Evolution Of Flow Patterns Research Articles

Experimental study on the evaporating progress of hexane lenses on water surface with different temperature

The evolution of an evaporating liquid on another nonmiscible fluid is important for many applications. An experimental study on the evaporation process of hexane lens on the surface of deionized water at different temperatures was carried out, and the surface temperature distribution characteristics and internal flow pattern evolution was analyzed. Results show that the surface temperature of hexane lens can be divided into two parts or three areas depending on the temperature of deionized water. The increasing temperature of deionized water gradually changes both vertical temperature gradient and radial temperature gradient inside hexane lens, and their competition determines the flow pattern inside the hexane lens. Flow along hexane edge caused by circumferential surface tension gradient forms a “low temperature channel” between the central low temperature region and the strong evaporation edge. The knowledge will benefit the fundamental understanding of evaporation characteristics of volatile droplets on a liquid surface.

Experimental and Numerical Simulation of Flow Pattern Evolution in a Flow Mixing Nozzle under a Moderate Inlet Flow Rate.

A flow mixing nozzle is a novel two-phase flow nozzle with a simple structure and has broad application prospects in the industrial field. However, the research on the working characteristics of this nozzle is mainly focused on a low inlet flow rate. There are few research studies on the working characteristics of the moderate and high flow rates which are more concerned in the industrial field, which limits the application of the flow mixing nozzle. In this paper, the experimental and simulation studies on the internal two-phase flow pattern and spray form of the flow mixing nozzle under a moderate inlet flow rate are carried out. The results show that the change in the two-phase inlet flow rate and the tube hole distance of the nozzle will affect the gas and liquid inertia force, thus affecting the working characteristics of the nozzle. The enhancement of gas inertia force will cause the flow pattern inside the nozzle to change to an unstable radially concave cone, and the spray form will be converted to atomization; the atomization effect of the liquid is better. The enhancement of the liquid inertial force will cause the flow pattern inside the nozzle to change to a radially convex cone, and the spray form will be converted to breakup; the atomization effect of the liquid is poor. The liquid surface tension maintains the existence of the cone inside the nozzle and plays a dominant role in jet atomization when the gas and liquid inertial forces are weak. In addition, the study also found that because of the compressibility of gas, the orifice has a reinforcing effect on gas inertia force.

Interfacial flow dynamic micro-response and spatiotemporal evolution of flow pattern for thermocapillary–buoyancy convection in a liquid bridge

Compared with the former studies, the perturbation behavior of thermocapillary–buoyancy convection caused by the simultaneous coupling response of the microscale surface flow, free surface deformation and spatiotemporal evolution of flow patterns is revealed by the combination of experimental and numerical methods for the first time. The free surface morphology transforms from the ‘Ƨ’-shape into the twisted ‘M’-shape in the corresponding balanced stage of thermocapillary–buoyancy convection (at t = 975, Bod = 251.5), and eventually becomes ‘Ƨ’-shape in the corresponding third stage (Bod = 229.9). Meanwhile, there is a weak response of the free surface flow during each transition stage accompanied by periodic hydrothermal waves. The perturbation characteristics of the velocity, the temperature and the transverse location of surface flow are the most prominent at the intermediate height of liquid bridge (y = 0.2). The characteristic of longitudinal velocity mainly presents as the pulsation, while there is also the pulsation inside the oscillation of transverse velocity with the large amplitude (the oscillating period of 2fu = 7.2 s and the amplitude of Au = 0.0057). The periodic characteristic of temperature oscillation is obvious (2fθ = 0.2 s, Au = 0.015).

