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Flow turbulization is one of the most commonly used techniques for improving heat transfer. This study uses numerical simulation and experimental tests to examine ball turbulators' effect on fluid friction and heat transfer characteristics in a circular pipe. Ball turbulators with different diameters of 10, 15, and 20 mm and spacer lengths of 20 cm are inserted in the circular pipes. The ratio of the diameter of the ball to the diameter of the internal flow path of the pipe, ball turbulent ratio (BTR) =D_b/D_p, becomes 0.41, 0.62, and 0.83. The water was a working fluid with Reynolds numbers 3,500 to 11,500. The findings demonstrate excellent agreement with deviations of less than 11%. The maximum thermal performance factor reached about 1.18, 1.24, and 1.4 for the BTR 0.41, 0.62, and 0.83, respectively. The ball turbulators increase friction factor; this increase in experimental findings is 52, 65, and 78% at BTR 0.41, 0.62, and 0.83, respectively. Also, it is noticeable that there is a gradual decrease in thermal performance when the Reynolds number range is higher than 7500. [ABSTRACT FROM AUTHOR]

Flow turbulization is one of the most commonly used techniques for improving heat transfer. This study uses numerical simulation and experimental tests to examine ball turbulators' effect on fluid friction and heat transfer characteristics in a circular pipe. Ball turbulators with different diameters of 10, 15, and 20 mm and spacer lengths of 20 cm are inserted in the circular pipes. The ratio of the diameter of the ball to the diameter of the internal flow path of the pipe, ball turbulent ratio (BTR) =D_b/D_p, becomes 0.41, 0.62, and 0.83. The water was a working fluid with Reynolds numbers 3,500 to 11,500. The findings demonstrate excellent agreement with deviations of less than 11%. The maximum thermal performance factor reached about 1.18, 1.24, and 1.4 for the BTR 0.41, 0.62, and 0.83, respectively. The ball turbulators increase friction factor; this increase in experimental findings is 52, 65, and 78% at BTR 0.41, 0.62, and 0.83, respectively. Also, it is noticeable that there is a gradual decrease in thermal performance when the Reynolds number range is higher than 7500.

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

The analysis of heat transfer by forced convection steady, laminar flow around three triangle cylinders. A numerical investigation is conducted to analyze the variation in cross-section from a triangle to a circle by altering the edges of a triangular cylinder within a two-dimensional channel in the air. The effect of Reynolds number and transverse pitch (ST) on the heat transfer characteristics are studied. Five Reynolds numbers (based on inlet height channel) are studied (Re = 100, 200, 400, 800, 1200) with a constant heat flux on cylinder surface (q''= 5000 W/m²), and ST = 1.5D, 2D, 3D, the D is triangle side length. The governing partial differential equations, which include the continuity, momentum, and energy equations, are resolved by computational methods using workbench 2022R2 program. The results reveal that heat transfer rises as the transverse distance and the diameter of the rounded edge change. [ABSTRACT FROM AUTHOR]

The working gas type had an important effect on the heat transfer performance of the turbulence device in the heater of Stirling engine, but the related researches were scarce, especially under the oscillatory flow. This work investigated the effects of four working gases (i.e., H2, He, N2, and CO2) on the heat transfer characteristics of a heating tube with spiral insert compared with a smooth tube under oscillating flow for a Stirling engine. The transient physical fields under different phase angles in an oscillatory cycle were analyzed, and the Nusselt (Nu) number, pressure loss and outlet temperature of working gas were studied. The results show that the cycle average Nu number of the enhanced tube with H2, He, N2, and CO2 as working gas increased to 1.67, 1.62, 1.61 and 1.72 times when comparing with those of the smooth tube. The cycle average friction coefficient increased to 1.96, 2.37, 2.36 and 2.62 times, respectively. Moreover, the performance evaluation criterion (PEC) values of the enhanced tube using the four types of working gas were all greater than 1 (1.21∼1.33). This implies that the comprehensive heat transfer performance of the heating tube with spiral insert was all improved with the four types of working gas. Moreover, the heat transfer enhancement effect was best when hydrogen was used. While considering the thermodynamic performance and safety reliability, the helium was more recommended. The findings are beneficial to enhance the operating efficiency of a Stirling engine.

