Blade Suction Surface Research Articles

Effects of water ingestion on the aerodynamic performance of an axial compressor

Liquid water in the atmosphere affects the safety and stability of aero-engines in their operation and threatens flight safety because when liquid water is ingested by compressors, it breaks into smaller droplets, which then bounce in, collide with, or splash onto the blades of compressors, increasing profile loss and aerodynamic drag. As a result, the performance of compressors would deteriorate after water ingestion. To investigate the mechanisms underlying such performance changes and flow differences within the passage due to water ingestion, we simulate the gas-liquid two-phase flow within National Aeronautics and Space Administration Rotor 67 by employing the Eulerian–Lagrangian approach. This paper investigates water–air ratios ranging from 3% to 9% of the total mass and droplet diameters between 500and 1500 μm. Numerical simulations are used to examine the flow and performance of the compressor under different inlet conditions. The results demonstrate that higher water ingestion rates and smaller droplet diameters decrease the compressor efficiency, due to increased entropy generation. They also reduce the mass flow rate, due to droplet accumulation blocking the passage. Specifically, water droplets reduce the axial velocity and significantly increase boundary layer separation losses on the blade suction surface. In addition, water ingestion partially mitigates tip clearance leakage flow losses, slightly weakens the shock intensity, and shifts the shock position forward, while delivering minimal impact on wake losses.

Unsteady Flow Mechanism of Impeller Stall Inception in a High-Pressure Ratio Centrifugal Compressor With Bleed Slots

Abstract The impeller stall inception in a high-pressure ratio centrifugal compressor with bleed slots was investigated by experiments and unsteady numerical analysis. This study reported that the flow instability of the internal flow was first initiated around the leading edge of the splitter blades and finally resulted in tip leakage flow instability at the leading edge of the main blades. The impeller stall consisted of a stall cell and rotated about 70% of the impeller rotational speed. Furthermore, a pressure disturbance occurs at the splitter blade leading edge before the main blade leading edge. The blockage within the passage of the suction side at the leading edge of the splitter blade was first increased because the leading-edge separation on the suction surface of the splitter blade was enlarged. The blockage within the passage of the pressure side was increased after that of the suction side stopped increasing. Therefore, the blockage development of both pressure and suction passage at the leading edge of the splitter blade was the main cause of the stall inception. Then, a longitudinal vortex was formed near the bleed slots and blocked the mainstream near tip side. Subsequently, the reverse flow region in the inducer was developed and extended to the impeller inlet. Finally, the flow angle to the main blade increased, the spillage occurred at a leading edge of a main blade, and the rotating stall was generated. The cause of impeller instability was the separation of the suction surface of the splitter blade and the formation of a stagnation region on the pressure surface at the splitter leading edge.

Effects of variable blowouts on aerodynamic loss reduction performance of compressor bionic blades

Abstract A passive flow control method inspired by blowouts was employed to eliminate flow separation and reduce the losses associated with higher compressor loads. The flow characteristics of a dimple-shaped cascade were investigated by implementing bionic blowout dimples on a blade suction surface and the loss-generation mechanism was analyzed. First, the reliability of the numerical simulation was confirmed via experimental validation. Subsequently, biomimetic principles were applied to arrange dimples on the suction surface of cascade blades, and the effects of several blowouts were analyzed. The analysis revealed that vortices within the dimples induced external fluid into the dimples, thereby increasing the turbulent kinetic energy of the external fluid and improving wall-adjacent flow adherence. The bionic-blowout variant dimples created a ‘rolling bearing’ effect that reduced frictional losses and effectively controlled flow separation. Within a certain blowout range, the bionic blowout variant dimples significantly improved the flow characteristics. At a −6° angle of attack, the total pressure loss of the dimple-structured cascade decreased by 35.85%, and the pressure ratio increased by 2.35%. The bionic blowout-variant dimples on the blade suction surface exhibited a three-dimensional disturbance effect. The induced vortex structures regulated the boundary layer transition and suppressed the formation of laminar separation bubbles, thereby enhancing the flow conditions near the corner region.