Experimental study of air-water two-phase flow pattern evolution in a mini tube: Influence of tube orientation with respect to gravity

The evolution of two-phase flow patterns is investigated experimentally in a 1500 mm long capillary tube of 3 mm inner diameter with different orientations with respect to gravity. The flow patterns are recorded with a fast camera (1000 fps) at two locations along the tube under the same operating conditions. The first is close to the inlet zone at L/D = 10 and the second is far downstream at L/D = 420. Co-flowing configurations downward and upward are considered where air is injected through a nozzle of 110 μm of inner diameter and centered on the tube axis. Unlike many existing studies in the literature, the present work explores ranges of small superficial velocities of the two fluids, from 0.78 × 10−3 m/s to 79 × 10−3 m/s for the liquid phase and from 2.3 × 10−3 m/s to 3.54 m/s for the gas phase, in order to deal with smooth interfaces. The experiments show that the flow patterns are very sensitive to inlet conditions. Although, the superficial velocities considered are very small, several two-phase flow patterns are visualized namely bubbly flow, slug flow, unstable annular and annular flow. In the upward flow, the annular pattern is unstable, while for the downward flow, the bubbly pattern is difficult to obtain. Phenomena like coalescence and breakup yield to the evolution of the flow pattern from the entrance of the tube to the downstream location. The visualizations highlight also that certain flow patterns evolve from one structure to another along the tube from the location L/D = 10 to L/D = 420. Furthermore, the two-phase flow maps are drawn and the effect of gravity on the flow maps as well as on the flow pattern transitions is discussed.

Batch transportation of oil and water for reducing pipeline corrosion

Different from the conventional transportation of oil-water mixtures, we proposed a method to reduce pipeline corrosion based on a concept of flow management, where the mixed fluids were firstly separated into oil and water and then a batch transportation of oil and water was introduced to tune the water wetting state at pipeline bottom. In this way, the corrosion risk of pipe bottom can be properly reduced with a minor modification of the transportation procedure. In this paper, the idea was theoretically and experimentally verified, and the modified procedure was effective in corrosion mitigation compared with the conventional oil gathering system in a CO2-enhanced oil recovery (EOR) production system. It was found that the corrosion mitigation efficiency held a positive relationship with the dimensionless length of the oil phase, which was determined by the input length of oil and water batch and the evolution of flow pattern at the oil/water interface. This is of special benefits for the case where a large amount of free water is unable to be entrained into oil phase. Such a concept of corrosion mitigation can be applied in the transportation of corrosive oil-water mixtures in petroleum industry, such as in CO2-EOR and offshore oil production.

Hydrodynamic Pattern in Drying Saline Droplet with Suspended Nanoparticles

Evaporation of multicomponent droplets has gained much attention nowadays because of their complex flow fields and various deposition patterns. Here we observe strong flows in evaporating sodium chloride saline droplets with suspended alumina oxide nanoparticles. The evolution of flow pattern was studied by tracking the trajectories of particles and the velocity field was investigated with Particle Image Velocimetry analysis. The non-uniform evaporation rate along the droplet surface leads to a concentration gradient which induces the convection flow. During the evaporation process before crystallization happens, evolution of the flow can be divided into two regimes. In Regime I, a centrosymmetric convection recirculation is formed gradually. In Regime II, the convection recirculation migrates to the droplet edge and evolves into several small vortexes. At the late evaporation stage, crystallization could induce strong convection flows. It is shown that the flow tends to become more chaotic with a lower salt concentration.

Direct numerical simulations of Rayleigh-Bénard convection of a gas-liquid medium near its density maximum

Oxidation of hydrocarbons is a commonly used chemical reaction due to the prominent economic value of its products in plasticizers, resins and other intermediate manufacturing processes. However, there is a significant explosion danger of the gas-liquid medium in this chemical reaction. Thus, it is necessary to make sense of the flow and thermal fields in the gas-liquid medium to manage the convection, avoid overheating and prevent explosion in the industry. A comprehensive numerical work of Rayleigh-Bénard convection of a gas-liquid medium near its density maximum was presented in this paper. The influences of the density inversion parameter and Rayleigh number on the flow stability, flow pattern evolution, and heat transfer ability of Rayleigh-Bénard convection were analyzed. The results show that the critical Rayleigh numbers for onsets of a steady convection and an unsteady convection rise with the increase of the density inversion parameter. The flow pattern evolution strongly relies on the density inversion parameter. The strength and heat transfer ability of Rayleigh-Bénard convection are enhanced as the Rayleigh number rises, while they are suppressed along with the density inversion parameter. A scaling relationship for the average Nusselt number on the bottom wall in terms of the Rayleigh member and density inversion parameter is successfully proposed. In addition, the predicted results based on the scaling correlation are consistent with those from simulations.

