Observed Flow Regimes Research Articles

Confinement and vapour production rate influences in closed two-phase reflux thermosyphons Part B: Heat transfer

This work investigates boiling heat transfer for a small scale thermosyphon. Heat transfer results were analysed for three fluids; water, ethanol and HFE-7000. The influence of confinement and vapour mass flux on thermosyphon heat transfer was studied using a fully transparent two-phase closed thermosyphon. By considering the thermal performance of the thermosyphon together with the observed flow regimes, diabatic flow maps were developed. Of particular importance is the reduced pressure of the system which influences the size of the generated vapour bubbles in the evaporator. At low reduced pressures, bubble diameters may be large enough to span the tube diameter and the thermosyphon exhibits confined flow behaviour similar to that of micro-scale convective two-phase flows. Flows of this nature result in oscillatory behaviour and the boiling heat transfer is less effective. The phenomenon can be avoided by increasing the reduced pressure or tube diameter of the system. The most favourable flow regime in the context of thermosyphon heat transfer was found to be churn-type flow. It was found that slug/plug-type flow was not conducive to heat transfer where vapour production rates, and thus latent heat transfer, were relatively low. The flow regime maps relating phase superficial momentum flux the heat transfer coefficient may be used as an aid to the design of thermosyphons for specific applications.

Confinement and vapour production rate influences in closed two-phase reflux thermosyphons Part A: Flow regimes

This study investigates the boiling regimes in a small diameter (D = 8 mm) transparent thermosyphon. The influence of confinement on the boiling regimes was studied using a range of fluids. The confinement of the vapour phase leads to boiling regimes that differ from those traditionally described for thermosyphon evaporator boiling physics, widely considered as a combination of pool boiling and film evaporation. The boiling behaviour of small dimension thermosyphons was investigated by designing and constructing a fully transparent thermosyphon, enabling simultaneous thermal and visual analysis. Three working fluids, water, ethanol and HFE-7000, were used to characterise the thermosyphon behaviour with varied characteristic bubble length scales. The observed flow regimes could be characterised in terms of the degree of confinement and rate of vapour production. A flow regime map was developed based on these observations to predict thermosyphon flow in terms of both confinement and the rate of vapour production. It was determined that for low confinement and high rates of vapour production, the boiling regimes resemble those of pool boiling. In contrast, at high levels of confinement and high heat flux, an unsteady regime exists where relatively large bubbles and vapour generation rates result in a pulsatile geyser-type flow regime.

A mixture theory approach to model co- and counter-current two-phase flow in porous media accounting for viscous coupling

Abstract It is well known that relative permeabilities can depend on the flow configuration and they are commonly lower during counter-current flow as compared to co-current flow. Conventional models must deal with this by manually changing the relative permeability curves depending on the observed flow regime. In this paper we use a novel two-phase momentum-equation-approach based on general mixture theory to generate effective relative permeabilities where this dependence (and others) is automatically captured. In particular, this formulation includes two viscous coupling effects: (i) Viscous drag between the flowing phases and the stagnant porous rock; (ii) viscous drag caused by momentum transfer between the flowing phases. The resulting generalized model will predict that during co-current flow the faster moving fluid accelerates the slow fluid, but is itself decelerated, while for counter-current flow they are both decelerated. The implications of these mechanisms are demonstrated by investigating recovery of oil from a matrix block surrounded by water due to a combination of gravity drainage and spontaneous imbibition, a situation highly relevant for naturally fractured reservoirs. We implement relative permeability data obtained experimentally through co-current flooding experiments and then explore the model behavior for different flow cases ranging from counter-current dominated to co-current dominated. In particular, it is demonstrated how the proposed model seems to offer some possible interesting improvements over conventional modeling by providing generalized mobility functions that automatically are able to capture more correctly different flow regimes for one and the same parameter set.

