Kinds Of Flow Patterns Research Articles

The Change of Flow Pattern from Stratified to Stratified-Wavy for Condensation in Wire on Tube Heat Exchangers

Flow patterns inside wire-on-tube condensers with different refrigerant mass flow rates were studied in a theoretical study. In this study, tubes with diameters of 3.25 mm (3/16"), 4.83 mm (1/4") and 6.29 mm (5/16") were used. R-134a and R-600a cooling fluids were used at condensing temperatures of 54.4°C, 45°C, and 35°C. The results of this study were obtained using Equal Equation Solver (EES) software. The proposed model was able to predict the type of refrigerant flow pattern based on the limitations reported in previous studies. It was possible to distinguish four kinds of flow patterns: laminar, wavy laminar, plugged, and spiral. The first variation in flow pattern from laminar to wavy laminar flow found between 0.8 and 0.39, and a second variation in flow pattern found from wavy laminar flow to plug or slug flow between 0.15 and 0.05. For the refrigerant conditions, the condensation temperature did not affect the flow pattern. When using R-134a, the inner tube diameter had no effect on the flow pattern. Change occurs with R-600a as inner diameter was increased.

A Comparative Study of Flow Boiling Heat Transfer and Pressure Drop Characteristics in a Pin-Finned Heat Sink at Horizontal/Vertical Upward Flow Orientations

Abstract With the continuous development of power electronic devices toward miniaturization and compactness, it is necessary to develop more efficient flow boiling heat transfer technologies. In this work, the flow boiling heat transfer and pressure drop characteristics of Novec649 in a pin finned channel under two kinds of flow orientations (horizontal and vertical upward) are experimentally investigated. Heat flux, inlet flow velocity, and inlet subcooling are considered as the variable parameters. The results show that among all boiling operating conditions, the heat transfer performances between two orientations are basically consistent, while the pressure drop of vertical upward pin finned channel is relatively lower, indicating that the comprehensive flow boiling heat transfer performance of vertical oriented channel is better. Subsequently, a series of flow visualization experiments are performed in vertical upward pin finned channel. With the increase of heat flux, four kinds of flow pattern are discovered in the order of dispersed bubble flow, bubble flow, homogeneous flow, and annular flow. In the region of annular flow, although a vapor flow has already formed in the channel, there is still a large amount of liquid phase surrounding the wall and pin fins. Therefore, no obvious heat transfer deterioration was observed in the pin finned channel. Along the flow direction, the diameter of bubbles will increase first, and then present obvious oscillation. As the heat flux increases, both the average bubble detachment diameter and the frequency increase correspondingly. As the fluid velocity increases, the average bubble detachment diameter presents a downward trend, while the average bubble detachment frequency presents an upward trend.

ST-ExpertNet: A Deep Expert Framework for Traffic Prediction

Recently, forecasting the crowd flows has become an important research topic, and plentiful technologies have achieved good performances. As we all know, the flow at a citywide level is in a mixed state with several basic patterns (e.g., commuting, working, and commercial) caused by the city area functional distributions (e.g., developed commercial areas, educational areas and parks). However, existing technologies have been criticized for their lack of considering the differences in the flow patterns among regions since they want to build only one comprehensive model to learn the mixed flow tensors. Recognizing this limitation, we present a new perspective on flow prediction and propose an explainable framework named ST-ExpertNet, which can adopt every spatial-temporal model and train a set of functional experts devoted to specific flow patterns. Technically, we train a bunch of experts based on the Mixture of Experts (MoE), which guides each expert to specialize in different kinds of flow patterns in sample spaces by using the gating network. We define several criteria, including comprehensiveness, sparsity, and preciseness, to construct the experts for better interpretability and performances. We conduct experiments on a wide range of real-world taxi and bike datasets in Beijing and NYC. The visualizations of the expert's intermediate results demonstrate that our ST-ExpertNet successfully disentangles the city's mixed flow tensors along with the city layout, e.g., the urban ring road structure. Different network architectures, such as ST-ResNet, ConvLSTM, and CNN, have been adopted into our ST-ExpertNet framework for experiments and the results demonstrates the superiority of our framework in both interpretability and performances.

