Tip Cavity Research Articles

Effect of Triangular Grooved Tip on Blade Tip Region Heat Transfer

A numerical analysis was conducted to investigate the effect of the triangular grooved tip shape on the flow and heat transfer characteristics near the tip region. To study the effect of the triangular grooved tip shape, five triangular grooved tips were designed by changing the relative apex location of the triangular groove. Numerical computations were conducted for a total of six tip shapes, including a squealer tip. The contour plot of the secondary velocity and streamline were analyzed to understand the flow over a blade tip in detail, and the effect of the leakage vortex was discussed by comparing the total pressure loss. To compare the heat transfer characteristics, the Nusselt number and heat load over the tip surface, including the cavity sidewall, were analyzed. Results showed that the origin and trace of the vortex in the tip cavity depended on the apex location of the triangular groove, and because of this, the Nusselt number on the tip and blade suction side surface were also affected. Among the considered tips, the triangular grooved suction side tip showed comparable flow loss to that of the squealer tip. It also showed a lower heat transfer than the other tip shapes.

Aeromechanical Optimization of a Winglet-Squealer Tip for an Axial Turbine

The possibility of reducing the over tip leakage loss of unshrouded axial turbine rotors has been investigated in an experiment using a linear cascade of turbine blades and by using CFD. A numerical optimization of a winglet-squealer geometry was performed. The optimization involved the structural analysis alongside the CFD. Significant effects of the tip design on the tip gap flow pattern, loss generation and mechanical deformation under centrifugal loads were found. The results of the optimization process were verified by low speed cascade testing. The measurements showed that the optimized winglet-squealer design had a lower loss than the flat tip at all of the tested tip gaps. At the same time, it offered a 37% reduction in the rate of change of the aerodynamic loss with the tip gap size. The optimized tip geometry was used to experimentally assess the effects of the opening of the tip cavity in the leading edge part of the blade and the inclination of the pressure side squealer from the radial direction. The opening of the cavity had a negligible effect on the aerodynamic performance of the cascade. The squealer lean resulted in a small reduction of the aerodynamic loss at all the tested tip gaps. It was shown that a careful consideration of the mechanical aspects of the winglet is required during the design process. Mechanically unconstrained designs could result in unacceptable deformation of the winglet due to centrifugal loads. An example winglet geometry is presented that produced a similar aerodynamic loss to that of the optimized tip but had a much worse mechanical performance. The mechanisms leading to the reduction of the tip leakage loss were identified. Using this knowledge, a simple method for designing the tip geometry of a shroudless turbine rotor is proposed. Numerical calculations indicated that the optimized low-speed winglet-squealer geometry maintained its aerodynamic superiority over the flat tip blade with the exit Mach number increased from 0.1 to 0.8.

The Reduction of Over Tip Leakage Loss in Unshrouded Axial Turbines Using Winglets and Squealers

The possibilities of reducing the over tip leakage loss of unshrouded rotors have been investigated using a linear cascade of turbine blades and computational fluid dynamics (CFD). The large-scale blade profile is the same as that of the tip profile of a low-speed high-pressure research turbine facility. The impact of various combinations of squealer and winglet geometries on the turbine performance has been investigated. The influence of the thickness of the squealers has also been assessed. It was found that a 22% reduction in loss slope was possible, when compared to the flat tip blade, using simple tip modifications. The results obtained with the suction side squealer and cavity tip agreed well with the work of other researchers. Three winglet-based tip geometries were tested. One was a plain winglet, the other two had squealers applied. A significant impact of the squealers and their shape on the tip gap flow pattern and loss generation was found. The physical processes occurring within the tip gap region for the tested geometries are explained using both numerical and experimental results. The impact of the flow pattern within the tip gap on the loss generation is described. Good agreement between CFD and the experimental data was found. This shows that CFD can be used with confidence in the design process of shroudless turbines.

