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The effective control of the tip-leakage flow and loss is of great significance to improve the aerodynamic performance of the turbine. In this paper, the evolution mechanism of tip-leakage flow in a transonic high pressure turbine with a squealer tip is investigated with numerical simulation methods. The impacts of squealer geometric, such as the inclined pressure side rim and squealer rim width, on the vortex structure in the gap and tip-leakage loss are discussed. The results show that the scraping vortex inside the cavity plays the role of aero-labyrinth seal, and forms interlocking sealing labyrinth structure with the rims on both sides, which has an effective sealing effect on the tip-leakage flow. The inclined pressure side squealer rim inhibits the development of the pressure side squealer corner vortex, which is beneficial to expand the influence range of the scraping vortex and enhance the sealing effect on the tip-leakage flow. The increase of the suction side squealer rim width reduces the effective flow area at the gap exit, which is conducive to reduction of the tip-leakage flow rate and tip-leakage loss. However, the increase of the pressure side squealer rim width strengthens the pressure side squealer corner vortex and limits the development space of the scraping vortex, causing the adverse effects on the control of tip-leakage flow.

As an effective method to influence end wall flow field, non-axisymmetric profiled end wall can improve the aerodynamic performance of compressor cascades. For a highly loaded low pressure compressor cascade, called V103, the study found the optimal non-axisymmetric profiled end wall decreases total pressure loss coefficient by 4.57%, 5.48% and 3.04% under incidences of −3°, 0°, and 3°, respectively, compared with those of the planar end wall. The optimal non-axisymmetric profiled end wall changes the structure of secondary flow in hub region, generating a corner vortex near suction surface, inhibiting the development of the passage vortex towards suction surface and reducing flow separation. When the inlet Mach numbers are 0.62 and 0.72, the total pressure loss coefficient decreases by 3.19% and 4.58% for optimal non-axisymmetric profiled end wall compared with those of the planar end wall. Though optimal non-axisymmetric profiled end wall increases total pressure loss near hub region in blade passage under different inlet Mach numbers, the peak value and region of high loss coefficient above 10% span in blade passage significantly decrease. In addition, different incidences affect the secondary flow streamlines and vortex structure near the cascade hub region, however, different inlet Mach numbers hardly change the secondary flow streamlines and vortex structure. In short, the optimal non-axisymmetric profiled end wall shows better aerodynamic performance than the planar end wall for the highly loaded compressor cascade in multi-conditions.

This paper describes film cooling characteristics of the novel combined configuration employing interrupted slot and trench holes on a vane endwall. Interrupted slot, formed by uneven thermal expansion between combustor and high-pressure turbine vane, can improve adiabatic film cooling effectiveness of the leading edge and pressure side-endwall junction by inhibiting the development of horseshoes vortex. Holes embedded into a straight trench were introduced to improve film efficiency comparing to cylindrical holes on vane passage endwall. The influences of lateral pressure gradient, slot coolant and step on crossflow of the hole coolant in trench were mainly discussed. Three dimensional Reynolds-averaged Navier-Stokes equations with shear stress turbulence model (SST k-ω) were used to obtain the flowfields and adiabatic film cooling effectiveness of a cascade model. Four hole blowing ratios M=0.5, 1.0, 1.5, 2.0 and two axial positions X/Cax= −0.05, 0 were considered. The coolant crossflow in trench on flat endwall of cascade passage trends to flow towards suction side due to the lateral pressure gradient rather than both sides on flat plate. For combined configuration of interrupted slot and trench holes, the step vortex rolls up hole coolant upstream of trench which changes η distributions comparing to that on flat passage endwall. Comparing to the cylindrical holes, better film cooling performance can still be obtained when arranging trench holes under high blowing ratios. The influence of hole’s axial positions was also discussed.

The tip leakage flow in different axial gaps is numerically investigated based on the Reynolds-Averaged Navier-Stokes equations and k-ω SST turbulent model using commercial CFD software ANSYS Fluent. The characteristics of vortexes in the flow field of tip seal and the distribution law of rotor tip leakage loss are analyzed under the change of axial gap. The results show that the flow field in the blade tip seal undergoes complex changes with the increase of axial gap. There are two opposite vortices in the seal cavity with small axial gap, namely, the large vortex in the cavity and the small vortex near the wall of the shroud. The small vortex gradually disappears with the increase of the axial gap. When the axial gap is increased to a certain value, the reverse large vortex is formed in the cavity behind the low teeth of seal, and the leakage rate increases significantly; the steam flow angle and the mixing loss also show different changes. Based on the traditional calculation formula of tip leakage rate, a more accurate calculation formula of tip leakage rate is constructed by introducing equivalent labyrinth seal teeth to consider the variation of axial gap.

