Dimple Geometry Research Articles

Stereoscopic Micro-PIV measurement of the flow dynamics in a spherical dimple

One way to increase the thermal efficiency of heat exchangers is to structure the heat transfer surfaces with dimples, resulting in an enlarged surface area and intensified turbulence in the fluid flow. The increased turbulence also causes higher wall shear stress, which potentially suppresses the deposition of particles and supports a self-cleaning of the surface. For a deeper understanding of these phenomena, the flow dynamics inside the dimple were observed experimentally with Stereoscopic Micro-Particle Image Velocimetry (Stereo µPIV). The formation of an unsteady oscillating vortex, which leads to an asymmetric trail downstream of the dimple, is visualized. The significant influence of the dimple geometry on heat transfer enhancement is shown, and the most beneficial geometric ratio of the spherical dimple regarding its ability to increase turbulence is identified. A comparison of the local flow velocities with the results of the numerically and experimentally observed patterns of the deposited particles caused by the dimple’s self-cleaning effect shows a good match.

Analysis of thermohydraulic flow and enhancement heat performance in 3D dimple tube based on varying geometrical configurations

AbstractThis research work investigates how different dimple designs affect the flow field and thermal performance of three‐dimensional pipes. The study focuses on the effect of the number of improved dimples NOD (3, 4, and 5), different groups numbers DGNs (1, 2, and 3 groups), arranged around the pipe, and different distances between dimples (DBDs). Dimple geometry affects flow: Changing dimple parameters alters the velocity and pressure distribution within the pipe. Performance evaluation factor (PEF) varies with dimple configuration: The PEF, which balances heat transfer enhancement and pressure drop penalty, ranges from 1.187 to 1.23 for NOD and from 1.292 to 1.31 for DGN, and also from 1.26 to 1.302 for DBD. Reynolds number range, Re = 4000–15,000; turbulence model, standard k–ε model; numerical scheme, second‐order upwind scheme; test tube conditions, inlet temperature (Tin) = 25°C; pipe diameter D = 23 mm; thickness = 2 mm; heat flux q = 25,500 W/m²; and material (Cu). This research focuses on improving heat transfer efficiency in pipes using dimples. Dimple size and arrangement significantly impact flow dynamics and heat transfer. PEF is used to evaluate the overall performance considering both heat transfer improvement and pressure drop penalty. The study found a specific range for PEF under various conditions for different dimple configurations. The average enhancement in Nusselt number for model 2 was 15.16% compared with a smooth pipe and the heat transfer performance by 10.028%–28.963% at the effect of NOD, the DGN has slightly higher Nu values than smooth pipes, indicating improved heat transfer due to the dimples (around 7%–58% at Re 4000–15,000 and 9%–13% at Re 12,000), and at DBD (13.5%) at a Reynolds number of 12,000 and 4.6%–59% at Re 4000–15,000.

An experimental investigation into the drag reduction performance of dimpled plates in a fully turbulent channel flow

Dimpled surfaces have gained increasing attention in recent years for their potential to reduce turbulent skin friction, a capability previously acknowledged for its beneficial implications on heat transfer. As a passive drag reduction method, dimpled surfaces offer significant advantages for marine applications due to their effectiveness and practical applicability. However, despite numerous studies, conflicting opinions and inconsistent drag reduction rates persist in the literature. This paper addresses these ambiguities and offers valuable insights into the effectiveness of dimpled surfaces for drag reduction in fully turbulent flows. We conducted an extensive experimental investigation involving various dimple configurations, including different depth-to-diameter ratios, diameters and orientations, utilising a specialised Fully Turbulent Flow Channel facility and a Particle Image Velocimetry system. Our findings demonstrated that circular dimple geometries, particularly those with low depth ratios, can achieve significant drag reduction of up to 27% as the Reynolds number increases. These results highlight the substantial potential of dimpled surfaces for improving energy efficiency in marine applications, where skin friction accounts for a significant portion of the total drag experienced by large vessels.

Geometry of dimples and its correlation with mechanical properties in high-strength bainitic steel

ABSTRACT The experimental measurements of 2 D micro/nanolayered dimples' dimensions (both geometry and pattern) on the published tensile fractographs of a series of austempered high-strength low-carbon bainitic steel are carried out and interpreted with the corresponding initial microstructural features and tensile mechanical responses as a function of austempering temperature.

