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The increase of turbine blade loading by the increase of blade turning angle is an effective method to improve the efficiency of gas turbines. However, it intensifies the secondary flows, such as passage vortex and leakage vortex. The squealer tip, which has a cavity on the blade tip surface, is one of the techniques to reduce the leakage flow. In this study, the performance tests were performed for the ultra-highly loaded turbine cascade (UHLTC) with three kinds of squealer tip, which were different in the cavity depth, by using the small sized wind tunnel test rig for an annular cascade. The test results showed that the application of the squealer tip with minimum cavity depth contributed to the enhancement of aerodynamic performance of the UHLTC. It increased the torque but had almost no effects on the efficiency due to the increase of the loss in the high flow coefficient region. On the other hand, its application improved the efficiency in the low flow coefficient region because of the reduction of the loss as well as the increase of the torque.

The ultra-high bypass ratio geared turbofan is a key concept to reduce the environmental impact of the next generation aircrafts. The increase in rotational speed of the low-pressure turbine (LPT) enabled by the geared architecture offers potential benefits in efficiency and considerable savings in engine weight, overall dimension, and cost. High-speed LPTs operate at transonic exit Mach numbers and low Reynolds numbers. For this combination of operating conditions, where compressibility influences the blade aerodynamics and loss mechanisms, there is a critical lack of openly available detailed experimental data concerning the combined effects of unsteady wakes, purge streams, leakages, and secondary flows for CFD models validation. The current research project focuses on the full characterization of an experimental test case concerning the impact of unsteady wakes and purge flows on the blade aerodynamics and loss mechanisms in a high-speed LPT cascade tested at engine-scaled conditions. The extensive experimental dataset is collected in an open-access database, providing novel validation material for comparison with numerical computations. In this two-part paper, a comprehensive study on the design, commissioning and testing of a transonic low-pressure turbine blade at on and off-design is presented. Part I describes the design of components and accurate instrumentation required to investigate steady quasi-3D flows in a linear cascade environment at low-Reynolds and transonic exit Mach numbers. A novel concept to perform blade radial traverses to obtain spanwise distributions of high-resolution blade measurements is presented. The repeatability and periodicity of the rig flow conditions are demonstrated. This work critically evaluates the methodology used for the design and adaptation of the existing rig for testing of unsteady three-dimensional flows in linear cascade experiments.

Axial-flow turbomachinery is widely used in low head water transfer and electricity generation projects. As there is a gap between the impeller and casing of the tubular flow unit, the fluid will cross the gap to form tip leakage flow, which may induce intense pressure pulsation, noise and mechanical vibration, and even threaten the safe operation of the unit. In order to ensure the efficient and stable operation of hydropower units, the influence factors of tip clearance flow and its formation and development mechanism have been deeply studied in this paper. In this paper, the impact of gap width, angle of attack and inlet velocity on tip leakage flow of hydrofoil with clearance are studied by orthogonal experiment method. The results suggest that the gap width has the greatest influence on tip clearance flow, the incidence angle takes the second place, and the inlet velocity has the least effect on tip clearance flow. Then the fractal characteristics of tip leakage vortices with different gap widths are studied. The results demonstrate that the fractal dimension of tip leakage vortices in large gaps was significantly larger than that in small gaps; The fractal dimension of the leakage vortex decreases gradually along the flow direction.

In this article, a numerical investigation of the effect of a miniaturized five-hole probe downstream of a transonic low-pressure turbine vane row is presented. First, a numerical calibration of the probe was performed in uniform flow conditions, as is the case for any traditional calibration, for a wide range of Mach number, yaw angle, and pitch angle conditions. The effect of the probe on the general flow field throughout the turbine vane segments was then evaluated by performing a comparison between a setup with vanes only (no probe) and with vanes and probes. It was found that, as the probe traverses downstream the vane, the probe impact on the vane isentropic Mach number depends on the probe circumferential position. The highest impact was observed when the probe is located at the upper mid-passage (θ = 0.5), consisting of a relatively small reduction of the isentropic Mach number on the vane suction side of just 0.02. To assess the accuracy of the quantities “measured” by the probe, the probe-determined flow field was compared to the flow field of the vanes-only setup. A nonnegligible modification of the probe-determined local distributions of Mach number, yaw angle, and pitch angle is revealed with respect to the undisturbed flow. Further investigation involving stagnation point tracking showed that the artificial high circumferential variation of the yaw angle is not caused by a modification of the vane outlet flow angle, but is induced by nonuniform flow conditions downstream of the vanes. With knowledge of the above, a two-step correction is used to account for the effects of the nonuniformity of the flow, and its impact is evaluated on 2D and 3D flow regions. A significative effect of the correction was found on the probe-determined yaw angle, in which the difference from the vanes-only data was reduced to below 1 deg, except near the endwalls where larger discrepancies remain due to probe–endwall interactions. A shortfall of the correction was instead observed on the probe-determined Mach numbers. Finally, the pitch-wise averaged quantities were evaluated. It was observed that the highest differences between probe-determined and undisturbed data occur where radial gradients of total pressure are stronger and that the two-step correction had almost negligible impact on the pitch-wise averaged quantities.

Compressed Air Energy Storage (CAES) System is an important power output component of the energy storage technology. Radial inflow turbine is the main power output device in CAES system, it is operated at extraordinary operation condition (inlet pressure ≥ 75 bar and inlet temperature

A large-eddy simulation is performed on a single nonrotating airfoil with tip gap in order to highlight the noise sources in the tip-leakage flow. The geometry is composed of a NACA 5510 airfoil (5...

