Backflow Phenomenon Research Articles

Experimental and numerical study on air flow uniformity in the isobaric windbox of a 600 MW supercritical CFB boiler

Airflow velocity profile of the primary air in the isobaric windbox of the first 600MW supercritical CFB boiler worldwide under cold state and various boiler loads was experimentally measured and numerically simulated to investigate uniformity of the primary air. Experimental measurements were conducted in the furnace under different boiler loads (100%, 85% and 70% BMCR for the right windbox; 100% BMCR for the left windbox) by a hot wire anemometer. Meanwhile, the effect of key structural parameters (including the pressure-stabilization section height W1, inlet area ratio AR, inclined angle θ and windbox depth) on the airflow uniformity of the isobaric windbox were analyzed based on two-dimension simulations of a series of isobaric windboxes with 43 different configurations, using the CFD software package FLUENT. The measured results showed that while the airflow velocity was basically uniform along the width direction of the windbox, it exhibited an obvious non-uniform profile along the depth direction. The airflow velocity was relatively higher in the front wall and obvious lower in the back wall. As the boiler loads increased, the airflow uniformity became worse because of intensified flow turbulence. Moreover, due to the jet flow at the entrance of the isobaric windbox, the airflow velocity in the initial a few simplified tubes was always lower than that of other parts regardless of the windbox depth. In addition, the airflow profile of the isobaric windbox within 12m was significantly different from those beyond 15m. The structural parameters of the isobaric windbox had obvious influence on the airflow uniformity, and this influence decreased with the increase of windbox dimension. The backflow phenomenon resulting from the interaction between W1 and AR was observed in some simulation cases. Finally, the optimal windbox structural parameters for primary air uniformity were determined by an optimization study of the isobaric windbox structural parameters.

Cuttings migration law in the CBM horizontal well

In this study, to explore the CBM cuttings migration law and influence of cuttings parameters and mechanical parameters on it, a CBM drilling horizontal wells visualisation test bench is built using field data. Through the device, cuttings bed transportation test, changing cuttings parameters test and variable mechanical parameters test were carried out. The content was expanded through CFD simulation analysis. The results showed that the cuttings sank to the bottom and formed into a cuttings bed at the connector, and there was a backflow phenomenon. With cuttings diameter and mass increase, the cuttings cleanup became more difficult. Under the best condition of equipment capacity, cuttings, which diameter is less than 5 mm or volume fraction is less than 5%, could be cleaned up. Improving pump delivery can improve transport efficiency, and increasing drilling pipe rotation can improve the cuttings grinding and cuttings migration. [Received: November 18, 2014; Accepted 23 November 2015]

Cuttings migration law in the CBM horizontal well

In this study, to explore the CBM cuttings migration law and influence of cuttings parameters and mechanical parameters on it, a CBM drilling horizontal wells visualisation test bench is built using field data. Through the device, cuttings bed transportation test, changing cuttings parameters test and variable mechanical parameters test were carried out. The content was expanded through CFD simulation analysis. The results showed that the cuttings sank to the bottom and formed into a cuttings bed at the connector, and there was a backflow phenomenon. With cuttings diameter and mass increase, the cuttings cleanup became more difficult. Under the best condition of equipment capacity, cuttings, which diameter is less than 5 mm or volume fraction is less than 5%, could be cleaned up. Improving pump delivery can improve transport efficiency, and increasing drilling pipe rotation can improve the cuttings grinding and cuttings migration. [Received: November 18, 2014; Accepted 23 November 2015]

Numerical investigation of condensing flow of low pressure steam turbine at different flow rate conditions

The numerical investigation on the wet steam flow in the last two stages of a 1000 MW fossil-fired low pressure steam turbine is presented in this paper. The non-equilibrium model via the classical nucleation theory is employed to simulate the condensing flow of the wet steam. The characteristics of the flow filed from design condition to low volume flow condition are calculated and the static performance of last stage moving blade is also obtained. The development of the backflow phenomenon is clearly captured through the analysis of the velocity triangle.

