Gas–Liquid Two-Phase Flow Evolution in a Long Microchannel

A pair of optical void sensors and a high-speed video camera were used to investigate the evolution of adiabatic gas–liquid two-phase flow in a long microchannel. Experiments were conducted with a 1676-mm-long, circular microchannel with an inner diameter of 100 μm. Two-phase flow patterns, void fraction, and velocities of gas plug/slug and liquid slugs were measured at different axial locations between the gas–liquid mixer and microchannel exit. The pressure decreased linearly in the first half of the microchannel, and more rapidly and nonlinearly in the second half of the test section. As a result, the flow accelerated significantly in the second half of the microchannel such that the void fraction and liquid slug velocity increased nonlinearly. The measured mean void fraction and mean velocity of liquid slugs also agreed well with the homogeneous flow model predictions when the liquid flow rate was constant and the mass velocity of the gas was low.     

Investigation of Ring Waves in Gas–Liquid Two-Phase Flow in a Microchannel

Gas–liquid two-phase flow in minichannels and microchannels displays a unique flow pattern called ring film, in which stable waves of relatively large amplitudes appear at seemingly regular intervals and propagate in the flow direction. In this paper, the behaviors of ring waves, which correspond to ring films that appeared in ring film flow and disturbed ring film flow regions, have been investigated experimentally in gas–liquid two-phase flows of nitrogen-distilled water and nitrogen/30 wt% ethanol–water solution in a 150-μm-diameter silica tube to elucidate their generation mechanism and propagation behavior. In order to clarify the existence region and characteristics of ring waves, the flow patterns observed in a microchannel were investigated and flow pattern maps were made. Furthermore, the velocity of the ring wave was also investigated and compared with the gas slug velocity. In these velocity measurements, high-speed video images were taken at 6,000 frames per second and the formation of ring films and the relationship between the wave amplitude and velocity were determined. The results indicate an interfacial instability leading to the formation and growth of ring waves with both low and high wave amplitudes. The wave velocity is correlated to the wave amplitude, with the large amplitude waves moving much faster than the low amplitude waves. As a result, coalescence of large and low amplitude waves has been observed.     

Effect of Channel Length on the Gas–Liquid Two-Phase Flow Phenomena in a Microchannel

An optical measurement system was used to investigate the effect of microchannel length and inlet geometery on adiabatic gas–liquid two-phase flow. Experiments were conducted with 146-mm- and 1571-mm-long, circular microchannels of 100 μm diameter. Void fraction and gas and liquid plug/slug lengths and their velocities were measured for two inlet configurations for gas–liquid mixing: (a) reducer and (b) T-junction. The superficial gas velocity was varied from 0.03 to 14 m/s, and superficial liquid velocity from 0.04 to 0.7 m/s. The test section length was found to have a significant effect on the two-phase flow characteristics measured at the same axial location (37 mm from the inlet) in both microchannels. The mean void fraction data for the short (146 mm) microchannel with the reducer inlet agreed well with the equation previously proposed by Kawahara et al. (2002). On the other hand, the mean void fraction data for the long (1571 mm) microchannel obeyed the homogeneous flow model and Armand's equation for both the reducer and T-junction inlet configurations. Many long and rapidly moving gas plugs/slugs and long, slowly moving liquid plugs/slugs were observed in the short microchannel compared to the long microchannel, leading to the differences in the time-averaged void fraction data. The mean velocity of liquid plugs/slugs generally agreed well with Hughmark's equation and the homogeneous flow model predictions, regardless of the inlet configurations and microchannel lengths. Thus, both the microchannel length and inlet geometry were found to significantly affect the two-phase flow characteristics in a microchannel.     

The Behavior and Characteristics of the InterfacialWaves in Gas-Liquid Two-Phase Separated FlowThrough Downward Inclined Rectangular Channel

INTaoDUCTIONInhorizontalandslightlyinclinedgas-liquidtwo-phaseflow,thestratifiedflowmnyoccurinthecaseofthelowgasandliquidflowratecombinations.Inthiskindofflowwhenthegasvelocityisincreasedwithintheregimeofthestratifledfiow,wavesappearonthegasandliquldinterfacel1'2].Forthesestrati-fiedwavyflowpatterns,thestructureanddynamicsoftheinterfacegreatlyinfiuencetheratesofmasslmo~umandheattransferaswellasthestabilityofthesysteml3'4l,thereforeanaccurateknowledgeofint~ialwavformationandwavparameterssuc…     

Effect of Void Fraction and Two-Phase Dynamic Viscosity Models on Prediction of Hydrostatic and Frictional Pressure Drop in Vertical Upward Gas–Liquid Two-Phase Flow

In gas–liquid two-phase flow, the prediction of two-phase density and hence the hydrostatic pressure drop relies on the void fraction and is sensitive to the error in prediction of void fraction. The objectives of this study are to analyze dependence of two-phase density on void fraction and to examine slip ratio and drift flux model-based correlations for their performance in prediction of void fraction and two-phase densities for the two extremes of two-phase flow conditions, that is, bubbly and annular flow or, alternatively, the low and high region of the void fraction. It is shown that the drift flux model-based correlations perform better than the slip ratio model-based correlations in prediction of void fraction and hence the two-phase mixture density. Another objective of this study is to verify performance of different two-phase dynamic viscosity models in prediction of two-phase frictional pressure drop. Fourteen two-phase dynamic viscosity models are assessed for their performance against 616 data points consisting of 10 different pipe diameters in annular flow regime. It is found that none of these two-phase dynamic viscosity models are able to predict the frictional pressure drop in annular flow regime for a range of pipe diameters. The correlations that are successful for small pipe diameters fail for large pipe diameters and vice versa.     

