Dimple Structure Research Articles

Research on Wear Resistance of AISI 9310 Steel with Micro-Laser Shock Peening

Improving the wear resistance of turbine engine drive components is crucial. This study presented a new Laser Shock Peening (LSP) technique: Micro-Laser Shock Peening (Micro-LSP) technology for surface modification and strengthening of AISI 9310 steel. The effects of different pulse energies (50 mJ, 150 mJ, 200 mJ) on surface morphology, mechanical properties, and wear behavior were investigated. The results showed that the Micro-LSP treatment reduced the wear rate by 56% to 74%. The dimpled structure induced during the strengthening process increased the surface roughness and reduced the contact area; moreover, the coefficient of friction (COF) was reduced. The treatment also had the effect of reducing the wear rate by collecting abrasive debris and changing some of the sliding wear into rolling wear. The reduced wear rate was a result of the combined effect of the dimpled structure and the hardened layer. In addition, a deeper hardened layer also slows down the onset of wear behavior. Micro-LSP technology offers completely new methods and possibilities for wear reduction.

Fabrication of oxide-free dimple structure on germanium via electrochemical jet machining enhanced by opposing laser irradiation

The electrochemical machining (ECM) of germanium (Ge) wafer can be significantly improved in terms of efficiency and localization by introducing laser irradiation. However, residual oxide particles usually exist when using neutral electrolyte. In this study, the NaNO3 electrolyte jet is formed via a cathodic nozzle and impacts perpendicularly on the bottom of the Ge wafer, at which position the nanosecond (ns) laser irradiates from opposing side to locally and precisely enhance the electrical conductivity. In this way, the dimple structure without any detectable oxidation products can be efficiently fabricated. Characterization of surface morphology is then amply conducted. The radial lines consisting of regular micro-waving structure are usually noticeable, especially near the dimple edge, which may be attributed to the rapid and radial electrolyte flow along the concave sidewall. In contrast, irregular corrugated pattern can be observed near the dimple center, which may be caused by the electrolyte turbulence after impingement. In addition, parametric experimental study has been carried out, and the effects of the applied voltage, laser power and electrolyte concentration on the dimple profile and surface characteristics have been respectively detailed discussed. Finally, simulation study of the electrolyte flow field is conducted to reveal the distribution regularities of velocity and pressure, to deepen the understanding of formation mechanism of the complex surface morphology.

Analysis of Discharge Caused by Soldering Defect in Wire-Bonding Power Module

Partial discharge (PD) assessment, widely applied in HV IGBT (insulated gate bipolar transistor) module, is not generally adopted for modules at low-to-medium voltage levels. In this article, a type of PD defect induced by the cavity in the solder layer of 1700-V IGBT modules. Using a zonal PD test method and the scanning acoustic microscope (SAM) analysis, we characterized the cavity defects in the solder layer and proposed a hypothesis for the underlying mechanism of the cavity formation. The hypothesis was then proved by adding two pretreatments during the attaching period of the substrates, which effectively suppressed the cavity defects. This suggests a method to settle the cavity defect by a half-etched dimple structure, which can improve the product pass rate during PD assessment.

Numerical study on heat transfer enhancement of a proton exchange membrane fuel cell with the dimpled cooling channel

As the power density of a proton exchange membrane fuel cell (PEMFC) increases, the problems of internal heat accumulation and non-uniform temperature distribution are becoming significant. In this paper, a novel cooling channel with dimple structures is designed and a three-dimensional PEMFC numerical model is established. When comparing to the conventional channels, the heat transfer performance of dimpled channel is 10% higher than the smooth one, and the pressure loss is almost 13% lower than that of wavy channel. In addition, the optimization of dimple structure parameters is investigated based on the index of uniformity temperature (IUT) and performance evaluation criteria (PEC) of heat transfer. It is found that a diameter-to-depth ratio of 4 is recommended when the dimple diameter is less than 0.80 mm. Furthermore, the clock-wise vortex observed inside the dimple is considered to be the main reason affecting heat exchange. This study will contribute to the design of cooling channels for high-power density PEMFCs in the future.

