Internal Flow Mechanism Research Articles

Investigation on the internal flow of ventilated partial cavity

This study presents investigations on the internal flow behaviors and mechanisms of the ventilated partial cavity created by air ventilation behind a backward-facing step. Both planar particle image velocimetry experiments and numerical simulations have been conducted over a range of freestream velocities (U∞) for varying ventilation rates (QAs). The experimental and numerical internal flows are reasonably concordance, revealing three distinct internal flow regions: the ventilation region, the entrained internal boundary layer region, and the reversed region sandwiched in-between. The three-dimensional internal flow structures and the internal pressure gradients are numerically revealed. The internal recirculation vortex in the ventilation region is found to be induced by the stream-wise adverse pressure gradients, while the transverse airflow near the closure of the cavity is attributed to the presentence of the span-wise pressure gradient. Based on the internal velocity and air flux profiles, and the corresponding internal boundary layer thickness, the entrained and reversed air fluxes are revealed to initially increase, attain a maximum, and then decrease along the cavity. For the increment of U∞, both entrained and reversed air fluxes increase. For the increment of QAs, the entrained air flux rises, while the reversed air flux declines. Such feature is further verified and explained by applying the Couette flow model with adverse pressure gradients, which provides a reasonable estimation of internal flow profiles and offers a theoretical explanation on the change of internal air flux due to the variation of U∞ and QAs.

Internal flow mechanism in a supersonic expander-rotor

This article proposes a numerical investigation into the internal flow structure in the supersonic expander-rotor (SER). In order to reveal internal flow mechanism, the significant influencing factors in the flow structure are identified, and the solutions to improving the integrated performance of the SER are developed. According to the numerical results, the wave structure of the expansion wave and the oblique shock wave is what characterizes the flow in the mainstream region of the SER. In addition, the expansion wave and the oblique shock wave impose control on the pattern of static pressure distribution in the 3-D channel and then the 3-D flow structure. The formation and breakdown of the tip leakage vortex are the main form that the motion of vortex takes in the SER. The concentration, recirculation, and separation of the boundary layer; the low energy fluid mixing with mainstream; and the interaction between the oblique shock waves and the boundary layer are the crucial motion tracing near the endwall. Compared with the traditional turbines, the flow structures in the tip region of the SER are relatively simpler; the essential motion tracing is the airflow near the leading edge of the strake wall moving from the PS through the tip gap to the SS as a result of the transverse pressure difference.

Can customer perceived service quality fluctuations predict the performance of retail service supply chain?

PurposeBased on the analysis of the dissipative structure of the retail service supply chain (RSSC), this paper divides the system into two internal and external dissipative mechanisms, including the internal performance dissipation mechanism and the perceived quality dissipation mechanism outside the system. Based on the prediction of RSSC performance, this paper aims to discuss the application of Hidden Markov Model (HMM) in this field and puts forward a set of complete process of forecasting the service supply chain (SSC) performance based on HMM model.Design/methodology/approachBased on the theory of dissipative structure, this paper selects the RSSC as the research object, analyzes the system characteristics of the dissipation structure of RSSC from three aspects, such as system opening type, distance from equilibrium state and nonlinear order and describes the quality fluctuation process of RSSC as a Hidden Markov process. Taking the RSSC of J Company as an example, this paper makes use of the observed state value of customer perceived service quality from 1997 to 2016, predicts the performance status of the enterprise's RSSC.FindingsThe research results show that: RSSC is a dissipative structure system, and its performance is the internal entropy flow of the system, and the customer perceived service quality is external, their interaction determines the dynamic evolution of the system dissipation structure, and the Markov property between supply chain performance and perceived service quality. There is a Markov property between supply chain performance and perceived service quality. Using the perceived service quality observation state data of the external consumers of the system can effectively predict the implicit state of RSSC performance. Based on this prediction result, the strategy adjustment and optimization of the action mechanism of internal and external entropy flow in the dissipative structure system can be carried out to promote the sustainable development of the RSSC.Originality/valueThis paper thinks that RSSC is a dissipative structure system and the SSC performance and customer perceived service quality are the internal and external entropy flow of the system, which determines the dynamic evolution of the system dissipation structure. There is a Markov property between supply chain performance and perceived service quality. The hidden state of SSC performance can be predicted effectively by using a hidden Markov model and observing state data of perceived service quality from consumers outside the system.

