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Heat Transfer and Fluid Flow in Microstructures

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A special issue of Micromachines (ISSN 2072-666X). This special issue belongs to the section "E：Engineering and Technology".

Deadline for manuscript submissions: 31 August 2024 | Viewed by 13005

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Dr. Xuan Zhang

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Department of Energy and Power Engineering, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China
Interests: icing; condensation; phase change heat transfer and flow; droplet dynamics; surface and interface science; micro-/nano-scale heat and mass transfer
Special Issues, Collections and Topics in MDPI journals

Dr. Steven Wang

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Department of Mechanical Engineering, City University of Hong Kong, Hong Kong
Interests: energy and environmental engineering; fluid mechanics; advanced materials for engineering applications; indoor air and ventilation; multiphase flows for engineering applications

Dr. Jinyao Ho

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School of Mechanical and Aerospace Engineering, Nanyang Technological University, Singapore 639798, Singapore
Interests: condensation; boiling; phase change; interfacial transport; micro/nanofabrication; additive manufacturing

Dr. Bingqiang Ji

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Department of Mechanical Engineering, City University of Hong Kong, Hong Kong
Interests: fluid mechanics; free surface flow; bubble dynamics; interface phenomena; surface rheology

Dr. Long Zhang

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Guest Editor

Department of Energy and Power Engineering, School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100811, China
Interests: frosting mechanism and characteristics on cold surfaces; defrosting performances for air source heat pumps; energy efficiency; advanced heat pump technologies
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Heat transfer and fluid flow are seen in many mechanical and electronic equipment and systems, and thus have attracted a great deal of attention. With advances in manufacturing technologies, an increasing number of microstructures have been used to control the heat transfer and fluid flow in recent decades. For example, micro-/nano-structured surfaces are applied to enhance boiling heat transfer and achieve a potentially higher critical heat flux, superhydrophobic surfaces with two-stage structures show promising applications in anti-icing/frosting, and microchannels are used in microfluidics chips. A lot of effort has been devoted to deepening our understanding of the heat transfer and fluid flow and advancing the development of relevant applications. The aim of this Special Issue of Micromachines, “Heat Transfer and Fluid Flow in Microstructures”, is to present recent advances in heat transfer and fluid flow in microstructures, including but not limited to associated theoretical analyses, experimental measurements, numerical simulations, and practical applications.

Dr. Xuan Zhang
Dr. Steven Wang
Dr. Jinyao Ho
Dr. Bingqiang Ji
Dr. Long Zhang
Guest Editors

Manuscript Submission Information

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Keywords

heat and mass transfer
fluid glow
micro-/nano-structure
phase change (boiling, evaporation, condensation, frosting, icing, etc.)
multiphase flow (gas-/solid–liquid, bubble, droplet, etc.)
heat exchanger
micro-/nano-fluid
micro-electromechanical system (MEMS)
chip

Published Papers (12 papers)

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Research

16 pages, 13482 KiB

Open AccessFeature PaperArticle

Experimental and Numerical Investigations on Thermal-Hydraulic Performance of Three-Dimensional Overall Jagged Internal Finned Tubes

by Shufeng Huang, Mingjiang Deng, Zhixin Chen, Dayong Yang, Yingshuai Xu and Ning Lan

Micromachines 2024, 15(4), 513; https://doi.org/10.3390/mi15040513 - 11 Apr 2024

Viewed by 312

Abstract

To satisfy the demand for efficient heat transfer, a novel three-dimensional overall jagged internal finned tube (3D-OJIFT) was fabricated, using the rolling–ploughing/extruding method. The thermal performance of the 3D-OJIFT were studied and compared in experiments and three-dimensional numerical simulations. The RNG k-ε turbulence model is well verified with the experimental results. By analyzing the distributions of velocity, temperature, and turbulence kinetic energy, it was found that the 3D-OJIFT destroyed the development of the velocity and thermal boundary layers, increased the turbulence disturbance, and reduced the temperature gradient, thus improving the heat transfer. The influences of the jagged height and jagged spiral angle of the 3D-OJIFT are discussed. The Nu and f increased as the jagged height of the 3D-OJIFT increased. The Nusselt number of the 3D-OJIFT was 1.67–2.04 times the value for the smooth tube. In addition, the comprehensive heat transfer performance of the 3D-OJIFT improved after increasing the jagged spiral angle. Compared with conventional internal helical-finned tubes and other reinforcement structures reported in the literature, the 3D-OJIFT demonstrated better comprehensive heat transfer performance. Finally, empirical correlations of the 3D-OJIFT were obtained. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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20 pages, 9586 KiB