Heightened interests have been laid at the preliminary design and optimization of the centrifugal compressor for the fuel cell vehicle. The centrifugal compressor for fuel cell vehicle is driven by a high-speed motor; however, the limit of the motor speed makes the flow passage of the impeller long and narrow, which leads to a serious tip leakage loss. Serious tip leakage loss deteriorates the compressor performance. In this paper, 3-D numerical simulations were carried out with the aim of investigating the tip leakage loss in a prototype centrifugal compressor for a 100 kW fuel cell stack. The results revealed that the mixing loss caused by the interaction between the tip leakage vortex and the downstream tip leakage flow contributed to the major part of the tip leakage loss. The path of the tip leakage vortex almost followed the streamwise direction, while the downstream tip leakage flow exhibited strong circumferential momentum, which referred to the fact that they were nearly orthogonal. Therefore, a flow control approach, which was realized by enhancing the blade loading around the leading edge of blade tips in this paper, was proposed to decrease the interaction angle between the tip leakage vortex and the downstream tip leakage flow and then mitigate mixing loss by changing the flow direction of the tip leakage vortex. The results showed a smaller interaction angle was achieved in the optimized impeller compared with the baseline one. Meanwhile, the efficiency was also improved by 1.30% at design condition and the maximum efficiency improvement could be up to 10% at large mass flow condition of 92 000 r/min. Being manufactured and tested, the optimized compressor was proved to achieve an isentropic efficiency of 75.84% at design condition.

The aerodynamic forces and vortex characteristics of an H-type Vertical Axis Wind Turbine (VAWT) become complicated because of dynamic stall, particularly in the three-dimensional impact on the blade spanwise direction. This study focused on the evaluation of the aerodynamic performance and vortex characteristics of an H-type VAWT in the spanwise direction by numerical simulations and wind tunnel experiments. Pressure acting on the blade surface was obtained from multiport pressure measurement devices by wind tunnel. Meanwhile, the vortex field around different blade sections was investigated through numerical simulations. The stall behavior was analysed by comparing the results of numerical simulations and experiments. As a result, the tangential force of single blade was mainly contributed at the chordwise position of x/c≤0.4c and the power of single blade was mainly contributed at the azimuthal angle range of 60°≤θ≤150° in the blade section position region of 0≤z/(H/2)≤0.7. At the section position of z/(H/2)=0.5, the initial flow separation was found at the suction side and progressed forward to the leading edge. With the increase of the tip speed ratios, the decreasing position of the averaged local power coefficient of each section was closer to the middle section of z/(H/2)=0, and the attenuation speed became faster. The power coefficient reductions at the blade section position of z/(H/2)=0.9 were 38.29%, 46.78% and 56.42% when the tip speed ratios were 1.38, 2.19 and 2.58, respectively. The results of this study provided a better understanding of the development of the performance characteristics and vortex characteristics of H-type VAWT.

The turbulence kinetic energy and heat transfer performance of air in spirally fluted tube were numerically studied at a constant wall temperature with Reynolds number (Re) between 5000 and 45 000. Furthermore, the flow dynamics and heat transfer performance of spirally fluted tubes with five different geometric parameters as well as the effects of separation vortex and swirling wake flow on heat transfer and flow resistance were analyzed. According to the results, heat transfer is enhanced mainly because the fluid hit the windward side of the flute, thus generating a strong turbulence kinetic energy to further reconstruct the boundary layer. The second reason is that the formation of the recirculation zone between the flutes disturbs the boundary layer caused by the flow separation. With the increase of flute depth ratio (Ld/D), the separation vortex will become stronger and larger on the leeward side of flute. The separation vortex will break the boundary layer and improve the heat transfer capacity which is accompanied with the increase of fluid resistance. As the flute pitch length ratio (Lp/D) decreases, the spiral flow is strengthened, and meanwhile more wake flow is generated. The spiral flow causes little impact on enhancing heat transfer but inhibits the development of the separation vortex and fluid pulsation; in addition, the fluid resistance is reduced at the same time. The maximum value of the average Nusselt number appears when Re=5000, Ld/D=0.25 and Lp/D=1.00, which is 2.53 times the value of smooth tube. In view of the whole range of Reynolds number, the thermal performance enhancement factor indicates that Ld/D=0.15 and Lp/D=1.00 are the optimal geometric design parameters.

Centrifugal compressors for the fuel cell vehicles often operate near the surge line compared with the turbocharger compressors. Low solidity and half vaned diffusers are recognized as good ways to improve the stability of the centrifugal compressor. The presented work investigated four diffuser configurations (i.e., the vaneless diffuser (VLD), full-height low solidity vaned diffuser (LSVD), hub-side half vaned diffuser (HVD) and shroud-side half vaned diffuser (SVD)) through steady-state and unsteady numerical simulations. The results show that the best performance is achieved by the LSVD, HVD and SVD at the design, surge and choke conditions. The flow rate at the surge operating point of the HVD has decreased by 15.53% compared with the LSVD, and 9.21% compared with the VLD. At near surge operating point, a longitudinal suction side passage vortex is formed on the hub of the LSVD and rotates as circumferential stall cells. A hairpin vortex is formed along the leading edge and is dragged by the main flow along the suction side as a local vortex shedding. The mechanism of the stability improvement by half vaned diffusers is that the tip leakage vortex migrates from the clearance side to the vane mounting side and replenishes the low-momentum zone on the mounting side. The best position where the half vaned diffuser should be mounted is based on the impeller outlet flow conditions, namely, the location of the wake region, where the meridional velocity and relative stagnation pressure is low.

In order to avoid collision with the end wall of the casing when the guide vane rotates around its own rotating axis in a centrifugal compressor, a circular gap will be left at the top of guide vane. Part of the gas flow through the clearance cannot be effectively regulated, and the tip clearance leakage vortex is formed, which has an impact on the mainstream and makes the compressor performance decline. In order to solve this problem, in this paper, three improved schemes of guide vane channel structure, including rectangular channel, cambered channel, and globular channel, were put forward, which can effectively improve the regulation ability of guide vane. Comparing these three improved channels with the original one, the stable operating range of the compressor was enlarged; the power consumption of the compressor was reduced and the efficiency of the compressor was improved. By comprehensive comparison, the cambered channel has better performance and is easier to process, which has potential to be widely used in practice.