Modeling study on drop breakup time in turbulent dispersions

Drop breakup time describes the time scale of the drop breakup process. In this work, a mathematical model for drop breakup time is constructed by the dimensional analysis method. The adjustable parameters in the model are determined and further validated by comparing with the experimental data in the literature. Results show that the model accurately predicts the drop breakup time when Weber number (WeL) is low. Further, we analyzed the effect of drop diameter, WeL, interfacial tension, and drop viscosity on the breakup time. It is noted that the breakup time increases with the increase of drop diameter and viscosity, while decreases with the increase of WeL and interfacial tension. The internal flow mechanism was proposed to illustrate the influence of the interfacial tension on the breakup time. The breakup time model can be further used in the construction of breakup kernels in a population balance model.

Effect of Rotation Speed and Flow Rate on Slip Factor in a Centrifugal Pump

This work analyzes the causes of the slip phenomenon in the impeller on the basis of the internal flow mechanism. Detailed optical measurements of the flow inside the rotation passages of a five-bladed centrifugal pump impeller are obtained through particle image velocimetry (PIV). On the basis of experimental data, the deviation coefficient of slip velocity is proposed and then revised according to the slip factor calculation formula of Stechkin. Results show that, at the same rotation speed, the slip factor increases with the flow rate and reaches the maximum value at 1.0 QBEP flow rate. At different rotation speeds, the slip factor increases with the rotation speed and shows a relatively large variation range. Moreover, a revised slip factor formula is proposed. The modified model is suitable for the correction of slip factor at part-load flow rates and serves as a guide for the hydraulic performance design and prediction of centrifugal pumps.

On aerodynamic droplet breakup

Abstract

Research Progress and Prospects of Multi-Stage Centrifugal Pump Capability for Handling Gas–Liquid Multiphase Flow: Comparison and Empirical Model Validation

The working capability of multi-stage pumps, such as electrical submersible pumps (ESPs) handling multiphase flow, has always been a big challenge for petroleum industries. The major problem is associated with the agglomeration of gas bubbles inside ESP-impellers, causing pump performance degradation ranging from mild to severe deterioration (surging/gas pockets). Previous literature showed that the two-phase performance of ESPs is greatly affected by gas involvement, rotational speed, bubble size, and fluid viscosity. Thus, it is necessary to understand which parameter is actually accountable for performance degradation and different flow patterns in ESP, and how it can be controlled. The present study is mainly focused on (1) the main parameters that impede two-phase performance of different ESPs; (2) comparison of existing empirical models (established for two-phase performance prediction and surging initiation) with our single-stage centrifugal pump results to determine their validity and working-range; (3) gas-handling techniques applied to enhance the multiphase performance of ESPs. Firstly, it aims at understanding the internal flow mechanism in different ESP designs, followed by test studies based on empirical models, visualization techniques, bubble-size measurements, and viscosity analysis. The CFD-based (computational fluid dynamics) numerical analysis concerning multiphase flow is described as well. Furthermore, gas-handling design methods are discussed that are helpful in developing the petroleum industry by enhancing the multiphase performance of ESPs.

Analysis, modeling and simulations of an innovative sliding vane rotary compressor with a rotating cylinder

A novel sliding vane rotary compressor with a rotating cylinder is proposed in this paper. The sliding vane dynamics analysis reveals that, compared with the traditional compressor, the friction loss at the sliding vane tips and between rotor and cylinder can be reduced to 0.25% and 10.23%. To numerically investigate the internal flow mechanism, a reliable three-dimensional simulation method is developed via Fluent 2020R1. Three pressure ratios are considered. During transient calculations, all parameters begin to fluctuate periodically after the fourth round. Both temperature and pressure for a single chamber reach the maximum near the exhaust port. As the pressure ratio εp increases from 5, 10 to 15, the outlet temperature increases from 496.9, 623.0 to 754.4 K and the mass flow rate decreases from 0.03089, 0.02946 to 0.02764 kg/s, the volumetric efficiency and isentropic compression efficiency decrease, with the maximum values of 97.14% and 90.33%. The gaps between the rotor and the cylinder and at the vane tips are 0.03 and 0.01 mm, but the leakage mass of the former is one order higher than that of the latter. The research can provide guidance for the design and practical application of sliding vane rotary compressors.

