Enhanced Heat Transfer Effect Research Articles

Experimental study of nucleate boiling heat transfer of refrigerant-oil mixture on various modified surfaces

As a crucial component of refrigeration and heat pump system, the evaporator performance directly affects system efficiency. Due to the lubricating oil in the compressor circulating into the evaporator, studying the nucleate boiling heat transfer (NBHT) of refrigerant-oil mixture is of greater practical and guiding significance. Surface modification technology has opened up a new avenue for enhancing NBHT of refrigerant-oil mixture. Therefore, experimental study was conducted on NBHT characteristics of R134a-POE mixture and R1234ze(E)-POE mixture on smooth surface, initial laser-ablated surface, stabilized laser-ablated surface, machined surface, and composite processed surface. The effects of oil concentration (ω), surface microstructure and surface wettability on NBHT of refrigerant were discussed and analyzed, and the underlying heat transfer mechanisms were elucidated. The research indicates that for NBHT of refrigerant-oil mixture, surface microstructure has a greater impact than surface wettability. To enhance heat transfer coefficient (HTC), it is crucial to establish a surface pattern comprising alternating regions of high and low nucleation sites to mitigate bubble interaction and coalescence. Once this objective is attained, enhancing surface wettability can be contemplated. The addition of oil promotes foaming and induces tornado-shaped bubble group, which facilitate heat transfer. However, the rise in mixture viscosity and surface tension exerts a more pronounced adverse effect on heat transfer, outweighing the positive impacts of foaming and tornado-shaped bubble group. Consequently, the presence of oil diminishes HTC. And as ω and heat flux increase, HTC further decreases. Additionally, the increase in vaporization nuclei on modified surface leads to a higher local ω, the increase in ω further weakens the enhanced heat transfer effect of modified surface, and thus narrows the difference in HTC among various modified surfaces. The research results aim to uncover the mechanisms by which surface wettability, surface microstructure, and lubricating oil affect NBHT of refrigerant.

Experimental study of heat transfer performance in rectangular microchannels enhanced by ultrasound

Ultrasound is recognized as an effective technology to enhance heat transfer, which is widely used in industrial fields such as microelectronics and heat storage. However, the heat transfer characteristics in rectangular microchannels assisted with ultrasound has little revealed in present research. In this work, the external ultrasonic was added in a rectangular microchannel to reveal the influence of ultrasound on the heat transfer characteristics of micro-channels. Hence, the comparative experiments were completed to explore the heat transfer performance with and without ultrasonic. Moreover, a large number of experiments were carried out within rectangular microchannels by varying the velocity of the working fluid and the heat flux at the bottom of the microchannels. The results indicate that the heat transfer performance of rectangular microchannels can be enhanced by ultrasound. But the enhanced heat transfer effect could be limited with the increase of the flow velocity and heat flux in microchannel correspondingly. For rectangular microchannels with adding ultrasound, the lower the flow rate and heat flow density are, the better the enhancement effect will be. During the measurement, when the fluid flow velocity is 0.2 m/s and the heat flux is 408.2 kW/m2, the heat transfer performance of the ultrasound-enhanced microchannel can be improved by a maximum of 12.9 %. Additionally, the tests indicate that the pressure drop mainly comes from the viscous loss along rectangular channel, yet the ultrasound has no negative impact on pressure drop in microchannel. This work could be beneficial to the further application of ultrasonic enhancement technology.

Study of ultrasonic influence on heat transfer and resistance performance of round tube with twisted belt

Abstract In this study, ultrasonic technology is combined with twisted belts to explore the comprehensive performance, and this study also investigated the effect of different Reynolds numbers, ultrasonic frequencies, and number of transducers on the performance of circular and twisted band tubes. It was found that ultrasonic waves applied on the tube plate enhanced the heat transfer performance of the heat exchanger tubes, reduced the flow resistance, and improved the overall performance, and the lower the ultrasonic frequency, the better the heat transfer and resistance reduction ability, and at the experimental condition frequency of 21 kHz, the maximum increase of Nu is 19.06%. With the increase of Reynolds number, the better the ultrasonic enhancement heat transfer performance, but the worse the resistance reduction performance. For different heat exchanger tube structures, the synergistic enhanced heat transfer effect of ultrasonic waves with the twisted belt is better than round tubes, and the synergistic drag reduction effect with the round tube is better than the twisted tape round tube. When the installed ultrasonic transducers are two, the heat transfer performance of the heat exchanger tube is the best, and the maximum increase in the value of Nu was 28.06%.

