Groove Tip Research Articles

Analysis of Wear Characteristics of Hydrodynamic Seals During Startup Using 3D Fractal Characterization and Study of Opening Performance Degradation Mechanism

An experimental study was conducted to investigate the degradation mechanism of the opening performance of hydrodynamic seals during multiple startups, and a model was proposed to analyze the wear characteristics during the startup process. The study that the state evolution of the hydrodynamic seal during the single startup process occurs in five stages: low-speed and heavy-load stage, mid-speed and mid-load stage, high-speed and light-load stage, intermittent contact stage, and noncontact stage. The main factor that influences the deterioration of the initial performance of hydrodynamic seals during multiple startups is the decrease in the depth of the root groove. To prevent the heavy deterioration of the opening performance of the hydrodynamic seal, it is important that the remaining depth of the tip of spiral groove (TSG) is greater than 47.67% of the initial value and the dimensionless startup time should not exceed 0.42 for a complete passage through the mid-speed and mid-load stage.

Exploiting graded triangular gratings for optimal nano-focusing: A novel approach to enhance SERS efficiency

Plasmonic graded nano-gratings enable rainbow trapping of multiple resonant modes over a wide wavelength spectrum, useful for multi-channel Surface Enhanced Raman Spectroscopy (SERS) of molecular species. However, rectangular nano-gratings have limitations in achieving efficient rainbow trapping and localizing a wide spectrum of plasmonic modes due to their stepwise geometry, which induces high dissipation of surface plasmon polaritons into the substrate. An alternative platform of graded triangular nano-gratings enables increased localization and more efficient adiabatic transformation between neighboring grooves. Varying groove angles, depths, and periods in the tapered geometry allow for smooth adjustment of the surface plasmon polariton propagation constant, reducing losses and maximizing nano-focusing inside the groove tips. To overcome the limitation of low aspect ratio in wet-etching silicon, we employed a multi-step process of reactive ion etching of a SiO2 barrier layer to generate aperture width, followed by anisotropic wet-etching. The resulting graded triangular nano-gratings showed excellent SERS enhancement along three laser wavelength excitations. The enhancement factors of 638 and 785 nm wavelengths are 8.5 × 109 and 9 × 108, respectively, for the detection of 1 µM Rhodamine 6G. In addition, graded triangular nano-gratings show similar enhancement factors for other species, specifically the lipid DPEE-PEG, at the 532 nm laser excitation wavelength with an excellent SERS enhancement factor of 1.5 × 109. Owing to the ability of the graded triangular gratings to elicit pronounced SERS responses across three distinct laser excitations, they unequivocally qualify as “rainbow trapping” structures. Wider apertures, lower ohmic losses, and the ability to tune the groove angle beyond conventional etching methods bode well for graded triangular gratings as a superior platform for miniature sensors.

The effect of groove and notch tip angles on testing fracture toughness by SEVNB method: models and experimental validation

ABSTRACT The single-edged V-notch beam (SEVNB) method is considered as an effective method for evaluating the fracture toughness values of brittle materials. In this method, it is assumed that the V-notch is a natural crack. However, this assumption may cause an overestimation of the fracture toughness due to the “notch passivation effect”. To investigate the effects of the V-notch and groove tip angles on the fracture toughness testing of ceramic materials, three typical models were established in this work. The stress intensity factors of these models were calculated using a J-integral based on the linear finite element method (LFEM). The results indicated that the measured fracture toughness values could be overestimated by 0.5%- 13.7% when the angle of the V-notch tip increased from 10° to 60°. Increasing the angle formed by the V-notch and groove from 10° to 60°, fracture toughness was overevaluated by about 0% – 2.0%. When the angle formed by the V-notch and groove increased to 120°, the fracture toughness was overevaluated by about 31%. Finally, two equations were fitted to assess the angles effects on fracture toughness, and the results have been validated by experiments. An important reference for the SEVNB method can be found in this work.