Unsteady Internal Flow and Loss Mechanism of a Turbocharger Turbine Under Pulsating Flow Condition

Unsteady Internal Flow and Loss Mechanism of a Turbocharger Turbine Under Pulsating Flow Condition

Synergy Methodology for Internal Flow of Turbomachinery

The complex curvature of turbomachinery rotor blade channels combined with strong rotational effect and clearance leakage brings on intricate internal flow phenomenon. It is necessary to study the internal flow and energy loss mechanism to reveal the influence law of the key parameters and to achieve its optimal design. Considering features of flow and temperature fields in rotor passage, the concept of synergy analysis derived from equation of energy conservation was put forward. Typical NASA low-speed centrifugal compressor (LSCC) rotor was chosen for analysis using CFD. Numerical results showed remarkable agreement with experiment datum in both the tendency of the performance characteristics and quantitative pressure values. Under different flow rates and inlet total temperatures conditions, thermal-fluid interaction effect and losses were studied by synergy analysis. Results showed that peak synergy positive value zones located around blade leading edge, across the shroud wall and hub wall, and at the position where tip-leakage flow was mixing with the bulk flow and high entropy zones existed. Increasing flow rate from design condition, positive and negative synergy areas both changed tiny around leading edge and trailing edge. Reducing flow rate, positive synergy areas tended to increase and negative areas decreased at same positions. The relationship between flow separation, heat transfer and losses in turbomachinery rotor can be revealed based on synergy analyses.

Design optimization of medium specific speed Francis turbines with unshrouded runners

In present, many undeveloped locations of small hydropower generation exist in Japan. To promote the use of small hydropower generation it is required to reduce costs and increase efficiency. Unshrouded runners are proposed to simplify the manufacturing process and also to reduce the manufacturing costs. In this paper, the computation results of optimized unshrouded runners are presented. This research was conducted to minimize the shroud tip leakage loss, which is the main loss of unshrouded runners. In the author’s laboratory, medium specific speed (160 [m-kW]) Francis turbines was developed. For this 160 [m-kW] Francis turbines, investigation of internal flow and loss mechanism were done by computation. Using this loss mechanism which was revealed by rothalpy, the design of the same specific speed Francis turbines with unshrouded runners was optimized. For the new runners, shroud tip clearance and the pressure distribution of vane surfaces was changed. Due to this, shroud tip leakage flow has weakened. In the simulation of whole domain, 2.45% of improvement was confirmed on the design point.

Characteristics and internal flow of a low specific speed pump used as a turbine

PAT (Pump as Turbine) has gradually been a popular solution for the small-scale hydropower generation. Because of the rich assortment and the low initial investment. However, PAT is designed as a pump but not a turbine. Therefore, a lot of research was conducted about the performance prediction and the loss distribution of pump and turbine mode. In this paper, we focused on the internal flow and loss mechanisms of the best efficient point of both modes. The characteristic and internal flow were investigated by experimental and computational approaches. The reasons of efficiency reduction in turbine mode and loss mechanisms were clarified by the computational results. The separation flow region occurs on the suction side or pressure side in non-BEP, and the swirling flow counter-rotates the PAT occurring at the outlet in BEP might be the reasons of the efficiency reduction. Moreover, because of the camber line shape of PAT, the separation flow occurs near the inlet could not reattach soon which induces the loss region. Furthermore, the secondary flow near the blade surface and the flow from the pressure side to suction side interfere with each other which induced the loss region.

The influence of axial-flow fan trailing edge structure on internal flow

Axial-flow fan with advantages such as large air volume, high head pressure, and low noise is commonly used in the work of air-conditioner outdoor unit. In order to investigate the internal flow mechanism of the axial-flow fan with different trailing edge structures of impellers, four kinds of impellers were designed, and numerical simulation and experiment were deployed in this article. The pressure distribution on the blades surface and distribution of vorticity in impellers were obtained using numerical simulation. Distribution of blade loading and velocity at the circumference are discussed. The relationship between the wideband noise and the trailing edge was established based on the experiment results. The results show that after the optimization of the trailing edge structure, the distribution of vorticity near the trailing edge of the blade is more uniform, especially at the trailing edge of 80% of the chord length of the suction surface. From the blade height position of 70% to the impeller tip, the pressure on the surface rapidly increases due to the tip vortex and the vortex shedding on the blade edge occurred in the top region of impeller. The pressure fluctuation amplitude at the trailing edge structure of the tail-edge optimization structure is smaller. In the distribution of blade loading, the three tail-edge optimization structures have smaller pressure fluctuations and pressure differences at the trailing edge structure. It is extremely important to control the fluctuation amplitude at the trailing edge. The amplitude of low-frequency sound pressure level of optimizing the trailing edge structure decreases obviously in the range of 50–125 Hz, and the optimization structure of trailing edge has an obvious effect on low-frequency wideband noise.

