Stable Flow Regimes Research Articles

Uncertainty of isokinetic sampling of the phase composition of a gas-oil-water mixture at different regimes of a developed horizontal pipe flow

The paper presents ten well-known developed stable flow regimes of an air/water/oil mixture in a horizontal pipe simulated numerically. The results have been tested by comparing the obtained fields of phase distribution and velocity with the results of experimental observations published in the literature. For sampling, four classical designs of isokinetic sampling probes guided by the existing standards for two-phase flows have been considered. The obtained results showed that the reliability of the estimation of the phase composition in the three-phase mixture by isokinetic sampling probes strongly depends on the flow regime. Therefore, in real oil production conditions with the presence of unstable and transient regimes, using the isokinetic probes without preliminary preparation of the flow in front of the sampling section is not recommended.

Structure of the natural convective flow above to the horizontal surface with localized heating

Pure thermal plumes have been the subject of study for the past decades. The active development of experimental and numerical research methods allowed to identify the flow characteristics in the entire region of the plume formation. Nevertheless, the bifurcation discovered in the last ten years remains to be studied, in the authors' opinion, poorly. Until now, there is no general conception of the existence of several stable flow regimes and the key features of the unsteady regime. In this paper results of an experimental and numerical study of pure thermal plume structure in the near-hot-wall region at moderate Grashof numbers are presented. Experimental data on the general flow structure and temperature field are supplemented by the results of numerical simulation. The analysis of changes in instantaneous and average values is carried out, in particular, the geometric characteristics of the near-wall layer are determined. Additionally, data on the characteristics of heat transfer, information on which in modern studies is usually limited and is fragmentary, are presented.

Effect of PEM fuel cell porous media compression on in-plane transport phenomena

Abstract Liquid-gas two-phase flow in the gas diffusion layer (GDL) of proton exchange membrane fuel cells is investigated using an ex-situ experimental setup. The mass transport phenomena is investigated in carbon paper and carbon cloth GDLs and at different compressions. Water percolation within the plane of the GDL is visualized with a CCD camera while its injection pressure is measured. Similarly, air percolation within the plane of GDL samples which were initially saturated with water is investigated. Experiments are conducted for the three flow regimes of stable displacement, capillary fingering, and viscous fingering. Images are analyzed to obtain the normalized wetted area. It is observed that while the GDL compression directly affects normalized wetted area for stable displacement and viscous fingering flow regimes, it has no impact on this parameter for capillary fingering flow regime. For stable displacement flow regime and for both carbon paper and carbon cloth samples, water percolation pressure increases with GDL compression. However, the water percolation pressure data obtained for capillary fingering flow regime does not suggest any discernible trend as a function of GDL compression. The findings in this study can be used to validate percolation models proposed by different schemes such as pore-network models.

Non-invasive arterial pressure monitoring by a new pneumatic sensor and on-line analysis of pulse waveforms for a modern medical home care systems.

The development of the third-generation devices for medical home care systems is in the center of discussion. The results of developing non-invasive arterial pressure monitoring channel on the basis of the unique pneumatic sensor are discussed in detail. In particular, the main principles of operation, design features and testing results of a pneumatic sensor are considered. Namely, the issues of the stable air flow regime in the sensor’s working chamber are discussed. It is shown that the miniature size of the sensor and the possibility of its precise positioning directly in the measurement zone on small (< 1 mm) areas of the skin elastic surfaces result in improved quality of pulse wave shape recovery, the parameters continuity measured and the disturbances reduction. Measurement examples for some human superficial arteries are given. The possibility of continuous measurement of the actual value of blood pressure for radial and temporal arteries is confirmed.

