Channel Vortex Research Articles

Investigation on formation mechanism of cavitation channel vortex in the runner of Francis turbine

Abstract Cavitation channel vortices generally cause flow state more complex in the runner, leading to the work efficiency is reduced and the cavitation erosion on flow components is enhanced. In this paper, simulation of cavitation flow is carried out for a Francis turbine model, and the evolution of vortices in the runner during cavitation development is analyzed under partial load conditions. The results show that cavitation has a great influence on the morphology and distribution of attached vortices between blades, trailing vortices at the outlet edge of the blades, and vortex near the cone in the runner. With the gradual development of cavitation, the invasion effect of vortex rope in draft tube on the runner is enhanced, and the vortex near runner cone is gradually connected as a whole. Under the effect of the downstream reverse pressure gradient in local low-pressure area, the flow separation area gradually expands to the blade and upstream, and more singular points evolve on flow separation lines, which leads to more trailing vortices at the outlet of the blade and cavitation channel vortices are formed in serious cavitation stage.

Influence of guide vane opening on channel vortex and pressure pulsation in Francis turbine runners

In order to investigate the impact of guide vane opening on channel vortex and pressure pulsation within the runner of a Francis turbine, this research examines the cavitation phenomenon, channel vortex phenomenon, and flow field characteristics of a power station's turbine in China. The investigation is conducted using computational fluid dynamics theory. Furthermore, the analysis focuses on the influence of changes in guide vane opening on the flow state within the turbine runner. The findings indicate that the occurrence of rotor cavitation is severe when the opening is small, and increasing the opening reduces the cavitation capacity of the rotor. The presence of a lobe vortex phenomenon is mainly observed within a small range of guide vane openings, and altering the opening modifies both the range of influence and flow characteristics of the lobe vortex. In the middle range of guide vane openings, vortex pressure pulsation dominates within the rotor, with the highest amplitude of pressure pulsation occurring at a 60% opening. In the large opening range, pressure pulsation mainly occurs in the lobe frequency pressure pulsation of the rotor within the lobe-less area. The amplitude of blade frequency is typically small, indicating good hydraulic stability of the unit. These study results elucidate the interval of stable operating conditions for the unit and offer valuable insights for maintaining its stable operation.

Improving the hydraulic performance of a high-speed submersible axial flow pump based on CFD technology

The hydraulic performance of a high-speed submersible axial flow pump is investigated to reduce its energy consumption. A more efficient and stable optimization method that combines parametric design, computational fluid dynamics, and a computer algorithm is proposed. The main aim is to broaden the high-efficiency operating zone, so the average efficiency under multiple conditions is optimized while considering rotor–stator matching. The design-of-experiments method and a radial-basis-function neural network are combined to form the optimization platform, and automatic optimization of the pump design is realized through repeated execution of design and simulation. The flow loss mechanism inside the pump is studied in depth via the entropy generation rate, and regression analysis shows that the pump efficiency is influenced mainly by the blade angles. After optimization, the target efficiency is increased by 8.34%, and the flow field distribution shows that the channel vortex and hydraulic loss are controlled effectively. Finally, the results are validated by experiment. The proposed optimization approach has advantages in saving manpower and obtaining globally optimal solutions.

Study on mechanism of unsteady gas-liquid two-phase flow in liquid-ring vacuum pump

To investigate the gas-liquid two-phase flow mechanism in liquid-ring vacuum pump, the large-eddy simulation coupled with volume-of-fluid method (LES-VOF) was used to numerically simulate the unsteady gas-liquid flow characteristics in pump. The structure of gas-liquid flow within pump and its dynamic evolution characteristics were studied, and the hydraulic excitation characteristics induced by gas-liquid flow were analyzed. The result shows that the channel vortex inside the impeller on the pump suction side gradually flows out of the flow passage with the rotation of the impeller, and continuously mixes with the wake vortex at the outlet of the impeller and the separation vortex at the inner wall of the casing. The evolution and development of vortex structures with different intensities and scales near the inner wall of the suction side casing have a relatively large contribution to pressure fluctuation. The wake vortex at the impeller outlet of the exhaust side gradually enters the flow passage and dissipates gradually with time. There is an obvious high-intensity backflow vortex in the exhaust section due to the pressure difference between the flow channels near the leading edge of the exhaust port, which leads to obvious rotor-stator interaction in the exhaust section.