Numerical study of the unsteady behaviors and rotating stall inception process in a counter-rotating axial compressor

Unsteady computations of a counter-rotating axial compressor are performed and analyzed to investigate the unsteady behaviors in the compressor and the role of the tip leakage flow together with the rotating stall inception process. The results show that the oscillation on the pressure side is much stronger than that on the suction surface for both rotors, especially near the tip region where the trajectory of the tip leakage vortex (TLV) interacts with the blades most often. There exists a periodical leading edge spillage of the interface in rotor2 due to the unsteadiness of tip leakage flow (TLF) at near-stall condition. The blockage generated by the TLV increases dramatically due to the increasing strength of the TLV and the backflow phenomenon as the mass flow decreased. The appearance of the frequency components of 0.5 blade passing frequency (BPF) and 1.5BPF from 0.64BPF can be viewed as the rotating stall inception warning. The fluctuation strength of oscillation frequencies of 0.5BPF and 1.5BPF decreases rapidly from leading edge to trailing edge in rotor2, which indicates that the unsteady fluctuation of TLF at the leading edge in rotor2 is responsible for the stall inception of the compressor. Additionally, both the leading edge spillage and trailing edge backflow phenomena are observed for spike initiated rotating stall at stall point.

CFD evaluation on the thermohydraulic characteristics of tube support plates in steam generator

Abstract The integrity and thermal hydraulic characteristics of steam generator are of great concern in the nuclear industry. The tube support plates (TSP), one of the most important components of the steam generator, not only support the heat transfer tubes, but also affect the flow dynamic and thermal hydraulic characteristics of the secondary-side flow inside the steam generator. Different working conditions, ranging from single-phase adiabatic condition to two-phase high-void boiling condition, are simulated and analyzed. Calculated void fraction, under simple geometry, agrees well with the experiment data whilst the simulated heat transfer coefficient is tremendously close to the empirical correlation. Temperature, void fraction, and velocity distributions in different locations show reasonable distribution. The simulation results indicate that TSP can enhance the heat transfer in the secondary side of the steam generator. On the top of TSP, with the increase in cross-section flow area, the back-flow phenomenon occurs, which might lead to the contamination of precipitation.

Eulerian and Lagrangian stagnation plane behavior of moderate Reynolds number round opposed-jets flow

Turbulent opposed jet (TOJ) flow is characterized by an unsteady stagnation plane formed by two counter-flowing jets. The complicated behavior of the stagnation plane significantly affects the turbulence characteristics in the impinging zone. In this study, direct numerical simulations (DNS) were performed to study the dynamic behavior of the stagnation plane in a three-dimensional round opposed-jet flow with a moderate Reynolds number (Re=4050). Proper orthogonal decomposition (POD) analysis was used to study the behavior of the Eulerian stagnation plane (ESP) and vortex structures. The results show that the first POD mode dominates the axial movement of the stagnation plane, and it contributes most of the axial velocity fluctuation. The stagnation plane is found to tilt relative to the normal direction of the jet flow by the second to fourth POD modes. The shape of the stagnation plane can be reconstructed to a large extent through the dominant POD modes (namely the first four modes). The Lagrangian property of the stagnation plane was investigated through calculation of the field of finite-time Lyapunov exponents (FTLE), in which the ridges can identify the Lagrangian coherent structures (LCSs) in the turbulent opposed jet flow. Throughout the domain three different types of LCSs were found, corresponding to the axial jet region, impinging zone and radial jet area. A Lagrangian stagnation plane (LSP) is defined as the LCS in the impinging zone, which indicates the boundary of the fluid parcels from the two nozzles. A discrepancy between the ESP and LSP was displayed, and this finding implies a backflow phenomenon, which means the moving impermeable LSP always forces the fluid parcels ahead of the LSP to move toward the ESP. As a result of this analysis, the unstable behavior of the stagnation plane is demonstrated.