Boiling of multicomponent working fluids used in refrigeration and cryogenic systems

Working fluids based on mixtures are widely used in cryogenic and refrigeration engineering. One of the main elements of low-temperature units is a recuperative heat exchanger where the return flow cools the direct (cold regeneration is carrying out) resulting in continuous boiling and condensation of the multicomponent working fluid in the channels. The temperature difference between the inlet and outlet of the heat exchanger can be more than 100K, which leads to a strong change in thermophysical properties along its length. In addition, the fraction of the liquid and vapor phases in the flow varies very much, which affects the observed flow regimes in the heat exchanger channels. At the moment there are not so many experimental data and analytical correlations that would allow to estimate the heat transfer coefficient during the flow of a two-phase mixture flow at low temperatures. The work is devoted to the study of the boiling process of multicomponent working fluids used in refrigeration and cryogenic engineering. The description of the method of determination of heat transfer coefficient during boiling of mixtures in horizontal heated channel is given as well as the design of the experimental stand allowing to make such measurements. This stand is designed on the basis of a refrigeration unit operating on the Joule-Thomson throttle cycle and makes it possible to measure the heat transfer coefficient with a good accuracy. Also, the calculated values of the heat transfer coefficient, obtained with the use of various correlations, are compared with the existing experimental data. Knowing of the heat transfer coefficient will be very useful in the design of heat exchangers for low-temperature units operating on a mixture refrigerant.

A model for non-Newtonian flow in porous media at different flow regimes

Polymeric liquids are of great practical importance for porous media flow as they can be used to improve the sweep of water in the reservoir and therefore improve the recovery of oil. Due to the non-Newtonian behavior of these liquids, they are extremely challenging to model. In this paper, we present a model that is capable of describing the most commonly observed flow regimes in porous media: (i) Newtonian, (ii) Shear thinning, (iii) Shear thickening, and (iv) Mechanical degradation. The novel feature of our model is that the time constants for the shear thinning and shear thickening behavior are related to variations in reservoir properties and conditions, thus making it possible to translate lab results to larger scale without introducing new fitting parameters. Furthermore, we present a way to estimate polymer mechanical degradation in porous media. In our model, the polymer degradation rate is linked to the effective pore radius (using a Kozeny-Carman type equation), shear stress, and polymer molecular weight, M w . The degradation results in a lower M w , while the polymer volumetric concentration is unaffected. The model is applied to a series of laboratory core flood experiments conducted with partially hydrolyzed polyacrylamide, HPAM, of different initial M w ranging from 5 to 20 MDa in seawater, and core permeability varied from 137 to 2019 mD. The flow rate is varied approximately three orders of magnitude and covers the shear thinning, shear thickening, and degradation flow regimes. We show that our model is able to reproduce experimental rate-dependent flow resistance, as well as viscosity of effluent samples. An important aspect supporting the use of the model as a predictive tool is that all the simulations with a given brine have made use of a single set of input parameters to describe the observed shear thickening and degradation behavior. Simulation of a second experimental series using low salinity brine required a separate set of input parameters for the shear thickening and shear degradation. The onset of shear thickening was not affected while shear thickening was reduced and degradation appeared to be slower.

Identification of flow regime in a cocurrent gas – Liquid upflow moving packed bed reactor using gamma ray densitometry

Flow regime identification in upflow moving packed bed reactors with the conical bottom was investigated using non-invasive gamma-ray densitometry (GRD) measurement technique. Time domain (Standard Deviation), and state space or chaotic methods (Kolmogorov Entropy) are employed on photon count time series of GRD to determine flow regime. The experiments were performed on a pilot plant scale upflow packed bed reactor made of Plexiglas column of 27.94cm I.D and a total height of 118cm including plena, and it is packed randomly with 0.3cm diameter catalyst till 79cm including the conical bottom. Two axial and various radial position are selected to conduct GRD scanning. The measurements are conducted at superficial liquid (water) velocity 0.017cm/s and superficial gas (air) velocity in the range of 0.6–7.7cm/s under these conditions catalyst bed behaves as packed bed. All analysis showed similar flow regime trend, with observed flow regime as bubbly and pulse flow, when compared with flow regime map for upflow packed bed.