Transition mechanism and loss analysis of a separated flow over herringbone riblets in a compressor cascade using the lattice Boltzmann method

Herringbone riblets were regarded as a promising approach to control the separation bubble on the compressor blade. However, the underlying mechanism requires further elucidation. And numerical simulations with body-fitted meshes often face challenges in mesh generation due to the tiny and complex geometries involved. In the present research, high-fidelity simulations using the Lattice Boltzmann Method and Immersed Boundary Method were performed to investigate the effects of herringbone riblets on separated flow in a compressor cascade. At a low Reynolds number of 90 000, a separation bubble appears on the blade suction surface. The application of herringbone riblets on the suction side surface shows that it effectively reduces the bubble length from 0.24c to 0.12c and reduces the loss coefficient by 11%. A counter-rotating mode of secondary flow occurs before the separation, with a near-wall spanwise motion from the divergent region to the convergent region and a compensating flow from the convergent region to the divergent region in the outer layer of the boundary layer. Transition occurs earlier on the suction side surface due to the complex flow patterns. Four different mechanisms are responsible for the earlier transition. Over the divergent region, engulfing of a high momentum fluid from the outer layer to the inner layer of the boundary layer suppresses the separation bubble, forcing a high-momentum passage where an attached boundary layer is observed. This thinner boundary layer leads to an earlier natural transition. Second, the discharge of fluid from the herringbone cusp causes the overflow from the riblet channel beside the divergent line, i.e., overflow transition. Meanwhile, the transition over the converging region is attributed to the accumulation of disturbance. Finally, in the middle region with yawed riblets, transition in a separated shear layer occurs earlier under the influence of adjacent transition mechanisms over the divergent/convergent region. These mechanisms also bring about a serrated structure in the downstream wake. Overall, this research confirms the role of the counter-rotating mode produced by herringbone riblets in separation control and reveals the transition mechanisms for loss production. The findings suggest that proper utilization of herringbone riblets can provide significant improvement on the compressor blade performance.

Analysis of Flow Loss Characteristics of a Multistage Pump Based on Entropy Production

To reveal the internal flow loss characteristics of a multi-stage pump, the unsteady calculation of the internal flow field of a seven-stage centrifugal pump was carried out, and the entropy production theory and Q criterion were utilized to analyze the unsteady flow characteristics of each flow component under different flow rates. The research results show that as the flow rate increases, the entropy production value and the energy loss inside the flow components also increase accordingly. The viscous dissipation entropy production caused by fluid viscosity is very small, and the turbulent dissipation entropy production caused by turbulent fluctuations and wall dissipation entropy production are the main sources of energy loss. The impellers, diffusers, and outlet chamber are the main regions of energy loss in the multistage pump. The entropy production value of the first-stage impeller is significantly higher than that of other impellers, while the entropy production value of the first-stage diffuser is significantly lower than that of other diffusers. Through vortex structure analysis, it is found that the high entropy production regions in the impeller are concentrated in the impeller inlet area, the blade suction surface, and the impeller outlet area.

Bionic microstructure design of cross flow fan for high aerodynamic and low noise performances

In this paper, a novel approach was introduced for cross flow fan design by incorporating bionic microstructures on the blade surface. Two types of bionic microstructures, riblet and convex hull, were constructed and studied to achieve high aerodynamic efficiency and low noise design. Numerical simulations were conducted to understand the influence of bionic microstructures on the fan’s aerodynamic and acoustic performance, and the impact of design parameters on these performances was investigated. After printing the bionic microstructure fan samples, the experimental tests were carried out. The flow field and sound field data of the fans with bionic microstructures and the original fan were compared. The results demonstrated that both riblet and convex hull microstructures effectively improved the fan’s aerodynamic performance by reducing turbulent kinetic energy on the blade surface. Additionally, the microstructures inhibited boundary layer separation on the suction surface of adjacent blades, which contributed to reducing vortex noise. However, the pressure gradient formed near the convex hull and the edge of the small-diameter riblet resulted in deterioration in the noise of the corresponding fan. By optimizing the diameter and spacing of the riblet microstructure, the best noise reduction performance was achieved for the riblet fan. The optimized fan showed a 5.3% increase in maximum air volume flow rate and a 2.4 dB(A) reduction in noise. Overall, the cross flow fan design method based on bionic microstructures has valuable application potential in improving the aerodynamic and acoustic performance of various rotating machinery.