Thermal engineering experiments were carried out with laboratory water-to-water tube-in-tube heat exchangers of the same design parameters with smooth and profiled inner tubes. The tubes were profiled by confuser–diffuser constrictions of the flow section of the inner channel, which were formed by the deformation of their walls and placed along the length at a step that was constant and equal for all profiled tubes. The obtained results showed a dependence of the heat transfer enhancement in the tube channel on the Reynolds and Prandtl numbers, with the dependence on the latter being much stronger than that on the former, at least in cases where the heat carrier is water. [ABSTRACT FROM AUTHOR]

Conjugate heat transfer is a complex problem because heat is transferred from a solid medium to a liquid medium through their interfaces. The steady-state laminar flow formed inside the microtubules is subjected to a constant temperature at the outer sidewall surface. These images cover a wide range of wall-to-fluid thermal conductivity ratios (ksf = 1, 2, 3, 4, and 5) and wall thickness-to-inner diameter ratios (δ/Ri = 0.25, 0.5, 0.75, 1, 1.25, and 1.5) and Reynolds numbers (Re = 200, 400, 600, 800, and 1,000). The results are processed by a Fluent program based on the finite volume method to numerically integrate the driver’s differential equations. The results show that increasing the wall-to-fluid thermal conductivity ratio ksf increases the inner wall dimensionless temperature and decreases the average Nusselt number. Conversely, an increase in the ratio of wall thickness to inner diameter results in a decrease in the dimensionless temperature of the inner wall and an increase in the average Nusselt number.

Improving the performance of heat sinks is very important in the development of cooling systems. In this study, the use of a novel combination method [magnetic field impingement jet (MF-IJ)] to improve the convective heat transfer coefficient in a designed heat sink is numerically investigated. To model heat transfer, a steady three-dimensional computational fluid dynamics (CFD) approach is employed. Numerical results including velocity and temperature contours, as well as the distribution of wall temperature of the heat sink and also the convective heat transfer coefficient are analyzed. The results show that the use of ferrofluid (Fe3O4/water) flow with an external magnetic field alone increases the heat transfer coefficient by 10%, while the use of an air impingement jet with pure water and without a magnetic field increases it by 22.4%. By using the MF-IJ method, a 32% enhancement of heat transfer coefficient is achieved compared to the case of pure water flow and without MF-IJ. Based on results, at a Reynolds number of 600, by applying the magnetic field intensities of 400, 800, and 1600 G, the average heat transfer coefficient increases by 5.35, 11.77, and 16.11%, respectively. It is also found that the cooling of the heat sink and temperature distribution is improved by increasing the Reynolds number and the inlet mass flow rate of the impingement jet. For instance, at z = 0.02 m, the application of an impingement jet with mass flow rates of 0.001, 0.004, and 0.005 kg/s results in a respective decrease of 0.36, 1.62, and 1.82% in wall temperature. The results of the current study suggest that the combination method of MF-IJ can be utilized for heat sinks with high heat flux generation as a flow control device.

The performance of an electronic radiator filled with metal foam with a porosity of 96% was studied. The effect of the factors including the flow rates, the pores per linear inch (PPI) and the numbers of fins was analyzed. The results show that the electronic radiator with metal foam reflects a stronger ability of the heat transfer compared to the electronic radiator without metal foam. With the increase in the flow rate between 10 L/h and 60 L/h, the heat transfer coefficient of both of the two electronic radiators will be improved, but it is also dependent on the number of fins. In this study, we find that the heat transfer coefficient first increases and then decreases with the number of fins. The optimum number is three. As for the effect of the PPI, the higher the PPI, the larger the heat transfer coefficient, while the pressure drop always increases with the flow rates' increase, the pores per linear inch (PPI) and the numbers of fins. [ABSTRACT FROM AUTHOR]

In order to meet the needs of the development of multi-layer thin-walled turbine blades, this work optimizes the jet impingement cooling structure in a limited space. Firstly, the NSGA-II algorithm is used to optimize the basic structure of the target surface with the maximum heat transfer coefficient and the minimum total pressure difference as the objectives. The structural parameters considered include: the number of rows of jet holes N , jet hole diameter D j , jet-to-target distance H / D j and transverse distance between jet holes S / W. The interaction between the impact factors is obtained by analyzing the response surface, and the Pareto front solution set is obtained. In this solution set, design point 12 is recommended when considering both heat transfer intensity and resistance, and design point 18 is recommended when pursuing heat transfer intensity. In addition, in order to further enhance the heat transfer intensity with a small resistance cost on the basis of design point 18, a circular protrusion array enhanced structure is arranged on the target surface. Using NSGA-II algorithm, multi-objective optimization is carried out with the goal of maximum heat transfer intensity and maximum comprehensive heat transfer intensity. Compared with the design point 18, the heat transfer intensity of the optimized enhanced structure is increased by 19.4 %, the comprehensive heat transfer performance is increased by 16.6 %, and the resistance is only increased by 7.4 %. • The object is applied between the double thin walls of the turbine blade. • The response surface of jet impingement cooling in a limited space is analyzed. • The Pareto front of the basic structure that is worth promoting is given. • A circular protrusion array enhanced target surface structure is proposed. • The enhanced structure increases Nu ave by 19.4 % and Δ P by 7.4 %. [ABSTRACT FROM AUTHOR]