Control of the Formation of a Transonic Region in an Axisymmetric Supersonic Flow with the Use of a Jet and a Near-Wall Energy Supply

The control action of a throttling air jet produced by a gas generator and of the energy sources positioned at the walls of an axisymmetric channel, which models the combustion-chamber unit of a ramjet in which a gas flow is decelerated, on the shock-wave structure of a supersonic gas flow in the channel was investigated for the purpose of formation of a lengthy transonic region in it. A heat energy was supplied to the gas flow in the indicated channel in the pulse-periodic regime, and the pressure in the gas generator was also changed periodically. The range of energies corresponding to a stable transonic flow regime in the channel was determined on the basis of solution of the nonstationary Euler equations in dimensionless variables. For comparison, the combustion of a gaseous fuel inflowing to the channel through a slot in the channel wall upstream of the control air jet was considered. A nonstationary gas flow in the channel was calculated on the basis of the Reynolds-averaged Navier–Stokes equations with the use of the k–ω SST model of turbulence. It is shown that a controllable energy release in the channel, providing the obtaining of a transonic flow regime in it analogous to that obtained on the basis of solution of the Euler equations, can be realized with the use of a control transverse air jet.

Rheological behavior of binary polymer compositions

The article presents the results of a study of the rheological behavior of binary composites based on serial polyethylene grade LDPE 15803-020 with different content of microcellulose grade Filtracell, in a wide range of temperatures and shear rates during deformation through a capillary diameter of 1 mm and a length of 5 and 30 mm. Stable flow regime is manifested in the temperature range from 160 to 200°C for composites containing microcellulose in an amount of 30 wt.%, and its partial replacement with spent microcellulose (waste production of vegetable oils) can reduce the effective viscosity to 25%, but the upper limit of the temperature range is limited to the exudation of impurities (190°C). The Bagley correction is carried out, the coefficients of the equations describing the dependences of the true shear stress, regardless of the capillary length, are calculated.

Control of pellets coating in a Wurster fluidised bed by means of electrical capacitance tomography

Wurster fluidised bed is commonly used for coating pellets in the pharmaceutical industry. Normally, the control in a Wurster fluidised bed for coating is based on point-based measurement, that is, pressure and optical probes. However, the point-based measurement methods only provide local flow information and cannot reveal the cross-sectional flow dynamics characteristics and it is difficult to control the whole process with limited data. In this paper, electrical capacitance tomography (ECT) is applied to reconstruct the solids distribution in a lab-scale Wurster fluidised bed for coating pellets and the measurements are used in a control loop of the process. The flow regime is identified from the ECT images using a simple but efficient approach. The objective of control is to keep stable minimum fluidisation and avoid undesired flow regime in the region between the outside wall and the Wurster tube, such as intermittent, plug and defluidisation. To achieve this target, a PID controller is applied to keep a low volume fraction in the annular region by adjusting the fluidisation air rate, and bang-bang control is applied to a peristaltic pump used for spraying coating solutions based on the detected flow regime. It has been found that the proposed control scheme based on the ECT measurement is effective for keeping a stable flow regime, reducing the degree of pellets agglomeration and avoiding defluidisation.

Inertial focusing of a neutrally buoyant particle in stratified flows

Particles in microfluidic channels experience two dominant lift forces in the direction transverse to the flow—the shear gradient lift force and the wall lift force. These forces contribute to the lift experienced by the particle and cause their cross stream migration until they attain an equilibrium position where the net lift force in the transverse direction is zero. Stratified coflow of two liquids with different viscosities is a stable flow-regime observed under some operating conditions. The presence of the second fluid alters the shear gradient induced lift force and the wall force acting on the particle at each point, changing the final equilibrium position. These positions can be tuned and controlled by altering the viscosity or the flow rates of the two fluids so that the particles focus in one fluid. A numerical method based on the combined Immersed Boundary-Lattice Boltzmann Method is used to study inertial focusing of neutrally buoyant particles in stratified Couette flows and pressure driven flows. We analyze how different factors such as the Reynolds number, flow rate ratio, viscosity ratio of the fluids, and particle size affect the particle migration in two-dimensional (2D) and three-dimensional (3D) geometries. Our study shows that in Couette flows, the particle focuses in the low viscosity fluid when the interface is at the center. We also found that a critical viscosity ratio exists beyond which particle focusing in low viscous fluid is guaranteed, for a given flow rate ratio in pressure driven flows.