Determination of the CO2 Laser Parameters on Dimple Geometry on Al2O3 Ceramic Surface

Dimples on Al2O3 ceramic plates were created with a CO2 laser using different laser parameters. The effects of the laser parameters used on the dimple geometry were investigated and the necessary laser parameters were optimized to obtain the desired dimple geometry. Taguchi method was used in the optimization process. The effects of laser power, scan speed and laser frequency from laser parameters were investigated. Optimum laser parameters were determined as a result of the Taguchi Optimization method. In addition, the laser parameter with the highest effect on the result was determined. Optimum laser parameters were obtained as 60 W for laser power, 35 s for laser exposure duration and 50 kHz for laser frequency.

Applicability of Drag Equation on a Dimpled Golf Ball using Changing Wind Velocities

The drag equation is commonly used to relate the resistance force experienced by an object traveling through fluids with its velocity. The pattern might vary based on the specific geometry of the objects (e.g., a dimpled golf ball). The dimples are known to reduce the amount of drag during the ball’s flight through a series of complex aerodynamic processes. To investigate the ball’s drag pattern, pressure and air velocity on the golf ball, a CFD using Ansys fluent is performed to simulate its aerodynamic features under nine different wind velocities. After the experiment, it was found that the relationship between the drag and velocity of the dimpled golf ball still generally conforms to the drag equation. The pressure and velocity distributions are consistent with the hypothesis, while the aerodynamic details are presented. The effect of dimples is also visualized, contributing to the flow demonstration. The investigation is valuable because the aerodynamic process of the dimpled sphere structure in changing wind velocities is well presented and analyzed, and it inspires further explorations on the aerodynamic effects of the dimple geometry.

Performance improvement for chilled water air conditioning cooling coils with various dimple fin geometries

The present research inspected the thermal and flow behavior of finned coiled tube heat exchangers in crossflow for chilled water air conditioning systems. The investigation variables cover the impacts of several variables, such as fin dimple shape, the number of fins, and the Reynolds number of water and air sides on the thermal-hydraulic aspects of a finned cooling coiled tube heat exchanger. The goal is to enhance this famous finned cooling coiled tube heat exchanger using dimples concerning the same coiled tube heat exchanger without dimples. Six test specimens were fabricated, three with fin dimple geometries as sharp-sided triangle dimples, curved-sided triangle dimples, and teardrop dimples. The other specimens with three different fin numbers 140, 155, and 170. The test runs were experimented in a transparent air duct with air Reynolds numbers ranging from 1540 to 6205, while the chilled water flow side has experimented with Reynolds numbers ranging from 3350 to 16250. The findings revealed that a higher Nusselt number and flow area goodness factor is achieved by a heat exchanger (D) with a curved-sided triangle fin dimple shape. The energy efficiency ratio for a cooling coil with a fin number of 170 is higher than that of cooling coils with fin numbers 140 and 155 by approximately 24.68% and 12.94%, respectively. Correlations of energy efficiency ratio, effectiveness, flow area goodness factor, and friction factor are extracted according to the experimental results with deviations.

Modelling and analysis of the micro-milling of micro-dimpled structures with copying method on glow discharge polymer (GDP)

The surface with micro-dimpled structures on the target ball is helpful for improving nuclear fusion efficiency in inertial confinement fusion. Micro ball-end milling with copying method can provide a simple and effective option in machining micro dimples with high surface finish and geometry (dimension and shape) accuracy. In this paper, based on the established geometric error model, the influence of geometric errors and structural parameters of the machine tool on the dimple dimension and shape was first studied. Next, a model expressing the trajectory of cutting edge for simulating dimple geometry generated by ball-end milling was built. The model was used to analyze the effect of the spindle attitude, tool run-out and deflection deformation on the dimple dimension and shape. Micro ball-end milling tests were carried out, and the dimension, shape and cross-section profile of simulated and milled dimples were compared. A good correspondence between the simulated and milled dimple was observed, the largest error is 6.87%. Finally, from surface morphologies and surface roughness of the milled dimples, it was found that spindle attitude, axial feed rate are the main factors affecting the surface quality of micro dimple machined using micro ball-end milling with copying method. In particular, the axial feed rate need being optimized, reducing the effect of cutting edge quality and avoiding the deterioration of dimple surface and contour caused by a high feed rate. This research is of great significance for fabricating micro-dimpled functional surfaces by surface texturing based on milling methods.

