Dimple Arrangement Research Articles

Design and Analysis of Dimple Arrangement on a Small Wind Turbine Blade

This article predominantly focuses on the performance estimation of a small wind turbine blade when a dimple arrangement is made along its upper surface. The dimple arrangement is grooved at two locations: 0.25c and 0.5c, where c is the chord length of the turbine blade. A CFD analysis using the k-ε turbulence model is carried out on the selected blade sections NREL S823 and S822. The continuity and momentum equations are solved using ANSYS Fluent Solver to assess the aerodynamic performance of the proposed design. The effect of introducing a dimple on the blade surface has shown to delay the flow separation, with the formation of vortices. Further, the overall performance of the blade is simulated using GH BLADED and the results acquired are discussed.

Investigation of flow structure and heat transfer enhancement in rectangular channels with dimples and protrusions using large eddy simulation

Large-eddy simulations are used to investigate flow structures and heat-transfer enhancements in rectangular channels with dimples and protrusions as Reynolds number varies from 5600 to 22,000. Roughness elements adopt sharp edges and are arranged inline, the ratios of their depths or heights to their respective print diameters are 0.1, and the ratio of channel height to dimple print diameter is 1.0. Comparisons are carried out between effects of Reynolds number and the gap between roughness elements, and the data presented include thermal performances, flow structures, local Nusselt numbers, local spatial correlation coefficients, characteristic frequencies, and turbulent kinetic energy. Results show that thermal performances can be increased considerably by either increasing Reynolds number or decreasing the gap ratio, but the type of flow structure is only related to Reynolds number. And inline arrangements for dimples and protrusions in many ways are equivalent to deepening the depth of the dimple, especially along the centerline of the arrangement, such that they can achieve higher thermal performances than a denser arrangement of dimples with a corresponding depth ratio. Meanwhile, sampling grids are deployed in the spacings between roughness elements to obtain local spatial correlation coefficients and characteristic frequencies. Frequent passing of vortical structures from roughness elements results in local minima of spatial correlation coefficients and protruding characteristic frequencies, and results show that contours of peak Nusselt numbers are in good correlation with them and thus directly resulting from these vortical structures and their enhancing effects. And it is found that the asymmetry in Nusselt number contours in the wake of the dimple is resulted from the combined influences of sizes of primary passages for vortical structures from dimples, shedding frequencies, and the turbulent kinetic energy carried by these vortical structures.

Multiphase Eulerian–Lagrangian LES of particulate fouling on structured heat transfer surfaces

In this study, multiphase Eulerian–Lagrangian large-eddy simulation (LES) is used to analyze the interaction between local flow structures, convective heat transfer and particle depositions (fouling) for a turbulent channel flow with a dimpled surface. Thus, eddy-resolving LES is applied for calculation of the turbulent working fluid (carrier flow), combined with an efficient Lagrangian particle tracking (LPT) algorithm, suitable to predict the trajectories of the suspended foulant particles precisely in time and space. In order to decrease the computational effort and to enhance the applicability of the proposed approach, deposited particles are deactivated and converted into a second continuous phase (fouling layer), which is modeled as a porous medium and accounts for the hydraulic losses as well as for the additional thermal resistance due to the fouling deposits. Prior to the fouling simulations, the envisaged method is validated for a turbulent particle-laden channel flow at Reτ=150, based on DNS results available in the literature, which shows the capability of the Eulerian–Lagriangian LES of capturing the flow physics within turbulent particle-laden, wall-bounded flows. Based on this observations, fouling simulations were carried out for a turbulent particle-laden channel flow over a staggered arrangement of sharp-edged spherical dimples, whereby two different dimple depth-to-diameter ratios are considered (d/D=0.26 and 0.35). The thermo-hydraulic performance is analyzed for the clean and contaminated dimpled surfaces.

