Main Flow Regimes Research Articles

Hydraulic performance of sluice gate with unloaded upstream rotor

This study presents video analysis of the hydraulic performance of a sluice gate with an unloaded upstream built-in rotor. A number of laboratory experiments were conducted using two unloaded rotor shapes. The first was the cross-shaped rotor and the second was the Savonius-like rotor. A new video analysis technique was introduced for measuring rotor angular speed and its perturbation. Swift speed cameras and Tracker software were used to measure the upstream backwater depth and to estimate the instantaneous variation of the rotor speed. The study shows that adding a rotor upstream of the gate caused the upstream water level to increase such that the averaged normalized afflux increased to 1.72 and 0.9 for the cross-shaped and the Savonius rotors, respectively. Lab experiments indicated that the water flow–structure interaction for the sluice-rotor is quite complex and nonlinear. Two main flow regimes were distinguished. The flow regimes are: the flow through a rotor with possible weir flow conditions and the orifice flow conditions. The time-averaged angular speed of the tested Savonius-like rotor ranged between 0 and 300 r/min. As the upstream backwater depth increased, the angular speed increased; however, the rate was significantly lower for the orifice flow condition compared to the flow under rotor and weir flow conditions. The video analysis also indicated that significant perturbation exists for the rotor angular speed. The normalized perturbation intensity varied from a minimum of 8% to a maximum of 60%. Keywords : sluice gate, rotor, angular speed, video analysis, hydropower

Effect of gas sparger design on bubble column hydrodynamics using pure and binary liquid phases

It is known that the fluid dynamics and transport phenomena in bubble columns depend mainly on the bubble column design (i.e., the column diameter, aspect ratio, and gas sparger openings) and the liquid phase properties. In this communication, we contribute to present-day discussion through an experimental study concerning the combined effects of the gas sparger design and liquid phase properties on both the gas holdup and the main flow regime transition. The experimental study concerning gas holdup measurements was conducted in a large-diameter and large-scale bubble column (with a height of 5.3 m and inner diameter of 0.24 m) operated in the batch mode. Air was used as the dispersed phase (using gas superficial velocities in the range 0.004–0.20 m/s), and various water–monoethylene glycol (MEG) solutions were employed as binary liquid phases. The water–MEG solutions tested have viscosities between 0.9 mPa·s and 7.97 mPa·s, densities between 997.086 kg/m3 and 1094.801 kg/m3, and surface tension between 0.0715 N/m and 0.0502 N/m. Two gas spargers were tested: (a) a spider sparger (“coarse gas sparger”) and (b) a needle sparger (“fine gas sparger”). The former produced a poly-dispersed homogeneous flow regime resulting in a concave gas holdup curve, whereas the latter produced a mono-dispersed homogeneous flow regime resulting in an S-shaped gas holdup curve. It was observed that the mono-dispersed bubble size distribution stabilized the homogeneous flow regime. The addition of MEG produced different effects depending on the gas sparger design. The addition of MEG in the “coarse gas sparger” configuration produced what is usually referred to as “dual effect of viscosity”: depending on the MEG concentration, the homogeneous flow regime was stabilized/destabilized, and thus, the gas holdup increased/decreased. Conversely, the addition of MEG in the “fine gas sparger” changed the shape of the gas holdup curve from an S-shape to concave, thus rendering it similar to the ones produced by “coarse gas sparger”. We speculate that viscous solutions reduce the influence of the inlet conditions in large-diameter and large-scale bubble columns; this is a matter of future research.

Experiment investigation of two-phase flow pattern transition and frictional pressure drop of R600a in a horizontal tube