Effects of Winglet Geometry on the Aerodynamic Performance of Tip Leakage Flow in a Turbine Cascade

Experimental and numerical methods were used to investigate the aerodynamic performance of a winglet tip in a linear cascade. A flat tip and a cavity tip were studied as baseline cases. The flow patterns over the three tips were studied. For the cavity tip and the winglet tip, vortices appear in the cavity and the gutter. These vortices reduce the discharge coefficient of the tip leakage flow. The purpose of using a winglet tip is to reduce the driving pressure difference. The pressure side winglet of the winglet geometry studied in this paper has little effect in reducing the driving pressure difference. It is found that the suction side winglet reduces the driving pressure difference of the tip leakage flow near the leading edge, but increases the driving pressure difference from midchord to the trailing edge. This is also used to explain the findings and discrepancies in other studies. Compared with the flat tip, the cavity tip and the winglet tip achieve a reduction of loss. The effects of the rounding of the pressure side edge of the tips were studied to simulate the effects of deterioration. As the size of the pressure side edge radius increases, the tip leakage mass flow rate and the loss increase. The improvement of the aerodynamic performance by using a winglet remains similar when comparing with a flat tip or a cavity tip with the same pressure side radius.

Experimental and Numerical Study of Gap Size and Cooling Flow Rate Effects on Tip Flow of Gas Turbine Blade

In this paper the flow near blade tip of gas turbines was investigated for blowing ratios M = 0, 0.9, 1.7 and 3, and gap 0.3, 1 and 1.5 percent of blade span. The structure of the flow near blade tip was identified using PIV. Velocity profiles from vertical PIV planes around the blade show the effects of blowing ratio on flow field distribution. Vortex flows were observed in the pressure side of leading edge for M = 0 or larger. Instead, the trailing edge region has high velocity magnitude but attached to blade wall. Numerical simulations were conducted in order to investigate the effects of varying the tip gap. Results show that leakage flow in suction side of trailing edge does not perturb too much the flow structure as in leading edge. It is observed that part of cooling flow redistributes as leakage from the tip cavity towards the suction side as expected. However, leakage on pressure side with high momentum rates was found as well, mainly on leading edge. It is confirmed that leakage mass flow distributes unevenly through blade chord as a function of the tip-gap size, cooling flow and passage conditions of the mainstream with maximum value on trailing edge. The paper shows that increments of blowing ratio reduce the contribution of coolant to leakage flow.

Thermal Performance of Different Cooled Tips in A High-Pressure Turbine Cascade

The thermal performance of the three turbine tips was investigated. The results are presented in terms of heattransfer coefficient (h), cooling effectiveness (η), net heat-flux reduction (NHFR), and heat load. The calculations were performed using a cooled flat tip, a cooled cavity tip, and a cooled suction-side squealer tip in a cascade at a tip gap of 1.6%C. The results were validated using experimental data where possible. For an uncooled flat tip, the fluid dynamics are dominated by flow separation at the pressure-side edge. A significant benefit of ejecting coolant inside this separation bubble is shown. At the coolant mass-flow ratio M c = 0:52%, both the cooled flat tip and the cooled cavity tip are well-protected by the coolant. For the cooled suction-side squealer tip, the coolant lifts off from the tip floor and leads to not only low cooling effectiveness, but also to high heat-transfer coefficients. The effects of the coolant mass-flow ratio on the thermal performance of the tip were presented. The effects of end-wall motion on the thermal performance of the blade tip are found to be small in the current study.

Jet attrition in a fluidized bed. Part II: Effect of fluidized bed hydrodynamics

The effect of fluidized bed hydrodynamics on jet attrition with a supersonic attrition nozzle has been studied in this paper. The aim is to determine a hydrodynamic condition that can help improve particle attrition. Two hydrodynamic zones were created by a specially designed fluidized bed. Experiments were conducted with a supersonic attrition jet either straddling both hydrodynamic zones, or completely enclosed within one hydrodynamic zone. It is found that jet grinding is most effective when the nozzle tip is located in a high fluidization velocity region and jet tip located in a low fluidization velocity region. Also, a larger nozzle is more effective in terms of the new surface area created. A baffle is found to improve the jet attrition process when it was inserted underneath the jet tip cavity.