Experimental study of flow boiling in a hybrid microchannel-microgap heat sink

A stable flow boiling operation is key to enhancing the two-phase cooling performance of microchannel heat sinks. To this end, a novel heat sink is developed which integrates a 300 μm × 600 μm straight microchannel array in the upstream region with a 25 mm × 600 μm microgap channel in the downstream region of a 25 mm × 25 mm copper heat sink. Flow boiling experiments are conducted using de-ionized water at 5 different mass fluxes in the range of 100–399 kg/m2 s, supplied at a fixed inlet temperature of 85.5 °C. The downstream heat transfer coefficient in the microgap section shows an M-shaped profile with increasing heat flux and vapor quality. Stable boiling conditions are prevalent across a large span of operating heat flux. Instabilities are experienced only for a short range of low heat flux following ONB. The stabilizing effect is attributed to the larger flow cross-section area offered by the downstream microgap section that allows expanding vapor bubbles to evacuate with lesser hindrance. Flow visualization reveals that a stable annular flow regime is established at moderate to high heat flux during which the stable boiling operation is observed. Thin film evaporation taking place during annular flow conditions results in an increasing trend in the downstream heat transfer coefficient from moderate to high heat flux. Pressure drop associated with the hybrid heat sink is found to be modest and reaches a maximum of 6 kPa at the highest mass flux of 399 kg/m2 s. A brief comparison of this heat sink is made with its straight microchannel and microgap heat sink counterparts. The hybrid heat sink shows a heat transfer performance that is superior to the microgap heat sink while poorer than the straight microchannel heat sink although it offers a better boiling stability than the straight microchannel heat sink. It however lowers the pressure drop compared to the straight microchannel heat sink.

Surface-groundwater interaction in unconfined sedimentary aquifer system in the Brazil’s tropical wet region

ABSTRACT Since groundwater and surface waters are important components of the hydrological system, determining their interaction is essential for the efficient management of water resources by predicting the consequences of interference, whether due to the growth of demand or due to climate change. However, integrated scientific studies on these water resources are scarce, including in the Guariroba’s Environmental Protection Area, responsible for supplying 31.3% of the Campo Grandem/MS’s population, representing a local water security element. Thus, this work had as objective to evaluate the interaction between surface-groundwater in an unconfined sedimentary aquifer system, based on hydrograph separation methodologies of base flow, Flow Duration Curve (FDC) analysis, Master Recession Curve (MRC) evaluation and verification of the relationship between the surface flow, piezometric levels (PL) of the wells and the monthly precipitation. The results indicates a proportional relationship between rainfall, superficial flow and PL variations; the FDC smooth slope suggests that the baseflow is sustained by the groundwater discharge, corresponding to 89% of the total flow; the low-flow index indicates that the groundwater’s storage capacity is about 80%; the Base-Flow Index (BFI) ranging from 0.804 to 0.921, indicates a stable flow regime, aquifer’s high permeability conditions, though not uniform, and low runoff.

Gas flow rate measurement in low-quality multiphase flows using Venturi and gamma ray

Online flow rate measurements are encountered in many areas with a great need for high accuracy, especially in low-quality (0–0.1) flows. This paper concerns gas flow rate measurements using a Venturi meter and gamma-ray attenuation technique. A linear correlation is developed to predict the gas flow rates with wide ranges of void fractions (0–95%). The correlation predictions were compared to experimental data in which the root mean square errors of the oil-air-water multiphase flow prediction results are 7.72%, 8.93% and 9.11% for the gas flow rate, the quality and the gas-liquid slip ratio, respectively. The effect of the Venturi size was tested, and the metering accuracy is found to increase with the inlet diameter. The prediction accuracy is improved for three-phase flows than for the corresponding two-phase flows due to the oil-water stirring in the three-phase flows. The analyses of the results indicate that the measurement accuracy is higher with stable flow regimes than intermittent flow regimes. The present method can be widely applied to both two-phase and three-phase flows in the oil industry even for different Venturis and fluid media.