With the growing concern of environment problems, there are urgent demands for environment-friendly refrigerants in the refrigeration industry. As a natural refrigerant, R600a not only has zero ozone depletion potential and quite low global warming potential, but also has better cooling performance than other refrigerants. Due to its good cooling performance and environment-friendly characteristics, R600a has been used as an alternative refrigerant in heat transfer applications such as refrigerators, freezers, and heat pumps. It is known that accurate prediction of two phase flow pattern transition and frictional pressure drop of R600a in a horizontal tube play an important role in design and optimization of refrigerators, freezers, and heat pumps. According to this issue, a detailed experimental study was carried out to investigate the two-phase flow pattern transition and frictional pressure drop characteristics of R600a in a smooth horizontal tube with an inner diameter of 6 mm. The experiments were performed at conditions covering saturation temperature from 282.4 to 304 K, mass fluxes from 67 to 194 kg/(m2 s). Based on a high speed camera, four main flow regimes can be observed: plug flow, stratified-wavy flow, slug flow and annular flow. The flow map of R600a has been drawn and comparisons between the experimental data of flow patterns and transition lines in the literature have been made. The results showed that with the increase of mass flux, the inception of the annular flow regime occurs earlier. The intermittent flow regime takes place over a narrow range of vapor quality and annular flow regime occupies a wide range of vapor quality. In addition, the trend of Barbieri and Ong and Thome I / A transitions lines are consistent with the experimental data. However, both of them overestimate the inception of the annular flow regime. After taking the influence of vapor inertia, liquid viscous and surface tension forces into account, a modified Weber number We * has been introduced to the new flow pattern transition model. Based on the relationship of X tt and We *, a new correlation has been proposed and it can well fit with the experimental data of flow pattern. Furthermore, a detailed discuss of frictional pressure drop of annular flow and non-annular flow has been made. The results indicated that frictional pressure drop increases with the increases of vapor quality. The tendency of frictional pressure drop presents different the changing laws in different flow regime. The frictional pressure drop increases slowly in the non-annular flow regime while it increases rapidly when the flow pattern turns into the annular flow regime. The reason is that the vapor-phase pressure drop often dominates the total two-phase frictional pressure drop. The annular flow regime owns higher void fraction than the non-annular flow regime. Nine classic prediction correlations of pressure drop were evaluated and the results showed that the correlation of Gronnerud and Muller-Steinhagen and Heck give the best fit to the experimental frictional pressure drop of annular flow and non-annular flow with a mean absolute relative deviation of 38.5% and 19.7% respectively.

Viscoplastic fluid displacement flows in horizontal channels: Numerical simulations

We study numerically isoviscous displacement flows of a Bingham fluid by a Newtonian fluid in a horizontal plane channel. We consider a heavy-light displacement flow with a laminar imposed flow. We analyze the flow from various perspectives, using the three dimensionless parameters that largely describe the flow: the Bingham number (Bn), the densimetric Froude number (Fr) and the Reynolds number (Re). In particular, we are able to categorize three classes for the main flow regimes, viz., center-type and slump-type, no-back-flow and temporary-back-flow, and stable and unstable displacements, for which we also quantify their transition boundaries versus the aforementioned dimensionless numbers. We show that the appearance of unstable flows can eventually lead to a number of different flow patterns, such as sinusoidal shaped wave and periodic-detachment forms. In addition, we study in depth an interesting feature exclusively associated to viscoplastic fluid displacements, i.e., the formation of static residual layers on the upper/lower walls of the channel. We also study the evolution of the key displacement flow features, i.e., the trailing and leading fronts, and classify certain secondary flow features related to the leading front, such as plug-like, inertial tip, semi-detached and fully-detached fronts. Finally, we quantify the velocity of the leading front at long times.

Condensation flow patterns and heat transfer in horizontal microchannels

An experimental investigation was carried out to study the effect of refrigerant mass flux, local vapour quality, coolant flow rate and inlet coolant temperature on the local condensation heat transfer coefficient. Flow visualization was also conducted to capture flow patterns during flow condensation using a high-speed camera integrated with a microscope. HFE-7100, a dielectric and eco-friendly refrigerant was used in rectangular multi-microchannels with a hydraulic diameter of 0.57mm. Experiments were performed at a saturation temperature of 60°C, mass flux range 48–126kg/(m2s), coolant flow rate range 0.5–1.1L/min and inlet coolant temperature range 20–40°C. The results showed that the local condensation heat transfer coefficient increases with increasing mass flux and decreases with decreasing local vapour quality. A negligible effect of the coolant side conditions, saturation-to-wall temperature difference, on the local condensation heat transfer coefficient was found. The main flow regime was annular flow, while slug and bubbly flow were found at some operating conditions. The experimental results were compared with the existing correlations for heat transfer rates. Also, two existing flow pattern maps, for conventional and mini/microchannels, were used to compare the current flow pattern results.