Both the compressor performance and the 3D flows inside the stator passage are significantly impacted by the stator hub gap. The interplay between leakage flow and corner separation within a cantilevered stator of a highly loaded, low-speed axial compressor with a succession of stator hub gaps was examined numerically in this paper. Firstly, the simulated results were compared with the measured results, including the compressor characteristics, the 3D flow structures, and the flow fields at the stator outlet. The results revealed that the used CFD solver, as well as the corresponding setup, can reproduce the flow not only in terms of the trend along with the stator hub gap, but also in terms of the specific scale of the 3D flow structure. Hence, it is feasible enough to be applied in the present investigation. Secondly, the flow mechanisms of the interplay between the corner separation and the leakage flow with different stator hub gaps were analyzed. It was found that the velocity of the leakage flow is the key parameter that dominates the flow structures as well as the compressor performance. Additionally, a simple metric was proposed to be used to choose the optimum stator hub gap. By comparing our results with those from published research, this metric was proven to be feasible. Finally, it is also discussed how the stator hub gap affected the stator inlet flow and rotor performance. It is demonstrated that the stator passage flow blockage can affect the upstream flow field. As a result, the performance of the rotor tends to vary in the opposite direction to that of the stator.

A phenomenon called rotating instability was rarely reported in a centrifugal compressor, although some associated discussions can be found in axial compressors. This paper presents a numerical investigation on the unsteadiness of the tip leakage flow and rotating instability in an isolated centrifugal compressor impeller. A three-dimensional, unsteady full annuls simulation is performed under three different operating conditions. The tip leakage flow unsteadiness at the impeller inlet and inside the impeller passage is detected by the pressure fluctuation. During the throttling process, the temporal and spatial propagation characteristic of the tip leakage flow unsteadiness under the near stall operating condition is revealed based on the wavelet analysis, Fourier transform, and dynamic mode decomposition. Furthermore, the relationship between the tip leakage flow unsteadiness and the rotating instability is also discussed. The results show that the pressure fluctuation mainly concentrates in the tip region at the near stall operating condition, which indicates the occurrence and circumferential propagation of tip leakage flow unsteadiness. The circumferential propagation of the tip leakage flow unsteadiness induces a significant circumferential rotating pressure wave with a wave number of 9 and approximately 48.2% of the rotational speed of the impeller. The characteristic frequency of the tip leakage flow unsteadiness agrees well with the dominant mode frequency of rotating instability. Therefore, the tip leakage flow unsteadiness is assumed to be responsible for the rotating instability. During the throttling process, the formation and development of the rotating instability at near stall operating condition can be divided into three stages based on signal analysis: prior to rotating instability, generation and development, and rotating instability.

Growing performance requirements for gas turbines have led to a continuous increase in gas temperature and pressure ratios. Together with the resulting increase in cooling flows, this requires more and more minimization and control of internal gas leaks. To meet future performance goals, the application of a new seal design and an improved understanding of leakage flow characteristics are of particular importance. The air mass flow through a labyrinth seal designed for a low-pressure turbine has been determined both through analytical calculus and CFD modeling. Different radial clearances and different air temperatures have been considered. In the next stage, the results will be validated through experiments.

Several linearly nonuniform clearances have been designed to explore a novel strategy for attenuation of leakage flow in tip region of high loading transonic rotor and the effects of axially nonuniform clearance on detailed tip flow structure and stable operating range of rotor have been discussed as well. The results showed that the tip flow characteristic of rotor is affected mainly by the combined effects of two parts of low-velocity flow, which are produced by interaction of leakage flow with passage shock and boundary layer separation near suction side, respectively. However, the stall margin of rotor and isentropic efficiency near tip region is dominated significantly by the former part, and the local changes of size and shape of tip clearance have a large influence on it. Once the strength of leakage flow decreases due to clearance size variation, the boundary layer separation near suction side of blade tip worsens gradually and increases additional aerodynamic losses in passage. Both the mass flow rate and mixing losses of the tip leakage flow can be reduced due to a smaller size of clearance existing in front part of clearance of rotor with a linearly divergent clearance shape, and the area of low-velocity region near pressure side has reduced accordingly. By contrast, a linearly convergent shape of tip clearance can increase both the area of low-velocity region and the mixing loss of leakage flow as a result of a larger size of clearance existing over the front part of blade tip of rotor. Eventually, a divergent shape of tip clearance with a reasonable minimum size near leading edge of blade tip is preferred for transonic rotor in consideration of the benefit in stall margin improvement with nearly no penalty in efficiency.

The tip-leakage vortex (TLV) cavitation is a challenging issue for a variety of axial hydraulic turbines and pumps from both technical and scientific viewpoints. The flow characteristics of the TLV cavitation were widely studied in the past decades, but the knowledge about the tip-leakage cavitating flow is still limited. The present paper reviews the progresses in the researches of the TLV cavitation, including the numerical methods for the TLV cavitation, the flow characteristics of the TLV, the influences of the TLV cavitation on the local flow field and the control strategies of the TLV cavitation. It is indicated that the non-condensable gas may play an important role in the development of the TLV cavitation, and this fact should be considered during a careful simulation of the TLV cavitation. It is also suggested that the development of the TLV cavitation will significantly influence the distributions of the vorticity and the turbulence kinetic energy. Due to the complexity of the TLV cavitation, it is still an open question how to suppress the TLV cavitation in a simple but effective way. Finally, based on these understandings, some advanced topics for the future work are suggested to further promote the study of the TLV cavitation, for a deeper knowledge about the TLV cavitation.

The influence of axial-slot casing treatment (ASCT) on the performance and stability enhancement mechanisms of ASCT were experimentally and numerically investigated in a high-speed mixed-flow compressor under three different tip clearances. Unsteady simulations showed the compressor stalled through end-wall stall route, i.e. the spike stall inception originating from rotor tip region, which was validated by dynamical measurements. When the ASCT was applied, greater than 20% of stall margin improvement (SMI) could be achieved for the compressor under each tip clearance size. The streamwise velocity contours and flow structures in the tip region and axial slots were deeply analyzed to explore how the so called “suction and injection effects” generated by the ASCT manipulate tip clearance flow and enhance the stability of compressor under different tip clearances. It was found that the dominant stability enhancement mechanisms of ASCT varies with tip clearance size for the mixed-flow compressor. (1) For the small tip clearance, the dominant mechanism of stability enhancement is the blockage reduction generated in the blade passage by the suction effect of ASCT. (2) For the large tip clearance, the injection effect of the ASCT is the dominant mechanism of stability enhancement with ASCT, which plays the leading role in delaying the spillage of incoming/tip leakage flow interface at the rotor leading edge (LE) plane.

