Intermittent Flow Regime Research Articles

Effects of urbanization on macroinvertebrates in tributaries of the St. Johns River, Florida, USA

The effects of urbanization on hydrology, water quality and macroinvertebrates were examined in 7 headwater tributaries of the St. Johns River in the Atlantic Coastal Plain of northeast Florida. All streams had sandy substrata and drained small catchments (24–231 ha) that ranged from 0 to 51% total impervious area (TIA). Streams unaffected by urbanization had intermittent stream flow and completely dry channels for several weeks in autumn and spring. Urbanized streams always possessed channel water, but 2 streams ceased flowing and became stagnant in autumn and spring. Principle components analysis of chemical and physical measures (i.e. conductivity, nutrients, pH, metals, and stream flow) produced one axis (PCA1) that explained 54% of the total variation among the streams. The variables that loaded negatively on this axis were associated with low flows, while the variables that loaded positively were associated with urban land-use. PCA1 was also positively associated with %TIA. Macroinvertebrate richness ranged from 27 to 45 taxa and was positively associated with %TIA. Macroinvertebrate biomass ranged from 3 to 45 g AFDM/m2 and showed a significant, exponential relationship with PCA1 (r2 = 0.93) with greatest biomass occurring at intermediate %TIA. Invertebrate community structure in the urbanized streams appeared to be mainly influenced by hydrologic factors (perennial vs. intermittent flow regimes). The effects of urbanization on both hydrological and biological variables among the study streams were apparent, but also influenced by site-specific conditions.

Void fractions for condensing refrigerant flow in small channels. Part II: Void fraction measurement and modeling

In this paper, (Part II of a two-part study), an investigation of void fractions for condensing flows of refrigerant R-134a in minichannels is presented. Custom digital image processing techniques were used to quantitatively analyze these flows in transparent minchannel geometries (square, rectangular and circular) with 2 < Dh < 4.91mm. A total of 140 void fraction values in the intermittent flow regime (slug and plug flow), the wavy flow regime, and an intermittent-wavy overlap region were measured. A new model was developed based on these measured values and predicts void fraction better than correlations from the literature.

Two-phase flow of refrigerants in 85 μm-wide multi-microchannels: Part II – Heat transfer with 35 local heaters

This article is the second part of a study on flow boiling of R236fa and R245fa. This part presents the heat transfer coefficients obtained in a 12.7 mm silicon evaporator composed of 135 microchannels with 85 μm wide and 560 μm high channels separated by 46 μm wide fins. There were 35 local heaters and temperature measurements arranged in a 5 × 7 array. The heat transfer results were uniform in the lateral direction to the flow (attributable to the inlet restriction) and a function of the heat flux, vapor quality and mass flux. The steady-state standard deviation of the local base temperature was less than 0.2 °C, inferring that the boiling process was very stable. For wall heat fluxes over 45 kW/m 2, the heat transfer coefficient curves were V-shaped, decreasing for intermittent flow regimes and increasing for annular flow. The three-zone model of Thome et al. (2004) was the best heat transfer prediction method when setting the dryout thickness equal to the channel roughness.

Modelling the impacts of flow regulation on fish distributions in naturally intermittent lowland streams: an approach for predicting restoration responses

Summary1. Understanding the relationships between flow regime and the distribution of biota is critical for managing flows in regulated rivers. In northern Victoria, Australia, efforts are presently underway to restore a natural, intermittent flow regime to several streams which, for over 100 years, have received perennial diversions as part of a stock, irrigation and domestic water supply.2. Bayesian, model‐averaged, binomial regression was used to predict probabilities of occurrence for 13 fish species, including five non‐native species, based on hydrologic variables characterising both the current and modelled future flow regimes at 10 sites representing a range of hydrologic regimes (categorised here as heavily regulated, moderately regulated and unregulated).3. Regression models accurately predicted present probabilities of occurrence for most species across all sites. The models predicted a reduced likelihood of large, native, flow‐dependent species occurring at regulated sites following flow restoration. Predictions regarding the future distribution of widespread species including two small‐bodied native and four exotic species were less certain as current distributions of these widespread species were unrelated to hydrologic variables we examined and thus unlikely to be significantly affected by flow restoration. The distributions of two small native species currently restricted to unregulated sites are predicted to increase throughout the system.4. This study illustrates the effects of artificially induced perennial flow on lowland fish distributions. Furthermore, the combination of pre‐restoration data together with predictive modelling provides valuable insights into the likely outcomes of flow regime shifts.5. This study clearly demonstrates the value of combining empirical research and modelling in guiding environmental flow and ecosystem restoration decisions. Knowledge from the study is now helping guide management decisions and the development of mitigation strategies to protect highly valued species in the system from potential future threats.

