Peter M.-Y. Chung

Investigation of two-phase flow pattern, void fraction and pressure drop in a microchannel

Abstract An experimental investigation has been carried out on two-phase flow characteristics in a 100 μm diameter circular tube. Two-phase flow patterns were determined by video recording the flow in the transparent capillary tube made of fused silica, in which de-ionized water and nitrogen gas were injected at superficial velocities of jG=0.1–60 m/s for gas, and jL=0.02–4 m/s for liquid. Time-averaged void fraction and two-phase friction pressure drop data were also obtained and analyzed. The flow patterns observed were intermittent and semi-annular flows, but a closer study of the liquid film structure revealed gas core flows with a smooth or ring-shaped film and a serpentine-like gas core surrounded by a deformed liquid film. Bubbly and churn flow patterns were not observed. A flow pattern map was developed based on the probability of appearance of each type of flow, and compared to the existing flow pattern maps obtained for ∼1-mm diameter channels. Void fraction remained low even at high gas flow rates, indicating large slip ratios and weak momentum coupling between the phases. The single-phase friction factor and two-phase friction multiplier data were shown to be in good agreement with the conventional correlations.

Two-Phase Flow Through Square and Circular Microchannels—Effects of Channel Geometry

An adiabatic experiment was conducted to investigate the effect of channel geometry on gas-liquid two-phase flow characteristics in horizontal microchannels. A water-nitrogen gas mixture was pumped through a 96 μm square microchannel and the resulting flow pattern, void fraction and frictional pressure drop data were compared with those previously reported by the authors for a 100 μm circular microchannel. The pressure drop data were best estimated using a separated-flow model and the void fraction increased non-linearly with volumetric quality, regardless of the channel shape. However, the flow maps exhibited transition boundaries that were shifted depending on the channel shape

Effects of Channel Diameter and Liquid Properties on Void Fraction in Adiabatic Two-Phase Flow Through Microchannels

Abstract Adiabatic two-phase flow experiments have been conducted to investigate the effects of channel diameter and liquid properties on void fraction in horizontal microchannels. Water/nitrogen gas and ethanol–water/nitrogen gas mixtures were pumped through circular microchannels of 50, 75, 100, and 251μm diameter. The ethanol concentration in water was varied to change the surface tension and liquid viscosity. The void fraction data were obtained by an image analysis technique and correlated as a function of homogeneous void fraction. The void fraction data obtained in 50, 75, and 100μm channels conformed well to the correlation in Kawahara et al. [1], but the data for a 251μm diameter channel agreed with the Armand [2] correlation suitable for minichannels. There was no significant effect of liquid properties on void fraction for all channel sizes. These results suggest that the boundary between microchannels and minichannels would lie between 100 and 251μm.

Characteristics of Single-Phase Flow in Microchannels

Experiments were performed to study the flow behaviour of de-ionized water and nitrogen gas through round capillary rubes having an inner diameter of 100µm. At steady state, the single-phase pressure drop along the glass microchannel was measured and analysed. To compare with conventional flow theory, an evaluation was made of the friction factor constant for laminar flow and critical Reynolds number for the transition from laminar to turbulent flow. The liquid flow data were well predicted by the conventional friction factor equations for larger channels, and the critical Reynolds number was close to the traditional macro-scale value. For single-phase gas flow, the measured friction factors were found to agree with theory if compressibility effects are taken into account. The addition of compressibility yields a non-linear pressure distribution that arises from the density change of the gas in the channel. Unlike liquid flow in microchannels, the gas friction factor constant depends on the Reynolds number, which changes along the channel length. Moreover, compressibility caused the velocity to vary all along the length of the channel and prevented the flow from being fully-developed. The neglect of the slip-flow boundary condition and compressibility may account for the discrepancy between the experimental results of various researchers.Copyright

Two-phase flow through square and circular microchannels - Effect of channel geometry