It is believed that the rotating instability phenomenon originating in the compressor tip region is due to leakage flow, which is closely associated with the blade tip clearance. In this work, we have studied the correlation between the dynamic characteristics of blade tip flow and the size of tip clearance for a single-stage low-speed compressor rotor, so as to unveil the mechanism of rotating instability. The full-passage numerical simulations were carried out to obtain the variations in frequency, circumferential mode, and spatial flow field associated with rotating instability. The results of spatial mode decomposition with open clearance show the number of predominate instability modes identified are 25 and 30, respectively. By diminishing the blade tip clearance, all these unstable modes greatly diminished. The formation and propagation of the tip leakage vortex were described in detail to show the development of rotating instability. Two flow field reduced-order methods, proper orthogonal decomposition and dynamic mode decomposition, were used to analyze the flow field, energy proportion, and stability of related modes under different tip clearances. The results show that the first several modes with strong stability account for a large proportion of energy and make a major contribution to flow unsteadiness. The energy proportion and stability of rotating instability decrease as the tip clearance becomes smaller. The blade-passing frequency and its multiples emerge as the main components of the flow field.

To investigate the influence of tip winglets on the tip leakage flow in a compressor cascade with different incidences, the experimental measurement combined with numerical simulation are used to study the conventional cascade and cascades with three different pressure surface tip winglets at five incidences of −6°, −3°, 0°, +3° and +6°. The results indicate that three different tip winglets at five incidences all restrain the occurrence of leakage flow, delay the mixing of leakage flow and the mainstream, change the formation path of leakage vortex and weaken its intensity, reduce the flow loss and improve the uniformity of flow field. The sensitivity of the flow field to variable incidences is reduced. The optimization degree of the flow field is proportional to the width of the blade tip winglet. The improvements are more obvious at positive angles. When the incidence reaches +6°, the flow loss is reduced by 12.4%.

The clearance gaps in twin-screw compressors are critical for their performance and reliable operation, as the leakage flows through these clearances influence the volumetric and adiabatic efficiencies. The amount of leakage flows depends on the clearance size and shape as well as various geometric and operating parameters. Usually, the isentropic nozzle equations along with appropriate flow coefficients are used for more accurate estimation of the leakage flow rates through the clearance gaps. Hence, a proper understanding of the flow coefficients and their relationship with the dimensionless parameters (such as pressure ratio, Reynolds number, and aspect ratios) is critical for an accurate prediction of the leakage flows. In the present study, considering the interlobe clearance gap in screw compressor in terms of rectangular openings, the interlobe leakage flow rates are estimated for various opening sizes and pressure conditions using isentropic nozzle equations and an iterative method. The flow coefficients are determined by comparing the experimental values obtained using a specialized test rig and the flow rates obtained from the analytical methods. The dimensionless parameters are varied to see their individual effect on the leakage mass flow rates and on the flow coefficients. The mean deviation from the experimental results when using an analytical iterative procedure (-8.5%) is substantially lower in comparison to the mean deviation (+26.8%) using the isentropic nozzle equations. The study validates that the iterative method can be preferred (for an interlobe leakage flow rate prediction) over the isentropic nozzle equation method.

This thesis deals with Conjugate Heat Transfer analysis of an oil-free Roots blower which is a Positive Displacement Machine used for low pressure ratio applications. Prediction of temperature distribution is critical in the design of rotor-to-rotor and rotor-to-casing clearance gaps inside these compressors. The aim of the study is to numerically analyse the heat transfer from pressurized air to rotors and casing and provide the temperature distribution over structural elements. These problems are too complex to be solved analytically so this thesis will employ High Performance Computing to perform CFD simulations and find an approximate numerical solution of the heat transfer problem. Internal flow inside the blower is highly transient and to predict its characteristic we need to tackle one of the most challenging deforming mesh problems where working medium is displaced during simulation. To allow for mesh deformation, due to intermeshing of the rotors, the moving mesh technique will be employed in this thesis. A numerical framework for 3D simulation of complex flow field inside a Roots blower compressor is established and presented. The framework employs Finite Volume Method. Five cases are investigated, each for different operating condition. In order to evaluate the accuracy of the developed numerical model, extensive validation is performed for several operating conditions. The numerical solution is compared to the overall performance of an equivalent experimentally measured compressor. The experimental data were provided by City University London and they include measurements of surface and air temperature, mass flow rate, pressure ratio, and effective power measurements. The comparison between numerical results has shown good agreement with the experimental data. The outcome of this research contribute to the understanding of the effects of leakage flow phenomena and can help design more efficient compressors. Ovaj diplomski rad bavi se analizom spregnutog prijelaza topline Roots-ovog puhala koji radi bez podmazivanja te se primjenjuje za niske kompresijske omjere. Procjena temperaturne raspodjele ključna je pri projektiranju zazora koji se nalaze u kućištu ovih kompresora. Cilj rada je numerički analizirati prijenos topline s komprimiranog zraka na rotore i kućište te dati temperaturnu raspodjelu na površinama rotora i kućišta. U radu će se koristiti računalni klasteri s visokom učinkovitošču za izvođenje CFD simulacija i pronalaženje približnog numeričkog rješenja problema prijenosa topline. Predstavljena je numerička metoda za simulaciju kompleksnog strujanja radnog medija unutar Roots-ovog puhala. Metoda koristi numeričku metodu kontrolnih volumena. Provedeno je 5 simulacija, svaka za jedan ispitni uvjet. Kako bi se procjenila točnost numeričkog modela, u radu je provedena opsežna validacija za nekoliko ispitnih uvjeta. Numeričko rješenje uspoređeno je s eksperimentalnim rezultatima ekvivalentnih ispitnih točki mjerenog kompresora. Eksperimentalne podatke je dao na raspolaganje fakultet City, Sveučilišta u Londonu. Podaci uključuju mjerenja temperature površine i zraka, masenog protoka, kompresijskog omjera te mjerenja efektivne snage. Usporedba je pokazala dobro slaganje numeričkog rješenja s eksperimentalnim podacima. Ishod ovog istraživanja doprinosi razumijevanju gubitaka u volumetrijskim kompresorima te može doprinijeti povećanju njihove efikasnosti.