Structural and functional characteristics of natural and constructed channels draining a reclaimed mountaintop removal and valley fill coal mine

AbstractMountaintop removal and valley fill (MTR/VF) coal mining has altered the landscape of the Central Appalachian region in the USA. Among the changes are large-scale topographic recontouring, burial of headwater streams, and degradation of downstream water quality. The goals of our study were to: 1) compare the structure and function of natural and constructed stream channels in forested and MTR/VF catchments across ephemeral, intermittent, and perennial flow regimes and 2) assess the relationship between leaf litter breakdown and structural measures, such as the habitat assessments currently used by regulatory agencies. Specific conductance of stream water was, on average, 36 to 57× higher at perennial reaches below valley fills than at perennial reaches in forested catchments, whereas pH was circumneutral in both catchment types. Channel habitat and invertebrate assemblages in litter bags differed between forested streams and constructed channels in VF catchments. Invertebrate density, diversity, a...

Improved flow pattern map for accurate prediction of the heat transfer coefficients during condensation of R-134a in smooth horizontal tubes and within the low-mass flux range

This paper presents an improved flow pattern map for predicting the heat transfer coefficients during condensation of R-134a inside a smooth horizontal tube. Experimental tests were conducted over the low-mass flux range of 75–300 kg/m 2 s, at a nominal saturation temperature of 40 °C, and with the test section vapour qualities ranging from 0.76 down to 0.03. This represents points within the annular, intermittent and stratified flow regimes. The results were used to modify the Thome–El Hajal flow pattern map to include a transition region between the stratified-wavy and annular or intermittent flow regimes. The revised flow pattern-based heat transfer correlation predicted the experimental data to a mean deviation of less than 6%.

The Cenozoic CH 4-derived carbonates of Monferrato (NW Italy): A solid evidence of fluid circulation in the sedimentary column

Two types of methane-derived carbonates, each with different lithological and paleontological signatures, have been distinguished in the Oligocene–Miocene sediments of Monferrato (NW Italy): 1) Irregularly-shaped, metre-sized rock bodies characterized by the widespread occurrence of remains of large bivalves ( Lucina sp.). These bodies include two subtypes, differing in the lithological characteristics of the cemented sediments: a) Lucina-rich marly limestones, encased in Langhian outer shelf marls. They document a slow seepage of CH 4-rich fluids at the paleo-sea floor; b) Lucina-bearing cemented mud breccias, found either as cemented masses in the original stratigraphic position or as redeposited blocks within Upper Messinian chaotic (block-in-matrix) sediments. These carbonate bodies document the activity of submarine mud volcanoes during the late Messinian. In particular, they document intermittent fluid emission ranging from violent eruptive phases, responsible for the extrusion of soft mud breccias at the sea floor, to quiet degassing phases, that promoted colonization by chemosymbiotic communities and localized cementation. 2) Very irregularly shaped strongly cemented, mainly coarse-grained, clastic sediments, reaching the lateral extent of few hundred meters and the thickness of few tens of meters. They occur within Oligocene to Messinian sediments, lack remains of chemosymbiotic taxa and are highly fractured, with fractures filled with sediments and cements. These carbonate-cemented masses, here referred to as macroconcretions, result from the precipitation of carbonate cements within buried sediments flushed by ascending CH 4-rich fluids. They document a complex fluid circulation pattern in the sedimentary column, in which SO 4-rich waters were drawn from surrounding sediments by the fast and prolonged flow of CH 4-rich fluids and promoted anaerobic oxidation of methane and the precipitation of carbonate cements. The polyphase network of clastic dykes and veins that characterize macroconcretions is related to an intermittent fluid flow regime: dykes formed during fast upward flows of sediment-rich fluids whereas carbonate-filled veins formed during slower, degassing phases. The local lithostratigraphic and structural framework allow to correlate genetically and chronologically the macroconcretions to Lucina-bearing mud breccias. Consequently, macroconcretions are here regarded as the result of cementation along fluid conduits feeding Messinian mud volcanoes at the sea floor. The massive release of fluids could have played a role in triggering large-scale mass wasting events responsible for the emplacement of sedimentary mélanges including CH 4-derived rocks in the Messinian.

