Heat Transfer Techniques Research Articles

Experimental and theoretical assessments on the systematic performance of a single-stage air-source heat pump using ternary mixture in cold regions

The performance of air-source heat pump is strongly influenced by ambient temperature variation. Improving its stability and efficiency during wintertime is important for promoting its applications, while existing studies paid few attentions to flexible parameter regulation of heat pump using mixture. In this paper, a single-stage compound air-source heat pump using ternary mixture was theoretically simulated and experimentally tested, in which operating parameters including pressure and concentration could be regulated with the cooperation of multiple valves and reservoirs. Using CO2/R134a/R600, operating pressure and mixture concentration were successfully regulated at ambient temperature from −30 ℃ to 0 ℃, achieving a coefficient of performance from 1.834 to 2.546 when producing hot water at 75 ℃. Whether R134a inclusion was favorable to energy performance depended on its influence on the temperature matching in recuperative process, especially the heat transfer process in recuperator. The addition of R134a significantly alleviated the increase of operating pressure and thus enlarged the operation range of this heat pump. The flammability range of CO2/R134a/R600 was calculated, indicating the insignificant influence of R134a on flammability retardance in the studied range of this paper. The significance of this study is further revealed with the potential advances of heat transfer technique and new-generation refrigerant. The findings in this paper contribute to: 1) the modification of air-source heat pump for operation at low temperature; 2) the regulation method for mixed-refrigerant system; 3) the exploration of new mixed-refrigerant in thermodynamic cycle.

Numerical analysis of the magnetic field impact on hydrothermal characteristics of a microchannel heatsink with Fe3O4 ferrofluid and various pin-fin shapes

Enhancing the hydrothermal proficiency of thermal management devices by integrating active and passive heat transfer techniques is possible. Under magnetic field (MF), pin–fin heat sinks have rarely been studied in terms of frictional losses and coefficient of heat transfer (h). Here, with a nanoparticle concentration ratio of 2%, Fe3O4 ferrofluid flow inside a pin–fin heatsink was analyzed numerically considering various pin–fin shapes (rhumbas, squares, circles, and triangles) for absenteeism (WO) and attendance (W) of MF. In the results, the greatest and lowest gains for h were 13.12% and 8.8%, respectively, utilizing MF and Re = 200, for the circular and triangular heatsinks, which would be reduced to 5.36% and 2.41% by increasing Re to 500. In addition, h (and thermal resistance factor) in triangular configuration is 7.82–10.86% (and 4.98–8.68%) and 3.23–8.08% (1.94–6.22%) greater (and smaller) than that in the circular design under two scenarios of MF. Moreover, MF brings about a reduction of 1.86–0.60%, 1.61–0.46%, 1.71–0.54%, and 1.16–0.24% in Tm,CPU for the circular, square, rhumbas, and triangular configurations, respectively, within the Re range of 200–500. Although, when MF is present, pressure loss escalates by almost 6.39%, 4.97%, 3.60%, and 2.92% at Re numbers of 200, 300, 400, and 500 for each configuration, with the maximum pressure loss for triangular heatsink compared to the other arrangements. PEC factors at different Re numbers range from 1.51 to 1.87, suggesting an improved heatsink hydrothermal performance under MF impact compared to absenteeism of that. The highest/lowest PEC values at Re numbers 200 and 500 are associated with the rhumbas/triangular (1.871/1.782) and Triangular/square (1.547/1.496) configurations, respectively.

Local thermal equilibrium analysis of complete phase change process inside porous diffuser using NanoFluids

This works implements an investigation on two-phase flow inside porous diffuser using Cu/water nanofluid instead of pure water. Interesting has been focused on how the using Cu/water affect the predictions of complete boiling process. Two-phase mixture model (TPMM), based on the assumptions of Local Thermal Equilibrium (LTE), has been converted to implement the actions of combining nano-fluid rather only water. The governing equations are solved by finite volume method. For the purpose of comparison, the solutions of temperature and liquid saturation, obtained employing Cu/water and only water as working fluid, have been displayed for a wide ranges of geometric parameters, operating parameters and properties of porous media to investigate impacts of using nanofluid. The findings illustrated that the adding Cu particle to water has a high influences on the heat diffusivity inside the diffuser as compared to the solutions of only water due to enhancing the axial diffusion. By employing Cu/water, the boiling and condensation fronts are considerably moved to downstream and upstream sections, respectively. The exit temperature is significantly reduced when utilizing Cu/water as compared to the only water solution. The findings of Cu/water show that the volume fraction of nano-fluid, operating conditions, geometric conditions and porous structure properties have large impacts on the locations of liquid and steam fronts. Accordingly, the nano-fluid is supposed as a safety key in the boiling applications since it improves the technique of heat transfer while simulating the complete boiling process.