In this paper, the performance of the turbine blade squealer tip has been studied detailed aimed to highlight the impact of the upstream guide vane passing wake. The first stage of GE-E3 high-pressure turbine has been employed to perform the three-dimensional simulation and the computational domain has been scaled based on the domain scaling method. Boundary conditions are consistent with operating conditions of the annular cascade testing. Circumferential averaged and realistic non-uniform interface conditions have been used to obtain steady and unsteady characteristics respectively. The validation of the turbulent model and mesh independent test has been conducted detailed in previous work. Three squealer tips, including two widths and heights, have been designed and investigated to understand its influence. Results show that the aerothermal performance of the squealer tip is remarkably influenced by the upstream passing wake. Although steady and time-averaged results have a good agreement, the variation of instantaneous heat transfer coefficient (HTC) would be over 30%, especially on the Cavity Floor region. Changing the geometry of the squealer also has different impacts on both steady and unsteady performance. The unsteady aerodynamic has relatively small fluctuation within 10%, and the distribution of steady and time-averaged leakage flow as well as total pressure loss coefficient still have a satisfactory agreement.

In the present study, the research progress of tip winglets that control tip clearance leakage flow in compressors is reviewed. Firstly, the effects of tip leakage flow on the aerodynamic performance of the compressor are presented. Subsequently, the development of tip winglet technology is reviewed. Next, a series of studies on compressor tip winglet technology are conducted. Besides, the effects of tip winglets on the aerodynamic performance of rectangular cascades of low-speed and high-subsonic compressors, subsonic compressor rotor and transonic compressor rotor are discussed, respectively, and the control effect of tip winglet technology combined with tip groove design on tip leakage is investigated. Lastly, the subsequent development direction and research prospect of compressor tip winglet technology are presented.

Tooth bending damage resulting from an intense impact by the rotor sometimes occurs in the transient operation. To investigate the influence of after-damage clearance and tooth bending length on the leakage performance and rotordynamic coefficients of labyrinth seals, three tooth bending damages were taken into consideration, including the unbent tooth damage (abbreviated as Unbent), the partial tooth bending damage (abbreviated as Pbent) and the complete tooth bending damage (abbreviated as Cbent). The transient CFD solution was utilized to calculate the leakage flow rates and rotordynamic coefficients of labyrinth seals with clearances of 0.3, 0.4, 0.5, 0.6 mm for three tooth bending damages. The obtained result shows that the Unbent tooth damage leaks least while the Pbent tooth bending damage leaks most, and an increase of 6.1% for Cbent tooth bending damage and an increase of 19.4% for Pbent tooth bending damage are discovered at the tooth clearance of 0.6 mm in comparison with the Unbent tooth damage. Compared to the Unbent tooth damage, the effective damping for Pbent tooth bending damage and Cbent tooth bending damage is lower and drops by 9.7%–33.6% and 8.5%–22.6% respectively at the tooth clearance of 0.6 mm, suggesting that Pbent tooth bending damage or Cbent tooth bending damage tends to weaken the seal stability when compared to the Unbent tooth damage.

The steady simulation with flow field and heat transfer performance for different rib layouts is investigated. By referring the GE-E3 turbine blade tip (Case 1) profile as a prototype model, four kinds of rib layouts of full rib structure (Case 2), half rib structure connected with suction side (Case 3), half rib structure connected with pressure side (Case 4), and half rib structure in the rear squealer cavity (Case 5) was designed. The availability of k-ω turbulence model was validated through comparison of heat transfer coefficient distribution with the experimental data. The area-averaged heat transfer coefficients of Case 5 decreases up to 11.3%, 3.1%, 11.3%, and 2.8% in comparison to Cases 1, 2, 3, and 4. The total pressure coefficient of Case 5 reduces by 1.4%, 2.7%, and 4.0% compared to Cases 1, 2, and 3. The results reveal that, among the five novel rib layout cases, Case 5 obtains the optimal comprehensive performance under the performance of considering tip heat transfer and cascade loss. Further investigation on the influence of Cases 1 and 5 at different rotational speeds on the flow field and heat transfer characteristics of the tip clearance is also illustrated and discussed.

To control secondary flow effects and enhance the aerodynamic performance of the compressor, the flow control effects of the flow suction at the endwall with different circumferential positions and at the blade tip were numerically investigated in the cantilever stator of an axial single-stage transonic compressor. The main purpose was to gain a better understanding of the application of boundary layer suction and the associated control mechanisms in the cantilever stator. The studies show that the optimal position of the endwall suction slot should be located up the stator blade, in terms of the leakage flow structures and the blade tip unloading effect. In addition, the flow control effects of the suction at the blade tip on leakage flow upstream is better than that of the endwall flow suction with the same structure. Further, the studies of compressor aerodynamic performance curves illustrate that the efficiency and pressure ratio is increased by 0.34% and 1.09% at the peak efficiency point, and are increased by 0.39% and 0.14% at the near stall point, respectively.

The interaction of tip leakage flow and main flow in an axial flow pump can induce the tip leakage vortex, which causes the unstable flow and complex cavitation structures. In the present research, the modified shear stress transport k–ω turbulence model was utilized to predict the cavitating flow of the model pump under design and off-design conditions. Validations were carried out using high-speed photography techniques. Results show that the simulation results about cavitation performance based on the modified shear stress transport k–ω turbulence model agree well with the experimental results. Cavitation inception occurs more possible at part-load conditions, and with the increase of the flow rate, the starting point of tip leakage vortex gradually moves to the back edge of the blade chord. Both the sheet cavitation and the triangular cavitation cloud that formed in the blade tip grow in size and intensity gradually with the decrease of cavitation number. Distributions of the cavity area fraction and axial velocity show that the tip leakage vortex cavitation locates at the radial coefficient r* = 0.95–1.0 with peaks at about r* = 0.97, while the sheet cavitation is found at r* = 0.5–0.9 on the suction side (SS). The cavitation structures lead to a significant decrease in the axial velocity, especially in the tip region of the blade.