Study of dimple restraining heat transfer deterioration of supercritical methane under high heat flux in mini channel

To attenuate the heat transfer deterioration of supercritical methane under high heat flux in mini channel, dimple structure is employed and studied numerically. Flow friction factor (f), heat transfer coefficient (HTC), Nusselt number (Nu) and performance evaluation criteria (PEC) are adopted to compare the heat transfer and flow characteristics between dimple and smooth channel. Besides, local velocity, turbulence kinetic energy (TKE), temperature and HTC are discussed. The results indicate that higher heat flux could deteriorate the heat transfer of supercritical methane resulted from the buoyancy and flow acceleration effect. While dimple enhances tremendously the heat transfer performance under high heat flux with enhanced factor up to 2.2, and there exists an optimal inlet temperature for the enhanced factor. On the other hand, the friction factor is worsened. However, PEC is positive up to 1.9 increased by the heat-mass ratio and mass flux. For enhanced mechanism, dimple induces vortex and raises the TKE, which strengthens the heat transfer between wall and bulk fluid.

Numerical and experimental study on power extraction performance of a semi-active flapping airfoil with bioinspired dimple

Inspired from the special structure of dragonfly wings (cambered and corrugated groove), in this work, the bioinspired dimple structure is introduced into the design of the airfoil to improve the power extraction performance of a semi-active flapping airfoil, and the characteristic parameters of the bioinspired dimple structure are optimized by the combination of numerical analysis and Taguchi method. The results show that the bioinspired dimple can effectively improve the performance of the flapping airfoil. The average power coefficient and power extraction efficiency of the optimized dimple flapping airfoil are 31.73% and 31.23% higher than that of the baseline smooth airfoil respectively, and the influence of the three considered characteristic parameters on the power extraction performance is ranked as: lx (the location of bioinspired dimple) >ly (the depth of bioinspired dimple)>w (the inlet width of bioinspired dimple). Further analysis the flow field around the flapping airfoil shows that the bioinspired dimple can promote the separation of the leading edge vortex from the airfoil surface during stroke reversal, meanwhile at the process of upstroke and downstroke, the bioinspired dimple can also delay the leading edge vortex to separate, which is the reason for the dimple airfoil extract more power from the fluid. Based on the numerical results, experiments test are also carried out to evaluate the power extraction performance of the optimized bioinspired dimple airfoil, and the results show that at the same experiment condition, more 12.81% average power coefficient is extracted by the optimized dimple flapping airfoil, which again verifies the effectiveness of the numerical analysis conclusion obtained in this work.

Computational Fluid Dynamics and Experimental Analysis of a Wind Turbine Blade’s Frontal Section with and without Arrays of Dimpled Structures

Horizontal axis wind turbines are used for energy generation at domestic as well as industrial levels. In the wind turbines, a reduction in drag force and an increase in lift force are desired to increase the energy efficiency. In this research work, computational fluid dynamics (CFD) analysis has been performed on a turbine blade’s frontal section with an NACA S814 profile. The drag force has been reduced by introducing an array of dimpled structures at the blade surface. The dimpled structures generate a turbulent boundary layer flow on its surface that reduces the drag force and modifies the lift force because it has greater momentum than the laminar flow. The simulation results are verified by the experimental results performed in a wind tunnel and are in close harmony with the simulated results. For accurate results, CFD is performed on the blade’s frontal section at the angle of attack (AOA) with a domain of 0° to 80° and at multiple Reynolds numbers. The local attributes, lift force, drag force and pressure coefficient are numerically computed by using the three models on Ansys fluent: the Spalart-Allmaras, the k-epsilon (RNG) and the k-omega shear stress transport (SST).

Asymmetric Jetting during the Impact of Liquid Drops on Superhydrophobic Concave Surfaces.