Investigation on heat transfer enhancement of supercritical hydrocarbon fuel in rotating channel interpolated with spiral wire

Hypersonic vehicles carry their hydrocarbon fuels as the only regenerative fluid because the existing air-cooled turbine blades cannot meet the stringent heat transfer requirements, including aerodynamic heating in overload environments. In this study, supercritical-pressure n-decane was used as the working medium, and a spiral wire with a wire diameter and pitch of 0.1 and 10 mm was inserted into a smooth circular tube (inner diameter = 2 mm; length = 200 mm) for studying the convective heat transfer characteristics under different rotating speeds, mass flow rates, inlet temperatures, and heat fluxes and investigating the enhanced heat transfer effect of the spiral wire. Experimental results indicate that the overall heat transfer performance of the spiral-wire structure tube was improved by more than 1.3 times compared to that of the smooth tube. Further, at the trailing edge surface at 300 rpm, the Nusselt number increases by an average of two times; in the meanwhile, the PEC value for the reinforced tube is obtained as 1.25–1.68, indicating a better enhanced heat transfer capability in the rotating channel. The experimental results are compared and verified using a computational fluid dynamics method with SST k-ω turbulence model, revealing the maximum relative deviation to be within 13.56 %. The numerical simulation results explain the mechanism of the temperature differences between the leading- and trailing-edge surfaces within the centrifugal section and the reinforcing effect of the interpolated spiral wire. This study provides a novel orientation towards the application of aviation fuel for turbine blade cooling and the efficient heat transfer performance of cooling channels under accelerated conditions.

Study on Enhanced Heat Transfer of the Convex Columns in the Cooling Channel of Motorized Spindle Based on Field Synergy

The cooling performance of motorized spindles plays an important role in accuracy in high-speed machining. Aiming at improving the cooling performance of traditional motorized spindles, convex columns were built in the cooling channel. Based on field synergy, the effects of quadrilateral, circular and triangular convex columns on the heat transfer performance of the cooling channel were analyzed numerically. We also compared the pressure drop between the inlet and outlet under the same conditions. The results show that the cooling channels with triangular convex columns provide the best cooling effect with the smallest increase in area compared to quadrilateral convex columns and circular convex columns. The pressure drop in the cooling channels with a circular convex column is minimized. By optimizing the spacing of the convex column, the best effect was found at a spacing of 7 mm. By optimizing the angle of the top angle of the triangular column, it is found that the enhanced heat transfer effect is best at 120° when the heat transfer area is the same. In addition, when considering the addition of convex columns, it is important to ensure sufficient pressure drop to achieve a good cooling effect.

Thermal performance of cold thermal energy storage system with fin and fin–foam structures

The heat transfer performance of most cold thermal energy storage (CTES) devices is limited by the low thermal conductivity of phase change materials (PCMs) and the increase in the thickness of PCMs. A comparative work was performed to explore the heat transfer performance of CTES systems with a fin structure (Fin-CTES) and a fin–foam structure (Fin–foam-CTES). The heat transfer performance, temperature distribution, and thermal effectiveness of Fin-CTES and Fin–foam-CTES at different inlet temperatures and volume flow rates of heat transfer fluid were investigated and compared. Results demonstrated that the overall heat transfer performance of Fin–foam-CTES is better than that of Fin-CTES. However, compared with the PCM in Fin-CTES, that in Fin–foam-CTES has a greater degree of supercooling, reaching 4.35 °C at the maximum. In the discharging (melting) process, Fin-CTES and Fin–foam-CTES have almost similar heat transfer effectiveness, in which the maximum difference is only 0.0107. That is, the enhanced heat transfer effect of the natural convection of the liquid PCM and the metal foam is basically the same during the discharging process.

Quasi-analytical study of dynamic performance of the thermocline heat storage system under time-varying inlet flow and temperature environment