Control of pellets coating in a Wurster type fluidized bed based on electrical capacitance tomography measurement

Wurster type fluidized bed is commonly used for pellets coating in many industrial sectors. However, there are few reports on the optimization and automatic control of these processes in literature. The main reason is that it is difficult to apply reliable in-line measurement. In this paper, electrical capacitance tomography (ECT) is applied to reconstruct the solids distribution in a lab-scale Wurster type fluidized bed and the results were used in a control loop of pellets coating process. The features of different flow regimes exist in fluidized bed are extracted from the ECT images using a simple but efficient approach. The objective of control is to keep a stable minimum fluidization state of the solids flow and avoid the occurrence of undesirable flow regimes in the annular zone in the bed including intermittent, plug and defluidization. A PID controller was applied to keep a low volume fraction of solids in the annulus zone by manipulating the air flow rate. The peristaltic pump was also manipulated by a simple control logic based on the detected flow regimes. It was found that the control strategy based on ECT measurement was useful for keeping a stable flow regime, and avoiding the occurrence of defluidization.

Experimental investigation of stratorotational instability using a thermally stratified system: instability, waves and associated momentum flux

ABSTRACTStratorotational instability (SRI) has been proposed as a mechanism for outward angular momentum transport in Keplerian accretion disks. A particular designed Taylor–Couette laboratory experiment with axial stratification is suitable for studying the instability. Bottom endplate is cooled and top endplate is heated to achieve axial stratification. Due to constructive constraints, endplates are visually unamenable and quantitative measurement techniques in the co-rotating frame can only be done by looking through the outer cylinder. For this purpose, we built a co-rotating mini-PIV (Particle Image Velocimetry) system with a camera having a tilted viewing angle regarding the horizontal laser sheet. The aim of this study is (i) to quantify the uncertainty of the mini-PIV together with the used calibration technique and (ii) to compare experimental findings on SRI with theoretical predictions.We perform measurements of the azimuthal and radial component of the velocity in axial stably stratified Taylor–Couette flows, consider velocity profiles and do frequency-filtering and flow decomposition. The absolute error of the mini-PIV system is 2% and we realised that stratified Taylor–Couette flows have smaller Ekman endwall effects than homogeneous ones. Still, Ekman pumping has an impact of the flow and might be responsible for differences between the data and theoretical models ignoring the endwalls. Here we focus on the flow structure during transition to SRI, the drift rate of SRI modes and the radial momentum flux as a function of the Reynolds number. Whereas the structure in form of trapped boundary Kelvin modes and the drift rate corresponds well with earlier predictions, the momentum flux shows a nonlinear dependency with respect to the Reynolds number. Away from the region of transition, theoretical models show a linear relationship. Several possible reasons for the mismatch between the experimental and theoretical models are discussed. Most important, we experimentally demonstrated that in the Rayleigh stable flow regime the SRI can provide a significant amount of outward momentum flux which makes this instability interesting in the context of accretion disks and also of atmospheric vortices where rotation and stratification also play a significant role.

On the Flow Stability in a Circular Cylinder based Open Volumetric Air Receiver for Solar Convective Furnace

The arid desert regions are considered for installing the concentrated solar thermal based systems owing to the available direct normal irradiance. For process heat applications, like the solar convective furnace that requires an air temperature of 750 K the open volumetric air receiver is preferred. Such innovative concepts are envisaged for achieving high thermal energy conversion efficiency. For the prolonged operation of the solar convective furnace system the stable flow is necessary. On the other hand, the thermally induced flow instability is reported in the open volumetric air receivers at a high temperature. This may limit the operating temperature range of a receiver. In this paper the flow stability analysis in a circular cylinder based open volumetric air receiver with the straight pores is presented. The findings allow expecting a stable flow regime in such an absorber pore at a high flux concentration and temperature.

