Stability Of Flow Field Research Articles

Self-ignition characteristics of the high-speed ramjet kinetic energy projectile in the launch process

To research the self-ignition characteristics of high-speed ramjet kinetic energy projectile in the launch process, the self-ignition process based on the solid fuel of polyethylene was numerically simulated by using the dynamic grid technology. The effect of different muzzle velocity on the self-ignition performance, and the effect of opening the blockage at different times on the flow field stability of the combustion chamber and the flow field characteristics after the solid fuel ramjet stabilized were analyzed. The results show that the occurrence of self-ignition is not only related to the pressure, temperature in the combustion chamber, and the muzzle velocity, but also to the content of C2H4 and its degree of mixing with air in the combustion chamber. After the kinetic energy projectile gets out of the muzzle and before the blockage opens, there is oscillation occurring in the combustion chamber. The higher the muzzle velocity of the kinetic energy projectile, the more prone to the occurrence of the self-ignition and the negative effects can be avoided due to the pressure oscillation in the combustion chamber. The effect of opening the blockage at different times on the flow field stability after the self-ignition occurs in a period of time is weak. After the blockage opens, the solid fuel ramjet can reach a stable working condition quickly.

Aerodynamic Optimization of a Centrifugal Fan Using Response Surface Methodology

In this paper, the optimization scheme of a centrifugal fan under the constraints of the total pressure difference, efficiency and shaft power was carried out. The blade inlet angle ß1A and the blade outlet angle ß2A of the centrifugal impeller were optimized using the Response Surface Methodology (RSM). Nine optimization cases were presented. The aerodynamic performance and the flow field of the optimized model were carefully compared with the original model. The results show that the total pressure difference and the total-pressure efficiency increase by 5.7% and 4.2% respectively after the optimization. Compared with the original fan, the flow field inside the impeller of the optimized fan has been improved. The flow separation existing around the blade suction surface outlet is suppressed significantly. For fields in the volute, the optimization scheme has reduced the local vortex intensity and weakened the vortex scale. The flow field stability around the volute tongue is also correspondingly improved.

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.

Cathode cross tank and return hole optimization design and experiment verification of electrochemical machining closed integral impeller outside flow channels

Electrochemical machining (ECM) cathode cross tank and return hole design is very crucial to machining aerospace closed integral impeller. In order to improve the precision of electrochemical machining closed integral impeller outside flow channels, the 3D cathode geometry model is developed by UG software, and the cathode structure flow field is simulated by COMSOL software. Through the flow field simulation, the width of cathode cross tank, the diameter, and the number of the electrolyte return holes were optimized. From the velocity cloud diagram, the simulation results show that the flow field is more stable and uniform under the parameters of electrolyte inlet pressure 0.4 MPa, 1.0-mm cross tank width, and the number of electrolyte return hole is 36 with 1.0 mm diameter. To verify the accuracy of the simulation result, the verified experiment was carried out. Finally, the experiment results were consistent with the simulation data. It indicates that electrolyte flow field simulation is an effective method to optimize cathode structure design. Utilizing this method can optimize the cathode structure and enhance the flow field stability during the processing of ECM. It can also reduce the cathode design cycle and cost.

Uniform, stable supply of medium for in vitro cell culture using a robust chamber

A uniform, stable supply of medium is important for in vitro cell culture. In this paper, a microfluidic device is presented for culturing cells inside a robust chamber with continuous perfusion of medium. The device consists of a main channel, two bifurcated channels and a culture chamber. The culture chamber connects to the bifurcated channels via multiple paths, and distributes symmetrically on the main channel, to improve the efficiency of medium exchange. Furthermore, regular polygonal chambers with various numbers of edges have been designed, to study the effects of chamber shape on flow fields. The finite element method has been employed to predict the effects of multiple paths on the uniformity and stability of flow fields in the culture chamber. Particle tracking technology has been used to evaluate the flow fields in the chambers, and PC-12 cells have been cultured using the microfluidic device, to test its validity. The results of simulation and experiment indicate that the microfluidic design could provide a continuous interstitial-like flow microenvironment, with a relatively stable and uniform supply of medium.

