Changes In Flow Field Research Articles

The Influences of the Nozzle Throat Length and the Orifice Grooving Degree on Internal Flow Field for a Multi-Entry Fan Nozzle Based on FLUENT4

Spray nozzle is a key component in equipment for plant protection and water-saving irrigation. The fan nozzle is a kind of spray nozzle, which is widely used in agriculture and forestry for irrigation and control of diseases, insects, and weeds. In consideration of the increasing velocity of the flow field, when the hydraulic pressure remains unchanged and the flow path becomes narrow, and because the increase of the velocity of spray drops can increase the penetrability of spray drops into the plant canopy, a kind of new fan nozzle with multi entries and simple inner structure was designed and the influences of its structure parameters on the inner flow field were analyzed using FLUENT software. The experimental results showed that the influence of the throat length on the inner flow field of the nozzle was insignificant, while the orifice grooving degree had a significant effect on inner flow field of the nozzle. The larger the grooving degree was, the smaller the pressure and velocity of internal flow field of the nozzle. The nozzle throat length had little influence on the velocity change of internal flow field. Positive correlation was shown between throat length and flow field velocity.

Effects of Lip Thickness on the Flowfield Structures of Supersonic Film Cooling

Schlieren visualizations of flowfield structures of supersonic film cooling in a backward-facing step were conducted in a Mach 2.95 suction wind tunnel with the film coolant tangentially ejected through half-Laval nozzles, all of which were designed in Mach 1.5 but with different lip thicknesses. The film coolants were fed with pressures matching the mainstream, and the flowfield changes of supersonic film cooling under different lip thicknesses between the slot and mainstream were clearly visualized under the conditions of the same step and slot heights. When the step height was kept the same, an increase in lip thickness decreased the angle of the shock wave emanating from the upper tip of the lip, and it decreased the strength of the reattachment shock wave when it was larger than a certain value. When the slot height was kept the same, the increase in lip thickness tended to decrease the angles of the shock waves emanating from the upper and lower tips of the lip and reattachment shock waves. This work provides an in-depth understanding of the flowfields of supersonic film cooling near the backward-facing step and may be used as a reference for the design of supersonic film cooling.

The Effects of Turbulent Burning Velocity Models in a Swirl-Stabilized Lean Premixed Combustor

Abstract The effects of turbulent burning velocities in a turbulent premixed combustion simulation with a G-equation are investigated using the 3D LES technique. Two turbulent burning velocity models – Kobayashi model, which takes into account the burning velocity pressure effect, and the Pitsch model, which considers the flame regions on the premixed flame structure – are implemented. An LM6000 combustor is employed to validate the turbulent premixed combustion model. The results show that the flame structures in front of the injector have different shapes in each model because of the different turbulent burning velocities. These different flame structures induce changes in the entire combustor flow field, including in the recirculation zone. The dynamic mode decomposition (DMD) method and linear acoustic analysis provide the dominant acoustic mode.

Dynamic and static sealing performance of elastic check valve spool

The non-metallic valve spool has the advantages of safety, stability, and biocompatibility. In order to expand the application of non-metallic elastic valve spool in medical water jet, a check valve spool structure with double-protrusion structure of polyurethane is designed to replace the metal valve spool and the spring thus achieves control of the fluid. Based on the non-linear mechanical response of elastic materials, ANSYS Workbench was used to calculate the stress and deformation under the different friction coefficient and pre-compression. The results show that the static sealing effect of the valve spool is best when the pre-compression is 0.1 mm and the friction coefficient between the spool and the valve sleeve is 0.2. Then, based on bidirectional fluid–solid coupling analysis through Fluent, the flow field changes during the interaction between fluid and elastic material and the key bearing position of the valve spool are clarified. The results shows that size, processing quality, and material properties of the inner ring protrusion will determine the dynamic sealing performance of elastic valve spool.

