Dimpled Wall Research Articles

Erosion Investigation of Dimple Wall using Erosion-Coupled Dynamic Mesh.

The erosion of the dimple walls is investigated experimentally and numerically. A mathematical simulation framework was proposed to describe quantitatively the morphological evolution of the dimple wall quantitatively. As the wall shape continues to evolve, the wall shear stress, mesh deformation, and erosion rate would decrease and gradually tend to be constant. Two distinct regions have been identified along the dimple's windward wall surface: the wall's central area and the lateral area. In the central region, the wall profile flare occurs mainly in the early stage. In the lateral region, profile flare occurs mainly in the later stages of erosion. The microhardness of the wall surface shows a positive correlation with the erosion rate. The liquid-solid two-phase impinges on the wall at a smaller angle, and the wall material removal process is mainly based on the microcutting and slip mechanism. The results provide theoretical implications for the design of dimple-shaped, wide-channel welded plate heat exchangers.

Experimental and numerical studies on enhanced effusion cooling with shallowly dimpled film holes on double-wall structure surface

This paper presents a flow and heat transfer investigation on multi-row dimpled film cooling in the double-wall configuration, considering important flow and geometrical characteristics including internal narrow jet impingement channel and outer film cooling with shallowly dimpled holes. The film cooling hole has dimpled exit, with the dimple depth to diameter ratio of 0.2. The inclined angle of the film hole is 30 degree and film hole length to diameter ratio is 3.0. The adiabatic film cooling effectiveness and heat transfer characteristics on the external surface are obtained by using transient infrared thermography measurement technique, which is based on the Impulse Response Method. And steady-state numerical computations using the Shear Stress Transport (SST) k-ω turbulence model were carried out respectively with the experimentally related adiabatic wall boundary conditions and with the fluid–solid conjugated boundary conditions, which provides the turbulent flow structure and overall cooling performance of the double-wall structure. The experimental results indicate that compared to the counterpart cylindrical film holes, the dimpled film holes can expand the spanwise coverage of coolant film, and the adiabatic film cooling effectiveness was improved by 66 %, 82 % and 180 % respectively at BR = 0.3, 0.6 and 1.0. Therefore, the dimpled holes can improve the film cooling effectiveness significantly. The numerical simulations indicated that the dimpled film holes influence the jet-in-crossflow structure and film cooling effectiveness significantly through complex interactions of the coolant jets, mainstream flow and dimpled wall. The overall cooling effectiveness of the double-wall structure with dimpled film holes on the thermal barrier coating can be distinctively improved by about 12.8 % over the case with conventional cylindrical film holes, which was indicated in the conjugated simulations under the blowing ratio of 1.0 and the mainstream-coolant temperature ratio of 2.0.

Evaluation of thermal hydraulic flow and enhancement of heat performance in different 3D dimpled tube configurations according to design of experiment analysis

ABSTRACT The existing numerical research is concentrated on studying pressure drop, various velocity components, and heat performance behavior of heat exchanger pipe fitted with dimple on the inner pipe wall. Three-dimensional computational calculations using the CFD technique are conducted to study the influence of four geometrical parameters, including the distance between dimples, number of dimples, dimple diameter, and dimple pitch on the enhancement of thermo-hydraulic heat transfer. Also, the effects of the following parameters are optimized using the design of experiments (DOE) approaches combined with both Taguchi and Response Surface Methods. The results revealed different patterns of flow field and heat performance due to the use of dimples on the inner pipe surface. Moreover, utilized dimples can rise heat performance due to the interactions between the dimpled wall surfaces and swirling flow; hence, that can rise in area of heat transfer. Detailed flow analysis between dimples and pipe wall is demonstrated to describe the mechanisms of heat performance and pressure drop change. The orthogonal experiment outcomes revealed that the optimal design of the dimpled pipe has improved by about 35.75% and 36.1%. The numerical outcomes indicate that high value for the overall evaluation factor (PEF) is higher than 1. Based on the above findings, it can be found that hydrodynamic analysis flow, enhancement of heat transfer performance, and dimple optimization are required for different design applications.