Phase transfer catalysis in a microchannel: Paradoxical effect of transition from kinetic control to mass transfer control

The high surface area per unit volume available in microfluidic devices has led to their use in process intensification of mass transfer limited reactions. The commonly observed flow regimes parallel and slug flows, play a major role in determining the performance of these systems. The strong internal circulations and high interfacial area per unit volume makes the slug flow regime a preferred regime for mass transfer limited reactions. The reaction between benzyl alcohol and sodium hypochlorite to yield benzaldehyde via phase transfer catalysis (PTC) is chosen as a test reaction system. Batch experiments were carried out at three different pH values viz. 10.4, 8.4 and 7.4. The rate of reaction decreased as the medium became alkaline. A mathematical model where the species in the aqueous phase is considered to be in equilibrium was developed. In the slug flow regime of the microchannel the yield of the reaction is found to increase with an increase in organic phase residence time at high pH. The yield however decreased with residence time at low pH. This paradoxical behavior is attributed to a shift in the system from the kinetic control regime to a mass transfer control regime. The effect of residence time, mass transfer time and reaction time on the test reaction is analyzed in this work. Criteria in terms of dimensionless numbers have been proposed to determine if a system is kinetically controlled or mass transfer controlled.

Assessing impacts of climate change and river regulation on flow regimes in cold climate: A study of a pristine and a regulated river in the sub-arctic setting of Northern Europe

River regulation and climate change are two major causes of river regime alteration that are difficult to separate, and thus the individual effect of each is not always clear. To demonstrate the relative effect of regulation practices and observed climate change, we present results from a spatio-temporal study of two adjacent rivers in Northern Europe, the Kemijoki (regulated) and the Tornionjoki (pristine), with similar climate and catchment conditions. Spatial and temporal analysis was performed using daily hydrologic data from 11 gauging stations on these rivers. Flow regime alteration analysis showed that both rivers had altered significantly on a daily scale, but that there were low impacts on flow regime on a monthly scale. Daily variation in natural stream flow regime for time period of 2001–2015 was altered in both rivers when compared to the discharge during 1952–1966, but the degree of hydrologic alteration (DHA) of daily flow values for the winter months was significantly higher in the case of the regulated Kemijoki river. For river Tornionjoki, changes and variations related to climate seems to be the reason for the observed flow regime change with increased mean monthly flow for August and December. An increase in annual maximum and decrease in minimum flows was also observed in Tornionjoki, these changes however were smaller than overall changes in the regulated river Kemijoki. Based on the degree of alteration (Indicators of Hydrologic Alteration (IHA) analysis), both climate change and regulation operations affected the river regimes. About 50% of total change in observed range variability approach (RVA) in the river Kemijoki was estimated to result from regulation practices. This was confirmed by analysing data from Ounasjoki which is the pristine tributary of Kemijoki. The extent of alteration in both rivers was dependent on the spatial position of the gauging station within the river network. The impact of regulation on downstream reaches of the regulated river decreased with the increase in catchment area around that part of the river. Thus integrating daily and monthly flow data from varied spatial location on a river network, offers a useful tool for analysing flow regime conditions.

On the pressure-saturation path in infinite-acting unconventional liquid-rich gas reservoirs