The effect of rotation on the thermal-solutal capillary-buoyancy flow in a shallow annular pool with various capillary ratios

In order to understand the effect of pool rotation on the thermal-solutal capillary-buoyancy convection at various capillary ratios, a series of three-dimensional simulations are performed in a rotating shallow annular pool filled with silicon-germanium melt and subjected to radial temperature and solute concentration gradients under normal gravity. The capillary ratios are Rσ=−0.5, −0.8, −1.25 and −2, while Taylor number ranges from Ta = 0 to 1200. Three types of basic flows are obtained at different capillary ratios. The pool rotation diversely affects the structures of the basic flow and further varies the destabilization mechanisms. The pool rotation enhances the flow stability at Ta < 400 and reduces the flow stability at Ta > 400 with Rσ=−0.5 and −0.8, while it always stabilizes the basic flow at Rσ=−1.25 and −2. The evolution of flow pattern depends remarkably on the Taylor number and the capillary ratio. Multiple flow bifurcations occur at small capillary ratios or high Taylor numbers, which results from the coupling of various driven forces. The reverse transition process is observed at Rσ=−0.5 and −0.8 with Taylor numbers slightly larger than the turning point. Meanwhile, abundant oscillatory flow patterns are shown. The effects of pool rotation on their characteristics are discussed.

Transient flow patterns in an evaporating sessile drop: A numerical study on the effect of volatility and contact angle

Abstract In this paper, we numerically investigated the evolution of flow pattern inside an evaporating sessile drop using a transient numerical model based on the Arbitrary Lagrangian–Eulerian (ALE) frame that fully couples fluid flow and heat and mass transfer. Effects of both liquid volatility and initial contact angle on the transient flow pattern were explored. The results show that stronger volatile drops have more vortices whereas multi-vortices circulation lasts for shorter time. With the increasing of contact angle, the maximum number of vortices at first increases gently and then rapidly, while the duration of the multi-vortices circulation at first decreases rapidly and then increases a little bit. Marangoni flow is found to dominate the formation of vortices in most of the simulated cases, whereas natural convection plays a role in drops of large contact angles. These conclusions agree with the judgement criteria based on Marangoni number and Grashof number. Besides the classical symmetrical vortices, encompassing vortices are observed during the transient development of internal flow. These encompassing vortices normally form either through the merging of a big vortex with its adjacent small one or through the splitting of a big vortex into two smaller ones.

Identification of gas-liquid two-phase flow patterns in dust scrubber based on wavelet energy entropy and recurrence analysis characteristics

For a wet dust scrubber, dust collection efficiency is tightly connected with the gas-liquid two-phase flow pattern. Using the characteristic parameters selected by current flow pattern identification methods, regions of significant coincidence exist among different patterns, thereby leading to a decline in the identification efficiency. In this study, a new method for processing the wavelet decomposition signal of the dust collector pressure was proposed to obtain the characteristic parameters that distinguish the flow pattern. Firstly, detailed information regarding the different frequency bands of the pressure signal was extracted via wavelet analysis. Then, in combination with the information entropy theory, wavelet energy entropy (WEE) was proposed to evaluate the uniformity of energy distribution in different frequency bands. The results show that WEE is sensitive to the change in gas-liquid two-phase flow patterns, and the corresponding distinguishing efficiency of flow patterns is 92.5%. There was only a small amount of crossover between the shear liquid curtain and entrainment air bubble pattern. For this, using the recursive analysis method (RAM), the characteristic recurrence plots (RP) and recurrence quantification analysis (RQA) of the original pressure signals and the wavelet decomposition signal with different frequency bands were obtained. Results show that the RP characteristics can intuitively reflect the gas-liquid flow state of different flow patterns. Although RQA characteristics are not sensitive to the change in low-level gas/liquid resonance flow pattern in the dust scrubber, it exhibits strong distinction to the evolution of other flow patterns. It effectively compensates for the crossover at the flow pattern distinguish between the shear liquid curtain and entrainment of air bubble using the parameters of the wavelet energy entropy. It is the highlight of this article that the combination of WEE and recurrence characteristics can effectively address the problems of high coincidence of flow pattern features in the dust scrubber.