Analysis on influence of self-fan cooling motor on fan aerodynamics

With the development of multi electric/all electric aircraft, the degree of aircraft secondary energy electrification is becoming higher and higher. As the core component of the electrical system, the motor presents the trend of small volume and high-power density. The heat dissipation problem of compact motor is becoming more and more serious. At present, the self-driven fan cooling system is widely used in aircraft motors. In this paper, the axial fan cooling system of an aero generator is taken as the research object, and the changes of fan flow field and aerodynamic characteristics in the fan motor coupling model of the isolated fan and the installed motor are analyzed by using three-dimensional computational fluid simulation. At the same time, the influence of the change of the axial distance between the fan and the motor on the aerodynamic performance is also studied. The analysis of the simulation results shows that the close coupling between the motor and the fan components greatly affects the flow structure in the fan, resulting in the distortion potential effect on the fan outlet section. The fan flow field appears the blade root reflux, the channel vortex in the blade and the expansion of the corner separation area. With the increase of the axial distance between the motor and the fan, the influence of the motor on the flow in the fan gradually approaches the flow blocking effect.

Study on Flow Characteristics of Francis Turbine Based on Large-Eddy Simulation

The research object was a Francis turbine, and the working conditions at 100%, 75%, 50%, 25%, and 1% opening were determined by the opening size of the guide vane. Large-Eddy Simulation (LES) was adopted as a turbulence model method to conduct three-dimensional unsteady turbulent numerical simulation of the entire flow channel of a Francis turbine, and the flow situation of various parts of the turbine under different working conditions was obtained. The flow characteristics of each component under different working conditions were analyzed, and the hydraulic performance of each part was evaluated. The factors that affected the stability of hydraulic turbines were identified, and their formation mechanisms and evolution laws were explored. The results show that the guide vane placement angle was reasonable in the guide vane area, and the hydraulic performance was fine. The impact on the stability of the hydraulic turbine was small. Further research showed that the hydraulic performance was poor in the runner area, and there were flow separation and detachment phenomena in the flow field. This created a channel vortex in the runner blade channel. The channel vortex promoted the lateral flow of water and had a significant impact on the stability of the hydraulic turbine. The diffusion section of the draft tube can dissipate most of the kinetic energy of the water flow in the draft tube area, and it had a good energy dissipation effect. However, the was a large pressure difference between the upper and lower regions of the diffusion section, and it generated a backflow phenomenon. It created vortex structures in the draft tube, and the stability of the hydraulic turbine was greatly affected.

Valide Tests zur Erstellung eines Assessments zum Screening für Risikofaktoren von Verletzungen der unteren Extremität in Mannschaftsballsportarten

In this paper, a novel butterfly wing type hydrocarbon fuel cooling channel structure is proposed and applied for the first time in the cooling of rotating power generation turbine blades of hypersonic vehicles. The effects of rotating butterfly wing-type channel root height and cross-critical temperature on the friction factor and heat transfer performance of hydrocarbon fuel are investigated in depth. Numerical results show that the thermal performance of the butterfly wing type channel corresponding to the root height of 0.8 D is the highest. Compared to the 1 D channel, the thermal performance of the butterfly wing type channel corresponding to the root height of 0.8 D is increased by a maximum of 1.8 times. Compared to the 1 D channel, the Nusselt number of the wing type channel corresponding to the root height 0.2 D is increased by a maximum of 1.03 times. Compared to the inlet temperature greater than critical temperature, the thermal performance of the wing type channel at inlet temperature less than critical temperature is increased by a maximum of 2.1 times. Compared to the 1 D channel, the wing type channel reduces the first channel backflow and vortex area by almost 50%.

Novel butterfly wing type rotating channel supercritical pressure hydrocarbon fuel heat transfer performance investigation

In this paper, a novel butterfly wing type hydrocarbon fuel cooling channel structure is proposed and applied for the first time in the cooling of rotating power generation turbine blades of hypersonic vehicles. The effects of rotating butterfly wing-type channel root height and cross-critical temperature on the friction factor and heat transfer performance of hydrocarbon fuel are investigated in depth. Numerical results show that the thermal performance of the butterfly wing type channel corresponding to the root height of 0.8 D is the highest. Compared to the 1 D channel, the thermal performance of the butterfly wing type channel corresponding to the root height of 0.8 D is increased by a maximum of 1.8 times. Compared to the 1 D channel, the Nusselt number of the wing type channel corresponding to the root height 0.2 D is increased by a maximum of 1.03 times. Compared to the inlet temperature greater than critical temperature, the thermal performance of the wing type channel at inlet temperature less than critical temperature is increased by a maximum of 2.1 times. Compared to the 1 D channel, the wing type channel reduces the first channel backflow and vortex area by almost 50%.