Experimental research on process stability in pulsed electrochemical machining of deep narrow grooves with high length-width ratio

With respect to deep narrow groove structures with high length-width ratio and high depth-width ratio (particularly when the ratio is greater than 10), the cathode rigidity and flow-field stability deteriorate during the electrochemical machining (ECM) process. To improve the machining efficiency and process stability of deep narrow grooves with high length-width ratio, a hollow slice cathode with stiffeners was designed and optimized to meet the rigid requirements using the method of one-way fluid-solid coupling finite element analysis. Additionally, the effects of cathode stiffener size, stiffener distribution, and cathode vibrating feed mode on the flow field in the machining gap were investigated using the flow-field simulation analysis method. The simulation results indicate that cavitation and electrolyte starvation in the machining gap are avoided with a reasonable stiffener design, and the renewal of electrolyte in the machining gap is improved with a reasonable vibration motion. Contrast experiments were performed based on the simulation results. The experimental results verified that the optimal design of stiffeners and the reasonable cathode vibration improve the stability of the flow field. Moreover, process stability and machining efficiency of ECM of deep narrow grooves with high length-width ratio improve with the adoption of the optimal vibrating feed parameters.

Phase holdup distribution and dispersion performance in a novel liquid–liquid cyclone reactor of isobutane alkylation catalyzed by ionic liquid

To improve the existing problems of traditional reactors for isobutane alkylation catalyzed by ionic liquid，a novel liquid–liquid cyclone reactor (LLCR) was designed for the liquid–liquid heterogeneous reaction. The LLCR mainly includes two parts: a reaction chamber and a separation chamber, so the isobutene alkylation reaction and separation between the products and catalyst could occur in the same reactor. Compared with the traditional hydrocyclone, the LLCR consists of two kinds of inlets, one for the light phase and one for the heavy phase. The light phase is injected into the reactor through two symmetric tangential slots in the inlet, while the heavy phase inlet is an axial entry with a guided vane. The phase holdup distribution and dispersion uniformity of light phase in the LLCR were investigated using a novel sampling device and software MATLAB R2012b. One special radius was proposed, rs, which separated the reaction chamber into two parts: upward flow and mixing area. Experimental results indicate that when the total inlet flow is 2m3/h, the flow field in the LLCR is irregular. Moreover, the discharge of the upward flow timely benefits the stability of flow field in the LLCR. Besides, a new parameter, dispersion uniformity of the light phase, β, was used to evaluate the dispersion performance of the light phase in LLCR. By analyzing the dispersion uniformity under different operational parameters, the dispersion of the light phase becomes more desirable under high total inlet flow, feed ratio and overflow ratio.

Predicting the optimum range of feed flow rate in a hydrocyclone using the method combined flow pattern and equation model

Existing design equations can hardly provide reliable guidance in hydrocyclone designs and give explanations for the selected optimum range. Therefore, a new method combined flow patterns and equation models (FP-EM) for hydrocyclone designs is proposed in this paper. The feed flow rate is used as a parameter to evaluate the FP-EM method. Results show that under a lower feed flow rate, the inner flow field is extremely unsteady, which is characterized by higher turbulence intensity and an unsteady air core. When the flowrate becomes extremely large, the value of the exponent n for the tangential velocity inside the free vortex region decreases. Furthermore, before flow field achieving stability, the separation accuracy is extremely low. In the optimum range, the separation efficiency and the accuracy increases with the increase of the feed flow rate, while the cut size decreases. Thus, for the φ50mm hydrocyclone used in this study, the lower limit (15L/min) can be determined by the flow field stability and the upper limit (55L/min) is largely ascertained by energy consumption. Three power-type fitting equations are deduced in the optimum range to quantify the relationships between separation indexes (split ratio, pressure drop and cut size) and the flowrate. Correlation coefficients (R2>0.97) of three equations verify that equation models can predict the separation performance successfully. Therefore, all above results confirm that the FP-EM method is capable of providing an optimum range, quantifying the relationships between separation indexes and the feed parameter, and predicting separation performances.

