An Experimental Investigation on Liquid Film Thickness Characteristics of Gas–Liquid Taylor Flow in Square/Rectangular Microchannels Applied in Microreactors

Research Overview

This study experimentally investigates the liquid film thickness of gas–liquid Taylor flow in square and rectangular microchannels applied to microreactors. Liquid film thickness was measured using a laser focus displacement meter (LFDM), while bubble shapes and velocities were captured by high-speed visualization. The work clarifies the effects of channel hydraulic diameter, aspect ratio, capillary number, and surface tension on liquid-film distribution in air–water and Tween-20-added systems.

Graphical Abstract

Figure: One-page summary of the research background, experimental method, measurement definitions, main findings, empirical correlations, and potential applications for microreactor design.

Background and Objective

Gas–liquid Taylor flow in microreactors provides a large interfacial area and stable segmented-flow structure, which are highly beneficial for reactions, absorption, separation, and heat and mass transfer. In processes such as CO2 absorption and chemical reactions, liquid film thickness controls the transport distance and strongly affects process efficiency.

Although liquid film thickness in circular microchannels has been widely studied, square and rectangular microchannels are more commonly manufactured in MEMS-based devices. Their corners and asymmetric cross-sections cause non-uniform liquid-film distribution, which remains insufficiently understood. This study aims to quantify the liquid-film characteristics in square and rectangular microchannels and provide reliable correlations for design.

Key Features of This Study

High-precision measurement: Liquid film thickness around Taylor bubbles was measured non-contact using LFDM.
Synchronized visualization: Bubble shape and bubble velocity were recorded using a high-speed camera.
Various channel geometries: Square channels with hydraulic diameters of 0.3, 0.5, 0.7, and 1.0 mm and rectangular channels with a depth of 0.5 mm were tested.
Surfactant effect: The influence of Tween-20 addition and reduced surface tension was evaluated.
Empirical correlations: Dimensionless bubble diameter in square and rectangular channels was correlated using Ca, Re, Bo, and Li/Dh.

Proposed Method and Working Mechanism

1. Generation and visualization of Taylor flow

Air and liquid were supplied by syringe pumps and mixed at a T-junction to generate gas–liquid Taylor flow in microchannels. A high-speed camera was used to observe bubble shape, bubble length, and bubble velocity.

2. LFDM measurement of liquid film thickness

The inner wall position x1 was measured in an air-only state, and the gas–liquid interface position x2 was measured when a bubble passed. The liquid film thickness was calculated as δ = |x2 − x1|.

3. Dimensionless analysis of liquid-film distribution

For square channels, the dimensionless bubble diameter was defined as D = 1 − 2δ/Dh. For rectangular channels, Di = 1 − 2δi/Li was used depending on the measurement direction.

4. Development of empirical correlations

Empirical correlations were developed using dimensionless groups such as Ca, Re, Bo, and Li/Dh to describe liquid-film behavior in square and rectangular microchannels.

Main Findings

Bubble velocity vs. superficial velocityBubble velocity in square and rectangular microchannels increased almost linearly with superficial velocity.

Applicable velocity modelsFor Dh < 0.5 mm and α ≤ 1.0, Kawahara et al.’s square/rectangular model described the bubble velocity well, while for Dh > 1 mm Gregory et al.’s model became suitable.

Three-stage liquid-film behaviorThe dimensionless bubble diameter D showed three stages with increasing Ca: initial stage, reduction stage, and stable stage.

Transition Ca in square channelsFor Dh = 0.3, 0.5, 0.7, and 1.0 mm, D entered the reduction stage at Ca = 0.019, 0.015, 0.011, and 0.009, respectively.

Directional non-uniformityIn rectangular channels, liquid-film behavior differed in the depth and width directions, showing a strong aspect-ratio effect.

Prediction accuracyThe proposed correlations predicted 90.3% of the experimental data within a ±5% error band.

Future Prospects

This study provides reliable experimental data and empirical correlations for liquid film thickness in gas–liquid Taylor flow in square and rectangular microchannels. These results can support the design of microreactors and micro heat exchangers where heat and mass transfer strongly depend on liquid-film behavior.

Future work can further investigate the effects of fluid properties, wall contact angle, surface wettability, and channel structure on liquid-film formation and distribution.

Combining these experimental correlations with numerical simulations and reaction or absorption models will help establish practical design guidelines for high-efficiency microreactors and scale-up strategies.

Potential Applications

The findings can be applied to micro-scale reaction, separation, and heat-transfer devices using gas–liquid two-phase flow.

Microreactor designCO2 absorption and separationEnhanced gas–liquid mass transferMicro heat exchangersProcess intensificationChemical reaction processes

Summary

This study combined LFDM measurement with high-speed visualization to quantitatively measure liquid film thickness in gas–liquid Taylor flow in square and rectangular microchannels.

The results show that liquid film thickness is governed by capillary number, hydraulic diameter, aspect ratio, and surface tension, and that square and rectangular microchannels exhibit geometry-dependent non-uniformity.

          Conclusion：
          Liquid film thickness in square and rectangular microchannel Taylor flow is governed by Ca, Dh, aspect ratio, and surface tension, and can be described accurately by the proposed empirical correlations.
        