Numerical simulation and experimental study on an innovative vortex eliminator using a modified SST turbulence model for gas-liquid two-phase flow

Vortices in the turbine and pump threatens the safe operation of the unit. The purpose of this paper is to design an antivortex guide cone (AVGC) based on the dynamic characteristics of the vortices. A new approach with modified SAS-SST turbulence model was adopted to simulate the vortex, which is verified by model experiments. The velocity gradient is the key factor of vortex generation, so the antivortex device with vortex elimination blades adopt streamline structure to reduce the velocity gradient in vortex area. Three schemes with N0 (N0 is the number of vortex elimination blades) of 4, 6 and 8 are designed respectively. The study results indicate that the vortex elimination effect of AVGC varies greatly with different N0, resulting in great differences in the performance of the unit. Some local small-scale vortices still exist near AVGC in Scheme 1 (N0 = 4). In Scheme 2 (N0 = 6) is 6, vortices can be eliminated thoroughly with the least hydraulic loss and the unit efficiency improved by 1.8%. In Scheme 3 (N0 = 8), some new channel vortex will be generated in the blade channel, resulting in more hydraulic loss. Only the hydraulic loss produced by AVGC less than that produced by vortex that indicate the scheme is feasible. The research work can provide important academic value and engineering insights to eliminate vortex.

Influence of grooved rib tip structure on tip loss and heat transfer in a gas turbine blade

This study focuses on the effects of three groove tip structures (full rib groove tip, partial rib tip on the suction side, and partial rib tip on the pressure side) on tip leakage flow, aerodynamic characteristics of a cascade, and heat transfer in a gas turbine blade. The groove’s width B = 1.6 mm, while the tip clearance is τ = 1.2 mm. Results of the flow parameters, fluid flow, and heat transfer in the recessed channel are discussed. The results show that all ribbed tips obtain more uniform outlet flow angle distribution and higher aerodynamic performance than the plane tips. The total aerodynamic pressure loss of the ribbed tips on the pressure side is the same as that of complete ribbed tips. The evolution mechanisms are different, although both can improve the turbine efficiency. Although the partial rib tip on the pressure side weakens the mixing of the channel vortex and leakage vortex near the trailing edge and has the best control effect on the leakage vortex, the lack of the suction side rib will make it easier for the low-energy fluid to flow into the gap from the front of the suction side, which is not conducive to reducing the leakage flow inside the gap; the full rib tip not only minimizes the tip relative leakage flow and leakage loss but also increases the channel vortex loss. With the complex vortex system in the groove and the rib blocking effect at the leakage outlet, the suction-side rib tip becomes the tip structure with the best leakage flow control effect under the same clearance, but the leakage vortex loss is the highest.

Helical Flow Channel and Helical Vortices in Abdominal Aortic Aneurysm: Are They Twirling Towards Rupture?

Despite the fact that aortic diameter remains the main parameter for considering elective abdominal aortic aneurysm (AAA) repair,1 many studies have recently aimed at identifying other parameters that could better predict the risk of rupture along with aortic diameter. Computational fluid dynamics (CFD) is one of the most promising fields of research in predicting AAA growth and rupture. CFD is a complex mathematical and computational analysis based on the finite element method, used to solve a problem by subdividing a large system into smaller, simpler parts, called finite elements.

Numerical simulation of the runner blade channel vortex in Francis turbine

The runner blade channel vortex could induce cavitation erosion, noise and vibration, when Francis turbines operate at part load condition. It's usually difficult to simulate the channel vortex exactly in runner passage, this paper attempts to predict the runner channel vortex in some operating conditions with different discharge and speed factors.The results show that the structure and intensity of runner channel vortex are various between the vortex incipient and development line. The velocity distribution indicates that there is a relationship between the channel vortex and backflow region near the hub with different specific discharge factors, while the flow incident angle has a distinct variation with different specific speed factors. Furthermore, this work enables the researchers to understand the flow field and carry out optimization design in runner hydraulic research.