Unconventional liquid rich natural gas reservoirs exhibit a number of unique technical challenges, including ultra-low rock permeability, which can lead to prolonged periods of infinite-acting flow. Despite this, traditional production data analysis techniques often assume boundary-dominated conditions, which may not be experienced by unconventional systems for very long stretches in their productive life. Analysis of two-phase flow and well production often relies on the use of two-phase pseudo-variables calculations for which the fluid saturation-pressure relationship must be estimated a priori. The de-facto assumption is that such a priori reservoir saturation-pressure dependency can be acquired from available constant volume depletion (CVD) or constant composition expansion (CCE) laboratory test results. As a result, the influence of reservoir characteristics or drawdown conditions on saturation-pressure path has not been rigorously investigated in general; and even less for the particular case of infinite-acting reservoir systems under linear flow regime, which is the most commonly observed flow regime in multi-fractured horizontal wells. In the present work, a recently developed semi-analytical solution for the multiphase black-oil formulation, which considers both pressure and liquid saturation as independent variables, is employed to investigate the influence of reservoir and well conditions on saturation-pressure path in linear infinite-acting liquid rich gas systems. Here, the effects of constant bottomhole flowing pressure, initial reservoir pressure, and relative permeability characteristics on pressure-saturation path are explored in a reservoir system. Results are compared to constant composition expansion (CCE) and constant volume depletion (CVD) tests, which are routinely invoked to estimate saturation-pressure path, and it is demonstrated that these tests provide very poor estimations of saturation-pressure path for all cases considered. In addition, saturation-pressure paths exhibit significant sensitivity to the parameters investigated, as a result of the significant interplay between liquid dropout and phase mobility. Finally, following the results of the sensitivity study, we use the concept of the compositionally-extended black-oil formulation to further study the physical mechanisms that control saturation-pressure path behavior, which are elucidated through an investigation of the in situ and flowing compositions of surface gas and surface oil pseudocomponents. The results of this paper indicate that traditional methods for estimation of the saturation-pressure path in liquid-rich unconventional systems cannot account for the influence of system parameters on liquid-phase dropout and condensate accumulation. Furthermore, production data analysis techniques must account for these dependencies to develop reliable reservoir forecasts and reserve estimations.

Multi-metric calibration of hydrological model to capture overall flow regimes

Summary Flow regimes (e.g., magnitude, frequency, variation, duration, timing and rating of change) play a critical role in water supply and flood control, environmental processes, as well as biodiversity and life history patterns in the aquatic ecosystem. The traditional flow magnitude-oriented calibration of hydrological model was usually inadequate to well capture all the characteristics of observed flow regimes. In this study, we simulated multiple flow regime metrics simultaneously by coupling a distributed hydrological model with an equally weighted multi-objective optimization algorithm. Two headwater watersheds in the arid Hexi Corridor were selected for the case study. Sixteen metrics were selected as optimization objectives, which could represent the major characteristics of flow regimes. Model performance was compared with that of the single objective calibration. Results showed that most metrics were better simulated by the multi-objective approach than those of the single objective calibration, especially the low and high flow magnitudes, frequency and variation, duration, maximum flow timing and rating. However, the model performance of middle flow magnitude was not significantly improved because this metric was usually well captured by single objective calibration. The timing of minimum flow was poorly predicted by both the multi-metric and single calibrations due to the uncertainties in model structure and input data. The sensitive parameter values of the hydrological model changed remarkably and the simulated hydrological processes by the multi-metric calibration became more reliable, because more flow characteristics were considered. The study is expected to provide more detailed flow information by hydrological simulation for the integrated water resources management, and to improve the simulation performances of overall flow regimes.

Modelling population responses to flow: The development of a generic fish population model

Many fish species are dependent upon flows to trigger breeding, facilitate high recruitment of offspring, and to maintain adult survival rates. Understanding how fish populations respond to different flow regimes is important in regulated waterways as subtle changes in regimes have the potential to influence both fish breeding and survival. In this paper, we describe an age-structured population response model that explores how quantitative changes in the flow regime can lead to changes in fish population size and structure through time. We use three large bodied fish species (golden perch, Murray cod and the invasive common carp) from the mid Murray River near Barmah-Millewa Forest to explore the possible responses to the observed flow regime over a 30-year period. The model links flow volumes, seasonality, temperature and rates of fall to the fecundity and survival rates for the different fish species to project population change through time.