Experimental and numerical investigation on the pressure pulsation and instantaneous flow structure in a nuclear reactor coolant pump

Abstract The instantaneous vortical flow structure is one of the typical flow structures inside the nuclear reactor coolant pump (RCP), which would cause the unsteady pressure pulsations, vibrations of the unit and fatigue of components. Due to high safety requirement of the RCP during the actual operation of the nuclear power plants (NPPs), revealing instantaneous nature of vortical flow structure and its pressure pulsation becomes a crucial issue for studying the internal flow mechanism of the RCP. The purpose of this study is to shed comprehensive light on the pressure pulsation and instantaneous vortical flow structure in the RCP by using the experimental method and the numerical simulation method. Based on the result of comparison with the experiment and numerical simulation, it can be considered that the LES method can better identify instantaneous nature of vortical flow structures in the low frequency band. The instantaneous vortical flow structure is attached to the impeller RBPS (rotor blade pressure surface) and transient jet wake vortex flow structures between the impeller and diffuser under the rotor-stator interaction effect are fairly obvious by Q-criterion. On the other hand, in the axial-vorticity component distribution, the rotor-stator interaction effect between the impeller and diffuser cannot be clearly revealed. From the three-dimensional structures, the main vortex structures are in the front and back cavity of the right-hand side near the discharge nozzle and the right-hand side below the discharge nozzle in the spherical casing. Meanwhile, combined with pressure spectrum, it is convinced that their unsteady characteristic is the main reason that causes complex excitation frequencies in the low frequency band of the right-hand side near the discharge nozzle.

Internal flow and loss mechanisms of specific speed 160 m-kW shroudless hydro turbine

High efficiency and low-cost hydro turbines are becoming more popular for medium and small-scale hydropower generation in underdeveloped locations. Because of this, a shroudless hydro turbine of medium specific speed 160 [m-kW] was developed based on a low specific speed 80 [m-kW] design concept previously studied in the author’s laboratory. We are aiming to develop an optimal design method and further improve performances of the medium- specific speed 160 [m-kW] shroudless hydro turbine. The most important factors are the internal flow and the loss mechanism of each flow path component. In this study, the internal flow and loss mechanisms were investigated and clarified by experimental and computational approaches. The main loss mechanisms in the double circular cascade is the high swirling strength vortices extending from the guide vane inlet to the outlet near the end wall of guide vanes. Furthermore, it has been found that the interference of the secondary flow and leakage flow, and the tip leakage vortex are the predominant loss mechanisms of the shroudless runner.

Preliminary study on internal flow simulation of centrifugal dredge pump by SPH algorithm

In order to simulate the internal flow of centrifugal dredge pump accurately and efficiently, a meshfree simulation method is proposed, which is a method of smooth particle hydrodynamics (SPH). The fluid domain is discretized into Lagrangian particles, the blades, the pump body are discretized as solid particles, the governing equations for fluid, and solid mechanics are transformed into corresponding particle force. When the initial state is started, the particles move and continue to evolve under the action of these forces. The motion parameters of the flow field and solid can be obtained by interpolating the kernel function with the information of particle. The correctness of the control equations and the boundary conditions is verified by simulating the standard examples. The preliminary analysis of using this method to simulate the movement of the two-dimensional centrifugal dredge pump shows that the simulation results are reasonable. It lays theoretical basis for 3-dimensional simulation of internal flow and transition process of centrifugal dredge pump by SPH algorithm. It will be very conducive to reveal the internal transient flow mechanism of flow system transition process in centrifugal dredge pump, which is of great theoretical significance to ensure the reliable and stable operation of dredge pump system.