Pressure transient behavior of a fractured well in multi-region composite reservoirs

Although there has been some research on the pressure transient behavior of fractured wells in radial composite reservoirs, the analysis and modeling of such systems remain limited. Most existing solutions only consider the case of a fractured well located at the center of an infinite bi-zonal radial composite reservoir. In addition, corresponding research on a partially penetration fractured well in a radial composite reservoir is also rare. In this paper, we present an analytical three-dimensional solution for a point source located at an arbitrary position in the first circular region of a multi-region radial composite reservoir with three side boundary conditions. Then, using the principles of discretization and superposition, a general pressure transient solution of a fully penetrating fractured well or a partially penetrating fractured well in reservoirs is obtained. The standard log–log type curves of the transient pressure are plotted. Sequentially, the main flow regimes are identified and the effects of relevant parameters are analyzed. There is a special flow regime (the first transition flow regime) in the type curves due to the well not being at the center of the circular region. The well location has a significant effect on the mid-term response of the transient pressure. The presented solution can also be used in well testing to obtain reservoir parameters.

Beta-plane turbulence above monoscale topography

Using a one-layer quasi-geostrophic model, we study the effect of random monoscale topography on forced beta-plane turbulence. The forcing is a uniform steady wind stress that produces both a uniform large-scale zonal flow $U(t)$ and smaller-scale macroturbulence characterized by standing and transient eddies. The large-scale flow $U$ is retarded by a combination of Ekman drag and the domain-averaged topographic form stress produced by the eddies. The topographic form stress typically balances most of the applied wind stress, while the Ekman drag provides all of the energy dissipation required to balance the wind work. A collection of statistically equilibrated numerical solutions delineate the main flow regimes and the dependence of the time average of $U$ on parameters such as the planetary potential vorticity (PV) gradient $\unicode[STIX]{x1D6FD}$ and the statistical properties of the topography. We obtain asymptotic scaling laws for the strength of the large-scale flow $U$ in the limiting cases of weak and strong forcing. If $\unicode[STIX]{x1D6FD}$ is significantly smaller than the topographic PV gradient, the flow consists of stagnant pools attached to pockets of closed geostrophic contours. The stagnant dead zones are bordered by jets and the flow through the domain is concentrated into a narrow channel of open geostrophic contours. In most of the domain, the flow is weak and thus the large-scale flow $U$ is an unoccupied mean. If $\unicode[STIX]{x1D6FD}$ is comparable to, or larger than, the topographic PV gradient, then all geostrophic contours are open and the flow is uniformly distributed throughout the domain. In this open-contour case, there is an ‘eddy saturation’ regime in which $U$ is insensitive to large changes in the wind stress. We show that eddy saturation requires strong transient eddies that act effectively as PV diffusion. This PV diffusion does not alter the kinetic energy of the standing eddies, but it does increase the topographic form stress by enhancing the correlation between the topographic slope and the standing-eddy pressure field. Using bounds based on the energy and enstrophy power integrals, we show that as the strength of the wind stress increases, the flow transitions from a regime in which the form stress balances most of the wind stress to a regime in which the form stress is very small and large transport ensues.

Buoyant miscible displacement flows in rectangular channels

Buoyant displacement flows of two miscible fluids in rectangular channels are studied, theoretically and experimentally. The scenario considered involves the displacement of a fluid by a slightly heavier one at nearly horizontal channel inclinations, where inertial effects are weak and laminar stratified flows may be expected. In the theoretical part, a lubrication approximation model is developed to simplify the displacement flow governing equations and furnish a semi-analytical solution for the heavy and light fluid flux functions. Three key dimensionless parameters govern the fluid flow motion, i.e. a buoyancy number, the viscosity ratio and the channel cross-section aspect ratio. When these parameters are specified, the reduced model can deliver the interface propagation in time, leading and trailing front heights, shapes and speeds, cross-sectional velocity fields, etc. In addition, the model can be exploited to provide various classifications such as single or multiple fronts as well as main displacement flow regimes at long times such as no sustained backflows, stationary interface flows and sustained backflows. Focusing on the variation of the buoyancy number, a large number of iso-viscous displacement experiments are performed in a square duct and the results are compared with those of the lubrication model. Qualitative displacement flow features observed in the theory and experiments are in good agreement, in particular, in terms of the main displacement flow regimes. The quantitative comparisons are also reasonable for small and moderate imposed displacement flow velocities. However, at large flow rates, a deviation of the experimental results from the model results is observed, which may be due to the presence of non-negligible inertial effects.