Turbulent flow acceleration and abnormal intensification of the separated flow in a channel with dense arrangement of inclined single-row oval-trench dimples

On the basis of the Reynolds-averaged Navier-Stokes equations closed by the shear stress transport model with the consideration of streamline curvature, turbulent flow acceleration in the channel with inclined single-row oval-trench dimples, earlier found in the laminar regime, has been revealed. It is shown that the velocity in the flow core increases up to 1.4 times when dimples are densely packed. The influence of the dense arrangement of dimples on abnormal intensification of the separated flow in the stabilized area of the channel has been assessed. Absolute friction in the backflow zone in the dimple is found to grow up to 5.5 times compared with friction at the smooth channel wall.

The analysis of the dimple arrangement of the artificial hip joint to the performance of lubrication

Artificial hip joint surgery is one of the most successful methods used to restore the functioning of damaged hip bones. But there are obstacles to the use of artificial hip bone, which is the amount of friction occurring and wears. To overcome these obstacles, a surface of the artificial hip joint is modified by adding dimples in order to minimize the contact pressure of solid and to reduce friction and wear. The purpose of this study is to determine a better of lubrication performance with the variations of the dimple arrangements under the normal walking condition. Simulation results have already exited the point of convergence studies, and the obtained results are such as hydrodynamic pressure, contact pressure, and the film thickness of the lubricant with the variations of the number and pitch dimples. The results of the CSM method under dry condition, it shows that the addition of surface with dimples has a positive effect to reduce the contact pressure and indirectly reduce wear. The maximum surface contact pressure is 54.84 MPa with dimple and 94.22 MPa without a dimple. The results of the FSI method under lubrication condition, it was found that the variation of 7 dimples with a dimple pitch of 500 μm has the best lubrication performance. The hydrodynamic pressure is 0.73 Pa, the contact pressure is 0.42 Pa, and the film thickness of the lubricant is 29.59 μm. The increase of film thickness that occurs due to hydrodynamic pressure will cause the fluid lift force to withstand the loading of the solid structure.

Heat transfer analysis on dimple geometries and arrangements in dimple jacketed heat exchanger

Purpose The purpose of this paper is to study the effects of dimple geometries and arrangements on the heat transfer enhancement in a dimple jacketed heat exchanger. Design/methodology/approach For the purpose of this paper, with the experimental validated numerical model, this paper carries out numerical simulations of both single dimples with different geometries and the whole dimple jacketed heat exchanger with different dimple arrangements. For a single dimple, its secondary vortex flow, temperature differences and the pressure drop performance for different geometries are analyzed. For the whole dimple jacketed heat exchanger, the heat transfer and pressure drop performances are investigated by comparing the no dimple, triangular and rectangular dimple arrangements. Findings Results show that dimples can improve the heat transfer efficiency compared with conventional jacketed heat exchanger, and specific geometries and arrangement of dimples for better heat transfer performance are figured out. Originality/value This paper considers both dimple geometries and arrangements, which can be useful for further applications in specific integrated devices or similar applications.

Multi-objective optimization of micron-scale surface textures for the cylinder/valve plate interface in axial piston pumps

Abstract An optimization model is established for the multi-scale textured cylinder/valve plate interface in axial piston pumps by coupling the lubrication model of the oil film, the cylinder dynamic model and the multi-objective optimization model based on the Non-dominated Sorting Genetic Algorithm (NSGA-II). The time-varying cylinder micro-motions are considered. Aiming to minimize both the mechanical and volumetric losses, optimal dimple shapes are obtained for an industrial pump and an electro-hydrostatic actuator (EHA) pump. The millimetre-scale interfaces which are large-scale textured with hundreds of the optimal micron-scale dimples are evaluated. After comparing two dimple arrangements for the EHA pump, the athletic-track-shaped and water-drop-shaped dimples are found to reduce the friction torque and leakage by 8.2% and 9.9% on average, respectively.

Experimental investigation of disc partial surface texture on the generation of hydrodynamic lubricating performance under conformal contact condition

A series of experiments is conducted to investigate the hydrodynamic lubricant characteristics of partial surface textured specimens. The specimens are fabricated with partial surface texture. Two dimple shapes are adopted: circular and elliptic. Four kinds of texture arrangements of elliptic dimples are designed. During each test, different rotating speeds and gaps between the tested specimen and the geometric probe are set to explain the relationship between the operation condition and load-carrying capacity. We found that the load-carrying capacity is strongly affected by the operation condition, i.e. by both speed and gap. The surface textured specimens generate large hydrodynamic lift force compared with the untextured specimen. In addition, the texture arrangements exhibit a significant impact on the lubrication property.