Heat transfer augmentation by plate motion in a wall jet flow over a heated plate: A conjugate heat transfer technique

The present study is an initiation to investigate the effect of plate movement on heat transfer characteristics in the solid plate and the fluid domain due to the turbulent wall jet flow over the plate in a quiescent surrounding. A conjugate technique is used to carry out the heat transfer solution that involves conduction in solid and convection in fluid coupled at the interface surface. The plate is heated continuously with a constant flux at its lower surface. A low-Reynolds number k − ε turbulence model developed by Yang and Shih (YS) is applied to simulate the turbulent flow. The parameters considered for the present study are the plate-jet velocity ratio ( 0 − 2 ) , the plate thickness ratio ( 0.5 − 1.5 ) , the conduction ratio ( 500 − 2000 ) , and the Reynolds number ( 7700 − 25 , 000 ) . The temperature distribution diagrams for various plate velocities, the interface temperature profile, and the averaged Nusselt number show a higher cooling rate at high plate-jet velocity ratios. The effect of plate thickness and conductivity ratios is visible in the temperature distribution diagrams. A better heat transfer is achieved at a higher Reynolds number, evident from the averaged and local Nusselt number distribution. A correlation is developed for average Nusselt number with the Reynolds number and plate-jet velocity ratios as the varying parameters.

An investigation of multistream plate-fin heat exchanger modelling and design: a review

Abstract In line with population expansion and industrial development, the world’s energy consumption has been rising gradually over the past three decades. As a result, methods for energy conservation have been sought. One of the most common strategies is heat recovery, which is efficient and cost-effective to the extent possible. Heat recovery is not just about saving energy for primary consumption; it is also about lowering emissions and protecting the environment. In this respect, one of the most important strategies for heat recovery is to develop heat exchangers and exploit the energy associated with many of the processes’ output products in order to use it in new processes. Many researchers working in the field of heat engineering are now looking into novel heat transfer techniques. Use of the heat exchanger as a compact is one of these ways that might be considered. The current review therefore concentrates on the design of plate-fin heat exchangers (PFHE) and multi-stream plate-fin heat exchangers (MSPFHE) based on various models. The current review offers some suggestions for upcoming studies on improving heat transfer and minimizing power use.

Optical visualization of heat transfer in supercritical carbon dioxide under near-critical, liquid-like, and gas-like conditions

Heat transfer in supercritical carbon dioxide (sCO2) is experimentally visualized, and a measurement method is proposed for evaluating the transport phenomena in near-critical, liquid-like, and gas-like conditions. There are various uses for sCO2 in engineering applications, such as abstraction, material processing, and soil remediation. However, the heat and mass transfer under supercritical conditions have not been fully revealed, and innovative measurement techniques with higher spatial and temporal resolutions are required. This study focuses on the evaluation of heat transfer in sCO2 using a high-speed phase-shifting interferometer. The density distribution of sCO2 under different temperature and pressure conditions is successfully visualized with the proposed interferometer. Characteristics of the density field patterns are observed near the critical point and in liquid-like and gas-like conditions. It is demonstrated that the sensitivity of the density (i.e., refractive index) to temperature changes is different for each condition. The transient heat transfer under gas-like condition is evaluated by the interferometer, and numerical simulations with 3D model are performed to evaluate the experimental results. Finally, the interference fringes pattern obtained by the interferometer is shown to be qualitatively in good agreement with the numerical temperature field change. Additionally, transient variations of optical path length difference obtained in the experiment, which means apparent temperature distribution, were compared with numerical simulations. Experimental results are quantitatively in good agreement with the numerical results under a thermal diffusivity of order 10−8 m2/s, confirming the feasibility of the proposed measurement technique for the transient heat transfer in sCO2.