Effect of radial inlet distortion on aerodynamic stability in a high load axial flow compressor

Radial inlet distortion induced by boundary layer separation can significantly affect the aerodynamic performance of the compressor. The effect of radial inlet distortion on the flow structure is numerically investigated in a high load axial flow compressor. The inlet boundary is determined by the experimental results at the downstream of the distortion generator. The results reveal that tip distortion leads to a reduction in the stall margin, whereas hub distortion extends it. Radial distortion redistributes the main flow toward undistorted region due to the obstructive effect of lattice ring. The deficit in axial velocity with tip radial distortion causes the rotor to operate at a higher incidence angle near the casing, while hub radial distortion alleviates the tip blade loading. The detailed three-dimensional flow field analysis indicates that increased blade loading with tip distortion shifts the trajectories of both primary and secondary tip leakage flow toward the leading edge, thereby expanding the blockage region. Conversely, hub radial distortion unloads the rotor tip region, thereby reducing the blockage region induced by tip leakage flow. Additionally, with hub distortion, the location of separation line on the blade suction surface moves closer to the leading edge, and the flow separation around the trailing edge is intensified.

Prediction of Progressive Erosion Characteristics of Centrifugal Pump Blades Based on Dynamic Boundaries

Abstract The conventional method of fixed boundaries for predicting erosion inadequately reflects the erosion characteristics of centrifugal pumps under realistic operating conditions. In this research, we employ the dynamic boundary method, considering the influence of changes in wall morphology due to erosion on the flow field. We utilize the Euler-Lagrange approach and the E/CRC erosion model, incorporating a dynamic boundary geometry reconstruction strategy, to predict the erosion characteristics of the IS80-50-315 single-stage single-suction centrifugal pump. Solid-liquid two-phase flow calculations are conducted on the pump to predict erosion characteristics under varied operating conditions. The results show that the dynamic boundary-based progressive erosion prediction method realistically forecasts the erosion characteristics of the overflow wall and captures the evolution of blade morphology with erosion time. Erosion intensity on the blade suction surface significantly surpasses that on the pressure surface, guided by particle distribution characteristics in the centrifugal pump flow channel. Particle diameter primarily influences the erosion position of centrifugal pump blades; smaller particles induce erosion in the middle region of the blade, while an increase in particle diameter shifts the severe erosion position towards the impeller outlet direction. At a consistent particle diameter, higher particle concentrations correspond to elevated erosion rates in the blade area. We establish a relationship between the blade mass loss rate and erosion time during 2500 hours of pump operation at the design condition, introducing the blade mass loss rate for convenient assessment and prediction of blade damage in the centrifugal pump.

Numerical investigation of energy dissipation and vortex characteristics on the S-shaped region of a reversible pump-turbine

In order to meet the regulation needs of power system, the pump-turbine is prone to operate in the S-shaped region during frequent start-up, leading to unit vibration and grid connection failure. In this paper, the S characteristic experiment was carried out, and the entropy production, unstable flow and vortex structure under turbine, runaway, turbine breaking, reverse pump conditions with optimum guide vane opening are detailed analyzed. The relationship between the energy dissipation and flow field characteristics is clarified. The results show that the runner entropy production is the largest in all components, and the wall entropy production is greater than the fluctuating entropy production under turbine condition. With the decrease in discharge, the fluctuating entropy production and wall entropy production first increase and then decrease, reaching the maximum values in the runaway condition. The circumferential movement trend of water enhances, and forms circumferential flow in the vaneless region. The separation vortex formed on the blade suction surface extends from the blade inlet to the middle of the flow channel, inducing a high entropy production. Under the turbine braking condition, the tangential velocity increases rapidly in the vaneless region. The circumferential flow develops into water ring, blocking the water flows into the runner. Under the reverse pump condition, the water flows in the opposite direction, with negative tangential and streamwise velocities. The absolute flow angle becomes obtuse, and a large-scale reverse vortex is formed. The runner inlet is blocked by the water ring, and the flow channel is almost completely filled with the low-speed region. The proportion of the guide vanes entropy production reaches maximum. Unstable flow leads to the increase in vorticity. Separation vortex, bypassed vortex, and reverse flow vortex leads to an increase in VST, forming a high VST region at corresponding positions. Due to the rotational motion accompanying the development of separated vortex, it leads to the increase in CFT. In addition, the circumferential flow and water ring have higher tangential velocities, which can lead to high CFT regions.

Internal flow stability analysis of vertical mixed-flow pump device based on EGT and FFT

With the purpose of researching the internal flow stability of the vertical mixed-flow pump device (VMFPD), the entire flow field was solved through the very large-eddy simulation (VLES) k- ω model and the internal flow characteristics of the pump device were quantitatively analyzed with the use of the energy gradient theory (EGT) along with the physical parameters of the flow field. The research elucidates the variety rule of the pressure along the VMFPD and the energy gradient function K as well as the time-frequency features of the pressure pulsation (PP) in the impeller and guide vane. The results demonstrate that the solid wall restriction of the guide vane and the impeller rotation cause a significant shift in the velocity and direction of the water flow. This causes a significant pressure difference between the water bodies, which makes it easier to create an unstable flow. The suction surface of the impeller blade possesses the great concentration of the area with energy gradient function K > 107, which is the critical area influencing the internal flow stability of the pump device.