Flow regime mapping of aqueous two-phase system droplets in flow-focusing geometries

The recent advent of aqueous two-phase system (ATPS) has shown great potential to rapidly generate microscale aqueous droplets without tedious post-processing. ATPS provides a more biologically friendly and straightforward method to manufacture aqueous droplets compared with conventional oil-water systems and as such, it has been employed in many biomedical applications. Although the cost-effective manufacturing of aqueous droplets has been feasible by direct generation in ATPS, an understanding of the underlying physics of droplet formation in ATPS is still in its infancy. In this paper, we investigated the hydrodynamic behavior and mechanisms of all-aqueous droplet formation in two flow-focusing droplet generators. This study specifically tests whether ATPS in different geometries generates aqueous droplets with different sizes and properties. To achieve these goals, two incompatible polymers, namely polyethylene glycol (PEG) and dextran (DEX), were mixed in water to make ATPS. The influences of inlet pressures and flow-focusing configurations on droplet size, droplet generation frequency, and thread breakup length were quantified. In addition, flow regime mapping for the two different droplet generators at 30° and 90° junction angles was obtained. The results show that droplet size is very susceptible to the junction angle. On the other hand, inlet pressures of the PEG and DEX flows readily control five main flow regimes including PEG Back Flow, DEX Back Flow, Dripping Flow, Jetting Flow and Stratified Flow. It is observed that generated droplets in the Jetting Flow regime are larger in the case of 30°, whereas larger droplets are obtained in the Dripping Flow regime at 90° configuration. The frequency of droplet generation increases and decreases by increasing PPEG and PDEX, respectively. Finally, we characterized flow regimes by the Capillary number (Ca) of the PEG flow.

Analysis of pressure behaviors and flow regimes of naturally and hydraulically fractured unconventional gas reservoirs using multi-linear flow regimes approach

This paper targets the applicability of multi-linear flow regimes approach in gas reservoirs having multiple porous media: matrix, natural fractures, and hydraulic fractures. The approach attempts to understand pressure profiles and flow regimes developed in some of unconventional shale layers that have already been naturally fractured. In this paper, the main focus is given to fluid flow phenomenon from the matrix towards naturally induced fractures and from these fractures to the hydraulic fractures. Studying the impact of matrix, natural fracture and hydraulic fracture parameters on reservoir performance is another objective for this paper.Purposefully, several mathematical models have been developed to investigate transportation phenomenon of reservoir fluid in porous medium. The porous medium is assumed to be finite rectangular reservoir having different configurations. It is divided into two zones; inner porous medium close to the horizontal wellbore where multiple-hydraulic fractures propagate to some distance and outer porous medium where naturally induced fractures exist in the drainage area close to the outer boundary. Reservoir fluids, in the outer zone, are assumed to flow from the matrix to natural fractures and then from natural fractures to the inner zone. In the inner porous medium, fluids flow from matrix to natural fractures in addition to fluids flowing from the outer porous medium. Then all fluids flow by natural fractures to hydraulic fractures and finally from hydraulic fractures to the wellbore.The mathematical models for fluid flow in the two zones of the reservoir have been solved and three main flow regimes have been characterized. For each flow regime, an analytical model is used to describe its pressure profile with time and production performance. Bilinear flow regime, typically observed at early time, refers to simultaneous linear flow inside hydraulic fractures and flow from inner porous medium to the hydraulic fractures while linear flow describes fluids flow from inner zone to hydraulic fractures and late linear or boundary dominated flow regime describes fluids flow from the outer to the inner porous media.The main outcomes of this study are: 1) The ability to characterize reservoirs with triple porous media using multi-linear flow regimes approach. 2) Understanding the impact on pressure distribution of inner porous medium caused by the characteristics of hydraulic fractures such as fracture width and conductivity in addition to hydraulic fracture storativity. 3) Understanding the effect of reservoir configuration, matrix, and natural fractures characteristics on pressure distribution in the outer porous media. 4) Pressure distribution and flow regimes obtained by applying transient flow approach are not similar to those obtained by applying pseudo-steady state flow approach. 5) Characterization of different linear flow regimes, controlled by different parameters, with respect to production time.