Examining the ecological consequences of restoring flow intermittency to artificially perennial lowland streams: Patterns and predictions from the Broken—Boosey creek system in northern Victoria, Australia

AbstractThe reinstatement of natural flow regimes is a rapidly emerging issue in river restoration worldwide. In northern Victoria, Australia, efforts are presently underway to restore a natural, intermittent flow regime to several streams which have received perennial diversions for both irrigation and stock and domestic water‐supplies for over 100 years. A pipeline to deliver water to landholders will significantly reduce transmission losses throughout the system allowing irrigation canals and diversion weirs to be decommissioned. The motivation for flow alteration in this system lies primarily in reducing inefficiencies in water delivery which, in turn, will be used to meet escalating demands on water resources. The ecological impact of the flow regime shift on these streams is likely to be substantial. This study utilized an existing artificial hydrological gradient (from perennial to intermittent) in two creek systems, to explore relationships between flow regime and a range of ecological variables. These data provide a benchmark against which to assess ecological changes once flow has been altered and form the basis for predicting changes that can assist future management decisions. Data collected from 10 sites across a strong hydrological gradient detected clear differences in geomorphology, water quality and biotic assemblages (macrophytes, macroinvertebrates and fish). By examining the relationship between flow regime and the distribution of biota we identify both the positive and negative outcomes of restoring naturally intermittent flow regimes within artificially perennial lowland streams. The reinstatement of intermittent flow regimes in artificially perennial streams will continue in many parts of the world as water delivery via these systems becomes increasingly uneconomical. While flow restoration may in principle be regarded as a positive step, these findings emphasize the need to consider fully the ecological consequences of restoring historical hydrological regimes to streams within the context of other human induced catchment disturbances. Copyright © 2009 John Wiley &amp; Sons, Ltd.

Flow characteristics of a small moving bed downcomer with an orifice under negative pressure gradient

Behaviour of gas–solid mixtures in a small moving bed downcomer operating under negative pressure gradient has been investigated experimentally, because flow characteristics in a downcomer are important factors in powder handling processes, such as a multi-stage spouted bed or a circulating fluidized bed. In this study, a small downcomer in which the moving bed height could be controlled was employed with the intention of using a downcomer as a connecting tube in a multi-stage spouted bed. Based on experimental observation, the moving bed flow changed from smooth flow (Region(I)) to slip-stick flow (Region(II)) and then changed from slip-stick flow to intermittent flow (Regime(III)), as the gas pressure gradient in the downcomer was increased. In this study, the critical point for steady-state (Region(I)) operation was also examined experimentally and theoretically, by using eight kinds of solid particles (Geldart B, D and A powders) and six different orifice sizes which were used to restrict solids flow at the base of the downcomer. As a result, a correlation of solids flow rate in the stable moving bed downcomer under negative pressure gradient was derived and the critical point between stable and unstable flow was explained by the proposed model.

Modeling of Pressure Drop During Condensation in Circular and Noncircular Microchannels