URANS computation of cavitating flows around skewed propellers

Cavitating flows around skewed propellers are investigated numerically by means of the unsteady Reynolds Averaged Navier-Stokes (RANS) Equation method. The standard k – ɛ turbulence and the modified Z-G-B cavitation models are employed. A measured nominal wake is used for the inlet velocity boundary condition. Predicted cavitating evolution processes and tip cavity patterns are compared with experimental observations. In addition, the influence of the skew angles on the cavitation and unsteadiness performances of propellers operating in a non-uniform wake is also studied. Results show that the modified Z-G-B cavitation model performs better to simulate the cavitating flow cases studied in this paper. Comparisons demonstrate that the skewed propeller with a skew angle of 20° is the best choice for a given stern wake with a assigned thrust and the minimum force fluctuations.

Pulse current auxiliary bulging and deformation mechanism of AZ31 magnesium alloy

The effects of pulse current on the deformation behavior, dislocation structure, microstructural evolution and cavity morphology of the coarse-grained (25 μm) AZ31 magnesium alloy during gas bulging process were investigated. The tests of pulse current auxiliary bulging (PCAB) showed that the formability of the alloy was improved as the increase in the peak current density. The results of TEM and SEM analysis showed that the dislocation motion was mainly glide, and the dislocation lines were approximate parallel with few dislocation tangles observed. Moreover, a part of cavity tips turned into circular bead by the concentration of the pulse current, and the rounding effect would lead to the relaxing of the high stress concentrated near the cavity tips. These phenomena indicated that the pulse current not only increased the dislocation motion but also restrained the cavity growing during PCAB process. In addition, the asymmetrical contours of the free bulging samples were observed and their forming mechanism was analyzed.

Heat Transfer and Film Cooling of Blade Tips and Endwalls

This paper investigates the flow, heat transfer, and film cooling effectiveness of advanced high pressure turbine blade tips and endwalls. Two blade tip configurations have been studied, including a full rim squealer and a partial squealer with leading edge and trailing edge cutouts. Both blade tip configurations have pressure side film cooling and cooling air extraction through dust holes, which are positioned along the airfoil camber line on the tip cavity floor. The investigated clearance gap and the blade tip geometry are typical of that commonly found in the high pressure turbine blades of heavy-duty gas turbines. Numerical studies and experimental investigations in a linear cascade have been conducted at a blade exit isentropic Mach number of 0.8 and a Reynolds number of 9×105. The influence of the coolant flow ejected from the tip dust holes and the tip pressure side film holes has also been investigated. Both the numerical and experimental results showed that there is a complex aerothermal interaction within the tip cavity and along the endwall. This was evident for both tip configurations. Although the global heat transfer and film cooling characteristics of both blade tip configurations were similar, there were distinct local differences. The partial squealer exhibited higher local film cooling effectiveness at the trailing edge but also low values at the leading edge. For both tip configurations, the highest heat transfer coefficients were located on the suction side rim within the midchord region. However, on the endwall, the highest heat transfer rates were located close to the pressure side rim and along most of the blade chord. Additionally, the numerical results also showed that the coolant ejected from the blade tip dust holes partially impinges onto the endwall.