Viscous fingering regimes in elasto-visco-plastic fluids

We experimentally study the Saffman–Taylor instability of air invasion into a non-Newtonian fluid (i.e., Carbopol solution) in a rectangular Hele-Shaw cell. In addition to viscous features, the non-Newtonian fluid used exhibits yield stress, shear-thinning as well as elastic behaviors. The key dimensionless parameters that govern the various flow regimes are the Bingham number (Bn), the capillary number (Ca), the Weber number (We), the Weissenberg number (Wi), the channel aspect ratio (δ ≫ 1), and the shear-thinning power-law index (n). Three main flow regimes are observed, i.e., a yield stress regime, a viscous regime and an elasto-inertial regime. We present a detailed description for each regime and quantify their transition boundaries versus dimensionless groups. Some of the secondary flow aspects, e.g., the wall residual layer thickness and a network structure regime, have been also studied.

Two-phase flow patterns, heat transfer and pressure drop characteristics of R600a during flow boiling inside a horizontal tube

A detailed experimental study was carried out to investigate the flow boiling heat transfer and pressure drop characteristics of R600a in a smooth horizontal tube with an inner diameter of 6mm. The experiments were performed at conditions covering saturation pressures from 0.215 to 0.415MPa, mass fluxes from 67 to 194kgm−2s−1 and heat fluxes from 10.6 to 75.0kWm−2. Based on an high speed camera, four main flow regimes can be observed: plug flow, stratified-wavy flow, slug flow and annular flow. Intermittent to annular flow transition was detected and plotted on flow pattern maps. Comparisons with available transition lines in the literature have been made. Furthermore, the influences of saturation pressure, mass flux and heat flux on heat transfer coefficient were analyzed. The experimental data was compared with the calculated data of seven well-known correlations. The results indicated that, the correlation of Liu and Winterton showed the best agreement with a mean absolute relative deviation of 11.5%. For two-phase frictional pressure drop, mass flux had obviously positive effect on frictional pressure drop, while negative effect was found for saturation pressure. Eight correlations were evaluated and the correlation of Müller-Steinhagen and Heck gave the best fit to the experimental data with a mean absolute relative deviation of 32.9%. A new correlation of pressure drop which accounted for the influence of surface tension and gravitational force was developed on the basis of Müller-Steinhagen and Heck correlation and its mean absolute relative deviation was about 16.6% for the experimental data.

Solid-Particles Flow Regimes in Air/Water Stratified Flow in a Horizontal Pipeline

Summary The entrainment of solid particles in crude oil occurs during production from reservoirs with low formation strength. The stationary solid-particles bed at the horizontal pipe bottom can cause operational problems such as production decline, excessive pressure loss, equipment failure, erosion, and corrosion. Solid-particles deposition can be managed by operating above the critical solid-particles-deposition velocity, which is the velocity that maintains the continuous movement of particles at the pipe bottom. Here, a comprehensive analysis of solid-particle flow regimes in stratified flow in a horizontal pipeline is presented, which is a novel contribution because it is applied to multiphase flow. The effect of concentration on the solid-particle flow regimes and identification of the critical solid-particles-deposition velocities for various particle concentrations are also investigated. The understanding of solid-particle flow regimes in pipelines for any given set of operational conditions is important for identifying the nature of particle interaction and movement. Experimental studies are conducted in a 4-in. horizontal pipeline for a stratified flow regime that uses air, water, and glass beads at relatively low solid-particles concentrations (<10,000 ppm). The effects of different experimental conditions, such as gas velocity, solid-particles concentration, and particle size, are investigated in this study. Six main solid-particles flow regimes in horizontal air/water flow are identified, and can be distinguished visually: fully dispersed solid flow, dilute solids at wall, concentrated solids at wall, moving dunes, stationary dunes, and stationary bed. Therefore, the critical solid-particles-deposition velocities are determined on the basis of the transition between moving (concentrated solids at wall/moving dunes, as appropriate) and stationary (stationary dunes/bed, as appropriate) solid particles. The experimental data show that with small particle size, the critical solid-particles-deposition velocity is almost independent of concentration, while with larger particle sizes, the critical velocity increases with the concentration.