Flows produced by the combined oscillatory rotation and translation of a circular cylinder in a quiescent fluid

AbstractFlows produced by a circular cylinder undergoing oscillatory rotation and translation in a quiescent fluid have been studied via direct numerical simulations. The incompressible Navier–Stokes equations were solved for large dimensionless time windows using an immersed boundary method with adaptive Cartesian grid refinement. Parametric studies were conducted in two dimensions on the Reynolds number, Keulegan–Carpenter number and phase shift. In addition to the previously reported net thrust case (Blackburn et al., Phys. Fluids, vol. 11, 1999, pp. 4–6), the study catalogued the appearance of several streaming jet regimes with varying deflection angles, deflected and horizontal vortex shedding regimes, and a double mirrored jet regime with varying inter-jet angles, as well as several chaotic cases. Visualizations are presented to clarify each observed flow regime and to illustrate the parameter space. Connections are drawn between these canonical bluff-body deflected wakes and a similar phenomenon observed in aerofoils oscillating at high reduced frequencies in a cross-flow. Also, the discovery of the streaming jet regimes with varying deflection angles opens the door for using these flows as a low-Reynolds-number propulsive mechanism requiring only a two-degree-of-freedom actuator. Simulation results suggest that the flow phenomena observed in two dimensions persist in three dimensions, despite spanwise fluctuations.

Turbulent ‘stopping plumes’ and plume pinch-off in uniform surroundings

Observations of turbulent convection in the environment are of variously sustained plume-like flows or intermittent thermal-like flows. At different times of the day the prevailing conditions may change and consequently the observed flow regimes may change. Understanding the link between these flows is of practical importance meteorologically, and here we focus our interest upon plume-like regimes that break up to form thermal-like regimes. It has been shown that when a plume rises from a boundary with low conductivity, such as arable land, the inability to maintain a rapid enough supply of buoyancy to the plume source can result in the turbulent base of the plume separating and rising away from the source. This plume ‘pinch-off’ marks the onset of the intermittent thermal-like behavior. The dynamics of turbulent plumes in a uniform environment are explored in order to investigate the phenomenon of plume pinch-off. The special case of a turbulent plume having its source completely removed, a ‘stopping plume’, is considered in particular. The effects of forcing a plume to pinch-off, by rapidly reducing the source buoyancy flux to zero, are shown experimentally. We release saline solution into a tank filled with fresh water generating downward propagating steady turbulent plumes. By rapidly closing the plume nozzle, the plumes are forced to pinch-off. The plumes are then observed to detach from the source and descend into the ambient. The unsteady buoyant region produced after pinch-off, cannot be described by the power-law behavior of either classical plumes or thermals, and so the terminology ‘stopping plume’ (analogous to a ‘starting plume’) is adopted for this type of flow. The propagation of the stopping plume is shown to be approximately linearly dependent on time, and we speculate therefore that the closure of the nozzle introduces some vorticity into the ambient, that may roll up to form a vortex ring dominating the dynamics of the base of a stopping plume.

New Techniques in Interpretation of Closure Pressure in the Montney Formation

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 163825, ’Interpretation of Closure Pressure in the Unconventional Montney Using PTA Techniques,’ by Robert V. Hawkes, Pure Energy Services; Irene Anderson, Talisman Energy; R.C. Bachman, Taurus Reservoir Solutions; and A. Settari, SPE, University of Calgary, prepared for the 2013 SPE Hydraulic Fracturing Technology Conference, The Woodlands, Texas, USA, 4-6 February. The paper has not been peer reviewed. The Montney formation in Canada is one of the largest resource plays in North America. Horizontal multistaged fracturing is the best method for developing this vast resource. Before hydraulic fracturing of the wellbore, the toe stage is frequently minifractured to obtain reservoir and geomechanical properties. Various pressure-transient-analysis (PTA) -based interpretation techniques have been introduced to the industry over the last few years for the determination of closure pressure. From a theoretical viewpoint, unification of the fields of traditional PTA and minifracture interpretation has been achieved. Introduction Despite low natural-gas prices, the Montney- play region of northeast British Columbia remains one of the most active of such plays in North America. Operators continue to push drilling activity in the Montney, with increased liquids production within many portions of the trend. Characterization techniques and stimulation and production mechanisms continue to make economics favorable. Prefracture diagnostic testing is an approach that regulators and operators are now more commonly using in unconventional and tight formations. Industry’s current use of minifracture analysis for the determination of fracture closure and after-closure reservoir properties, however, still presents several challenges. The conventional approach to all minifracture analysis is the 1D Carter leakoff model, which leads directly to the concept of G time. For more than 30 years, G time (or the G-function) has played the dominant role for the determination of closure stress, but there remains ambiguity in performing minifracture analysis. Part of the problem is that the recommended plots do not rigorously identify the various flow regimes that occur during a minifracture/falloff. Minifracture analysis requires a general theory that accounts for all of the actual observed flow regimes (both before and after closure). The objective of this paper is to create and perform a consistent workflow process of four Montney diagnostic injection tests (one vertical and three horizontal wells) on the same pad, with a systematic approach highlighting the characteristic flow-regime slopes when using the Bourdet log-log derivative plot. In brief, the Bourdet derivative function used in the PTA-based log-log derivative approach accounts for rate variation before the analyzed shut-in period. The workflow we present is also dependent upon plotting the primary-pressure derivative (PPD) in combination with the Bourdet log-log derivative plot.