Experimental and numerical study on an enhanced swirl cooling with convergent tube wall and local dimple arrangements

This paper experimentally and numerically investigates an enhanced swirl cooling structure, featuring multiple connected convergent swirl tubes and dimpled wall. The dimpled swirl tube adopts a novel local dimpled wall scheme, only arranging dimple arrays under the tangential jets, which takes advantage of the heat transfer potential of the individual dimple. A counterpart multistage convergent swirl tube without dimples is also studied as a comparison baseline to clarify the action of local dimple arrays on swirling flow. The spatially resolved heat transfer data are measured by applying transient thermochromic liquid crystal (TLC) technique. Four channel Reynolds numbers (Re = 20,000 to 50,000) are tested based on the maximum tube parameters. The results show that the enhanced dimpled swirl tube performs a synergistic reinforcement of the high-speed swirling flow over the dimple arrays, which produces many local high Nusselt number patterns. Under the individual injection slot, the upstream dimples have a better enhanced heat transfer performance. For the studied working conditions, the globally averaged Nusselt number of the dimpled swirl tube can be increased by 6.0%–11.5% in comparison with the baseline convergent swirl tube. Besides, the dimpled wall also suppresses the swirl intensity, which leads to the weakening of wall-fluid shear stress action and the disappearance of downstream vortex breakdown, and consequently reduces the pressure loss by 2.0%–10.2%. Based on the well-validated CFD results, the dimpled wall under incoming jets redistributes the contribution of the near-wall turbulence and the forced convection due to the near-wall flow motion to the heat transfer enhancement in the swirl tube, which achieve an ideal cooling performance of high heat transfer in combination with low pressure loss. This provides a new inspiration for the design and optimization of swirl cooling device.

Drag and heat transfer in turbulent channel flow over shallow circular dimples: The shift of the deepest point of dimples

In this study, the flow characteristics and enhanced heat transfer performance of circular dimples in a channel flow were numerically analyzed using DNS and compared with a flat channel. The effect of shifting the deepest point of the dimples in a turbulent channel flow on their drag and heat transfer performance are discussed. The strength and extent of the induced recirculating flow is suppressed significantly when the deepest point is shifted downstream, enhancing the heat transfer performance of the dimpled wall. At the same time, the flow structure above the dimpled wall is also manipulated by the geometry changes. The flow impingement on the dimpled wall increases the drag; consequently, the power required to drive the flow is increased. A study is conducted to investigate the effect of the shift in the deepest point of circular dimples on its heat transfer performance while minimizing the drag increase.

Numerical study of heat transfer and flow structure over channel surfaces featuring miniature V rib-dimples with various configurations

In order to enhance the heat transfer performance of the internal cooling for gas turbine blades, miniature V rib-dimple hybrid structures on cooling channel surfaces were studied numerically at Re = 50,500, and the effects of parameters of rib heights, dimple depths, rib pitches and rib-to-dimple pitches were detailly investigated on the turbulent flow structure, pressure loss and heat transfer characteristics. The Abe Kondoh Nagano (AKN) k−ε turbulence model was utilized for the numerical simulation work. The results showed that as the rib height and dimple depth increase, the globally averaged heat transfer augmentation increases significantly at first, and then becomes stable. And the maximum heat transfer enhancement is reached when the rib height-to-channel hydraulic diameter ratio is 0.075 and the dimple depth-to-diameter ratio is 0.2. Besides, the more densely fabricated hybrid structures induce higher heat transfer, while a relatively larger spacing between the rib and the downstream dimple also obtains higher heat transfer. Within the parameters of this study, the total heat transfer enhancement of the hybrid structures can be up to 95.3% and 43.2% respectively higher than that of the dimple-only and the rib-only structures. The optimal hybrid structure with the overall thermal performance factor of 1.73, with e/Dh=0.075, δ/d = 0.2, P1/e = 7.2 and L/P1 = 0.83, has a total Nusselt number ratio of 3.9 and a friction ratio of 11.6. It is evident that with the miniature V rib-dimple hybrid structure, the turbulent kinetic energy of the flow passing over the V ribs is enhanced, thereby reducing the recirculation zone in the front of the dimples. Additionally, the vortex flow induced by the V rib strongly interacts with the downstream dimpled wall which significantly augments the overall momentum and heat transport of the near-wall flow field, and contributes to enhanced and more uniformly distributed heat transfer on the channel surface.