Integrated intelligent neuro computing technique for mixed convective flow and heat transfer in heterogeneous permeability media

This article examines the flow and heat transfer characteristics of variable permeability such as linear, exponential and quadratic permeability functions with Brinkmann–Forchheimer extended Darcy model by utilizing the intelligent computing paradigm via artificial Levenberg–Marquardt back propagated neural networks (ALM – BPNNs). The vertical porous channel walls are considered as electrically conducting and non-conducting. The variable permeability and induced magnetic field effects are significant on flow and heat transfer. To solve the obtained governing equations, finite element method is adopted. It is observed that for the three cases of permeability functions, increase in Hartmann number drops the velocity profile and is accompanied with an increase in the fluid thermal state level. The significance of the Brinkmann number on the fluid temperature in all the cases of permeability indicates a rise in temperature due to increased heat generation. But, the reverse effect is observed when the mixed convection parameter rises, and the boundary layer thickness reduces due to which temperature drops for distinct cases of permeability under consideration. The heat transfer characteristics are analyzed through an artificial neural network and multiple linear regression models. The present work finds its applications in studying packed bed heat exchangers and fixed-bed catalytic reactors.

Design and performance optimization of solar water heating system with perforated obstacle using hybrid multi-criteria decision-making approach

Solar water heating system (SWHS) is a cost-effective technology with high household adoption rates worldwide. The performance of SWHS significantly deteriorates due to several factors, including design parameters as well as radiative and convective heat losses from the system, which considerably impair the system's efficiency. Novel designs and unique heat transfer techniques are increasingly being explored to improve the efficiency of SWHS. In this framework, the following research focuses on designing and optimizing an SWHS integrated with perforated delta obstacles. The influence of Reynolds number (400, 800, 1200), angle of attack (15°, 30°, 45°), and pitch ratio (0.5, 1, 1.5, 2.0) on the friction factor, Nusselt number, and thermo-hydraulic performance was investigated and optimized. The combination of Reynolds number = 1200, angle of attack = 45°, and pitch ratio = 1 yielded the most significant Nusselt number (90.55). In contrast, the combination of Reynolds number = 1200, angle of attack = 15°, and pitch ratio = 0.5 yielded the lowest friction factor (0.38). The largest thermo-hydraulic efficiency (2.75) was obtained using Reynolds number = 400, angle of attack = 45°, and pitch ratio = 1. Since no single SWHS design alternative can meet all desired performance criteria, selecting the best among SWHS design alternatives is not easy. Therefore, a hybrid multi-criteria decision-making approach called AHP (analytic hierarchy process)-ARAS (additive ratio assessment) was implemented, suggesting the SWHS design alternative having the angle of attack = 45°, Reynolds number = 1200, and pitch ratio = 1 satisfies the preset performance criteria. Finally, sensitivity analysis and validation with other decision-making approaches were performed to verify the proposed decision-making framework's effectiveness, proving the results' robustness.

A Comprehensive Assessment of Two-Phase Flow Boiling Heat Transfer in Micro-Fin Tubes Using Pure and Blended Eco-Friendly Refrigerants

This review study examines flow boiling heat transfer in micro-fin tubes using mixed and pure refrigerants with zero ozone-depleting potential (ODP) and minimal global warming potential (GWP). This investigation focuses on the extraordinary relationship between heat transfer coefficients (HTCs) and vapor quality. Since the introduction of micro-fin heat exchanger tubes over 30 years ago, refrigerant-based cooling has improved significantly. Air conditioning and refrigeration companies are replacing widely used refrigerants, with substantial global warming impacts. When space, weight, or efficiency are limited, micro-fin heat exchangers with improved dependability are preferred. This review article discusses flow boiling concepts. The researchers used several refrigerants under different testing conditions and with varying micro-fin tube parameters. Micro-fin tubes are promising for improved heat transfer techniques. This tube increases the heat transfer area, fluid disturbance, flow speed, and direction owing to centrifugal force and HTC. As the focus shifts to improving heat transfer, pressure drop, mean vapor quality, and practical devices, this subject will grow more intriguing. A radical shift will reduce equipment size for certain traditional heat transfer systems and bring new products using micro-scale technologies. This suggested review effort helps comprehend saturation flow boiling through micro-fin tubes and find the right correlation for a given application. This domain’s challenges and future relevance are also discussed.