Flow around the Guide vanes (GV) in Francis turbine differs with the shape of hydrofoils. The difference in the pressure of fluid travelling to pressure side and suction side of GV contributes to flow behavior. This study presents the numerical technique using alternative clearance gap method to predict the flow around GV and its consequent effect on turbine performance. GV profile has a significant effect on the performance of the turbine with sediment contained fluid flow. In this paper, symmetrical NACA 0012 and cambered NACA 2412, NACA 4412 hydrofoils are studied introducing 0 mm, 2 mm, and 4 mm clearance gaps. Vortex filament can be seen when fluid leaves the clearance gap due to the leakage flow occurring through the gap. The intensity of vortex leaving clearance gap rises with an increase in the size of the clearance gap. However, in the case of asymmetrical GV profile, the velocity of fluid travelling along the vortex compared to that of symmetrical hydrofoil is lower. In case of low specific speed Francis turbines, this vortex is found to be a major reason to erode the runner surface due to high velocity of a sand particle travelling with them. With the alternative clearance gap approach, this paper compares the pressure pulsation downstream of GVs contributed by leakage flow for three NACA profiles, whose frequency is half of blade passing frequency.

Three-dimensional blading is an efficient technique in compressor aerodynamic design, and its function mechanism in the cantilevered stator needs to be addressed. This paper focuses on the sweep and dihedral in the cantilevered stator and seeks to expose their effects through detailed flow field analysis. Results show that the forward sweep could alleviate the corner flow separation by preventing the accumulation of the secondary flow toward the corner region, resulting in stronger flow separation at the blade trailing edge; in summary, forward sweep with appropriate parameters could increase static pressure rise by 14.3%. The positive dihedral will carry the endwall flow to the upper-span sections, thereby reducing blade corner separation; hence, as much as 23.5% improvement in static pressure rise could be obtained with the appropriate dihedral. Moreover, the combination of a relatively large sweep height and a moderate sweep angle with a low dihedral height and a moderate sweep angle provides optimum aerodynamic performance; the static pressure rise coefficient sees an increment of 25.5% at the near stall point. An experiment is then performed to further validate the theory, which shows a 2% improvement in efficiency of 3D blading at small mass flow rates. However, the secondary leakage should be given attention at high mass flow coefficients, while the corner separation needs further elimination at small mass flow rates.

Computational Fluid Dynamics (CFD) simulations of a leakage in a pressurized pipe were undertaken to determine the empirical effects of hydraulic and geometric factors on the leakage flow rate. The results showed that pressure, leakage area and leakage form, influenced the leakage flow rate significantly, while pipe thickness and mean velocity did not influence the leakage flow rate. With relation to the interactions, the effect of pressure upon leakage flow rate depends on leakage area, being stronger for great leakage areas; the effects of leakage area and pressure on leakage flow rate is more pronounced for longitudinal leakages than for circular leakages. Finally, our results suggest that the equations that predict leakage flow rate in pressurized pipes may need a revision. RESUMO Simulações com Fluidodinâmica Computacional (CFD) de um vazamento em um conduto pressurizado foram realizadas para determinar os efeitos empíricos de fatores hidráulicos e geométricos na vazão através do vazamento. Os resultados mostram que a pressão, área de vazamento e forma do vazamento, influenciaram a vazão significativamente, enquanto que a espessura do tubo e a velocidade média não influenciaram a vazão através do vazamento. Com relação às interações, o efeito da pressão sobre a vazão depende da área do vazamento, sendo maior para áreas de vazamento maiores; os efeitos da área de vazamento e pressão na vazão através do vazamento é mais pronunciada para vazamentos longitudinais em comparação a vazamentos circulares. Finalmente, nossos resultados sugerem que as equações que predizem vazão através de vazamentos em tubos pressurizados precisam passar por uma revisão.

The present paper reports a 2D-PIV (particle image velocimetry) study of the effect of the operating point on the leakage flow in a low-speed ring fan. First, the flow pattern has been studied at 12 operating points covering the whole characteristic curve. At very low loading, the leakage flow streams along the rotor ring and is directly reingested, then, a separation bubble attached to the ring forms that, approaching the design point, modifies in a flow streaming radially outward. As the loading further increases, a separated flow region appears in the blade tip region that finally merges with the leakage flow. A further, more detailed study has been performed at eight operating points in the neighborhood of the design one. Very small loading variations may yield the leakage flow pattern modification, but no intermittence is present during the transition, as instantaneous flow patterns of any intermediate type continuously alternate. These results provide a consistent explanation for the ones of previous acoustic measurements.

The computation of the gas velocity in leaking paths which may contain labyrinth elements of screw compressors is developed from the continuity, energy and momentum equations. It accounts for the effects of the local loss of flow kinetic energy and line fluid-wall friction by assuming that the fluid is an ideal gas at constant temperature to eliminate gas density. This is straightforward situation if the clearance leakage flow is considered to happen in a single leakage gap element, however, if a series of leakage elements are applied, as for example, it happens in a labyrinth seal components, there will be a pressure drop in each groove which in sum will give the overall pressure drop. A sensitivity analysis of the compressor design parameters that influence such a sequence of the labyrinth seal performance upon that the leakage flow strongly depends upon the gap clearance and only weakly depends on the clearance length and the number of labyrinth grooves. The influence of the number of grooves becomes even weaker if the entire kinetic energy of the leakage flow is not fully dissipated in the labyrinth grooves.