The impact of liquid drops on superhydrophobic solid surfaces is ubiquitous and of practical importance in many industrial processes. Here, we study the impingement of droplets on superhydrophobic surfaces with a macroscopic dimple structure, during which the droplet exhibits asymmetric jetting. Systematic experimental investigations and numerical simulations provide insight into the dynamics and underlying mechanisms of the observed phenomenon. The observation is a result of the interaction between the spreading droplet and the dimple. An upward internal flow is induced by the dimple, which is then superimposed on the horizontal flow inside the spreading droplet. As such, an inclined jet is issued asymmetrically into the air. This work would be conducive to the development of an open-space microfluidic platform for droplet manipulation and generation.

Responsive friction modulation of 3D printed elastomeric bioinspired structures

Elastomeric bioinspired structures with desirable friction performances are attractive for engineering applications. However, friction-tuning mechanisms through hybrid designs and shape fabrication techniques remain challenging. Herein, we developed a scalable elastomeric bioinspired structure using 3D printing inspired by claws, toe pads and overhang granules found in nature. Friction modulations are demonstrated with pillar and dimple hybrid structures in dry and wet conditions, including different structure arrangements, mechanical properties (strength ~13.8 MPa, elongation ~410%) and numerical simulations. We identified pillar hybrid design has strong friction features (~2.5 times higher than smooth) on wet surfaces, by evaluating through strong edge boundary and spontaneous liquid splitting effects. Pillar structures are used to mimic grip experiments and simulations to determine gripping strategies.

Combination of Scanning Strategies and Optimization Experiments for Laser Beam Powder Bed Fusion of Ti-6Al-4V Titanium Alloys

This paper studies the effects of different combinations of scanning strategies between layers on the surface quality, tensile properties, and microstructure of samples in a laser beam powder bed fusion (L-PBF) formation experiment of Ti-6Al-4V titanium alloy. The purpose of this experiment was to improve the comprehensive performance of the piece by selecting the optimal combination of scanning strategies. The results show that the surface roughness of the L-PBF specimen was the lowest under the combination of the CHESS scanning strategy, reaching 14 μm. The surface hardness of the samples was generally higher with the LINE scanning strategy and the angle offset of 90°, reaching 409 HV. The overall density of the samples was higher under the combination of CHESS scanning strategies, reaching 99.88%. Among them, the CHESS&45° sample had the best comprehensive properties, with a density of 99.85%, a tensile strength of up to 1125 MPa, a yield strength of 912 MPa, and an elongation of 8.2%. The fractured form was a ductile fracture, with many dimple structures. Compared with the CHESS scanning strategy, the tensile properties of the CHESS&45° samples were improved by 12.8%. The microstructure of the L-PBF sample was mainly composed of the primary β phase and α’ martensite phase. The upper surface of the CHESS scanning strategy combination sample had a clear melt channel, and the distribution of each phase was uniform. A certain number of columnar β crystals were distributed in the longitudinal section of the sample, which was paralleled to the build direction. The columnar β crystals of CHESS&45° were relatively coarse, which enhanced the tensile properties of the sample.

Numerical study on flow and heat transfer characteristics of swirling jet on a dimpled surface with effusion holes at turbine blade leading edge