The Tip Leakage Flow of an Unshrouded High Pressure Turbine Blade With Tip Cooling

Experimental, analytical, and numerical methods have been employed to study the aerodynamic performance of four different cooled tips with coolant mass ratios between 0% and 1.2% at three tip gaps of 1%, 1.6%, and 2.2% of the chord. The four cooled tips are two flat tips with different coolant holes, a cooled suction side squealer tip and a cooled cavity tip. Each tip has ten coolant holes with the same diameter. The uncooled cavity tip produces the smallest loss among all uncooled tips. On the cooled flat tip, the coolant is injected normally into the tip gap and mixes directly with flow inside the tip gap. The momentum exchange between the coolant and the flow that enters the tip gap creates significant blockage. As the coolant mass flow ratio increases, the tip leakage loss of the cooled flat tip first decreases and then increases. For the cooled cavity tip, the blockage effect of the coolant is not as big as that on the cooled flat tip. This is because after the coolant exits the coolant holes, it mixes with flow in the cavity first and then mixes with tip flow in the tip gap. The tip leakage loss of the cooled cavity tip increases as the coolant mass flow ratio increase. As a result, at a tip gap of 1.6% of the chord, the cooled cavity tip gives the lowest loss. At the smallest tip gap of 1% of the chord, the cooled flat tip produces less loss than the cooled cavity tip when the coolant mass flow ratios larger than 0.23%. This is because with the same coolant mass flow ratio, a proportionally larger blockage is created at the smallest tip gap. At the largest tip gap of 2.2% of the chord, the cavity tip achieves the best aerodynamic performance. This is because the effect of the coolant is reduced and the benefits of the cavity tip geometry dominate. At a coolant mass flow ratio of 0.55%, the cooled flat tips produce a lower loss than the cavity tip at tip gaps less than 1.3% of the chord. The cooled cavity tip produces the least loss for tip gaps larger than 1.3% of the chord. The cooled suction side squealer has the worst aerodynamic performance for all tip gaps studied.

The impact of a translating plunging jet on a pool of the same liquid

An experimental study on the impact of a translating two-dimensional transient jet on an initially quiescent liquid pool is studied experimentally using high-speed cinematic visualization and particle image velocimetry methods. Six jet conditions (covering a range of jet thicknesses, velocities and inclination angles relative to vertical) are considered, with measurements performed over a range of horizontal translation speeds for each jet condition. For all conditions studied herein, the jet penetrates into the pool and forms two craters – one upstream and one downstream of the jet. Gravity acts to close these craters, which after a short time pinch off at intermediate depths, thereby entrapping cavities of air. The translation speed of the jet is found to have a dramatic effect on the cavity shapes, pinch-off depths and pinch-off times. A simple theory based on a potential flow and a hydrostatically driven collapse is used to model this flow, and the resulting jet tip trajectories and cavity shapes compare favourably with the experimental data.

The impact of a translating plunging jet on a pool of the same liquid

An experimental study on the impact of a translating two-dimensional transient jet on an initially quiescent liquid pool is studied experimentally using high-speed cinematic visualization and particle image velocimetry methods. Six jet conditions (covering a range of jet thicknesses, velocities and inclination angles relative to vertical) are considered, with measurements performed over a range of horizontal translation speeds for each jet condition. For all conditions studied herein, the jet penetrates into the pool and forms two craters – one upstream and one downstream of the jet. Gravity acts to close these craters, which after a short time pinch off at intermediate depths, thereby entrapping cavities of air. The translation speed of the jet is found to have a dramatic effect on the cavity shapes, pinch-off depths and pinch-off times. A simple theory based on a potential flow and a hydrostatically driven collapse is used to model this flow, and the resulting jet tip trajectories and cavity shapes compare favourably with the experimental data.

Tip‐Enhanced Raman Spectroscopic Studies of the Hydrogen Bonding between Adenine and Thymine Adsorbed on Au (111)