URANS Models for the Simulation of Full Load Pressure Surge in Francis Turbines Validated by Particle Image Velocimetry

Due to the penetration of alternative renewable energies, the stabilization of the electrical power network relies on the off-design operation of turbines and pump-turbines in hydro-power plants. The occurrence of cavitation is however a common phenomenon at such operating conditions, often leading to critical flow instabilities which undercut the grid stabilizing capacity of the power plant. In order to predict and extend the stable operating range of hydraulic machines, a better understanding of the cavitating flows and mainly of the transition between stable and unstable flow regimes is required. In the case of Francis turbines operating at full load, an axisymmetric cavitation vortex rope develops at the runner outlet. The cavity may enter self-oscillation, with violent periodic pressure pulsations. The flow fluctuations lead to dangerous electrical power swings and mechanical vibrations, dictating an inconvenient and costly restriction of the operating range. The present paper reports an extensive numerical and experimental investigation on a reduced scale model of a Francis turbine at full load. For a given operating point, three pressure levels in the draft tube are considered, two of them featuring a stable flow configuration and one of them displaying a self-excited oscillation of the cavitation vortex rope. The velocity field is measured by two-dimensional (2D) particle image velocimetry (PIV) and systematically compared to the results of a simulation based on a homogeneous unsteady Reynolds-averaged Navier–Stokes (URANS) model. The validation of the numerical approach enables a first comprehensive analysis of the flow transition as well as an attempt to explain the onset mechanism.

Effect of tilted electrostatic field on the Kelvin–Helmholtz instability in a liquid dielectric and gas flow

The effect of surface ponderomotive forces on the Kelvin–Helmholtz instability is studied in the linear formulation based on the equations and boundary conditions of the electrostatics and fluid dynamics of an ideal incompressible fluid. Conditions to be satisfied by the values of determining parameters of the problem for the transition of an unstable flow in zero electric field into a stable regime after the application of a horizontal electric field have been written in the form of inequalities. It has been shown that, at the stability bound, the wavelength of the most instable mode is independent of the ponderomotive forces. In case of a liquid with large permittivity a stable flow regime exists for which the stability condition only differs in small dimensionless values from the stability condition for the charged surface of a quiescent liquid conductor in contact with a gas at rest.

Two-phase operational maps, pressure drop, and heat transfer for flow boiling of R236fa in a micro-pin fin evaporator

Cooling the new generation of 3D high power electronic chips is one of the leading challenges in microelectronics, as it is a key to achieve high computational performance at lower cooling system power consumption, thus reducing the operating cost. Two-phase flow boiling in two micro-pin fin heat sinks is studied here for this cooling process. Each micro-evaporator has a heated area of 1cm2 and contains 66 rows of cylindrical micro-pin fins with an in-line configuration and diameter, height and pitch of respectively 50μm,100μm and 91.7μm. The fluid tested is refrigerant R236fa. Channel entrances with and without inlet restrictions are tested in order to evaluate their relative effect on the stability of the flow. The inlet restrictions consist of an extra row of micro-pin fins with a larger diameter of 100μm placed at the inlet of the heated area. The present study investigates operational stable and unstable flow regimes, pressure drop and heat transfer performance for the two micro-evaporators (one with and one without inlet restrictions) by coupling high speed visualization of the micro-pin fins flow area with fine resolution infrared temperature measurements of the micro-evaporator base, which yields a 2D map of local heat transfer coefficients of the whole heated area. Working conditions tested have mass flux varying from 500kgm−2s−1 to 2500kgm−2s−1, heat flux ranging from 20Wcm−2 to 48Wcm−2, and a constant outlet saturation temperature of 30.5°C. In agreement with previous studies for multi-microchannel evaporators, it is here observed that inlet restrictions extend the map of stable operational regimes, in particular toward lower values of the mass flux, without any appreciable increase of the pressure drop. Unlike evaporation through parallel microchannels, the heat transfer coefficient trends and magnitudes vary dramatically with the flow conditions (mass flux and heat flux), thus suggesting that micro-pin fins have a strong impact on the two-phase flow pattern development, in particular delaying the transition to annular flow.