This paper presents a multiple flow-regime model for pressure drop during condensation of refrigerant R134a in horizontal microchannels. Condensation pressure drops measured in two circular and six noncircular channels ranging in hydraulic diameter from 0.42mmto0.8mm are considered here. For each tube under consideration, pressure drop measurements were taken over the entire range of qualities from 100% vapor to 100% liquid for five different refrigerant mass fluxes between 150kg∕m2s and 750kg∕m2s. Results from previous work by the authors on condensation flow mechanisms in microchannel geometries were used to assign the applicable flow regime to the data points. Garimella et al. (2005, “Condensation Pressure Drop in Circular Microchannels,” Heat Transfer Eng., 26(3) pp. 1–8) reported a comprehensive model for circular tubes that addresses the progression of the condensation process from the vapor phase to the liquid phase by modifying and combining the pressure drop models for intermittent (Garimella et al., 2002, “An Experimentally Validated Model for Two-Phase Pressure Drop in the Intermittent Flow Regime for Circular Microchannels,” ASME J. Fluids Eng., 124(1), pp. 205–214) and annular (Garimella et al., 2003, “Two-Phase Pressure Drops in the Annular Flow Regime in Circular Microchannels,” 21st IIR International Congress of Refrigeration, International Institute of Refrigeration, p. ICR0360) flows reported earlier by them. This paper presents new condensation pressure drop data on six noncircular channels over the same flow conditions as the previous work on circular channels. In addition, a multiple flow-regime model similar to that developed earlier by Garimella et al. for circular microchannels is developed here for these new cross sections. This combined model accurately predicts condensation pressure drops in the annular, disperse-wave, mist, discrete-wave, and intermittent flow regimes for both circular and noncircular microchannels of similar hydraulic diameters. Overlap and transition regions between the respective regimes are also addressed to yield relatively smooth transitions between the predicted pressure drops. The resulting model predicts 80% of the data within ±25%. The effect of tube shape on pressure drop is also demonstrated.

Adiabatic Two-Phase Flow in Scaled Microchannel Heat Sinks with Cross Links

channels.Furthermore,theresultsshowedthatthereisasignificantimpacton flowdistributionwhencomparedwith thestraightchanneldesign.Flowpatternsobtainedfrom flowvisualizationwerepresentedintermsoffractionaltime function, and provided further insight into flow characteristics. Compared with a single channel flow regime map, the expected intermittent flow regime was observed 84–90% of the time for the cross-linked designs, and 65–80% of the time for the straight channel design. The effects of cross links on two-phase pressure drop are also documented.

Fourier Representation of Intermittent Flow in a Porous Medium

The field measurements and numerical results for intermittent flow regime in a sandy soil show that the time distributions of the soil water flux q(z, t), and the soil water content θ(z, t)at various depths are periodic in nature, where t is time and z is the depth (i.e., at the surface z = 0 and at depths z = − 5, − 10, − 15 cm, etc). The period of q(z, t) and θ(z, t) variations are generally determined by the sum of the duration of pulse and the duration between the initiation of two consecutive pulses of water at the soil surface. Fourier series models have been given for q(z, t) and θ(z, t) variations. The predicted Fourier results for these variations have been compared with the experimentally verified numerical results—designated as observed values. The results show that the amplitudes of these variations were damped exponentially with depth, and the phase shift increased linearly with depth.

Adiabatic two phase flow distribution and visualization in scaled microchannel heat sinks

The present work investigates the characteristics of adiabatic two-phase flow, and compares the flow distribution in two different multi-channel configurations. One is a standard straight channel design, and the second is similar, except that it has two cross-linked paths located at one-third and two-thirds of the channel length. Each test section consists of 45 rectangular channels with a hydraulic diameter of 1.59 mm and is fabricated out of acrylic to enable flow visualization. The test facility operates with water and air as the working fluids. Gas and water superficial velocities range from 0.03 to 9.93 and 0.04 to 0.83 m/s, respectively, corresponding to a flow quality between 0 and 0.25, whereby the mass flux range is from 41 to 834 kg/m2s. Four channels were selected for flow distribution tracking, and the results showed that the straight and cross-linked designs both have unequal flow distributions. Flow patterns are presented in terms of a fractional time function to gain insight into the flow characteristics. Compared to a single channel flow regime map, the intermittent flow regime was observed 65 to 85% of the times expected, whereas the dispersed flow regime only occurred ∼40% of the times expected for both designs. Further discussion on flow distribution and flow patterns are also documented.