Mechanism Analysis of Falling Film Evaporation Two-Phase Flow in Narrow Channels

Abstract As an efficient heat and mass transfer technology, falling film evaporation has attracted more and more scholars' attention. In this paper, a falling film evaporation platform was established to discuss the mechanism of two-phase flow in narrow channels. The width of the channel varied from 0.5 to 2.0 mm. We focused on the relationship between heat transfer characteristics with flow patterns, channel width, and pressure loss. Three kinds of flow patterns were observed: bubble flow, restricted bubble flow, and dry area. In channels with the same width, the initial heat flux of subcooled evaporation increased linearly with the growth of flow rate. But at the same flow rate, the initial heat flux of subcooled evaporation decreased when the channel width became larger. The heat transfer coefficient on the wall was influenced by the bubble generation frequency and bubble separation diameter. Higher bubble generation frequency led to smaller bubble separation diameter. Through mathematical analysis, it was found that the heat transfer coefficient changed with the channel size. And a boundary was observed between the strengthening and weakening status. The comparison of theoretical and experimental values showed good agreement.

How to electromagnetically stir in the slab continuous casting mould

Fluid flow plays a significantly important role in the continuous casting process which has a great influence on the steel quality. Electromagnetic stirring (EMS) has become a fairly established technology to control the flow behaviour within the mould. However, there still exist two important questions need to be answered, that is, what kind of flow pattern is the best one to achieve the high steel quality under EMS, and how to determine the optimal EMS parameters. Therefore, in order to clarify these questions, a methodology based on numerical simulation in an actual slab continuous casting mould has been developed to select an optimal EMS current, and three evaluation criteria are proposed in this paper, including: (1) a free surface rational criterion, (2) a uniformity criterion of the flow velocity distribution, (3) an inclusion removal criterion. By synthetically considering these evaluation criteria, the optimal EMS current is determined under different operating parameters. Meanwhile, this methodology can also be applied to optimize other operating conditions of continuous casting. Tables 1, Figs 8, Refs 13.

Image Reconstruction Based on Sequential Monte Carlo Principle for EMT

Due to the single and limited prior information in electromagnetic tomography (EMT), the existing reconstruction methods are often unable to reconstruct the image with high accuracy for different kinds of flow patterns. In this article, an image reconstruction algorithm based on the sequential Monte Carlo principle for EMT is proposed. It can increase the diversity of samples to extract more image features for EMT images reconstruction and maintain a high image reconstruction effect for different flow patterns. First, the EMT image reconstruction process is modeled as a dynamic time-varying system to search for the optimal estimation in state space. At the same time, controllable adjustment of probability density distribution can improve the reconstruction effect under different flow patterns. Second, samples are obtained from the initial sample space, and their weights are updated according to the samples’ information. The numbers of effective samples and the diversity of samples are increased by resampling. Finally, the system state estimation is calculated according to the samples and their updated weights, and the system state is updated iteratively until the optimal solution is obtained. Simulation results have shown that the proposed method is superior to other existing methods in image error and correlation coefficient, and has better imaging quality.

Review of measurement techniques for void fraction of two-phase flow through annulus

This paper presents a state of the art measurement and imaging systems used to determine the Gas Void Fraction (GVF) of two-phase flow through annuli. Several research works were conducted for GVF measurement of two and three-phase fluids flowing through full circular pipelines but only few of them targeted the annuli. The highly restricted access and the low volume within which the multiphase fluid flows through in annuli make the instrumentation design challenging. This is especially true if non-intrusive sensors are required to not disturb the flow, which is the case, for instance, if an accurate CFD modeling of annulus flow is sought out of these measurements. Motivated by the numerous industrial applications of annuli, several researchers have suggested various corresponding non-invasive GVF measurement devices to better understand the behavior of the annulus flow and also to determine the production rate. Many of these techniques are based on electrical conductivity measurements using one or several pairs of electrodes. Various configurations of the electrodes were suggested and this paper highlights that none of them can cope with all kinds of flow patterns. Imaging techniques using wire mesh and electrical resistance tomography were also suggested and the initial corresponding results were very encouraging. This paper provides a critical review of each of these techniques and highlights the advantages, disadvantages, and limitations of each one. Hence, it can be a valuable research reference for either new scientists in the area of instrumentation to design a sensor targeting annulus flow or to experienced engineers and researchers to keep in touch with the most recent research advances in the area.