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.

Effect of nanosecond-pulsed plasma actuation on a separated laminar flow

An experimental investigation was carried out on the effect of unsteady periodic control on a separated laminar shear layer. Time resolved Particle Image Velocimetry (tr-PIV) was used to characterize a backward-facing step (BFS) flow (Reh=3600), periodically perturbed by a nanosecond Dielectric Barrier Discharge (ns-DBD) plasma actuator. Ensemble averaged vector fields indicate a decrease of reattachment length with increasing actuation non-dimensional frequency, reaching a minimum at a Sth=0.32. Further increase of forcing non-dimensional frequency, up to 0.4, resulted into an increase of the reattachment length, which nevertheless remained shorter than the non-actuated case. Spectral analysis of the fluctuating fields revealed a change of the amplified frequency range for the actuated cases with respect to the base flow. Proper Orthogonal Decomposition (POD) analysis showed that actuation leads to a redistribution of energy among coherent spatial modes. Stability diagrams were calculated from mean velocity field data for each case via Linear Stability Theory (LST). Results indicate that stability of each actuated case changes with respect to the non-actuated case. Moreover, looking into the calculated growth rate for all the cases a more stable flow regime is observed for the cases of most successful reduction in terms of reattachment length. The effect of a pulsed periodic perturbation on the control of a laminar shear layer promotes the development of large structures, i.e. K-H vortices, due to inviscid-viscous interaction. These convect downstream resulting in a mean flow deformation (MFD) which causes a change of stability. New unstable frequencies are excited and promote the redistribution of energy among modes. This ultimately affects the efficiency of actuation in promoting transition from laminar to turbulent flow.

Numerical studies on the dynamics of an open triangle in a vertically oscillatory flow

A direct-forcing fictitious domain method is employed to study the dynamics of an open triangle in a vertically oscillatory flow. The flow structures, the vertical force and the torque on the fixed body are analysed for the stable flow regime in which the flow structures form and evolve exactly in the same way in each period and the unstable regime, respectively. Our results indicate that in the stable flow regime for the body with upright orientation, the steady streaming structure mainly comprises two vortex pairs located respectively above and below the body. Due to up–down asymmetry of the body, the pair below the body produces a larger vertical force on the body than the upper pair, which is mainly responsible for the non-zero average force at relatively high Reynolds numbers. The average force increases with increasing Reynolds number or increasing dimensionless period for the parameter range studied, due to the vortex effects. In the unstable regime, a vortex pair is ejected downward from each body edge. The irregular motion of the emitted vortices below the body leads to the irregular fluctuation of the vertical force. Regarding the torque on a tilted body, in the stable regime, the body experiences a restoring torque when its vertex angle is larger than a critical value being close to (and smaller than) 60°, and otherwise a destructive torque, irrespective of the value of tilt angle. For a fixed vertex angle, the torque magnitude is largest when the tilt angle is around 45°. In the unstable regime, the persistent ejection of the vortex pair during upward flow and corresponding restoring torque are observed for a large tilt angle with one edge aligned close to the horizontal direction, as in the experiment of Liu et al. (Phys. Rev. Lett., vol. 108, 2012, 068103). For a relatively small tilt angle, the emission direction of the vortex pair has intermittency, leading to the intermittency in the direction of torque. The reasons for the above observations are discussed. The predictions on the stable orientation for a hovering body in the stable flow regime and the irregular orientation in the unstable regime are confirmed in the dynamic simulation of a freely moving body. The body with the stable horizontal orientation in case of small vertex angle migrates along the body-shape-diverging direction.