Holdup Characteristics of Gas−Non-Newtonian Liquid Flow through Helical Coils in Vertical Orientation

Experimental investigations have been carried out to evaluate the holdup for gas−non-Newtonian liquid flow through helical coils in vertical orientation. The experiments performed using 36 different helical coils and 4 different concentrations of sodium salt of carboxymethyl cellulose (SCMC) as non-Newtonian liquids. The intermittent flow regime was observed in the experiments. The effects of gas and liquid flow rate, coil diameter, helix angle, and liquid properties on the holdup were illustrated. An attempt has been made to fit the experimental holdup data by the Lockhart and Martinelli (Chem. Eng. Prog. 1949, 45, 39−48) correlation and the modified Lockhart and Martinelli correlation as presented by different authors. In another approach, a correlation was developed to predict the holdup as functions of the physical and dynamic variables of the system. Statistical analysis of the correlation suggests that they are of acceptable accuracy.

06/01291 Fault diagnosis and refrigerant leak detection in vapour compression refrigeration systems: Tassou, S. A. and Grace, I. N. International Journal of Refrigeration, 2005, 28, (5), 680–688

Micro Cryogenic Coolers (MCCs) can achieve very small sizes and high efficiencies when operating with a refrigerant mixture, but micro-scale compressors have a limited pressure output. Four refrigerant mixtures were designed to operate between 0.4 MPa and 0.1 MPa, and tested in a MCC system both with and without pre-cooling. For comparison, two pure refrigerants were tested as well. Without pre-cooling, each mixture exhibited considerably lower cooling power than the design value. With pre-cooling, the mixtures exhibited unsteady cooling temperatures accompanied by flow pulsations after a period of time. The low cooling power, unsteady temperatures, and time required for the pulsations to occur are analyzed in terms of composition change due to liquid hold-up in the annular and intermittent flow regimes.

Experimental study on two-phase flow and pressure drop in millimeter-size channels

The present paper provides considerable insight into the structure of two-phase flow as well as its characteristics inside mini- and micro-channels. A new test facility was designed and constructed to investigate the two-phase flow regimes and pressure drop in mini- and micro-channels. The two-phase frictional pressure drop measurements and flow regimes experiments were performed with three different test sections, which are 3 mm, 1 mm, and 800 μm in diameter. The experimental pressure drop data was compared with the homogenous model, the Friedel [L. Friedel, Improved friction pressure drop correlations for horizontal and vertical two-phase pipe flow, Paper E2, European Two-Phase Group Meeting, Ispra, Italy, 1979] model, and the Chisholm [D. Chisholm, A theoretical basis for the Lockhart Martinelli Correlation for two-phase flow, Int. J. Heat Mass Transfer, 10, 1967, pp. 1767–1778] model, in order to determine their validity range in mini- and micro-channel flows. The homogenous model and the Chisholm [D. Chisholm, A theoretical basis for the Lockhart Martinelli Correlation for two-phase flow, Int. J. Heat Mass Transfer, 10, 1967, pp. 1767–1778] model showed the most similarities with the data acquired for the 1 mm and 0.8 mm test sections, while the Friedel [L. Friedel, Improved friction pressure drop correlations for horizontal and vertical two-phase pipe flow, Paper E2, European Two-Phase Group Meeting, Ispra, Italy, 1979] model over-predicted the pressure drop for every test section. The two-phase flow patterns were observed for high gas superficial velocities ( U GS ⩾ 10 m/s) using a 3CCD analog camera, and comparisons were made with the existing flow regime maps. The present data, as well as the available data in literature, were used to develop a simplified flow regime map for horizontal micro-channels. The flow regimes in this map were separated into surface-tension-dominated regimes and inertia-dominated regimes. The regimes were named bubbly, intermittent, churn and annular. The intermittent flow regime regroups all slug and plug flows, as well as all the transition flows occurring in the vicinity of the intermittent region. It was found that the simplified transition lines provide a good approximation of the regime transitions for all existing studies within the allowable range of the map.