Simplified and highly stable thermal Lattice Boltzmann method simulation of hybrid nanofluid thermal convection at high Rayleigh numbers

A new method called simplified and highly stable thermal lattice Boltzmann method, which is based on the lattice Boltzmann framework, was used to simulate the hybrid nanofluid natural convection and heat transfer in a square enclosure with a heating obstacle at high Rayleigh numbers. Four fins protrude from the heating obstacle to affect the flow pattern and heat transfer performance. The effects of the Rayleigh number (106 ≤ Ra ≤ 109), nanoparticle volume fraction (0 ≤ ϕ ≤ 0.05), and length of the fin (0.1 ≤ h ≤ 0.3) on the flow pattern, temperature distribution, and heat transfer characteristics were illustrated and analyzed. To validate the present method, the benchmark simulation results were performed. Three kinds of flow patterns (steady symmetry, unsteady symmetry, and unsteady asymmetry) can be identified at various Rayleigh numbers. At different lengths of fins, the critical Rayleigh number of flow pattern transition (from steady symmetry to steady asymmetry and from steady asymmetry to unsteady asymmetry) is different. ϕ and h also significantly affect the flow pattern. At higher ϕ, the flow inside the enclosure is steadier and the effect of h on the flow pattern varies at different Ra.

Experimental study on the effect of step-changing heating on water flow boiling in a horizontal long mini channel

In this study, experiments were performed to investigate the flow boiling of deionized water in a rectangle (1 mm × 2 mm) horizontal long mini channel, and visualization experiments were carried out to study the transformations of flow patterns. Uniform heating (UH for short) and step-changing heating (SCH for short) were studied. The local heat transfer coefficients during both heating methods were compared and analyzed. Heat transfer coefficient curves showed same trends, but the slope of local heat transfer coefficient under of SCH was steeper and the occurrence of the dry-out flow would make the local heat transfer coefficient decrease. Besides, six kinds of flow patterns were observed, and an amendment correlation was proposed for a better prediction of heat transfer coefficient.

Numerical investigation into the characteristics of a vortex in an argon inductively coupled plasma

A numerical simulation of a technical radio frequency inductively coupled plasma was made with three coils and coupled power in the range of 2–10 kW. The generator frequency varied from 1.76 to 13.56 MHz. The mechanisms of forming a vortex on the upstream side of the discharge and the influences of coupled power, generator frequency and flow rate of central gas on the vortex dimensions were studied. Special attention was paid to investigate two different kinds of vortex flow pattern—Benard and toroidal—as well as the vortex intensity under those two kinds of flow pattern. A critical flow rate of central gas, above which the flow pattern would transform from a Benard to a toroidal vortex, was found on the basis of force balance between the dynamic head of central cool gas and magneto-hydrodynamic and Stokes drag forces of plasma. The influences of coupled power, generator frequency, diameter of the injection probe and the compositions of central gas on the critical flow rate of the central gas were discussed. The dependences of the vortex intensity and maximum vorticity on the coupled power under different generator frequencies and flow rates of central gas were established.

Gas⁻Solid Two-Phase Flow Pattern Identification Based on Artificial Neural Network and Electrostatic Sensor Array.

A method for gas–solid two-phase flow pattern identification in horizontal pneumatic conveying pipelines is proposed based on an electrostatic sensor array (ESA) and artificial neural network (ANN). The ESA contains eight identical arc shaped electrodes. Numerical simulation is conducted to discuss the contributions of the electrostatic signals to the flow patterns according to the error recognition rate, and the results show that the amplitudes of the output signals from each electrode of the ESA can give important information on the particle distribution and further infer the flow patterns. In experiments, the average values and standard deviations of the eight output signals’ amplitudes are respectively extracted as the inputs of the ANN to identify four kinds of flow patterns in a pneumatic conveying pipeline, which are fully suspended flow, stratified flow, dune flow and slug flow. Results show that for any one of those two input values, the correct rates of the ANN model are all 100%.

Vortex-induced vibrations of a square cylinder under linear shear flow

This paper investigates the numerical vortex-induced vibration (VIV) of a square cylinder which is connected to a 2-DOF mass-spring system and is immersed in the planar shear flow by employing a characteristic-based split (CBS) finite element method (FEM). The reduced mass of the square cylinder is Mr = 2, while the reduced velocity, Ur, is changed from 3 to 12 with an increment of ΔUr = 1. The effects of some key parameters on the cylinder dynamic responses, vibrating frequencies, the flow patterns as well as the energy transferred between the fluid and cylinder are revealed. In this study, the key parameters are selected as follows: shear ratio (k = 0, 0.05 and 0.1) and Reynolds numbers (Re = 80 and 160). Numerical results demonstrate that the X–Y trajectories of the cylinder mainly appear as a symmetrical figure ‘8’ in uniform flow (k = 0) and an unsymmetrical figure ‘8’ and ‘O’ in shear flows (k = 0.05 and 0.1). The maximum oscillation amplitudes of the square cylinder in both the inline and transverse directions have distinct characteristics compared to that of a circular cylinder. Two kinds of flow patterns, ‘2S’ and ‘P + S’, are mainly observed under the shear flow. Also, the mean values of the energy of the cylinder system increase with the reduced velocity, while the root mean square (rms) of the energy reaches its peak value at reduced velocity Ur = 5.

