Heat Transfer Enhancement Methods Research Articles

Advancing Thermal Performance in PCM-Based Energy Storage: A Comparative Study with Fins, Expanded Graphite, and Combined Configurations

Phase Change Material (PCM) thermal energy storage systems have emerged as a promising solution for efficient thermal energy storage from low to very high-temperature applications. This paper presents an investigation into the utilization of medium temperature range PCM-based systems for domestic hot water application, focusing on different techniques to overcome the low thermal conductivity of the PCM. Five shell and tube heat exchangers were fabricated employing different heat transfer enhancement methods including fin, expanded graphite (EG), and a combination of fin and EG. The combination of EG and circular fins exhibited the best performance in terms of charging and discharging, maintaining a uniform temperature distribution throughout the system due to extensive conductive network provided by the combination of EG and circular fins. When the PCM/EG/fin heat exchanger system is fully charged, the energy stored in the system is 109% higher than that of the PCM heat exchanger at the same elapsed time. Furthermore, the PCM/EG/fin system demonstrated a faster discharging response compared to other thermal energy storage (TES) configurations, with over 160% higher discharging power than a system without any enhancement methods. These findings emphasize the practical viability of integrating PCM/EG composite materials into thermal energy storage systems, offering a viable solution for meeting high heat demand requirements in domestic hot water applications. Furthermore, the enhanced discharging response observed in the PCM/EG/fin system has significant implications for improving energy efficiency and reducing operational costs in real-world applications.

Prediction of heat transfer coefficient and pressure drop of flow boiling and condensation using machine learning

Heat transfer coefficient and pressure drop are two critical parameters in diverse thermal engineering applications, and the prediction of these parameters plays a crucial role in the development of heat exchangers. Traditional methods are often reliant on empirical correlations based on limited refrigerants and experimental conditions. The present research explores the application of machine learning methods to predict the heat transfer coefficient and pressure drop of a wide range of pure and mixed refrigerants. The methodology involves the compilation of over 20, 000 experimental observations encompassing an extensive range of operating conditions, heat exchanger geometries, and refrigerants. A total of 27 machine learning algorithm models, such as regression tree, support vector machines, and deep neural networks, are trained on this diverse dataset to discern intricate patterns and dependencies. After dividing the dataset into 80% for training and the rest for validation, the machine learning prediction shows an ability to predict the heat transfer coefficient and the pressure drop with high accuracy in most of the refrigerants. Notably, the accuracy achieved by the machine learning models surpassed that of conventional correlations, highlighting their superior predictive capabilities. Furthermore, the accurate prediction of heat transfer enhancement methods added another layer of validation to the model’s effectiveness. It is noteworthy that the dataset employed in this study has been made publicly accessible online.

Thermohydraulic analysis of nanofluid flow in tubular heat exchangers with multi-blade turbulators: The adverse effects

Based on the significance of heat transfer in tubular flows, various methods of heat transfer enhancement have been developed by scholars. The use of turbulator inserts like twisted tapes is widely discussed and suggested by researchers, and many studies have concentrated on the positive influence of these devices. However, the question is whether these devices always positively impact heat transfer and fluid flow. In this study, efforts were made to find possible adverse impacts of using twisted tapes on the average Nusselt number (Nu), friction factor (f), flow behavior, and performance evaluation criterion (PEC) of water-titania nanofluid. Three-dimensional (3D) numerical methods were used to assess a combination of three different configurations of 156 cases with/without turbulators with different numbers of blades and pitch ratios (PR). Results suggest that at Reynolds number (Re) = 4000, 6000, and 8000, only 25 %, 25 %, and 22.9 % of the examined cases led to PEC values over 1. Based on the results, while twisted tapes raised the Nu by up to 65.1 %, the f can be increased by up to more than six times. Furthermore, streamlines and velocity magnitude contours were employed to discuss the fluid flow behavior in the presence of the turbulators. According to the findings, while with the best turbulator, the PEC value was increased by only 6.3 %, some of the turbulators reduced this parameter by up to 11.8 %, which is more severe. The worst performance was observed with the Case C (three-bladed) turbulator at a PR value of 11, which reduced the PEC by 11.8 %.