• The combined effect of the jet offset ( e/d ) and dimpled target surface on heat transfer of swirling flow are investigated. • Spherical dimples (SDs) and oval-trench dimples (OTDs) in an in-line and staggered arrangement are considered. • The thermal-hydraulic performance of the target chamber demonstrates a maximum improvement of 24%. • Under the same e/d , the OTD structure is superior to the SD structure. In this paper, a numerical study has been conducted on the effect of the offset of jet holes on the flow structure and heat transfer of swirling flow in a concave target chamber with various dimple structures and effusion holes at the turbine blade leading edge. The ratio of jet holes off the centerline distance to jet hole diameter (e/d) varies from 0 to 2.0, and the ratio of jet-to-target plate spacing to jet diameter (H/d) is 4. The effusion hole diameters of 0.5d are arranged in a staggered pattern relative to the jet holes. Four types of dimple structure with a large depth ratio (δ/D) of 0.25, including spherical dimples (SDs) and oval-trench dimples (OTDs) in an in-line and staggered arrangement, are considered. The heat transfer characteristic and pressure loss of the different leading-edge are evaluated and compared at a Reynolds number of 30,000 based on the jet hole diameter. The results show that the offset of the jet holes provides 15%-higher overall heat transfer performance and more uniform heat transfer of the target surface, while having little impact on the friction factor within the e/d range of 0–2.0. The introduction of the dimple structures on the target surface decreases the overall averaged Nusselt number but enhances the total heat transfer rate from the target chamber due to the clear increase of heat transfer areas, for which a maximum of a 24% improvement of the thermal-hydraulic performance is obtained for e/d = 2.0 with OTDs in a staggered arrangement. Under the same e/d, the OTD structure, especially with the staggered arrangement, is superior to the SD structure.

Mechanical reinforcement of UV-curable polymer nanocomposite for nanopatterned mold

This study proposed a method for manufacturing nanopatterned molds with high strength and durability using UV-curable polymer nanocomposites. Zirconia (ZrO2) nanoparticles (NPs) were blended with Ormostamp, a commercial UV-curable resin, using the solution blending method. The elastic modulus and hardness of the cured nanocomposite layer were investigated through a nanoindentation test for different volume ratios of NPs in the Ormostamp composite from 0 to 53.8%. The modulus and hardness of pure resin were 0.075 ± 0.002 GPa and 9.7 ± 0.164 MPa, respectively, and they increased to 11.9 GPa and 146.9 MPa at an NP concentration of 58.3%. Nanocomposite molds were fabricated by nanoimprinting with a PDMS stamp, which is replicated from the nanodimple array master. The fabricated nanocomposite mold can efficiently replicate other UV-curable resins. Hot embossing was then performed for a PMMA film using the fabricated molds. Unlike a pure UV-curable resin, which showed damage and fracture of the dimple structure after the first replication, the nanocomposites maintained the nanodimple shape even up to 30 replications. Uniform nanopatterns can be formed on even cylindrical surfaces by using a soft replica stamp. Polymer nanocomposites with a high NP concentration exhibited significantly improved mechanical reinforcement when compared to conventional pure polymers for mold making. Therefore, they are expected to be extremely useful for manufacturing nanopatterned molds using UV resin.

Investigation of Optimum Milling Parameters in Micro Dimples Machining

This research used a carbide ball nose end mill cutting tool to fabricate a micro dimple on the aluminium block. The problem is that the micro dimple should be fabricated with a better surface finish which is related to surface integrity and form accuracy of the dimple. The objective is to study the optimum CNC milling parameters for the micro dimple fabrication and analyse the surface integrity and form accuracy of the dimple. Based on three different parameters which is depth of cut, feed rate, and spindle speed, the better surface roughness and the diameter, depth of micro dimple are experimented using the carbide cutting tools to fabricate micro dimple and analyse the data using 3D confocal microscope. The better surface roughness, high form accuracy and surface integrity of micro dimple structure were achieved with the optimum CNC milling parameters. The high spindle speed, low depth of cut and low feed rate showed the best results for the surface roughness and form accuracy. The proposed parameters were high spindle speed of 3600 RPM, low depth of cut 0.25mm and low feed rate of 50mm/min.