To analyze the internal flow characteristics of each subchannel in a low-specific-speed centrifugal pump with splitter blades, the time histories and frequency spectra of pressure fluctuations and the distributions of the corresponding flow states in one impeller channel were investigated through the numerical analysis. Performance experiments and particle image velocimetry (PIV) tests were carried out to verify the results of the numerical calculations. The results suggested that the simulation analysis agreed well with the test results. The time histories and frequency spectra of pressure fluctuations depending on its location (close to or away from the volute tongue) present different changes. The predominant frequency of each monitor point equals to five or ten times shaft frequency. The jet-wake flow pattern at each subchannel separated by splitter blade in one impeller channel is not circumferentially uniform. For the channel away from volute tongue, the magnitude of turbulence kinetic energy in pressure side subchannel is well larger than that in suction side subchannel. With the increase in flow rate, the region close to the elbow of the volute outlet emerges a large-scale vortex.

In modern high-load high-pressure turbine, the secondary flow in the blade channel is very strong and occupies a large spanwise region. Although high-quality experimental data at the stage interfaces have been obtained in previous research, the influence of the clocking position on the secondary flow patterns is not fully understood. This paper investigates the clocking effect in a 1.5-stage high-pressure turbine and focuses on the variations of secondary flow patterns and their effect on the turbine performance. The detailed flow fields of various clocking positions were obtained by carrying out unsteady flow simulations using an in-house code. Among the four clocking positions in this work, the highest entropy generation was observed when the wakes from stator 1 hit the leading edges of stator 2, which is opposite to the well-known conclusion for the turbine with high-aspect-ratio blades. Detailed flow analysis showed that the wakes and the near tip secondary vortices from stator 1 showed different traces when entering the stator 2 channel and the secondary vortices clearly have a more important influence in determining the performance. The different behaviors of the secondary vortices explained the performance variations due to the clocking effect.

In this paper, the rotating instability (RI) in an axial compressor has been investigated numerically in order to examine the capability of URANS method to simulate its typical characteristics of RI broadband humps with multi-peak frequencies (MPFs) and further to uncover the underlying flow mechanism. A full-annulus modeling solution has been adopted to fully capture the wide range of different length-scale flow disturbances that circumferentially propagating around the compressor rotor. During the transient computing process, long-term monitoring up to 50 revolutions has been carried out to achieve a fine frequency resolution, and that would be essential for resolving the MPFs with small frequency interval. It is shown that the MPFs feature of RI has been successfully captured by the full-annulus URANS approach, and also its frequency range and spectral feature agree well with the experimental results. Further, with a circumferential mode decomposition on the MPFs characteristics of RI, it has been found that the MPFs result from the interaction between long- and short-scale flow disturbances which circumferentially propagate around the compressor rotor near the clearance region. Detailed examination on the numerical three dimensional flow field indicates that the short-scale disturbance is induced by the unsteady oscillation of tip clearance vortexes with inter-passage phase delay. The long-scale disturbance is caused by the mistuning of the wave number of the short-scale disturbance and the blade number within the whole annulus.

In order to understand and improve the oil and gas mixing performance of a dual-channel vortex chamber diesel engine, the BH175F dual-channel vortex chamber combustion system was used as the research foundation, and the oil-gas mixture process of the combustion system was numerically analyzed. By analyzing the cylinder temperature, cylinder pressure, mixing process and combustion process of the combustion system, the mixture performance of the combustion system was studied. Results indicated that: The mixture of the compression Top Dead Center (TDC) started to enter the main combustion chamber through the start-up hole; when the piston reached 4° After Top Dead Center (ATDC), the mixture started to enter the main combustion chamber through the connecting channels A and B, and the high-concentration mixture entered the main combustion from the start-up hole; when the piston continued running down to 20° and 25° ATDC, it could be seen that the main combustion chamber mixture was already relatively uniform; besides, when the equivalence ratio was between 0.8 and 1, the air-fuel mixture was unevenly distributed in the main combustion chamber. It provides guidance for further improvement of the combustion system.

Direct numerical simulation (DNS) of shock wave/turbulent boundary layer interaction (SWTBLI) with pulsed arc discharge is carried out in this paper. The subject in the study is a Ma=2.9 compression flow over a 24-degree ramp. The numerical approaches were validated by the experimental results in the same flow conditions. The heat source model was added to the Navier-Stokes equation to serve as the energy deposition of the pulsed arc discharge. Four streamwise locations are selected to apply energy deposition. The effect of the pulsed arc discharge on the ramp-induced flow separation has been studied in depth. The DNS results demonstrate the incentive locations play a dominant role in suppressing the separated flow. Results show that pulsed heating is characterized by a thermal blockage, which leads to streamwise deflection. The incentive locations upstream the interaction zone of the base flow have a better control effect. The separation bubble shape shows as “spikes”, and the downstream flow of the heated region is accelerated due to the momentum exchange between the upper boundary layer and the bottom boundary layer. The high-speed upper fluid is transferred to the bottom, and thus enhances its ability to resist the flow separation. More stripe vortex structures are also generated at the edge of the flat-plate. Furthermore, the turbulent kinetic disturbance energy is increased in the flow filed. The disturbances that originate from the pulsed heating are capable of increasing the turbulent intensity and then diminishing the trend of flow separation.