Effects of combined sweeping jet actuator and winglet tip on aerodynamic performance in a turbine cascade

Compared with steady injection, the unsteady flow control techniques have received more attention in recent years and are considered to be more hopeful for reducing secondary flow losses in turbine stages. In particular, the sweeping jet actuator (SJA) is gradually being applied to reduce losses in turbomachinery due to its larger sweeping range. The objective of this study is to investigate the effect of combined flow control of SJA and the winglet tip structure on the aerodynamic performance and tip leakage flow of the turbine cascade through experimental and numerical methods. The effect of blade tip geometry on aerodynamic performance enhancement is discussed in detail by comparing flat and groove tips. The effect of injection flow on aerodynamic performance and secondary flow of turbine cascade with different blade tip structures is discussed by comparing SJA and hole-style steady jet actuator (HSJA). Compared with the flat tip without injection case, the combination of the winglet tip and SJA results in a maximum reduction of 20.1% in the total pressure loss at the cascade outlet, which is significantly better than the combination of the winglet tip and HSJA (16.2%). The results of the validated CFD calculations are also reported to help analyze the flow field and secondary flow loss mechanisms.

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

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

Sulfide Stress Cracking (SSC) of Low Alloy Linepipe Steels in Low H<sub>2</sub>S Content Sour Environment

Resistance to Sulfide Stress Cracking (SSC) caused by local hard zones of pipe inner surface has been required in low alloy linepipe steel. In this study, using two samples with different surface hardness, the detailed SSC initiation behavior was clarified by four-point bend (4PB) SSC tests in which immersion time and applied stress were changed in a sour environment containing 0.15 bar hydrogen sulfide (H2S) gas. SSC cracks occurred when the applied stress was higher than 90% actual yield strength (AYS) in higher surface hardness samples over 270 HV0.1. From the fracture surface observation of SSC crack sample, it was found that the mechanism gradually shifted from active path corrosion (APC) to hydrogen embrittlement (HE), and that the influence of APC mechanism remained partially in the process of SSC initiation at the tip of corrosion pit or groove. The polarization measurement in the 4PB SSC test showed that the anodic and cathodic reactions (especially cathodic reactions) were activated when the applied stress was 90% AYS or higher. The FEM coupled analysis simulating the stress and strain concentration at the bottom tip of the corrosion groove and the hydrogen diffusion and accumulation was carried out. The principal stress in the tensile direction showed the maximum value at 0.04-0.06 mm away from the tip of the corrosion groove, and the hydrogen accumulation became the maximum. It was analytically found that the SSC crack initiated and propagated with HE mechanism dominated type when the threshold value of about 0.82 ppm is exceeded.

Influence of a Winglet Combined with a Groove Tip on the Performances of a Variable-Pitch Axial Flow Fan

Taking a two-stage variable-pitch axial fan as the research object, five schemes, including a single counter-flow rib layout grooved tip, are numerically simulated using the fluent software. The results indicate that, compared with the original blade tip, the total pressure rise and efficiency of the four proposed schemes have been improved to various degrees, with Scheme 4 (groove tip with double counterflow ribs) displaying the best performances. The total pressure and efficiency are increased by 113.44 Pa and 0.955%, respectively. The blade tip leakage flow is reduced to varying degrees under different schemes, according to the following order: Scheme 1, Scheme 2, Scheme 4, and Scheme 3 leading to a reduction of 7.44%, 6.46%, 5.36%, and 4.35%, respectively. Steady results are used as the initial condition for the ensuing strength check and modal analysis.

Deformation Simulation in Tube Expansion Processing of Inner Grooved Heat Transfer Tubes

In this paper, we proposed a simulation method based on finite element method utilizing cyclic symmetry model to evaluate the deformation of inner grooved heat transfer tubes during tube expansion. The validity of the simulation method was confirmed by comparing the simulation results and experimental results for the deformation of inner grooved aluminum heat transfer tube due to tube expansion. Using the simulation method, we carried out a case study based on the orthogonal table for design of experiments and clarified the effects of design parameters such as groove depth, groove tip width, number of grooves, and lead angle on the deformation of inner grooved aluminum heat transfer tubes due to tube expansion. This made it possible to optimize the design of the heat transfer tube considering the depth change and tilt angle of the inner grooves caused by tube expansion.