A Computational Analysis of Turbocharger Compressor Flow Field with a Focus on Impeller Stall

Understanding the flow instabilities encountered by the turbocharger compressor is an important step toward improving its overall design for performance and efficiency. While an experimental study using Particle Image Velocimetry was previously conducted to examine the flow field at the inlet of the turbocharger compressor, the present work complements that effort by analyzing the flow structures leading to stall instability within the same impeller. Experimentally validated three-dimensional computational fluid dynamics predictions are carried out at three discrete mass flow rates, including 77 g/s (stable, maximum flow condition), 57 g/s (near peak efficiency), and 30 g/s (with strong reverse flow from the impeller) at a fixed rotational speed of 80,000 rpm. Large stationary stall cells were observed deep within the impeller at 30 g/s, occupying a significant portion of the blade passage near the shroud between the suction surface of the main blades and the pressure surface of the splitter blades. These stall cells are mainly created when a substantial portion of the inlet core flow is unable to follow the impeller’s axial to radial bend against the adverse pressure gradient and becomes entrained by the reverse flow and the tip leakage flow, giving rise to a region of low-momentum fluid in its wake. This phenomenon was observed to a lesser extent at 57 g/s and was completely absent at 77 g/s. On the other hand, the inducer rotating stall was found to be most dominant at 57 g/s. The entrainment of the tip leakage flow by the core flow moving into the impeller, leading to the generation of an unstable, wavy shear layer at the inducer plane, was instrumental in the generation of rotating stall. The present analyses provide a detailed characterization of both stationary and rotating stall cells and demonstrate the physics behind their formation, as well as their effect on compressor efficiency. The study also characterizes the entropy generation within the impeller under different operating conditions. While at 77 g/s, the entropy generation is mostly concentrated near the shroud of the impeller with the core flow being almost isentropic, at 30 g/s, there is a significant increase in the area within the blade passage that shows elevated entropy production. The tip leakage flow, its interaction with the blades and the core forward flow, and the reverse flow within the impeller are found to be the major sources of irreversibilities.

Numerical Analysis of a Novel Casing Treatment in an Axial Flow Fan

The present paper reports the use of a slotted casing treatment of trapezoidal shape to enhance the stall margin of an axial flow fan. The Taguchi method was used to optimize the rectangular slotted casing treatment based on that which the trapezoidal slots were designed. The unsteady numerical simulations were carried out using the shear-stress-transport turbulence model. The rotor with trapezoidal slots led to the stall margin improvement of 23.4% and mild pressure rise of 0.54%. Without the casing treatment, the tip leakage trajectory was directed from the suction surface of the blade toward the pressure surface of the adjacent blade. After incorporating the trapezoidal slots, the tip leakage trajectory was significantly shifted toward the suction surface of the same blade. This alteration in flow characteristics near the blade-tip region may be attributed to the exchange of momentum due to the simultaneous suction and blowing through the trapezoidal slots. It was also found that the trapezoidal slots improved the axial flow velocity and reduced the diffusion factor in the blade-tip region as compared to the baseline case.

Analysis of the Bubble Diameter Effect on the Gas-liquid Flow Characteristics of a Multiphase Rotodynamic Pump

Abstract In order to study the effect of bubble diameter on the gas-liquid flow in the multiphase rotodynamic pump, steady simulations with different bubble diameters were conducted using ANSYS CFX17.0. When the inlet gas content IGVF is 10%, the pump head and efficiency decrease with the increased inlet bubble diameters, and there are different downward trends at different stages of bubble diameter increase. The pump head and efficiency with small bubble diameter (d≤0.3 mm) are much higher than those of large bubble diameter(d≥0.4 mm). A small amount of gas accumulates on both the blade suction and pressure surfaces near the hub under conditions of small bubble diameters. When the bubble diameter is large, A large amount of gas accumulates on the blade suction surface, and almost no gas accumulates on the blade pressure surface. Due to the density difference between gas and liquid phases, gas accumulates near the hub. The gas content rapidly decreases when away from the hub under small bubble diameter conditions, resulting in good gas-liquid fluidity and weak gas-liquid separation.

