Experimental characterization of two-phase flow patterns in a slit microchannel

https://doi.org/10.1016/j.expthermflusci.2019.01.022Get rights and content

Highlights

  • A new technique for measuring characteristics of a two-phase flow in microchannels has been developed.

  • Experiments in a wide (width of 10 mm) microchannel with the height of 50 μm were carried out for the first time.

  • A new method for determining the boundaries of two-phase flow regimes based on quantitative criteria is proposed.

  • The following flow regimes have been distinguished: jet, bubble, stratified, annular, and churn ones.

Abstract

At present, there is revolutionary development of microchannel systems. In such systems, it is extremely important to determine two-phase flow patterns depending on the gas and liquid flow rates. However, in most works there are no clear criteria for turning from one regime to another. In the present work, we have developed a new technique that makes it possible to measure the local void fraction, size of characteristic regions of the liquid films on the upper and lower walls of microchannel, frequency of bubble formation, and other quantitative characteristics. The two-phase flow patterns in a microchannel of 50-μm height and 10-mm width have been studied experimentally. Based on the developed methodology, criteria that accurately determine the boundaries between the two-phase flow regimes have been created. The features of a two-phase flow in a microchannel have been determined, and a new flow regime map has been created.

Introduction

Today one of the urgent problems of thermal physics is the problem of cooling microelectronic equipment. Microelectronic components, microelectromechanical systems (MEMS, microsystems, micromachines), which are the three-dimensional mini- and micro-objects with high heat fluxes (up to 1 kW/cm2), where characteristic dimensions of elements are millimeters or microns, are being developed. The use of microchannels can significantly reduce the average film thickness in two-phase flows, which leads to intensification of heat transfer during evaporation. The efficiency of microchannels was shown in [1], [2], where the heat fluxes of more than 5 kW/cm2 were removed. The most effective flow regimes in the channel (from the point of view of heat removal and minimization of pressure drop) are the annular and stratified flows [3]. In this regard, for a wide range of technical applications, it is important to understand hydrodynamics in microchannels with the most efficient heat and mass transfer processes [3].

A significant number of techniques for studying the two-phase flow have been developed. People have been observing the flow of liquid and gas for more than 300 years. One of the main methods for visualizing flow irregularities is the schlieren method. The airflow from the candle flame was first visualized in the 17th century by Hooke [4]. The system for determining inhomogeneities in optical glasses was modified in 1864 [5]. Using a modified system, the shock waves generated by an electric discharge in air were observed [6], and the experiments of [5] were repeated. A combination of the schlieren-system and interferometry principles was used, when an optical filter with variable bandwidth was used instead of the usual filter-knife edge. The obtained images are processed after reflection using the principles of interferometry and the 3-D profile is restored [7]. The thickness of the water film flowing over the tube surface is measured using interference [8]. A thin film of n-butanol condensate on a quartz surface was measured in [9]. To study the two-phase flow in the microchannels, the schlieren method was used in [10]; it showed its effectiveness for visualizing the deformation of liquid films of up to 1-μm thickness.

The optical method for observing deformation and measuring the water film thickness is based on the principles of fluorescence: LIF (Laser Induced Fluorescence) [11], [12], [13]. The intensity of light emitted by a fluorescent substance in water is measured. The intensity of light depends on two parameters: layer thickness and fluorescent substance concentration. This method is well used in the absence of evaporation, which increases concentration of the fluorescent substance. The regime maps of the two-phase flow of water and air in micro- and mini-channels were plotted using the LIF method [14], [15], [16]. However, with a decrease in the channel height, the LIF method loses its effectiveness, since calibration of µ-LIF systems for visualization of liquid films with a thickness of 1 µm or less is very difficult.

To determine the liquid film thickness, a non-optical method is used: the capacitance one. Its principle is the measurement of electric capacity between two electrodes located on different sides of the channel with a two-phase flow. Electric capacitance varies with the percentage of liquid in a gap [17]. Currently, the capacitance method is used in electrical capacitance tomography (ECT) [18]. The authors were able to reconstruct distribution of oil in a cylindrical tube for simple types of flow: annular, stratified, flow of a liquid jet in the tube center when gas moves along the channel walls, and movement of only one phase inside the tube. Application of this method is also limited by the size of the channels.