Output System of a 42 84 GHz 0.5 MW Dual Regime Gyrotron

In this paper, the design studies of the output system are carried out for a dual regime Gyrotron in the context of India’s requirement of clean energy. This design study consists of a dimpled wall quasi-optical launcher (QOL) and RF window. After proper coupling of energy from beam to RF wave, the amplified wave need to be down-convert in a Gaussian-like beam (much simplified lower order). The launcher is designed with a commercial software LOT/Surf3d. This Gaussian-like mode coming out from the gyrotron vacuum system through the RF window is coupled to the corrugated waveguide using Matching Optic Unit (MOU) section. The complete design of the RF window is carried out using Gyrotron Design Suite Version 4.0 (GDSv4.0 2016).

Numerical investigation of heat transfer enhancement in a dimpled cooling channel with different angles of the vortex generator

This study carried out numerical simulations to investigate the flow and heat transfer characteristics for air flowing in a dimpled cooling channel with a vortex generator. A crescent-shaped protrusion was mounted as a vortex generator on the downstream of the dimple. A direct numerical simulation (DNS) was conducted in a dimpled cooling channel with a Reynolds number of 2800. Seven different cases were considered, including one general dimpled wall and dimpled wall with six different angles inside the vortex generator. For the vortex generator mounted cases, the main design variable is the angle inside the vortex generator, which varies from 0 to 75°. The normalized thermo-performance factors were calculated to estimate the performance of the cooling channel. The normalized thermo-performance factors f/f0, j/j0, and the volume goodness factor decreases as θ increases. The dimpled channel with a vortex generator shows better normalized thermo-performances than the general dimpled channel.

Computation of flow and heat transfer through channels with periodic dimple/protrusion walls using low-Reynolds number turbulence models

PurposeThis paper aims to predict turbulent flow and heat transfer through different channels with periodic dimple/protrusion walls. More specifically, the performance of various low-Rek-ε turbulence models in prediction of local heat transfer coefficient is evaluated.Design/methodology/approachThree low-Re numberk-εturbulence models (the zonalk-ε, the lineark-εand the nonlineark-ε) are used. Computations are performed for three geometries, namely, a channel with a single dimpled wall, a channel with double dimpled walls and a channel with a single dimple/protrusion wall. The predictions are obtained using an in house finite volume code.FindingsThe numerical predictions indicate that the nonlineark-εmodel predicts a larger recirculation bubble inside the dimple with stronger impingement and upwash flow than the zonal and lineark-εmodels. The heat transfer results show that the zonalk-εmodel returns weak thermal predictions in all test cases in comparison to other turbulence models. Use of the lineark-εmodel leads to improvement in heat transfer predictions inside the dimples and their back rim. However, the most accurate thermal predictions are obtained via the nonlineark-εmodel. As expected, the replacement of the algebraic length-scale correction term with the differential version improves the heat transfer predictions of both linear and nonlineark-εmodels.Originality/valueThe most reliable turbulence model of the current study (i.e. nonlineark-εmodel) may be used for design and optimization of various thermal systems using dimples for heat transfer enhancement (e.g. heat exchangers and internal cooling system of gas turbine blades).

Heat transfer and pressure loss characteristics in a swirl cooling tube with dimples on the tube inner surface

This paper presents a numerical study of heat transfer and pressure loss characteristics in a swirl cooling tube with five tangential inlet jets and with dimples on the tube inner surface. The swirl tube has a length of 20 times the tube diameter, and the Reynolds number based on the tube diameter ranges from 10,000 to 40,000. Polyhedral meshes as well as the SST k-ω turbulence model were chosen for the numerical calculations. A swirl cooling system with a smooth tube was investigated as baseline, and comparatively with dimples on the tube inner surface. The results showed different swirling flow and heat transfer patterns in the swirl tubes. Dimples on the tube inner surface can reduce the wetted-area averaged heat transfer coefficient on the tube inner surface. However, the total heat transferred in the dimpled swirl cooling tubes can be increased by up to 7.2% due to the increased heat transfer area and the interactions between the swirling flow and the dimpled wall. On the other hand, the pressure loss in the dimpled tube can be appreciably reduced by up to 17.6% compared with that of the smooth swirl tube. Detailed flow interactions between the jets, the tube wall and the dimples are illustrated to explain the heat transfer change and pressure loss reduction mechanisms.