Experimental and Numerical Studies on Microbubble Generation and Flow Behavior in a Microchannel with Double Flow Junctions

Experimental visualizations and numerical simulations were carried out to investigate the bubble generation mechanisms in a microchannel with double flow junctions. Two kinds of flow patterns could be observed from the experiments and simulations: slug-bubble flow and split flow. When 0.00554 < Ca2 < 0.025 and 0.25 < QG/QL < 6, slug-bubble flow can be observed. When Ca2 < 0.00554 and 6 ≤ QG/QL ≤ 8, split flow will occur. Bubble length and velocities were also analyzed experimentally and theoretically. The relationship between bubble length, Ca and flow rate ratio QG/QL was obtained by regression, and the correlation could predict the data within an RMS error of 20%. Bubble velocity data correlated with the drift flux model showed that the distribution parameter of the two flow junctions C01 andC02 had values of 1.0654 and 1.1123, respectively.

Thermal-solutal capillary-buoyancy flow of a low Prandtl number binary mixture with a −1 capillary ratio in an annular pool

A series of three-dimensional numerical simulations on thermal-solutal capillary-buoyancy flow in an annular pool were carried out. The pool was filled with silicon-germanium melt with an initial silicon mass fraction of 1.99%. The Prandtl number and the Lewis number of the working fluid are 6.37 × 10−3 and 2197.8, respectively. Both the radial temperature gradient and the solute concentration gradient were applied to the annular pool. The capillary ratio was assumed to be −1, which means that the solutal and thermal capillary effects were equal and opposite. Results show that the thermal-solutal capillary-buoyancy flow always occurs at this special case with the capillary ratio of −1, and even in a shallow annular pool with an aspect ratio of 0.05. With the increase of the thermal Marangoni number, four kinds of flow patterns appear orderly, including concentric rolls, petal-like, spoke, and rosebud-like patterns. These flow patterns are strongly influenced by the local interaction between the solutal and thermal capillary effects and the vertical solute concentration gradient near the outer cylinder. A small vortex driven by the dominant solutal capillary effect emerges near the inner cylinder, which is different from the flow pattern in a pure fluid. In addition, the critical thermal Marangoni number of the initial three-dimensional flow decreases with the increase of the aspect ratio of the annular pool.

Turbulence effects on the wake characteristics and aerodynamic performance of a straight-bladed vertical axis wind turbine by wind tunnel tests and large eddy simulations

The wake characteristics of wind turbines are of crucial importance for the optimum placement of multiple turbines. In this study, the wake development of a straight-bladed VAWT (vertical axis wind turbine) was investigated and compared in low-turbulence smooth and grid-turbulence flows. The wake fields in both flow regimes were measured with WTTs (wind tunnel tests). To further examine the flow physics, LES (large eddy simulation) was performed with a structured mesh. An algebraic wall-modeled LES capable of overcoming the Reynolds number scaling problem was used. The LES models accurately captured the velocities in both flow regimes from the WTTs. It was shown in the WTTs that the grid turbulence benefited both self-starting and wake recovery. The LES results also suggested delayed dynamic stall and greater power production in the turbulent flows. Moreover, it was revealed by the WTTs and LES that the wake exhibited great asymmetry in the horizontal direction, whereas it was approximately symmetrical against the blade mid-span plane. Further, vortex-ring structures consisting of counter-rotating vortices in the wake were discovered in both flow regimes. This kind of flow pattern was assumed to contribute to fast wake recovery.