Open AccessArticle

Gradient-Pattern Micro-Grooved Wicks Fabricated by the Ultraviolet Nanosecond Laser Method and Their Enhanced Capillary Performance

by Guanghan Huang, Jiawei Liao, Chao Fan, Shuang Liu, Wenjie Miao, Yu Zhang, Shiwo Ta, Guannan Yang and Chengqiang Cui

Micromachines 2024, 15(1), 165; https://doi.org/10.3390/mi15010165 - 22 Jan 2024

Viewed by 807

Abstract

Capillary-gradient wicks can achieve fast or directional liquid transport, but they face fabrication challenges by traditional methods in terms of precise patterns. Laser processing is a potential solution due to its high pattern accuracy, but there are a few studies on laser-processed capillary-gradient wicks. In this paper, capillary step-gradient micro-grooved wicks (CSMWs) were fabricated by an ultraviolet nanosecond pulsed laser, and their capillary performance was studied experimentally. The CSMWs could be divided into three regions with a decreasing capillary radius. The equilibrium rising height of the CSMWs was enhanced by 124% compared to the non-gradient parallel wick. Different from the classical Lucas–Washburn model describing a uniform non-gradient wick, secondary capillary acceleration was observed in the negative gradient direction of the CSMWs. With the increase in laser power and the decrease in scanning speed, the capillary performance was promoted, and the optimal laser processing parameters were 4 W-10 mm/s. The laser-enhanced capillary performance was attributed to the improved hydrophilicity and reduced capillary radius, which resulted from the increased surface roughness, protrusion morphology, and deep-narrow V-shaped grooves induced by the high energy density of the laser. Our study demonstrates that ultraviolet pulsed laser processing is a highly efficient and low-cost method for fabricating high-performance capillary gradient wicks. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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28 pages, 9956 KiB

Open AccessArticle

Experimental Study of Flow Boiling Regimes Occurring in a Microfluidic T-Junction

by Xiangzhong Bao, Fei Yang and Xuan Zhang

Micromachines 2023, 14(12), 2235; https://doi.org/10.3390/mi14122235 - 13 Dec 2023

Viewed by 743

Abstract

Microchannel flow boiling is an efficient cooling method for high-heat-flux electronic devices. To understand the flow boiling regime in a T-shaped microchannel, this paper prepared T-shaped microchannels of different sizes and designed an experimental platform for the visualization of flow boiling in microchannels, and aimed to study the evolution characteristics of two-phase flow patterns in T-shaped microchannels. The influences of the flow rate and channel size on the boiling flow pattern inside a T-shaped microchannel were experimentally observed and quantitatively described. The results indicate that the occurrence position of the vaporization core gradually migrates from branch channel to main channel as the wall temperature increases. The flow boiling at the bifurcation of the T-shaped microchannel mainly includes the extrusion fracture flow, bubble flow, plug–annular alternating flow and annular flow, in which the annular flow can be further divided into the intermittent annular flow and the stable annular flow. In addition, a high flow rate and small channel size can lead to the disappearance of the bubble flow, and the presence of the bubble flow delays the appearance of the annular flow. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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15 pages, 6425 KiB

Open AccessArticle

A Study on the Simulation and Experiment of Evaporative Condensers in an R744 Air Conditioning System

by Thanhtrung Dang and Hoangtuan Nguyen

Micromachines 2023, 14(10), 1826; https://doi.org/10.3390/mi14101826 - 25 Sep 2023

Viewed by 1141

Abstract

The heat transfer characteristics of evaporative condensers in an R744 air conditioning system were evaluated using the numerical and the experimental methods. Two configurations of condensers were studied: Case 1 with five layers of tubes and Case 2 with eight layers of tubes. In order to evaluate the heat transfer characteristics, the temperature field, the phase change, the pressure distribution, and thermodynamic parameters were considered. For Case 2, it indicated the capability of R744 condensation from the superheated status to the liquified status by analyzing the outlet temperature of the condenser changed from 28.7 °C to 30.3 °C with a change in condensation pressure from 72.6 bar to 68.5 bar. In this study, R744 mass flow rate increases from 14.34 kg/h to 46.08 kg/h, and the pressure drop also increases from 0.23 bar to 0.47 bar for the simulation and 0.4 bar to 0.5 bar for the experiment, respectively. The results indicate that the five-layer configuration causes a higher pressure drop and lower COP than those obtained from the eight-layer one (splitting into two sets for smaller pressure drop). Furthermore, the evaporative condensers using mini tubes that are flooded in the cooling water tank are suitable for the subcritical R744 air conditioning system. In addition, the results obtained from the experimental data are in good agreement with those obtained from the numerical results, with a deviation of less than 5%. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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21 pages, 14695 KiB