Heat transfer around a flat plate fin integrated with piezoelectric actuator used as oscillated fin in laminar flow has been studied experimentally utilizing thermal image camera. This study is performed for fixed and oscillated single and triple fins. Different substrate-fin models have been tested, using fins of (35mm and 50mm) height, two sets of triple fins of (3mm and 6mm) spacing and three frequencies applied to piezoelectric actuator (5, 30 and 50HZ). All tests are carried out for (0.5 m/s and 3m/s) in subsonic open type wind tunnel to evaluate temperature distribution, local and average Nusselt number (Nu) along the fin. It is observed, that the heat transfer enhancement with oscillation is significant compared to without oscillation for low air inlet velocity. Higher thermal performance of triple fins is obtained compared to the single rectangular fin, also triple fins with (height=50mm and fin spacing=3mm) gives better enhancement as compared to other cases. This work shows that the piezoelectric actuator when mounted on the rectangular fins shows great promise for enhancing the heat transfer rate.

• Microchannel two-phase loop integrated with latent heat storage is proposed. • Delayed response and real-time response are characterized by heat load and flow rate. • Thermal response behaviors of the two-phase loop under various heating models are studied. • A venation-finned porous network LHE is proposed to enhance melting heat transfer. • The melting heat storage rate is significantly enhanced and heat transfer hysteresis is broken. High-power equipment frequently faces the specific challenge, such as operating with intermittent high heat flux under limited cooling resources. However, traditional single-cooling approaches are inadequate to meet this specialized heat dissipation requirement. To address this issue, a novel microchannel two-phase loop with latent heat storage has been proposed for dissipating high heat flux in a microchannel evaporator and storing heat in a latent heat exchanger (LHE). Dynamic thermal performance of the two-phase loop is systematically explored, with a special focus on thermal response behaviors of microchannel evaporator and LHE unit at three heating modes. It indicates that the novel system can effectively deal with thermal shock of instantaneous high thermal load. Two characteristic thermal response modes have been found, i.e. delayed response mode and real-time response mode. The delayed response mode would quickly utilize the heat storage capacity of LHE and shorten melting duration, accompanying with an increase of 147.2 % in the peak storage rate and a decrease of 60.2 % in working time. To enhance the thermal performance of LHE, a novel venation-finned porous network LHE is employed and experimentally tested. Due to its efficient heat transfer pathways, this design improves the melting heat storage rate and eliminates heat transfer lag. As a result, the maximum temperature difference between two typical vertical testing points is only 0.9 °C, which demonstrated the more uniform temperature distribution. [ABSTRACT FROM AUTHOR]

The pulsating flow is one of the techniques that can enhance heat transfer, therefore leading to energy saving in tubular heat exchangers. This paper investigated the heat transfer and flow characteristics in a two-dimensional in-line tube bundle with the pulsating flow by a numerical method using the Ansys Fluent. Numerical simulation was performed for the Reynolds number Re = 500 with different frequencies and amplitude of pulsation. Heat transfer enhancement was estimated from the central tube of the tube bundle. Pulsation velocity had an asymmetrical character with a reciprocating flow. The technique developed by the authors to obtain asymmetric pulsations was used. This technique allows simulating an asymmetric flow in heat exchangers equipped with a pulsation generation system. Increase in both the amplitude and the frequency of the pulsations had a significant effect on the heat transfer enhancement. Heat transfer enhancement is mainly observed in the front and back of the cylinder. At a steady flow in these areas, heat transfer is minimal due to the weak circulation of the flow. The increase in heat transfer in the front and back of the cylinder is associated with increased velocity and additional flow mixing in these areas. The maximum increase in the Nusselt number averaged over space and time in the entire studied range was 106%, at a pulsation frequency of 0.5 Hz and pulsation amplitude of 4.5. A minimum enhancement of 25% was observed at a frequency of 0.166 Hz and amplitude of 1.25. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering for Thermal Energy & Power / Reneng Dongli Gongcheng is the property of Journal of Engineering for Thermal Energy & Power and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Consideration is given to the possibility of enhancing heat transfer in circular and plane nonseparating diffusers with small opening angles. Numerical modeling of the heat transfer was performed with the three-parameter differential turbulence model supplemented with the transfer equation for the turbulent heat flux. It has been show that at the same opening angle, the Nusselt number in the circular diffuser is much higher than that in the plane diffuser, with this excess growing with opening angle. However, the Reynolds analogy factor for the circular diffuser is only slightly higher than that for the plane one. A study has been made of the influence of the Reynolds number, the diffuser length, and the Prandtl number of the heat-transfer agent. It has been shown that the minimum effect of heat-transfer enhancement is attained for gases with a small Prandtl number and depends weakly on the Reynolds number and the diffuser length. [ABSTRACT FROM AUTHOR]