Research of Flow Field of Alkaline/Surfactant/Polymer Solution in the Annular Depressurization Slot by PIV Experiment

In order to study the flow field changes of ASP (alkaline/surfactant/polymer) solutions with different molecular weights and flow rates when flowing through the annular depressurization slot in the laboratory, the annulus pipeline model was designed based on the principle of similarity. The particle image velocimetry (PIV) system was used to continuously capture the transient images when the ASP solution flows in the model under different experimental conditions. Tecplot software was used to analyze and process the nephrograms of the velocity distribution of ASP solution when flowing through the partially pressured injection tool with different depressurization slots. The experimental results showed that the vortex occurred at the bottom of the depressurization slots; the greater the flow rate, the closer the vortex center to the outer wall; higher molecular weight of the polymer in the ASP solution caused larger velocity gradient towards the wall. The number of the slots has no significant effect on the position of the vortex. This experiment provides a new research method for the velocity distribution of the internal flow field in the partially pressured injection tool.

Microfluidic device for generating regionalized concentration gradients under a stable and uniform fluid microenvironment

A relative stable microenvironment is essential for in vitro cell culture. This paper presents a novel cell culture device with multiple arrays of perfusion channels (PCs) to maintain the stability and uniformity of flow fields. Besides, a series of regionalized concentration gradients can be generated by adjusting the relative positions of inlets. The number of circular arrays of PCs have been optimized based on the simulation results. The particle tracking technology has been used to evaluate the changes of flow fields inside the culture chamber as the variation of velocity in medium inlet. The Hela cells, L929 mouse fibroblast cells and HUVEC cells are cultured dynamically in the optimized device. The culture device without PC is used as a comparison. The results of normalized cell density and cell viability indicate that the device with multiple arrays of PCs is more favorable to cell growth. The effects of adjacent position and opposite position of two inlets on generating regionalized concentrations have been demonstrated by FEM simulation analysis and fluorescence distribution. The densities and attachment areas of L929 cells inside one chamber with regionalized concentration gradient have also shown the validity of the designed device.

AN EXAMPLE OF THE APPLICATION OF A FILTRATION MODEL TO THE PREDICTION OF THE INFLUENCE OF A DEEP-FOUNDED BUILDING ON GROUNDWATER LEVEL CHANGES

Nowadays, for economical aspect, most of large cubature objects are constructed with underground storeys or car park areas. During the planning phase, it is important to design an optimal, economical and safe dewatering system and technical solutions aimed to eliminate the influence of underground structure on groundwater levels in the neighbourhood. Mathematical modelling of groundwater filtration allows creating a prognosis of excavation dewatering process and influence of new buildings on groundwater levels in subject areas during both execution and operation of structure. The paper presents an example of groundwater filtration modelling for a shopping mall in the centre of Budapest, using SPRING software. The project includes the construction of a large building with an underground story for cars. With filtration models, it was possible to assess the changes in groundwater flow fields caused by projected structure and its excavation dewatering system and the prognosis of groundwater levels around the building, taking into consideration the influence of the flood wave from the Danube on groundwater filtration in the investment area. Based on modelling results the suitable technical solutions were proposed, which allow limiting the effect on the surrounding buildings and the potential changes of water levels in the investment area.

PIV Experimental Study of the Axial Flow Blood Pump’s Internal Flow Field Under Pulsatile Condition

Taking the axial flow blood pump as the research object, we used the particle image velocimetry (PIV) to measure the flow distribution of inlet, outlet and impeller at the changing rotation speed. Effects of the acceleration and deceleration process on the change of internal flow field were compared and analyzed. The experimental results show that the flow field of inlet and outlet area of the blood pump is stable at the constant working speed, but the vortex and reflux exist in the impeller area. However, acceleration of the impeller increases the flow field’s volocity and disturbance. Deceleration of the impeller rotation speed reduces the flow velocity, and the flow state of overall flow field is relatively stable. Design of the pulsatile blood pump should be optimized in order to reduce the occurrence of hemolysis in the process of acceleration and deceleration.