Output System of A 220-/247.5-/275-GHz, 1.0-MW, Triple-Frequency Regime Gyrotron

In this paper, the design studies of an output system [comprising a nonlinear taper, a quasi-optical launcher (QOL), and an RF window] for 220-/247.5-/275-GHz, triple-frequency gyrotron are presented. The precisely chosen mode-triplet is TE39,14/TE43,16/TE47,18 to support adequate power transfer at their corresponding frequencies of operation. The nonlinear uptaper is optimized using our in-house code Gyrotron Design Suite version 4.0 (GDSv4.0 2016) [7] in order to reduce the mode conversion for all the frequencies. A common dimpled wall QOL is designed and simulated using commercially available launcher optimization tool. A chemical vapor-deposited-diamond RF window is recommended with a decent transmission bandwidth for all the operating frequencies. The knowledge of correct phase of the Gaussian-like beam coming out of the RF window of any high-power gyrotron is most important for scheming a correct matching optic unit. Hence, the theoretical phase of the beams at a window plane is retrieved using an iterative phase retrieval algorithm.

Numerical investigation on heat transfer performance and flow characteristics in a rectangular air cooling channel (AR = 2) with ridged dimples

As an innovative type of structure, ridged dimples exhibit great potential for heat transfer enhancement. In this study, the effects of ridged dimples on heat transfer, flow characteristics and entropy generation in a rectangular air cooling channel with aspect ratio of 2:1 are numerically investigated. A comparative and comprehensive analysis is accomplished, considering the influences of extending ratio r and Reynolds number Re, as well as a comparison with the results of spherical dimple case. It is shown that heat transfer is further enhanced when ridged dimples are employed compared with spherical dimple case, and the effect is markedly promoted when r increases. The spherical dimple primarily affects the dimpled wall, while the influence of ridged dimple on heat transfer enhancement is mainly reflected on the side walls and top wall. In addition, the areas of the high temperature regions at the side boundaries of all walls significantly decrease with an increasing r. Besides, the overall turbulent kinetic energy TKE in the domain of spherical dimple case is in a relatively low level. When ridged dimples are adopted, a considerably large area of high TKE is detected in the domain above the dimpled wall. And the proportion of the high TKE region in the domain rises with the increase in r. In ridged dimple cases, the low TKE regions found in spherical dimple case at the side boundaries are not detected. The overall TKE level in ridged dimple case is higher than that of spherical dimple case. Furthermore, the average heat transfer entropy generation rate and average total entropy generation rate are prominently reduced with the employment of ridged dimples, while a slight increase in average friction entropy generation rate is detected. The employment of ridged dimple significantly reduces the maximum wall temperature and distinctly improves the wall temperature uniformity, with a further enhanced effect when r ascends, greatly beneficial for safe and stable operations of engineering devices.

I/O System for A 77/154-GHz, 0.5-MW Dual Regime Gyrotron

This paper contributes to the design of a common input/output (I/O) system to support a dual regime gyrotron operation. I/O system comprises of different subassemblies such as magnetron injection gun (MIG), quasi-optical launcher, and RF window. The operating frequencies selected are 77/154 GHz with the desired mode-pair TE15,4/TE28,8, respectively. The necessary selection criteria of such unified design for these subassemblies has been briefly discussed. The initial design parameters have been selected using our in-house code Gyrotron Design Suite version 3.0 2014 (GDSv3.0 2014). Final optimization of MIG to generate electron beam with proper parameters to support the maximum power transfer has been carried out using a particle trajectory code ESRAY. A dimpled wall, helical cut launcher has been designed to convert the complex higher order modes to a simple Gaussian-like beam and has been optimized with a commercial software launcher optimization tool. The design and optimization of RF window has been done using GDSv3.0, which will allow the chosen frequency beam to be extracted out from the gyrotron.