Pellet heat transfer has always been one of the biggest limiting factors in its direct reduction process in the rotary hearth furnace (RHF). Therefore, strengthening its heat transfer process is very important for productivity in RHF. In this study, a coupling model of the carbon‐containing pellet reduction process considering porosity and chemical heat variation with intermediate media in its center was established. With the pellet radius R0 = 11 mm, the influence of the intermediate spherical medium radius r0, the offset position of the spherical medium ξ, and the materials used on the overall pellet reduction process were analyzed. The results showed that the presence of a spherical medium in the pellet center can increase the temperature inside the pellet, leading to it being more advantageous for reduction of the pellet. With the increase of the medium radius r0, it becomes more favorable for the pellet reduction process, as the spherical medium is more efficient for heat conduction. With the increase of the offset value ξ, the spherical medium acting as heat‐conducting material can allow the heat transferred to the pellet inside faster, which is also more favorable to the reduction process. In addition, when the spherical medium has a smaller effective volume heat capacity ρ∁p and a larger thermal conductivity k, it is more favorable for the pellet reduction process too. These results can serve as a reference for optimum pellet design in RHF. [ABSTRACT FROM AUTHOR]

A classification of means of enhancing heat exchange from the outer surface of individual and grouped pipes is proposed. The most characteristic designs proposed by domestic and foreign designers and inventors along with their advantages and drawbacks are considered. [ABSTRACT FROM AUTHOR]

In the air conditioning industry, the use of compact heat exchangers is very common. The main objective of this equipment is to transfer energy from one fluid to another with a different temperature. To achieve this, it is intended to use the smallest possible heat transfer area that guarantees efficient operation of the equipment. The numerical study was carried out in compact heat exchangers with circular tubes and flat fins, using parallelogram-shaped vortex generators. Metaheuristic techniques were applied to obtain the geometry of a model, which transferred the maximum amount of heat possible and generated the least energy losses due to friction. The numerical model was validated against previously published experimental results. The numerical study was performed after developing an analysis of mesh independence. The main result was a geometry capable of intensifying heat transfer by more than 6%. [ABSTRACT FROM AUTHOR]

A mathematical investigation on the combined effect of oscillation and conjugation on the enhancement of heat transfer in a heat pipe called Dream Pipe is carried out, when viscoelastic fluids (CPyCl/NaSal) are employed as the heat carriers. Closed-form solutions for the momentum and heat equations are presented. The physical and thermal properties of the polymer solution used are obtained by experiments. The effects of thermal conductivity and thickness of the wall, fluid thickness, Womersley number (α), Deborah number and Prandtl number on the enhancement of heat transfer are examined. Results obtained in the present analysis are in excellent agreement with those of the existing literature. The effective thermal diffusivity (κe) is maximized at optimum α where the fluid flow exhibits a resonant behavior. Several maxima occur in κe for several resonant frequencies and the dramatic increase in κe due to oscillation for the viscoelastic fluid is 5.63 x 109 times higher than that obtained by the molecular motion. This increase is much higher than that (1.84 x 104 times) obtained for the Newtonian fluid. κe is increased with increasing wall thermal conductivity and thickness in the viscous regime whereas in the elastic regime the effect of conjugation is saturated. In the viscous regime, a maximum increase of 50.63% in κe is obtained by optimizing the wall thickness. Also κe increases with increasing molar ratio of concentrations of counterion to surfactant. A maximum heat flux of 4.54 x 1010 W/m2 is achieved using a viscoelastic fluid with thermally conducting wall and this highest heat flux is 207 times higher than that (2.19 x 108 W/m2) obtained with the Newtonian fluid (liquid metal). Hence, viscoelastic fluids are preferable to liquid metals as working fluids in the Dream Pipe. The new insights gained by the present investigation are useful while designing viscoelastic Dream Pipes and micro channel heat exchangers.