Large eddy simulations of the turbulent channel flow over dimpled surfaces

Reducing skin friction has a key role in the efficiency of rail, highway, and airway transport vehicles or naval systems such as ships and underwater vehicles. In recent years, there is a growing interest in investigating turbulent drag-reducing capabilities of dimpled surfaces, which have great potential as a passive solution, while there still exists highly conflicting views and drag reduction rates reported in the literature as well as a lack of information about the drag reduction mechanism. In this study, large-eddy simulations (LES) were performed to investigate the characteristics and physical mechanism of the fluid flow over dimpled surfaces in a fully developed channel flow. The Reynolds number based on the channel height and the mean bulk velocity was nearly 5600 for all cases examined. Within the framework of the study, various dimple depth to diameter ratios as well as different dimple arrangements and geometries were considered. The detailed mean and instantaneous flow fields, turbulent kinetic energy budget and spectral characteristics of the flow are presented. The study revealed the potential of the dimpled surface in reducing skin friction and provided critical information about the flow features affecting the performance of the dimples.

Experimental and numerical investigation into the drag performance of dimpled surfaces in a turbulent boundary layer

Although several previous studies have reported a potential drag-reducing effect of dimpled surfaces in turbulent boundary layers, there is a lack of replicability across experiments performed by different research groups. To contribute to the dialogue, we scrutinize one of the most studied dimple geometries reported in the literature, which has a dimple diameter of 20 mm and a depth of 0.5 mm. There is no general consensus in literature on the drag-reduction performance of this particular dimple geometry, with some studies suggesting a drag reduction, while others report a drag increase. The present combined experimental and numerical study comprises two sets of wind tunnel experiments and a well-resolved large-eddy simulation. The wind tunnel experiments and the large-eddy simulation both depict a total drag increase of around 1%–2% compared to the flat reference case. This finding agrees with a recent study by Spalart et al. (2019). Furthermore, the present wind tunnel experiments have shed light on a plausible reason behind the discrepancy between the study by Spalart et al. (2019) and earlier results from van Nesselrooij et al. (2016). Lastly, the large-eddy simulation results reveal that the pressure drag is the main contributor to the increase in the total drag of the dimpled surface. We believe that these results will contribute to a new consensus on the drag performance of this dimple geometry.

Effects of pyrolysis on heat transfer enhancement for hydrocarbon fuel flow in unilateral heated channels with dimples

• Pyrolysis aggravates the non-uniformity of secondary flow intensity by two times. • Pyrolysis causes nonlinear correspondence between heat flux and wall temperature. • Dimples weaken the stratification of secondary flow intensity for pyrolysis fuel. • Dimple cannot disturb aggravated non-uniformity of properties for pyrolysis fuel. • Dimple cannot effectively enhance heat transfer for cracking fuel at high heat flux. Heat transfer enhancement of hydrocarbon fuel is essential to improve the regenerative cooling performance for scramjet engines, especially under hypersonic flights ( Ma >6). Aiming to investigate the effects of fuel pyrolysis on heat transfer enhancement design in the channel, this study exploits the local secondary flow intensities for non-pyrolysis and pyrolysis fuel flow. Flow and heat transfer characteristics of pyrolysis n-Decane in smooth and dimpled channel are compared at 0∼4 MW/m 2 . Parametric analysis of dimple geometry is conducted. Among five dfferent dimensions dimples, h p =0.3 mm, d p =1.5 mm, p =3 mm obtains the best heat transfer performance but it still cannot effectively reduce wall temperature compared with smooth channel. Fuel pyrolysis intensively aggravates the non-uniformity of fluid properties and secondary flow intensities. Besides, the deep-degree pyrolysis in near wall region causes nonlinear correspondence between fluid temperature and heat flux. Above two factors brings enormous difficulties for heat transfer enhancement design in the cooling channel. Dimples weaken the secondary flow intensities stratification and significantly improve the adaptability for the heat transfer of pyrolysis fuel to the changing heat conditions. But dimples cannot sufficiently disturb the strengthened properties stratification induced by severe pyrolysis non-uniformity. Therefore, under high heat flux, dimples cannot perform good heat transfer enhancement for pyrolysis hydrocarbon fuel.

