Heat Transfer Enhancement Methods Research Articles

Evaluation and optimisation of hybrid sensible-latent heat thermal energy storage unit with natural stones to enhance heat transfer

Latent heat thermal energy storage improves the utilization efficiency of renewable energy. Phase change materials (PCMs) commonly suffer from low thermal conductivity and many heat transfer enhancement methods have been developed. However, conventional methods need additional material preparation and processing, which increases the cost and makes them less environmentally friendly. In the current study, natural stones are used to enhance the heat transfer of the PCM in a shell-and-tube unit, forming a hybrid sensible-latent heat storage configuration. Namely, stones, which are widely accessible, low-cost and environmentally friendly, not only act as sensible heat storage media but as the thermal enhancer of the PCM. Results indicate that the energy storage rate of cases with 25 mm-sized stones increased by 8.3%–92.6%. The case with a filling height of 72.8 mm is superior owing to the high energy storage rate and large total stored energy. The stone size rarely influences the total stored energy that increases almost linearly with the void fraction, while it affects the energy storage rate significantly. Cases with 20 mm and 40 mm-sized stones generally have a higher storage rate. Finally, the mechanism is analysed.

A novel convective heat transfer enhancement method based on precise control of Gyroid-type TPMS lattice structure

Innovative single-phase flow heat transfer enhancement technologies can improve heat dissipation efficiency and reduce the weight and volume of heat dissipation equipment. Triply Periodic Minimal Surfaces (TPMS) hold promise as a outstanding single-phase heat transfer enhancement structure. To further improve the convective heat transfer performance of TPMS, this study independently presents a novel lattice structure control method specifically designed for Gyroid-type TPMS, along with the corresponding mathematical equation. By adjusting the parameter “α” in the equation, the Gyroid lattice geometry can be controlled. This study named “α” as the “Lattice deformation control parameter of Gyroid”. Numerical simulations indicate that increasing α from 0 to 0.25 within the Reynolds number range of 282–2579 can lead to a 19.9–28.9% increase in the average convective heat transfer coefficient (h¯) of the Gyroid, a 0.3%-4.5% increase in the Nusselt number (Nu), and a 13.0%-20.3% decrease in the average friction factor (f¯). The research results suggest that optimizing α is a promising new technology for enhancing convective heat transfer of Gyroid-type TPMS, as it not only improves its heat transfer performance but also reduces its flow resistance. The main reason for the convective heat transfer enhancement of Gyroid-type TPMS with increasing α is the increase in the heat transfer area, while the reduction of flow resistance is mainly due to the formation of the “flow-guiding structure” inside the Gyroid.

Augmentation of heat transfer through passive techniques

AbstractThe thermal performance of energy preservation systems is greatly improved by increasing miniaturization and boosting. These are imaginative (or Promethean) techniques to enhance heat transfer. Enhancement methods of heat transfer draw great attention in front of the industrial sector because of their ability to provide energy savings and raise the economic efficiency of thermal systems. Three techniques these methods are categorized; those are active, passive, and compound. Different types of components are used in passive methods because of the transfer/working fluid flow path to the enhancement of the heat transfer rate. In this article, the subject of the review was the passive heat transfer enhancement methods including inserts (conical strips, winglets, twisted tapes, baffles), porous materials, coil/helical/spiral tubes, rough surfaces (corrugated/ribbed surfaces), extended surfaces (fins) and nanofluids (mono and hybrid nanofluid). Recent passive heat transfer enhancement techniques are studied in this article as they are cost‐effective and reliable, and also comparably passive methods do not need any extra power to promote the energy conversion systems' thermal efficiency than 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 nanofluid). From the pioneers' research work, it is clear that a lower twist ratio and lower pitch, lesser winglet angles can provide more heat transfer rate and a little bit more friction factor. In the case of nanofluids, a little bit of pumping power is enhanced. Finally, heat transfer enhancement is compared with the thermal performance factor, which is more than unity.