Experimental investigation of natural convection about drift-emplaced waste canisters

Cylindrical waste canisters placed horizontally in the underground drifts of a nuclear repository will emit decay heat by convection to the surrounding air and by radiation heat transfer. In drifts with no forced air circulation, natural convection about the waste canisters will play an important role in controlling the temperature distribution inside the canisters and on the drift wall. Experiments were conducted to visualize the flow about model canisters and to measure the steady-state temperature distribution about the canister. The results show an enhancement in heat transfer when compared to heat flow between infinitely long eccentric horizontal cylinders. A discussion of the observed flow regimes as a function of Rayleigh number is provided to describe the various experimental results.

Experimental Study of Two-Phase Flow Regime and Pressure Drop in a Particulate Bed Packed with Multidiameter Particles

This paper documents an experimental study on two-phase flow regimes and frictional pressure drop characteristics in a particulate (porous) bed packed with multidiameter (1.5-, 3-, and 6-mm) glass spheres. The experimental results provide new data to validate/develop hydrodynamic models for coolability analysis of debris beds formed in fuel-coolant interactions during a postulated severe accident. The POMECO-FL test facility is employed to perform the experiment, with the spheres packed in a test section of 90 mm diameter and 635 mm height. The pressure drops are measured for air/water two-phase flow through the packed bed, and flow patterns are obtained by means of visual observations. Meanwhile, local void fraction in the center of the bed is measured by a microconductive probe.The experimental results show that the frictional pressure drop of single-phase flow through the bed can be predicted by the Ergun equation, if the area mean diameter of the particles is chosen in the calculation. Given the so-determined effective particle diameter, the estimation of the Reed model for two-phase flow pressure gradient in the bed has a good agreement with the experimental data. The characteristics of the local void fraction can be used to predict flow pattern and mean void fraction. It is observed that slug flow prevails when the mean void fraction is <0.5, whereas annular flow dominates after the mean void fraction is >0.7. If the effective particle diameter is further used as an influential parameter in flow pattern identification, the observed flow regimes of two-phase flow in porous media are well predicted by the existing flow pattern map.

Comparison of Co-Current and Counter-Current Flow Fields on Extraction Performance in Micro-Channels

Several applications such as liquid-liquid extraction in micro-fluidic devices are concerned with the flow of two immiscible liquid phases. The commonly observed flow regimes in these systems are slug-flow and stratified flow. The latter regime in micro-channels has the inherent advantage that separation of the two liquids at the exit is efficient. Recently extraction in a stratified counter-current flow has been studied experimentally and it has been shown to be more efficient than co-current flow. An analytical as well as a numerical method to determine the steady-state solution of the corresponding convection-diffusion equation for the two flow-fields is presented. It is shown that the counter-current process is superior to the co-current process for the same set of parameters and operating conditions. A simplified model is proposed to analyse the process when diffusion in the transverse direction is not rate limiting. Different approaches to determining mass transfer coefficient are compared. The concept of log mean temperature difference used in design of heat exchangers is extended to describe mass transfer in the system.