In the current study, thermal boundary conditions are considered in a rotating smooth channel with a square cross-section to investigate the secondary flow and compare it to that of the same channel without heating. The measurement is conducted at three streamwise planes (X=445 mm, 525 mm, 605 mm). The flow parameters are the Reynolds number (Re=4750, which was based on the average longitudinal or primary velocity U and the hydraulic diameter D of the channel cross-section), the rotation number (Ro=ΩD/U, where Ω is the rotational velocity, ranging from 0 to 0.26), and the aspect ratio of the channel cross-section (AR=1, which is calculated by dividing the channel height by the channel width). The leading and trailing walls are heated under a constant heat flux qw=380 W/m2, and the top and bottom walls are isothermal at room temperature. This work is in a series with our previous work without thermal boundary conditions. Based on the experimental data, we obtained a four-vortex regime. There is a counter-rotating vortex pair near the leading side and the trailing side. Because the leading and trailing walls are heated, the buoyancy force increases the relative vertical position of the vortex pair near the trailing side from 5% to 12.5% of the hydraulic diameter. When moving upstream along the streamwise direction, the upper vortex near the trailing wall becomes weaker, whereas the lower vortex becomes stronger. As the rotational speed increases, the vortex pair near the trailing side is inhibited by the Coriolis force. Under heated thermal boundary conditions, the vortex pair near the trailing side reappears due to the effect of buoyancy force. These results indicate that the buoyancy force has a substantial effect on the secondary flow regime under thermal boundary conditions.

At present, off-design centrifugal compressor suffers from great instabilities, such as rotating stall and surge, which lead to work failure, strong pressure fluctuations, and violent vibrations of the entire system. One cause of these instabilities is the rotating stall occurring in vaneless diffuser. In this review, recent researches on rotating stall of vaneless diffuser in centrifugal compressor are reviewed, and its inductive mechanism and characteristic description are emphatically introduced. In vaneless diffuser, the rotating stall starts up and fully develops under non-design conditions with the characteristic of increasing pressure fluctuation amplitude. The negative effects of rotating stall include reverse flow and vortex, both of which have significant effects on the instability of centrifugal compressors. Some improvement methods of rotating stall are also introduced, and a lot of examples are given. Finally, some suggestions and discussions are put forward for the future work.

The combustion and emission formation process of liquid fuel in the trapped vortex combustor (TVC) are very complicated. A trapped vortex combustor with replaceable bluff-bodies was designed to investigate these processes. The bluff-body widths varied from 0.021 m to 0.036 m. Experimental tests were carried out. Liquid RP-3 aviation kerosene was used in the tests. Emissions were measured under atmospheric pressure. The combustion process was analyzed theoretically in the viewpoints of relative evaporation time, mixing time and reaction time. Numerical simulations were also conducted to help analyze the formation and depletion processes of pollutants in TVC. The results reveal that atomization was a critical factor for formation and depletion processes of pollutants. By controlling mixing speed of burned and unburned gases and thus fuel-air uniformity, turbulence intensity could also affect emission levels. In addition, residence time also affected the emissions by affecting combustion completeness and the time for NOx formation. All these factors were combined in a complicated way to affect combustion process and pollutant emissions.

In this paper, the flow field characteristics of a double-swirl low-emission combustor were analyzed by using Particle Imaging Velocimetry (PIV) technology in an optical three-sector combustor test rig. The interactions between sectors and the flow field structure were explained. The results illustrated that there was a big difference between the flow field structures of the middle sector and the side sector under the same pressure drop, which was mainly induced by the interactions between sectors. The interactions made the swirl intensity of the middle sector weaker than that of the side sector, which made the recirculation zone of the middle sector be smaller than that of the side sector. With the increase of swirler pressure drop, the jet velocity at the exit of the swirler, the jet expansion angle, the width of the recirculation zone and the recirculating speed of the central axis became larger, enhancing the interactions between air streams from middle sector and side sector. The flow velocity in the central plane between sectors was small, especially the radial velocity, mainly because of the loss of the swirl intensity by the interactions between flow field of adjacent sectors. The expansion angle determined the position of the vortex in the primary recirculation zone; the axial and radial position of the vortex move downstream and radial outward with the increase of the jet expansion angle. The results of the mechanism of flow field organization in this study can be used to support the design of new low-emission combustor.

In this paper, a computational investigation of circumferential groove casing treatment in a highly-loaded low-reaction transonic compressor rotor is conducted, in which the stage reaction is significantly reduced due to a larger meridional contraction with respect to conventional transonic compressors. Steady computation at near-stall point is performed first to capture the stall inception of the rotor with smooth casing. Detailed observations, which mainly focus on the tip leakage flow behavior, obstruction and vortical structures in the tip region, determine the reason for the compressor stall. There is tip leakage vortex breakdown in the tip region. Moreover, it yields passage obstruction, and finally leads to the compressor stall. Then, attempts are made to investigate how the circumferential grooves can be applied for the compressor’s stall margin enhancement without compromising efficiency. Three configurations are obtained and analyzed by changing axial position and the number of the circumferential grooves. The results of computational parametric study indicate the optimal location of the groove is near the leading edge and the downstream grooves combine their influence on the compressor’s stabilization and performance in a cumulative manner. The optimal circumferential groove configuration produces an increase of 1% in total pressure ratio at the near-stall point and a gain of 3.7% in stall margin, without any penalty in efficiency. Furthermore, the impact the grooves will exert on the flow mechanisms between the grooves and the main flow is also considered.

