Main Flow Patterns Research Articles

Experimental investigation on flow boiling characteristics of radial expanding minichannel heat sinks applied for two-phase flow inlet

Generally, two-phase (TP) flow inlet condition is not allowed for conventional microchannel heat exchangers, and a certain subcooled degree of liquid is strictly required to guarantee the efficient operation of system. To solve multi-heat sources cooling issue, a type of circular radial expanding minichannels heat sink (REMHS) utilizing flow boiling of deionized water is proposed in present work. Flow boiling heat transfer of REMHS is investigated with the inlet vapor mass qualities in the range of 0.01–0.53. Under TP flow inlet, slug-annular and annular flows are the main flow patterns observed in REMHS in this work, where the low-medium mass flux (22–132 kg/m2s) and medium-high heat flux (33.1–176.4 kW/m2) conditions are applied. The experimental results illustrate that the proposed REMHS under TP flow inlet maintains a stable and high flow boiling heat transfer performance without occurring heat deterioration. When xin increases from 0.04 to 0.53, the maximum local heat transfer coefficient increases by 22% from 42.5 to 51.9 kW/m2K. The flow distribution among the expanding channel array is relatively uniform and the heat transfer capability in the REMHS displays a good symmetry under TP flow inlet. The maximum discrepancy in heat transfer coefficient is below 19% within our experimental range. Six classic flow boiling heat transfer correlations are compared with the experimental results, and Li & Wu's correlation gives a good prediction on all the measured data with MAE of 21.1%. Besides, the thermal independence of REMHS connected in series is also examined, and the mutual interaction in heat transfer between the REMHS in series is trivial, the proposed REMHS is expected to be applied to multi-heat source cooling.

Research on dynamical overflow characteristics of a vertical H2S‐containing natural gas well

AbstractThis work aims to explore the overflow characteristics of a vertical H2S‐containing natural gas well. A two‐phase flow model for H2S‐containing natural gas well combining with a transient temperature prediction model was established to simulate the overflow process of a vertical H2S‐containing natural gas well. The model was validated by reproducing the field data of Well Longhui #2. The effects of H2S content, mud displacement, drilling fluid density, geothermal gradient, and reservoir permeability on the overflow characteristics of a vertical H2S‐containing natural gas well were studied and analyzed in this work. Results indicate that bubble, slug, and churn flows constitute the main flow patterns in the whole overflow process. The higher the H2S content is, the more obviously the gas void fraction increases. The phase change position of H2S is closer to the wellhead at lower H2S content. An increase in mud displacement indicates the decreases in overflow time. As drilling fluid density increases, the release position of H2S moves up, and the overflow time and shut‐in casing pressure increase. The initial gas void fraction is higher and the gas invasion volume will be larger in gas reservoirs with higher permeability. As the reservoir permeability increases, the shut‐in casing pressure rises while the overflow time declines. With higher geothermal gradient, the wellbore temperature tends to be higher at the same depth, leading to an increase in the H2S solubility. The gasification starting position is further away from the wellhead at higher geothermal gradient. The results of this work could provide important theoretical basis and technical guidance for drilling engineers to reduce a blowout risk during drilling of H2S‐containing natural gas well.

Lattice-Boltzmann Simulation and Experimental Validation of a Microfluidic T-Junction for Slug Flow Generation

We investigate the interaction of two immiscible fluids in a head-on device geometry, where both fluids are streaming opposite to each other. The simulations are based on the two-dimensional (2D) lattice Boltzmann method (LBM) using the Rothman and Keller (RK) model. We validate the LBM code with several benchmarks such as the bubble test, static contact angle, and layered flow. For the first time, we simulate a head-on device by forcing periodicity and a volume force to induce the flow. From low to high flow rates, three main flow patterns are observed in the head-on device, which are dripping-squeezing, jetting-shearing, and threading. In the squeezing regime, the flow is steady and the droplets are equal. The jetting-shearing flow is not as stable as dripping-squeezing. Moreover, the formation of droplets is shifted downstream into the main channel. The last flow form is threading, in which the immiscible fluids flow parallel downstream to the outlet. In contrast to other studies, we select larger microfluidic channels with 1-mm channel width to achieve relatively high volumetric fluxes as used in chemical synthesis reactors. Consequently, the capillary number of the flow regimes is smaller than 10−5. In conclusion, the simulation compares well to experimental data.