Laminar flow and forced convective heat transfer of shear-thinning power-law fluids in dimpled and protruded microchannels

The three dimensional laminar flow structures and forced convective heat transfer of shear-thinning power-law fluids in dimpled and protruded microchannel were investigated in this study. The CMC (Carboxyl Methyl Cellulose) aqueous solutions with various concentrations were employed as the working substances. The dimples and protrusions with relative depth δ/D of 0.1, 0.2 and 0.3 were vertically aligned to arrange on the opposite walls. The results reveal that the modes of separation flow near protruded walls, observed at some cases with δ/D=0.2 and 0.3, obviously differ from that near dimpled walls. Both the scale and intensity of all the separation bubbles are strengthened with the increase of flow rate and CMC concentration, except 4000ppm. While, the results at cases of 4000ppm show the variable geometrical structure of protrusion can overcome the negative effect of large CMC concentration on heat and mass transfer. Moreover, the heat and mass transfer of main flow and near-wall flow, as well as the strengthened secondary flow in the passage, are enhanced by the dynamic viscosity distinction of working substance, differing from the Newtonian flow. Furthermore, relative Fanning friction factor f/f0 increases with the increase of depth of dimple/protrusion and flow rate and the decrease of CMC concentration. Relative Nusselt number Nu/Nu0 continuously increases with the increase of flow rate and depth of dimple/protrusion. And then the maximum and minimum thermal performance TP are observed at the case 2000ppm with δ/D=0.3 and CMC100 with δ/D=0.1, respectively. At last, new correlations of f/f0 and Nu/Nu0 are proposed based on the results in this study with a well-established data fitting.

Heat transfer in a rectangular duct with perforated blockages and dimpled side walls

Heat transfer and pressure drop in a duct with three serial perforated blockages equipped with staggered jets were experimentally investigated. Eight types of jet holes and three types of side walls, including dimpled walls, were tested. For heat transfer measurements, the transient liquid crystal technique was used. Reynolds numbers based on the hydraulic diameter of the duct and inlet velocity ranged from about 10,000–30,000. Experimental results showed that the Nusselt number ratios decreased as the Reynolds number increased, and the friction factor ratios increased as the Reynolds number increased. The heat transfer coefficient and the pressure loss were strongly affected by the number and the configuration of jets. Compared to the smooth side wall case, the cases with dimpled side walls showed large increases in heat transfer with slight increments in pressure loss. Therefore, it was determined that the thermal performance factor could be enhanced by up to 25% by using a dimpled side wall in the duct with perforated blockages.

Heat transfer of turbulent flow over surfaces with spherical dimples and teardrop dimples

A comparative experimental and numerical study was conducted to investigate the heat transfer of turbulent flow in channels with one dimpled wall with arrays of spherical dimples and teardrop dimples, and the dimples have the same depth with depth-to-diameter ratio of 0.2. The heat transfer and flow structure characteristics in the two dimpled channels have been obtained and compared with each other for the Reynolds numbers of 8500–60,000. The study showed that the globally averaged heat transfer enhancement of the spherical dimple channel can be 1.5–1.7 times the fully developed flow in a smooth duct with the friction factor of 1.2–2.0 times the smooth duct flow. Compared to the spherical dimples, the teardrop dimples have distinctively higher heat transfer performance by about 18.0% with increased pressure loss by 35–15% as the Reynolds number increases. Furthermore, the numerical computations showed that the standard k–ω turbulence model can reasonably well predict the heat transfer and pressure loss in the dimpled channels, which indicated different heat transfer and turbulent flow patterns in the spherical and teardrop dimples, and revealed the underlying mechanisms for heat transfer enhancement for the spherical and teardrop dimples.