The present study investigated the spectrum characteristics of unsteady disturbance and the tip leakage vortex evolution during pre-stall process for a contra-rotating axial compressor (CRAC). Transient numerical simulation was carried out in a single passage of the CRAC. The original transient fluctuation and oscillation of the tip leakage vortex structure with varying flow capacity of the CRAC were revealed using circle-like pattern figure and phase-locked root mean square (PLRMS). Additionally, the tip leakage flow in terms of vortex structure evolution was visualized for the sake of revealing the flow mechanism during pre-stall process. Results show that the unsteady fluctuation first appears at φ=0.3622, and the fluctuation frequency is 2.86 BPF. Unsteady disturbance source is mainly located at the tip side of the downstream rotor leading edge. From the choking point to the near stall condition, tip leakage vortex is always found in the tip leading edge of the upstream rotor. In addition, the tip leakage vortex of upstream rotor remains in the same place over time, i.e., no fluctuation, even when the downstream rotor entered into stall state. Such a phenomenon indicates that the stall point of the contra-rotating compressor is determined by the downstream rotor. Moreover, the maximum fluctuation position is mainly concentrated on the interface between the mainstream and the tip leakage vortex of the downstream rotor. By throttling the compressor, the angle between the main leakage vortex and the circumferential direction decreases gradually. When the main leakage vortex touches and continuously impacts on the leading edge of the adjacent blade, the unsteady disturbance, which is different from that of BPF, appears firstly.

This paper presents a numerical study on the aerodynamics loss reduction characteristics after the leading-edge (LE) optimization in a low-pressure turbine linear cascade. The LE was optimized with a simple and practical method of “Class Function/Shape Function Transformation Technique” (CST). The simulation conditions, covering the whole working range, were independently determined by incidence, Reynolds number and Mach number. Quantitative loss analyses were carried out with a loss breakdown method based on volumetric integration of entropy production rates. To understand the reason of loss reduction, the local sources at different operating points were identified with entropy production rates. The results showed that LE optimization with the CST method played a positive role in decreasing the total losses, and the working range with lower loss was extended. The profile loss and the endwall loss were significantly reduced by the LE optimization, which were also verified to be the major causes of the total loss reduction by loss breakdown. The decrease of profile loss can be attributed to the boundary layer near the LE region and the boundary layer of downstream at off-design incidence. The reduction mostly came from the pressure side at negative incidence, while came from the suction side at the positive incidence. The endwall loss was decreased markedly about 2.5%–5% by the LE optimization at the incidence of‒12°, which was 1% at the incidence of 12°. The mechanism for the endwall loss reduction at different incidences was different from each other. At negative incidence, the LE optimization diminished the corner separation vortex on the pressure side. While at positive incidence, the benefits came from three aspects, i.e., reduced suction LE separation bubbles close to the endwall, reduced passage vortex strength, and weakened shear process between passage vortex and trailing shed vortex. The loss of the downstream zone was relatively lower than that of the profile losses and the endwall losses. The effect of LE optimization on the loss of the downstream zone at different conditions was complex and it depended both on the profile boundary layer behavior at the suction trailing edge and on the passage vortex strength.

In order to reveal the regularity of unsteady flow of centrifugal pump under different cavitation stages, a visual closed test-bed is built to collect signals such as the distribution of cavitation bubbles at the impeller inlet and external characteristics, etc. in the process of cavitation of centrifugal pumps. Combined with the shape and distribution of bubbles captured by high-speed photography, the cavitation stage of the centrifugal pump is divided. In addition, the variation of vorticity distribution, pressure pulsation and radial force of centrifugal pump under different cavitation stages are studied using the standard k-e turbulence model and the Kunz cavitation model. Main contributions are as follows: The cavitation bubbles can absorb the energy of vortex core to a certain extent and increase the volume of vortex core. Cavitation bubbles can also block the flow-path and induce the distortion of the internal flow field, resulting in unstable pressure waves that cause a significant increase in pressure pulsation rate. Besides, with the development of cavitation, the radial force on the impeller tends to remain invariable first and then decrease, and trajectory of the radial force changes from closed to open.

Curved blade and end-bend blade are considered as effective passive control methods for improving the aerodynamic performance of a compressor cascade, and their applicability also needs to be further studied. A highly-loaded compressor linear cascade in the design condition was taken as the research objective in this paper. Numerical simulation was used to study the positive and negative effects of these methods on the flow loss of the cascade by redistributing the vortex structures. Results show that: the curved blade could reduce the flow loss to 2.55%, while the end-bend blade and the end-bend+curved blade increase it to 10.58% and 2.19%, respectively. The positive effect of the curved blade weakens the strength and scale of Concentrated Shedding Vortex (CSV), which accounts for 59.2% of the total flow loss. The negative effect of the end-bend blade exacerbates the low-energy fluid clusters from the end wall and wake into CSV. Finally, the end-bend+curved blade can take their own strengths, but it enhances the twisting motion of Passage Vortex (PV) and CSV, which makes it fail to reduce the flow loss. Therefore, the positive and negative effects of these methods on the flow loss of a compressor cascade are much clearer. In addition, we also predict the potential danger of the end-bend blade in the negative incidence conditions and provides some suggestion for future study.