Study on Flow and Heat Transfer Characteristics of Vapor–Liquid Two-Phase Flow in a Narrow Rectangular Channel With Longitudinal Vortex Generators

The heat transfer enhancement of the longitudinal vortex (LV) is a kind of technology with high efficiency and low thermal resistance. An LV is produced by longitudinal vortex generators (LVGs). Due to their relative long influence distance and simple structure, the LVGs may be used in narrow channels with a flat surface. In this paper, the dimension of a narrow rectangular channel is 600 mm (length) × 40 mm (width) × 3 mm (height), and one LVG is 14 mm (length) ×2.2 mm (width) × 1.8 mm (height). The rectangular blocked LVGs are periodically laid out in the heated plate, and the attack angle of LVGs is 44°, the longitudinal pitch between LVGs is 100 mm, and the transverse pitch between LVGs is 4 mm. The test section is visual with three surfaces and heated with one surface by direct current. The working fluid is water. The experimental results show that the boiling heat transfer coefficient on the heated surface is increased by 25.8%, while the pressure drop along the test section is increased by 50.6%. At the same time, the visual experimental data shows that the bubbles’ behavior has been intensively affected by LVs, the growth and gathering of bubbles have been depressed, and the thermal boundary layer in the test section has been greatly damaged and reduced; as results, the momentum and energy exchange in the test section have been strengthened. Thus, the heat transfer is obviously enhanced by LVs.     

Computational Fluid Dynamics–Population Balance Modeling of Gas–Liquid Two-Phase Flow in Bubble Column Reactors With an Improved Breakup Kernel Accounting for Bubble Shape Variations

Abstract

Traditionally, bubble shapes have been assumed to be spherical in breakup models such as the one developed by Luo and Svendsen in 1996. This particular breakup model has been widely accepted and implemented into computational fluid dynamics (CFD) modeling of gas–liquid two-phase flows. However, simulation results from this model usually provide unreliable predictions about the breakage of very small bubbles. The incorporation of bubble shape variation into breakup models has rarely been documented in literature but the bubble shape plays an important role in the interactions with the surrounding eddies, especially when the effects of bubble deformation, distortion, and bubble internal pressure change are considered during the events of eddy-bubble collision. Thus, the assumption of spherical bubbles seems to be no longer appropriate in reflecting this phenomenon. This study proposes and implements a modified bubble breakup model, which accounts for the variation of bubble shapes when solving the population balance equations for CFD simulation of gas–liquid two-phase flows in bubble columns. The key parameters predicted by the modified breakup model have been compared with the ones predicted by the original model. The simulation results of interfacial area and mass transfer coefficient for larger bubbles have been greatly enhanced by the modified breakup model.     

An evaluation and model of the Chinese Kang system to improve indoor thermal comfort in northeast rural China – Part-1: Model development

In the rural area of northern China, residents often use a traditional stove-Kang system to heat their homes, which is an effective way of using locally-produced biomass to solve the energy and environment problems. However, many of these homes are not heated sufficiently during the winter months. The current system is inefficient and requires high levels of fuel consumption. This paper researches the Kang system by creating a simplified and modular system modeling program with the aim to assist in accurate real-time design and optimization of the domestic Kang heating system. This part-1 introduces the structure, principles and the underlying equations of the modular simulation system. A variety of improvement measures were modeled and tested, upon the base model, including the use of a water heat exchanger and radiator system to the existing Kang set up, solar hot water system, bathing system, and the use of phase change material at the surface of the Kang. The program demonstrates good flexibility of implementing different technical solutions as well as fast simulation capability.     

An introduction to the special issue section on “The 7th International Conference on Photosynthesis and Hydrogen Energy Production in Honor of Nathan Nelson and T. Nejat Veziroğlu, 19–25 June 2016, Pushchino,Russia”

The research in photosynthesis and hydrogen energy production provides a unique opportunity for transforming sustainable solar energy into our energy system. This special issue presented the selected and invited papers from the International Conference on Photosynthesis Research for Sustainability in honor of Nathan Nelson and T. Nejat Veziro?lu which was held on June 19–25, 2016, in Pushchino, Russia. These papers offered readers with some of the most recent and exciting progresses in photosynthesis and hydrogen energy production. The potential limitations and future efforts with open questions were also offered to stimulate the further research endeavors in the field.