Analyzing Production Data From Tight Oil Wells

Summary Performance prediction of wells producing from tight (microdarcy) formations is a daunting task. Complexities of geology (the presence/absence of naturally occurring fractures and contribution from different lithological layers), completion and fracture geometry complexities (multiple transverse or longitudinal fractures in long horizontal boreholes), and two-phase flow are impediments to simple performance forecasting. We demonstrate the use of various analytical and numerical tools to learn about both short- and long-term reservoir behaviours. These tools include (a) traditional decline-curve analysis (Arps 1945), (b) Valko's stretched-exponential (SE) method (Valko 2009), (c) the Ilk et al. (2008, 2010) power-law exponential (PLE) method, (d) rate-transient-analysis (RTA) and transient-PI analyses to ascertain the stimulated-reservoir volume (SRV), and (e) numerical-simulation studies to gain insights into observed flow regimes. The benefits of collective use of analytical modelling tools in history matching and forecasting both short- and long-term production performance of tight oil reservoirs are demonstrated with the use of real and simulated data. Diagnosing natural fractures, quantifying stimulated-reservoir volume, and assessing reliability of future performance predictions all became feasible by using an ensemble of analytical tools.

Experimental investigation of flow boiling characteristics in a cross-linked microchannel heat sink

This paper experimentally investigates flow boiling characteristics in a cross-linked microchannel heat sink at low mass fluxes and high heat fluxes. The heat sink consists of 45 straight microchannels each with a hydraulic diameter of 248 μm and heated length of 16 mm. Three cross-links, of width 500 μm, are introduced in the present microchannel heat sink to achieve better temperature uniformity and to avoid flow mal-distribution. Flow visualization, flow instability, two-phase pressure drop, and two-phase heat transfer measurements are conducted using the dielectric coolant FC-72 over a range of heat flux from 7.2 to 104.2 kW/m 2, mass flux from 99 to 290 kg/m 2 s, and exit quality from 0.01 to 0.71. Thermochromic liquid crystals are used in the present study as full-field surface temperature sensors to map the temperature distribution on the heat sink surface. Flow visualization studies indicate that the observed flow regime is primarily slug. Visual observations of flow patterns in the cross-links demonstrate that bubbles nucleate and grow rapidly on the surface of the cross-links and in the tangential direction at the microchannels’ entrance due to the effect of circulations generated in those regions. The two-phase pressure drop strongly increases with the exit quality, at x e,o < 0.3, and the two-phase frictional pressure drop increases by a factor of 1.6–2 compared to the straight microchannel heat sink. The flow boiling heat transfer coefficient increases with increasing exit quality at a constant mass flux, which is caused by the dominance of the nucleation boiling mechanism in the cross-link region.

Experiments on the generation of internal waves over continental shelf topography

Experiments were performed to examine the generation of internal waves by a barotropic tide forcing a continuously stratified fluid over idealized continental shelf/slope topography. A range of responses was observed, including the generation of both internal wave beams and boundary layer boluses, primarily dependent on the values of both the Reynolds number and the topographic steepness parameter. The formation of beams required a critical bottom slope, whilst for bolus formation a large vertical fluid excursion was necessary. A bolus formed when the non-dimensional vertical excursion parameter ΔhN/W0 &gt; 3.2. Here Δh is the vertical excursion, N is the buoyancy frequency and W0 is the near-bottom vertical velocity associated with the local depth-averaged velocity. We simplified the classification of the observed flow regimes using a generation parameter G, defined as the ratio of a Reynolds number to the topographic steepness parameter. The estimated flow regime boundaries were: for G &lt; 3 only a beam was observed, for 3 &lt; G &lt; 50 there was a transitional regime with both a beam and a bolus observed, for 50 &lt; G &lt; 400 there was another transitional regime with no beam but a bolus observed, and finally for the regime with G &gt; 400 there was no bolus observed. We estimated that approximately 4% of the barotropic energy was converted to baroclinic energy when beams were generated.