Prediction of the Optimal Vortex in Synthetic Jets

This article presents three different low-order models to predict the main flow patterns in synthetic jets. The first model provides a simple theoretical approach based on experimental solutions explaining how to artificially generate the optimal vortex, which maximizes the production of thrust and system efficiency. The second model is a data-driven method that uses higher-order dynamic mode decomposition (HODMD). To construct this model, (i) Navier–Stokes equations are solved for a very short period of time providing a transient solution, (ii) a group of spatio-temporal data are collected containing the information of the transitory of the numerical simulations, and finally (iii) HODMD decomposes the solution as a Fourier-like expansion of modes that are extrapolated in time, providing accurate predictions of the large size structures describing the general flow dynamics, with a speed-up factor of 8.3 in the numerical solver. The third model is an extension of the second model, which combines HODMD with a low-rank approximation of the spatial domain, which is based on singular value decomposition (SVD). This novel approach reduces the memory requirements by 70% and reduces the computational time to generate the low-order model by 3, maintaining the speed-up factor to 8.3. This technique is suitable to predict the temporal flow patterns in a synthetic jet, showing that the general dynamics is driven by small amplitude variations along the streamwise direction. This new and efficient tool could also be potentially used for data forecasting or flow pattern identification in any type of big database.

Influence of various shapes of CuO nanomaterial on nanofluid forced convection within a sinusoidal channel with obstacles

Heat transfer intensification is significant for enhancement of the performance of various industrial uses such as condensers. In addition, the presence of nanofluid considerably increases the thermal features of carrier fluid. In current paper, CuO nanoparticles involving different shapes have been employed for heat transfer augmentation in wavy channel with obstacles. This work tries to focus on the main flow pattern and indicate the highest operative terms on the heat rate. In order to do this, computational simulation was applied to simulate hydrothermal behavior. Significant parameter studies were performed to investigate the influences of wavelength and height ratios and velocity inlet. According to obtained results, suitable formula for Nusselt number is offered. Outputs show that Nu goes up with augment of height of obstacle due to increasing in strength of intensity. Also, thinner temperature boundary layer thickness can be seen for greater wavelength ratio. Our finding reveals that heat transfer rate increases more than 55% as the spherical nanoparticles is replace by platelet ones.

An Analysis of LSPIV‐Based Surface Velocity Measurement Techniques for Stormwater Detention Basin Management

AbstractStormwater detention basins are significant components for the sustainable management of urban stormwater, aiming to control stormwater quantity (prevent from flooding) and quality (trap particulate pollutants by settling). The design and management of such facilities need knowledge of the hydrodynamic characteristics and the sedimentation zones for cleaning purposes. Large‐scale particle image velocimetry (LSPIV), mostly applied in riverine environments, has been proven as a low‐cost, highly efficient, and convenient image‐based methodology for surface velocity measurement. In this study, LSPIV technology is employed to investigate free surface velocity in a field stormwater detention basin. The method is first evaluated by comparing its results with those obtained by acoustic Doppler velocimeter at pilot scale. Then, the technique is applied to better understand the main flow field and to explain spatial sediment distribution within an extended stormwater detention basin. Results illustrate two main flow patterns, which are consistent with observations and preliminary simulated results. LSPIV‐based free surface velocity field measurements can be used to evaluate numerical models and as a support to predict sediment deposition zones in stormwater detention basins and to help for their retrofit and management.