Thermal-hydraulic analysis of a fluoride-salt-cooled pebble-bed reactor with CFD methodology

The Fluoride Salt Cooled High Temperature Reactor (FHR) is an innovative concept reactor that inherits the technical foundation and advantages of the six optional generation-IV reactors and pressurized water reactors, which is mainly in process in both China and the United States. In this paper, the porous and realistic modeling approaches are adopted to analyze the thermal hydraulic characteristics of a FHR core and a unit segment of pebbles in the core respectively. The distributions of temperature and pressure of the fluoride salt, as well as the reflector temperature profile, are obtained using the porous model. The detailed local flow and heat transfer are investigated by the realistic modeling method for the locations which may have the maximum coolant temperature based on the results of the porous model. The profiles of temperature, velocity, pressure and Nusselt number (Nu) of the coolant on the surface of the pebble are also obtained and analyzed. Numerical results showed that the flow field between the fuel pebbles is complex including secondary flow and back-flow phenomenon, which are hard to measure by experiments. This work can provide useful information for the experimental and mechanism research of FHRs.

Hydraulic optimization of a double-channel pump’s impeller based on multi-objective genetic algorithm

Computational fluid dynamics (CFD) can give a lot of potentially very useful information for hydraulic optimization design of pumps, however, it cannot directly state what kind of modification should be made to improve such hydrodynamic performance. In this paper, a more convenient and effective approach is proposed by combined using of CFD, multi-objective genetic algorithm (MOGA) and artificial neural networks (ANN) for a double-channel pump’s impeller, with maximum head and efficiency set as optimization objectives, four key geometrical parameters including inlet diameter, outlet diameter, exit width and midline wrap angle chosen as optimization parameters. Firstly, a multi-fidelity fitness assignment system in which fitness of impellers serving as training and comparison samples for ANN is evaluated by CFD, meanwhile fitness of impellers generated by MOGA is evaluated by ANN, is established and dramatically reduces the computational expense. Then, a modified MOGA optimization process, in which selection is performed independently in two sub-populations according to two optimization objectives, crossover and mutation is performed afterword in the merged population, is developed to ensure the global optimal solution to be found. Finally, Pareto optimal frontier is found after 500 steps of iterations, and two optimal design schemes are chosen according to the design requirements. The preliminary and optimal design schemes are compared, and the comparing results show that hydraulic performances of both pumps 1 and 2 are improved, with the head and efficiency of pump 1 increased by 5.7% and 5.2%, respectively in the design working conditions, meanwhile shaft power decreased in all working conditions, the head and efficiency of pump 2 increased by 11.7% and 5.9%, respectively while shaft power increased by 5.5%. Inner flow field analyses also show that the backflow phenomenon significantly diminishes at the entrance of the optimal impellers 1 and 2, both the area of vortex and intensity of vortex decreases in the whole flow channel. This paper provides a promising tool to solve the hydraulic optimization problem of pumps’ impellers.

Exceptional suppression of flux-flow resistivity in FeSe0.4Te0.6 by back-flow from excess Fe atoms and Se/Te substitutions

We measured the microwave surface impedance of FeSe$_{0.4}$Te$_{0.6}$ single crystals with- and without external magnetic fields. The superfluid density exhibited a quadratic temperature dependence, indicating a strong pair-breaking effect. The flux-flow resistivity behaved as $\rho_f(B\ll B_{\rm c2})/\rho_n=\alpha B/B_{\rm c2}$. The observed $\alpha$ value of $\approx0.66$ was considerably smaller than that of other Fe-based materials ($\alpha\geq1$) and was attributed to a back-flow of superfluids remarkable in disordered superconductors. This is the first-time observation of the back-flow phenomenon caused by an origin other than the vortex pinning in multiple-band systems.

Numerical simulation of the internal flow field of a new main nozzle in an air-jet loom based on Fluent

In order to decrease the turbulence influence and the backflow during weft insertion in an air-jet loom, a new main nozzle composed of two nozzle needles connected in series is designed. The internal flow fields of the commonly used nozzle and the new nozzle are numerically studied by means of a three-dimensional model implemented by the computational fluid dynamics technique. It is observed that the turbulence strength is effectively controlled with different air supply pressures and the backflow phenomenon at the upstream passage generated by high supply pressures can be eliminated when the supply pressure is reasonably distributed into two inlets of the new nozzle. The drag force exerted on the weft yarn is calculated when the velocity distribution of the internal airflow is obtained, and results show that the drag force can be improved. Also, the numerical simulation can give some guidelines for future prototyping and experiments.

