2,123 research outputs found

    Recent Advances in Heat Transfer Augmentation by using Twisted Tapes: A Review

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    In the past decade several studies on the passive techniques of heat transfer augmentation have been reported. The present paper is a review on progress with the passive augmentation techniques in the recent past and will be useful to designers implementing passive augmentation techniques in heat exchanger. In passive technique twisted tape insert is playing an important method to enhance the heat transfer characteristics of a heat exchanger without affecting much the overall performance of the system. The present review is organized in two different sections: augmentation of heat transfer in laminar flow and augmentation of heat transfer in turbulent flow. Key words: Heat transfer augmentation, Passive methods, Twisted Tape inserts, Reynolds number, Friction facto

    Thermohydraulic analysis of covalent and noncovalent functionalized graphene nanoplatelets in circular tube fitted with turbulators

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    © The Author(s) 2022. This article is licensed under a Creative Commons Attribution 4.0 International. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.Covalent and non-covalent nanofluids were tested inside a circular tube fitted with twisted tape inserts with 45° and 90° helix angles. Reynolds number was 7000 ≤ Re ≤ 17,000, and thermophysical properties were assessed at 308 K. The physical model was solved numerically via a two-equation eddy-viscosity model (SST k-omega turbulence). GNPs-SDBS@DW and GNPs-COOH@DW nanofluids with concentrations (0.025 wt.%, 0.05 wt.% and 0.1 wt.%) were considered in this study. The twisted pipes' walls were heated under a constant temperature of 330 K. The current study considered six parameters: outlet temperature, heat transfer coefficient, average Nusselt number, friction factor, pressure loss, and performance evaluation criterion. In both cases (45° and 90° helix angles), GNPs-SDBS@DW nanofluids presented higher thermohydraulic performance than GNPs-COOH@DW and increased by increasing the mass fractions such as 1.17 for 0.025 wt.%, 1.19 for 0.05 wt.% and 1.26 for 0.1 wt.%. Meanwhile, in both cases (45° and 90° helix angles), the value of thermohydraulic performance using GNPs-COOH@DW was 1.02 for 0.025 wt.%, 1.05 for 0.05 wt.% and 1.02 for 0.1 wt.%.Peer reviewe

    A Review of Recent Passive Heat Transfer Enhancement Methods

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    [EN] Improvements in miniaturization and boosting the thermal performance of energy conservation systems call for innovative techniques to enhance heat transfer. Heat transfer enhancement methods have attracted a great deal of attention in the industrial sector due to their ability to provide energy savings, encourage the proper use of energy sources, and increase the economic efficiency of thermal systems. These methods are categorized into active, passive, and compound techniques. This article reviews recent passive heat transfer enhancement techniques, since they are reliable, cost-effective, and they do not require any extra power to promote the energy conversion systems' thermal efficiency when compared to the active methods. In the passive approaches, various components are applied to the heat transfer/working fluid flow path to improve the heat transfer rate. The passive heat transfer enhancement methods studied in this article include inserts (twisted tapes, conical strips, baffles, winglets), extended surfaces (fins), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), and nanofluids (mono and hybrid nanofluids).Ajarostaghi, SSM.; Zaboli, M.; Javadi, H.; Badenes Badenes, B.; Urchueguía Schölzel, JF. (2022). A Review of Recent Passive Heat Transfer Enhancement Methods. Energies. 15(3):1-55. https://doi.org/10.3390/en1503098615515

    Combination of Co3O4 deposited rGO hybrid nanofluids and longitudinal strip inserts: thermal properties, heat transfer, friction factor, and thermal performance evaluations

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    The reduced-graphene oxide/cobalt oxide hybrid nanoparticles were prepared based on the in-situ/chemical co-precipitation technique, and they were analyzed by transmission electron microscope, x-ray diffraction, and magnetometer techniques. The hybrid nanofluids were prepared with particle loadings of 0.05%, 0.1%, and 0.2% by dispersing synthesized reduced-graphene oxide/cobalt oxide in distilled water and their physical properties were measured. The thermal performance of the nanofluids was studied, when they flow in the turbulent regime through a circular tube. The thermal performance was also evaluated when straight (longitudinal) strip inserts with aspect ratios of 1, 2, and 4, were used inside the circular. These straight strip inserts by increasing the flow turbulence intensity act as turbulators. Results indicate that with a dilution of 0.2% concentration of hybrid nanoparticles in water, the Nusselt number is enhanced by 25.65%, and it is further enhanced by 110.56% with a straight strip insert of aspect ratio 1. The use of hybrid nanofluids and straight strip inserts leads to a slight penalty in fluid friction. For 0.2% concentration of hybrid nanoparticles in water, the penalty in friction factor is 11%, and it is further increased to 69.8% with 0.2% particle loadings and a straight strip insert of aspect ratio1. Moreover, the thermal performance factor of hybrid nanofluids with and without straight strip inserts presents values higher than 1, which shows the benefit of the prepared hybrid nanofluids in a turbulent flow. A general form of regression equations are developed based on the experimental data.publishe