On the efficient topology of the exhaust heat exchangers equipped with thermoelectric generators for an internal combustion engine

The increasing energy demand and simultaneously growing concerns about pollution and climate change can clarify the importance of energy recovery, especially in the transportation section where a huge amount of fossil fuels is still consumed. Thermoelectric modules (TEMs) are one of the most stable means to extract energy from flue gases. However, the heat recovery by TEMs require a precise thermo-hydrodynamic design for heat exchangers to transfer more energy, owing to the low efficiency of the modules. Therefore, the current numerical study follows the ideas that can increase the efficiency of electrical power generation in vehicles. Three various simple or dimpled geometries, called Dual Flat, C-shaped, and U-shaped models, are introduced involving the ideas of chaotic flows, extending heat transferring surfaces, and intensifying turbulence levels. The number of TEMs is kept constant for all geometries and a trial procedure is performed to specify the thermo-physical properties of TEMs, directly connected to their hot and cold side temperature. The results are compared to the traditional Flat geometry, in terms of power generation, efficiency, and thermo-hydrodynamic evaluations. Dual Flat and Flat geometry are respectively labeled as the most and least power generating models by 45% relative difference. Despite the highest pressure drop, the chaotic flow in a C-shaped geometry provides the most efficient case if the pressure drop is freely provided by the engine exhaust inertia, while dimpled Dual Flat geometry is ranked the first for best efficiency when pressure drop compensation needs additional cost. The latter geometry can produce nearly 700 W power, as the maximum value.

Tessellated dimple geometry of high entropy alloy

This article focuses on the characterization and analysis of dimple geometry on the published tensile fracture surfaces of a HfNbTaTiZr refractory high entropy alloy of different matrix microstructures, generated through cold deformation and systematic annealing treatments. Changes in different microstructural states which led to different tessellated fracture complexions have been quantified, compared and correlated with the corresponding tensile responses of the alloy. This interpretation allows the use of fracture surface morphology and image texture in a quantitative way.

Studies on diesel engine exhaust gas for retrieving the waste heat through Triple Tube Heat Exchanger (TTHE) through different tubes

ABSTRACT This paper presents a methodology to recover the waste heat from the diesel engine exhaust gas via triple tube heat exchanger (TTHE) with various intermediate tubes like corrugated tube, helically finned tube, dimpled tube, protrusion tube, and then compared with double tube heat exchanger numerically and experimentally. Due to its practical catastrophic nature by application, only limited work has been done in TTHEs. In many studies, the dimpled tube is used for only fluid flows and not used for gaseous states due to boundary layer problems. In TTHE, the intermediate tube carries exhaust gases of diesel engine, while the inside and outside of the tube carries water, respectively, where the Reynolds number (Re) varyies from 4,000 to 11,000. The rate of heat transfer and fluid flow attributes of TTHE for counter flow and parallel flow orders with different mass flow rates are investigated. The temperature variations of plain tube and different intermediate tubes are analyzed with computational fluid dynamics (CFD) and also experimentally. The outer pipe’s outer diameter is 101 mm, intermediate pipe’s outer diameter is 76 mm, and inner pipe’s outer diameter is 50 mm and the thickness of each of the three tubes is 1.5 mm with length of 2,000 mm singly. The TTHE thermal performances and fluid flow characteristics are expressed by the Nusselt number (Nu), Effectiveness (e), and overall heat transfer coefficient (U). The results obtained numerically by CFD and the experimental value of TTHE were compared with the double tube heat exchanger. The counter flow kind of TTHE with dimpled tube geometry produces the best results. The greatest waste heat recovered using dimpled tube TTHE is roughly 6.279 kW at full load circumstances, according to the findings.