The combined effect of conjugation, external magnetic field and oscillation on the enhancement of heat transfer in the laminar flow of liquid metals between parallel plate channels is analyzed. In order to make our results useful to the design engineers, we have considered here only the wall materials that are widely employed in liquid metal heat exchangers. Indeed, all the results obtained through this mathematical investigation are in excellent agreement with the available experimental results. The effective thermal diffusivity κe is increased by 3 · 106 times due to oscillation and that the heat flux as high as 1.5 · 1010 (W/m2) can be achieved. Based on our investigation, we have recommended the best choice of liquid metal heat carrier, wall material and its optimum thickness along with the optimum value of the frequency to maximize the heat transfer rate. At the optimum frequency, by choosing a wall of high thermal conductivity and optimum thickness, an increase of 19.98% in κe can be achieved. Our results are directly relevant to the design of a heat transfer device known as electromagnetic dream pipe which is a very recent development.

The utilization of corrugation for improvement in heat transfer is increasingly becoming interesting recently due to its combined advantages such as extended surfaces, turbulators as well as roughness. This study employed the use of both numerical as well as experimental settings on the water flowing at lower Reynolds numbers in a corrugated tubes with spiral shape to evaluate the performance of heat in a newly designed corrugation style profile. The total performance of the heat for the corrugation tubes were determined and the mathematical information generated from both the Nusselt number and the factors of friction were equated with those of the experimentally generated outcome for both standard smooth as well as the corrugated tubes. Analysis of the dat generated revealed improvements in heat transfer ranges of (2.4–3.7) times those 0btained from the smooth tubes with significant increase in the friction factors of (1.7–2.3) times those of the smooth tubes. Based on the findings of study, it was concluded that for extended period and extensive range use, tubes with severity index values at 36.364×10–3 could produce better heat performance (1.8–3.4) at Reynolds numbers ranging from 100 to 1300. This was an indication that the geometric expression with spiral corrugation profile could significantly enhance the efficiency of heat transfer with significantly increased friction factors.

The solar energy is a renewable source that has a great potential for conversion into thermal energy or for generation of electric power through photovoltaic panels in Brazil. Concerns about environmental impacts and the fossil resources scarcity have motivated the technological development of renewable alternatives to fill out the energy matrix. The flat-plate solar water heater is an equipment used for domestic or commercial applications to heat fluids, which can minimize the demand for electric energy and, consequently, decrease the electrical bill. However, the development of technologies to increase the conversion of solar energy into thermal energy remains a challenge in order to increase the efficiency of these devices. Thus, passive techniques to enhance heat transfer have been applied and those results seem to be promissory. Among them, delta-winglet longitudinal vortex generator (VG) is a consolidated passive technique currently applied on compact heat exchangers, although few works have been applied this technique on the solar water heater. By a computational fluid dynamics approach, in this work, we analyze the augmentation of heat transfer through delta-winglet longitudinal vortex generator inside a tube of a flat-plate solar water heater. For the Reynolds numbers 300, 600, and 900, the better ratio between the heat transfer and the pressure drop penalty is found for the attack angle of the delta-winglet of 30 deg, while the highest heat transfer was to the attack angle of 45deg. Moreover, the first vortex generators showed significant impact only on the friction factor and could be eliminated of the solar water heater with no penalty to the heat transfer. [ABSTRACT FROM AUTHOR]

Consideration is given to the possibility of enhancing heat transfer in circular and plane nonseparating diffusers with small opening angles. Numerical modeling of the heat transfer was performed with the three-parameter differential turbulence model supplemented with the transfer equation for the turbulent heat flux. It has been show that at the same opening angle, the Nusselt number in the circular diffuser is much higher than that in the plane diffuser, with this excess growing with opening angle. However, the Reynolds analogy factor for the circular diffuser is only slightly higher than that for the plane one. A study has been made of the influence of the Reynolds number, the diffuser length, and the Prandtl number of the heat-transfer agent. It has been shown that the minimum effect of heat-transfer enhancement is attained for gases with a small Prandtl number and depends weakly on the Reynolds number and the diffuser length. [ABSTRACT FROM AUTHOR]