Influence of freestream turbulence on boundary layer transition over a controlled-diffusion compressor blade

The influence of inlet freestream turbulence (FST) on the boundary layer transition over the suction surface of a controlled-diffusion compressor blade is demonstrated here by employing a well-resolved large-eddy simulation. Inherent to low Reynolds number conditions, a laminar separation bubble (LSB) forms on the suction surface, attributing to substantial flow diffusion. Inlet FST levels ranging from 1.5% to 7.6% are systematically varied, while maintaining a constant Reynolds number based on axial chord and inlet velocity at 2.1 × 105. Transition of the shear layer is initiated via Kelvin–Helmholtz instability with the amplification of selective frequencies until an inlet FST of 2.3%. Secondary instability emerges in the second half of the LSB, attributed to the amplification of perturbations in the braid region, ultimately leading to breakdown near the reattachment. At a moderate FST level of 4.2%, longitudinal streaks in the first half of the blade elongate downstream, causing the LSB to disappear, while the flow becomes inflectional at the mid-chord. Thus, the boundary layer transition in the second half of the blade is attributed to the high receptivity of the inflectional layer and breakdown of streaks, leading to an exponential growth of disturbances. Finally, at an inlet FST of 7.6%, the boundary layer appears pre-transitional in the first half of the blade, exhibiting significant turbulence levels. In the latter half, excitation occurs primarily through the breakdown of streaks, reflecting an algebraic growth of disturbances. Flow features and oscillations in the Nusselt number in this case suggest the outer mode of streak-induced instability.

Investigation into the Vortex Evolution characteristics of the Francis Turbine During the Runaway Process

Abstract The runaway process is a dynamic transition process that gradually deviates from the optimal operating condition. The flow passage components dissipate all of the energy corresponding to the hydraulic turbine’s head, causing the internal flow of the hydraulic turbine to gradually deteriorate and produce a large-scale vortex structure during the runaway process. A model Francis turbine is used as the research object in this paper to understand the evolution characteristics of an unsteady vortex during a runaway. The runaway process is investigated using high-precision simulation technology of gas-liquid two-phase flow, and the runaway speed is obtained, consistent with the experimental results. The results show that the cavitation volume increases as the rotating speed increases. The large incidence angle between the inlet flow and the blade causes large-scale flow separation in the runner, which leads to forming of a sheet vortex band near the hub of the runner hub and a columnar vortex in the blade passage of the runner blade suction surface. As the runaway process continues, the flow separation in the runner intensifies and the induced vortex size increases, obstructing the runner’s passage. In addition, pressure signal analysis reveals that the pressure fluctuation caused by the sheet vortex band is low-frequency. On the other hand, the pressure fluctuation caused by the columnar vortex is frequent. These two types of vortex structures with high-energy pressure fields degrade the flow in the runner and reduce the hydraulic performance of the turbine.

Erosion characteristics of a centrifugal pump based on rosin-rammler distribution of particles

Abstract Centrifugal pumps are widely used in water conservancy, agricultural irrigation and petrochemical fields, but the transferred fluid usually contains solid particles. With prolonged operation in a solid-liquid flow, the components of centrifugal pump are easily eroded, resulting in damaged components, shortened life and reduced efficiency. It is crucial to study the erosion characteristics on centrifugal pump in solid-liquid flow. In this work, based on the particles with Rosin-Rammler distribution in a pump station, the transient operation and erosion process of the centrifugal pump under sandy flow are simulated by using the Eulerian-Lagrangian method and Oka model. Results show that the erosion of R-R distribution is characterized by both large and small particle sizes, but it is closer to the latter condition due to the higher proportion of small particle sizes. The distribution ratio of average erosion rate on hub, blade and shroud is 1.18: 1.68: 1.00, and the most eroded areas are in the leading edge of blade and the trailing edge of the blade suction surface.

Research on gas–liquid two-phase transient process of reactor coolant pump under stuck shaft accident

The stuck shaft accident (SSA) is a typical and severe accident of the reactor coolant pump (RCP). During this accident, significant friction and heat generation result in gas–liquid two-phase flow, which adversely affects the working performance and stability of the RCP. In order to explore the internal flow field and transient pressure fluctuation of the two-phase flow in the RCP under the SSA, this study adopts three methods of standard k-ε, Large Eddy Simulation (LES) and Lattice Boltzmann Method-Large Eddy Simulation (LBM-LES) for refined simulation and then are validated by experiments. It finds that the gas volume fraction (GVF) is relatively high in the inlet path of the guide vane during the initial stage, and then the gas disperses from the suction surface of the impeller blade to the pressure surface. When the GVF increases, the small gas masses accumulate into large masses and collapses in the RCP. At this time, the vorticity in the impeller path increases and then decreases at the outlet. The pressure fluctuation on the pressure surface at the impeller outlet increases with the increase of GVF. The impact of the GVF on the pressure surface is greater than that on the suction surface, particularly in the high-frequency band during the initial stages of the SSA. Among these three models, LBM-LES demonstrates the highest accuracy with a maximum deviation of only 8.84% when the inlet GVF is 10%. The research results improve the prevention and mitigation ability of related accidents, and provide theoretical basis and technical support for subsequent optimization and improvement.