The dynamic thermal performance of single-tank thermocline (STTC) thermal energy storage (TES) system under oscillating boundary conditions (BCs) during cloud cover or start-up period is critical to the safety and efficiency of the whole concentrating solar power (CSP) plant. In this study, the approximate analytical solution of the STTC TES system under oscillating velocity and temperature BCs are derived from the one dimensional-two phase (1D-2P) models, which can be applied in arbitrary realistic BCs with Fourier decomposition method. After model validation with dynamic experimental data, the effects of typical sinusoidal oscillating parameters on packed bed transport phenomena and TES efficiency were investigated. It is found both of the oscillating inlet velocity and temperature lead to degradation of thermal stratification, such as outlet temperature fluctuation, while it also causes enhanced heat transfer effect and increased charging/discharging cut-off efficiency. Cut-off efficiency increases with amplitude of oscillating inlet velocity or oscillating inlet temperature, while for oscillating frequency there is optimum value. On that basis, the general optimum equation of cut-off efficiency and oscillating frequency is proposed, and the relative error is smaller than 1.0% when outlet temperature is larger than 0.011 for the discharging process with oscillating inlet velocity BC and smaller than 3.2% for oscillating inlet temperature BC. Moreover, the thermocline region movement velocity increases with inlet velocity for oscillating inlet velocity BC and decreases with oscillating frequency for oscillating inlet temperature BC.

Development and performance research of a helical longitudinal-groove tube with secondary flow reinforcement for heat transfer

The secondary flow has a strong promotion effect on heat transfer in the tube. In order to further utilize the secondary flow phenomenon to strengthen heat transfer of the helical tube, in this paper, a secondary flow enhanced heat transfer helical tube is developed, and stable secondary flow with high strength is obtained. The structure parameters are optimized with the highest comprehensive evaluation factor as the goal. The results show that compared with helical transverse-groove tube and helical grooved tube, helical longitudinal-groove tube has the best heat transfer performance. The new structure's enhanced heat transfer effect for high-Pr number fluid is better than that of low-Pr number fluid, and the enhanced heat transfer effect is not affected by the inlet Reynolds number. The helical angle of helical groove has a greater influence on the comprehensive evaluation factor, while the groove depth and the number of heads have a smaller influence on the comprehensive evaluation factor. The optimal parameter scheme is: The groove depth is 0.5 mm, the helical angle of helical groove is 82°, the head number of helical groove is 1, the comprehensive evaluation factor of helical longitudinal-groove tube compared with ordinary helical tube is 1.173

Nanoparticle Sphericity Investigation of Cu-Al2O3-H2O Hybrid Nanofluid Flows between Inclined Channels Filled with a Porous Medium.

With the porous medium-filling inclined channels, we investigate the nanoparticle sphericity of Cu-Al2O3-H2O hybrid nanofluid flows. We consider the constant flow rate through the channels as well as the uniform heat flux on wall channels. We provide analytical solutions for both the velocity and temperature fields. Several parameters are considered in the analytical solutions, including the mixed convection variable, the Peclet number, the channel tilt angle, and nanoparticle sphericity and volume fractions. The significant findings of this study are that the effective thermal conductivity increases when increasing the temperature in the same nanoparticle volume fractions. Nanoparticles with a smaller average sphericity size have a greater specific surface area and contain a greater concentration of small particles, which enhances the internal heat transfer of nanofluids. The other noteworthy observation of this study is that when the nanoparticle volume fraction increases from 0.1 to 0.2, although the heat transfer enhancement rate has slowed down, it has also increased by about 25%. The hybrid nanofluids have suitable stability, and the enhanced heat transfer effect is better with the increase in nanoparticle compositions.

Experimental and numerical studies on heat transfer enhancement in the silicothermic reduction process

Currently, the silicon thermal method is the main process to produce primary magnesium, which is controlled by chemical reaction kinetics and the heat transfer process in the retort. However, the unbelievably low thermal conductivity and point contact thermal resistance of the pellets greatly reduce the heat transfer efficiency, resulting in low magnesium production efficiency. To increase productivity, the enhanced heat transfer mechanism was studied in detail by experiments in this paper. The results show that temperature rise may not accurately reflect the enhanced heat transfer effect, thus a synthetical valuation method which took total heat transfer, effective heat transfer ratio and temperature rise into account is proposed. On this basis, a new recycling technique which uses high temperature silicon carbide particles to enhance the heat transfer in the retort is developed. Then a three-dimensional unsteady numerical model incorporating the chemical reaction kinetics and heat transfer was established and used to prove the superiority of this new technique in terms of heat transfer efficiency, magnesium production rate and production cycle. Numerical results show that when the initial temperature of silicon carbide is 1373 K, the instantaneous magnesium production rate can reach 13.2 kg/h within 30 min of heating, which is three times that of the traditional silicothermic process. The production cycle could also be shortened to 240 min, which is half of the original, and the production efficiency can be doubled. This new technique can significantly improve heat transfer efficiency, shorten the production cycle and reduce the production cost.