A mathematical investigation on the combined effect of oscillation and conjugation on the enhancement of heat transfer in a heat pipe called Dream Pipe is carried out, when viscoelastic fluids (CPyCl/NaSal) are employed as the heat carriers. Closed-form solutions for the momentum and heat equations are presented. The physical and thermal properties of the polymer solution used are obtained by experiments. The effects of thermal conductivity and thickness of the wall, fluid thickness, Womersley number (α), Deborah number and Prandtl number on the enhancement of heat transfer are examined. Results obtained in the present analysis are in excellent agreement with those of the existing literature. The effective thermal diffusivity (κe) is maximized at optimum α where the fluid flow exhibits a resonant behavior. Several maxima occur in κe for several resonant frequencies and the dramatic increase in κe due to oscillation for the viscoelastic fluid is 5.63 x 109 times higher than that obtained by the molecular motion. This increase is much higher than that (1.84 x 104 times) obtained for the Newtonian fluid. κe is increased with increasing wall thermal conductivity and thickness in the viscous regime whereas in the elastic regime the effect of conjugation is saturated. In the viscous regime, a maximum increase of 50.63% in κe is obtained by optimizing the wall thickness. Also κe increases with increasing molar ratio of concentrations of counterion to surfactant. A maximum heat flux of 4.54 x 1010 W/m2 is achieved using a viscoelastic fluid with thermally conducting wall and this highest heat flux is 207 times higher than that (2.19 x 108 W/m2) obtained with the Newtonian fluid (liquid metal). Hence, viscoelastic fluids are preferable to liquid metals as working fluids in the Dream Pipe. The new insights gained by the present investigation are useful while designing viscoelastic Dream Pipes and micro channel heat exchangers. [ABSTRACT FROM AUTHOR]

Copyright of Journal of Engineering & Sustainable Development is the property of Republic of Iraq Ministry of Higher Education & Scientific Research (MOHESR) and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

• Tesla-type microchannel configuration is designed to inhibit two-phase backflow. • Overall HTC and CHF are significantly increased owing to effective suppression of vapor backflow in forward direction and enhanced two-phase mixing in backward direction. • CHF and HTC are reached to 274 Wcm-2 and 144 kWm-2K-1 at G = 360 kgm-2s-1 in the forward direction. • Two-phase flow instability regarding pressure fluctuation is dramatically reduced up to 7-fold. • These enhancements are achieved without the cost of pressure drops. Improving CHF and HTC of microchannel flow boiling is highly desirable for high-power density electronics cooling. However, two-phase flow instabilities strongly influence the enhancement of flow boiling in confined domains. Although extensive surface modifications and advanced functional structures have significant effects on two-phase heat transfer, gradual suppression of vapor backflow along in the entire channel length still lacks. A novel tesla-type copper microchannel configuration was explored with aim to inhibit vapor backflow and promote two-phase transport simultaneously in the forward direction in this work. Also, the effect of this configuration on two-phase mixing is investigated in the backward direction. Experiments are conducted on DI-water with mass velocity varying from 60 to 360 kgm−2s−1. Flow boiling and two-phase regimes are comprehensively investigated and compared to plain-wall microchannel. Overall HTC and CHF are significantly increased owing to effective suppression of vapor backflow in forward direction and enhanced two-phase mixing in backward direction. For example, at G = 360 kgm−2s−1, CHF and HTC are up to 274 Wcm−2 and 144 kWm−2K−1, respectively, accompanying with enhancements of 88.4% and 86.8% in the forward direction. Interestingly, the thermal performances in the forward direction outperform that in backward direction as the further increase of mass velocities, indicating the successful regulation of backflow. Upon the management of vapor backflow, two-phase flow instability regarding pressure fluctuation is dramatically reduced up to 7-fold. [ABSTRACT FROM AUTHOR]

An experimental study has been made of the heat transfer and hydraulic resistance in air flow across a single-row bundle of tubes with spiral grooves on the exterior surface and of a bundle of smooth tubes of the same diameter. Cutting a spiral groove of width 3 mm and depth 2 mm with a pitch of 20 mm on a tube with an outside diameter of 22 mm increased the heat-transfer coefficient in the range of Reynolds numbers 6200-16,000 by 40-53% compared to the smooth-tube bundle. The hydraulic resistance of the tube bundle decreased by 22%. The Reynolds analogy factor increased, on the average, by 41%. Tubes with spiral grooves, in the investigated range of the velocities of flow, have shown a certain increase in the Strouhal number to values of 0.24-0.36. Computer simulation has been carried out of the hydrodynamics of the single-row bundle of smooth-wall tubes and the tubes with spiral grooves. It has been shown that the enhancement of heat transfer and the decrease in the hydraulic resistance alike are attributable to the appearance of nonsymmetric vortex structures induced by the flow across the spiral grooves. [ABSTRACT FROM AUTHOR]

