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Theoretical analysis, numerical simulations, and experimental studies are used to comprehensively investigate the changes in the heat transfer characteristics of Fe3O4–water magnetic nanofluid under different types of magnetic fields and the heat transfer mechanism, providing new ideas for its practical application. By combining the fundamental conservation theorem and the Boussinesq approximation, dimensionless control equations have been established. The analysis of the variation pattern of the calculated values of the source terms under different boundary conditions, as well as the order of magnitude analysis, concludes that: the enhanced trend of Brownian motion and thermophoretic motion is the main reason for the enhanced heat transfer capability of the magnetic nanofluid; where the thermophoretic motion contributes slightly more to the heat transfer; the increase in Joule heat is the reason for the further enhancement of the heat transfer capability by the magnetic field. Volume fraction, temperature, and magnetic field strength are positively correlated with the average convective heat transfer coefficient. The reason for magnetic fields to enhance heat transfer is revealed by the deflection of the magnetic nanofluid by the magnetic body flow under local magnetic fields to form localized reflux. The alternating magnetic field further enhances the ability of magnetic fields to enhance heat transfer in magnetic nanofluids. Overall, the effect of the applied magnetic field on the motion of the magnetic nanoparticles causes the disruption of the thermal boundary layer, which is responsible for the enhanced heat transfer capacity of magnetic nanofluids. [ABSTRACT FROM AUTHOR]

This article numerically investigates the behavior of nonuniform distributed serrated fins including equaled, shortened, lengthened, short on both sides, and long on both sides. In outputs, not only the temperature field and velocity contour but also the synergy field were examined. The results show significant advantages of nonuniform fins, having certain characteristics, in improving the overall heat transfer performance. At the near entrance of nonuniform fin type, heat transfer can be improved by increasing fin length gradually, while it becomes worsened by decreasing fin length. The length of the first fin in the near inlet region has a great influence on the distribution of the temperature field. For the best array compared with the uniform case, the Colburn j factor increased by 9.1 − 16.8%. Either uniform or nonuniform fin type deteriorates the flow resistance while heat transfer capability is enhanced. From the perspective of synergy between temperature gradient and velocity, the local field synergy angle at the near inlet region, and the windward side of the fin has a decisive effect on the heat transfer performance. [ABSTRACT FROM AUTHOR]

For the serrated heat transfer surface, the heat transfer and flow properties are investigated with external vibration excitation using numerical simulation method. By using the dynamic grid to realize the vibration of the fin wall, the impacts of frequency, amplitude, wind velocity and ambient temperature on vibration-strengthened heat transfer are investigated. The results show that vibration impacts the temperature distribution around the fin, interferes with the flow of air, causes the air to form a certain angle with the fin wall, thus preventing the development of the boundary layer and achieving strengthened heat transfer. The impact of amplitude on vibration-strengthened heat transfer is smaller than that of frequency, and low wind velocity is more favorable to the role of vibration-strengthened heat transfer, and the change of ambient temperature has less impact. Compared with the stationary condition, wall vibration can improve the average heat transfer coefficient by up to 66.6%. [ABSTRACT FROM AUTHOR]

Nanofluids are important heat transfer mediums with great potential in industry applications because of its high thermal conductivity when adding metal or metalloid nanoparticles, which has better heat transfer characteristics and controllability than the base liquid. However, there are great uncertainties, deviations and anomalies in the computational model, experimental results and theoretical analysis when conducting research on the thermal physical properties, convective heat transfer performance and heat transfer mechanism of nanofluids. This poses a great challenge to the precise and active controllability of nanofluids when considering their own parameters and external excitation. Also, these inevitable problems may cause certain confusions on the follow-up study of nanofluids. Based on these phenomena, this paper systematically reviews the recent researches on heat transfer performance of nanofluids. The thermophysical characteristics, convective heat transfer coefficient, stability and heat transfer mechanism of suspension mixed nanofluids were summarized and compared. It is hoped that it can provide effective references and guidance for researchers to study the heat transfer performance of nanofluids, and also provide new ideas for future researches on the heat transfer performance of nanofluids. [ABSTRACT FROM AUTHOR]