Abrasive jet machining for the microprofile control patterning of herringbone grooves

Fluid bearings have features of high speed and high rotation accuracy, and therefore, they are used in spindle motors of hard disk drives, cooling fans of central processing units, and other devices. Further, these bearings have microherringbone grooves on the shaft or sleeve inner surface that help generate dynamic pressure in the lubricant fluid around the shaft. Although the depth of the groove is constant, dynamic pressure can be increased by decreasing the depth from both ends to the central corner of each groove on a micron scale. This study aims to verify the effect of sloped herringbone grooves using computational fluid dynamics (CFD) analysis and to develop a new microfabrication method for manufacturing microsloped herringbone grooves on the shaft surface using abrasive jet machining. The generated dynamic pressure is analyzed using CFD; the results indicated that the sloped herringbone grooves result in an increase in the dynamic pressure at the groove tips and cause a decrease in the fluctuations in dynamic pressure in the circumferential direction.

Thermal stress cleavage of a single-crystal round sapphire bar by carbon dioxide laser

This study is the first attempt of applying thermal stress cleavage to a single-crystal round sapphire bar having a morphologically continuous circumference surface. Previously, thermal stress cleavage was only applied to brittle material substrates or wafers owing to the crack propagation behaviour induced by thermal stress distribution. To overcome this issue, a V-shaped groove was prepared on the circumference as an initial point for crack propagation, and a continuous-wave carbon dioxide (CO2) laser was used as the heat source. The thermal stress behaviour was simulated via finite element analysis, and the effect of the initial crack morphology on the thermal stress behaviour was evaluated. In addition, the crack propagation was monitored using an acoustic emission sensor, and the separation mechanism of the round sapphire bar was investigated. Results indicated that the thermal stress cleavage using the CO2 laser beam allowed the complete separation of the round sapphire bar with the V-shaped groove without any final edge remaining. The crack propagation behaviour was determined from the relationship between the thermal stress distribution and morphology change during crack propagation. The fabricated sharp V-shaped groove improved the quality of the cleaved surface, and the high aspect ratio of the groove enabled a high-quality cleavage. The initial crack propagation immediately after the start of laser beam irradiation was affected by the tip of the V-shaped groove, and the crack propagation direction could be controlled through the tip morphology of the V-shaped groove.

Study of the Cavitation Phenomenon Inside Roller Vane Pump

The roller vane pumps are widely used in the automobile industry as steering pumps. However, the cavitation damage near the outlet relief groove could directly decrease serve life and efficiency of the roller vane pump. In this paper, the mechanism of the cavitation is discussed, especially the cavitation damage near the tip of the relief groove. The computational fluid dynamics simulation method (CFD) based on dynamic meshing technique and user defined function (UDF) is applied to calculate flow dynamics and multi-phase flow characteristics, especially the flows inside the fluid film between the stator and valve plate. The simulated results show that the cavitation region is the same with the experiment one, and the cavitation inside the fluid film is caused by the shear action of the stator and valve plate. The negative pressure region near the relief groove has an extreme low pressure at 0.08 MPa, and the maximum flow velocity is 99.3 m/s. Besides, the cavitation region near the relief groove has the highest gas volume fraction of 2.23% and the lowest density of 560 kg/m3. Furthermore, the reason why cavitation happened at this position in a roller vane pump is analyzed by theoretical analysis.

Influence of displacement constraints to the surface reconstruction of stressed bicrystal thin films

In this work, surface morphology evolution of bicrystal thin films under the combined action of grain boundary and surface diffusion is investigated by considering different mechanical constraints. 2D surface topographies of thin films, that are (a) freestanding, (b) strongly bonded to its substrate and (c) strongly bonded to its substrate and one of sidewalls, are simulated using a numerical implementation of an irreversible thermo-kinetics model. Relationships which give the groove depth as a function of time are obtained. Results show that mechanical loading conditions are effective in determining the morphology and kinetics of grooving. For the three scenarios that had been investigated, it was found that the groove depth evolves linearly with different tip velocities under the same level of uniaxial tension. In freestanding films groove tip evolves faster; i.e. as the film gets constrained from its substrate and/or one of its sidewalls, the tip velocity slows down. It was also observed that high triple junction mobilities at low levels of applied stress hinder the effects of displacement constraints to groove shape, even in the case of asymmetric stress distributions inside the film. On the other hand, low triple junction mobilities at moderate applied stresses allow formation of asymmetric grain boundary grooves due to the induced asymmetry in the driving force for surface diffusion with respect to the grain boundary.