Three-dimensional wake transition in the flow over four square cylinders at low Reynolds numbers

The three-dimensional characteristics of the flow past four square cylinders in an in-line square configuration, with five spacing ratios ranging from 1.4 to 5, were studied in depth in this study. Direct numerical simulation of the spectral/hp element method was employed at Re = 150 and 200. The onset and evolution of various unstable modes were expounded in detail by means of three-dimensional vortices, energy curves, wake patterns, and force coefficients. At each spacing, the three-dimensional instability and the corresponding flow pattern were comprehensively analyzed to illustrate transitional features. Except for the existence of unstable mode A and mode B when spacing was considerably small and large, for most of the intermediate spacing ratios, the vortex structures were dominated by mode C instability, whose flow patterns all appeared as anti-phase synchronization. Through the evolution of flow patterns over time, the three-dimensional effects were already observed at a low Reynolds number of 150 because of the influence of the gap flow and the mutual interference of the wake. Under the transitional spacing for Re = 200, multiple modes were interfering fiercely with each other and appeared as chaotic states. Compared with other bluff body forms, the four square cylinders generated numerous discrepancies and new modal transitions in three-dimensional cases.

Numerical analysis of nitrogen condensation heat transfer enhancement with liquid film fluctuation at cryogenic temperature

Film condensation is the main condensation mode for cryogenic fluids with extremely low surface tension. Deep understanding of how fluctuating liquid film affects heat transfer is of great significance to the design and optimization of cryogenic condensers. In this study, a numerical model of nitrogen condensation on a vertical plate is established which adopts an accurate condensation mass transfer model, and the simulation results match well with the experimental data. The transient flow behaviors like thickness, temperature field, velocity field of liquid nitrogen film and various heat transfer coefficients are analyzed. Results show that the condensed liquid nitrogen film undergoes a flow pattern evolution from laminar flow to sinusoidal-like wave flow and then solitary wave flow. Compared to water condensation, nitrogen condensation has a more intense fluctuating flow pattern due to the lower latent heat. Large local heat transfer coefficient could be found at both the trough and peak areas of the film waves, while the lowest value occurs at the front of the wave peak. The analysis shows that the liquid film in the laminar flow zone between the two peaks is thinned to below Nusselt's theoretical value, and the convection effect at the peak is enhanced due to a sharp increase in the wave velocity. Finally, the time domain characteristics of the local heat transfer coefficients reveal that the synergistic effect of intermittent thinning of film thickness and enhanced convection in solitary waves improves the heat transfer coefficient up to 30% compared with the Nusselt's theoretical value for laminar flow.

Synergistic effect of flow pattern evolution of dispersed and continuous phases in direct-contact heat transfer process

The marker-controlled multiple watershed segmentation are achieved to distinguish darker continuous phase in gas-liquid two-phase flow patterns efficiently. Each plot of the Betti numbers βi is curve-fitted using a four-parameter logistics model, for characterizing mixing effects. Similarity between adjacent pixels can be quantified by the distance. The β1 of continuous phase decreases linearly at distance ≥ 2, which can be used to determine the threshold for segmentation. Repeated tests with different pixels and methods are conducted to ensure the repeatability and effectiveness of this model. More interestingly, we find that the rapid increase of β1 of bubbles swarm coincides with the evolution of β1 of continuous phase, and the median of difference of β1 between the two phases in the visible window, as a novel metric of flow regime control is obtained and correlated with average volumetric heat transfer coefficient. This dynamic image analysis method, equipped with computational homology, provides the ability of evaluation of the spatial flow structure from a two-dimensional image and can be used to control the process.

Effects of flow pattern on hydraulic performance and energy conversion characterisation in a centrifugal pump

An experimental investigation based on particle image velocimetry (PIV) technology was used to measure the internal flow in a low-specific-speed centrifugal pump impeller. The main purpose of this paper is to quantitatively study the influence of the internal flow patterns on the hydraulic performance and energy conversion characteristics through this visualization experiment. The PIV measuring region covers full impeller channels. Phase-averaged relative velocity distributions were calculated to investigate the evolution of internal flow pattern with the decrease in flow rate. The effect of flow pattern on pump head has also been investigated qualitatively. Results show that a clockwise vortex on the blade suction side and a counterclockwise vortex on the blade pressure side will appear in different impeller passages with the decrease in flow rate. The counterclockwise vortex moves towards the impeller outlet, but the clockwise vortex moves to the opposite side. Detailed studies show that the motion of the vortices considerably impacts pump hydraulic performance. The counterclockwise vortex will increase energy losses, whereas the clockwise vortex positively affects the pump head. Moreover, the blade loading distribution and its variation with flow rate are proposed to analyse the energy conversion mechanism in the centrifugal pump impeller.

