in this research, and computational method; 2) Results and discussion; and 3) Conclusion.

Methodology

This chapter outlines the methodology chosen for this work. The model assumption and verification will discuss the theory that will be the basis on model assumption, flow parameters, and boundary condition.

Software Description

Simulation is done on CFDSOF, a CFD software developed by CCIT Group. CFDSOF has been widely used, especially in Indonesia, by several universities and state-owned enterprises projects. Numerical fluid dynamics research has also been done using CFDSOF. The mesh generation and CFD simulation is conducted using CFDSOF. The result is then post-processed and presented using Paraview.

Model Assumption and Verification

Interfacial forces add complexities in micro-channel flow and thus will be considered in making model assumption. Review on the flow dynamics in micro-channel is presented in depth by Gunther and Jensen (2006). Generally, multiphase flow is also transient. Multiphase flow is characterized by three additional non-dimensional number, namely Bond number, capillary number, and Weber number.

Figure 2 Operating conditions in gas-liquid micro-channel experiments (Gunther and Jensen, 2006)

Bond number is the ratio of interfacial forces to gravity. On microscale, interfacial forces often dominate over the influence of geometry. In channels narrower than 1mm interfacial forces begin to dominate and when it reaches 1µm they exceed six orders of magnitude. This is the case when the velocity of the flow is relatively low. On the other hand, if the flow exceeds several meters per second, inertial effect dominates over gravity and viscous forces, in which bubbles and droplets smaller than the hydraulic diameter can be formed and causes interfacial instabilities. Instabilities is caused in a rather complex manner. Based on the review of Gunther and Jensen, instability is categorized to three, namely capillary instability, pressure-drop induced breakup, and instabilities at higher flow rates, which will not be discussed in depth one by one. Different flow regime will also produced depending on the phase in the flow domain, whether it is gas-liquid or liquid-liquid multiphase flow. Mixing characteristics has also been measured in several research. In his review, Gunther and Jensen mentioned that dispersion in capillaries display a pronounced tailing effect in the RTD curve in which is caused by relatively short channel length, considering the high velocities, for the capillary measurements.

Based on the discussion above the following simulation model is constructed:

1) Bond number << 1 to ensure interfacial forces dominates the flow behavior, and thus neglecting the gravity.

2) Laminar flow model because the low Reynolds number given by small channel size low fluid flow velocities.

3) Transient flow to capture the instabilities.

4) Constant mass-flow rate.

5) Viscous flow to capture the effect of interfacial forces.

6) Calculation will not involve energy equation.

7) Air-water flow system will be used. Air and water density is assumed to be constant.

As an addition, the simulation will be conducted in a 2D model to reduce computation time and simulate more variation in geometry. The selected boundary conditions in each simulation will be based on Fig 2. which produces slug flow regime. JG and JF, the superficial gas and liquid velocity, respectively, will be selected at which produces Bo << 1. Details on the parameters are presented in Table 1.

Table 1 Summary of simulation parameters

Parameters	Value
Gas superficial velocity (JG) (m/s)	Example
Liquid superficial velocity (JL) (m/s)	Example
Mass-flow rate (kg/s)	Example
ρair (kg/m-3)	1.2
μair (Pa.s)	Example
ρwater (kg/m-3)	1000
μwater (Pa.s)	Example

Micro-channel Geometry

The geometries that are going to be simulated are T-junction microchannel with differentiation in 1) aspect ratio with constant hydrodynamic diameter with circular cross-section; and 2) hydrodynamic diameter in circular cross-section micro-channel with constant aspect ratio. The details of the geometrical parameters are shown in Table 2.

Table 2 Summary of geometrical parameters

Parameters	Value
Gas superficial velocity (JG) (m/s)	Example
Liquid superficial velocity (JL) (m/s)	Example
Mass-flow rate (kg/s)	Example
ρair (kg/m-3)	1.2
μair (Pa.s)	Example
ρwater (kg/m-3)	1000
μwater (Pa.s)	Example

Parameters to be Analyzed

Gas-liquid micro-channel flow regime has been studies systematically and reviewed by Gunther and Jensen (2006), summarized in Fig. 2 which also shows the typical operating conditions. Most of the experiment were conducted in a air-water flow system. Past research has observed slip velocities, pressure profile, and other flow characteristics with respect to the corresponding non-dimensional quantities, such as Re, We, Ca, etc. In this work, the main parameters to be observed is the pressure drop and RTD, each will be analyzed with respect to the geometrical parameters. Commonly observed parameters, such as slip velocities, pressure profile, and flow regime, which will be observed qualitatively, will also be presented.

Acknowledgement

This research was conducted to complete the computational fluid dynamics class, in Mechanical Engineering Department, Universitas Indonesia, as the final project. The author thank Dr. Ahmad Indra as our advisor, and also Bang Edo, Bang Bani, Ales, Abi, Elvin, Mas Agus, Josiah, and all other students for the valuable discussion in creating this research paper.

References

Aubin, J., Prat, L., Xuereb, C. and Gourdon, C., 2009. Effect of microchannel aspect ratio on residence time distributions and the axial dispersion coefficient. Chemical Engineering and Processing: Process Intensification, 48(1), pp.554-559.

Günther, A. and Jensen, K., 2006. Multiphase microfluidics: from flow characteristics to chemical and materials synthesis. Lab Chip, 6(12), pp.1487-1503.

Kawahara, A., Chung, P. and Kawaji, M., 2002. Investigation of two-phase flow pattern, void fraction and pressure drop in a microchannel. International Journal of Multiphase Flow, 28(9), pp.1411-1435.

Kawaji, M., Mori, K. and Bolintineanu, D., 2009. The Effects of Inlet Geometry and Gas-Liquid Mixing on Two-Phase Flow in Microchannels. Journal of Fluids Engineering, 131(4).

Moreau, M., Di Miceli Raimondi, N., Le Sauze, N., Gourdon, C. and Cabassud, M., 2017. A new numerical method for axial dispersion characterization in microreactors. Chemical Engineering Science, 168, pp.178-188.

Naher, S., Orpen, D., Brabazon, D., Poulsen, C. and Morshed, M., 2011. Effect of micro-channel geometry on fluid flow and mixing. Simulation Modelling Practice and Theory, 19(4), pp.1088-1095.

Sattari-Najafabadi, M., Nasr Esfahany, M., Wu, Z. and Sunden, B., 2018. Mass transfer between phases in microchannels: A review. Chemical Engineering and Processing - Process Intensification, 127, pp.213-237.