One of the most interesting modern methods of flow analysis in micro-dimensional systems is the modification of the Particle Image Velocimetry (PIV) method: micro-PIV. One of the first applications of the micro-PIV method was used in [19] to study the Hele – Shaw flow with a cylindrical obstacle in the center of a square cross-section of 120 μm. A two-dimensional velocity field with a spatial resolution of 6.9 × 6.9 μm in one plane was obtained, and the efficiency of using micro-PIV for characterizing micro-dimensional flows was shown. This work was continued by Meinhart et al. [20]; they achieved a spatial resolution of 0.9 × 13.6 μm inside the glass channel of 30 × 300 μm. Sharp and Adrian [21] measured the velocity field of the liquid flow in the tubes with the diameter of 50–247 μm, using a Nd:YAG laser, 12-bit CCD camera and tracers with the diameter from 3.8 to 19 μm. Kovalev et al. [22] investigated the flow in the liquid-liquid system with extremely low viscosity ratio (10–3) in a T-shaped microchannel with a 120 × 120 μm inlets and 240 × 120 μm outlet channel using the Particle Tracing Velocimetry (PTV) method. The main feature of PTV is that the algorithm can track the individual particles for velocity calculations in contrast to the frequently used PIV (Particle Image Velocimetry) technique, based on correlations between intensity values of interrogation regions. Therefore, PTV can better distinguish the higher flow gradients than PIV. Despite the enormous prospects of micro-PIV methods, it is quite difficult to apply for the study of gas-liquid flow in a microchannel, because the thickness of the liquid layer can be below 1 µm.

There were many works on the study of the liquid film thickness under the bubble flow regime [22]. Fairbrother and Stubbs [23] found that the thickness of a liquid film surrounding a gas bubble does not depend on the bubble length, but only on capillary number Ca=μU/σ, where µ is the dynamic viscosity of liquid, U is the velocity of bubbles, σ is the surface tension. Taylor [24] continued this work using round tubes of 2 mm and 3 mm with Ca values of up to 1.9 and suggested that the asymptotic value of the fraction of liquid remaining on the wall is 0.55. At the same time, Bretherton [25] developed a model for Ca<0.01. Irandoust and Andersson [26] studied the film thickness of an ascending bubbly flow in the channels with the diameter of 1–2 mm. Their correlation was confirmed by experimental results for Ca < 2.0. Like Fairbrother and Stubbs, they found that the film thickness does not depend on the bubble length for lengths exceeding the channel diameter; this result was also observed by Ratulowski and Chang [27], who studied the transfer of bubbles in capillaries. Aussillous and Quere [28] developed a phenomenological model that described well the experimental data of Taylor [24] (Taylor law). They also investigated the effect of channel size on the film thickness and found that a higher channel diameter leads to a greater film thickness. Bartkus and Kuznetsov [29] investigated experimentally the film thickness in a microchannel with a cross-section of 420 × 280 μm2 and compared it with the work of Aussillous and Quere [28]. The data obtained on local film thickness confirm the failure of the Taylor law at long distances from the bubble head due to transverse capillary flows in a rectangular microchannel. Han and Shikazono [30] measured the film thickness in circular microchannels using a new method of laser focus displacement and developed an empirical model that could predict the film thickness depending on Ca values, Reynolds number (Re=ρUD/μ) and Weber number We=ρU2D/σ, where ρ is the density of liquid, and D is the diameter of channel. They stated that their model could predict their experimental data with an accuracy of up to 15% for Ca < 0.25. For the non-circular channels, Kolb and Cerro [31] studied the shape of a liquid film on the wall of a square capillary, filling the capillary with viscous liquid and then injecting air. They stated that the shape of the interface in the radial plane can be either round or non-round, depending on the Ca value. It was found that the non-circular shape is noticeable at small values of Ca. As the Ca value increases, the film thickness in the corners increases, and the thickness on the sides remains almost unchanged. As Ca increases further, the shape of the interface becomes circular when Ca exceeds a critical value. Han and Shikazono [30] measured the film thickness of air bubbles in the square microchannels with hydraulic diameter of 0.3–1.0 mm and confirmed the work of Kolb and Cerro [31] on interface evolution. They developed an empirical correlation, depending on Ca and We, to predict the film thickness both on the walls and in the corners of the channel with the accuracy of 5% for Ca < 0.4.