The authors have presented results of experimental investigation into the hydrodynamics and heat transfer during the water flow in a tube with internal spiral finning at angles of twist from 14 to 87°, and also of visualization of the flow structure from numerical-simulation results. [ABSTRACT FROM AUTHOR]

The present paper summarizes the review of numerical and experimental works done by researchers to study various types of fin configurations under natural convection conditions to increase heat transfer with different fin surface roughness. Compared to plane fin, the fin with varying surface roughness influence heat transfer in varying degrees. Fins with perforations of rectangular, circular, triangular shape etc. and fins with extensions or protrusions of various shapes have been studied. Such perforations / protrusions induce flow discontinuity over fins and hence help to alter flow quantities near the wall. Thus generated instability in boundaiy layer increase heat transfer from fins. In some studies, it was found that roughness height plays an important role and is a decisive factor in increasing or decreasing heat transfer from such surfaces. In other studies, roughness patterns considerably enhance heat transfer from 10 to 40% compared to plane fin. As a future scope, fins with various knurling patterns and slit openings of various sizes and shapes seem promising in assessing heat transfer increase. [ABSTRACT FROM AUTHOR]

This paper presents a mathematical model of the refrigeration system based on one-dimensional heat transfer theory, and analyses respectively the operating characteristics of thermoelectric refrigerator under the condition of maximum cooling capacity and maximum cooling efficiency. Besides, it sets up an experimental platform for mini thermoelectric cooling system coupling with heat pipe, and designs a spoiler duct to enhance performance of the cooling system. The cooling performance has been improved by considering the coolers operating voltage, the cold side ventilation duct and spoilers. The text results show that the performance of the thermoelectric cooling system coupled with heat pipe affected by the above control parameters. The ventilation rate of the cold side increases with the increasing of operating voltage and the cooling effect will be more obviously. The application of spoiler duct is possible significantly to enhance cooling system performance. [ABSTRACT FROM AUTHOR]

An unsteady two-dimensional laminar forced convection heat transfer around a square cylinder with the rounded corner edge is numerically investigated for Re = 80-180 and non-dimensional corner radius, r = 0.50-0.71 at Pr = 0.71 (Air). A structured non-uniform mesh is used for the computational domain discretization, and the finite-volume-method-based commercial code FLUENT is used for numerical simulation. The heat transfer characteristics over the rounded corner square cylinder are analyzed with average Nusselt number ( Nu ) at various Re and various corner radii. The heat transfer characteristic is predicted by gene expression programming (GEP), and the GEP-generated explicit equation of Nu is utilized in particle swarm optimization to optimize the corner radii for maximum heat transfer rate. The data required for the training the GEP model have been collected from the authors' recent published article (Neural Comput Appl, 2015. doi:). It is found that the heat transfer rate of a circular cylinder can be enhanced 12 % by introducing a new cylinder geometry of corner radius r = 0.51. [ABSTRACT FROM AUTHOR]

• Sodium-potassium alloy LMHOHPs with different filling ratios were developed. • The effects on the heat transfer performance of LMHOHP were studied with the filling ratio range of 22–80%. • The heat transfer performance increased firstly and then decreased with the increase of the filling ratio. The best filling ratio was 48%. The liquid metal high-temperature oscillating heat pipe (LMHOHP) is a kind of oscillating heat pipe with liquid metal as the working fluid, which can work efficiently in high-temperature environments over 500 ℃. In this paper, in order to master the effects of the filling ratio on the performance of the LMHOHPs, the start-up performance, the flow state, and the heat transfer performance of LMHOHPs were studied by experiments. The working fluid of LMHOHPs was sodium-potassium alloy with the potassium mass fraction of 78%. Under the input powers of 1500–3500 W and the inclination angles of 0–90°, LMHOHPs with filling ratios of 11–88% were tested. The experimental results showed that: (1) The LMHOHPs with filling ratios of 22–80% could successfully start up, and the heat transfer process was carried out by oscillation flow of the working fluid. The start-up mode varies with the filling ratio, and the start-up temperature increased with the increasing filling ratio. (2) With the filling ratios of 35–64%, the flow states of the LMHOHPs could change from the oscillation flow to the unidirectional circulation flow, and the heat transfer performance was significantly enhanced. With the increase of filling ratio, the required input power at the transition of the flow state decreased. (3) When the inclination angles were 30–90°, the thermal resistance of LMHOHPs decreased firstly and then increased with the filling ratio, and the achieved minimum thermal resistance was 0.081 ℃/W when the filling ratio was 48%. However, when the inclination angle was 0°, the thermal resistance of LMHOHPs monotonically increased with the filling ratio. [ABSTRACT FROM AUTHOR]