Uncertainty quantification of the effects of squealer tip geometry deviation on aerothermal performance

The first-stage rotor squealer tip is a key area in gas turbine for both aerodynamic performance and blade cooling tasks, which should be carefully designed. However, harsh operating conditions near the rotor squealer tip can cause the geometry of the squealer tip to degrade, and manufacturing inaccuracies can also cause the squealer tip geometry to deviate from the ideal design. These geometry deviations would change the flow field near the blade tip, which will influence the thermal and aerodynamic performance. Thus, it is important to quantitatively investigate the effects of squealer tip geometry deviation on the aerothermal performance. In this paper, a typical transonic first-stage turbine is employed, and three important geometric features of squealer tip, the tip clearance height (H), the squealer depth (D), and the squealer edge chamfer (R), are selected. An uncertainty quantification process is performed to study the effect of deviation of H, D, and R on the aerothermal performance. Many cases with different geometry features are checked in the current study using 3D Reynolds-averaged Navier–Stokes simulations. A parameter sensitivity analysis using Sobol Indice method is carried out to identify the key parameters to aerothermal performance of the squealer tip. The uncertainty quantification results show that the existence of the tip chamfer reduces the size of separation bubble and the dwelling range of the scraping vortex, thus, the blockage effect of the leakage flow is weakened, which results in larger amount of leakage flow and more mixing loss of squealer tips with edge chamfer than those without edge chamfer. The results of the parameter sensitivity analysis show that the height of tip clearance is the main factor that affects the aerodynamic performance of the squealer tip. This work provides a certain guiding direction for the optimization design of the turbine groove tip.

Effect of Casing Movement on Flow and Heat Transfer Characteristics of Grooved Blade Tip

Using numerical simulation method, the influence of casing movement on the flow and heat transfer characteristics in the groove tip has analysed, and the influence of rotation speed and blowing ratio was also considered. The results show that the casing movement reduces the casing temperature at the top of the rib and the Nu peak number at the bottom of the groove, it also reduces the mixing loss from the interaction of coolant and leakage flow. The higher the casing motion speed, the lower the average Nu on the wall surface and the smaller the mixing loss in the gap. Large blowing ratio cooling jet can effectively improve wall heat transfer.

Study on the Cracking Time in Low-stress Precision Cropping Based on Acoustic Emission Technology

The high-speed centrifuge and low-stress precision cropping is the technology which makes use of the stress concentration effect of the prefabricated V-shaped groove on the surface of the metal bar and relies on the expansion of the crack to achieve precision cropping. This paper applies the acoustic emission testing technology to monitor the entire process of low-stress precision cropping in order to solve the problem that the cracking time of V-shaped groove tip cannot be precisely determined during the low-stress cropping. It is concluded that the acoustic emission signal can be used to describe the crack damage process of V-shaped groove tip, providing better cross-section quality. Based on this, the cracking time of V-shaped groove tip is further determined, which better meets the requirements of low-stress precision cropping.

Flow measurements near the open surfaces of single circumferential grooves in a low-speed axial compressor

Circumferential grooves (CG) is an effective method to enhance the compressor stall margin with minor efficiency disturbances. The interaction between the groove and the blade passage is found to be responsible for the stall margin improvement (SMI) by many studies. The momentum transport across open surface of an individual groove could be an indicator to assess its effectiveness. These findings are all derived by numerical means. Two single grooves are chosen as the examples to demonstrate the needs of the experimental validation. In experiments, one groove, CG5 at 27% axial chord, produced less SMI than the other one, CG7 at 43% axial chord. Yet, many CFD simulations predicted just the opposite. In order to validate the numerical simulations, detailed flow structures are needed near the open surfaces of the casing grooves. However, such experimental results are still missing due to the difficulties of measurement, especially for the flow measurement inside the grooves. Aiming at providing reasonable experimental results for these casing grooves, a measuring technique is proposed in this paper. A special algorithm is utilized to obtain the time-averaged magnitudes of velocity components by using three hot-wire probes. The groove–passage interaction is recorded using the fluctuations of each velocity component and the frequency spectrum. Intensive interaction is detected for CG7. The experimental results demonstrate that the flow interaction between the groove and blade tip is crucial to the stability enhancement.