The complex structure at trailing edge reduces the manufacturing precision, which results in an error in the size of the trailing edge structure. In this study, the performance of a stage high-pressure turbine (HP turbine) is calculated out in three dimensions. In the HP turbine guide vane, the trailing edge cutback configuration is adopted. Through three-dimensional simulation, the complex flow around the trailing edge with cutback cooling configuration is presented in this study, and the manufacturing precision reduction due to the complex structure at trailing edge is considered. Furthermore, the effect of trailing edge lip thickness and deflection of the stator on the turbine performance is discussed. Overall, as the press-side lip thickness increasing, the turbine efficiency and turbine inlet flow are reduced. However, the changes in the turbine work output are relatively complex. On the other hand, as the spacing between suction-side lip and press-side lip increases, turbine performance becomes worse. Both of the turbine efficiency and the turbine work output become smaller, while the turbine inlet flow becomes bigger. The effect of the spacing between suction-side lip and press-side lip is obviously greater than that of the press-side lip thickness. The change of the press-side lip thickness has little effect on the relation between the turbine performance and the spacing between suction-side lip and press-side lip. However, when the spacing between suction-side lip and press-side lip deviates from the baseline value, the effect law of the press-side lip thickness on the turbine performance will be affected. As the press-side lip thickness increases, it leads to an increase in the low-velocity zone at both of the pressure-side and suction-side trailing edge. And more main stream is affected or mixed into the wake flow. When the spacing between suction-side lip and press-side lip becomes smaller, the low-velocity zone at the trailing edge is smaller, and the change of vortex with the press-side lip thickness is affected. With a bigger spacing between suction-side lip and press-side lip, the variation is contrary.

In order to further understand the aerothermodynamic performance and flow loss mechanism of SCO2 turbines, RANS equations and an SST Turbulence Model were chosen for a numerical study on the secondary flow and vortex structure of cascades using the commercial software CFX. The dimensionless vorticity analysis method was used to study the flow characteristics of the logarithmic layer and viscous sublayer in high pressure guide vane cascades. The new vortex structure and formation mechanism of the vortices were given and analyzed. Simulation results indicated that during the motion of the boundary layer in the cascades, the logarithmic layer and viscous sublayer obtain the different rotational direction vorticity, respectively. The endwall logarithmic layer and pressure side leg of the horseshoe votex gradually develop into the passage vortex, with the endwall viscous sublayer gradually developing into the corner viscous sublayer vortex II and the endwall viscous sublayer vortex I. The endwall viscous sublayer that rolled by the passage vortex is encountered with the upstream-side and radial boundary layer of the vane at the suction separation line, forming the suction separation line vortex beside the passage vortex. A pair of radial transition vortices are formed between the wake and the main stream.

In nuclear reactor fuel assemblies, spacer grids are installed among the rod bundles to support the fuel rods and affect the flow field between rods. Mixing vanes, as a swirling device, are set on the upper apex of the spacer grid. Vortexes produced by mixing vanes move along the axial direction in subchannels and enhance the forced convection heat transfer between the rods and cooling-fluid medium. In this paper, a numerical simulation method was used to investigate vortex motion produced by typical AFA-3G spacer grids in a 5×5-rod bundle by Star-CCM+ software. The shear-stress transport k-ω model was used to simulate turbulence phenomena. A dimensionless parameter, Se, based on the absolute vorticity flux, was reported to specify the intensity of secondary flow. Its physical meaning is the ratio of inertial force to viscous force induced by secondary flow. The results are helpful to take advantage of spacer grids in a much more effective way in pressurized water reactors.

Flow measurement is important in the fluid process and transmission system. For the need of accuracy measurement of fluid, stable flow is acquired. However, the elbows and devices as valves and rotary machines may produce swirling flow in the natural gas pipeline networks system and many other industry fields. In order to reveal the influence of upstream swirling flow on internal flow fields and the metrological characteristics, numerical simulations are carried out on the swirl meter. Using RNG k-e turbulent model and SIMPLE algorithm, the flow field is numerically simulated under swirling flows generated from co-swirl and counter-swirl flow. Simulation results show fluctuation is enhanced or weakened depending on the rotating direction of swirling flow. A counter- swirl flow increases the entropy production rate at the inlet and outlet of the swirler, the junction region between throat and divergent section, and then the pressure loss is increased. The vortex precession dominates the static pressure distributions on the solid walls and in the channel, especially at the end region of the throat.

In this paper, the feasibility of controlling the subsonic jet flow and its noise using pores of blind holes added on the nozzle inner wall is explored numerically. These pores are intended to introduce disturbances to the shear layer so as to change the flow mixing. This passive strategy has not been attempted so far. A convergent nozzle with a cylindrical extension is selected as the baseline case. Three nozzles with pores on the inner wall are set up. Validations of the numerical settings are carried out, then the compressible turbulent jets at the exit Mach number M j = 0.6 in the four nozzles are calculated by large eddy simulations (LES), while the radiated sounds are predicted by the FW-H acoustic analogy. The results show that the blind holes have produced some effects on weakening the turbulence intensity in the shear layer. Comparison reveals that both temporal and spatial correlations of the turbulent fluctuations in the modified cases are suppressed to some extent. Meanwhile, the porous nozzles are shown to suppress the pairing of vortices and enhance the flow mixing, and therefore, the development of shear layer and the fragmentation of large scale vortices are accelerated.

