S.M. Ghiaasiaan

Two-phase flow in microchannels

Publisher Summary This chapter reviews recently published research dealing with gas-liquid two-phase flow in microchannels. Only microchannels with hydraulic diameters of the order of 0.1 to 1 mm and with long length-to-hydraulic diameter ratios are considered here. The hydrodynamic phenomena reviewed includes the two-phase flow regimes, void fraction, and frictional pressure drop in narrow rectangular and annular passages, a micro-rod bundle, and microchannels under conditions where surface tension and inertial forces are both significant. In these systems the channel characteristic dimension is of the same order of magnitude, or smaller than, the neutral interfacial wavelengths predicted by the Taylor stability analysis. The review is also restricted to situations where the fluid inertia is significant in comparison with surface tension. Such microchannels and flow conditions are encountered in miniature heat exchangers, research nuclear reactors, biotechnology systems, the cooling of high-power electronic systems, the cooling of the plasma-facing components in fusion reactors, and the heat rejection systems of spacecraft, to name a few. The flow through cracks and slits when such cracks develop in vessels and piping systems containing high-pressure liquids is another application of two-phase flow in microchannels of interest here.

Boiling incipience in microchannels

Abstract The available data dealing with boiling incipience of water in microtubes (tubes with diameters in the 0.1–1 mm range) are analyzed. Macroscale models and correlations appear to under predict the heat fluxes that lead to the boiling incipience in microtubes. It is suggested that boiling incipience in microchannels may be controlled by the thermocapillary force that tends to suppress the microbubbles that form on wall cavities. Accordingly, a semi-empirical method is proposed for predicting the boiling incipience in microtubes. The effect of the turbulence characteristics of the microtube on the proposed method is also examined.

Flow regimes and gas holdup in paper pulp–water–gas three-phase slurry flow

Abstract Hydrodynamic flow characteristics of solid–liquid–gas slurry made by intimately mixing fibrous paper pulp with water and air were investigated in a short, vertical circular column. The pulp consistency (weight fraction of pulp in the pulp–water mixture) was varied in the low consistency range of 0.0–1.5%. The test section was 1.8 m long, with 5.08 cm inner diameter. Mixing of the slurry prior to entering the test section was done using a patented mixer with controlled cavitation that generated finely dispersed micro-bubbles. Flow structures, gas holdup, and the geometric and population characteristics of gas bubbles in the gas–pulp–liquid three-phase flow were experimentally investigated, using visual observation, Gamma-ray densitometry, and flash X-ray photography. Superficial velocities of the gas and liquid/pulp mixture covered the ranges 0– 26 cm / s and 21– 51 cm / s , respectively. Five distinct flow regimes could be visually identified. These included dispersed bubbly, characterized by isolated micro-bubbles entrapped in fiber networks; layered bubbly, characterized by bubbles rising in a low consistency annular zone near the channel wall; plug; churn-turbulent; and slug. The dispersed and layered bubbly regimes could be maintained only at very low gas superficial velocities or gas holdups. Flow regime maps were constructed using phasic superficial velocities as coordinates, and the regime transition lines were found to be sensitive to consistency. The cross section-average gas holdup data showed that both the dispersed and the layered bubbly regimes could best be represented by the homogeneous mixture model. The drift flux model could best be applied to the reminder of the data when the plug and churn-turbulent flow regimes were treated together, and the slug flow was treated separately. The drift flux parameters depended on the pulp consistency.

Low-Flow Critical Heat Flux in Heated Microchannels

Critical heat flux (CHF) associated with the flow of subcooled water in heated microchannels is experimentally investigated. Four different channels, all 16 cm in length, are used: two are circular and uniformly heated and have 1.17- and 1.45-mm diameters, and the other two represent flow channels in a microrod bundle with a triangular array and 1.131-mm hydraulic diameter, with one uniformly heated over its entire surface and the other heated only over the surfaces of the surrounding rods. The test section parameter ranges are as follows: 250 to 1000 kg/m 2 . S mass flux, 344- to 1043-kPa exit pressure, 407- to 1204-kPa inlet pressure, and 49 to 72.5°C inlet temperature. The effect of noncondensables (air) on CHF is also examined by repeating some of the experiments with degassed water and with water saturated with air at test section inlet pressure and temperature. Critical heat flux occurs at very high flow qualities (0.36 and higher) in all the tests and indicates the occurrence of dryout. Furthermore, the CHF appears to monotonically increase with increasing mass flux or pressure. The CHF depends on channel cross-section geometry, and unlike high mass flux data, it increases with increasing channel diameter, The dissolved air slightly increases the CHF for the smaller circular channel and reduces the CHF for the other test sections. The experimental data are compared with the predictions of three widely used empirical correlations. The Bowring-1972 correlation could predict the data with reasonable accuracy.