Flow around the wing models with straight and swept leading edge in case of contact with turbulent wake

The paper presents the results of experimental studies of the flow around wing models with a straight and swept leading edge in contact with a turbulent wake. The experiments were carried out in subsonic wind tunnels at low Reynolds numbers. A fluffy thread was used to simulate a turbulent wake in the flow. The flow patterns on the surface of the wings were obtained before and after the impact of the turbulent wake at various angles of attack, wing sweep angles and free-stream velocity values. It was found that the wake has a significant effect on flow separation and on vortex structures arising above the wings. The evolution of the vortex flow patterns in the areas of separation of the boundary layer and with a change in the flow regime is shown.

Experimental study of transient friction characteristics and velocity distribution of pulsatile flow in rod bundles

Rod bundles are widely used in various devices and energy engineering including heat exchangers, nuclear fuel assembly and so on. The transient fluctuation of flow rate in the thermal devices is a common phenomenon under accident scenarios or ocean condition, the study of pulsatile flows has significant practical relevance for the design and safety of thermal devices. In order to determine the flow characteristics of pulsatile flows in rod bundles, the experimental investigations for transient friction loss and evolution of flow field of pulsatile flows of a 5 × 5 rod bundle (P/D = 1.326) are performed. The experiments were conducted under the ranges of time-averaged Reynolds number Reta = 2000–7000, pulsatile amplitude Au = 0.2–0.6 and pulsatile period T = 10 s–100 s. A comparison of friction factors under steady flows and pulsatile flows is performed to determine the influence of pulsatile flows on frictional characteristics. The results show that the transient friction factors versus flow rate under pulsatile flow presents an “oval” shape, and the “oval” shape has a variation with pulsatile parameters changing. To further reveal the details of pulsatile flow fields, the evolution of flow pattern in the pulsatile flow of Reta = 5200, Au = 0.6, T = 10 s is visualized in rod bundles by Particle Image Velocimetry (PIV). The velocity distribution in different stages of pulsatile flows are displayed and discussed. The influence of velocity gradient and turbulent shear stress in the whole pulsatile period for frictional loss is analyzed, and the variation of transient friction factors under different pulsatile parameters is revealed. The results describe transient behaviors to contribute a better understanding for transient flows in fuel assemblies.

Visualization experiment and numerical simulation of nitrogen-neon mixture condensation

The condensation at cryogenic temperatures is rich in physics and challenging to explore, in particular for mixtures. The flow pattern evolution of liquid film for both pure nitrogen and nitrogen-neon mixture condensation is visualized. An improved mass transfer model is proposed to simulate the mixture condensation process, and the diffusion coefficient of nitrogen-neon mixed vapor and saturation temperature of nitrogen are modified. The new model agrees well with the experimental data. The complete transition process of three typical flow patterns, namely, laminar flow, two-dimensional wave flow and three-dimensional wave flow, is observed for the pure nitrogen condensation, while the Reynold number of liquid film decreases greatly during the condensation of nitrogen-neon mixture at the same condensation heat, and the fluctuation effect of interfacial waves shows attenuation. The simulation illustrates that the neon has an accumulation effect near the wall and greatly reduces the mass transfer rate of nitrogen vapor. Besides, it indicates that liquid film is the main thermal resistance for pure condensation, while the non-condensable gas layer at gas-liquid interface becomes a crucial factor affecting the mass transfer performance for mixture condensation. The mechanism of mixture condensation process is revealed through liquid film flow pattern analysis and CFD simulation results.