Pressure fluctuation around the tongue has large effect on the stable operation of a centrifugal pump. In this paper, the Reynolds averaged Navier-Stokes equations (RANS) and the RNG k-epsilon turbulence model is employed to simulate the flow in a pump. The flow field in the centrifugal pump is computed for a range of flow rate. The simulation results have been compared with the experimental data and good agreement has been achieved. In order to study the interaction of the tongue with the impeller, fifteen monitor probes are evenly distributed circumferentially at three radii around the tongue. Pressure distribution is investigated at various blade positions while the blade approaches to and leaves the tongue region. Results show that pressure signal fluctuates largely around the tongue, and it is more intense near the tongue surface. At design condition, standard deviation of pressure fluctuation is the minimum. At large flow rate, the increased low pressure region at the blade trailing edge results in the increases of pressure fluctuation amplitude and pressure spectra at the monitor probes. Minimum pressure is obtained when the blade is facing to the tongue. It is found that the amplitude of pressure fluctuation strongly depends on the blade positions at large flow rate, and pressure fluctuation is caused by the relative movement between blades and tongue. At small flow rate, the rule of pressure fluctuation is mainly depending on the structure of vortex flow at blade passage exit besides the influence from the relative position between the blade and the tongue.

Free surface vortex control is vital in a pump sump system because the air absorbed by free surface vortex induces noise, vibration, and cavitation corrosion on the pumping system. In this study, the change of free surface vortex and air absorption in a pump intake has been investigated by the Volume of Fraction (VOF) method with steady multiphase flow model in order to represent the behavior of the free surface vortex exactly. The homogeneous free surface model is used to apply interactions of air and water. The results show that air intake by the free surface vortex motion can be visualized using the iso-surface of air volume fraction. The vortices make an air column from the free surface to the pump intake. Also, it was found that the free surface vortex can be controlled by installing curtain walls.

With the aid of numerical method, both flow field and its accompanied loss mechanism within the rotating cavity are investigated in detail in the 1st part of the two parts paper. For ease of comparison, rotating cavity is further classified as the rotor-stator cavity case and the rotor-rotor cavity case. Results indicate that flow within both kinds of the cavity act as the inviscid flow except that the flow near walls, neighboring the lower G region and in the vicinity of the rotating orifices. In the regions except such inviscid-flow-dominate domains, the theoretical core rotation factor can be safely used to predict the swirl ratio within the cavity. When detailed flow pattern is considered, Ekman-type flow exists near periphery of the surface’s boundary layer where viscous effect is non-negligible. However, due to the complex profile of the simulated cavity case, vortices structure is varied within the cavity. By comparison, swirl ratio can be used to predict the magnitude of loss. Due to the relatively evident rotating effects of the rotor-rotor cavity, swirl ratio even increases to 1.4 in the current model, which means that flow is moving faster than the surrounding disc. Further investigation finds that this kind of highly rotating flow is accompanied with serious undesirable pressure loss. Parenthetically, unlike its counterpart, swirl ratio above 1.0 doesn’t happen when fluid passes through the rotor-stator cavity. So it is suggested that rotor-rotor flow cavity with the superimposed inward throughflow should be avoided in the engine design or certain measurements should be provided when such structure design is unavoidable. Simulation done in the current paper is meaningful since these dimensional parameters are typical in the design of state-of-art. Relatively lower range of Re φ and C w is not considered in the current two parts paper.

To explore the effects of airfoil-probe tubes and its installment position on the flow field of the compressor cascade, and find out the mechanism that how the airfoil-probes affect the aerodynamic characteristics of the compressor cascade, this paper performed both numerical and experimental works on the same compressor cascade. The experiment mainly focused on the cases of low Mach number (Ma = 0.1), and cases with different Mach numbers (0.1, 0.3, 0.7) and different incidence angles (-5, 0, 5) are investigated by the numerical method. The case without the airfoil-probe tube was referenced as the baseline, and other three cases with the airfoil-probe tubes installed in different chordwise positions (30%, 50%, 70% of the chord length) were studied. The diameter of the airfoil-probe tube is 3mm, which is configured as 300% amplification of some particular airfoil-probe according to the geometrical similarity principle. The results show that the airfoil-probe tubes have a negative influence on the flow capacity of the cascade at all investigation points. The separations and the large scale streamwise vortices that induced by the airfoil-probe tube on the pressure side cause most the losses at the high Mach number. The influence of the airfoil-probe tube on the flow field in the vicinity of the pressure side surface is local separation at the low Mach number. The airfoil-probe tubes also have a clearly effect on the leakage flow. It decreases the mass flow of the leakage flow and weakens the intensity of the leakage vortex, but enlarges the influence area. The total pressure loss of the case that the tube is installed at the half chordwise position is generally lower than other cases especially at the high Mach number, it can even decrease the losses compared with the basic case.

When two cavitation bubbles exist in a confined space, the interaction between the bubbles significantly affects the characteristics of bubble dynamic behaviors. In this paper, a three-dimensional (3D) model is established to study the growth and collapse of two cavitation bubbles in a heated tube and its effects on heat transfer. The liquid and gas phases throughout the calculation domain are solved by a set of Navier-Stokes equations. It is assumed that the gas inside the bubble is compressible vapor, and the surrounding liquid is incompressible water. The mass transfer between two phases is ignored. The calculated bubble profiles were compared to the available experimental data, and a good agreement has been achieved. Then, the relationship among the bubble motion, flow field and pressure distributions was analyzed. On this basis, the effects of bubble interaction on the heat transfer between the wall surface and sounding liquid were discussed. It is found that heat transfer in the centre wall region is enhanced owing to the vortex flow and micro-jet induced by the bubble contraction and collapse. In contrast, the highest surface temperature appears in the surrounding region, which is mainly attributed to the thermal resistance induced by the bubble. The present study is helpful to understand the heat transfer phenomenon with cavitation in the liquid.