Vortex Distribution and Energy Loss in S-Shaped Region of Pump Turbine

Research on the S-shaped region of pump turbines requires a detailed understanding of the vortex distribution law and energy losses under various working conditions. In this study, numerical simulations of a pump turbine model were conducted, and the results were consistent with the experimental results. The |ω|-criterion in the vortex analysis method was combined with the Q-criterion to reveal vortex distribution in the S-shaped region for each working condition along the Q11-n11 curve for all the conditions. Under the runaway and turbine break conditions, the flow field vortices were mainly the leaf channel vortex and separation vortex. Under zero-flow-rate and reverse-pump conditions, the vortices developed towards the stay-guide vanes, obstructing the flow path. Combined with the entropy production rate distribution, vorticity is closely related to energy loss. Compared to the rotation, the vorticity generated by the strong shear effect is significant.

Analysis of Internal Flow Characteristics of a Startup Pump Turbine at the Lowest Head under No-Load Conditions

The instability of the no-load working condition of the pump turbine directly affects the grid connection of the unit, and will cause vibration and damage to the components of the unit in severe cases. In this paper, a three-dimensional full flow numerical model including the runner gap and the pressure-balance pipe was established. The method SST k-ω model was used to predict the internal flow characteristics of the pump turbine. The pressure pulsation of the runner under different operating conditions during the no-load process was compared. Because the rotation speed, flow rate, and guide vane opening of the unit change in a small range during the no-load process, the pressure pulsation characteristics of the runner are basically the same. Therefore, a working condition was selected to analyze the transient characteristics of the flow field, and it was found that there was a high-speed ring in the vaneless zone, and a stable channel vortex was generated in the runner flow passage. Analyzing the axial water thrust of each part of the runner, it was found that the axial water thrust of the runner gap was much larger than the axial water thrust of the runner blades, and it changed with time periodically. It was affected by rotor stator interaction. The main frequency was expressed as a multiple of the number of guide vanes, that is, vanes passing frequency, 22fn. During the entire no-load process, the axial water thrust of the runner changed slowly with time and fluctuated slightly.

Analysis of Channel Vortex and Cavitation Performance of the Francis Turbine under Partial Flow Conditions

To realize a multienergy complementary system involving hydropower and other energy sources, hydraulic turbines frequently run under partial flow conditions in which a unique flow phenomenon, the channel vortex, occurs in the runner, causing fatigue failure and even cavitation to the turbine blade. Cavitation severely shortens the service life of the unit and terribly limits the output of the turbine under partial flow conditions. In this paper, a numerical model of a Francis turbine was created with tetrahedral grids; the large eddy simulation (LES) method based on the WALE subgrid scale model and the Schnerr–Sauer cavitation model was adopted to carry out numerical simulation of the Francis turbine; and a vortex identification method based on the Q criterion was used to capture and analyze the channel vortex. The calculation results showed that a negative impact angle at the inlet of the runner occurred when the turbine ran under partial flow conditions, leading to three different types of channel vortexes in the blade channel. Also, different channel vortexes caused cavitation on different positions on the runner, and the volume change of cavitation showed periodic properties.

A Three-Dimensional Simulation Analysis of Fluid Flow and Heat Transfer in Microchannel Heat Sinks with Different Structures

Abstract Numerical studies have been performed to analyze the fluid flow and heat transfer characteristics of nine microchannel heat sinks (MCHS) with different shapes and different arrangements of the ribs and cavities on the sidewalls, using three common shapes (square, triangle, and circular) of ribs or cavities as the basic structure in this work. The boundary conditions, governing equations, friction factor (f), Nusselt number (Nu), and performance evaluation criteria (ξ) were considered to determine which design was the best in terms of the heat transfer, the pressure drop, and the overall performance. It was observed that no matter how the circular ribs or cavities were arranged, its heat sink performance was better than the other two shapes for Reynolds number of 200–1000. Therefore, circular ribs or cavities can be considered as the best structure to improve the performance of MCHS. In addition, the heat sink performance of the microchannel heat sink with symmetrical circular ribs (MCHS-SCR) was improved by 31.2 % compared with the conventional microchannel heat sink at Re = 667. This was because in addition to the formation of transverse vortices in the channel, four symmetrical and reverse longitudinal vortices are formed to improve the mixing efficiency of the central fluid (low temperature) and the near-wall fluid (high temperature). Then, as the Reynolds number increases, the heat sink performance of MCHS-SCR dropped sharply. The heat sink performance of microchannel heat sinks with staggered ribs and cavities (MCHS-SCRC, MCHS-STRC, and MCHS-SSRC) exceeded that of MCHS-SCR. This indicated that the microchannel heat sink with staggered ribs and cavities was more suitable for high Reynolds number (Re > 800).