Gas-liquid two-phase flow in narrow horizontal annuli

Abstract Experimental data associated with the two-phase flow regimes, void fraction and pressure drop in horizontal, narrow, concentric annuli are presented. Two transparent test sections, one with inner and outer diameters of 6.6 and 8.6 mm, and an overall length of 46.0 cm; the other with 33.2 and 35.2 mm diameters and 43.0 cm length, respectively, were used. Near-atmospheric air and water constituted the gas and liquid phases, respectively. The gas and liquid superficial velocities were varied in the 0.02–57 and 0.1–6.1 m s −1 ranges, respectively. The major two-phase flow patterns observed included bubbly, slug/plug, churn, stratified, and annular. Transitional regimes, where the characteristics of two distinct flow regimes could be observed in the test sections, included bubbly-plug, stratified-slug and annular-slug. The obtained flow regime maps were different than flow regime maps typical of large horizontal channels and microchannels with circular cross-sections. They were also different from the flow regimes in rectangular thin channels. The measured average void fractions for the two test sections were compared with predictions of several empirical correlations. Overall, a correlation proposed by Butterworth [Butterworth, D., 1975. A comparison of some void fraction relationships for co-current gas–liquid flow. Int. J. Multiphase Flow 1, 845–850] based on the results of Lockhart and Martinelli (1949) provided the most accurate prediction of the measured void fractions. The measured pressure drops were compared with predictions of several empirical correlations. The correlation of Friedel [Friedel, L., 1979. Improved friction pressure drop correlations for horizontal and vertical two-phase pipe flow. 3R Int. 18, 485–492] was found to provide the best overall agreement with the data.

Onset of flow instability and critical heat flux in thin horizontal annuli

Abstract The onset of flow instability (OFI) and critical heat flux (CHF) in heated thin horizontal annular flow passages cooled by subcooled water were investigated. For OFI, six different test sections, with inner diameter of 6.4 mm, annular gap widths of 0.724–1.001 mm, and heated lengths of 174–197 mm were used. The test parameter ranges were: 85– 1428 kg/m 2 s mass flux, 0.344–1.034 MPa exit pressure, 50–150 °C inlet temperature, 0.124– 1.0 MW/m 2 surface heat flux and 0–∞ inner-to-outer surface heat flux ratio. In addition, the effect of dissolved non-condensables on OFI was examined by performing similar experiments with degassed water and water saturated with air at test section inlet temperature and exit pressure. A total of 138 OFI test were run addressing important parametric trends. A theoretical model based on the solution of one-dimensional fluid conservation equations, which assumes no voidage upstream the onset of significant void (OSV) point, and accounts for thermodynamic non-equilibrium beyond the OSV point using an empirical quality profile fit, was shown to predict the conditions leading to OFI reasonably well. For CHF, the test section was an annulus with 6.45 and 7.77 mm inner and outer diameters, respectively (0.66 mm gap width), and an 18.5-cm long heated section. The experimental parameters investigated covered the following ranges: test section exit pressure: 0.344–1.034 MPa; coolant (water) mass flux: 100– 380 kg/m 2 s ; wall heat flux: 0.231– 1.068 MW/m 2 ; water inlet temperature: 30–65 °C. The measured CHF values were considerably lower than the expected CHF values for vertical test section configuration. In all the tests CHF occurred at relatively high equilibrium qualities, and was preceded by flow stratification which caused dry-out of the upper surface of the flow channel. The data were correlated in two ways: by introducing empirical correction multipliers into three widely used correlations for vertical channels; and based on the compensated distortions method.