Recent advances of selected passive heat transfer intensification methods for phase change material-based latent heat energy storage units: A review

The following article overviews recent studies regarding heat transfer enhancement methods, explicitly focusing on fins and coils utilization, in phase change material-based latent heat thermal energy storage systems. It discusses the influence of various geometrical and material parameters on the melting and solidification processes, as well as the orientation of the heat transfer surface within the storage tank. Additionally, the article examines the use of a range of phase change materials regarding their melting temperature. Results show that there are research gaps regarding a few ranges of phase change materials of certain previously studied melting points. This paper's main goal was to detect possible research gaps within the phase change studies field. It should be highlighted that a vast amount of numerical studies of both finned and coiled geometries is in need of experimental verification. More than 62% of analyzed studies were performed numerically, while only 37% were performed experimentally. What is more, there were only a few studies concerning experimental investigations for melting temperatures higher than 60 °C. Furthermore, the majority of experimental as well as numerical studies were concerned only with melting phenomena. This paper also advocates for more standardized studies regarding coil geometries using non-dimensional parameters.

A Review of Gas Capture and Liquid Separation Technologies by CO2 Gas Hydrate

Gas hydrates, being promising energy sources, also have good prospects for application in gas separation and capture technologies (e.g., CO2 sequestration), as well as for seawater desalination. However, the widespread use of these technologies is hindered due to their high cost associated with high power consumption and the low growth rates of gas hydrates. Previous studies do not comprehensively disclose the combined effect of several surfactants. In addition, issues related to the kinetics of CO2 hydrate dissociation in the annealing temperature range remain poorly investigated. The presented review suggests promising ways to improve efficiency of gas capture and liquid separation technologies. Various methods of heat and mass transfer enhancement and the use of surfactants allow the growth rate to be significantly increased and the degree of water transformation into gas hydrate, which gives impetus to further advancement of these technologies. Taking the kinetics of this into account is important for improving the efficiency of gas hydrate storage and transportation technologies, as well as for enhancing models of global climate warming considering the increase in temperatures in the permafrost region.

Numerical Evaluation of Novel Curved Rib Turbulators for Thermal Performance Improvement in Circular Geometries

Internal heating/cooling using roughness elements such as ribs are a promising passive heat-transfer enhancement technique for many thermal applications. The curved ribs are roughness elements that boost the fluid residence time and enhance the thermal-hydraulic performance of the system. In the present study, curved ribs of different cross-sections were fitted at the inner wall of the circular tube to intensify convection heat transfer to the fluid flowing through it. Novel curved ribs of inline configurations of different shapes and geometrical parameters were numerically studied for convection heat transfer using a three-dimensional computational model. The finite volume method discretizes the tube fluid flow domain’s mass, momentum, and energy equations. The local heat transfer enhancement over the tube surface is 2.75 times the smooth tube flow, with Reynolds numbers ranging from 10,000 to 50,000. The cost-effectiveness of this heat transfer enhancement method was quantified using a thermohydraulic performance evaluation factor. This factor accounts for the increased heat transfer at the same pumping power and the same available surface area for heat dissipation. The average heat transfer enhancement at the same Reynolds number and pumping power was found to be 120% to 450% and 98% to 260% of the smooth tube flow.

Detailed numerical analysis of heat transfer enhancement in horizontal shell-and-tube-type n-eicosane-based latent thermal energy storage system

A numerical analysis is conducted for both the outward and inward phase change (melting/solidification) processes in a horizontal shell-and-tube-type latent thermal energy storage (LTES) system filled with an n-eicosane phase change material. The analysis includes—for the first time—direct comparisons of the effects of heat transfer enhancement (HTE) methods on the solidification and melting processes, considering five parameters (with three levels for each) and at four phase change stages (70 %, 80 %, 90 %, and 100 %). The HTE, defined by the reduction in the overall phase change time, was found to differ between melting and solidification. In certain situations, the HTE method induces a smaller improvement in melting but a significant deterioration in solidification. The solidification-to-melting-time ratios (SMTRs) for all analysis cases are compared to clearly identify the common HTE methods between the melting and solidification processes. Maximal SMTR values of 11 and 10.85 (corresponding to the worst cases) were observed at the maximal tube eccentricity during the outward phase change and at the maximal tube diameter during the inward phase change, respectively. This research should aid engineers in designing novel technologies capable of efficiently and safely storing thermal energy.