Characteristics of heat transfer and flow resistance of magnetic fluid flow through porous media combined with magnetic field effect

The heat transfer and flow resistance of magnetic fluid flow under a magnetic field in the absence and presence of porous media were investigated experimentally in a rectangular duct. A ceramic foam, with a pore density of 13PPI (pores per inch), was used as a porous media. The mainstream flow rate varied between 0.1 and 3.7 l/min, while the magnetic intensity was adjusted to 0, 100, 300, and 500 mT. The results showed heat transfer was enhanced in both the non-porous and porous ceramic by applying the magnetic field to the magnetic fluid flow. Under a magnetic intensity of 500 mT, the heat transfer increments in the absence and presence of the foam were approximately 16.8% and 15.3%, respectively. By applying a magnetic fluid flow through the foam, the maximum heat transfer increment reached 171.7% compared to the absence of foam at the same flow rate. Notably, combining the use of a magnetic field and the ceramic foam resulted in a synergetic enhancement in heat transfer at low flow rates. The magnetic fluid flowing through the foam had a substantially higher flow resistance than the non-foam magnetic fluid flow. The pressure drop increased by applying a magnetic field to the magnetic fluid in the absence and presence of foam. The performance of the combined heat transfer techniques was evaluated in this research based on a performance evaluation criterion value. The results revealed that by applying the magnetic field, the performance evaluation criterion value of the magnetic fluid flow in the presence of foam was higher than in the absence of foam. Finally, the predictive formulas for Nusselt number and performance evaluation criterion as a function of Magnetic fluid heat transfer number were proposed.

DESIGN OF THERMAL CONDUCTING EQUIPMENT

This research presents the design and manufacture of thermal conductivity test equipment using heat transfer techniques (steady state) with the ability to test materials at high temperatures. The design of this test equipment is an extension of the ASTM D5470-06 standard, where the same is used to calculate the surface temperature and heat transfer through the sample during the test, where at each reference rod point, 10 K-type thermocouples are embedded. The maximum heater power required by the reference rod with a heating time of 1 hour is 30.4 KW. This test tool uses 3 pieces of 500-Watt power, and Thermoelectric TEC1-12706 12 Volt 6 Ampere 40x40 mm, Heatsink, 12 Volt DC Fan 0.15Ampere 38x38 mm, to keep the test at a steady state. And the use of Thermal Paste to minimize thermal contact resistance. From the calculations, it can be seen that the faster the time needed to heat the reference rod will require greater heater power, and vice versa. For high temperatures, this tool can reach a test temperature of 100 ℃. This research was conducted to measure the thermal conductivity of stainless steel, iron, wood, rubber, and Styrofoam. After the measurement, the thermal conductivity of stainless steel was 15.14037 W / mK Reference 14.00 W / mK, iron 41.07 W / mK Reference 79.5 W / mK, wood 1.54 W / mK Reference 0.21 W / mK, rubber 0.6 Reference 0.2 W / mK, Styrofoam 0.062 W / mK Reference 0.095 W / mK. Thus, further research is needed on the method of measuring the thermal conductivity of metals by transient means.

Effect of design parameters on passive control of heat transfer enhancement phenomenon in heat exchangers–A brief review

The development of innovative heat exchangers with improved heat transfer performance is an important concern for heat transfer applications. The use of passive techniques such as vortex generators, surface roughness, augmentation of heat transfer area, or the combination of passive techniques is effective to improve the performance of heat exchangers. This review briefly summarizes the passive heat transfer enhancement techniques and the most important types of heat exchangers. Many effective design parameters have been briefly discussed to improve the performance of passive heat transfer techniques for heat exchangers.

Evaporation of sessile droplet on surfaces with various wettability

The evaporation kinetic mechanisms of sessile droplets are investigated with numerical simulations. A sessile droplet evaporation model, combining flow, heat transfer, and vapor transport equations, is developed based on Arbitrary Lagrange-Euler (ALE) method. The numerical results are validated to be in good accordance with the experimental results in the literature. The results show the formation and evolution mechanisms of the internal flow in an evaporating droplet are controlled by the thermal conduction distance and evaporative cooling effect. As the contact angle decreases, the single-vortex flow inside an evaporating droplet transforms into a multi-vortex flow. Furthermore, the maintenance of the multi-vortex flow inside a flat droplet requires an appropriate volume. The results presented in this work provide a theoretical basis and practical guidance for exploring more efficient heat transfer techniques.