Effect of a Submerged Vane-Field on the Flow Pattern of a Movable Bed Channel with a 90° Lateral Diversion

This laboratory study focused on the effect of a submerged vane-field on the flow pattern and bed morphology near and inside the entrance reach of a movable bed 90° lateral diversion. The system was modelled under live bed conditions for a water discharge ratio of ≈0.2. Two experiments were run until bed equilibrium was reached: with and without a vane-field installed close to the diversion entrance to control the transfer of sediments into the diversion channel. The equilibrium bed morphology and the associated 3D flow field were measured in great detail. The bed load diverted into the diversion was reduced by approximately one quarter due to the action of the vane-field. The vanes prevented the formation of the diversion vortex in the main channel, upstream of the diversion’s entrance, thus contributing to that decrease. They also created a main channel vortex that started at the most upstream vanes and further decreased the amount of bed load entering the diversion. The flow separation zone inside the diversion was larger with vanes, but conveyance was balanced through a slightly deeper scour trench therein. The flow structures described were confirmed through the measurements of the turbulent kinetic energy.

Multi-condition optimization of a cross-flow fan based on the maximum entropy method

Cross-flow fans are widely used in heating, wind-curtains, and air-conditionings, as well as other ventilation systems. A single or double arc is generally used as the camber line of cross-flow fans, but this design leads to constraints in the geometry of the blade profiles. In this study, the camber line of a cross-flow fan blade was parameterized by five parameters based on the fourth-order Bezier curve. A two-dimensional computational fluid dynamics (CFD) simulation was conducted to predict the aerodynamic characteristics and the internal flow field. It is necessary in multi-condition optimization, to evaluate the relative importance of the performance parameters under different working conditions and determine their weight factors. Here, a novel maximum entropy method (MEM) was proposed to quantify of volume flow rate, because the method avoids the subjectivity in the selection of the weights. Subsequently, a multi-island genetic algorithm (MIGA), combined with numerical simulation, was used to search the global optimum in the given design space. The results indicated that the optimum combination of the structural parameters reduced the blade channel vortex in a particular location of the impeller and changed the position and size of the eccentric vortex. The volume flow rate of the optimized impeller was 4.28% higher at the minimum rotation speeds and 12.87% higher at the maximum rotation speeds.

Analysis of Unsteady Flow Field in Rotating Detonation Turbine Engine

The rotating detonation turbine engine is a research hotspot in recent years, but the research is mainly at the phase of theoretical analysis. In this paper, the influence of the circumferentially rotating pulsating flow field on the performance of the turbine is studied by numerical simulation. The results show that the uneven turbine inlet flow will increase the working load of the stator vane, but will produce greater flow loss. At the same time, the strength of the channel vortex and the tip leakage vortex inside the rotor is also increasing. The mass flow rate and work efficiency of the turbine decrease with the increase of the unevenness of the flow field.

Numerical Study on the Blade Channel Vorticity in a Francis Turbine

A relevant way to promote the sustainable development of energy is to use hydropower. Related systems heavily rely on the use of turbines, which require careful analysis and optimization. In the present study a mixed experimental-numerical approach is implemented to investigate the related mixed water flow. In particular, particle image velocimetry (PIV) is initially used to verify the effectiveness of the numerical model. Then numerical results are produced for various conditions. It is shown that an increase in the guide vane opening can reduce the extension of the region where the fluid velocity is 0 at the inlet of the runner blade, i.e., it can counteract the generation of the channel vortex; an increase in the guide vane opening also contributes to mitigate the pressure acting on the runner blade; no matter what the working conditions are, the surface pressure is usually higher than that on the suction surface, and there is a cliff-like drop of pressure at the tail of the blade, which indicates that the runner blade tail is more prone to develop backflow.