Although the hydrogen bond is one of the weakest chemical interactions, it plays a key role in forming Watson–Crick base pairs (adenine–thymine and guanine–cytosine) that hold together the two helical chains of nucleotides in DNA molecules and form the basis of the genetic code. [1–3] The bonding process is easily influenced by molecular structures, steric hindrance, [4] the base-pair geometry, [5] and the surrounding environment. [5–7] X-ray crystal structure analysis and NMR spectroscopy, [8] infrared spectroscopy, [9–11] Raman spectroscopy, [12] electrochemistry [13] and the quartz crystal microbalance technique [14] have been employed to explain the basic biological, chemical and physical mechanisms of hydrogen bonding. Most of the methods are based on the data collected from large amounts of molecules. Due to its extremely weak chemical interaction, hydrogen bonding gives rather subtle signals that are easily covered by signals from other stronger interactions between the molecules. Highly sensitive and selective methods are needed to get more accurate insight into hydrogen bonding. Tip-enhanced Raman spectroscopy (TERS) is a recently developed near-field spectroscopic method, which can simultaneously achieve high spatial resolution from the scanning probe microscopy (SPM) images and rich chemical information from the Raman spectra. [15, 16] Benefitting from the greatly enhanced electromagnefic field generated in the cavity of the SPM tip and the substrate by illuminating the SPM tip, TERS has been successfully applied to Raman studies on atomically smooth surfaces with high reproducibility. [16] The sample volume in surface-enhanced Raman spectroscopy (SERS) is defined by the laser focal spot projected on a rough coinage metal surface, that is, about 1 to 2 mm 2 ( 10 12 m 2 ) from where large amounts of molecules are excited. By using an STM tip to focus the enhanced electromagnetic field into an area of ~ nm range (~ 10 15 m 2 for a tip of 40 nm in diameter) [17] at an atomically smooth surface, the number of the excited molecules is dramatically reduced, allowing one to study molecular events at approximately the single-molecule level for resonant molecules [17] and picomole level for non-resonant molecules, such as DNA bases. [18] Very recently, a single brilliant cresyl blue molecule has been spectroscopically detected and imaged by using the UHV–TERS setup in our group. [19] A sensitive study of hydrogen bonding can therefore be achieved at single crystalline surfaces without information-averaging between large amounts of molecules and interference from ill-defined substrates.

Effects of Tip Cavity Depth, Width, and Location on the Leakage Flow in an Axial Turbine Cascade

In this study, effects of the tip cavity with various depths, widths, and locations on the leakage flow and performance of an axial turbine cascade have been investigated numerically. The blade was a linear model of the tip section of the GE-E3 high-pressure turbine first-stage rotor blade. The Delayed Detached Eddy Simulation (DDES) model was used in the simulations. The computational results showed that the leakage mass flow rate and mass-averaged total pressure loss decreased as the depth and width of the tip cavity increased. And, it was shown that the cavity near the pressure side is more effective than that near the suction side. These effects depend on a vortex generated behind the pressure side in the cavity, which is changed with the depth, width, and location. The vortex entrains the leakage flow through the clearance toward the bottom of the cavity and in the chord wise direction, which reduces the flow leaking out.

Mathematical Approach to Investigate the Behaviour of the Principal Parameters in Axisymmetric Supercavitating Flows, Using Boundary Element Method

ABSTRACTIn this paper, a direct boundary element method (DBEM) is formulated numerically for the problems of the unbounded potential flows past supercavitating bodies of revolution (cones and also disks which are special case of cones with tip vertex angle of 180 degree) at zero degree angle of attack. In the analysis of potential flows past supercavitating cones and disks, a cavity closure model must be employed in order to make the mathematical formulation close and the solution unique. In the present study, we employ Riabouchinsky closure model. Since the location of the cavity surface is unknown at prior, an iterative scheme is used. Where, for the first stage, an arbitrary cavity surface is assumed. The flow field is then solved and by an iterative process, the location of the cavity surface is corrected. Upon convergence, the exact boundary conditions are satisfied on the body-cavity boundary. For this work, powerful software, based on CFD code, is developed in CAE center of IUST. The predictions of the software are compared with those generated by analytical solution and with the experimental data. The predictions of software for supercavitating cones and disks are seen to be excellent. Using the obtained data from software, we investigate the mathematical behavior of axisymmetric supercavitating flow parameters including drag coefficients of supercavitating cones and disks, cavitation number and maximum cavity width for a wide range of cone and disk diameters, cone tip angles and cavity lengths. The main objective of this study is to propose appropriate mathematical functions describing the behavior of these parameters. As a result, among all available functions such as linear, polynomial, logarithmic, power and exponential, only power functions can describe the behavior of mentioned parameters, very well.