Productivity enhancement of solar still through heat transfer enhancement techniques in latent heat storage system: a review.

Solar still is one of the sustainable and renewable technology which converts brackish or salty water into fresh water. The technology helps in CO2 mitigation, global warming effect, and the use of solar desalination contributes towards decarbonization, mitigation of CO2 and other adverse global warming effect, and it contributes to the sustainable development goals (SDG). However, due to the low production rate of the distillate, the performance of solar still gets affected. The phase change materials (PCMs) as latent heat storage systems can enhance the thermal performance of solar still (SS). Further, techniques like increasing the area of contact and thermal conductivity can be practiced to enhance the heat transfer in PCM-SS. The article reviewed the performance of various designs of solar still integrated with PCM. Furthermore, the effect of nanoparticles enhanced PCM-integrated solar still with different absorber designs and configurations was seen. Compared to conventional solar still (CSS), the heat transfer techniques in PCM's SS can significantly improve the overall distillate productivity of Tubular SS by 218%, followed by single basin single slope SS 149%, pyramidal 125%, hemispherical 94%, and stepped 68%, respectively. In addition, the night time productivity was increased by 235%. Also, it was observed that in comparison to tubular PCM-SS, the nanodisbanded tubular PCM-SS increases the productivity by 68%, whereas in stepped solar still by using external condenser arrangement the productivity was increased by 48%. In single basin single slope, the nanoparticle disbanded PCMSS increases the productivity from 11 to 33%.

Research Progress on Process-Intensified Water Treatment Applications

Process intensification (PI) is aimed towards essentially smaller, cleaner, safer, energy-efficient sustainable technologies involving the application of a number of strategies, including reducing the number of devices, miniaturization, process integration, improving mass and heat transfer, novel energy and separation techniques and combined optimization and control methodologies. Over the recent years, PI has attracted attention in the domain of aqueous medium adsorptive separations and wastewater treatment as well. Thus far, a limited number of investigations have appeared in the literature; in addition, there is yet a lack of published methods to follow the intensified solutions for processes in wastewater treatment. In this connection, this article aims to present an overview of the recent applications and advances in process-intensified decolorization of dyes; removal of aromatic hydrocarbons from wastewaters; and recovery of proteins, heavy metals and rare earth elements from aqueous media. Selected applications have been identified in terms of the PI techniques, and the corresponding process improvements have been discussed for a variety of examples with the aim of contributing to the future progress of applications. It has been confirmed that considerable process improvements could be possible, such as intensified process efficiency, improved adsorption and separation performance, and minimized sorbent requirement and processing time. Even though there have been considerable developments in the field, there is still a need for further developments for the enhancement of the technologies in adsorption wastewater treatment using a systems approach.

Three-dimensional simulation of conjugate heat transfer using the hybrid lattice Boltzmann-finite difference method

This paper presents a three-dimensional numerical study of natural convection and conduction thermal phenomena using MRT-lattice Boltzmann and finite difference approaches. Conduction is examined using a simple conjugate heat transfer technique, which is predicated on the assumption of continuity of heat flux and temperature at the solid/fluid interfaces, instead of utilizing the conventional method based on the solution of the conduction equation. Several parameters such as Rayleigh number and thermal conductivity related to common construction materials are also investigated to explore their effects on the evolution of heat transfer. The obtained results show that augmentation of the Rayleigh number increases the quality of heat transfer by around 35–293% for various values of thermal conductivity ratio (solid/fluid). While the increase in thermal conductivity ratio enhances the heat transfer rate by about 212–685%, where the maximum rate of heat exchange is still the same for the thermal conductivity ratio ≥1000. Entropy generation is also examined in this paper to evaluate the efficiency of our studied system. The performed investigations indicate that the rate of entropy generation due to heat transfer, friction, and total entropy generation varies significantly with increasing Rayleigh number and thermal conductivity of the solid.