Vertical Section Observation of the Solid Flow in a Blast Furnace with a Cutting Method

The solid flow plays an important role in blast furnace (BF) ironmaking. In the paper, the descending behavior of solid flow in BFs was investigated by a cold experimental BF model and numerical simulation via the discrete element method (DEM). To eliminate the flat wall effect on the structure of solid flow in lab observations, a cutting method was developed to observe the vertical section of the solid flow by inserting a transparent plate into the experimental BF model. Both the experimental and numerical results indicated that plug flow is the main solid flow pattern in the upper and middle zones of BFs during burden descending. Meanwhile, a slight convergence flow and a deadman zone form at the lower part of the bosh. In addition, the boundary between the plug flow and convergence flow in BFs was determined by analyzing the velocity of the burden in vertical directions and the Wilcox–Swailes coefficient (Uws). The results indicated that the Uws can be defined as a critical value to determine the solid flow patterns. When Uws ≥ 0.65, the plug flow is dominant. When Uws < 0.65, the convergence flow is dominant. The findings may have important implications to understand the structure of the solid flow in BFs.

NUMERICAL AND EXPERIMENTAL ANALYSIS OF THE FLOW OVER A COMMERCIAL VEHICLE - PICKUP

This work presents an ongoing numerical and experimental study of the flow around a pickup vehicle by means of CFD simulations and wind tunnel experiments. The model was based on the light-pickup market in Brazil and it was designed with flat surfaces and sharp edges. One of the objectives of this research was the understanding about the flow pattern around the vehicle, especially in the region behind the cabin and the wake. Another goal was to obtain original data from experimental measurements which could be used on further computational investigations. The experiments were carried out in a low-speed wind tunnel at Reynolds number of 5 x 105. Hot-wire anemometry was used to obtain the velocity profiles. Wall tufts were applied to describe the flow direction and regions of attached/detached and recirculation zones. Acceleration due to the underbody and the shear layer formed on the cabin were well defined, also indicating a region of reverse flow behind the tailgate. The flow visualization allowed the identification of recirculation regions inside the trunk and regions of detached flow. These flow patterns were also reproduced in the CFD simulations resulting satisfactory information to describe the main flow pattern over the pickup vehicle.

Flow patterns and slug scaling of liquid-liquid flow in square microchannels

Liquid-liquid flow regimes in three square microchannels were visualized simultaneously both at the cross-shaped junction and in the microchannel “far” from the junction in order to reveal flow regime evolutions along the microchannel. At the inlet junction, three major flow regimes including tubing/threading, dripping and jetting were mapped using the aqueous capillary number versus the organic Weber number. Correspondingly, in the main microchannel, annular, slug and droplet flow patterns were mapped using two dimensionless numbers (Weber number times Ohnesorge number) of both phases. Both dripping and jetting regimes at the inlet junction can evolve into either slug or droplet flows in the main microchannel. Besides, it was realized that as the organic flow rate increases, the transitional aqueous flow rate at the slug-droplet transition firstly increases, then decreases and then increases again. The droplet formation mechanism has transited from dripping to jetting, which causes the slug-droplet transition to occur at a much lower aqueous flow rate. Moreover, a scaling relation for the slug size in dripping was developed, which can predict the slug length for five different liquid-liquid systems. It applies for liquid-liquid microfluidic devices with a cross-shaped inlet junction in the dripping regime, for slug sizes longer than 1.5 times the channel depth. The slug velocity has been correlated as functions of the capillary number Caj (µcj/γ) by using the continuous phase viscosity and the bulk velocity.

Destination Prediction by Trajectory Distribution-Based Model

In this paper, we propose a new method to predict the final destination of vehicle trips based on their initial partial trajectories. We first review how we obtained clustering of trajectories that describes user behavior. Then, we explain how we model main traffic flow patterns by a mixture of 2-D Gaussian distributions. This yielded a density-based clustering of locations, which produces a data driven grid of similar points within each pattern. We present how this model can be used to predict the final destination of a new trajectory based on their first locations using a two-step procedure: we first assign the new trajectory to the clusters it most likely belongs. Second, we use characteristics from trajectories inside these clusters to predict the final destination. Finally, we present experimental results of our methods for classification of trajectories and final destination prediction on data sets of timestamped GPS-Location of taxi trips. We test our methods on two different data sets, to assess the capacity of our method to adapt automatically to different subsets.