Hydrodynamic characteristics of counter-current two-phase flow in vertical and inclined channels: effects of liquid properties

Abstract Flow patterns, counter-current flow limitation (flooding), and gas hold-up (void fraction) in counter-current flow in vertical and inclined channels were experimentally investigated. Tests were performed in a 2 m-long channel with 1.9 cm inner diameter, using air, and demineralized water, mineral and paraffinic oils, covering a surface tension range of 0.0128–0.072 N/m, and a liquid viscosity range of 1 × 10 −3 −1.85 × 10 −1 Ns/m 2 . The liquid and gas superficial velocity ranges for the tests with demineralized water were 0 ⩽ U GS ⩽ 54 cm/s and 1 ⩽ U GS ⩽ 299 cm/s, for tests with mineral oil were 0 ⩽ U LS ⩽ 23 cm/s and 1 ⩽ U GS ⩽ 248 cm/s, and for paraffinic oil were 0.15 ⩽ U LS ⩽ 11.6 cm/s and 1 ⩽ U GS ⩽ 224 cm/s, respectively. The examined channel angles of inclination with respect to the vertical line were 0, 30 and 68°. Flooding data were significantly different from pure water results only at very high liquid viscosities. The effect of liquid viscosity on gas hold-up and flow patterns was significant, furthermore, and several existing models and correlations were unable to correctly predict the data trends. With increasing the liquid viscosity the parameter range of the slug flow pattern expanded for all angles of inclination, and froth flow replaced the churn flow pattern in the vertical configuration and it replaced the churn/stratified and semi-stratified patterns in inclined configurations. The churn-stratified flow pattern is predominantly wavy stratified and is interrupted by upward-moving flooding-type waves. Semi-stratified is a periodic pattern where in each period the flow regime is initially wavy stratified while liquid accumulates in the bottom portion of the test section and forms a large liquid slug which subsequently moves upwards in the channel.

Liquid-side interphase mass transfer in cocurrent vertical two-phase channel flows

Volumetric liquid-side interphase mass transfer coefficients were experimentally measured in vertical channels supporting cocurrent, upward two-phase flows. Deionized water and an aqueous solution of sucrose constituted the liquid phase, pure nitrogen was the gas phase, and oxygen was the transferred species. In each test, oxygen concentrations in the liquid at two measurement stations near the two ends of the test section were measured on-line. The channel entrance effects were eliminated by performing hydrodynamically-identical tests with two different test section lengths, and using the shorter test section results for quantification of entrance effects in the longer test section. The channel average gas holdups representing developed flows were also measured using two simultaneous, quick-closing valves. The obtained data were compared with predictions of several widely-referenced correlations, with significant disagreements among the correlations, and between the correlations and the data. New empirical correlations were developed based on the obtained data representing the fully-developed slug and churn flow regimes.

Turbulent forced convection in microtubes

Abstract The anomalous and opposing trends in the published data dealing with turbulent flow friction factor and heat transfer coefficient in microchannels, and their apparent disagreement with macroscale correlations, are discussed. It is shown that the modification of turbulent eddy diffusivities, consistent with the way suspended particles may modify turbulence, can explain the observed higher-than-expected heat transfer coefficients in some data. It is thus suggested that suspended microscopic particles may be a major contributor to the aforementioned inconsistencies and disagreements in some of the published data.

Low-flow onset of flow instability in heated microchannels

Onset offlow instability (OFI) in uniformly heated microchannels cooled with subcooled water at very low flow rates was experimentally investigated. Four different microchannels, all of which were 22 cm long with a 16-cm-long heated section, were used. Two were circular with 1.17- and 1.45-mm diameters. The other two represented flow channels in a microrod bundle with triangular array and had a hydraulic diameter of 1.13 mm; one was uniformly heated over its entire surface, and the other heated only over the surfaces of the surrounding rods. The test parameter ranges were as follows: 220 to 790 kg/m 2 .s mass flux, 240- to 933-kPa channel exit pressure, 30 to 74°C inlet temperature, and 0.1 to 0.5 MW/m 2 heat flux. In addition, the effect of dissolved noncondensables on OFI was examined by performing similar experiments with degassed water and water saturated with air with respect to the test section inlet temperature and exit pressure. Conditions leading to OFI were different from those reported for larger channels and for microchannels subject to higher coolant mass flow rates. In all the experiments, OFI occurred when equilibrium quality at channel exit was close to zero or positive, indicating the possibility of insignificant subcooled voidage in the channel and indicating that the widely used models and correlations that are based on the OFI phenomenology representing larger channels may not apply to microchannels at low-flow rates. The channel total pressure drops were significantly greater in tests with air-saturated water as compared with similar tests with degassed water. The impact of the dissolved noncondensable on the conditions leading to OFI was relatively small, however. With all parameters including heat flux unchanged, the presence of dissolved air changed the mass fluxes that led to OFI typically by only a few percent.