Open AccessArticle

Pseudo-Desublimation of AdBlue Microdroplets through Selective Catalytic Reduction System Microchannels and Surfaces

by Claudiu Marian Picus, Ioan Mihai and Cornel Suciu

Micromachines 2023, 14(9), 1807; https://doi.org/10.3390/mi14091807 - 21 Sep 2023

Viewed by 761

Abstract

In the present paper, the occurrence and development of the pseudo-desublimation process of AdBlue microdroplets in the microchannels and surfaces of catalytic reduction systems (SCR) are reported. In order to understand how the pseudo-desublimation process develops, the influence of heat flux values on the heat transfer of AdBlue injection was analysed, taking into account the structure of the microchannels inside the SCR and the overall configuration of the installation. The evolution of the AdBlue vapour flow in the SCR system was simulated, as well as the temperature variation along an SCR microchannel through which the mixture flows. An experimental set-up was designed in order to visualise and interpret the processes at the onset of pseudo-desublimation. The results described in this paper confirm the existence of a pseudo-desublimation process that occurs only under certain temperature conditions when AdBlue is injected into SCR systems. The characteristics of the crystals formed and their growth rate depend on the working temperature, which could be controlled by efficient preheating methods immediately after engine start. A better understanding of the process will allow the development of methods of avoiding solid depositions on SCR system components, which has a direct impact on SCR catalyst performance and durability. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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23 pages, 14136 KiB

Open AccessArticle

Flow Boiling of Liquid n-Heptane in Microtube with Various Fuel Flow Rate: Experimental and Numerical Study

by Muhammad Tahir Rashid, Naseem Ahmad, Raees Fida Swati and Muhammad Bilal Khan

Micromachines 2023, 14(9), 1760; https://doi.org/10.3390/mi14091760 - 12 Sep 2023

Viewed by 854

Abstract

The evaporation of liquid hydrocarbon n-heptane is discussed in detail with experimentation and numerical techniques. A maximum wall temperature of 1050 K was reported during an experimental process with a two-phase flow that was stable and had a prominent meniscus at a small fuel flow rate (FFR) ≤ 10 µL/min. At medium to high FFR (30–70 µL/min), the flow field was unstable, with nucleating bubbles and liquid droplets inside the microtube and the maximum temperature recorded was 850 K for 70 µL/min. For the numerical model, the temperature of the wall was used as a boundary condition. Using the numerical model, the evaporative flux at the meniscus, pressure drop, pressure oscillation, and heat transfer coefficient (HTC) were investigated. A single peak in HTC was obtained at a low fuel flow rate, while multiple peaks were obtained for high FFR. At low FFR, the pressure peak was observed to be 102.4 KPa, whereas at high FFR, the pressure peak increased to 105.5 KPa. This shows a 2% increase in pressure peak with an increase in FFR. Similarly, when the FFR increased from 5 µL/min to 70 µL/min, the pressure drop increased from 500 Pa to 2800 Pa. The high amplitude of pressure drops and a high peak of HTC were found, which depend on the mass flow rate. The coefficient of variation for pressure drop depends mainly on the fuel flow rate. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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18 pages, 17854 KiB

Open AccessArticle

Experimental Investigation on the Flow Boiling of Two Microchannel Heat Sinks Connected in Parallel and Series for Cooling of Multiple Heat Sources

by Zhengyong Jiang, Mengjie Song, Jun Shen, Long Zhang, Xuan Zhang and Shenglun Lin

Micromachines 2023, 14(8), 1580; https://doi.org/10.3390/mi14081580 - 10 Aug 2023