Characterization of the Endwall Flow in a Low-Pressure Turbine Cascade Perturbed by Periodically Incoming Wakes, Part 2: Unsteady Blade Surface Measurements Using Pressure-Sensitive Paint

Unsteady pressure-sensitive paint (i-PSP) measurements were performed at a sampling rate of 30 kHz to investigate the near-endwall blade suction surface flow inside a low-pressure turbine cascade operating at engine-relevant high-speed and low-Re conditions. The investigation focuses on the interaction of periodically incoming bar wakes at 500 Hz with the secondary flow and the blade suction surface. The results build on extensive PIV measurements presented in the first part of this two-part publication, which captured the ’negative-jet-effect’ of the wakes throughout the blade passage. The surface pressure distributions are combined with CFD to analyze the flow topology, such as the passage vortex separation line. By analyzing data from phase-locked PIV and PSP measurements, a wake-induced moving pressure gradient negative in space and positive in time is found, which is intensified in the secondary flow region by 33% with respect to midspan. Furthermore, two methods of frequency-filtering based on FFT and SPOD are compared and utilized to associate a pressure fluctuation peak around 678 Hz with separation bubble oscillation.

Characterization of the Endwall Flow in a Low-Pressure Turbine Cascade Perturbed by Periodically Incoming Wakes, Part 1: Flow Field Investigations with Phase-Locked Particle Image Velocimetry

Particle image velocimetry (PIV) measurements were performed inside a low-pressure turbine cascade operating at engine-relevant high-speed and low-Re conditions to investigate the near-endwall flow. Of particular research interest was the dominant periodic disturbance of the flow field by incoming wakes, which were generated by moving cylindrical bars at a frequency of 500 Hz. Two PIV setups were utilized to resolve both (1) a large blade-to-blade plane close to the endwall as well as midspan and (2) the wake effects in an axial flow field downstream of the blade passage. The measurements were performed using a phase-locked approach in order to align and compare the results with comprehensive CFD data that are also available for this test case. The experimental results not only support a better understanding and even a quantification of the wake-induced over/under-turning inside and downstream of the passage, they also enable the tracing of the ‘negative-jet-effect’, which is widely known in the CFD branch of the turbomachinery community but is seldom visualized in experiments. The results also reveal that the bar wake periodically widens the blade wake by up to 165%, while the secondary flow is less affected and exhibits a phase lag with respect to the 2D-flow effects. The results presented here are an essential basis for the subsequent investigation of the near-endwall blade suction surface effects using unsteady pressure-sensitive paint in the second part of this two-part publication.

Numerical investigation on vortex characteristics in a low-head Francis turbine operating of adjustable-speed at part load conditions

In this paper, the vortex characteristics in a low-head Francis turbine operating of adjustable-speed at part load conditions are numerically investigated. First, the rules of fixed-speed and adjustable-speed operations are introduced in detail, aiming at operating limits extension. Second, comparative analysis on vortex rope is conducted, with emphasis on vortex rope volume, tail water excitation and spontaneous power swing. Finally, inter-blade vortex has been analyzed and successfully demonstrated, and its induced high amplitude pressure fluctuation characteristic is explored. It is found that at deep part load, vortex rope volume and power swing coefficient of adjustable-speed operation decreased by 69 % and 61 % respectively. The amplitude of Rheingans frequency decreased by 17 %, while the amplitude of blade passing frequency increased by 83 %. After decreasing runner speed, inter-blade vortex is stranded at blade suction surface leading-edge. In addition to low-frequency disturbance, pressure fluctuation shows high-frequency wide-band characteristics. It is concluded that adjustable-speed operation has beneficial effects on improvement of cavity cavitation and energy stability. As for hydraulic stability, the Rotor-Stator Interaction effect cannot be ignored. After changing runner speed, the inter-blade vortex becomes more serious, which could provoke low-cycle and high-cycle fatigue of the blades.