Inducer Design to Avoid Cavitation Instabilities

Three inducers were designed to avoid cavitation instabilities. This was accomplished by avoiding the interaction of tip cavity with the leading edge of the next blade. The first one was designed with extremely larger leading edge sweep, the second and third ones were designed with smaller incidence angle by reducing the inlet blade angle or increasing the design flow rate, respectively. The inducer with larger design flow rate has larger outlet blade angle to obtain sufficient pressure rise. The inducer with larger sweep could suppress the cavitation instabilities in higher flow rates more than 95% of design flow coefficient, owing to weaker tip leakage vortex cavity with stronger disturbance by backflow vortices. The inducer with larger outlet blade angle could avoid the cavitation instabilities at higher flow rates, owing to the extension of the tip cavity along the suction surface of the blade. The inducer with smaller inlet blade angle could avoid the cavitation instabilities at higher flow rates, owing to the occurrence of the cavity first in the blade passage and its extension upstream. The cavity shape and suction performance were reasonably simulated by three dimensional CFD computations under the steady cavitating condition, except for the backflow vortex cavity. The difference in the growth of cavity for each inducer is explained from the difference of the pressure distribution on the suction side of the blades.

Rotating Choke and Choked Surge in an Axial Pump Impeller

Unlike usual turbopump inducers, the axial flow pump tested operates very stably at design flow rate without rotating cavitation nor cavitation surge. Flow visualization suggests that this is because the tip cavity smoothly extends into the flow passage without the interaction with the leading edge of the next blade. However, at low flow rate and low cavitation number, choked surge and rotating choke were observed. Their correlation with the performance curve under cavitation is discussed and their instantaneous flow fields are shown.

Cause of Cavitation Instabilities in Three Dimensional Inducer

Alternate blade cavitation, rotating cavitation and cavitation surge in rocket turbopump inducers were simulated by a three dimensional commercial CFD code. In order to clarify the cause of cavitation instabilities, the velocity disturbance caused by cavitation was obtained by subtracting the velocity vector under non-cavitating condition from that under cavitating condition. It was found that there exists a disturbance flow towards the trailing edge of the tip cavity. This flow has an axial flow component towards downstream which reduces the incidence angle to the next blade. It was found that all of the cavitation instabilities start to occur when this flow starts to interact with the leading edge of the next blade. The existence of the disturbance flow was validated by experiments.

Comparison of advantages of three-dimensional and two-dimensional ultrasound in embryo transfer

Objective To compare the advantages of three-dimensional(3D) and two-dimensional(2D) ultrasound in embryo transfer. Methods A total of 319 patients accepted embryo transfter were included in this study. 2D and 3D ultrasound were used to investigate the uterine cavity and transfer distance from the fundus (TDF),respectivly. They were divided into four groups according to TDF difference(D-TDF) between 2D and 3D ultrasound(group of DTDF<3mm,group of DTDF3～5mm,group of 6～9 mm,group of DTDF≥10 mm. Pregnancy outcomes among the four groups were compared. Results Of the 319 patients, 41 were observed to have abnormal uterine cavity. For 140 patients, the TDF measured by 2D ultrasound were different from that measured by 3D ultrasound. Clinical pregnancy rate and implantation rate were found lowest in group of TDF≥10 mm mm (7.7% vs 34.1%,38.1% ,40.0% and 3.6% vs 18.2% ,21.2% ,20.0%, P <0.05). Conclusions 2D ultrasound is limit and deficient for embryo transfer, especially for the visualization of uterine cavity and location of catheter tip, however, it may be better achieved with 3D ultrasound. It is helpful to use the 3D ultrasound to place the catheter tip accurately and improve the pregnancy rate of embryo transfer. Key words: Ultrasonography; Embryo transfer