Experimental investigation of combined transpiration and film cooling for sintered metal porous struts

A combined transpiration and film cooling method was evaluated experimentally for protecting struts made of sintered stainless steel porous media with film holes on the leading edge in a supersonic wind tunnel. The combined cooling results were compared to standard transpiration cooling of the strut. Schlieren figures show that the film cooling and transpiration cooling had little effect on the flow field stability and the shock wave profiles around the struts for the present conditions. Standard transpiration cooling can protect most of the strut but cannot effectively cool the leading edge even with increased coolant injection pressures. The combined film and transpiration cooling effectively cools both the leading edge and other parts of the strut. Non-uniform coolant injection with higher injection pressures in the front cavity and lower injection pressures in the back cavity more effectively utilized the limited coolant flow. The average cooling efficiency of the front part of the strut increased from 25.7% for standard transpiration cooling to 37.9% for combined transpiration and film cooling with the same coolant consumption using the optimal non-uniform coolant flow distribution.

A multi-sensor study of the impact of ground-based glaciogenic seeding on clouds and precipitation over mountains in Wyoming. Part I: Project description

The AgI Seeding Cloud Impact Investigation (ASCII) campaign was conducted in early 2012 and 2013 over two mountain ranges in southern Wyoming to examine the impact of ground-based glaciogenic seeding on snow growth in winter orographic clouds. The campaign was supported by a network of ground-based instruments, including microwave radiometers, two profiling Ka-band Micro-Rain Radars (MRRs), a Doppler on Wheels (DOW) X-band radar, and a Parsivel disdrometer. The University of Wyoming King Air operated the profiling Wyoming Cloud Radar, the Wyoming Cloud Lidar, and in situ cloud and precipitation particle probes. The characteristics of the orographic clouds, flow field, and upstream stability profiles in 27 intensive observation periods (IOPs) are described here. A composite analysis of the impact of seeding on snow growth is presented in Part II of this study (Pokharel et al., 2017).Clouds were stratiform in most IOPs, but in 10 IOPs convective clouds were present. Most clouds were shallow (~2km deep), all had some liquid water but generally with a liquid water path <0.3mm, and all were naturally precipitating but generally at a rate <1mmh−1.

Towards positivity preservation for monolithic two-way solid–fluid coupling

We consider complex scenarios involving two-way coupled interactions between compressible fluids and solid bodies under extreme conditions where monolithic, as opposed to partitioned, schemes are preferred for maintaining stability. When considering such problems, spurious numerical cavitation can be quite common and have deleterious consequences on the flow field stability, accuracy, etc. Thus, it is desirable to devise numerical methods that maintain the positivity of important physical quantities such as density, internal energy and pressure. We begin by showing that for an arbitrary flux function, one can put conditions on the time step in order to preserve positivity by solving a linear equation for density fluxes and a quadratic equation for energy fluxes. Our formulation is independent of the underlying equation of state. After deriving the method for forward Euler time integration, we further extend it to higher order accurate Runge–Kutta methods. Although the scheme works well in general, there are some cases where no lower bound on the size of the allowable time step exists. Thus, to prevent the size of the time step from becoming arbitrarily small, we introduce a conservative flux clamping scheme which is also positivity preserving. Exploiting the generality of our formulation, we then design a positivity preserving scheme for a semi-implicit approach to time integration that solves a symmetric positive definite linear system to determine the pressure associated with an equation of state. Finally, this modified semi-implicit approach is extended to monolithic two-way solid–fluid coupling problems for modeling fluid structure interactions such as those generated by blast waves impacting complex solid objects.

Numerical simulation of slit wall effect on the Taylor vortex flow with radial temperature gradient

Abstract Numerical simulation was applied to investigate the Taylor vortex flow inside the concentric cylinders with a constant radial temperature gradient. The reliability of numerical simulation method was verified by the experimental results of PIV. The radial velocity and temperature distribution in plain and 12-slit model at different axial locations were compared, and the heat flux distributions along the inner cylinder wall at different work conditions were obtained. In the plain model, the average surface heat flux of inner cylinder increased with the inner cylinder rotation speed. In slit model, the slit wall significantly changed the distribution of flow field and temperature in the annulus gap, and the radial flow was strengthen obviously , whichpromote d the heat transfer process at the same working condition. Keywords : Taylor vortex flow, Slit wall, temperature gradient, heat transfer, numerical simulation 1. Introduction In 1923, Taylor[1] discovered the secondary flow existed in the annulus gap between two relatively rotating concentric cylinders, which is called Taylor-Couette flow. Since then, a lot of scholars carried out in-depth research on it. Yang[2] studied the transition process of Taylor-Couette flow by flow visualization method, the aluminum particles were added to investigate the stability of flow field, they found as the rotation speed increased, the aluminum particles have the axial motion before the transition to Taylor vortex flow. Kedia et al.[3] studied the effects of heat transfer on Taylor-Couette flow, and found the heat transfer decreased with Grashof number in axisymmetric Taylor vortex flow regime, and increased with Grashof number after the flow becomes non-axisymmetric. Lee and Minkowycz [4] studied heat transfer characteristics by using the naphthalene sublimation technique in the annular gap between two short concentric cylinders which had either two walls or one plain plain and one axially slit wall, and yielded qualitative information regarding heat transfer but did not address the flow phenomena inside the annular gap. Liu[5] studied the axial wall slits effect on the Taylor vortex flow in the gap between two concentric cylinders by Particle Image Velocimetry, and found the existence of slit wall accelerated the transition process.