Performance enhancement of triplex tube latent heat storage using fins, metal foam and nanoparticles

The inherent low thermal conductivity of latent heat storage materials hinders its effective use due to quite long charging/discharging process. Heat transfer enhancement techniques are promising solutions to overcome this major drawback. In this study, the effect of combining heat transfer techniques including fins of different arrangements, fins-nanoparticles, and fins-metal foam is numerically examined and compared for latent thermal energy storage in a triplex tube. Two fin configurations are examined uniform as well as non-uniform. The predictions of the numerical model are validated by comparisons with experimental data available in the literature. The transient liquid fraction, temperature distribution, and interface position are analyzed for the considered cases. Results show that the non-uniform fin arrangement contributes to improving the thermal energy storage. The non-uniform fin distribution expedites the melting rate by 56.45%. Furthermore, it is shown that the combination of fins-metal foam outperforms the fins-nanoparticles configuration. The time needed for full charging and discharging reduces with fin-metal foam by up to 80.12% and 90.04%, respectively, depending on the fin arrangement. The proposed storage model and results are useful for designing efficient thermal energy storage units.

Effect of a Triangular Configuration of Heat Exchanging Tubes on Local Heat Transfer Coefficient in Bubble Column Reactor

The effect of a triangular configuration of heat exchanging tubes (HETs) on the instantaneous and local heat transfer coefficient (LHTC) has been investigated and quantified in an air-water bubble column (BC) utilising an advanced heat transfer technique. The bundle of heat exchange tubes was designed to cover 25% of the column's cross-sectional area (CSA) to simulate the heat exchanger tubes used in the industrial Fischer Tropsch (FT) process. The local heat flux, surface temperature, bulk temperature, and heat transfer coefficient (HTC) were investigated instantaneously in a Plexiglass BC under a wide range of superficial gas velocity (5-45 cm/s). The obtained results reveal that the presence of a bundle of (HETs) and their arrangement effect the (HTC) in the bubble column, where unsymmetrical profiles for (HTC) were obtained with triangular tube pitch arrangement. Additionally, steeper (HTC) profiles notice were when the triangular tube pitch arrangement was utilized. High (HTC) values were obtained at the core region of the column relative to the wall region under all the studied operating conditions. It was discovered that the local and (HTC) increase as superficial gas velocity increases despite the fact that the bubble column is heavily packed with a bundle of (HETs). The rise in HTCs was higher at the core than at the wall region of the column. For example, at a superficial gas velocity of 45 cm/s and at an axial height of 65 cm (Z/D=5), compared to the wall, the HTC in the core of the column increased by 131 %.

Microstructure-guided computational model for predicting effective thermal conductivity of cementitious composites filled with phase change particles

Functional composite phase change materials (CPCMs) are particularly emphasized by integrating PCM particles in building materials for energy-saving applications, due to PCM's excellent temperature adjustment capacity. However, the inherent drawback of low thermal conductivity of PCM decreases its phase-changing efficiency. Finding highly-efficient heat transfer technique becomes quite necessary. This work utilizes small hollow steel balls (HSBs) to encapsulate paraffin to provide a simple and practical solution, not only ensuring sufficient protection to PCM core in alkaline environment but also enhancing heat transfer into PCM. Then the PCM-HSBs aggregates mix with cementitious matrix to fabricate CPCM. The composite thermal conductivity (CTC) is respectively determined by experiment, theory and computational microstructure model precisely representing random morphology of PCM-HSBs in matrix, and a good agreement is observed between them. Then the dependence of CTC on microstructure features is illustrated to identify their roles. Finally, the composite compressive strength (CCS) and failure mode is discussed briefly to establish comprehensive understanding of composite properties. The results show that CTC has a significant increase of 25.84%, while CCS decreases 12.7%, when 15.28% PCM-HSBs are used. The degradation of CCS is mainly attributed to the smooth surface of HSB, which can be roughened by abrasive blasting.

Prospects of heat transfer approaches to dissipate high heat fluxes: Opportunities and challenges

• High heat fluxes cooling needs are representing the bottleneck for development of many technologies. • Thermal managements has focused on two-phase flows cooling techniques and enhancement methods. • Discussion of boiling in micro-channels and spray cooling highlights the trends and limitation in these two cooling approaches. • Discussion of nanofluids as working fluids and surface modifications highlights the needs for further cooperation between disciplines to make progress. The present paper presents a brief discussion of selected promising approaches adopted to achieve high heat fluxes dissipation. The selected techniques include flow boiling in microchannels, spray cooling, use of nanofluids and surface modification. Whilst the two first approaches are established two-phase heat transfer techniques, the use of nanofluids and surface modification are comparatively new approaches used to enhance heat transfer. Achievements, challenges and new research directions in the selected techniques are covered to provide an overview and guideline for researchers and engineers working in this field.