Effect of large-scale vortex induced by a cylinder on the drag and heat transfer coefficients of smooth and dimpled surfaces

The drag and heat transfer coefficients of smooth and dimpled surfaces located in the wake of a cylinder are experimentally determined. The cylinder, 8 mm in diameter, was placed in a 30 mm-high slot channel. In the experiments the cylinder position along the channel height and the dimpled surface geometry varied: the gap between the cylinder and the wall under consideration ranged from 0 to 21 mm and the in-line arrangement of spherical dimples and the staggered arrangements of spherical and oval dimples were considered. In determining the drag coefficients the Reynolds number based on the cylinder diameter varied in the range Red = 10.9–53.3∙103. In determining the heat transfer coefficients the Reynolds number was Red = 53.3∙103. The cylinder was unheated. The drag coefficients of the smooth and dimpled surfaces were determined by directly weighing the models on a single-component strain-gauge balance. The local values of the heat transfer coefficients were determined by means of recording the rate of the model surface cooling using an IR imager. The heat transfer and drag coefficients in the cylinder's wake divided by their values on the smooth wall in the undisturbed flow varied in the ranges 0.98–2.64 and (−1.65)-(1.95) for the smooth and 0.75–3.34 and (−3.77)-(4.5) for the dimpled surfaces. The Reynolds analogy factor for the above-mentioned models ranged from 0.45 to 7.73 depending on the particular surface and the gap between the cylinder and the wall.

Heat transfer and friction factor characteristic of spherical and inclined teardrop dimple channel subjected to forced convection

ABSTRACT An experimental and numerical investigation is performed in order to determine the outcome of dimple geometries on the heat transfer and friction factor in a dimple cooling channel subjected to turbulent flow. Two geometries taken into consideration are spherical and inclined teardrop. In order to have a better comparison between the two different dimple channel, the dimple depth, total wetted area of dimple, and dimple pitch have been kept constant. In case of spherical and inclined teardrop dimple channels, heat transfer augmentation, friction losses, and flow pattern have been obtained for a Reynolds Number range from 14,000 to 65,000. The investigation shows that the dimple geometry has a significant contribution to increasing the heat transfer augmentation and determining the flow pattern. The inclined teardrop dimple arrangement shows the maximum heat transfer that is 17% higher than the spherical dimple channel, whereas inclined teardrop dimple results in the rise of friction factor of about 5.93–16.14% times as compared to the spherical dimple within the specified Reynolds number. The inclined teardrop and spherical dimple channel show the heat transfer enhancement of 2.74 to 3.20 times and 2.38 to 2.68 times than that of smooth channels provided thermal boundary conditions and flow conditions are kept same. The numerical study has been performed, which provided a detailed insight into the flow structures and vortex formations in spherical and inclined teardrop dimple channel.

Comparative study on the influence of depth, number and arrangement of dimples on the flow and heat transfer characteristics at turbulent flow regimes

The ensuing study is dedicated to a series of numerical investigations concerning the effects of various geometric parameters of dimpled plates on the flow structure and heat transfer performance in a rectangular duct compared to the smooth plate. These parameters are the arrangement, number and depth of dimples. Two widely used staggered and square patterns in addition to a triangular arrangement, and three dimple depths (Δ = δ/d = 0.25, 0.375 and 0.5) have been chosen for this particular study. All studies have been conducted at three different Reynolds numbers Re = 25,000, 50,000 and 100,000. In order to capture the flow structures in the vicinity of dimples and contributing phenomena related to the boundary layer interactions, fully structured grids with y+ < 1 have been generated for all the cases. The realizable kt-e two-layer model was selected as a proper turbulent model. It can be observed from the obtained results that higher effective area for heat transfer and a myriad of turbulent vortices mixing the hot fluid near the surface with the passing cold fluid generated from the downwind rims of dimples are the causes for improved average Nusselt number in the dimpled surface in comparison to the smooth plate. However, more pressure loss due to the higher friction drag and recirculation zones inside dimples will exist as a drawback in this system. Moreover, for all arrangements increasing dimple ratio Δ has a negative impact on the heat transfer augmentation and also deteriorates the pressure loss, which leads to this fact that Δ = 0.25 serves as the best option for the dimple depth.