    CFD Study of Behavior of Transition Flow in Distinct Tubes of Miscellaneous Tape Insertions

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    Application of transition flow can be found in several processes and systems. It has been revealed through findings from various researchers that the values of Reynolds numbers at which transition flow occurs vary. In the current work, investigations were numerically conducted by Fluent on transition of water flow in three assorted plain tubes fitted with miscellaneous tape insertions. They are plain tube with crossed-axes-circle-cut tape insert (C-C tube), plain tube with crossed-axes-triangle-cut tape insert (C-T tube), and plain tube with crossed-axes-ellipse-cut tape insert (C-E tube). The focus of the work is to explore the influence of the tape insertion on commencement and finish of transition flow in the tubes with respect to the Reynolds number of the flow. The Reynolds number (Re) taken into account for the transition flow is 2,150≤Re≤4,650, and the variation of Shear-Stress Transport κ-ω model that deals with transition flow was utilized. The results showed that transition flow starts at Re=2,300 and finishes at Re=4,400 in C-T tube, starts at Re=2,780 and finishes at Re=4,610 in C-C tube, but starts at Re=2,550 and finishes at Re=4,500 in C-E tube. The Nusselt number in C-T tube is 19.3% to 45.6% higher than that in C-C tube, but the Nusselt number in C-T tube is 3.6% to 28.3% more than that in C-E tube. The friction factor in C-T tube is 2.15% to 4.56% higher than that in C-C tube; the friction factor in C-T tube is 0.83% to 3.33% more than that in C-E tube. These results indicate that for the case of the tubes considered in this work, the C-T tube, which is the first one in which transition flow commences and ends, has the highest Nusselt number, but C-C tube, in which transition flow commences and finishes last, has the least Nusselt number. Interestingly, the same phenomenon applies to the friction factor. Doi: 10.28991/HIJ-2022-03-02-02 Full Text: PD

    Adsorption behaviour of molecularly imprinted-beta-cyclodextrin polymers prepared by reversible addition-fragmentation chain transfer (RAFT) polymerization for selective recognition of benzylparaben

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    Molecularly imprinted polymers (MIPs) are kinds of powerful materials with promising selective molecule recognition abilities. However, the conventional MIPs have relatively low binding capacity. In order to improve this characteristic of MIPs, the modification monomer based on β-cyclodextrin (β-CD) and the essential of reversible addition�fragmentation chain transfer (RAFT) polymerization process were studied to generate potential MIPs. The study focuses on the characterization and adsorption behaviour of MIPs for selective recognition of benzylparaben (BzP) analyte. The potential of β-CD in MIP was investigated by synthesizing a reversible addition-fragmentation chain transfer molecularly imprinted methacrylic acid functionalized β-cyclodextrin polymer; RAFT�MIP(MAA-β-CD) based on methacrylic acid functionalized β-cyclodextrin (MAA-β-CD) monomer, which was then compared to a reversible addition-fragmentation chain transfer molecularly imprinted methacrylic acid polymer; RAFT-MIP(MAA) synthesized without β-CD. Both MIPs were prepared by the RAFT polymerization process in bulk polymerization method. The resulting MIPs were characterized using Fourier Transform Infrared Spectroscopy (FTIR), Field Scanning Electron Microscope (FESEM) and Brunauer-Emmett-Teller (BET) analysis. The batch adsorption study that includes studying of the pH, kinetic, isotherm and thermodynamic was conducted. The essential of RAFT polymerization on MIP was studied by comparing RAFT-MIP(MAA-β-CD) with the molecularly imprinted methacrylic acid functionalized β-cyclodextrin polymer; MIP(MAA-β-CD) was synthesized without RAFT agent, and characterized by using FTIR, elemental analysis, FESEM and BET. The binding experiments demonstrated that the RAFT-MIP(MAA-β-CD) has a higher binding capacity and higher accessibility compared to RAFT-MIP(MAA) and MIP(MAA-β-CD) for selective of BzP, respectively. The β-CD and RAFT polymerization process improved the MIP’s physical properties and iv enhanced its recognition capacity, thus affecting the adsorption behaviour of RAFT�MIP(MAA-β-CD). The effects of RAFT polymerization process were also investigated by a reversible addition-fragmentation transfer molecularly imprinted hydroxylethyl methacrylate functionalized β-cyclodextrin polymer; RAFT-MIP(HEMA-β-CD). The RAFT-MIP(HEMA-β-CD) was synthesized based on the hydroxylethyl-methacrylate functionalized β-cyclodextrin (HEMA-β-CD) monomer and was prepared by the RAFT polymerization process in bulk polymerization method. The molecularly imprinted hydroxylethyl-methacrylate functionalized β-cyclodextrin polymer; MIP(HEMA-β-CD) without a RAFT agent was synthesized as comparison. A similar study to RAFT�MIP(MAA-β-CD) had also been carried out for RAFT-MIP(HEMA-β-CD).The effects of RAFT polymerization on RAFT-MIP(HEMA-β-CD) were contrasted with RAFT�MIP(MAA-β-CD). The compact and non-porous morphology of RAFT-MIP(HEMA-β�CD) reduces its binding capacity performance compared to MIP(HEMA-β-CD). Thus, this directly affected the RAFT-MIP(HEMA-β-CD) adsorption behaviour towards BzP. It was resulted that the RAFT polymerization had not improved the synthesis of RAFT�MIP(HEMA-β-CD). Careful choice of RAFT agent and monomer is essential in realizing good control over the RAFT-MIP polymerization process, and generating potential MIP