A review on nanofluids coupled with extended surfaces for heat transfer enhancement

Recently, due to the increasing demands for effective and efficient devices, the thermal sections of modern machinery deserve considerable attention. Researchers in the field of thermal engineering have since devised strategies of combined passive methods for further heat transfer enhancement. From the available literature, the employment of nanofluids, coupled with extended surfaces is scarce, with limited studies optimizing both techniques. The current investigation summarizes the combined effects of nanofluids coupled with various types of extended surfaces, within numerous applications. Parameters involving the nanofluid type, volume concentration, fin geometry, and flow conditions were examined, whereby their respective effects were investigated on the thermal performances. It was deduced that the effects of surfactants, nanoparticle volume concentration, and Reynolds number significantly contributed to the heat transfer enhancement. The review presents a trend finding that nanoparticle volume concentration would positively contribute to heat transfer enhancement, up to an extent. Surpassing the optimal concentration would lead to lower Brownian motions associated with higher viscosity and density. In terms of extended surfaces, the state-of-the-art review denoted that each fin design possesses a unique attribute that alters the thermal and hydraulic performances. Nevertheless, researchers fabricating novel fin designs should highly prioritize the effects on pressure drop when employing fins. Combining both techniques, the nanofluids and extended surfaces achieved remarkable results. The outstanding results could potentially achieve the standards of active methods, for a fraction of the operational cost. Concluding, the information surrounding the present review may be of aid to researchers developing new approaches utilizing nanofluids and extended surfaces.

Thermal Energy Storage with PCMs in Shell-and-Tube Units: A Review

The paper presents a survey of the experimental and numerical studies of shell-and-tube systems in which phase change material (PCM) is used. Due to the multitude of design solutions for shell-and-tube systems, the emphasis is placed on double-tube (DT), triplex-tube (TT), and multi-tube (MT) units. Additionally, only single-pass systems are considered. Particular attention is paid to the method of heat transfer intensification. The analysis of the research results begins with the classification of each of the three mentioned systems. The systems are divided according to the angle of inclination, the method of heat transfer enhancement (HTE), the flow direction of heat transfer fluid (HTF), and the arrangement of tubes in the bundle. Moreover, the simplified schemes of the particular research cases are proposed. Then, the works on each of the mentioned systems, i.e., DT, TT, and MT, are discussed chronologically. Finally, in the corresponding tables, details of the discussed cases are presented, such as geometric dimensions, and the type of PCM or HTF used. A novelty in the present work is the precise classification of PCM TESUs as DT, TTH, and MTH. In the literature, there is a lot of discretion in this regard. Second, the methods of heat transfer intensification in the presented PCM TESUs are listed and discussed. Third, unified schemes of design solutions for the discussed PCM TESUs are proposed. The review shows that development directions for shell-and-tube TESUs include systems with high conductivity fins of different shapes, heights, and spacing, several PCMs, and modified shells.

Experimental study on the melting heat transfer of octadecane with passively adding graphene and actively applying an electric field

The low thermal conductivity of organic phase change materials (PCMs) has seriously obstructed the performance of thermal latent-heat thermal storage (LHTS) system. For the first attempt, we experimentally studied the phase-change heat transfer characteristics of octadecane inside a horizontal cavity with the passive addition of nano graphene and the active application of an electric field. The effects of nano graphene weight ratio, direct current (DC) voltage magnitude and polarity on melting heat transfer and their corresponding mechanism are then examined. Results are reported for the real-time melting process, the liquid fraction, and the evolution of absorbed energy. The velocity fields obtained by particle image velocimetry (PIV), the temperature fields measured by fiber Bragg grating (FBG) sensors, and the current-voltage curves are reported to reveal the heat transfer enhancement mechanism. The absorbed thermal energy is increased by 33.6% when nano graphene is added alone. A single electric field promotes the energy absorption by 153.5% where electric field driven flow develops. The net charge is generated by the conduction mechanism. For composite PCMs’ melting, the absorbed energy is enhanced by 124.8% at most when the cavity's bottom wall is exposed to -10.0 kV. However, when +10.0 kV is applied to the bottom wall the electrophoretic deposition takes place, and the combined augmentation effect on absorbed thermal energy is 111.2%. The present results provide a reference for improving the LHTS system's performance through sole or synergistic heat transfer enhancement method.