Clinical value of fluid bolus contrast flow meter during hysterosalpingography

The purpose of this study was to investigate the clinical value of the fluid bolus contrast flow meter during hysterosalpingography. Hysterosalpingography information of 342 cases, which included a manual handset group of 213 cases and a bolus instrument group of 129 cases were reviewed. Comparative analysis was used to compare the two groups in order to assess the clinical adverse reactions, contrast agent reflux, and image quality. In the instrument bolus group compared with the manual handset group, the clinical adverse reactions decreased from 75.12 to 31.78% (P < 0.001); the backflow phenomenon of the contrast agent decreased from 13.62 to 3.10% (P < 0.01); and image quality significantly improved, with the A class film rate increasing from 54.46 to 68.99% (P < 0.01) and the C class film rate decreasing from 8.92 to 2.33% (P < 0.05). The use of a contrast bolus through the liquid inlet of the hysterosalpingography instrument can provide fully dynamic observation, reducing the contrast agent reflux and adverse reactions as well as improving the image quality and diagnostic accuracy. In addition, the medical staff is not subjected to radiographic radiation. Therefore, it is a safe and reliable imaging method.

Modeling Study of Molten Steel Turbulence Control by SEN Design Improvement in a Conventional Slab Caster

A research study to improve the fluidynamics of a typical slab caster mould by redesigning the submerged entry nozzle (SEN) ports through a complete mathematical analysis was carried out, selected geometries are deeply tested and analyzed by physical modeling. For that a 1:1 scale analogue water model was constructed; PIV techniques, red dye tracer injections, and ultrasonic sensors are employed to study the molten steel flow patterns behavior. A comparison between the original and the improved SEN designs indicate that both geometries show weak double roll flow pattern at shallow position, but the two‐roll flow pattern is well developed at a deeper position. The original design induces vortexes formation at either, shallow or deep immersion. In the other hand, the proposed improved design, where the backflow is eradicated, maintains good free surface stability without any vortex formation. The physical results indicate that the backflow phenomenon induces jet fluctuations, which are reflected in a more turbulent free surface, and its elimination provides more stable flow patterns in the mould.

Fast polarisation-insensitive optical shutter supported by backflow in dichroic dye-doped dual-frequency liquid crystal

This paper describes a novel implementation of a dual-frequency liquid crystal optical shutter of the guest–host type. The transmissive state of the filter is obtained by applying a low-frequency electric field that brings the dichroic dye in a homeotropic orientation. The light-absorbing state is realised by a twisted planar configuration for which the absorption is quasi-independent of the polarisation. Switching between the two states occurs in about 1 ms and the devices show no scattering for wavelengths inside or outside the absorption band of the dichroic dye. Simulations and experiments reveal how a twisted state is obtained through the backflow phenomenon.

Pyelolymphatic backflow demonstrated by abdominal CT: A case report

SummaryBackgroundPyelolymphatic backflow phenomenon, which is a subtype of pyelorenal backflow, is a rare condition that occurs during the acute phase of urinary obstruction. Pyelorenal backflow has already been described in humans with retrograde pyelography. Our report presents a rare case of pyelolyphatic backflow demonstrated by a computed tomography.Case ReportA 67-year-old man with a history of bladder carcinoma was admitted to the emergency department due to right-sided flank pain and hematuria. Hematuria resolved after insertion of a 3-way urinary catheter, but flank pain persisted. As a result, an abdominopelvic CT was performed. CT revealed numerous tiny, serpiginous tubular structures connected with each other and filled with urine. They began intrarenally and extended caudally surrounding the ureter in the retroperitoneum. Subsequently, the patient underwent an ultrasound-guided nephrostomy to decompress the collecting system of the right kidney. Antegrade pyelography revealed minimal hydroneprosis. However, no leakage from the ureter to the retroperitoneum was observed, proving that the changes demonstrated by a CT were due to pyelolymphatic reflux caused by increased pressure in the collecting tubules filling the lymphatics with opaque urine.ConclusionsThis report presents a very rare case of pyelolymphatic reflux demonstrated by a CT. We present this case report as a reminder that although rare, pyelolymphatic reflux can occur as a result of obstruction without manifestations of hydronephrosis and it can be confused with leakage from the ureter.