• A comprehensive review of the current research status of microchannel heat sinks was conducted from the perspectives of channel configuration, secondary structure, combined structure, and special cooling agents. • An analysis of the heat transfer and flow characteristics of various microchannel heat sinks was conducted. • A comparison of the advantages and disadvantages of various microchannel heat sinks was made. Enhanced heat transfer technology can be divided into active technology and passive technology based on whether external power is consumed. Due to the absence of external energy consumption, passive technology is more reliable than active cooling technology. Microchannel heat sink (MCHS) are widely used for heat dissipation in the field of microelectronics. In recent years, due to the continuous miniaturization of electronic chips and the high heat flux per unit area (about 300 W/cm2), the passive enhanced heat transfer technology of microchannel heat sinks has developed rapidly. This article provides a comprehensive review of passive enhanced heat transfer technology in MCHS. It introduces the influence of channel configuration, secondary structure, combined structure, and different heat transfer media on the heat transfer capacity and pressure drop of microchannel heat sinks. Finally, a summary is given based on the collected data. [ABSTRACT FROM AUTHOR]

The steady state heat transfer and flow resistance performance in cosine style runners with different amplitudes are studied numerically and experimentally in this paper. The results show that: When the Reynolds numbers (Re) range from 1210 to 5080, the core volume goodness factor (ηohstdα) is used to compare the overall heat transfer performance of the two runners, and the ηohstdα value in the cosine style runner is 7–25% larger than that of the equal cross section runner, so that the cosine style runner has better overall heat transfer enhancement performance. When the amplitudes (2A) range from 5 to 9 mm, with the decrease of amplitude, the overall heat transfer performance is getting better. At the same amplitude, the convective heat transfer performance gradually increases as the inlet height (Fh) decreases; with the increase of Re, the thickness of the thermal and velocity boundary layers are both decreasing. Based on the field synergy principle, the heat transfer enhancement mechanisms with different parameters are evaluated, and we conclude that the smaller the amplitude is, its field synergy is better. [ABSTRACT FROM AUTHOR]

The laminar convective flow and heat transfer in trapezoidal ducts with a rectangular cross-section are studied numerically. Under certain conditions, the temperature difference distribution and pressure drop in trapezoidal ducts are better than that of in rectangular ducts, which leads to an advantage of heat transfer enhancement. At the same heat transfer area, the maximum Colburn (j) factor in trapezoidal ducts increases by 8% when compared with a rectangular duct, and the minimum friction (f) factor decreases by 22.6%. The numerical data also shows that the thermal-hydraulic characteristics for trapezoidal ducts first increases and then decreases while slope angle (β) increases. The airflow is disturbed at the wall showing recirculation flow and separation when β = 30°, but the thermal-hydraulic performance starts to decrease when β > 40°. In a word, the thermal-hydraulic characteristics of trapezoidal ducts are better than that of rectangular ducts when β ≤ 40°. On the contrary, When β > 40°, the rectangular duct has a better performance than that of trapezoidal ducts. The applicability of the above conclusions is proved by using the taguchi method. [ABSTRACT FROM AUTHOR]

Convective heat and mass transfer are common phenomena in the field of engineering. In recent years, the research on convective heat and mass transfer is getting deeper and deeper. People are constantly seeking new methods to improve the efficiency of heat and mass transfer, but the research on the mechanism of heat and mass transfer is not yet systematic and mature. The theory of the field synergy has made up for this research gap, but the research progress and application of its heat and mass transfer theory lack systematic summary and evaluation. This paper summarizes the theory of enhanced heat and mass transfer based on field synergy theory, analyzes the field synergy equation of heat transfer, mass transfer, and heat-mass cooperation under laminar and turbulent conditions, and gives the evaluation standard of field synergy equation. Field synergy theory shows that improving the synergy between the velocity field and temperature gradient field, that is, reducing the angle between them, can effectively improve the heat transfer efficiency, and improving the synergy between the velocity field and concentration gradient field can effectively improve the mass transfer efficiency. Subsequently, the research progress of field synergy theory in different engineering fields is summarized. It is shown that field synergy theory can be used to analyze the cooperation between fields in heat exchangers, micro-components, nanofluids, reactors and other fields to guide the optimal design of equipment, which proves the universality of field cooperation theory. Finally, in view of the universality of the damp-heat exchange in the cooling exchangers and the research object of field synergy theory, a new idea of using the field synergy theory to guide the research on the structural design of cooling exchangers is put forward, which provides new ideas and methods for effectively improving the underground humid and thermal environment. [ABSTRACT FROM AUTHOR]