Melting performance analysis of finned metal foam thermal energy storage tube under steady rotation

The latent heat thermal energy storage (LHTES) technology based on solid-liquid phase change material (PCM) is of great significance for the efficient utilization of thermal energy. To address the issues of slow thermal response and non-uniform melting of the LHTES technology, a hybrid heat transfer enhancement method combined with finned metal foam and steady rotation is proposed in this work. An enthalpy-porosity model considering non-Darcy porous effects and mechanical rotation is established based on the local thermal equilibrium assumption and the fixed grid system. Four different structures (uniform metal foam, graded metal foam, finned metal foam with uniform porosity, and finned metal foam with graded porosity) are investigated firstly to identify the optimal structural arrangement under the same volume of PCM. Numerical results demonstrate that the LHTES units strengthened by the finned metal foam with graded porosity achieve the shortest melting time and largest thermal energy storage rate (TESR). The graded porous structure reduces thermal resistance, rotation enhances flow and heat transfer inside the container, and the fins expand the heat source area. Moreover, the effect of different fin types, graded porosities, and rotational speeds are further considered. The findings suggest that the increase of these three parameters does not correspond to better thermal performance; instead, there is an optimal value. Compared with the base case, the optimal configuration (fin length of 20 mm, fin width of 1.5 mm, porosity gradient of 2%, and rotational speed of 0.5 rpm) can shorten the melting time by 46.68%, increase the TESR and Nu by 74.06% and 69.02%, respectively. This paper validates the feasibility of the hybrid heat transfer enhancement method with finned metal foam and steady rotation, which can offer new insights into the engineering practice of the LHTES technology.

Hydrothermal performance of a wavy minichannel heatsink with longitudinal vortex generators

Wavy channels and vortex generators (VGs) are potential heat transfer enhancement methods, and their combination can provide a remarkable hydrothermal performance. In this study, a water-cooled top and bottom walls sinusoidal wavy minichannel equipped with 14 VGs on each of its side walls is experimentally and numerically investigated. The pressure drops and average base temperatures are recorded for the Reynolds numbers (Re) between 500 and 1300 and the obtained experimental data are employed to validate the numerical conjugate heat transfer model. 3 configurations of the wavy channel with inline VGs and 3 with staggered VGs which differ by the attack angle of VGs are numerically investigated. The hydrothermal performances of the cases are studied and compared with a wavy channel without VGs. The Nusselt number (Nu), friction factor (f), and thermal performance factor (TPF) are utilized to evaluate the hydrothermal performances. The most significant TPF is found to belong to the wavy channel with staggered VGs, periodically titled for 45° in the direction of the first part of the local curve of the wavy walls. Its TPF is found to be in the range of 1.4–1.46, while the TPF of the wavy channel without VGs is in the range of 1.17–1.36 for the studied Re range.

Review of passive methods of heat transfer enhancement in helical tube heat exchanger

Review of passive methods of heat transfer enhancement in helical tube heat exchanger

Enhancing heat exchanger efficiency with novel perforated cone-shaped turbulators and nanofluids: a computational study

Abstract The present paper presents a numerical investigation of heat transfer in an exchanger fitted with a modified conical-shaped turbulator containing water/Fe2O3 nanofluid. The study aims to address the critical need for improved heat exchanger efficiency, a vital component in various industries, including the chemical, power generation, and food industries. The work focuses on achieving enhanced heat transfer performance within a smaller volume, a primary goal of modern technology and industrial processes. The innovation in this study lies in the design and analysis of a novel conical turbulator, which has not been explored extensively in the context of heat exchangers fitted with nanofluids. Unlike traditional methods, which often rely on active or semi-active means to enhance heat transfer, this research introduces a passive approach through the incorporation of turbulators. Specifically, the study investigates the use of perforated cone-shaped turbulators in conjunction with nanofluids to boost heat transfer performance. The research employs state-of-the-art computational fluid dynamics (CFD) models, allowing for a comprehensive evaluation of the turbulator’s performance across a wide range of Reynolds numbers (Re = 4000–20,000). It further examines the influence of various turbulator parameters, nanoparticle content, and geometry on heat transfer efficiency. Key findings indicate that the modified turbulator exhibits exceptional performance, increasing Nusselt numbers by 3.4–5.4 times and friction coefficients by 2.3–1.8 times compared to smooth pipes. Particularly noteworthy is the 92 % increase in the Nusselt number achieved with a mere 2 % increase in the Fe2O3 nanoparticle content. The present study introduces a novel passive heat transfer enhancement method using perforated cone-shaped turbulators and nanofluids, filling a significant gap in existing research. The innovative turbulator design and its substantial performance improvements offer promising prospects for achieving higher heat exchanger efficiency, making it a valuable contribution to thermal systems and heat transfer engineering.