Computational Research of the Main Rotor Hover and Vertical Descent States Based on the Nonlinear Blade Vortex Model

The research is conducted of aerodynamic performance of the Mi-8 helicopter main rotor in the hover state and the vertical descent state, including vortex ring states (VRSs). Total and distributed aerodynamic performance, wake patterns, and main rotor flow patterns are calculated. The findings and results were compared with those provided by other researchers.

Benchmark for Numerical Models of Stented Coronary Bifurcation Flow.

In-stent restenosis ails many patients who have undergone stenting. When the stented artery is a bifurcation, the intervention is particularly critical because of the complex stent geometry involved in these structures. Computational fluid dynamics (CFD) has been shown to be an effective approach when modeling blood flow behavior and understanding the mechanisms that underlie in-stent restenosis. However, these CFD models require validation through experimental data in order to be reliable. It is with this purpose in mind that we performed particle image velocimetry (PIV) measurements of velocity fields within flows through a simplified coronary bifurcation. Although the flow in this simplified bifurcation differs from the actual blood flow, it emulates the main fluid dynamic mechanisms found in hemodynamic flow. Experimental measurements were performed for several stenting techniques in both steady and unsteady flow conditions. The test conditions were strictly controlled, and uncertainty was accurately predicted. The results obtained in this research represent readily accessible, easy to emulate, detailed velocity fields and geometry, and they have been successfully used to validate our numerical model. These data can be used as a benchmark for further development of numerical CFD modeling in terms of comparison of the main flow pattern characteristics.

Dimensionless analysis on liquid-liquid flow patterns and scaling law on slug hydrodynamics in cross-junction microchannels

Liquid-liquid flow patterns and slug hydrodynamics were experimentally studied in non-circular glass microchannels with water-butanol, water-toluene, water–oil and water-hexane systems, considering various hydraulic diameters (600 µm, 400 µm, 200 µm), inlet junctions (crossed, T) and aspect ratios (0.5, 1). The aqueous phase was the continuous phase due to the hydrophilic microchannel walls and the organic phase was the dispersed phase. Three main flow patterns were observed, i.e., annular flow, slug flow and droplet flow. The mechanism of flow pattern formation was explained by a force analysis, based on which, a dimensionless analysis regarding Weber number and Reynolds number was performed to develop general flow pattern transition criteria. Additionally, slug velocities and slug length were investigated. A new scaling law was proposed to predict the slug length and it showed a good agreement with the experimental results. A linear relationship between the slug velocity and the bulk velocity of the two phases was obtained.

Flow boiling heat transfer of propane in 1.0 mm tube

Flow patterns and local heat transfer coefficients were investigated experimentally during flow boiling of R-290 in a tube with inner diameter equal to 1.0 mm. The tests were conducted at heat fluxes ranging from 5 to 60 kW/m2, mass flux from 240 to 480 kg/(m2s) at 25 °C saturation temperature. Both mass flux and heat flux have a significant influence on the heat transfer coefficient. A new flow pattern identification method is proposed, using non-dimensional parameters based on phase velocities and heat flux. The main flow patterns identified were bubbly, plug, slug, churn, wavy- and smooth-annular flow. Convective boiling was found to be predominant for most flow patterns. Moreover, the resulting heat transfer coefficients were compared with predicting correlations available in the literature.