Study on Vibration and Heat Transfer Performances of a Modified Elastic Tube Bundle Heat Exchanger

Based on the bi-directional fluid-structure coupling method, the vibration and heat transfer performances of a modified elastic tube bundle heat exchanger are studied. The vibration frequency and amplitude of tube bundle are compared between different directions and tube row spacing. Furthermore, the Nusselt number of tube bundle are studied in vibrational and non-vibrational cases. The numerical results represent that the vibration amplitude of tube bundle is the largest in the z direction, which is 7.25 times that in the y direction (direction of the smallest amplitude). In the calculation scope, the vibration frequency and amplitude promote with tube row spacing. From 50 mm to 90 mm spacing, the vibration frequency and amplitude increase by 21.88% and 10.65%, respectively. The Nusselt number under the fluid-induced vibration increases up to 11.67% compared with non-vibration. Besides, the Nusselt number of tube bundle in the middle part is lower than that at the two ends. Within 50-90 mm interval, the enhanced heat transfer effect rises along with tube row spacing. The rising rate of the Nusselt number increases by 52.4%. in the range of computation.

CBN grain wear during eco-benign grinding of nickel-based superalloy with oscillating heat pipe abrasive wheel

Cubic boron nitride (CBN) has high hardness and superior thermal conductivity, and it was used to fabricate an oscillating heat pipe abrasive wheel (OHPGW) for the eco-benign grinding of hard-to-machine materials. Furthermore, the wear behavior of the CBN grains and the related lifecycle of the OHPGW are important. In this study, a comparative analysis was conducted to reveal the wear behavior of CBN grains on OHPGWs and conventional wheels. Under the enhanced heat transfer effect of the OHPGW, the CBN grain wear behavior, development of the CBN protrusion heights, and the ground surface quality were investigated. The CBN grains on the abrasive wheel experience initial, stable, and fast wear periods in conditions of attrition, micro-fracture, and macro-fracture. Owing to the low grinding heat and temperature acting on the OHPGW, the CBN grains mostly undergo attrition during the stable period, while CBN grains on the conventional wheel mainly undergo micro and macro-fractures. As the wear of the CBN grains progressed, the average protrusion height of the CBN grains decreased. The scattering of the protrusion height distribution decreased at first and then increased when entering the fast wear period. Consequently, the surface topography and roughness initially improved and then deteriorated with the development of the protrusion height distribution of the CBN grains.

Amelioration of boiling heat transfer by 3D deposition structure of graphene-silver hybrid nanoparticle

• The novel hybrid coating solves the thermal barrier problem of graphene. • Modification makes the coating have rich chemical bonds and complex microstructure. • The oxygen-containing functional groups promote the capillary effect of coating. • Concentration has little effect on structure and element content of hybrid coating. During the boiling process, the heating surface has a substantial influence on the growth of the bubbles, the wetting of dry spots, and the deposition of the particles. Therefore, improving the surface properties is one of the main ways to enhance heat transfer. In this study the heating coatings were prepared by boiling deposition using GNP nanofluids, GNP/Ag mixed nanofluids, and GNP-Ag hybrid nanofluids with different mass fractions (wt% = 0.001,0.002,0.003) as raw materials. The boiling heat transfer characteristics of DI water on the bare copper surface and heating coatings were studied, and the heat transfer mechanism was analyzed. The results show that GNP-Ag hybrid nanoparticle coating has the most outstanding enhanced heat transfer effect. DI water achieves the highest critical heat flux (CHF) of 163.65 W/cm 2 and heat transfer coefficient (HTC) of 9.69 W/cm 2 K on the coating, which are respectively 45.84% and 82.14% higher than the smooth copper surface. The wettability was analyzed by measuring the contact angle. Scanning electron microscope image was used to analyze the microstructure of the coating. The microstructure of Ag modified functionalized GNP nanoparticle has a great change compared with mixed nanoparticles. It was observed that the hybrid nanoparticle deposit into a large number of bulges and cavies, and the bulges accumulate in clusters and form self-assembled flower-shaped bulges. Combined with the energy spectrum test results of the heating coating, the coverage of the nanoparticles in the coating on the copper substrate and the variation trend of each deposited element with the concentration are analyzed. The novel heating coating enhances heat transfer under the combined action of nanoparticle functional groups, coating microstructure, and wettability. GNP-Ag hybrid nanoparticle coating was applied towards the heat dissipation of electronic devices. When the input power is 300 W, the maximum temperature of electronic devices is 108.52 °C, which is much lower than that of other coatings.