Numerical investigation of the hydrothermal performance of novel vortex generators in a rectangular channel by employing inclination and rotational angles

Vortex generators are passive methods of heat transfer enhancement in any thermal system. However, more research is needed to compare the effects of vortex generators having unique shapes on the flow distribution for high heat transmission. Also, a research gap exists in analyzing how different angular orientations of the vortex generator (VG) affect thermohydraulic performance. Therefore, this CFD study aims to propose a total of five novel vortex generator shapes applying modification on a rectangular VG shape, analyzing and comparing the heat transfer and pressure drop properties in a rectangular channel for the Reynolds number varying in the range of Re=4000–11,000. VG-1 (vortex generator-1) with three triangles on its top side, VG-2 with concave shape arc, VG-3 combined with a rectangle and a triangle, VG-4 consists of a rectangle and two triangles at two corners, while VG-5 consists of a rectangle with a triangular inside cut. Among the five designs, VG-1 gives the most optimal hydrothermal performance, incrementing Nusselt number and friction factor of about 38.2 % and 80.38 %, respectively. Combining these two dimensionless parameters, we get the maximum thermal performance factor, TPF, for VG-1, whereas VG-2 performs the least in terms of TPF. Moreover, compared with conventional rectangular VG, VG-1 performed the best with a 1.63 % increase in TPF. Considering the effect of different angular orientations, VG-1 was further studied by applying five vertical inclination angles and four horizontal rotation angle configurations to investigate the impact of inclination and rotational angles. Minimizing the pressure drop penalty, the 30˚ inclination design had the best hydrothermal performance overall, with the highest TPF range of 1.19–1.41, whereas 120˚ variants had the lowest TPF, at 1.09–1.31. Also, for the configuration of the horizontal rotations, 30˚ angle gives the best result, having a slight edge over 150˚case with 1.17–1.41 TPF. Finally, the best cases from vertical inclination and horizontal rotation are combined in a new hybrid configuration, which yielded the maximum TPF of 1.22–1.45, indicating that this configuration is the most effective among all the cases.

Investigating heat transfer enhancement for PCM melting in a novel multi-tube heat exchanger with external fins

The low thermal conductivity coefficient is the limiting parameter of phase change materials (PCMs) for using in thermal energy storage systems. Various heat transfer enhancement methods such as utilizing extended surfaces techniques which include finned tubes and multi-tubes are studied. In this paper the melting of a PCM is investigated in a novel multi-tube heat exchanger (MTHX). RT35 and water are used in the outer and inner tube as the PCM and the heat transfer fluid (HTF), in turn. The main purpose of this study is to peruse the effect of installing external longitudinal fins on the inner tube (tubes) during the charging process. Two, three, and four tubes are engaged to check the effect of inner tubes number on the melting process with a constant amount of PCM and the HTF mass flow rate. A two-dimensional finite volume method based on the enthalpy–porosity technique has been developed using FLUENT software. A comparison between MTHX without fins and with external fins shows that adding of fins improves the heat transfer rate, more uniform temperature distribution, and reduced melting time. The PCM melting time was reduced by 27.84 %, 33.8 %, 29.84 %, and 29.55 % in cases A, B, C, and D, respectively, by installing external fins on the MTHX.

Hydrothermal and energy-saving performances in a mini-channel heat sink under different wire coil inserts induced swirling flow

Energy conservation and emission reduction are crucial international policies. For micro/mini-channel heat exchangers, improving their thermal performances while consuming as little energy as possible is essential for implementing these policies. Swirling flow stands out as one of the most effective methods for heat transfer enhancement in channel. The wire coil insert, known as an effective swirling flow device, has been widely applied in traditional heat exchangers. However, the effects of different wire coil inserts on hydrothermal and energy-saving performances in the square micro/mini-channel remain unclear. Actually, some wonderful swirling flow generators achieve excellent thermal performance at the cost of sacrificing flow performance and energy conservation. This violates the primary intention of energy conservation and emission reduction. Therefore, this study designs several wire coil inserts with various shapes (square, circle, obverse or reverse trapezoidal, observe or reserve triangular, and hexagonal cross-section shapes) and different pitches (p=3–10.5 mm). Also, a numerical investigation on effects of these wire coil inserts on performances in mini-channels (a single mini-channel with 2×2 mm2 cross-section) at Reynolds number (Re) of 523–1120 is conducted. The results show that the square wire coil insert yields the best heat transfer and comprehensive thermal performance. This is attributed to the fact that wire coil inserts induce swirling flow, which promotes the separation and redevelopment of the flow and thermal boundary layers. Based on this, the effect of wire coil pitch is analyzed thoroughly. Increasing the pitch leads to an increasing first and then decreasing tendency in the comprehensive thermal performance. Overall, the mini-channel with a square wire coil insert at p=6 mm obtains the optimal comprehensive thermal performance factor (∼2.0) for Re = 1120. Moreover, it also presents an acceptable energy-saving performance. In addition, the comparison of this study and several other studies suggests that, square coil insert has great potential to improve comprehensive performance of mini-channel heat exchanger at higher Reynolds number.