Taylor-Couette-Poiseuille flow and heat transfer in an annular channel with a slotted rotor

This paper investigates a Taylor-Couette-Poiseuille flow in an annular channel of a slotted rotating inner cylinder, corresponding to a salient pole hydrogenerator. The purpose of this study is to improve the understanding of flow and thermal phenomena in electrical machines using a simplified scale model. The validation of the numerical model for a specific configuration is first shown by comparing the results with the experimental data. A parametric study is also performed to investigate all main flow regimes and to derive correlations in terms of the Nusselt number distribution on the rotor pole face and sides. The results show that the Nusselt number is proportional to the tangential Reynolds number to the power 1/7 in the pole and inductive faces trailing side. This relationship is similar to the one encountered in classical Taylor-Couette-Poiseuille flows between two concentric and smooth cylinders.

Buoyant miscible displacement flows in vertical pipe

The displacement flow of two miscible Newtonian fluids is investigated experimentally in a vertical pipe of long aspect ratio (δ−1 ≈ 210). The fluids have a small density difference and they have the same viscosity. The heavy displacing fluid is initially placed above the light displaced fluid. The displacement flow is downwards. The experiments cover a wide range of the two dimensionless parameters that largely describe the flow: the modified Reynolds number (0 ≤ Ret⪅800) and the densimetric Froude number (0 ≤ Fr ≤ 24). We report on the stabilizing effect of the imposed flow and uncover the existence of two main flow regimes at long times: a stable displacement flow and an unstable displacement flow. The transition between the two regimes occurs at a critical modified Reynolds number RetCritical, as a function of Fr. We study in depth the stable flow regime: First, a lubrication model combined with a simple initial acceleration formulation delivers a reasonable prediction to the time-dependent penetrating displacing front velocity. Second, we find two sub-regimes for stable displacements, namely, sustained-back-flows and no-sustained-back-flows. The transition between the two sub-regimes is a marginal stationary interface flow state, which is also well predicted by the lubrication model. The unstable regime is associated to instabilities and diffusive features of the flow. In addition, particular patterns such as front detachment phenomenon appear in the unstable flow regime, for which we quantify the regions of existence versus the dimensionless groups.

The heterogeneous to homogeneous transition for slurry flow in pipes

Models for modeling slurry flow present difficulties on long lines with large pipe diameters and with broad graded sands or gravels. In order to get more insight into the slurry flow process, the Delft Head Loss & Limit Deposit Velocity Framework has been developed that integrates the 5 main flow regimes of slurry transport: fixed or stationary bed transport, sliding bed transport, heterogeneous transport, homogeneous transport and sliding flow transport. Additional models for; the limit deposit velocity, the holdup function, the bed height, the concentration distribution and graded sands and gravels, complement the Framework. The Framework is based on constant spatial volumetric concentration curves for uniform sands and gravels. The models for the flow regimes and the limit deposit velocity are based on energy considerations.At line speeds near the transition of the heterogeneous and homogeneous flow regimes however, there is no sharp transition between the two flow regimes for medium sized particles. The limits of medium sized particles depend on the solid and liquid properties and on the pipe diameter. It is often observed that the hydraulic gradient lies in between the Equivalent Liquid Model (ELM) and the pure liquid Darcy Weisbach model at higher line speeds, resulting in the conclusion that at higher line speeds the pure liquid hydraulic gradient is approached. Based on energy considerations however, it is shown that the heterogeneous hydraulic gradient collapses due to turbulent near wall lift neutralizing the particle submerged weight and collisions with the pipe wall, while at higher line speeds the turbulent eddies integrate particles, resulting in a hydraulic gradient approaching a reduced ELM (RELM). For medium sized particles in large diameter pipes there is a gap between the moment the heterogeneous hydraulic gradient collapses and the homogeneous hydraulic gradient builds up, resulting in a hydraulic gradient approaching the RELM hydraulic gradient. The model for this transition is described and derived and experimental evidence is given.