Both LES and DDES are conducted in a low-Reynolds number tip leakage flow model. The DDES uses the SST kω model and employs the same grid with the LES, but the turbulence field diverges from the LES result. Referring to the comparison between LES and DDES, a modification of the zonal function in the DDES model is proposed, which enhances the dissipation of the modeled turbulence thus promote the transition to fully LES in the tip region when the mesh is fine enough. It can generate much finer vortex structure than the original model, including the primary streamwise vortex, induced vortices and the vortex fragments after breakdown. The modification fixes the underestimation of the vorticity and pressure drop at the formation stage of the tip leakage vortex, and generates more reasonable turbulence field and energy spectra. The modified model is introduced to a real rotor simulation at engineering Reynolds number. Compared with the original model on both mean flow field and turbulence field, the modified model shows favorable agreements with the measurements. The study also gives a practical example of using the tip leakage flow model in turbulence modeling.

In a boundary layer ingesting (BLI) propulsion system, the fan is continuously exposed to inflow distortions. The distorted inflows lead to nonuniform loss distributions along the radial and circumferential directions. Since the rotor tip suffers from higher intensive distortion, the local loss increment is a major contributor to the BLI fan performance penalty. To explore the effects of distorted inflows on tip leakage flow evolutions and associated mechanisms for increased loss in a BLI fan, three-dimensional full-annulus unsteady simulations are conducted. Results show that about 54% of total additional losses due to distortion are formed in the tip region and more than 80% of tip region entropy generation is related to the tip leakage flow. The intensities of leakage vortex–shock interactions vary at different annulus locations. When the rotor moves into the distorted region, the vortex–shock interaction is weaker than the undistorted locations due to attenuated leakage flow. At the locations where the rotor is moving out from the distorted region, the vortex–shock interaction is notably enhanced because the front part of the blade tip airfoil suffers a higher load, resulting in a rapid vortex core expansion and eventually vortex breakdown. The increase of flow blockage in the front section of blade tip passages at local circumferential positions leads to a corresponding rise of flow loss. The findings in this study highlight the impacts of tip leakage flow on aerodynamic loss of fan working under BLI inflow distortion and provide improved understandings of loss mechanisms in a BLI fan.

The shell side flow fields of both sextant and trisection helical baffle heat exchangers are presented on meridian and multilayer hexagon slices. It verifies that the performance of sextant schemes is better than those of the other kinds of helical baffle heat exchangers. The main mechanisms are due to the restricted leakage flow in the minimized gaps with increased baffle number and by one row of tubes dampen the leakage flow in the circumferential overlapped area of the adjacent helical baffles. The performance features were simulated on two different angled sextant helical heat exchangers and each compared with two trisection ones of either identical helical pitch or identical incline angle. The results verified that the performances of helical heat exchangers are mainly determined by the helical pitch rather than the baffle incline angle. The average values of comprehensive index hoΔpo−1/3 of the trisection helical schemes T-24.1° and T-29.7° are correspondingly 3.47% and 3.34% lower than those of the sextant ones X-20° and X-25° with identical helical pitches. The comparison results show that the average values of shell side h.t.c. ho and comprehensive index hoΔpo−1/3 of the optimal dual helix sextant scheme DX30° are respectively 7.22% and 23.56% higher than those of the segment scheme S100.

The current research of seawater hydraulic motor mainly focused on piston motor and vane motor, but seldom regarded low speed high torque seawater hydraulic motor. Low speed high torque seawater hydraulic motor as a kind of energy conversion device and actuator plays an important role in seawater hydraulic transmission system. However, the physical and chemical properties of seawater, such as low viscosity, high causticity and poor lubrication, result in numerous problems. In this paper, the flow distribution characteristics of port plate pairs for the seawater hydraulic motor are investigated, and the leakage flow and power loss models of port plate pairs are established. Numerical simulations are carried out to examine the effects of water film, inlet pressure and rotating speed on the pressure distribution and leakage flow. And the friction and wear tests of port plate pairs are also carried out. Moreover, the test system of the seawater hydraulic motor is constructed and the performance of prototype with no-load or loading is conducted. The results indicate that the clearance of port plate pairs and inlet pressure have a significant effect on distribution characteristics, but the effect of rotating speed is not very obvious. The experimental results show that the minimum error rate can be maintained within 0.3% by the proposed flow model and the counter materials of 316L against carbon-fiber-reinforced polyetheretherketone (CFRPEEK) are suitable for the port plate pairs of seawater hydraulic motor. Finally, based on the seawater hydraulic experiment platform, the volumetric efficiency of no-load and loading are obtained that the maximum can achieve 94.71% and 90.14%, respectively. This research work may improve the flow distribution performance, lubrication and the friction and wear properties, enhance energy converting efficiency of port plate pair and provide theoretical and technical support for the design of high-performance water hydraulic components.

A novel structure of anti-stagnation labyrinth seal was proposed in this paper. An opposed jet suppressing the circumferential flow was generated in seal cavities with pressure-driven micro-nozzles on adjacent seal teeth. The theoretical identification approach for rotordynamic coefficients based on the infinitesimal theory was applied to predict rotordynamic coefficients of annular gas seals. The effects of the layout of anti-stagnation nozzles, inlet pressure and rotational speed on rotordynamic characteristics were investigated using a computational fluid dynamics (CFD) method. Compared with that of the traditional labyrinth seal, the new seal possesses better stability with a 200% increase in the effective damping coefficients at high whirling frequency. Compared with those in the middle and downstream position, nozzles installed in the upstream of the seal can effectively improve the dynamic stability of the system. The anti-stagnation labyrinth seal shows better stability performance for higher inlet pressure and rotational speed. The leakage flow rate of the anti-stagnation labyrinth seal is slightly higher (an increase of less than 4%) than that of the original labyrinth seal.

The tip clearance flow could lead to work reduction and loss generation in turbomachines. However, the effect of separation at low Reynolds number on leakage flow is seldom studied. The previous method for evaluating tip leakage characteristics should also be further researched. Thus, numerical investigations on the tip clearance flow in an unshrouded axial-inflow turbine are conducted at low Reynolds number (3.5 × 104 of the rotor outlet at the designed condition) in the present study. The flow patterns and leakage mass flow rate of the clearance have been analyzed in detail. It is found that the tip clearance flow is greatly affected by the flow separation caused by low Reynolds number. The scraping ratio adopted in previous references does not accord with the clearance flow characteristics at low Reynolds number, especially in the front part of the clearance. A coefficient by −0.70 power of the Reynolds number is proposed to modify the scraping ratio in the present study. The synergy between the velocity and the pressure gradient is innovatively employed to research the tip clearance flow characteristics, and it gives a reliable criterion of indicating the flow patterns in the tip clearance.