Study on Application of CAE in a Centrifugal Compressor Impeller

CAE technology is the most common method to study properties of impeller in a centrifugal compressor. The fluid field was numerically simulated by CFX program to obtain the distribution rules of pressure, turbulence intensity and erosion wear. Based on fluid-solid interaction, stress and deformation were analyzed by Ansys program. According to the simulation results, the maximum deformation and equivalent stress of the impeller are all located on the junction between the blade trailing edge and the shroud. The most serious damaged part by erosion wear in impeller is on the pressure surface of long blade. The erosion wear area on blade pressure surface caused by particles impact primarily locates on blade trailing edge root and middle part. In the flow field, the turbulent intensity on suction surface is greater than that on pressure surface in the corresponding position and the greatest turbulent intensity is located on the leading edge of suction surface. There is backflow phenomenon around the suction surface of long blade and the short blade has significant effect to reduce backflow. The results of numerical simulation explain some actual impeller failure cases and can be applied to anti-wear impeller design and repair.

Visualization Experiment on the Steady Operating Process of the close Loop Pulsating Heat Pipe

A visualization experimental investigation for the pulsating heat pipe (PHP) was carried out to obtain the flow characteristics in the PHP steady operating process with controlling the input heat flux and condensing temperature. The experimental results show that the flow patterns are mainly slug flow with the intermittent pulsation at low heating power, churn flow, and annular flow with steady unidirectional circulation at high heating power. The flow direction switch phenomenon was also observed and analyzed. When the PHP heating section is occupied by vapor slugs for a long time, the backflow phenomenon provides liquid to the bottom of the heating section and reforms a new liquid slug.

Finite Element Model and Fluid-Structure Interaction Analysis of the Intravascular Stent

The 316L stainless steel stent was analyzed about the effect on interaction with plaque and vessels and blood flow during and after implantation using finite element method (FEM) and computational fluid dynamics (CFD). The results showed that tilted ends are likely to damage the intima which may stimulate thrombus formation and neointimal hyperplasia and will cause the restenosis. Stagnant zones formed by the stent may cause the backflow phenomenon, which also contribute to the important elements of in-stent restenosis (ISR).

A falling film down a slippery inclined plane

AbstractA gravity-driven film flow on a slippery inclined plane is considered within the framework of long-wave and boundary layer approximations. Two coupled depth-averaged equations are derived in terms of the local flow rate$q(x, t)$and the film thickness$h(x, t)$. Linear stability analysis of the averaged equations shows good agreement with the Orr–Sommerfeld analysis. The effect of a slip at the wall on the primary instability has been found to be non-trivial. Close to the instability onset, the effect is destabilising whereas it becomes stabilising at larger values of the Reynolds number. Nonlinear travelling waves are amplified by the presence of the slip. Comparisons to direct numerical simulations show a remarkable agreement for all tested values of parameters. The averaged equations capture satisfactorily the speed, shape and velocity distribution in the waves. The Navier slip condition is observed to significantly enhance the backflow phenomenon in the capillary region of the solitary waves with a possible effect on heat and mass transfer.

Analysis and Suppression of Back-Flow Lightning Surges in Onshore Wind Farms

The rapid expansion of wind power generation has brought problems involving lightning strikes to the fore. Many such incidents have damaged not only the wind turbine that was actually struck, but also other turbines that were not. The phenomenon of surge invasion to the distribution line is categorized as surge. It has been reported that this back-flow surge sometimes burns out surge arresters or breaks low-voltage circuits even far from the point of the lightning-struck. In this paper, an analysis of the back-flow lightning surge phenomenon is presented for two wind turbines connected to the grid using the PSCAD/EMTDC. This analysis is implemented for two different lightning surges. The analysis considers the nonlinearity of the grounding system of the wind farm and the point on voltage wave when the surge strikes the wind tower. This paper investigates the effect of a proposed addition grounding grid for each wind tower on the suppression of back-flow lightning surge in the onshore wind farm. The results demonstrate the effect of the grounding system nonlinearity and the effect of changing the point on voltage wave when the lightning surge strikes the wind tower on the back-flow surge phenomenon. The results show that the addition of a grounding grid for each wind tower highly decreases the bad effects of the back-flow lightning surge on the surge arresters.