The heat transfer and pressure drop performance of magnetic nanofluids (MNFs) under different alternating magnetic fields were investigated experimentally. The results showed that the effect of alternating magnetic field on local heat transfer coefficient is better than that of unidirectional and non-magnetic fields along flow direction. The local heat transfer coefficient in the rear of the flow channel has a greater promoting capacity than that in the front. The heat transfer performance is higher at a lower Reynolds number (Re), but worse at a higher Re. Heat transfer coefficient always increases with the increase of volume fraction with or without alternating magnetic fields. When the volume fraction is among 1 vol%-3 vol%, the greater the alternating frequency is, the better the heat transfer performance will be. When the volume fraction is greater than 3 vol%, increasing the frequency of alternating magnetic field has little effect on heat transfer performance. For magnetic nanofluids with different volume fractions and different magnetic fields, they also show quadratic variation rules. The larger the volume fraction and magnetic field frequency are, the greater the pressure drop will be. The improvement of the overall heat transfer performance increases first and then decreases when the alternating frequencies and Reynolds numbers increase. The reasons for the improvement of the heat transfer are the movement and accumulation of magnetic nanoparticles at the thermal boundary layer, and the eddy action of MNFs. • Effect of alternating magnetic field on heat transfer coefficient is better than that of unidirectional fields. • Heat transfer coefficient increases with the increase of alternating frequency and volume fraction. • Factors that enhance heat transfer are particle motion, accumulation, vortex flow, and thermal boundary layer. • The larger the volume fraction and magnetic field frequency are, the greater the pressure drop will be. • The overall heat transfer performance increases first and then decreases. [ABSTRACT FROM AUTHOR]

Numerical simulations were adopted to explore the heat transfer and flow characteristics of magnetic nanofluids under different magnetic field intensities, volume fractions, and magnetic field directions. Due to these parameters have a significant effect on thermal hydraulic performance and the mechanism of this enhancement are not yet clear, a turbulent model of Re-normalization group (RNG) k-ε is used to analyze the variation of thermal boundary layer and particle motion. The obtained results found that the effect of heat transfer enhancement was small under a weak magnetic field, but it increased considerably under a strong magnetic field. Besides, the convection heat transfer coefficient initially increased and then decreased with an increase of volume fractions of magnetic nanoparticles. Moreover, the directions of the magnetic field have a significant effect on convective heat transfer coefficient, which results in 8% increase when the direction is perpendicular to direction of flow. [ABSTRACT FROM AUTHOR]

With the increasing application of heat exchangers, the research has become increasingly comprehensive. Previous studies mainly focused on comparing the amplitudes to investigate the performance of heat exchangers, while the study of variable amplitude channel structures was limited. This study investigated the thermal-hydraulic performance of seven different sinusoidal channels with inclination angles of 20° and 30°, and their combinations. Using the CFD solver, simulations were conducted to assess the heat transfer capabilities of sinusoidal channels under various Reynolds numbers and different amplitude conditions, compared to straight channels and single amplitude sinusoidal channels. The results indicated that within one period, both the pressure drop and temperature difference exhibited a ripple trend of gradually decreasing amplitude. When evaluating the comprehensive performance of the channels, three trends emerged across the seven models, with the performance of the cold fluid and the hot fluid within the same group showing opposite trends. This suggested that more variations could enhance the overall performance of the hot fluid while reducing the performance of the cold fluid. For structures with the same amplitude, in terms of amplitude variation frequency, models with six periods per group exhibited higher overall heat transfer performance than those with three periods per group, with an average increase of 36.37%. For the arrangement of amplitudes (taking the hot fluid inlet as an example), configurations with a smaller amplitude at the inlet outperformed those with a larger amplitude, indicating that in combined amplitude models, a structure with a larger group and a smaller amplitude at the inlet (taking the hot fluid as an example) should be designed. [ABSTRACT FROM AUTHOR]