Thermophysical properties analysis of graphene-added phase change materials and evaluation of enhanced heat transfer effect in underwater thermal vehicles

Underwater vehicles are one of the most important tools for ocean observation and exploration, whose duration and range of underwater vehicles can be significantly extended by harvesting and utilizing ocean thermal energy. Generally, most underwater vehicles use the thermal engine to harvest ocean thermal energy. The phase change material (PCM)is the most important component of the thermal engine, the heat transfer performance of which directly affects the performance of the underwater vehicle. Enhancing the thermal conductivity of phase change materials is an effective way to improve the heat transfer rate of underwater thermal vehicles. The heat transfer performance of thermal engine can be effectively improved by incorporating graphene into PCM. In this paper, the molecular dynamic simulations of PCM with different addition ratio of graphene were carried out. Then the models of temperature–pressure-density, temperature–pressure-thermal conductivity and pressure-heat capacity were established for different simulation systems. On this basis, the internal connection between the additive ratio of graphene and the heat transfer rate of the thermal engine was revealed by using the phase change heat transfer model considering pressure conditions. The simulation shows that the appropriate addition of graphene is beneficial to the heat transfer rate of thermal engine at 0.1 MPa. Under the pressure condition of 20 MPa, the thermal engine can achieve the highest the heat transfer rate when the addition ratio is 1.2%, which also shows the best performance under simulated ocean conditions. The melting time can be reduced by 11.97% compared with the thermal engine without added graphene.

Design and investigation of single tank phase change thermal storage domestic hot water system

Thermal energy storage (TES) is extensively applied in production and daily life. As a basic work, we designed a single tank phase change TES domestic hot water system using night valley power. Based on the single tank TES experimental platform, a 2D numerical model of the system was established. To optimize the performance of a single tank TES system, the effect of heat transfer fluid (HTF) inlet velocity and temperature on the heat release of the storage tank was analyzed based on simulation. The enhanced heat transfer effect of the radial cascade system and axial cascade system was also compared. The results showed that 0.8 m/s is the optimal HTF inlet velocity in the calculation range. The rule of the heat release about the HTF inlet temperature just meets the different hot water requirements in winter and summer. The heat release capacity of the radial cascade system and axial cascade system is higher than that of the conventional single-stage system, and the proportion of latent heat release in radial cascade is larger. This study will provide theoretical guidance for the optimal design of the single tank TES domestic hot water system in the future.

Analysis and Experimental Research on the Fluid–Solid Coupled Heat Transfer of High-Speed Motorized Spindle Bearing Under Oil–Air Lubrication

Abstract For high-speed motorized spindle bearing, temperature rise is the primary factor that restricts the maximum speed of spindle and affects the stability of system. This paper addresses the lubrication and cooling of spindle bearing by exploiting the precise oil control and high cooling efficiency of oil–air lubrication. Enlightened by the bearing tribology and two-phase flow theory, a numerical model of oil–air two-phase flow heat transfer inside bearing cavity is created, with which the effects of operating condition and nozzle structure parameters on the temperature rise are studied. As the results show, with the elevation in speed, the heat generation increases rapidly, and despite the somewhat enhanced heat transfer effect, the temperature still tends to rise. Given the higher volume fraction of air than oil in the two-phase flow, the temperature rise of bearing is suppressed greatly as the air inlet velocity increases, revealing a remarkable cooling effect. When a single nozzle is used, the bearing temperature increases from the inlet to both sides, which peaks on the opposite side of the inlet. In case multiple evenly distributed nozzles are used, the high-temperature range narrows gradually, and the temperature distributions in the inner and outer rings tend to be consistent. With the increase in the nozzle aspect ratio, the airflow velocity drops evidently, which affects the heat dissipation, thereby resulting in an aggravated temperature rise. Finally, the simulation analysis is verified through experimentation, which provides a theoretical basis for selecting optimal parameters for the oil–air lubrication of high-speed bearing.