Innovative method for heat transfer enhancement through shell and coil side fluid flow in shell and helical coil heat exchanger

Innovative method for heat transfer enhancement through shell and coil side fluid flow in shell and helical coil heat exchanger

The thermo-aerodynamic performance of turbulent channel flow over dimples of different sizes

Dimples were introduced as a passive method for heat transfer enhancement and a potential drag reduction tool. However, besides the flow parameters, dimples have several design parameters that control their thermal and dynamic performance. To find the optimal design to improve dimples' performance, each parameter's effect has to be investigated. In this study, we show the effect of different dimples' sizes and flow Reynolds numbers on heat transfer and drag performance of a turbulent channel flow. Wall-resolved Large Eddy Simulation has been used to simulate the flow over the dimples fitted only on the bottom wall of the channel. The flow is simulated over three different sizes with dimensionless diameters D+=2.5, 4π/3, and 5 along with three different friction Reynolds numbers Reτ ≅180, 395, and 590. The dimples are arranged in a staggered form with smooth rounded edges of radius equal to half dimple's base radius along with constant maximum allowable coverage ratio and depth-to-diameter ratio of 5%. The thermo-aerodynamic performance of the cases under study shows an increase in drag for all the cases but accompanied by heat transfer enhancement. The thermo-aerodynamic efficiency, represented in area and volume goodness factors, shows an increase up to around 2%, and 6.8% compared to the flat channel, respectively, with the area goodness factor to be affected by the dimple size more than Reynolds number.

Experimental investigation of thermal characteristics of phase change material in finned heat exchangers

A significant challenge to the widespread use of practical latent heat energy storage systems based on phase change materials is the inherent low thermal conductivity of these materials. Inserting fins is considered one of the effective methods of heat transfer enhancement. In the present paper, an experimental investigation of the melting process of PCM in finned and unfinned heat exchangers is conducted. The experimental setup was constructed, and the measurement system was used to monitor the transient progress of the melting process. The heat exchanger comprises annular horizontal concentric double pipes, where the inner tube is heated by flowing water as heat transfer fluid (HTF), and the outer shell is thermally insulated. Four configurations of fins of the same fin/PCM volume ratio are attached to the inner tube. These configurations are longitudinal fin (LF), circular fin (CF), longitudinal perforated fin (LPF), and circular perforated fin (CPF). The effect of inlet HTF temperatures of 60, 70, and 80 °C is tested. The experimental findings indicate that the thermal performance of finned heat exchangers exceeds that of unfinned heat exchangers. Also, the perforated finned heat exchangers showed a higher thermal feature than the unperforated ones. In addition, the CPF achieved the lowest melting time and the highest melting rate, followed by LPF, CF, and LF. The maximum reductions of melting time are 71.1 %, 70.6 %, and 70.2 % due to using CPF HE instead of UF HE for inlet HTF temperatures of 60, 70, and 80 °C, respectively.

Review of Developments in Plate Heat Exchanger Heat Transfer Enhancement for Single-Phase Applications in Process Industries

A plate heat exchanger (PHE) is a modern, effective type of heat transfer equipment capable of increasing heat recuperation and energy efficiency. For PHEs, enhanced methods of heat transfer intensification can be further applied using the analysis and knowledge already available in the literature. A review of the main developments in the construction and exploration of PHEs and in the methods of heat transfer intensification is presented in this paper with an analysis of the main construction modifications, such as plate-and-frame, brazed and welded PHEs. The differences between these construction modifications and their influences on the thermal and hydraulic performance of PHEs are discussed. Most modern PHEs have plates with inclined corrugations on their surface that create a strong, rigid construction with multiple contact points between the plates. The methods of PHE exploration are mostly experimental studies and/or CFD modelling. The main corrugation parameters influencing PHE performance are the corrugation inclination angle in relation to the main flow direction and the corrugation aspect ratio. Optimisation of these parameters is one way to enhance PHE performance. Other methods of heat transfer enhancement, including improving the form of the plate corrugations, use of nanofluids and active methods, are considered. Future research directions are proposed, such as improving fundamental understanding, developing new corrugation shapes and optimisation methods and area and cost estimations.

Studying the performance of phase change heat storage enhanced by ultrasonic energy

The ultrasonic wave could propagate in elastic media and bring ultrasonic flow which could effectively strengthen the convective heat transfer. Thus, various research works have been carried out on the enhancement effect for its potential future application. This work studied the influence of the acoustic flow effect on the melting process of phase change material. For the first, the influence of ultrasonic acting position on enhancement effect was examined. Then, the effectiveness of ultrasonic energy on enhancing heat storage process was discussed. The present work was also compared with other representative studies. The results show that when the side surface of the container is a constant temperature heating surface, the ultrasonic vibration surface at the bottom of the container has best melting performance of phase change heat storage. Then, a new evaluation method of heat transfer enhancement was proposed. The new method cannot only measure the improvement of melting performance, but also clearly measure the relationship between the improvement of melting performance and the consumption of ultrasonic energy. Finally, by contrast and analysis with representative studies, the development potential and further research direction of ultrasonic enhanced phase change heat storage are clarified. This work can provide reference for the further application of active heat transfer enhancement technology in the field of phase change heat storage.

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.