The model of Taitel and Dukler is the classic model for finding the film thickness in case of a stratified flow [32]. The equilibrium liquid level in the channel is obtained by considering an integral momentum balance on the liquid and gas flow. The equilibrium level of liquid for the annular flow is determined in the same way [33]. Kanno et al. [34] measured the liquid film thickness and visualized the annular flow in circular microchannels of 0.3–0.5 mm at a mass velocity of 100–500 kg/m2s. Comparing their experimental values of the dimensionless liquid film thickness with the model of film thickness for the annular flow proposed by Revellin et al. [35], they concluded that the model overestimates the experimental results at low void fraction, which is explained by instability of the interface caused by ripples.

A review of publications on the two-phase flow in microchannels is presented in [36], [37], [38]. It is shown in [39] that in short channels the transition to the annular regime is observed at lower gas flow rates. Traditionally, researchers have identified the following regimes: slug, bubble, churn, jet, stratified, and annular. However, in most works there are no clear criteria for the transition from one regime to another. In many studies, these transitions are determined qualitatively and cannot provide sufficient accuracy, when repeating experiments. In this paper, we developed an original experimental technique for accurate determination of the boundaries between two-phase flow regimes based on quantitative criteria, and also studied the two-phase flow regimes in a microchannel with the height of 50 μm and width of 10 mm.

Section snippets

Flow circuit

The scheme of experimental setup is shown in Fig. 1. The gas mixture is fed into the central part of microchannel from cylinder (1) with reducer (2). The gas flow rate is regulated from 20 to 10 ml/min and kept constant with the help of Bronkhorst El-Flow flow regulator (3) with an accuracy of 0.5%. Gas is supplied to gas chamber (4), from this chamber it is sent to the microchannel through gas nozzle (5). The flow rate is varied from 0.5 to 100 ml/min by a high-precision syringe Cole-Parmer

Measurement methods

To visualize the two-phase flow, a Nikon D7000 digital camera was used in the schlieren photography mode. The applied method was calibrated as follows: first, only gas was fed into the microchannel and an image was recorded when all microchannel walls were dried, Fig. 4a. Then, liquid and a small gas flow were fed into the microchannel to prevent liquid entering into the gas chamber. In this pattern, the difference between the area of microchannel filled with liquid (4) and non-wetted area on

Results and discussion

A two-phase flow in short horizontal rectangular channels 50 μm high and 10 mm wide was experimentally investigated. Instant schlieren images of two-phase flow patterns in the initial part of the channel are obtained at a distance of up to 40 mm from the liquid inlet to the channel. The thin liquid films on the channel walls were registered based on the schlieren images. Post-processing of images made it possible to fix the structure of a two-phase flow in the channel accurately. Based on

Conclusions

A new technique for determining the regime boundaries based on quantitative criteria was developed in this work. The presented technique provides a powerful tool for analyzing the two-phase flow patterns. For the first time, experiments were carried out in wide (10 mm wide) microchannels 50 μm in height. The following flow regimes have been distinguished: jet, bubble, stratified, annular, and churn ones. It is shown that the criterion for transition from the jet to the bubble regime is a sharp

Conflict of interest

The authors declared that there is no conflict of interest.

Acknowledgments

The authors thank Yu. Dementyev and V. Cheverda for participating in the experiments and data processing. The work was financially supported by the grant of the Russian Science Foundation No. 18-19-00407.

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