    A Review of Heat Transfer Enhancement Methods Using Coiled Wire and Twisted Tape Inserts

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    Heat transfer enhancement is categorized into passive and active methods. Active methods need external power to input the process; in contrast, passive methods do not require any additional energy to improve the thermohydraulic performance of the system. Passive methods are widely used in both experimental and numerical applications when investigating heat transfer enhancement and friction losses to save energy and costs. The many passive methods for increasing heat transfer rate include various components located in the fluid flow path, such as twisted tapes, coiled or tangled wires, and nozzle turbulators. The present paper represents a comprehensive review that focused on heat transfer enhancement methods with coiled wire and twisted tape inserts since the installation of inserts is easier and more economical. The thermodynamic performance of heat exchange components is also affected by the flow conditions such as laminar or turbulence. The present review comprises investigations on the enhancement of heat transfer using twisted tape and coiled wire inserts in laminar and turbulent flow region

    Experimental investigation on an air tubular absorber enhanced with Raschig Rings porous medium in a solar furnace

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    An experimental study was carried out to assess the performance of a tubular absorber enhanced with Raschig Rings (RR) porous medium for CSP applications. Two alternative designs with different porous lengths of 20 and 40 mm were fabricated and compared with two conventional tube designs with and without surface coating. Several tests were conducted at the solar furnace SF60 of the Plataforma Solar de Almeria (PSA) within the international access program of the SFERA III project, financed by the EU. The main scope of the study was to provide comprehensive detail on the hydraulic and thermal characteristics of the modified tube for further optimization and deployment in point-focusing solar systems. Therefore, evaluations were directed to determine the effects of each design on the pressure losses and the tube wall temperature, as well as on the useful heat gain. Results indicated that although the porous inserts rise the pressure losses through the fluid flow, the higher wetted area in the porous zone for heat transfer between the air and the heated plate reduces the wall tem-perature significantly. Moreover, applying the PYROMARK 2500 as the surface coating has a high influence on increasing solar absorption and reducing thermal losses. Further investigations revealed that the integration of the porous medium changes the temperature profile formed all over the tube, transforming a Gaussian shape in the plain pipes to a spline shape with two peaks in the modified tubes. Increasing the energy and exergy effi-ciencies of the solar absorber up to 30-50% and 60-70%, respectively, demonstrated the improving effects of the proposed porous material for future applications in the solar industry

    Computational Heat Transfer and Fluid Mechanics

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    With the advances in high-speed computer technology, complex heat transfer and fluid flow problems can be solved computationally with high accuracy. Computational modeling techniques have found a wide range of applications in diverse fields of mechanical, aerospace, energy, environmental engineering, as well as numerous industrial systems. Computational modeling has also been used extensively for performance optimization of a variety of engineering designs. The purpose of this book is to present recent advances, as well as up-to-date progress in all areas of innovative computational heat transfer and fluid mechanics, including both fundamental and practical applications. The scope of the present book includes single and multiphase flows, laminar and turbulent flows, heat and mass transfer, energy storage, heat exchangers, respiratory flows and heat transfer, biomedical applications, porous media, and optimization. In addition, this book provides guidelines for engineers and researchers in computational modeling and simulations in fluid mechanics and heat transfer