Cited by 1 | Viewed by 998

Abstract

Cooling methods for multiple heat sources with high heat flux have rarely been reported, but such situations threaten the stable operation of electronic devices. Therefore, in this paper, the use of two microchannel heat sinks is proposed, with and without grooves, labeled Type A and Type B, respectively. Experimental investigations on the flow boiling of two microchannel heat sinks connected in parallel and in series are carried out under different mass fluxes. In addition, a high-speed camera is used to observe flow patterns in the microchannels. The cold plate wall temperature (T_w), heat transfer coefficient (HTC), and pressure drop (PD) are obtained with the use of two microchannel heat sinks. The flow patterns of the bubbly flow and elongated bubbles in the microchannels are observed. The results of the analysis indicated that the T_w, HTC, and PD of the two microchannel heat sinks connected in parallel were degraded, especially when using the Type A-B parallel connection. Compared to the use of a single heat sink, the maximum decrease in HTC was 9.44 kW/(m²K) for Type A heat sinks connected in parallel, which represents a decrease of 45.95%. The influence of the series connection on the T_w, HTC, and PD of the two heat sinks is obvious. The Type A-A series connection exerted the greatest positive effect on the performance of the two heat sinks, especially in the case of the postposition heat sink. The maximum increase in HTC was 12.77 kW/(m²K) for the postposition Type A heat sink, representing an increase of 72.88%. These results could provide a reference for a two-phase flow-cooling complex for multiple heat sources with high heat flux. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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16 pages, 13506 KiB

Open AccessFeature PaperEditor’s ChoiceArticle

Experimental Investigations upon Ultrasound Influence on Calefaction of AdBlue in Selective Catalytic Reduction Systems (SCR)

by Claudiu Marian Picus, Ioan Mihai and Cornel Suciu

Micromachines 2023, 14(8), 1488; https://doi.org/10.3390/mi14081488 - 25 Jul 2023

Viewed by 787

Abstract

The present paper intends to provide an analysis of how the process of calefaction occurs in a selective catalytic reduction (SCR) system and the mechanisms by which the deposition of AdBlue crystals on a hot surface evolve. Experimentally, two aluminium samples heated to 200 °C were used, over which AdBlue droplets with different atomisation rates were dropped, maintaining the same dynamic flow parameters, in order to observe the influence of temperature effects on the degree of deposition of crystallised sediment on the surface. The authors proposed the use of calefaction in an ultrasonic environment to prevent deposition and to increase droplet fragmentation by a break-up process. To prove the performance of this method one sample was subjected to a normal flow regime while a second sample was exposed to ultrasound. Both samples were assembled on a magneto-strictive concentrator operating at a frequency of 20 kHz. The obtained results indicated that the sample exposed to ultrasound demonstrated lower urea crystallisation compared to the sample that was not exposed to this treatment. Thus, it can be seen that the proposed method of injecting AdBlue into an ultrasonic zone gives the desired results. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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19 pages, 11671 KiB

Open AccessArticle

Assessment of Vapor Formation Rate and Phase Shift between Pressure Gradient and Liquid Velocity in Flat Mini Heat Pipes as a Function of Internal Structure

by Ioan Mihai, Cornel Suciu and Claudiu Marian Picus

Micromachines 2023, 14(7), 1468; https://doi.org/10.3390/mi14071468 - 21 Jul 2023

Viewed by 724

Abstract

Flat mini heat pipes (FMHPs) are often used in cooling systems for various power electronic components, as they rapidly dissipate high heat flux densities. The main objective of the present work is to experimentally investigate whether differences in the rate of vapor formation occur on an internal structure containing trapezoidal microchannels and porous sintered copper powder material. Several parameters, such as hydraulic diameter and fluid velocity through the material, as a function of the internal structure porosity, were determined by calculation for a steady state regime. Reynolds number was determined as a function of porosity, according to Darcy’s law, and the Nusselt number was calculated. Since the flow is Darcy-type through the porous medium inside the FMHP, the Darcy friction factor was calculated using five methods: Colebrook, Darcy–Weisbach, Swamee–Jain, Blasius, and Haaland. After experimental tests, it was found that when the porous and trapezoidal microchannel layers are wetted at the same time, the vaporization progresses at a faster rate in the porous material, and the duration of the process is shorter. This recommends the use of such an internal structure in FMHPs since the manufacturing technology is simpler, the materials are cheaper, and the heat flux transport capacity is higher. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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17 pages, 3753 KiB

Open AccessArticle

Numerical Investigation of Fluid Flow and Heat Transfer in High-Temperature Wavy Microchannels with Different Shaped Fins Cooled by Liquid Metal

by Tingfang Yu, Xing Guo, Yicun Tang, Xuan Zhang, Lizhi Wang and Tao Wu

Micromachines 2023, 14(7), 1366; https://doi.org/10.3390/mi14071366 - 02 Jul 2023