Investigation of using 60Co source and one detector for determining the flow regime and void fraction in gas–liquid two-phase flows

In this study, a simple detection system comprised of one 60Co source and just one NaI detector was investigated in order to identify flow regime and measure void fraction in gas–liquid two phase flows. For this purpose, 3 main flow regimes of two-phase flows including stratified, homogenous and annular with void fractions in the range of 5–95% were simulated by Monte-Carlo N Particle (MCNP) code. At first step, 3 features (count under full energy peaks of 1.173 and 1.333MeV, and count under Compton continuum) were extracted from registered gamma spectrum. These 3 extracted features were used as inputs of artificial neural network (ANNs). A primary network was trained for identifying the flow regimes, but after testing many different structures, it was found that just two regimes of stratified and annular could be completely identified from each other. After identifying the mentioned two flow regimes by the first ANN, two specific ANNs were also implemented for predicting the void fraction. Using the proposed method in this work, void fraction percentages were predicted with a mean relative error (MRE) of less than only 0.42%. Using fewer detectors is of advantage in industrial nuclear gauges, because of reducing economical expenses and also simplicity of working with these systems.

Experimental study of water vaporization occurring inside a channel of a smooth plate-type heat exchanger at subatmospheric pressure

In order to be able to design properly low-pressure evaporators for sorption chillers, expertise on vaporization of the refrigerant under conditions that might occur in these evaporators is fundamental. However, few studies focus on this subject and there is a lack of knowledge about vaporization (boiling or evaporation) phenomena occurring in compact evaporators at low pressure. The objective of this article is thus to go further in the understanding of phenomena occurring in compact plate-type evaporators. In that goal, an experimental test setup was designed and built. It allows the observation of the water vaporization in a channel mimicking that of a plate heat exchanger of standard dimension (0.2m width×0.5m height) under various operating conditions (working pressure ranging from 0.85kPa to 16kPa, secondary fluid temperature ranging from 10.9°C to 23.1°C, filling ratio ranging from 1/2 to 1/10 of the whole channel height). During these experiments, three main flow regimes were observed and three different working areas were identified: a pool boiling area, a vapor area, and a film evaporation area. In the latter, the creation of a liquid film due to the splashing of droplets is observed. These droplets result from the break-up of the membrane of a previously formed large bubble. The bubble formed is of several centimeters in diameter and appears in the pool boiling area, few centimeters below the free surface. It was shown that the major part of the cooling capacity is achieved in the film evaporation area. As a matter of fact, it is possible to predict the overall cooling capacity obtained experimentally with a maximum relative difference of 30% directly from the evaluation of the overall heat transfer coefficients in that area.

Prediction of segregation in funnel and mass flow discharge

In this paper we present a model to predict the onset and evolution of segregation during the discharge of binary mixtures of granular materials. The model accounts for the multi-phase and multi-component nature of the granular mixtures, to simulate the main flow regimes occurring in the discharge of silos (funnel and mass flow) and how they affect segregation. The new comprehensive model for segregation follows a continuum Eulerian approach and results from the coupling between an ad-hoc rheology for granular flow and a percolation model for multi-component mixtures. Predictions are compared with independent literature experimental data, for short and tall silos and prove to be quite accurate, after a tuning of the percolation flux sub-model. The larger segregation in short flow paths with smaller amount of fines reported by the experiments is quantitatively predicted. The model also predicts the three phases observed in experiments during the discharge of tall silos.

Global and local hydrodynamics of bubble columns – Effect of gas distributor

Global (level swell) and local (WMS – Wire Mesh Sensor) measurements were made on waters of different purities and air, in a cylindrical laboratory bubble column (2m tall, 0.127m dia) using two different gas distributors: a perforated plate (to produce homogeneous flow) and a spider sparger (to produce heterogeneous flow). The level swell method provided the steady space-averaged gas holdup/gas flow rate data. The WMS method provided the actual gas holdups and bubble sizes resolved in time and space at one cross-sectional horizontal plane (1m above distributor), whose integration yields the time-averaged data. The following results were obtained: The global and local data agree relatively well; there are distinct differences between the radial profiles and bubble size distributions between the two main flow regimes; the local information identifies why the predictions of published models, which account for the smaller and larger bubbles in the flow, may not perform well; the modelling approaches based on the hindrance and enhancement concepts prove to be suitable for the flow regime identification and description, including the transition range between the homogeneous and heterogeneous flows; based on the hydrodynamics, the specific interfacial area is obtained, together with the mass transfer coefficient.