Visual experimental research on bubble absorption in a vertical tube with R124–DMAC working pair

Visual experimental investigations were carried out on a vertical co-current glass tube absorber to observe the behavior of the R124 (2-chloro-l,l,l,2,-tetrafluoroethane)–DMAC (N′,N′-dimethylacetamide) bubble absorption process and study the effects of different operating conditions on absorption performance. The results show that the refrigerant vapor flow rate, absorption solution flow rate, concentration, temperature and nozzle orifice diameter significantly affect the flow pattern and absorption height. As the vapor flow rate, absorption solution concentration and temperature, and nozzle orifice diameter increases, the absorption height increases. As the absorption solution flow rate increase, the absorption height reduces. Under the conditions of a large refrigerant vapor flow rate, high solution inlet concentration and temperature, three kinds of flow pattern, namely churn, slug and bubbly flow, can be simultaneously observed in the absorption tube.

Flow boiling heat transfer in minichannels at high saturation temperatures: Part II – Assessment of predictive methods and impact of flow regimes

In order to evaluate the reliability of the current flow boiling heat transfer prediction methods for conditions of high saturation temperatures, this paper focuses on the comparison between experimental results (presented in the first part of this two-part article) and results predicted with the commonly used correlations or models from the literature. The dataset was obtained with R-245fa as working fluid in a 3.00mm inner diameter stainless steel tube. It is characterized by a saturation temperature ranging from 60°C to 120°C. The database is composed of 5964 data points covering four flow patterns: (i) intermittent flow, (ii) annular flow, (iii) dryout flow, and (iv) mist flow regimes. An extensive literature review was performed to select the flow boiling heat transfer prediction methods that were classified according to their theoretical background. Finally, thirty flow boiling prediction methods were assessed against our database. The results are presented graphically but also statistically. The effect of the saturation temperature and the kind of flow pattern on the ability of the methods to predict the flow boiling heat transfer coefficient were investigated. At 60°C, most of the prediction methods produce homogeneous results and are able to predict with accuracy the flow boiling heat transfer coefficient. On the contrary at 120°C, the existing methods fail to predict the heat transfer coefficient with accuracy. The only methods able to capture the experimental trends are those developed from carbon dioxide data with or without other fluids.

Flow boiling heat transfer in minichannels at high saturation temperatures: Part I – Experimental investigation and analysis of the heat transfer mechanisms

This paper presents new experimental data concerning flow boiling heat transfer in minichannel at high saturation temperatures. The experimental data were obtained in a horizontal 3.00mm inner diameter stainless steel tube with R-245fa as working fluid. The mass velocity ranges from 100 to 1500kg/m2s, the heat flux varies from 10 to 50kW/m2 and the inlet vapor quality from 0 to 1. This experimental work is characterized by a saturation temperature ranging from 100°C to 120°C. Flow boiling heat transfer coefficients in these conditions have not been reported in the open literature so far. Four flow patterns are likely to appear in these conditions: intermittent flow, annular flow, dryout flow and mist flow regimes. The kind of flow pattern has a major influence on the heat transfer mechanisms. The influence of the mass velocity and the heat flux was investigated to identify the dominant heat transfer mechanisms. At high saturation temperatures, the experimental results clearly show the dominance of nucleate boiling over a wide range of vapor quality.

GS16 音響計測による気液二相流動様式の判定についての研究

Identification method of flow pattern was studied by using sound data which were taken and analyzed to flow through pipe. Sound data of two-phase flow were taken on flow through vertical pipe. Five kinds of flow pattern were made for sound data and analyzed. Frequency spectrums could show the features in each flow, almost these features existed below 250Hz. By using auditory roughness which is one of the methods of psychoacoustic sound quality evaluation, we found specific feature on containing large amount gas flow.

Non-equilibrium thermodynamic analysis of quasi-static granular flows

Granular flow is usually divided into three kinds of flow pattern, namely quasi static flow, slow flow, and rapid flow. The core issue of the research is the constitutive relation. A series of constitutive relations of application value have been received up to now, however, the study on principal theory is insufficient. Granular flow has an emergent mesoscopic structure, such as force chain network and vortex, involving complex irreversible processes. This paper studies its mesoscopic structure and principal characters, introduces the concept of two granular temperatures Tconf and Tkin of the granular flow to characterize the degree of chaotic motion and disordered configuration evolution, sets them as the non-equilibrium variables to constitute the thermodynamic state variables set for granular flow with the classical irreversible thermodynamic (CIT) variables, also determines the granular flow law of energy conversion and the entropy production rate, etc., and develops the two granular temperatures (TGT) model. Taking the simple shear quasi-static granular flow in a constant volume as example, and combining it with the discrete element method (DEM), this work confirms the material parameters needed for the TGT model, and analyzes the law of developing period and the effective coefficient of friction of steady period of granular flow.