Case Study of Heat Transfer during Artificial Ground Freezing with Groundwater Flow

For the artificial ground freezing (AGF) projects in highly permeable formations, the effect of groundwater flow cannot be neglected. Based on the heat transfer and seepage theory in porous media with the finite element method, a fully coupled numerical model was established to simulate the changes of temperature field and groundwater flow field. Firstly, based on the classic analytical solution for the frozen temperature field, the model’s ability to solve phase change problems has been validated. In order to analyze the influences of different parameters on the closure time of the freezing wall, we performed the sensitivity analysis for three parameters of this numerical model. The analysis showed that, besides the head difference, the thermal conductivity of soil grain and pipe spacing are also the key factors that control the closure time of the frozen wall. Finally, a strengthening project of a metro tunnel with AGF method in South China was chosen as a field example. With the finite element software COMSOL Multiphysics® (Stockholm, Sweden), a three-dimensional (3D) numerical model was set up to simulate the change of frozen temperature field and groundwater flow field in the project area as well as the freezing process within 50 days. The simulation results show that the freezing wall appears in an asymmetrical shape with horizontal groundwater flow normal to the axial of the tunnel. Along the groundwater flow direction, freezing wall forms slowly and on the upstream side the thickness of the frozen wall is thinner than that on the downstream side. The actual pipe spacing has an important influence on the temperature field and closure time of the frozen wall. The larger the actual pipe spacing is, the slower the closing process will be. Besides this, the calculation for the average temperature of freezing body (not yet in the form of a wall) shows that the average temperature change of the freezing body coincides with that of the main frozen pipes with the same trend.

The effects of flood history on sediment transport in gravel-bed rivers

The transport of coarse sediment during floods often exhibits hysteresis patterns from changes in flow fields, channel geometry, bedforms, or sediment supply conditions. Flume experiments that simulate hydrographs tend to confirm that hysteresis is a consequence of the progressive organization of surface sediments in terms of grain protrusion, imbrication, orientation, and roughness. Hysteretic patterns are also highly dependent on the kind of sediment supply conditions, and the type of simulated hydrograph. A factor that has not been investigated extensively is the effect of the timing and sequencing of floods on bedload transport. Depending on its magnitude and duration, each flood leaves the channel bed in a different condition, which influences the bedload transport of the next event, representing the river bed's memory of past floods, which can determine future responses to natural disturbances. In this study, I investigated the effects of different sequences of events, i.e., the flood history, on sediment transport through a series of flume experiments that simulated three types of stepped and symmetrical hydrographs (ranging from short-duration/high-magnitude to long-duration/low-magnitude events) under sediment recirculation conditions. Hydrographs were simulated as a sequence of the same event, and with events in different sequences, in order to explore the effects of different antecedent conditions on sediment transport. The results show that a previous event decreases the rates of sediment transported by a certain hydrograph by around 40% if a high-magnitude event precedes another one and around 70% if a low-magnitude event precedes another event of similar magnitude. A low-magnitude event does not affect the rate of sediment transported by a subsequent high-magnitude flood, but a high-magnitude event reduces the sediment transported by a subsequent long-duration/low-magnitude event.

Corrosion of Ductile Cast Iron Pipeline in a Water Supply Network Based on the Flow Field Changing

In this work, a computational fluid dynamics (CFD) simulation and electrochemical measurements are performed to study corrosion at different positions of the elbow and its adjacent areas in water supply pipelines using two designed pipe fittings on a dynamic corrosion platform. The corrosion passive film on the ductile cast iron surface is the primary control factor in the corrosion process. Furthermore, although corrosion activity occurs more frequently at the innermost portion of the elbow and the outer portion of the downstream straight sections, corrosion at the top (or bottom) and outer side of the elbow occurs less often.

Flow characteristics in a partly vegetated channel with emergent vegetation and seepage

The flow characteristic in a partly vegetated channel with emergent condition is studied by covering a finite length of the channel with natural Oryza sativa (rice) stems with downward seepage. Spatial study of velocity profiles shows that vegetation reduces the flow velocity lengthways of vegetation zone even with the presence of downward seepage. Reynolds stress increases at the downstream end of the interface section because of higher velocity fluctuations between vegetated and un-vegetated zone. The increase in Reynolds stress at the downstream of interface section is attributed to higher lateral flow and momentum exchange in this transition zone between vegetated and un-vegetated area. For the vegetated section, vegetation serves as a barrier for the vegetated section in reducing the maximum Reynolds stress in the range of 17–20% along the flow direction. It is also observed that downward seepage increases the Reynolds stress as well as turbulence intensities. Integral scales increase with increase in percentage of downward seepage resulting in higher Reynolds stresses with downward seepage. Presented work will help in analyzing the changes in flow field with partially emergent vegetation in alluvial channel subjected to downward seepage. The overall results of this study may enhance the comprehensive understanding of the vegetation effects on flow characteristics in partly emerged vegetated channel and may also provide some guidance for the practice of river ecological restoration on river bed or banks.