This paper presents experimental measurements of the flow field in a Low-speed Turbine Cascade using a stereoscopic particle-image velocimetry (SPIV). During the measurements, a pair of frame-straddling-based CCD cameras were configured at different sides of the laser light sheet, and appropriate tracing particles (DEHS) were employed. The measurements were conducted at the incidence angle of 0 degree and exit Reynolds number of 1.7 × 105 with the tip clearance 1.18% of blade chord. The tip flow features, such as the evolution and breakdown of tip leakage vortex, the horseshoe vortex, turbulence characteristics of tip leakage flow, were studied for the flow field analysis. The results showed that the tip leakage flow/vortex mainly dominate flow fields in the tip region. The tip leakage vortex performs as a concentrated vortex before its breaking down and splitting into small vortices. The highest turbulence intensity mainly occurs in the tip region along with the trajectory of tip leakage vortex, and when the vortex breaks down, the turbulence intensity reduces rapidly. Additionally, the SPIV with this configuration also shows an advantage in investigating the flow structures and mechanism inside the turbine cascade.

The main source of the noise of an axial flow fan is the fluctuating pressure field on blade surfaces caused by the shedding of vortices at the trailing edge of blades. An analytical model to predict the vortex shedding noise generated at the trailing edge of blades of axial flow fans was proposed by Lee in 1993. In this model, for mathematical convenience, an idealized vortex street is considered. However, the agreement between the analytical results and the experimental data needs to be improved because of the simplification about the Karman vortex street in the wake of blade. In the present study, a modified model is proposed based on the prediction model by Lee. The boundary layer theory is used to analyze and calculate the boundary layer development on both the pressure and the suction sides of blades. Considering the effect of boundary layer separation on the location of noise source, the predicted overall sound pressure level compares favorably with the experimental data of an axial fan. In the calculation of A-weighted sound pressure level (LA), considering the effect of static pressure on radiate energy, the predicted broadband noise with the modified model compares favorably with the experimental data of a multiblade centrifugal fan.

The characteristics of a rotating stall of an impeller and diffuser and the evolution of a vortex generated at the diffuser leading-edge (i.e., the leading-edge vortex (LEV)) in a centrifugal compressor were investigated by experiments and numerical analysis. The results of the experiments revealed that both the impeller and diffuser rotating stalls occurred at 55 and 25 Hz during off-design flow operation. For both, stall cells existed only on the shroud side of the flow passages, which is very close to the source location of the LEV. According to the CFD results, the LEV is made up of multiple vortices. The LEV is a combination of a separated vortex near the leading- edge and a longitudinal vortex generated by the extended tip-leakage flow from the impeller. Therefore, the LEV is generated by the accumulation of vorticity caused by the velocity gradient of the impeller discharge flow. In partial-flow operation, the spanwise extent and the position of the LEV origin are temporarily transmuted. The LEV develops with a drop in the velocity in the diffuser passage and forms a significant blockage within the diffuser passage. Therefore, the LEV may be regarded as being one of the causes of a diffuser stall in a centrifugal compressor.

In the present work the aerodynamic performances of an innovative rotor blade row have been experimentally investigated. Measurements have been carried out in a large scale low speed single stage cold flow facility at a Reynolds number typical of aeroengine cruise, under nominal and off-design conditions. The time-mean blade aerodynamic loadings have been measured at three radial positions along the blade height through a pressure transducer installed inside the hollow shaft, by delivering the signal to the stationary frame with a slip ring. The time mean aerodynamic flow fields upstream and downstream of the rotor have been measured by means of a five-hole probe to investigate the losses associated with the rotor. The investigations in the single stage research turbine allow the reproduction of both wake-boundary layer interaction as well as vortex-vortex interaction. The detail of the present results clearly highlights the strong dissipative effects induced by the blade tip vortex and by the momentum defect as well as the turbulence production, which is generated during the migration of the stator wake in the rotor passage. Phase-locked hot-wire investigations have been also performed to analyze the time-varying flow during the wake passing period. In particular the interaction between stator and rotor structures has been investigated also under off-design conditions to further explain the mechanisms contributing to the loss generation for the different conditions.

In order to control the maximum tangential velocity Vθm(m/s) of the turbulent rotational air flow and the collection efficiency ηc (%) using the fly ash of the mean diameter XR50=5.57 µm, two secondary jet nozzles were installed to the body of the axial flow cyclone dust collector with the body diameter D1=99mm. Then in order to estimate Vθm (m/s), the conservation theory of the angular momentum flux with Ogawa combined vortex model was applied. The comparisons of the estimated results of Vθm(m/s) with the measured results by the cylindrical Pitot-tube were shown in good agreement. And also the estimated collection efficiencies ηcth (%) basing upon the cut-size Xc (µm) which was calculated by using the estimated Vθ m(m/s) and also the particle size distribution R(Xp) were shown a little higher values than the experimental results due to the re-entrainment of the collected dust. The best method for adjustment of ηc (%) related to the contribution of the secondary jet flow is principally to apply the centrifugal effect Φc (1). Above stated results are described in detail.