Tip leakage flow reduces both efficiency and performance of axial turbines and damages turbine blades as well. Therefore, it is of great importance to identify and control tip leakage flow. This study investigated the effect of flow injection (from the casing), alongside flow structure, on turbine performance. Additionally, the effect of different injection parameters, including injection mass flow rate, angle, location, and diameter on the turbine performance are evaluated. A numerical analysis of the flow in a two-stage axial turbine was employed by using CFX software. To ensure the accuracy of the results, turbine performance curves were compared with the experimental results, which are in good agreement. Analyses revealed that active control method reduces tip leakage flow, improves turbine performance, and increases the efficiency by 1% to 5% as well. A parametric investigation of the tip injection has sought to identify how various parameters affect the turbine performance. The cross-section diameter and the angle of injection had no significant increase on efficiency. Additionally, results showed that at a point 9 mm further from the leading edge, the injection degree of effectiveness is optimum. Finally, analysis of the flow structure in the tip clearance region supported the tip leakage flow reduction.

This article presents a procedure for designing rockfill toe protections with a highly permeable downstream shoulder to avoid mass sliding in dams during extremely high through-flow episodes. This accidental through-flow may be caused by reasons such as overtopping or leakage flow due to the loss of sealing at the impervious element of the dam or its foundation. The proposed protection is located at the downstream toe of the dam, and it is composed of highly permeable material, typically compacted rockfill. This material can be different from the rockfill forming the downstream shoulder of the main dam. The work is a result of a combination of numerical and experimental research carried out to analyze the influence of the geometry of the toe protection for given material properties of both dam shoulder and protection, when they can be considered highly isotropic. As a result of these studies, a design methodology is obtained. A series of validation tests are presented to support the reliability of the method.

The transient flow model of a scroll refrigeration compressor which included axial clearance was established in the paper using PUMPLINX based on the computational fluid dynamics and the dynamic grid methodology. According to the actual size in the physical compressor, the flank and the axial clearances were set equal to 0.030 mm and 0.015 mm, respectively. As the fluid domain of the axial clearance was too small to generate enough layers of grids in PUMPLINX, it was first magnified 100 times to generate grids of high quality, and then the fluid domain and grids were recompressed to the original size. The simulation results were verified experimentally. The influence of the radial leakage on the downstream flow field was investigated. Moreover, the characteristics of radial and tangential leakage flows were analyzed. It was concluded that the predictions of the compressor performance, except the discharge temperature, are improved after the axial clearance was included in the model. The radial leakage is dominant while a portion of involute leakage flow exists near the flank clearance. The influence of radial leakage on the downstream temperature is lower than that of the tangential leakage in flank clearance. The tangential leakage flow is turbulent basically while the radial leakage flow is laminar. The tangential leakage is severer at the beginning of the discharge process, and the radial leakage is more remarkable at the end of the compression process. When the pressure ratio across the leakage path is higher than the critical pressure ratio, the tangential leakage flow becomes supersonic for the converging–diverging clearance structure, while the radial leakage flow remains subsonic due to the larger flow losses. The results obtained through this research could be applied to modify the current leakage models and optimize the clearances of scroll compressors.

Wear-ring clearance leakage would affect performance of pump-turbine significantly. In this paper, the variation of the leakage and efficiency of flat ring seal and labyrinth seal are numerically studied on one pump-turbine when the width of clearance is 0.2 mm and 0.5 mm. The result shows that the effect of leakage flow cannot be neglected. The pump-turbine performance affected by leakage in turbine mode is more than that in pump mode at the same sealing structure and width of clearance. Each component’s proportion of total pressure loss hardly varies with flow rate at pump mode, which is opposite to that at turbine mode. Leakage does not change proportionally with system flow rate. When the width of clearance decreases to 0.2 mm, the leakage is reduced obviously because the maximum entropy occurs in the front pump chamber. The mixing of leakage flow and mainstream at impeller inlet at pump mode will increase the total pressure and decrease the flow angle and relative flow angle. Finally, it reduces the impeller’s work capacity.

Analysis of earth dams is generally conducted in three stages of stability, deformability and water penetration analysis. Lack of sufficient attention to leakage, as one of the most important issues, leads to erosion and destruction of slope stability. The aim of current paper is to analyze the earth dam leakage with respect to the existing uncertainty in soil hydraulic parameters. In this research, the Monte Carlo (MC) method was used to calculate soil hydraulic parameters. Using these parameters, analysis of Alborz earth dam leakage by means of SEEP/W model based on the finite elements method was investigated. Due to the hydraulic conditions of the core soil, the total head value, pore water pressure, and water flux in core region will change. The results indicate that uncertainty in the hydraulic parameters of Alborz earth dam are significant, thus risk is important in this dam. The application of the proposed methodology in estimation of leakage from Alborz earth dam in Mazandaran province reveals its efficiency and proper accuracy in predicting the amount of leakage flow in earth dams with respect to the possible changes in the hydraulic parameters of the soil. Moreover, it was found that the quantity of seepage increases considerably when the dam is without core, therefore, the core is very necessary to decrease the value of seepage through the earth dam.

In this paper a novel casing treatment structure i.e. leading edge suspended groove shroud end wall structure is proposed in order to reduce the tip clearance losses in highly loaded helium compressor. The highly loaded design method of helium compressor can effectively resolve the problem of helium compression in high temperature gas cooled reactor (HTGR) plants. However, smaller blade height and increase in blade loading cause higher tip clearance leakage losses. The result shows that the leading edge suspended groove shroud structure in highly loaded helium compressor rotor effectively controls the formation of the tip clearance leakage flow and reduces the tip clearance leakage loss. 30° groove slope minimizes the total clearance losses close to blade tip. Moreover, It increases the efficiency of the highly loaded helium compressor by 0.4% at 1 mm tip clearance gap.