Condensation heat transfer in annuli outside horizontal stainless steel enhanced tubes

The condensation heat transfer of R410A outside several enhanced heat transfer (EHT) tubes [dimple/herringbone (DIM/HB), herringbone (HB), and dimple (DIM)] and smooth tube were evaluated. All tubes are made of stainless steel; the test length is 2 m; the outer diameter is 12.7 mm; the inner diameter is 11.5 mm. Experimental conditions were set at a saturated temperature of 45 °C; condensed vapor mass decreased from 0.8 to 0.2; mass flux ranged between 75 and 225 kg m−2 s−1. As a result, the HB tube effectively improved flow condensation performance outside tubes; while the dimple-only structure is not effective since it produces a lower heat transfer coefficient (HTC) than a smooth tube (at some low flow rate conditions). Substantial enhancement is produced by the DIM/HB composite surface produced from dimple and herringbone microfin patterns, which is due to the combination of the liquid drainage effects by herringbone structure to remove condensate and causing strong turbulence of dimple structure. With the increase of mass flux and average vapor quality, the condensation heat transfer coefficient and pressure drop of each test tube increase as well. Additionally, an improved Nusselt equation for external condensation on a smooth tube was developed to predict the heat transfer coefficient for the experimental conditions considered here. Finally, correlations of enhanced heat transfer (EHT) tubes (microfin – HB, dimple - DIM, and composite surface – DIM/HB) to predict condensation heat transfer coefficient were developed and evaluated.

A Numerical Study on Heat Transfer Enhancement and Fluid Flow of Enhanced Tube With Ellipsoidal Dimples and Protrusions

Abstract Heat transfer enhancement is an important thermal–hydraulic phenomenon in heat exchangers. Previous investigations mostly focused on unidirectional heat transfer enhancement inside the tube. In order to realize the bidirectional heat transfer enhancement inside and outside the tube, an enhanced tube with ellipsoidal dimples and protrusions is proposed. Through comparison among three computational conditions, the non-conjugate and periodic condition is adopted in this paper. Flow characteristics and heat transfer performance of the enhanced tube are investigated by numerical method. The distributions of temperature and flow fields are carried out to illustrate the heat transfer enhancement mechanisms. The Nusselt number, friction coefficient, and performance evaluation criteria (PEC) are presented to reveal thermal–hydraulic performance. Moreover, the effects of the geometric parameters such as dimple depth, dimple pitch, dimples/protrusions numbers in the circumference, and arrangement mode on thermal–hydraulic characteristics are investigated. The results show that the structure of dimples and protrusions induces intensive impingement and interrupts the boundary layers, improving the heat transfer for the internal and external flow, respectively. Besides, the Nu increases with depth and dimples/protrusions numbers in the circumference and decreases with the pitch in most cases. The overall heat transfer performance of staggered arrangement is better than that of aligned arrangement. For the present parameter conditions, when D = 2 mm, P = 25 mm, n = 6, and Re = 5000, the largest PEC of 1.40 and 1.42 occurs for the internal and external flow, respectively.

EFFECT OF MULTIPLE FACTORS ON FLOW AND HEAT TRANSFER OF SUPERCRITICAL METHANE IN A MINICHANNEL WITH DIMPLED STRUCTURE

EFFECT OF MULTIPLE FACTORS ON FLOW AND HEAT TRANSFER OF SUPERCRITICAL METHANE IN A MINICHANNEL WITH DIMPLED STRUCTURE

Effect of welding processes on mechanical and metallurgical characteristics of carbon steel cylindrical components made by wire arc additive manufacturing (WAAM) technique

In recent decades, Additive Manufacturing (AM) has become a viable alternative to the manufacture of metal parts. Wire arc additive manufacturing (WAAM), a welding-based AM technique is an important research area since it permits the economical manufacture of large-scale parts with relatively high deposition rates. This article compares the effect of heat input on mechanical properties of carbon steel cylindrical components fabricated by Gas Metal Arc Welding (GMAW) and Cold Metal Transferred Arc Welding (CMTAW) processes. Firstly, the influence of heat input on the grain size was analysed. Subsequently, the effect of heat input on the tensile properties, impact toughness and hardness of the cylindrical components were studied along the building direction. The cylindrical component made by CMTAW process showed superior tensile properties and higher impact toughness than GMAW component. Similarly, CMTAW component exhibited higher hardness than GMAW cylindrical component. The variations in mechanical properties are mainly due to the appreciable variations that have occurred in the microstructural features and different grain sizes evolved at different heat input levels. The bottom and top regions of the fractured tensile and impact specimens of the components are characterised by dimple structures revealing the ductile fracture.