Experimental investigation on thermo hydraulic performance of ferronanofluid flow in a dimpled tube under magnetic field effect

ABSTRACT Active and passive techniques have been utilized together to enhance heat transfer in this study. The ferronanofluid, magnetic field, and dimpled tube have not been utilized together in the literature so far. Regarding this issue, this investigation is the first experimental study to specify the effect of use of these three effects simultaneously. The concept of this study is to determine the thermo-hydraulic performance of Fe3O4/H2O flow inside a dimpled tube under magnetic field effect. Constant and uniform heat flux of 4500 W/m2 has been applied on the surface of the tube. The work aims to gain data in the range of laminar flow (1131≤ Re≤2102) in the dimpled tube. Dimple geometry with pitch ratio of P/d = 3.75, magnetic field (B = 0.03 ≤ T ≤ 0.16), and nanoparticle volume fraction of 1.0% are the base variables. The results showed that Nusselt number increases with increasing Reynolds number and magnetic field intensity. The highest increase in Nusselt number is obtained as 115.31% compared with the distilled water flow in the smooth tube for the case of magnetic field intensity of 0.3 T. The highest Performance Evaluation Criteria value is also determined as 1.44 for the case of ferronanofluid flow in dimpled tube at Re = 1131 in absence of magnetic field.

Numerical investigation on turbulent flow and heat transfer characteristics of ferro-nanofluid flowing in dimpled tube under magnetic field effect

• This is the first study on dimpled tube under magnetic field effect. • Effects of dimple geometry and magnetic field on the thermal and hydrodynamic behaviour have been investigated. • The Nu enhances with increasing volume fraction of magnetic nanoparticle. • The highest PEC is obtained for Ha = 75, magnetic nanoparticle fraction of 2.5 vol% and dimpled tube with P 2 /d = 7.5. For the aim of increasing the heat transfer enhancement, a hybrid method in which active and passive heat recovery techniques have been used together. The usage of nanofluid, MHD and dimpled fins tube have not been utilized together so far. Regarding this issue, this study is the first numerical study to determine effect of usage of three effects together comprehensively. In this study, thermo-hydraulic performance of Fe 3 O 4 /H 2 O nanofluid (ferro-nanofluid) flow inside dimpled tube under magnetic field effect has been examined numerically. The main purpose of the study is to obtain numerical data for turbulent flow in the spherical dimpled tubes providing some aid to design a highly efficient thermal energy storage devices. Dimple geometry with nondimensional pitch ratio ( P/d = 3.75, 7.50 and 11.25 ), Hartmann number ( Ha = 75, 150, 225) and nanoparticle volume fraction ( φ = 0.5, 1.0 and 2.5 vol % ) are the parameters investigated in this study. The numerical analyses have been carried out Reynolds number ranging from 10,000 to 50,000 at a constant heat flux at 20 kW/m 2 . The simulations have been built up by Realizable k- ε turbulence model and single-phase approach. Also, “MagnetoHydroDynamic” ( MHD ) module has also been activated for defining magnetic field effect. The results showed that Nusselt number increases with increasing Reynolds number and decreasing pitch ratio. The dimple geometry type of P/d = 7.50 has been determined as the most efficient dimple geometry type. In the case of highest magnetic field intensity, the highest Nusselt number increment (72.48%) has been obtained for φ = 2.5 vol% compared to the base fluid of distilled water using as the working fluid for smooth tube. The highest PEC value was also obtained as 1.126 for the case of P/d = 7.5, φ = 2.5 vol% and Ha = 75. In addition, the effect of magnetic field intensity on velocity and temperature distributions has been presented with contour graphs.

Friction factor effect and heat transfer enhancement in combined dimple geometry arrange in different angle to flow direction

One of the most effective passive techniques for heat transfer enhancement in heat exchanger devices is the surface roughness flow augmentation model. In this study, the surface pressure and heat transfer are measured using experimental and computational methods in a rectangular channel. Three aspect ratios, AR, 0.05, 0.035, and 0.025, dimple channel with pitch to depth ratios, P/δ = 6 are tested in this study. The angled dimple surface comprises of two cases; case 1 0 °Circular - 45°Oval dimple (0 °C – 45°O) and Case2 45° Oval - 0° Circular dimple (45° O – 0 °C). The computational study is conducted for Reynolds number, Re 600–11000. The data from the compound-dimple channel is normalized with the smooth channel data of heat transfer and friction factor parameters. It is revealed that friction factors, f, friction factors ratio, f/fo, average Nusselt number, Nuavg, Nusselt number ratios, Nu/Nuo, are dependent on Re. The performance index on the compound dimple surface using the ratio of Nusselt numbers, Nu/Nuo, to friction factors, f/fo. The arrangement and location of the dimple have momentous consequences for the pressure drop parameters f and f/fo. The surface pressure and temperature show that the flow is uniformly distributed in the channel. The performance index improves and Re has strong influence on it. The compound dimple geometry does indicate a strong model for application in heat exchanger devices. Optimum performance of 49% for case 1 and 23% for case 2 are observed.