Numerical investigation on mixing and combustion of transverse hydrogen jet in a high-enthalpy supersonic crossflow

Mixing and combustion characteristics of a three-dimensional unsteady reacting flowfield generated by transverse hydrogen jet into high-enthalpy supersonic crossflow are investigated numerically. A hybrid RANS/LES (Reynolds-Averaged /Large Eddy Simulation) method acting as wall-modeled LES is adopted, where the ninth-order WENO (Weighted Essentially Non-oscillatory) scheme is introduced to discretize the inviscid fluxes. The validations of numerical results are performed for the jet penetration height, time-averaged and instantaneous structures of reacting flowfield. It is found that the Kelvin–Helmholtz instability of jet shear layer on the windward side of jet plume discontinuously induces large scale coherent structures, which promote the combustion by enhancing the fuel mixing and enlarging the reacting area. The chain reactions creating OH radical mainly occur in the lean-fuel region and the heat-releasing chain reactions consuming OH radical are in the rich-fuel region. In the boundary layer and windward shear layer where supersonic crossflow stagnates, a diffusion flame with autoignition first occurs and then propagates to the downstream of jet plume, which holds the flame stabilization of total flowfield.

Effects of axial inclined guide vanes on a turbo air classifier

For a turbo air classifier, the upward axial velocity in the annular region will cause the negative influence on the stability of flow field, and it is also detrimental to material dispersion. As a result, the classification accuracy will be reduced. In order to decrease the upward axial velocity, a downward force must be introduced to offset it. According to the guiding principle of guide vanes, the axial direction of airflow will be changed when the guide vanes are inclined. In this paper an axial inclined guide vane model was designed. Four models (T-0, T-2.5, T-5, T-7.5) were established with axial inclined angles of 0°, 2.5°, 5° and 7.5°. Fluent software was used to simulate the inner flow field of different structures. The simulation results show that axial inclined guide vanes can decrease the upward axial velocity in the annular region. Especially, when the inclined angle is 2.5°, the upward axial velocity is decreased and the tangential velocity is increased. This is favorable to keep the flow field stable. At the same time the classification force field is enhanced to improve the dispersion of the powders. Discrete phase simulation results reveal that particle residence time in the annular region of structure T-2.5 is shorter than it is in the annular region of structure T-0. This can reduce the collision probability of particles and the energy cost is reduced. The partial classification efficiencies of T-2.5 are higher than that of T-0. From the numerical Tromp curves, it is observed that the cut size of T-2.5 is smaller than that of T-0. The classification accuracy of T-2.5 is 90.7%, while that of T-0 is 88.5%. That means that the classification performance is improved with the new structure. Calcium carbonate classification experiment results also show that the cut size decreases by 0.97–8.42μm and the accuracy increases by 6%–9% for the structure T-2.5, compared to the structure T-0. Therefore, the structure T-2.5 is more favorable for classification than the structure T-0. These actual experimental results are in good agreement with the simulation results and the significance of this optimization is proved.

Investigation of Flow Control for the Hypersonic Inlets via Counter Flow

Experimental results show that there exist two flow fields in the hypersonic inlets when the forebody waves interact with the lip boundary, which is similar to the shock reflection ion hysteresis phenomenon. In order to improve the performance of the flow field, counterflow is applied to control the shock reflection configuration in the hypersonic inlets. For better understanding of the internal mechanism, inviscid numerical simulation is conducted. And the results demonstrate that it is feasible to realize the transition between the regular reflection configuration and the Mach reflection ion configuration in the hypersonic inlets. That is because the von Neumann criterion and detached criterion play a dominant role, respectively, in these transitions. In addition, the evolution process of Mach reflection ion in the hypersonic inlets can be divided into three stages: transmission of waves, emergence of Mach stem, and stabilization of flow field.