An adiabatic experiment was conducted to investigate the effect of channel geometry on gas-liquid two-phase flow characteristics in microchannels. A mixture of water and nitrogen gas was pumped through a 96 μm × 96 μm square microchannel and the flow pattern, void fraction and pressure drop data were obtained and compared with those previously obtained in a 100 μm circular microchannel. The frictional pressure drop was determined from the measured total pressure drop, and the two-phase flow pattern and void fraction were determined from image analysis of the video recordings. In the square channel, 136 runs were performed over a range of 0.09 ≤ jG,AVG ≤ 62 m/s for the average superficial gas velocity and 0.01 ≤ jL ≤ 4 m/s for the superficial liquid velocity. The frictional pressure drop data showed that the calculations based on a separated–flow model were best at estimating the frictional pressure drop for both microchannels. No particular effect of the channel shape was found for the two-phase frictional pressure drop. The void fraction-to-volumetric quality relationship was also found to be similar for both shapes of microchannels, exhibiting an exponential increase in void fraction with increasing volumetric quality. The empirical correlation that describes the void fraction-to-volumetric quality relationship for the square microchannel was developed earlier from the measured data for the circular microchannel. Observations of the recorded images indicated the two-phase flow patterns to be primarily intermittent with liquid and gas slugs. The liquid film surrounding the gas core displayed a smooth or ring-like structure. The probability of each interfacial structure occurring was examined in detail to develop a novel flow pattern map consisting of four regions named slug-ring flow, ring-slug flow, multiple flow and semiannular flow. Between the square and circular microchannels, the two-phase flow maps exhibited transition boundaries that were shifted depending on the channel shape. The region of ring-slug flow that appears in the circular microchannel collapsed in the square microchannel, possibly due to the suppression of the liquid-ring film in the corners of the square channel.Copyright

Study on Two-Phase Flow Through a Micro T-Junction

The field of microfluidics is developing rapidly with advances in MEMS (micro electro mechanical system) and μ TAS (micro total analysis system) technologies. In various devices, controlling the flow rate of liquid or gas accurately at micro or nanoliter volume levels is required. In this work, the gas-liquid two-phase flow patterns in a microchannel T-junction have been examined. The two-phase flow patterns were observed in the T-junction under two different gas and liquid injection conditions. The flow patterns of liquid and gas in the microchannel after the T-junction were also observed and classified into three types depending on the flow rate, and a two-phase flow pattern map was constructed. The mechanism of bubble breakup at a micro T-junction was also experimentally and analytically investigated.Copyright

Turbulence Modification Due to Wave Action at Low Reynolds Numbers in Horizontal Open-Channel Flow

Abstract Wave-turbulence interaction was experimentally investigated in turbulent open-channel flows with a shear-free wavy surface using a photochromic dye activation technique. In the experiments conducted, two-dimensional waves of different amplitudes, wavelengths, and frequencies were superimposed on a liquid surface via a mechanical wave maker. The range of Reynolds numbers varied from 3900 to 5000 based on the hydraulic diameter, with the corresponding aspect ratio of the channel width to liquid depth varying from 7.5 to 5. Within the range of Reynolds numbers investigated, the results showed that the streamwise turbulence intensity increased in the bulk and interfacial regions in comparison to the undisturbed flow. Furthermore, video sequences of the flow visualization experiments clearly revealed that the spanwise motion of the liquid was significantly suppressed; the traces did not immediately deform in the spanwise direction but retained their shape with increasing wave amplitude and frequency as compared to smooth interface flows. This suggests that waves may have suppressed longitudinal vortices generated near the smooth interface. The suppression of the longitudinal vortices in wavy open-channel flows has been proposed as a mechanism responsible for the turbulence energy redistribution, different from that for smooth open-channel flows.

Effect of Channel Diameter and Liquid Property on Void Fraction in Adiabatic Two-Phase Flow Through Microchannels

Adiabatic two-phase flow experiment have been conducted to investigate the effects of channel diameter and liquid property on void fraction in horizontal microchannels. Water/nitrogen gas and ethanol-water/nitrogen gas mixtures were pumped through circular microchannels of 50, 75, 100 and 251 μm diameter. The concentration of ethanol in water was varied to change the surface tension and liquid viscosity. The void fraction data were obtained by an image analysis technique and correlated as a function of homogeneous void fraction. The void fraction data in channels with a diameter between 50 and 100 μm conformed well to Kawahara et al.’s (2002) correlation, but the data for a 251 μm diameter channel agreed with the Armand correlation (1946) suitable for minichannels. There was no significant effect of liquid properties on void fraction for all the channel sizes. Thus, these results suggest that the boundary between microchannels and minichannels would lie between 100 and 251 μm, and not be sensitive to the fluid property.© 2004 ASME