Dynamic instability of a pump-turbine in load rejection transient process

Load rejection is one of the most crucial transient processes in pump-turbines. However, only a few achievements on the internal flow mechanism of pump-turbines in load rejection processes have been presented. In this study, firstly, the load rejection process in a pump-turbine was simulated with a three-dimensional unsteady turbulent numerical method using the technology of dynamic mesh and the user-defined functions in the FLUENT software. The rotational speed predicted through numerical simulation agrees well with experimental data. Secondly, based on numerical simulations, a dynamic instability in the load rejection process was found and presented that the pressure and performance characteristics, including hydraulic torque on the runner and the discharge, fluctuate in the overall trend. Meanwhile, all the performance characteristics and the pressure fluctuate sharply near the operating condition points, where hydraulic torque on the runner is equal to zero or reverse flow is maximum at reverse pump conditions. Finally, the time-frequency features and formation mechanism of the dynamic instability were analyzed emphatically. The analysis of the internal flow in the pump-turbine reveals that dynamic instability in the load rejection process are mainly caused by the vortex flow in the tandem cascades regions. Furthermore, the possible methods to improve the dynamic instability in the load rejection process were recommended.

Fluid Flow and Thin-Film Evolution near the Triple Line during Droplet Evaporation of Self-Rewetting Fluids.

The microscopic region near the triple line plays an important role in the heat and mass transfer of droplets, although the mechanisms of evaporation and internal flow remain unclear. This paper describes an experimental study of fluid flow and thin-film evolution near the triple line in sessile droplets of self-rewetting fluids, aqueous solutions of alcohols with the number of carbon atoms varying from 1 to 7, to analyze the influence of various factors on the mesoscale flows. The mechanism of internal flow for self-rewetting fluid droplets was different from that of conventional fluids, and hence, a novel expression of the in-plane average velocity was fitted for them. The temporal and spatial evolution of the thin-film thickness near the triple line during droplet evaporation was obtained by using a proposed subregion method, which was developed from an evanescent-wave-based multilayer nanoparticle image velocimetry technique. The self-rewetting fluids are conducive to increase the microscopic critical contact angle and the energy barrier of the contact line, which reduces the rate of thin-film thickness variation. The inhibited impact of self-rewetting fluids on evaporation increases gradually with an increasing number of carbon atoms.

Numerical Analysis of the Effect of Misaligned Guide Vanes on Improving S-Shaped Characteristics for a Pump-Turbine

The S-shaped characteristic curves in pump-turbines complicate synchronization with the electrical grid and affect system safety. Misaligned guide vanes (MGVs) are one of the most effective solutions to avoid S-shaped characteristics. The internal flow mechanism with the MGV for improving S-shaped characteristics was studied by numerical analysis. Six operating conditions were modeled in the S-shaped region. Four guide vanes were arranged as the MGVs to qualitatively and quantitatively analyze the flow behavior. The internal flow was quite complex at the four operating points without the MGV; here, the attack angle and the flow behavior had no obvious difference at each vane. For the similar conditions with MGVs, attack angles and internal flow fields varied clearly at each vane, especially in the vaneless region and in the runner blade passages. For the same discharge rates, total openings, and rotating speeds, the internal flows were quite different between with and without the MGVs. The MGVs disrupt the high-speed circumferential water ring (appreciably faster compared to the main flow) in the vaneless region and maintain operation with higher unit speeds. Consequently, the unit speed is larger at the same unit discharge in the S-shaped region. Therefore, the MGV method can reduce S-shaped characteristics.

Numerical simulation of the bubble column at elevated pressure with a CFD-PBM coupled model

Bubble column reactors are often operated under elevated pressure. However, the significant effects of pressure are not well understood. This work aimed to simulate the bubble column operated at elevated pressure by computational fluid dynamics (CFD) coupled with the population balance model (PBM). The effect of the pressure on the bubble breakup was considered based on the mechanism of internal flow during bubble deformation and breakup. For the drag coefficient of bubble swarms, both the hindering effect of small bubbles and the wake accelerating effect of large bubbles were taken into account. Good predictions were obtained concerning the effect of the operating pressure on the gas holdup in both the homogeneous and heterogeneous regimes, especially the significant increase of the gas holdup and decrease of the bubble size at elevated pressure. These results showed that the CFD-PBM coupled model was promising for further simulation of the bubble column under complex operating conditions.