Two-phase flow of air-water mixture in a minichannel

Two-phase systems have a huge potential for solving problems of removing large heat fluxes. To date, mini-channel and microchannel systems are widespread. Of particular interest are the stratified flow regime and the annular flow regime in miniand microchannel. It is necessary to know in detail the map of flow regimes for the realization of these flow regimes. In this paper, we present an investigation of flow regimes for a rectangular minichannel 10 mm wide and 1.1 mm high.


Introduction
Two-phase systems in mini and micro channels have received wide application in medicine, microelectronics, power engineering, aircraft and machine building. In medicine, microchannels are used to implement lab-on-chip. In chemistry, chemical reactors are created on the basis of mini and microchannels. In the field of microelectronics, boiling in microchannel is used to cool electronic components; heat exchange in heat exchangers in two-phase systems in mini and micro channels is the basis of the action of heat pipes.
One of the promising ways of removing large heat fluxes from the surface of heatstressed elements of electronic devices is the use of two-phase flows in microchannels [1,2]. The most efficient flow regimes in the channel (in terms of heat removal) is annular or stratified flow [3]. Flow regimes are of interest for study [4][5][6], investigation has been carried out for various channel parameters. Of great interest are mini and micro channels with a rectangular cross-section that can be used in cooling systems for microchips. It is important to understand hydrodynamics in such systems to create effective cooling systems. The hydrodynamics of two-phase flow in the minichannels height of 1 mm is investigated in [7]. Regimes in such channels significantly differ from the regimes in microchannels [8].

Experimental setup
A schematic diagram of the two-phase mini-channel system is shown in Fig. 1. The system has two working circuits: closed liquid circuit and a gas circuit. The liquid circuit, as seen from Fig. 1, contains pump Grundfos DDE for pumping working liquid. The gas circuit contains a membrane vacuum pump-compressor MVNK 3x4, which produces an output of up to 400 l/min of working gas. To control gas consumption, the Bronkhorst F-111AC-70K flow regulator is used, which has an operating gas flow range from 0 to 100 l/min. Gas and liquid is supplied into the test section, in which a two-phase system is realized. The test section has an inlet for liquid, gas and a common outlet. A rectangular channel with a width of 10 mm and a height of 1.1 mm is constructed. The channel is oriented horizontally. The temperatures of the the substrate, liquid and gas at the inlet are constant and equal to 20 °C. The working fluid is ultrapure water created with the Merck Millipore Direct-Q 3 UV water purification system. In the test section, there is a copper heater, but it is not used in this study.

Results and discussion
At very low superficial liquid and gas velocities determined as flow rate divided by channel cross-section area) we observed the jet regime. In fig. 2a    In the bubble flow regime, we observe many small bubbles (8). The bubble size and frequency depends on the gas and liquid flow rates. The typical bubble size is about several millimeters and not exceed the channel width. In fig 3a, we present characteristic image of the bubble flow at U SL = 0.17 m/s, U SG = 0.121 m/s. Gas bubbles with different sizes move along the channel. In this case, bubbles are formed directly near the liquid nozzle. With an increase in superficial gas velocity, the frequency of bubble formation increases. Further increasing the superficial gas velocity leads to formation of the gas jet in the inlet section, fig 3b,   The flow regime map for investigated channel is shown in fig. 4. More than 96 points with different flow rates have been investigated. The following two-phase flow regimes are shown in the regime map: jet (1), bubble (2), stratified (3), and annular (4). The transitions between regimes are investigated.

Conclusions
The flow pattern map has been created for wide range of flow parameters (liquid superficial velocity 0.005 -1 m/s, gas superficial velocity 0.05 -50 m/s). More than 96 points with different flow rates have been investigated. The four main regimes for minichannel have been identified: jet, bubble, stratified, and annular. In the future this experimental test cell will be used for heat transfer investigations with heating element created by additive technologies. A special structure will be created on the surface in order to intensify heat flux.
The reported study was funded by RFBR according to the research project № 18-48-543034.