The heat transfer and flow characteristics of the air-water cross flow over cambered ducts were experimental and numerical investigated. The sequence of their Core Volume Goodness Factor (CVGF) is cosinoidal, parabolic, circular, trapezoidal and rectangular ducts successively from superior to inferior. Cambered ducts have more uniform temperature difference distribution than the equal cross section duct, and it has the minimum temperature difference in inlet and outlet of the cosinoidal duct. With the optimal overall heat transfer performance, the cosinoidal duct is superior to that of the rectangular duct by 7.3%–28.1%. In the cosinoidal duct, the smaller the amplitude is, the better the heat transfer performance is. The thickness of the thermal and velocity boundary layers adjacent to the wall surface decreases constantly with increased Reynolds number. In the near wall region, n = 5 um , the main heat transfer area is the peak and middle areas, but with weaker heat transfer performance in the trough region. Although gradually expanding cambered duct slows down the flow velocity, the structure form decreases the pressure drop loss during the flow process. While improving the convective heat exchange capability of the upstream, the heat transfer area of the downstream is also improved to boost the overall heat transfer performance. • Performance in divergent cambered ducts are better than that of equal cross section duct. • The cambered ducts have uniform temperature difference distribution. • The smaller the amplitude is, the better overall heat transfer performance is. • Temperature gradient in the peak region is bigger than that in the trough region. [ABSTRACT FROM AUTHOR]

• The performance in parabolic duct is better than that of trapezoidal duct. • Divergent duct can improve heat transfer performance. • The overall heat transfer performance decreases with the increase of focal length. • The parabolic duct has uniform temperature difference distribution. In this paper, the heat transfer and flow characteristics of parabolic ducts with different focal lengths (P = 120 mm, 140 mm, and 160 mm) are analyzed and compared with rectangular and trapezoidal ducts respectively. The governing equations in the curved ducts, including the mass conservation equation, momentum equation and energy conservation equation, are established. The results show that the order of the overall heat transfer performance of the three ducts from the best to worst is parabolic duct, trapezoidal duct and rectangular duct. The parabolic duct has the best overall enhanced heat transfer performance, which is 5.1% and 25.4% larger than that of the trapezoidal and the rectangular ducts respectively. This means that, although the divergent type structure reduces the flow velocity and thus weakens the intensity of convective heat transfer in the duct, it improves the downstream heat transfer area and reduces the pressure drop loss in the flow process, thus promoting the improvement of overall heat transfer performance. Besides, the temperature difference in the parabolic flow duct has the most uniform distribution, which is an important factor to improve and strengthen heat transfer performance. From the perspective of field synergy theory, the average field synergy angle decreases with the increase of focal length, and the field synergy becomes better. [ABSTRACT FROM AUTHOR]

Most of the thermal management for the battery energy storage system (BESS) adopts air cooling with the air conditioning. However, the air-supply distance impacts the temperature uniformity. To improve the BESS temperature uniformity, this study analyzes a 2.5 MWh energy storage power station (ESPS) thermal management performance. It optimizes airflow organization with louver fins and simulates its heat transfer behavior. To improve the flow rate distribution along the airflow passage, the air-supply organization of louver fins for the overhead air conditioner is optimized. It indicates that the air-supply angle (A) for louver fins and the battery spacing (D) impacts greatly on BESS thermal management performance. The air supply optimal organization with the A of 90° and the D of 18mm was determined, of which the average temperature (TA) rise of the battery can be reduced by 18%, and the temperature difference (TD) is decreased by 23%. Furthermore, a multi-parametric sensitivity analysis is conducted, and the different heat-flow parameter influential sequence for the TA and TD are revealed. The air supply volume flow rate (L) and the air supply temperature (AST) has a significant effect on the TA reduction, and the AST shows significance on the TD decreasing. When AST is reduced to 290 K, the TA rise of batteries decreased by 71%. When the L is increased 10 times, the TA rise of batteries decreased by 227%. When the AST is decreased to 290 K, the TD is reduced by 39%. When the L for air supply is increased by 10 times, the TD will increase by 28%. In this research, the louver fins are firstly adopted for orifices air supply organization to improve thermal field uniformity for BESS. [ABSTRACT FROM AUTHOR]