An Experimental/Numerical Investigation on Drag Reduction by Dimples

AbstractThe paper is concerned with experimental and numerical investigations of the turbulent flow over dimpled surfaces. Shallow dimples distributed regularly over the wall of a plane channel with large aspect ratio are used to study their effect on the skin‐friction drag. The resulting pressure drop in the channel was measured for smooth and dimpled walls. In addition to these investigations on internal flows, an external flow study was performed and boundary‐layer profiles were measured using a Pitot‐tube rake. Complementary to the measurements, direct numerical simulations for the internal flow configuration with and without dimples were carried out for two different grid resolutions and analyzed in detail. The objective was to clarify whether or not dimples cause reduction of the skin‐friction drag. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Heat transfer and pressure drop in hexagonal ducts with surface dimples

Measurements of detailed Nusselt number ( Nu) distributions and pressure drop coefficients ( f) for four hexagonal ducts with smooth and dimpled walls are performed to comparatively examine the thermal performances of three sets of dimpled walls with concave–concave, convex–convex and concave–convex configurations at Reynolds numbers ( Re) in the range of 900–30,000. A set of selected experimental data illustrates the influences of dimple configuration and Re on the detailed Nu distributions, the area-averaged Nu over developed flow region ( Nu ¯ ) and the pressure drop coefficients. Relative enhancements of Nu and f from the smooth-walled references ( Nu ∞ and f ∞) along with the thermal performance factor ( η) defined as ( Nu ¯ / Nu ∞)/( f/ f ∞) 1/3 are examined. Nu ¯ and f correlations are individually obtained for each tested hexagonal duct using Re as the controlling parameter.

Three-Dimensional Morphological and Textural Complexity of Archean Putative Microfossils from the Northeastern Pilbara Craton: Indications of Biogenicity of Large (>15 μm) Spheroidal and Spindle-Like Structures

We recently reported a diverse assemblage of carbonaceous structures (thread-like, film-like, spheroidal, and spindle-like) from chert in the ca. 3.0 Ga Farrel Quartzite of the Gorge Creek Group in the Pilbara Craton, Western Australia. Results from a rigorous examination of occurrence, composition, morphological complexity, size distributions, and taphonomy provided presumptive evidence for biogenicity. In this study, we present new data of morphological and textural complexity of large (>15 microm) spheroidal and spindle-like structures, using an in-focus, 3-D image reconstruction system, which further raises the scale of credibility that these structures are microfossils. While many of the large spheroids are single-walled, and the wall is irregularly folded, a few specimens are partially blistered, double walled, or have a dimpled wall. The wall-surface texture varies from smooth and homogeneous (hyaline) to patchy, granular or reticulate. Such variation is best explained as resulting from taphonomic processes. Additionally, an inner solitary body, present in some large spheroids, is hollow and partially broken, which indicates a primary origin for this substructure. Spindle-like structures have two types of flange-like appendage; one is attached at the equatorial plane of the body, whereas the other appears to be attached peripherally. In both cases, the appendage tends to have a flat geometry, a tapering thickness, and constancy in shape, proportions, and dimensions. Spindle-wall surfaces are variously textured and heterogeneous. These morphological and textural complexities and heterogeneity refute potential abiogenic formation models for these structures, such as crystals coated with organic matter, fenestrae, and the diagenetic redistribution of carbonaceous matter. When coupled with other data from Raman spectroscopy, NanoSIMS analysis, and palynology, the evidence that these large carbonaceous structures are biogenic appears compelling, though it is still equivocal as to whether they are cells or outer envelopes of colonies of smaller cells.

Drag reduction by dimples? – A complementary experimental/numerical investigation

The paper is concerned with an experimental and numerical investigation of the turbulent flow over dimpled surfaces. Shallow dimples distributed regularly over the wall of a plane channel with large aspect ratio are used to study their effect on the friction drag. The resulting pressure drop in the channel was measured for smooth and dimpled walls. In addition to these investigations on internal flows, an external flow study was performed and boundary-layer profiles were measured using a Pitot-tube rake. Complementary to the measurements, direct numerical simulations for the internal flow configuration with and without dimples were carried out for two different grid resolutions and analyzed in detail. The objective was to clarify whether or not dimples cause reduction of the skin-friction drag.