Gas-liquid two-phase flow in minichannels has been the subject of increased research interest in the past few years. Evaluation, however, of today's state of the art regarding hydrodynamics of flow in minichannels shows significant differences between existing test results. In the literature there is no clear information regarding: defining the boundary between minichannels and conventional channels, labelling of flow patterns. The review of literature on the hydrodynamics of gas-liquid flow in minichannels shows that, despite the fact that many research works have been published, the problem of determining the effect of diameter of the minichannel on the hydrodynamics of the flow is still at an early stage. Therefore, the paper presents the results of research concerning determination of flow regime map for the vertical upward flow in minichannels. The research is based on a comprehensive analysis of the literature data and on the research that has been carried out. Such approach to the mentioned above problems concerning key issues of the two-phase flow in minichannels allowed to determine ranges of occurrence of flow structures with a relatively high accuracy.

Technology advancements entail a necessity to remove huge amounts of heat produced by today’s electronic devices based on highly integrated circuits, major generators of heat. Heat transfer to boiling liquid flowing through narrow minichannels is a modern solution to the problem of heat transfer enhancement. The study was conducted for FC-72 boiling in a rectangular, vertical and asymmetrically heated minichannel that had depths of 0.5-1.5 mm, a width of 20 mm and a length of 360 mm. The heat flux increased and decreased within the range of 58.3-132.0 kWm−2, the absolute pressure ranged from 0.116 to 0.184 MPa and the mass flux was 185-1139.2 kgm−2s−1. The boiling process took place on a flat vertical heating surface made of Haynes-230 0.1 mm thick acid-proof rolled plate with the surface roughness of 121 μm.

The aim of the study was the implementation of a numerical simulation of the air-water two-phase flow in the minichannel and comparing results obtained with the values obtained experimentally. To perform the numerical simulations commercial software ANSYS FLUENT 12 was used. The first step of the study was to reproduce the actual research installation as a three-dimensional model with appropriate and possible simplifications - future computational domain. The next step was discretisation of the computational domain and determination of the types of boundary conditions. ANSYS FLUENT 12 has three built-in basic models with which a two-phase flow can be described. However, in this work Volume-of-Fluid (VOF) model was selected as it meets the established requirements of research. Preliminary calculations were performed for a simplified geometry. The calculations were later verified whether or not the simplifications of geometry were chosen correctly and if they affected the calculation. The next stage was validation of the chosen model. After positive verification, a series of calculations was performed, in which the boundary conditions were the same as the starting conditions in laboratory experiments. A satisfactory description of the experimental data accuracy was attained.

Gas-liquid flows abound in a great variety of industrial processes. Correct recognition of the regimes of a gasliquid flow is one of the most formidable challenges in multiphase flow measurement. Here we put forward a novel approach to the classification of gas-liquid flow patterns. In this method a flow-pattern map is constructed based on the average energy of intrinsic mode function and the volumetric void fraction of gas-liquid mixture. The intrinsic mode function is extracted from the pressure fluctuation across a bluff body using the empirical mode decomposition technique. Experiments adopting air and water as the working fluids are conducted in the bubble, plug, slug, and annular flow patterns at ambient temperature and atmospheric pressure. Verification tests indicate that the identification rate of the flow-pattern map developed exceeds 90%. This approach is appropriate for the gas-liquid flow pattern identification in practical applications.

This research explored different types of two-phase flow patterns that influenced heat transfer rate by assessing rectangular two-phase closed thermosyphon (RTPCT) made from glass with the sides of equal length of 25.2 mm, aspect ratio 5 and 20, evaporation temperature of 50, 70, and 90 °C, working substance addition rate of 50% by volume of evaporator, and water inlet temperature at condensation of 20 °C. Upon testing with aspect ratios 5, three flow patterns emerged which were: bubble flow, slug flow and churn flow respectively. As per the aspect ratio 20, four flow patterns were discovered which were: bubble flow, slug flow, churn flow and annular flow, respectively. Aspect ratio 5 pertains characteristic which resulted in a shorter evaporation rate of the RTPCT than that of the aspect ratio 20, thus, a shorter flow distance from the evaporator section to heat releaser was observed. Therefore, flow patterns at aspect ratio 5 exhibited a faster flow velocity than that of the aspect ratio 20. Furthermore, changes of flow pattern to the one that is important for heat transfer rate can be easily achieved. Churn flow was the most important type of the flow for heat transfer, followed by slug flow. Moreover, with aspect ratio 20, annular flow was the most important flow for the heat transfer, followed by churn flow, respectively. Throughout the test, average heat flux as obtained from the aspect ratio 5 were 1.51 and 0.74 kW/m2 which were higher than those of the aspect ratio 20. The highest heat flux at the operating temperature of the evaporator section was 90 °C, which was equivalent to 2.60 and 1.52 kW/m2, respectively.

The article presents detailed two-phase adiabatic pressure drops data for refrigerant R134a. Study cases have been set for a mass flux varying from 200 to 400 kg/m2s, at the saturation temperature of 19.4°C. Obtained experimental data was compared with the available correlations from the literature for the frictional pressure drop during adiabatic flow. Influence of mixture preparation on pressure drop was investigated, for varying inlet subcooling temperature in the heated section. The flow patterns have also been obtained by means of a high-speed camera placed in the visualization section and compared with literature observations.

To find effective and practical methods to distinguish gas-liquid two-phase flow patterns, new flow pattern maps are established using the differential pressure through a classical Venturi tube. The differential pressure signal was first decomposed adaptively into a series of intrinsic mode functions (IMFs) by the ensemble empirical mode decomposition. Hilbert marginal spectra of the IMFs showed that the flow patterns are related to the amplitude of the pressure fluctuation. The cross-correlation method was employed to sift the characteristic IMF, and then the energy ratio of the characteristic IMF to the raw signal was proposed to construct flow pattern maps with the volumetric void fraction and with the two-phase Reynolds number, respectively. The identification rates of these two maps are verified to be 91.18% and 92.65%. This approach provides a cost-effective solution to the difficult problem of identifying gas-liquid flow patterns in the industrial field.