Purpose It is believed that non-physiological leakage flow through hinge gaps during diastole contributes to thrombus formation in Bileaflet Mechanical Heart Valves (BMHVs). Because of the small scale and difficulty of experimental access, fluid dynamics inside the hinge cavity has not yet been characterised in detail. The objective is to investigate small-scale structure inside the hinge experimentally, and gain insight into its role in stimulating cellular responses. Methods An optically accessible scaled-up model of a BMHV hinge was designed and built, preserving dynamic similarity to a clinical BMHV. Particle Image Velocimetry (PIV) was used to visualize and quantify the flow fields inside the hinge at physiological Reynolds number and dimensionless pressure drop. The flow was measured at in-plane and out-of-plane spatial resolution of 32 and 86 mu m, respectively, and temporal resolution of 297 mu s.\documentclass[12pt] Results Likely flow separation on the ventricular surface of the cavity has been observed for the first time, and is a source of unsteadiness and perhaps turbulence. The shear stress found in all planes exceeds the threshold of platelet activation, ranging up to 168 Pa. Conclusions The scale-up approach provided new insight into the nature of the hinge flow and enhanced understanding of its complexity. This study revealed flow features that may induce blood element damage. The authors gratefully acknowledge the support of Science Foundation Ireland (SFI) under the Research Frontiers Programme. This study was funded by Science Foundation Ireland (grant number 11/RFP.1/ENM/3310). peer-reviewed 2020-06-24

For water-lubricated twin-screw compressors, the estimation of leakage flow remains one of the most important problems related to screw compressor design, performance prediction and improvement. In this paper, a mathematical model of working process including detailed leakage and heat exchange models is constructed for this purpose, and the calculated results are verified by measured data. Based on the proposed model, the leakage in the water-lubricated twin-screw air compressor are analyzed. The effects of each leakage path on compressor performance including volumetric efficiency, indicated power and specific indicated power at different rotating speed are calculated and discussed. The prediction of leakage distribution could be used to optimize the compressor design.

Tip geometry modification is frequently used to suppress the tip leakage flow in the turbine cascade however a universally beneficial tip geometry modification design has not been fully discovered. In this paper, the two-surface coupling arbitrary blade tip design method in three-dimensional physical space which satisfies the simple trigonometric function law is proposed and the mathematical parametric description is presented. The effects of different arbitrary blade tips on tip leakage flow have been studied numerically in a highly loaded axial turbine cascade. The aerodynamic performance of different tips is assessed by the tip leakage mass flow rate and the total pressure loss coefficient at the exit section. The Kriging model and genetic optimization algorithm are used to optimize the arbitrary blade tips to obtain the optimal arbitrary blade tip. Compared with the flat tip, the tip leakage mass flow rate is decreased by 10.57% and the area-average total pressure loss coefficient at the exit section is reduced by 8.91% in the optimal arbitrary blade tip.

For a centrifugal compressor with volute, the flow field is circumferentially nonuniform because of the volute asymmetrical structure and leads to a circumferential difference in the tip leakage flow. In this work, the compressor performance and the casing wall static pressure distribution are measured, and the results are compared with the time-averaged results of the unsteady calculation to verify the reliability of the simulation. The results show a relationship between the tip leakage vortex trajectory and the high static pressure region in the diffuser, based on which a prediction model is established for the reverse propagation of pressure waves caused by a volute tongue. Influenced by the volute asymmetric structure, the trajectory, shape, and strength of the tip leakage vortex at different circumferential positions differs significantly. The tip leakage vortex trajectory affected by the high static pressure is more inclined to a circumferential direction because the tip leakage flow velocity flowing out of the suction surface is reduced, and the tip leakage flow with low velocity is subjected to the high-pressure gradient in a passage. Moreover, the tip leakage vortex breakdown in different passages differs significantly. A tip leakage vortex core more inclined towards the streamwise direction is more likely to break down than a tip leakage vortex core inclined towards the circumferential direction because of the larger reverse pressure gradient.

Control of the compressor tip leakage flow using plasma actuation is studied in this paper based on a compressor cascade model. Three types of plasma actuation layouts are designed: the axial plasma actuation which induces body force along the axial flow direction, the normal plasma actuation which induces body force normal to the interface between the tip approaching and leakage flow, and the stagger angle plasma actuation which induces body force normal to the blade chordwise direction. Influences of plasma actuation on the compressor cascade tip total pressure loss and blockage as well as the flow structures are first studied numerically and the corresponding flow control law is obtained. The numerical results are then verified through experiments. At last, the flow control mechanism behind the suppression of compressor tip leakage flow using plasma actuation is investigated. It is found that the key element of plasma actuation in suppressing the compressor tip leakage flow lays in improving the axial momentum of compressor cascade tip flow. The optimal flow control effect in suppressing tip leakage flow is achieved with the axial plasma actuation that is the most effective in improving the axial momentum of compressor tip flow.

To avoid friction, a clearance between the rotor blade tip and the cylinder is needed in a steam turbine. As a result, the leakage flow is formed under the pressure difference which mixes with the main flow and causes the mixing loss. So, a numerical calculation was conducted based on a high-pressure 1.5-stage steam turbine with the tip labyrinth seal. The leakage vortex system and the mixing progress at different tip clearances were analyzed. The related leakage losses were calculated. The results show that the leakage flow will lead to the steam deflecting at the rotor exit and cause incidence loss. Furthermore, in the downstream stator, the leakage flow near the suction side with high radial velocity causes the deflection of the outflow angles. The main influence region of leakage flow is distributed above 75% of blade height, whereas the most intense region is distributed at approximately 95% of blade height. It is found that the mixing loss is related to the size of the backflow vortex. The related leakage losses increase with tip clearance, in which the mixing loss is the major part and the mass-averaged entropy mixing loss coefficient is 7–11%.