Akimaro Kawahara

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

Prediction of the single-phase turbulent mixing rate between two parallel subchannels using a subchannel geometry factor

This paper presents a simple method for predicting the single-phase turbulent mixing rate between adjacent subchannels in nuclear fuel bundles. In this method, the mixing rate is computed as the sum of the two components of turbulent diffusion and convective transfer. Of these, the turbulent diffusion component is calculated using a newly defined subchannel geometry factor F* and the mean turbulent diffusivity for each subchannel which is computed from Elders equation. The convective transfer component is evaluated from a mixing Stanton number correlation obtained empirically in this study. In order to confirm the validity of the proposed method, experimental data on turbulent mixing rate were obtained using a tracer technique under adiabatic conditions with three test channels, each consisting of two subchannels. The range of Reynolds number covered was 5000–66 000. From comparisons of the predicted turbulent mixing rates with the experimental data of other investigators as well as the authors, it has been confirmed that the proposed method can predict the data in a range of gap clearance to rod diameter ratio of 0.02–0.4 within about ±25% for square array bundles and about ±35% for triangular array bundles.

Flow redistribution due to void drift in two-phase flow in a multiple channel consisting of two subchannels

Abstract Void drift in two-phase flow is studied experimentally using a geometrically simple, vertical channel consisting of two interconnected subchannels. Data on the flow redistributions of both air and water along the channel axis are obtained and presented for the following two multiple channels: one with two circular subchannels of different cross-sectional area and the other with two identical circular subchannels. The data are analysed by a simple one-dimensional subchannel code taking account of the effects of void drift and turbulent mixing between subchannels, i.e. incorporating both the void-settling model of Lahey et al. and a term similar to that in the COBRA code in the momentum equation. The flow redistribution process can be explained by the analysis.

Prediction of turbulent mixing rates of both gas and liquid phases between adjacent subchannels in a two-phase slug-churn flow

Abstract This paper presents a slug-churn flow model for predicting turbulent mixing rates of both gas and liquid phases between adjacent subchannels in a BWR fuel rod bundle. In the model, the mixing rate of the liquid phase is calculated as the sum of the three components, i.e. turbulent diffusion, convective transfer and pressure difference fluctuations between the subchannels. The components of turbulent diffusion and convective transfer are calculated from Sadatomi et al.s [Nucl. Eng. Des. 162 (1996) 245–256] method, applicable to single-phase turbulent mixing, by considering the effect of the increment of liquid velocity due to the presence of gas phase. The component of the pressure difference fluctuations is evaluated from a newly developed correlation. The mixing rate of the gas phase, on the other side, is calculated from a simple relation of mixing rate between gas and liquid phases. The validity of the proposed model has been confirmed with the turbulent mixing rates data of Rudzinski et al. [Can. J. Chem. Eng. 50 (1972) 297–299] as well as the present authors.

The turbulent mixing rate and the fluctuations of static pressure difference between adjacent subchannels in a two-phase subchannel flow

Turbulent mixing rate between adjacent subchannels in a two-phase flow has been known to be strongly dependent on the flow pattern. In this study, flow visualization was made to investigate the mechanism of the turbulent mixing between subchannels in a two-phase flow under hydrodynamic equilibrium conditions. The test channel was a vertical multiple channel consisting of two identical rectangular subchannels, and the working fluids were air and water. It was observed in slug-churn flows that a large scale inter-subchannel liquid flow occurs in front of the nose of a large gas bubble and behind the tail when the bubble axially passes through the subchannel, and thus a high turbulent mixing rate of the liquid phase results. In order to know driving force of such a large scale inter-subchannel flow, measurement of instantaneous static pressure difference between the subchannels was also made. The result showed that there is a close relationship between the liquid phase turbulent mixing rate and the magnitude of the pressure difference fluctuations.

Characteristics of Two-Phase Flows in a Rectangular Microchannel with a T-Junction Type Gas-Liquid Mixer

In this study, gas–liquid two-phase flows in a horizontal rectangular microchannel have been investigated. The rectangular microchannel has a hydraulic diameter of 0.235 mm, and a width and depth of 0.24 mm and 0.23 mm, respectively. A T-junction-type gas–liquid mixer was used to introduce gas and liquid in the channel. In order to know the effects of liquid properties, distilled water, ethanol, and HFE7200 were used as the test liquids, with nitrogen gas was used as the test gas. The flow pattern, the bubble length, the liquid slug length, and the bubble velocity in two-phase flow were measured with a high-speed video camera, and the void fraction was determined from the bubble velocity data and the superficial gas velocity data. In addition, the pressure drop was also measured with a calibrated differential pressure transducer. The bubble length data were compared with the calculation by the scaling law proposed by Garstecki et al. [7]. The bubble velocity data and/or the void fraction data were well correlated with the well-known drift flux model [12] with a new distribution parameter correlation developed in this study. The frictional pressure drop data were also well correlated with the Lockhart-Martinelli method with a correlation of the two-phase friction multiplier.

Assessment of void fraction correlations for adiabatic two-phase flows in microchannels

In this paper, firstly a review is presented on our previous study of void fraction for adiabatic gas-liquid two-phase flows in horizontal microchannels. Water/nitrogen gas and/or ethanol-water-solution/nitrogen gas were pumped through circular microchannels of 50, 75, 100, 176, 251 and 530 μm in diameter. The concentration of ethanol in water was varied to change the surface tension and the liquid viscosity. The void fraction data for the 50 to 100 μm diameter channels showed non-linear variation against a homogenous void fraction, but the data for 251 and 530 μm diameter channels varied linearly with the homogeneous void fraction. Secondly, the data have been compared with the predictions of various correlations usually applied to mini/micro-channels as well as conventional size channels. Since no correlation could predict well all of our data, a new correlation has been proposed based on our data. It was found that the calculated void fraction by the proposed correlation agreed with all the data within 0.1, irrespective of channel diameters and the liquid properties.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