Investigation of the effects of shot overlap and rigid body assumptions on surface layer characteristics in shot peening simulation

As an important mean of shot peening (SP) research, finite element simulation has been widely and successfully employed in estimating the surface characteristic and residual stress distribution after SP treatment. In this study, the effects of shot overlap and rigid body assumptions on dimple geometry and residual stress distribution in depth were discussed via the validated ordered dimple pattern and stochastic dimple pattern SP models for quenched and tempered 42CrMo steel. The results show that shot overlap assumption has a great influence on the surface plastic deformation of peened material. The simulated value of plastic deformation is much larger than the experimental value, which results in the residual stress of the overlapping shot SP model deviating from the actual result seriously. For the prediction of the residual stress distribution, the shot must be set as an elasto-plastic body when the number of shot is large. The shot can be set as a rigid body for the aim of reducing the calculation time when the number of shots is small. In addition, the influence of material hardness on residual stress and equivalent plastic strain (PEEQ) was investigated via simulating the residual stress and PEEQ of different hardness materials treated by SP process . • It is necessary to choose the non-overlapping shot in SP simulation when the dimple geometry and residual stress distribution is predicted. • It is more accurate to choose elasto-plastic shot in SP model for the dimple geometry prediction. • The shot can be set as a rigid body when the number of shot is small.

Laser process parameter optimization of dimple created on oriented carbon fiber reinforced epoxy composites

It was mainly aimed at the study to make the optimization of laser parameters to obtain dimples with the desired shape and size. Carbon Fiber EPOXY composite (CF-EPOXY) surfaces were ablated by Nd:YAG laser which has a 1064 nm wavelength. Some important laser process parameters such as focus position, pulse energy, duration and number were optimized to achieve maximum aspect ratio, circular shape and minimum thermal defect. In addition, it has been determined that which laser parameters are more effective to obtain the desired quality surface. These different shapes and geometry of dimples could be used to improve some properties such as wettability, friction, etc. The pulse energy with an effective rate of 55.97 % is the most effective parameter to achieve the larger aspect ratio. The focus position is the most effective parameter with the rates of 66.18 % and 47.94 % to obtain both perfect circularity and minimum thermal defects respectively. Confirmation experiments were performed and the highest aspect ratio was found as 1.14, the best circular dimple and the minimum thermal effects outside the spot area were found with the rates of 1.021. These are the optimum results of 9 experiment sets in this study for each output. The results were supported by confirmation experiments and regression analysis. It can be concluded that the Taguchi method is reliable and saves time and materials.

Experimental study of heat transfer and friction factor in a double pipe heat exchanger using twisted tape with dimple inserts

ABSTRACT The present experimental work deals with heat transfer and friction factor analysis of water flowing in double pipe heat exchanger with dimpled twisted tape inserts. The investigated parameters are twist ratio of 5.5, Reynolds number (Re) of 6000–14,000, dimple diameter (D) of 3 mm, 5 mm, and 7 mm and dimple diameter-to-depth ratio (D/H) of 1.5, 3, and 6. Nine dimpled twisted tapes were tested and a comparison was made between these twisted tapes and plain twisted tape along with plain tube. Nusselt number (Nu) is found to be maximum (113.68 at Re 14,000 and 59.56 at Re 6000) for D = 5 mm and D/H = 3. This highest value of Nu is 1.50–1.82, 1.37–1.46, 1.24–1.19 and 1.18–1.10 times greater than the values for Nu for plain tube, twisted tape without dimple, dimpled twisted tape with D = 3 mm (D/H = 3) and dimpled twisted tape with D = 7 mm (D/H = 3) respectively. Based on the analysis, the friction factor (f) is found directly proportional to the depth and diameter of the dimple, consequently, maximum value of friction factor (f) is found at 7 mm diameter and D/H 1.5. In addition, it is observed that maximum value of performance evaluation criteria (PEC) is found at dimple diameter of 5 mm (D/H = 3). The correlations for heat transfer and friction factor have been established in terms of dimple geometry (D and D/H) and Reynolds number (Re).