Analysis of the characteristics of two-phase flow pattern in Y-shape mixed microchannel based on double coupled Duffing vibration simulation detection system

In this work, a visualization experiments were carried out in the Y-shape mixed rectangular microchannels with the hydraulic diameter of 133 μm at atmospheric pressure and at room temperature (25 °C). In the experiment, the range of inlet gas superficial velocities was 0.01–16.668 m/s, and the inlet liquid superficial velocity was 0.01–6.945 m/s. The main flow patterns observed in the experiments were slug flow, elongated slug flow, slug-annular transition flow, annular flow. Flow pattern maps were drawn according to the experimental data. Unlike the traditional friction resistance based rationalization for two phase flow patterns, this study relates the two-phase flow pattern mainly to the surface wettability of the material, the shape of the cross section and the hydraulic diameter of the microchannel. The new correlation presents the flow pattern transition boundary according to these three factors. In addition, the coupled Duffing vibration simulation system method is presented as a way to detect signals determining gas-liquid two-phase flow patterns and to analyze the flow characteristics. Based on the instantaneous oscillator velocity and displacement, we can deduce the underlying dynamics of two-phase flow patterns. The Duffing detection method can serve as a model for other nonlinear system analysis and other multi-phase flow characteristics analyses.

Liquid-liquid flow patterns and slug hydrodynamics in square microchannels of cross-shaped junctions

Flow patterns for water-butanol, water-toluene, water-hexane, water-oil and water/glycerol (weight ratio 60:40) mixture-oil two-phase flows were visualized in the cross-shaped junctions of three square glass microchannels with hydraulic diameters of 200µm, 400µm and 600µm. The aqueous phase is the continuous phase contacting the channel walls while the organic phase is the dispersed phase in the experiments. Three main flow pattern groups were observed, including the tubing/threading regime group, the dripping regime and the jetting regime. The flow regimes were mapped based on the Capillary number of the continuous phase and the Weber number of the dispersed phase. The flow rate ratio and the Capillary number of the dispersed phase were also employed to present flow patterns. The effects of hydraulic diameter of the square microchannels, flow rates, and physical properties, e.g., the interfacial tension and the viscosities of the aqueous and organic phases on flow pattern transitions were clarified. Besides, in the dripping regime, the dimensionless slug length can be scaled asa function of the Capillary number of the continuous phase for cross-shaped junctions. The slug velocity is linearly dependent on the average flow velocity in the dripping regime.

Development of a fast evaluation tool for rotating detonation combustors

• Reduced modeling of rotating detonation combustors. • Prediction of the combustor performance in function of the inlet conditions. • Prediction of the radius to detonation height ratio for minimum entropy. Although rotating detonation engines may lead to improvements in the cycle thermal efficiency, their overall optimization is currently constrained by the need to perform time-consuming three-dimensional Navier–Stokes simulations of the combustor. Thus, there is a critical need to establish fast models to calculate the time-dependent temperature and pressure at the combustor outlet for further engine analysis and turbine integration. This manuscript describes a fast 2D simulation code of a rotating detonation combustor. This numerical methodology uses a 1D chemical kinetics solver, to reproduce the change of properties across the detonation wave front. The post-detonation flow is then resolved using a method of characteristics, which allows the prediction of pressure and temperature gain across the combustor. The main flow pattern, including the detonation front features, the shock wave angle and the outlet properties were compared with the solution of a two-dimensional Unsteady Reynolds Averaged Navier–Stokes solver. This approach is subsequently used to predict the detonation height based on the entropy generation across the combustor.

Ultrasound Vector Flow Imaging – could be a new tool in evaluation of arteriovenous fistulas for hemodialysis?