Evolution of Microstructure and Mechanical Properties of LM25-HEA Composite Processed through Stir Casting with a Bottom Pouring System.

Aluminum matrix composites reinforced by CoCrFeMnNi high entropy alloy (HEA) particulates were fabricated using the stir casting process. The as-cast specimens were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and transmission electron microscopy (TEM). The results indicated that flake-like silicon particles and HEA particles were distributed uniformly in the aluminum matrix. TEM micrographs revealed the presence of both the matrix and reinforcement phases, and no intermetallic phases were formed at the interface of the matrix and reinforcement phases. The mechanical properties of hardness and tensile strength increased with an increase in the HEA content. The Al 6063–5 wt.% HEA composite had a ultimate tensile strength (UTS) of approximately 197 MPa with a reasonable ductility (around 4.05%). The LM25–5 wt.% HEA composite had a UTS of approximately 195 Mpa. However, the percent elongation decreased to roughly 3.80%. When the reinforcement content increased to 10 wt.% in the LM25 composite, the UTS reached 210 MPpa, and the elongation was confined to roughly 3.40%. The fracture morphology changed from dimple structures to cleavage planes on the fracture surface with HEA weight percentage enhancement. The LM25 alloy reinforced with HEA particles showed enhanced mechanical strength without a significant loss of ductility; this composite may find application in marine and ship building industries.

Methodology for the assessment of cleanability and geometry optimization using flow simulation on the example of dimple-structured pipe surfaces

The hygienic design of machinery, e.g. in the food and pharmaceutical industries, has so far mainly been based on empirical guidelines. Recently developed, efficient models for the numerical simulation of cleaning processes promise a real optimization of the geometry with respect to cleaning. To realize this potential, an approach is provided, which takes into account the variety of cleaning tasks and allows for a generalized evaluation of the cleanability. It is demonstrated on the example of film-like soils in dimple-structured pipes. In this approach, the cleaning behaviour is investigated qualitatively as a function of the relevant cleaning mechanisms and relative to the reference case of a smooth pipe. The overall cleaning performance is calculated from a weighted average of the individual performances. The flow simulations in ANSYS fluent are based on the Reynolds averaged Navier–Stokes equations. The soil behaviour is modelled as a boundary condition to minimize calculation times. Only in the case of viscous shifting, a transient two-phase flow of immiscible fluids is simulated. Three different dimple structures are investigated, where the spherical dimple with sharp edges yields the best results. Through the subsequent optimization of its shape, the cleaning performance even exceeds the value of the smooth pipe.

Numerical study on heat transfer characteristics of swirling flow on dimpled surfaces with effusion holes at turbine blade leading edge

In this paper, we conducted a numerical study to investigate the effect of the offset of the jet holes on heat transfer of swirling flow in a concave target chamber with various dimple structures and effusion holes at the turbine blade leading edge. The distance of the jet holes off the centerline e/d varies from 0 to 2.0. Four types of dimple structure, including spherical dimples (SDs) and oval-trench dimples (OTDs) in the inline and staggered arrangement, are considered. The heat transfer performance of the different leading-edge, impingement-effusion cooling structures is evaluated and compared at a Reynolds number of 30,000 based on the jet hole diameter. Results show that the offset of the jet holes provides 15% higher overall heat transfer performance and more uniform heat transfer of the target surface within the e/d range of 0-2.0. The introduction of the dimple structures on the target surface slightly decreases the overall averaged Nusselt number but enhance the heat transfer quantity due to the clear increase of heat transfer areas. Under the same e/d, the OTD structure, especially with the staggered arrangement, is superior to SD structure.