Multi-physics Coupling Simulation of Precise Electrochemical Machining(PECM) with Pulsed Current and Vibrating Cathode

A new multi-physics model is presented for numerical simulation of the precise electrochemical machining process with pulsed current and vibrating cathode. The method includes the change of electric field, flow field, temperature field and geometric deformation field. Meanwhile, the effects of cathodic vibration on electrolyte velocity is obtained by transient simulation of flow field and the velocity distribution obtained from the multi-step steady state flow field is compared with that of transient flow field. To solve the problem that machining process simulation needs a lot of computing time, a multi-step steady state flow field solution and a loop iterative solution algorithm based on COMSOL are presented, and the error analysis was carried out. The results show that the algorithm can ensure the error within a reasonable range and greatly reduce the calculation time.

Review and optimization of main parameters of pump turbine in Xiangshuijian pumped-storage power station

Mixed transportation technology of oil and gas is an efficient transportation method of offshore oil fields, in which oil and gas multiphase pump is the key equipment. In this paper, aimed at a single compression stage unit of a helico-axial multiphase pump prototype, a numerical simulation of internal flow field is carried out. It is assumed that the flow is steady and the two-phase flow pattern is bubble flow. Water is the main phase and incompressible air is the second phase. The change of external characteristics is compared in different inlet gas volume fraction (GVF) condition; the internal flow field characteristics such as velocity fields, pressure fields, and two-phase distributions are discussed as well. The result shows that the differential pressure and the hydraulic efficiency decrease with the increase of GVF, and there are some significant changes in internal flow fields. Further, under the condition of 10% inlet GVF, the changes of external characteristics and internal flow field of the pump when the air bubble diameter varies are compared. The result shows that the differential pressure and the hydraulic efficiency decrease with the increase of bubble diameter.

The two-phase flow analysis of helico-axial oil-gas multiphase pump for offshore oil fields

Mixed transportation technology of oil and gas is an efficient transportation method of offshore oil fields, in which oil and gas multiphase pump is the key equipment. In this paper, aimed at a single compression stage unit of a helico-axial multiphase pump prototype, a numerical simulation of internal flow field is carried out. It is assumed that the flow is steady and the two-phase flow pattern is bubble flow. Water is the main phase and incompressible air is the second phase. The change of external characteristics is compared in different inlet gas volume fraction (GVF) condition; the internal flow field characteristics such as velocity fields, pressure fields, and two-phase distributions are discussed as well. The result shows that the differential pressure and the hydraulic efficiency decrease with the increase of GVF, and there are some significant changes in internal flow fields. Further, under the condition of 10% inlet GVF, the changes of external characteristics and internal flow field of the pump when the air bubble diameter varies are compared. The result shows that the differential pressure and the hydraulic efficiency decrease with the increase of bubble diameter.

Influence Analysis of Blade Fracture of Ducted Propellers of offshore platforms on Flow Field

The flow field of ducted propellers in viscous flow is analyzed and calculated with CFD technology, and the flow field and surface pressure distribution of the ducted propeller of offshore platforms with the fracture in different positions of a certain blade are simulated. Firstly, the geometrical models of the ducted propellers are established with UG software and put into the girding caved up by the Gambit. Then the numerical simulations are carried out with CFD, by which the surface pressure distribution of the ducted propellers in open water are calculated using Moving Mesh method. Lastly, the flow field characteristics of ducted propellers with the fracture in different positions of a certain blade are compared, and the result data are analyzed and summarized to gain flow field change laws. The results can be applied in project for reference.