Experimental study on the flow boiling oscillation characteristics in a rectangular multiple micro-channel

Multiple micro-channels can be used as heat sink components of many electronic devices due to high heat transfer capacity. In this paper, the flow boiling oscillation characteristics of a multiple micro-channel test section made of six rectangular micro-channels with an equivalent hydraulic diameter of 526.2 μm were experimentally studied. The experiments were operated with heat flux qeff 11.58–99.56 kW/m2, mass flux G 20.40–107.04 kg/m2·s and inlet temperature Tin = 55 °C. The flow pattern evolution process in the multiple micro-channel was visually studied and analyzed. Also, the pressure drop characteristics of the multiple micro-channel test section and two individual channels of the multiple micro-channel were measured and studied with time domain and frequency domain analyses, and the relationship between amplitude-frequency characteristics of test section pressure drop and qeff/G under typical working conditions was gained. The results showed that under oscillation conditions different flow pattern evolution modes may occur for different micro-channels due to the coupling effects of inertial force, the evaporation momentum, heating wall etc. During boiling, reverse flow of bubbles could be observed, and liquid may also be pushed upstream by the bubbles. The differences of oscillation waveform, frequency, phase position as well as the interaction effects of individual channels were found to get smaller with an increase of outlet equilibrium quality. Based on the amplitude-frequency characteristics, five flow boiling patterns were identified as boiling developed, i.e. the single phase pattern, the boiling onset pattern, the reverse flow onset pattern, the reverse flow infecting pattern, and the complete reverse flow pattern. The boundaries of the flow boiling patterns were very clear from a view of oscillation amplitude-frequency characteristics, and it was found that the boundaries can be roughly determined with parameter qeff/G.

Assessment of Climatology and Predictability of Mid-Atlantic Tropical Cyclone Landfalls in a High-Atmospheric-Resolution Seasonal Prediction System

Abstract Tropical cyclone (TC) landfalls over the U.S. mid-Atlantic region, which include the so-called Sandy-like, or westward-curving, tracks, are among the most infrequent landfalls along the U.S. East Coast. However, when these events do occur, the resulting economic and societal consequences can be devastating. A recent example is Hurricane Sandy in 2012. Multimodel ensemble seasonal hindcasts conducted with a high-atmospheric-resolution coupled prediction system based on the ECMWF operational model (Project Minerva) are used here to compile the statistics of these rare events. Minerva hindcasts are found to exhibit skill in reproducing climatological characteristics of the mid-Atlantic TC landfalls particularly at the highest atmospheric horizontal spectral resolution of T1279 (16-km grid spacing). Historical forecasts are further interrogated to identify regional and large-scale environmental conditions associated with these rare TC tracks to better quantify their predictability on synoptic time scales, and their dependence on model resolution. Evolution of the large-scale atmospheric flow patterns leading to mid-Atlantic TC landfalls is analyzed using local finite-amplitude wave activity (LWA). We have identified large-amplitude quasi-stationary features in the LWA and sea surface temperature (SST) anomaly distributions that persist up to about a week leading to these land-falling events. A statistical model utilizing indices based on the LWA and SST anomalies as predictors is developed that exhibits skill (mostly at T1279) in predicting mid-Atlantic TC landfalls several days in advance. Implications of these results for longer time-scale predictions of mid-Atlantic TC landfalls including climate change projections are discussed.

Numerical analysis of flow pattern transition in a conical silo with ellipsoid particles

Gravity-driven discharge flow in conical silos is ubiquitous in manufacturing processes for numerous industries such as food, pharmaceutical and chemical industries, wherein flow pattern is the key topic to be studied. In this work, discrete element method was used to study discharge flow behaviors in a special conical silo. This work aims to understand the dynamic evolution of discharge flow pattern and establish methods for regional determination of flow pattern transition. The results indicated that both mass flow and funnel flow patterns coexist in the silo at the initial stage of discharge, and there is a definite transition process from mass flow to funnel flow. Furthermore, the flow pattern transition is revealed by the change in discharge flow characteristics. Specifically, the change in force acting on particles leads to the change of particle orientation and particle velocity field, macroscopically resulting in the flow pattern transition. Finally, based on the kinetic stress field and shear rate, the height and radial region of flow pattern transition were determined, respectively. Understanding of the flow pattern transition is useful to the design, scale-up and optimization of silos and similar structural devices.