A numerical analysis of casing groove parameters on the performance of wave energy conversion device

Casing grooves in Wells turbine can enhance its performance. The present computational work has been carried out to understand the effect of casing-groove's width-to-chord (w/C) ratio on the performance of a wave energy conversion device. In this study, four different values of w/C ratio such as 0.24, 0.48, 0.64 and 0.96 were investigated. The circumferential casing groove depth (3% of the chord length) and tip clearance (1% of the chord length) were kept constant for all the cases. The flow field was analysed through discretisation of three-dimensional steady incompressible Reynolds-averaged Navier-stokes (RANS) equations with two-equation eddy viscosity turbulence closer model. The simulation results show that the w/C ratio has a significant impact on the turbine performance and the w/C = 0.64 was found to be the optimal one among the several values investigated in the present study.

Repairing the surface grooves of St37 structural steel using flame spray welding

In today’s industry, preventive measures for the preservation and repair of devices and equipment are extremely important. Various welding methods are used for repairing the surface crack of metal pieces in order to improve their mechanical properties and increase their useful life. In the present study, the performance of flame spray welding using pure iron powder is evaluated in repairing the surface grooves of St37 structural steel. First, five groups of specimen (one control group and four grooved steel groups with 0.5 mm groove tip, 10 mm length and 0.5, 0.8, 1 and 1.3 mm groove depths) were prepared. Then, the repair was performed. The results showed that after repairing the surface groove via flame spray welding using pure iron powder, the tensile strength and yield strength of the repaired specimens with the groove depths of 0.5 and 0.8 mm were achieved to control ones, but the elongation was decreased, which was still acceptable for engineering applications (module of toughness of 74.2 MJ) for the repaired specimen with the groove depth of 0.5 mm.

Direct numerical simulation of turbulent flow over wide‐rib rectangular grooves

The turbulent flow over the wide‐rib rectangular grooves, the motions and variations of near‐wall streamwise vortices with time, and the interaction between microgroove and near‐wall streamwise vortices were investigated by direct numerical simulation method (DNS). The distributions of radius, density, and swirling strength of streamwise vortex were also studied quantitatively by using swirling‐strength criterion. It was found that the distribution of vortex radius in smooth channels can approximately be divided into three parts. The vortex radii are smaller in grooved channels than in smooth channels and almost the same when y+ > 40 for all grooved cases. Moreover, a simple prediction method was proposed to estimate the optimal height and spacing of drag‐reducing microgrooves for different fluids, and they were about 10 and 17 wall units for water, respectively. Furthermore, using the same frictional velocity uτ to normalize the shear stress is more suitable for the quantitative comparison and analysis of different longitudinal microgrooves. The drag‐reducing mechanism of longitudinal microgrooves could be considered as the competition results between the “restriction or blockage effect” of microgroove on the near‐wall vortices (causing a drag‐reducing effect) and the “tip effect” of microgrooves caused by the scouring of higher speed fluid near the groove tip (causing a drag‐increasing effect). A large number of small streamwise secondary vortices with small swirling strength within the groove valley, which are induced by microgrooves, may be the essential reason of drag reduction by microgrooves.

Effect of blade tip pattern on performance of a twin-stage variable-pitch axial fan

The effect of five blade tip patterns on performance and acoustics of a twin-stage variable-pitch axial fan is investigated numerically. The best tip pattern is determined by comparing the overall performance under the condition of those patterns carving in stage I impeller, and the performance promotion is then examined under the selected tip grooving in stage I impeller, stage II impeller and both two stage impellers. Finally, flow dynamics and acoustic noise are explored. The results show that all proposed tip patterns can improve fan performance effectively, and the best scenario is the direct groove tip implementing in stage I impeller. After blade tip grooving, the internal dynamics in the tip clearance tends to be more complex, the static pressure reduces notably in the suction side and the sound power level in the tip increases significantly, while the leakage losses decline distinctly. Blade tip pattern changes the profile and amplitude of the sound pressure in time domain; the base frequency and first five−order harmonics are clearly observed in two stage impellers, but only the second−order harmonics are emerged in the other regions. The noise is generated largely by low and medium frequency sound with an obvious broadband feature.