Numerical investigation of heat transfer and flow characteristics of supercritical CO2 in U-duct

The present study investigated the heat transfer and flow characteristics of supercritical CO2 in U-ducts by numerical simulations. Analyses focused on the effect of the turn geometry and the gravity direction on the flow field and temperature field of supercritical CO2. Results show that, as the radius of the turn geometry increases from 0.5D to 3.0D, the flow and temperature distribution after the turn region changes a lot, due to the flow separation and reattachment. In the turn region for the vertical upward flow, the gravity plays a dominant role in the change of flow field, and a secondary flow structure is induced by gravity, which is completely opposite to the Dean Vortex in conventional fluid flow. The angle between the centrifugal force and gravity direction have a significant influence on the migration and deposition process of the low-temperature and high-density fluid when supercritical CO2 passing through the turn region, making the flow field and temperature distribution after the turn different completely.

Experimental study of the global flow-state transformation in a rectangular Rayleigh-Bénard sample

Large-scale flow structures were measured in a rectangular Rayleigh-Bénard sample of equal length and height of 10cm and a depth of 2.5cm. The working fluid was water at a Prandtl number of Pr=6.9. Aiming to capture flow structures to their full extent and throughout the entire sample, the three-dimensional (3D) Lagrangian measurement technique 3D-Particle Tracking Velocimetry (3D-PTV) was employed. The study provided direct confirmation that the global mean flow field changes significantly in dependence of the Rayleigh number Ra within the investigated range of 2.1·106⩽Ra⩽4.5·108. Several distinguishable flow states were observed in the laminar-turbulent transition regime complementing the well-known mean wind. A two-dimensional (2D)-mode decomposition revealed a breakdown of the mean wind during the transition phase and its new formation in the turbulent regime. Further, the global distribution of magnitudes of the instantaneous velocity fields was used to extract characteristic velocities in the flow field. It was shown, that the Reynolds-number-dependent scaling law Re∼Raγ can be unambiguously determined by means of directly measured Lagrangian velocities, since the same scaling behavior holds true for all chosen reference velocities. In this context, using the product of a polynomial and an exponential function, the global velocity distribution was described analytically in dependence of Ra and four independent fit parameters in the sub-range 2.8·107⩽Ra⩽4.5·108.

Study on the Electromagnetic Field, Fluid Flow, and Solidification in a Bloom Continuous Casting Mold by Numerical Simulation

Based on the Maxwell's equations and low‐Reynolds number turbulence model, a coupled three‐dimensional numerical model has been developed to describe the electromagnetic field, fluid flow, and solidification in a bloom continuous casting mold with electromagnetic stirring (M‐EMS). The optimum frequency is obtained by calculating the maximum electromagnetic torque of the strand with consideration of mold tube shield effect. The effect of stirring current intensity on the turbulent flow, temperature distribution, and shell growth has been investigated numerically. According to the simulation results, the electromagnetic force with a circumferential distribution on the strand transverse section, and a swirling flow field along the axial direction of the strand are observed in the mold region with the application of M‐EMS. It is shown that the melt maximum swirling flow velocity decreases from 0.420 to 0.226 m s−1 and the flow field changes remarkably when solidification is involved in the model. With the current intensity increasing from 260 to 500 A, the swirling flow intensifies significantly, which prevents the superheated molten steel moving downwards to the liquid core and promotes the superheat dissipation in mold.

Experimental and numerical investigation of flow forces in a seat valve using a damping sleeve with orifices

The power of hydraulic piston engines is much affected by the on-off valves which control the fuel injection of the piston assembly. Therefore, the opening time of the seat valve used as the on-off valve is optimized by minimizing the axial flow forces on the spool. A damping sleeve with orifices is proposed to change the valve internal geometry. Experimental and numerical investigations of the flow forces acting on the spool with and without the proposed damping sleeve are carried out to identify the differences in the flow field and to minimize the forces’ effect. The simulated results fit the experimental results well. Both results show that the proposed damping sleeve affects the pressure distribution along the spool cone surface and the jet stream direction significantly. The effects of the orifice’s width, height, and relative sleeve installation positions on the flow field and cavitation are assessed using simulation methods. As a result of the flow field changing, the damping sleeve can reduce the flow forces significantly and even reverse the forces’ direction at the cost of a little flow loss. The opening time of the seat valve can be reduced by 31% to 0.67 ms by using the proposed damping sleeve.