A Review of Cooling Studies on Gas Turbine Rotor Blades with Rotation

Increases in power density and thermal efficiency of a highly efficient gas turbine engine motivate an ever-mounting turbine entry temperature. The combined metallurgical and cooling advancements ensure the structural integrity of a gas turbine rotor blade that spins at high rotor speeds in a gas stream with temperatures above the melting point of the blade material. The cooling performances promoted by a variety of heat transfer enhancement methods typical of the coolant channels of the leading edge, the mid-chord region, and the trailing edge of a gas turbine rotor blade are reviewed. The manifested rotational effects on the aerothermal performances of impinging jets and swirl chambers for leading-edge cooling, multi-pass ribbed, dimpled, and/or wavy channels over the mid-chord region, as well as pin fin and latticework narrow ducts in the trailing edge of a gas turbine rotor blade, are summarized and cross-examined. Research orientations for future cooling studies aimed at preventing the development of hot spots in a gas turbine rotor blade are recommended.

Heat transfer enhancement in inserted extruded aluminum finned tubes

Heat transfer enhancement in tubes is widely applied in various industrial processes. The traditional method of heat transfer enhancement is to insert radial fins to expand the internal surface area and destroy the flow. However, there is a flue gas corridor in the central area of the tube, and the cost of expanding the inner fin is high. A centrally symmetrical bent extruded aluminum fin was developed to enhance the heat transfer in the tube. An experimental platform was constructed to test the performance of inner finned tubes and explore the process of combining extruded aluminum inner fin with steel tube. The bent extruded aluminum fins with two side fins extending to the central axis possesses the best comprehensive heat transfer performance. The heat transfer correlation of the optimal fin structure is Nu = 0.864Re0.4275Pry0.3818(Pry/Prw)?0.1004. With low cost and strong heat transfer performance, the bent extruded aluminum inner fin tube will gain wide applications in the engineering field.

A review on dryout and Post-dryout heat transfer inside tubes and tube bundles

• Detailed review on dryout mechanism and dryout and post-dryout heat transfer. • Summarizing the dryout criterions and models of dryout and post-dryout heat transfer. • Elaborating the methods of dryout and post-dryout heat transfer enhancement. • Proposing further research suggestions of dryout and post-dryout heat transfer. The working medium changes from subcooled liquid through flow boiling to superheated vapor in actual once-through thermal equipment (such as once-through steam generators, once-through boilers). Dryout heat transfer deterioration inevitably occur in this vapor-liquid two-phase flow and heat transfer process. After dryout occurs completely, vapor almost completely covers the wall, resulting in a rapid deteriorating heat transfer performance and a sharp rising wall temperature, which puts forward higher requirements on the design, manufacture, and operation of the equipment. The mechanism of dryout is elaborated, and the dryout criterion is summarized in this paper. The three methods that are used to describe the mathematical model of dryout and post-dryout heat transfer are arranged, and the dryout and post-dryout heat transfer in the straight channel is introduced. Furthermore, the methods of enhancing dryout and post-dryout heat transfer are introduced such as changing the geometric structure (including changing the shape of the flow channel and processing the surface of the flow channel) and injecting liquid droplets, etc.