Hydrocyclone Numerical Simulation and Separation Efficiency Optimization

For the hydrocyclone’ separation efficiency affects by many factors, this paper combined Reynolds stress model and SIMPLEC algorithm of Fluent software with orthogonal test to simulate hydrocyclone’s operating process and analysis the flow field. Different overflow pipe wall thickness values (4mm, 8mm, 12mm), volume fraction values (1%, 5%, 10%) and inlet velocities (3m/s, 4m/s, 5m/s) was considered as the separation efficiency affective factors. Results show that the overflow pipe wall thickness has greatest influence on separation efficiency. The inlet velocity is the second and the volume fraction value is the last. The optimal combination is the overflow pipe wall thickness value 8mm, the volume fraction 5% and the inlet velocity 5m/s. The overflow pipe wall thickness value increasing can decrease the turbulent kinetic energy and increase the stability of hydrocyclone flow field.

A Study of Composite Flow Meter Based on the Theory of Electromagnetic and Ultrasonic

This thesis, under the guiding of electromagnetic and ultrasonic theory, tries to do numerical simulation based on the Fluent simulation software. The composite flowmeter (CFM) improves the accuracy of metrology on water flow and equipment itself with self-calibration and inter-calibration function. The aim is to calculate the average velocity of fluid on the cross-section at the electrode and the stability of flow field by researching on the influence of transducer. The result shows when the distance between two transducers is close, influences the flow field near the electrode, but when the product of two transducers distance and pipe diameter is around 4000, the average fluid velocity is close to the pre-selected fluid velocity and without any swirl. After comprehensive thinking on the structure influence of excitation coil, the transducer should been equipped on the same longitudinal section with the electrode.

CFD numerical simulation of Archimedes spiral inlet hydrocyclone

For traditional linear type inlet, hydrocyclone has an unstable inner field, high turbulence intensity and low separation efficiency, this paper proposes an inlet mode that uses an Archimedes spiral hydrocyclone. A Mixture liquid-solid multiphase flow model combined with the kinetic theory of granular flow was used to simulate the high concentration water-sand-air three-phase flow in a hydrocyclone. We analyzed the pressure field, velocity field and turbulent kinetic energy and compared with traditional linear type inlet hydrocyclone inner field. The results show that Archimedes spiral inlet hydrocyclone's pressure field is evenly distributed. The Archimedes spiral inlet hydrocyclone can guide and accelerate the mixture flow and produce small forced vortex and less short circuit flow. The particles easily go to the outer vortex and are separated. The Archimedes spiral inlet hydrocyclone has effectively improved the stability of inner flow field and separation efficiency.

Low-Diffusion Preconditioning Roe’s Scheme on Simulation of Low-Speed Flows

Preconditioning technology is an effective method to simulate low Mach number flows, but the stability of the result is poor relatively. With the traditional preconditioning method on Roe’s scheme, the dissipation is too large in the boundary layer and extremely low-speed zone, and sometimes the result is unreasonable. Numerical experiments show the numerical dissipation of the scheme has a notable effect on low-speed flow numerical simulation with preconditioning method. A low-diffusion preconditioning Roe scheme by using an adjustable parameter in preconditioning Roe scheme to control the numerical dissipation and the adjustable parameter value rules are proposed. The low-diffusion preconditioning Roe scheme reflects the real physical dissipation in the extremely low-speed zone. In the region similar to the inflow speed, low-diffusion preconditioning Roe scheme can ensure the stability of the flow field. Numerical results show the efficiency of the new scheme and the low Reynolds number steady solutions of viscous flow past a circular cylinder are satisfied.

Influence of Guide Vanes' Angle on Flow Fields of Cylinder Cage Powder Classifier

To research guide vanes' influences on flow fields of cylinder cage powder classifier at different angles, a study of guide vanes under 3 different angles is therefore undertaken examining air flow behavior. The investigation of these flow field characteristics made use of the computational fluid dynamics (CFD) to simulate the air flow in the classifier. The results indicate that smaller angles of guide vanes can increase velocity but damage the stability of flow fields, and that those larger angles will reduce the velocity.