A Study on effects of the shape and arrangement of Dimples on Heat Transfer performance

A Study on effects of the shape and arrangement of Dimples on Heat Transfer performance

Engineering Fluid Dynamics

Over the last few decades, the use of computational fluid dynamics (CFD) and experimental fluid dynamics (EFD) methods has penetrated into all fields of engineering. [...]

Laser Textured Surfaces for Mixed Lubrication: Influence of Aspect Ratio, Textured Area and Dimple Arrangement

Unidirectional sliding experiments with polished and laser textured steel surfaces were carried out to investigate the effects of different textured area densities, aspect ratios and dimple arrangements. The system was lubricated with Polyalphaolefin (PAO) at 100 °C and the contact pressure was 3 MPa. For measuring Stribeck curves, the sliding speed was controlled between 40 and 2000 mm/s. The textured area density was varied between 5% and 30%, with the lowest friction values found for 10%. Aspect ratios ranging from 0.02 to 0.2 were investigated and for 0.1 the lowest friction values were measured. The dimple arrangements tested were cubic, hexagonal and a random distribution for a textured area density of 10% and an aspect ratio of 0.1. Our results demonstrate that the dimple arrangement does affect friction values, hinting to the fact that individual texture elements do influence each other. The optimum dimple arrangement was found in a hexagonal packing. This systematic variation of these three key texturing parameters for the morphological texturing of a tribological surface with dimples will allow a strategic choice of texturing parameters. This makes the most of the tremendous potential that laser surface texturing has for reducing friction forces and thereby CO2 emissions.

Multi-objective optimization of a solar receiver considering both the dimple/protrusion depth and delta-winglet vortex generators

In this study, multi-objective optimization of the performance of a solar receiver with dimple and delta-winglet vortex generators (DWVGs) is carried out. The thermal performance and outlet dimensionless turbulent kinetic energy (TKE) are chosen as the optimization objectives. The dimple depth, dimple arrangement, the length, height, angle and spacing of the DWVGs are selected as the optimization variables. The Non-Dominated Sorting Genetic Algorithm II (NSGA II) optimization algorithm and computational fluid dynamics (CFD) are adopted. The Re number is fixed as 15,000 in the optimization process. After the optimization, five cases including a Baseline are studied in detail. Results of the Pareto front, flow structure, heat transfer, TKE and thermal performance are included. The results show that the Pareto front is obtained by using this optimization platform. The mixing and thermal performance are significantly increased in the optimal cases. In addition, the solar receiver with inline arranged dimples and DWVGs provides better performance of both mixing and heat transfer than the stagger arrangement. The flow impingement on the protrusion leading edge and half downstream the dimple, the reattachment downstream the dimple are responsible for the thermal performance augmentation while the vortices generated by dimples and protrusions contribute to the increase of mixing. As a result, the heat transfer is enhanced by 75.78%, the friction factor is increased by 166.57% and the TKE is augmented by 5.2 times, respectively. The thermal performance analysis indicates that optimization increases the thermal performance by 72%.

ティアドロップディンプル面回転角度と配列がディンプル面伝熱総合性能へ与える影響の三次元熱伝導を考慮した過渡応答法による評価

ティアドロップディンプル面回転角度と配列がディンプル面伝熱総合性能へ与える影響の三次元熱伝導を考慮した過渡応答法による評価

Comparative Study of Shell and Helically-Coiled Tube Heat Exchangers with Various Dimple Arrangements in Condensers for Odor Control in a Pyrolysis System

This study performed evaluations of the shell and helically-coiled tube heat exchangers with various dimple arrangements, that is, flat, inline, staggered, and bulged, at different Dean numbers (De) and inlet temperatures of a hot channel. Conjugated heat transfer was analyzed to evaluate the heat transfer performance of the exchangers through temperature difference between the inlet and outlet, Nusselt number inside the coiled tube, and pressure drop of the coiled tube by using 3-D Reynolds-averaged Navier–Stokes (RANS) equations with shear stress transport turbulence closure. A grid dependency test was performed to determine the optimal number of the grid system. The numerical results were validated using the experimental data, and showed good agreement. The inline and staggered arrangements show the highest temperature differences through all De. The staggered arrangement shows the best heat transfer performance, whereas the inline arrangement shows the second highest performance with all ranges of De and the hot channel’s inlet temperature. The inline and staggered arrangements show the highest pressure drop among all inlet temperatures of the hot channel.