BORU YÜZEYİNDEKİ KAPSÜL TİPİ KABARTMANIN VE AL2O3 SU NANOAKIŞKANIN ISI TRANSFERİNE ETKİSİNİN SAYISAL ANALİZİ

This study aims to numerically investigate and evaluate the enhancement of heat transfer by new capsule dimples on tube surfaces for flow of water and Al2O3-water nanofluid with different concentrations, under uniform surface heat flux. The originality of this work lies in combining two passive heat transfer enhancement methods such as geometrical improvements and nanofluids together. Capsule dimples with different depths were considered. Al2O3-water nanofluid was modeled as a single-phase flow based on the mixture properties. The effects of dimple depth and nanoparticle concentrations on Nusselt number, friction factor and performance evaluation criteria (PEC) were studied. Numerical computations were performed using ANSYS Fluent commercial software for 2000 14000 Reynolds number range. It was found that when laminar, transient and fully developed turbulent flow cases are considered, increase in the dimple depth increases the Nusselt number and friction factor for both pure water and Al2O3-water nanofluids cases. Also, the friction factor increases as dimple depth increases. Results show that increase in PEC is more pronounced in the laminar region than in the transition region, it starts to decrease for turbulent flows. For nanofluid, PEC values are considerably higher than pure water cases. The variation of PEC for capsule dimpled tubes are dependent on flow regimes and dimple depths. Increasing the nano particle volume concentration and dimple depth in laminar flows increase the PEC significantly.

Heat Transfer Enhancement Methods Applied in Energy Conversion, Storage and Propulsion Systems

With the development of energy storage and conversion or advanced propulsion systems, heat transfer enhancement methods have become widely applied [...]

Transient cooling performance of R134a flash-evaporation spray enhanced by cold air jet: A novel heat transfer enhancement method in laser dermatology

Nevus of Ota is a dermal hyperplastic pigmented disease with a morbidity rate of 0.1–0.6% in the Asian population. Laser surgery with the assistance of cryogen spray cooling (CSC) is a potential clinical treatment for nevus of Ota. However, for Asians with high melanin content, it is necessary to enhance cooling capability without the increment of refrigerant amount. In this study, a R134a flash-evaporation cryogen spray cooling coupled with cold air jet (CSC-CAJ) was developed to improve the cooling capacity. Epoxy resin was employed as a skin phantom to investigate the heat transfer characteristics of CSC-CAJ with different air jet parameters. Air jet parameters of CSC-CAJ are recommended to be Ta = -7 ℃, Va = 3 m3/h Za = 10 mm and Φa = 30°. Compared to CSC, CSC-CAJ could reduce the minimum surface temperature from -39 to -42.1 ℃, and the average value of convective heat transfer coefficient (h) of CSC-CAJ could increase from 8 to 12.9 W/(m2·k) or 61.3% for the recommended parameters. The liquid film residence time (τ) decreases from 301 to 234 ms or 23.3%. In addition, based on the visualization observation and surface heat transfer measurement, the frost formation phenomenon on the surface and the heat transfer enhancement mechanisms are well understood. Four modes, including Leidonfrost mode, no runoff mode, runoff mode and evaporation mode, are proposed to describe the four stages of CSC-CAJ. A non-dimensional correlation of normalized Nusselt number (Nu*) and time (t*) is summarized. The primary heat transfer enhancement mechanism of CSC-CAJ is found to be the increase in cryogen evaporation rate and the ratio of heat removal from the phantom surface to phase change latent heat.

Numerical investigation of nanoparticles dispersion on forced / mixed convective flows and heat transfer in a lid-driven stepped cavity configurations