We report the use of a new ultrasound technique to evaluate the axial and lateral components of a complex flow in the arteriovenous fistula (AVF). Vector Flow Imaging (VFI) allows to identify different components of the flow in every direction, even orthogonal to the flow streamline, represented by many single vectors. VFI could help to identify flow alterations in AVF, probably responsible for its malfunction. From February to June 2016, 14 consecutive patients with upper-limb AVF were examined with a Resona 7 (Mindray, Shenzhen, China) ultrasound scanner equipped with VFI. An analysis of mean velocity, angular direction and mean number of vectors impacting the vessel wall was carried out. We also identified main flow patterns present in the arterial side, into the venous aneurysm and in correspondence of significant stenosis. A disturbed flow with the presence of vectors directed against the vessel walls was found in 9/14 patients (64.28%): in correspondence of the iuxta-anastomotic venous side (4/9; 44.4%), into the venous aneurysmal tracts (3/9; 33.3%) and in concomitance of stenosis (2/9; 22.2%). The mean velocity of the vectors was around 20-25 cm/s, except in presence of stenosis, where the velocities were much higher (45-50 cm/s). The vectors directed against the vessel walls presented high angle attack (from 45° to 90°, with a median angular deviation 65°). VFI was confirmed to be an innovative and intuitive imaging technology to study the flow complexity in the arteriovenous fistulas.

Quantitative predictions of cavitation presence and erosion-prone locations in a high-pressure cavitation test rig

Experiments and numerical simulations of cavitating flow inside a single-orifice nozzle are presented. The orifice is part of a closed flow circuit, with diesel fuel as the working fluid, designed to replicate the main flow pattern observed in high-pressure diesel injector nozzles. The focus of the present investigation is on cavitation structures appearing inside the orifice, their interaction with turbulence and the induced material erosion. Experimental investigations include high-speed shadowgraphy visualization, X-ray micro-computed tomography (micro-CT) of time-averaged volumetric cavitation distribution inside the orifice as well as pressure and flow rate measurements. The highly transient flow features that are taking place, such as cavity shedding, collapse and vortex cavitation (also known as ‘string cavitation’), have become evident from high-speed images. Additionally, micro-CT enabled the reconstruction of the orifice surface, which provided locations of cavitation erosion sites developed after sufficient operation time. The measurements are used to validate the presented numerical model, which is based on the numerical solution of the Navier–Stokes equation, taking into account compressibility of both the liquid and liquid–vapour mixture. Phase change is accounted for with a newly developed mass transfer rate model, capable of accurately predicting the collapse of vaporous structures. Turbulence is modelled using detached eddy simulation and unsteady features such as cavitating vortices and cavity shedding are observed and discussed. The numerical results show agreement within validation uncertainty with the obtained measurements.

Numerical simulation of gas-liquid two-phase flow in channel fracture pack

Channel fracturing technique (CFT) is a stimulation method which can promote the generation of high-fracture-conductivity open channels and get higher production and less cost in fracturing operation. This paper presents the numerical simulations of the gas-liquid two-phase flow in channel fracture packs by establishing a new model. To model the multi-phase flow in channel fracture pack, a two-fluid basic model is adopted and extended by including slip velocity and friction force in the closure relations. The flow in channel and in pillars are coupled by pressure and this model is solved using finite volume method. Also, a series of laboratory experiments were carried out to obtain the structure of channel fracture packs, and the measurement of fracture conductivity was carried out and the fracture productivity formula was used to validate the model. The simulation results of pressure, velocity and streamline are analyzed and discussed in this paper, and it shows that the main reason for the significant decrease in the actual production compared to the expected value in channel fracturing well is that ideal continuous channels which has high-fracture-conductivity are not formed, and the structure of pore-channels which complicates the flow laws are formed. The appearance of the structure of pore-channels will change the expected main flow pattern which is along the continuous open channels to a complicated flow which will bring much friction resistance. The flow laws in the channel fracture pack are primarily dominated by the number and distribution of pore-channels. Also, the factors such as bottom hole flowing pressure (BHFP), viscosity and density of the mixture in the formation, numbers of phase states and mass flow rate have impact on it, mainly in the aspect of velocity distribution. The natural gas and the formation water both have a similar behavior in pore-channels but have different flow trends in proppant packs, and their velocity in pore-channels are relatively higher than them in proppant packs, and compared to the velocity of natural gas, the velocity of formation water in pore-channels is relatively high. The original flowing trend in proppant pack changes and tends to show a sudden inrush to pore-channels which will bring a large amount of inlet, outlet and linear losses.