Flow-Regime-Based Model for Pressure Drop Predictions in Microchannels

A flow-regime-based pressure drop model is developed to correlate experimental data in the intermittent and annular flow regime. For the intermittent region, an alternative method for microchannel pressure drop predictions based on the average kinetic energy of the mixture is developed. A parameter based in the Weber number, the Lockhart-Martinelli parameter, and the liquid-to-vapor density ratio is proposed for pressure drop predictions in the annular flow region. The two correlations were built on experimental data of R-410A, R-134a, and air-water mixtures with mass fluxes ranging from 50 to 300 kg/(s·m2), with quality ranging from 0 to 1. Six-port and fourteen-port aluminum microchannel tubes with hydraulic diameters of 1.54 mm and 1.02 mm, respectively, were used for the two-phase pressure drop experiments. Entrance/exit pressure drops were directly measured with an additional experiment. The losses due to the entrance/exit zones have been found to exhibit homogeneous flow characteristics, and were correlated to a homogeneous flow parameter based on the kinetic energy of the flow field.

On the drag reduction for the two-phase horizontal pipe flow of highly viscous non-Newtonian liquid/air mixtures: Case of lubricating grease

The prediction of the pressure drop gradient and the evaluation of the drag reduction phenomenon observed during the piping multiphase flow of a lubricating grease/air mixture have been investigated. With this aim, viscous flow tests in rotational rheometers and pressure drop measurements in pipelines have been carried out using different geometries with both smooth and rough surfaces. The Sisko model has been used to predict the pressure drop gradient. The drag ratio, as a function of air flow rate, for highly viscous pastes such as lubricating greases, significantly differs, qualitative and quantitatively, from that found in the literature for other non-Newtonian fluids with viscosities of around 200 times lower. The pressure drop gradient in the intermittent multiphase flow regime can be predicted by modifying the classical approach of Lockhart and Martinelli with an empirical correction factor. An empirical model, with a combination of power-law and sigmoidal-type equations, has been proposed to describe the experimental evolution of the drag ratio as a function of Re L ′ / Re TP ′ . The accuracy of the proposed model has been tested by estimating the classical Fanning friction factor for a non-Newtonian fluid, f = 16/ Re′, once the pressure loss has been corrected with the drag ratio previously obtained.

Liquid fragmentation and intermittent flow regimes in unsaturated fractured media

Flow processes in unsaturated fractures considerably differ from flow in rock matrix because of the dominance of gravitational forces, accentuated by variations in fracture geometry. This gives rise to liquid fragmentation, fingering, and intermittent flow regimes that are not amenable to standard continuum representation. We develop an alternative modeling framework to describe the onset of liquid fragmentation and subsequent flow behavior of discrete liquid clusters. The transition from a slowly growing anchored liquid element to a finger‐forming mobile liquid element is estimated from the force balance between retarding capillary forces dominated by contact angle hysteresis and suspended liquid weight. A model for liquid fragmentation within the fracture plane (smooth and parallel walled fractures) for given a steady input flux and aperture size is developed and tested. Predictions of sizes and detachment intervals of liquid elements are in good agreement with experimental results. The results show that the mass of detached liquid element is only weakly related to flow rate but increases with fracture aperture size. Periodic discharge similar to that experimentally observed is a result of the interplay between capillary, viscous, and gravitational forces. We show that the presence of even a few irregularities in a fracture plane may induce complicated flux patterns downstream. Similar erratic fluxes are observed in studies involving gravity‐driven unsaturated flow.

Condensation Pressure Drop in Circular Microchannels

This paper presents a multiple flow-regime model for pressure drop during the condensation of refrigerant R134a in horizontal microchannels. Two-phase pressure drops were measured in five circular channels ranging in hydraulic diameter from 0.5 mm to 4.91 mm. For each tube under consideration, pressure drop measurements were first taken over the entire range of qualities from 100% vapor to 100% liquid for five different refrigerant mass fluxes between 150 kg/m2-s and 750 kg/m2-s. Results from the previous work by the authors on condensation flow mechanisms in microchannel geometries were used to assign the applicable flow regime to the data points. Pressure drop models for intermittent [1] and annular [2] flow reported earlier by the authors were modified and combined to develop a comprehensive model that addresses the entire progression of the condensation process from the vapor phase to the liquid phase. This combined model accurately predicts condensation pressure drops in the annular, disperse wave, mist, discrete wave, and intermittent flow regimes. Overlap and transition regions between the respective regimes are also addressed using an appropriate interpolation technique that results in relatively smooth transitions between the predicted pressure drops. The resulting model predicts 82% of the data within ± 20%.