With the recent trends in the manufacturing techniques of semiconductor chips, the compactness of electronic devices is getting improved. Though the computational speed is improved extensively, overcoming the heat dissipation remained the most challenging task. Buoyancy based convection in the heat sink alone is not sufficient to extract this huge amount of heat. Therefore, better heat transfer enhancement methods are essential to designing thermal efficient electronic devices. One of these techniques is the induction of forced convection by driving one of the lids of the sink. In this work, a 2-Dimensional steady-state thermal analysis of a moving lid enclosure provided with water/ (water + CuO) nanofluid for different Grashof numbers (5000, 13000, and 20000), Reynolds numbers (100, 200, 300, 400, and 500), and volume fractions (1%, 3%, and 5%) is carried out. The impact of Gr, Re, and volume concentrations on Nusselt number is studied. The design of the solid zone is studied by considering three different orientations (Frontward step, Zero step, and Rearward step). The maximum amount of heat transfer is achieved with the Frontward step for Gr = 20000, Re = 500, and 5% CuO volume concentration in pure water. For Re = 100 and Gr = 20000, Nuavg is improved by 5.49% for mixed mode of heat convection as compared to forced mode of heat convection. A heat transfer enhancement of 42.75% more than pure water is attained with a 5% CuO volume fraction of nanofluid at Re = 500 for frontward stepped geometry. However, there is a reduction in the rate of increment of (12.98% vs 16.54%) upon further addition of nanoparticles from 3 to 5% as compared to 1 to 3%.

Hydro-thermal and economic analyses of the air/water two-phase flow in a double tube heat exchanger equipped with wavy strip turbulator

For the first time in this study, three different heat transfer enhancement methods including bubble injection into the working fluid, wavy strip turbulator (WST) utilization in the inner tube, and the Nanofluid application as the working fluid, were used separately and simultaneously in a double tube heat exchanger (DTHEX). Different air flow rates were injected into the hot fluid. Various turbulators’ angles were examined, including 60, 90, 115, and 150°, with water and water-CuO nanofluid as working fluids. The results showed that bubble injection, application of Nanofluid, and the utilization of turbulator increased the heat transfer up to 66%, 74%, and 17%, respectively. Furthermore, the simultaneous use of these three methods increased heat transfer from 33% to 144%. Finally, the most optimal case was chosen utilizing the TEF factor.

Evaluation of discharging performance of molten salt/ceramic foam composite phase change material in a shell-and-tube latent heat thermal energy storage unit

Molten salts are widely used energy storage media in integrated solar power systems, however, due to their high corrosivity and the extreme high-temperature environment, many current methods of heat transfer enhancement do not apply. Herein, we proposed to use ceramic foam to enhance the discharging performance of molten salt to efficiently supply heat for power generation. The ceramic foam was prepared and its corrosion resistance was confirmed experimentally. The discharging performance in a shell-and-tube latent heat thermal energy storage unit was numerically studied. It is found that compared to the configuration without enhancement, the solidification time of the ceramic foam-enhanced unit is shortened by up to 52.0%. The unit with the upper foam insert shows better discharging performance than the one with the lower foam insert. Different foam filling height is also considered and results indicate that the thermal energy release rate always increases with the foam filling height and that of the fully foam-inserted unit is 118.1% higher than that of the none foam-inserted unit. For the first time, the discharging performance of ceramic foam-enhanced molten salt is quantitatively evaluated. This study provides guidance on designing thermal energy storage sub-system with excellent heat supply performance for solar power generation.

Heat transfer enhancement method for high-voltage cable joints in tunnels

A cable joint is regarded as a hot spot in the cable line. Especially for the case of high loads, a cable joint is prone to overheating, which can eventually cause electrical accidents such as explosions. There is still a lack of heat transfer enhancement methods to address this overheating. In this study, a forced convection method of applying cooling fan for cable joints in tunnels was proposed. Combined with the heat transfer characteristics and structural characteristics of cable joints, a forced convection thermal rating (FCTR) model was established. The effects of cooling fan positions and air velocity on cable joint heat transfer enhancement performance were determined using the FCTR model. The results show that the optimal position for a cooling fan is segment C1/C2 of a cable joint. At this position, the recommended air velocity is 4 m/s. This is because the heat transfer enhancement performance of cable joints at a higher air velocity is almost the same as that when the air velocity is 4 m/s. Based on the experimental measurements, the maximum error of the FCTR model is 2.2%. Compared with the previously reported model, the FCTR model is conducive to obtaining a more accurate conductor temperature with the accuracy improved by 4.8%. The results of this study can provide more accurate theoretical guidance for evaluating the heat transfer performance of cable joints in engineering.