Study on the flow and heat dissipation of water-based alumina nanofluids in microchannels

In this paper, the flow and heat dissipation performance of 0.1–0.5 vol per cent of Al2O3 -water nanofluid through serpentine micro-channel was experimentally studied in the Reynolds range 124–1000. By analyzing the experimental results of nanofluid flow and heat dissipation: In the experiment, the Nusselt number of nanofluids is 1.12–1.66 times that of deionized water, which indicates that Al2O3 -water nanofluids have significant heat transfer enhancement effect. Nanofluid flow resistance is 1.02–1.80 times that of deionized water, this indicates that Al2O3 -water nanofluid has the disadvantage of increasing energy consumption. From a comprehensive performance point of view: all the nanofluids used in the experiment have enhanced heat transfer factors more significant than one, and the enhanced heat transfer effect is better than that of deionized water, this suggests that Al2O3 -water nanofluid has the drawback of rising energy consumption. All the nanofluids used in the experiment have enhanced heat transfer factors more significant than one, and the enhanced heat transfer effect is better than that of deionized water. Nano-fluids, with a volume fraction of 0.4%, have an average increased heat transfer factor of 1.4, which is the best overall performance.

Performance and Exergy Transfer Analysis of Heat Exchangers with Graphene Nanofluids in Seawater Source Marine Heat Pump System

A marine seawater source heat pump is based on the relatively stable temperature of seawater, and uses it as the system’s cold and heat source to provide the ship with the necessary cold and heat energy. This technology is one of the important solutions to reduce ship energy consumption. Therefore, in this paper, the heat exchanger in the CO2 heat pump system with graphene nano-fluid refrigerant is experimentally studied, and the influence of related factors on its heat transfer enhancement performance is analyzed. First, the paper describes the transformation of the heat pump system experimental bench, the preparation of six different mass concentrations (0~1 wt.%) of graphene nanofluid and its thermophysical properties. Secondly, this paper defines graphene nanofluids as beneficiary fluids, the heat exchanger gains cold fluid heat exergy increase, and the consumption of hot fluid heat is heat exergy decrease. Based on the heat transfer efficiency and exergy efficiency of the heat exchanger, an exergy transfer model was established for a seawater source of tube heat exchanger. Finally, the article carried out a test of enhanced heat transfer of heat exchangers with different concentrations of graphene nanofluid refrigerants under simulated seawater constant temperature conditions and analyzed the test results using energy and an exergy transfer model. The results show that the enhanced heat transfer effect brought by the low concentration (0~0.1 wt.%) of graphene nanofluid is greater than the effect of its viscosity on the performance and has a good exergy transfer effectiveness. When the concentration of graphene nanofluid is too high, the resistance caused by the increase in viscosity will exceed the enhanced heat transfer gain brought by the nanofluid, which results in a significant decrease in the exergy transfer effectiveness.

Numerical simulation and analysis of enhanced heat transfer in corrugated tube heat exchanger

At present, in many fields such as petrochemical industry, in order to make better use of heat energy and reduce energy consumption in the process of equipment operation, heat transfer enhancement of heat exchange equipment has attracted more and more attention. Corrugated tube has many advantages compared with other smooth tubes, which are enhancing heat transfer, strong anti-fouling ability and easy to clean. In this paper, a heat exchanger with corrugated tube is studied. The geometry is meshed with the software GAMBIT. It is used with Fluent software and the initial and boundary conditions are set. Heat transfer performance between smooth tube and corrugated tube at the same inner wall temperature and velocity. Then, the influence from inlet velocity on heat transfer performance is investigated. The variation of temperature field, pressure field and velocity field in corrugated tube heat exchanger with different inlet velocity is discussed. The results show that the enhanced heat transfer effect of corrugated tube is much better than that of smooth tubes. At three different inlet velocities (0.5m/s, 1m/s, 3m/s), the enhanced heat transfer effects of the corrugated tube heat exchanger are different. With the increase of inlet velocity, the heat transfer effects of corrugated tube heat exchanger are enhanced, but the extent of enhancement decreases. In the range of this paper, the optimal inlet velocity is 0.5m/s in order to obtain the more heat transfer quantity and lower operation cost.

CONVECTION HEAT TRANSFER PERFORMANCE OF THE FRACTAL TUBE BANK UNDER CROSS FLOW

Convective heat transfer process between fractal tube bank and coolant water was simulated by using the finite element method. Both rectangular and circular tube banks were investigated in detail. For the relationship between Nusselt number and Reynolds number, the fractal circular tube bank shows the same trend as the rectangular tube one. Based on simulation results, the empirical heat transfer correlations for different stage fractals were regressed. The exponents of correlations increase with the fractal stage. Almost linear correlations between Nu and Re for fourth stage fractal were obtained, indicating a good heat transfer enhancement with the increase in fractal stages. Compared to the uniform tube bank, the fractal tube banks show better overall performance in enhanced heat transfer effect. This research would be helpful to understand the heat exchange mechanism of the multi-scale structure.