Viewed by 1605

Abstract

The microchannel heat sink has been recognized as an excellent solution in high-density heat flux devices for its high efficiency in heat removal with limited spaces; however, the most effective structure of microchannels for heat dissipation is still unknown. In this study, the fluid flow and heat transfer in high-temperature wavy microchannels with various shaped fins, including the bare wavy channel, and the wavy channel with circular, square, and diamond-shaped fins, are numerically investigated. The liquid metal-cooled characteristics of the proposed microchannels are compared with that of the smooth straight channel, with respect to the pressure drop, average Nusselt number, and overall performance factor. The results indicate that the wavy structure and fin shape have a significant effect on the heat sink performance. Heat transfer augmentation is observed in the wavy channels, especially coupled with different shaped fins; however, a large penalty of pressure drops is also found in these channels. The diamond-shaped fins yield the best heat transfer augmentation but the worst pumping performance, followed by the square-, and circular-shaped fins. When the Re number increases from 117 to 410, the Nu number increases by 61.7% for the diamond fins, while the ∆p increases as much as 7.5 times. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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18 pages, 10262 KiB

Open AccessArticle

Heat Transfer Enhancement Using Al₂O₃-MWCNT Hybrid-Nanofluid inside a Tube/Shell Heat Exchanger with Different Tube Shapes

by Maissa Bouselsal, Fateh Mebarek-Oudina, Nirmalendu Biswas and Abdel Aziz I. Ismail

Micromachines 2023, 14(5), 1072; https://doi.org/10.3390/mi14051072 - 18 May 2023

Cited by 39 | Viewed by 2141

Abstract

The high demand for compact heat exchangers has led researchers to develop high-quality and energy-efficient heat exchangers at a lower cost than conventional ones. To address this requirement, the present study focuses on improvements to the tube/shell heat exchanger to maximize the efficiency either by altering the tube’s geometrical shape and/or by adding nanoparticles in its heat transfer fluid. Water-based Al₂O₃-MWCNT hybrid nanofluid is utilized here as a heat transfer fluid. The fluid flows at a high temperature and constant velocity, and the tubes are maintained at a low temperature with various shapes of the tube. The involved transport equations are solved numerically by the finite-element-based computing tool. The results are presented using the streamlines, isotherms, entropy generation contours, and Nusselt number profiles for various nanoparticles volume fraction 0.01 ≤ φ ≤ 0.04 and Reynolds numbers 2400 ≤ Re ≤ 2700 for the different shaped tubes of the heat exchanger. The results indicate that the heat exchange rate is a growing function of the increasing nanoparticle concentration and velocity of the heat transfer fluid. The diamond-shaped tubes show a better geometric shape for obtaining the superior heat transfer of the heat exchanger. Heat transfer is further enhanced by using the hybrid nanofluid, and the enhancement goes up to 103.07% with a particle concentration of 2%. The corresponding entropy generation is also minimal with the diamond-shaped tubes. The outcome of the study is very significant in the industrial field and can solve many heat transfer problems. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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22 pages, 7300 KiB

Open AccessArticle

Mixed Convection Hybrid Nanofluid Flow Induced by an Inclined Cylinder with Lorentz Forces

by Farizza Haniem Sohut, Umair Khan, Anuar Ishak, Siti Khuzaimah Soid and Iskandar Waini

Micromachines 2023, 14(5), 982; https://doi.org/10.3390/mi14050982 - 29 Apr 2023

Cited by 3 | Viewed by 1316

Abstract

Hybrid nanofluids may exhibit higher thermal conductivity, chemical stability, mechanical resistance and physical strength compared to regular nanofluids. Our aim in this study is to investigate the flow of a water-based alumina-copper hybrid nanofluid in an inclined cylinder with the impact of buoyancy force and a magnetic field. The governing partial differential equations (PDEs) are transformed into a set of similarity ordinary differential equations (ODEs) using a dimensionless set of variables, and then solved numerically using the bvp4c package from MATLAB software. Two solutions exist for both buoyancy opposing (λ < 0) and assisting (λ > 0) flows, whereas a unique solution is found when the buoyancy force is absent (λ = 0). In addition, the impacts of the dimensionless parameters, such as curvature parameter, volume fraction of nanoparticles, inclination angle, mixed convention parameter, and magnetic parameter are analyzed. The results of this study compare well with previously published results. Compared to pure base fluid and regular nanofluid, hybrid nanofluid reduces drag and transfers heat more efficiently. Full article

(This article belongs to the Special Issue Heat Transfer and Fluid Flow in Microstructures)

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