On the solar receiver thermal enhancement by using the dimple combined with delta winglet vortex generator

In this study, effects of dimples and their arrangement on the flow structure, heat transfer and friction factor in a solar receiver heated channel with delta-winglet vortex generators (DWVGs) are numerically studied. The dimples are placed either in an inline or a staggered layout. A smooth channel with pure dimples and a smooth channel with pure DWVGs are studied, in which the smooth channel with DWVGs is considered as baseline. The Reynolds number is ranging from 4000 to 40,000. Results of the flow structure, heat plate Nu number, friction factor, temperature and turbulent kinetic energy (TKE) are included. The results show that the adoption of dimples significantly impacts the flow structure by interacting with the vortex which is generated by the DWVGs. A small part of the vortex moves downward and impinges on the inline arranged dimpled surface, which is beneficial for the heat transfer enhancement. However, for the staggered arrangement of dimples, the vortex flow is lifted upward before it impinges on the dimple surface. In this case, the inline arrangement dimples provides the best mixing and the highest heat transfer performance. The heat transfer is augmented by 36.23% while the friction factor is increased by 36.29% compared to the Baseline, respectively. The thermal performance analysis indicates that the use of dimples increases the thermal performance by 28.50%. It is also found that the adoption of inline dimples contributes to the optimal thermal performance and mixing of the cold and hot fluid.

Experimental investigation of heat transfer and drag on surfaces with spherical dimples

The results of an experimental investigation of the heat transfer and the hydraulic drag in air flow past models with different configurations of vortex reliefs in the form of spherical dimples in a plane surface are considered. To increase the reliability of the results and to reduce their uncertainty the experiments were performed on two aligned models, one of which was smooth (reference model), while the other was coated with the relief under study. Both the thermal and the hydraulic parameters of the two surfaces were simultaneously recorded. The drag coefficient was determined by directly weighing the models under study in the form of floating elements using a one-component strain-gauge balance. The heat transfer coefficient was determined by recording the unsteady heat transfer process and solving the time-dependent three-dimensional heat conduction equation using the measured temperature fields on the surfaces under study. The two-dimensional fields of the heat transfer coefficients on the model surfaces were obtained and the flow over the dimpled surfaces was visualized. The Reynolds number dependences of the drag, heat transfer, and heat-hydraulic efficiency were determined, Re being based on the boundary layer length. The dependences of the mean drag (cx/cx0) and heat transfer (St/St0) coefficients on the dimple arrangement density (streamwise and spanwise pitches) were obtained. For the given set of parameters a heat-hydraulic optimum geometry is determined; for this geometry the mean Reynolds analogy factor RAF=1.1 at St/St0=1.21 and cx/cx0=1.1.

STUDY OF FLOW AERODYNAMICS IN A CHANNEL WITH DUMBBELL-SHAPED DIMPLES

The paper deals with the study of aerodynamic resistance in a channel with dumbbell-shaped dimples on the surface and with smooth walls. It has been found that the hydraulic resistance of a channel with dumbbell-shaped dimples is higher than of a smooth one. The surface of the channel of the proposed design is also higher. It can be used for the intensification of heat transmission in recuperative heat exchangers of ventilation systems. The numerical study of the flow in a rectangular channel with one-sided arrangement of dimples of proposed geometry has been carried out. It has been determined that the use of such dimples at small flow rates near the inlet (1-0.5 m/s) will increase the rate of gas flow near the surface with dimples in comparison with a smooth wall to 20 %. When the flow rate near the inlet is 16.5 m/s, the areas with lower rates appear near the dimples.