The enhanced heat transfer in the oscillatory flow of liquid metals between two thermally insulated infinite parallel plates, when a constant axial temperature gradient superimposed, is investigated. The fluid is set to oscillation by oscillating both the plates axially along with an axial oscillatory body force, having the same frequency as that of the plates. The effective average thermal diffusivity is calculated and the effect of oscillation of the plates and the oscillatory body force on the enhancement of heat transfer are discussed and compared.

To explore the effect of metal foam(MF)pore density on the melting and heat transfer of phase-change materials(PCMs), a semi-cylindrical visualization experimental setup was established to experimentally study copper MFs with different pore densities coupled with paraffin wax. The results showed that as pore density increases, the melting time of the composite PCMs decreases compared to that of pure paraffin wax by 11.5%, 16.2% and 18.1% for pore densities of 5, 20, and 30 PPI, respectively. Composite PCMs with a pore density of 30 PPI exhibited fastest melting and lowest maximum temperature difference during melting. Considering the relative thermal energy storage(TES) capacity and relative TES rate, composite PCMs with a pore density of 30 PPI exhibited optimal overall thermal storage performance. Furthermore, heat conduction was the primary means of thermal transmission during the melting of the bottom paraffin. However, the melting of the upper paraffin was nevertheless influenced by natural convection, Hence, as pore size decreased, the influence on the weakening of natural convection became more significant. The results of the study provide guidance for the optimal design of latent heat storage devices for semi-cylinders. • The metal foam's pores are smaller the more effectively it transfers heat. • The copper foam with a 30 PPI pore density exhibited the best thermal storage performance. • The natural convection effect was less noticeable as pore size decreased. • The rate of heat storage increased as pore density continued to rise. [ABSTRACT FROM AUTHOR]

Neutral beam injection is an important auxiliary heating and current driving method for magnetic confinement fusion research. In order to better build the Neutral Beam Injector with Negative Ion Source (NNBI), the pre-research on key technologies has been carried out for the Comprehensive Research Facility for Fusion Technology (CRAFT). Combined with the theoretical design, the maximum heat deposition distribution on the surface of the polar plate in the case of normal operation is analyzed. Then, according to the actual heating load on the surface of the polar plate, the heat removal simulation under the optimized beam power distribution is carried out. The improvement of the heat transfer performance of the water-cooled structure by the spoiler structure is simulated. The results show that the water-cooled structure of the electrostatic residual ion dump can meet the heat removal requirements of the system. [ABSTRACT FROM AUTHOR]

In order to remove high heat fluxes for plasma facing components in International Thermonuclear Experimental Reactor (ITER) divertor, a numerical simulation of subcooled water flow boiling heat transfer in a vertically upward smooth tube was conducted in this paper on the condition of one-sided high heat fluxes. The Eulerian multiphase model coupled with Non-equilibrium Boiling model was adopted in numerical simulation of the subcooled boiling two-phase flow. The heat transfer regions, thermodynamic vapor quality ( x th ), void fraction and temperatures of three components on the condition of the different heat fluxes were analyzed. Numerical results indicate that the onset of nucleate boiling (ONB) and fully developed boiling (FDB) appear earlier and earlier with increasing heat flux. With the increase of heat fluxes, the inner CuCrZr tube will deteriorate earlier than the outer tungsten layer and the middle oxygen-free high-conductivity (OFHC) copper layer. These results provide a valuable reference for the thermal-hydraulic design of a water-cooled W/Cu divertor. [ABSTRACT FROM AUTHOR]

An unsteady two-dimensional laminar forced convection heat transfer around a square cylinder with rounded corner edge is numerically investigated for Prandtl Number, Pr=0.01-1,000 and non-dimensional corner radius, r=0.50-0.71 at low Reynolds number, Re=100. The effect of gradual transformation of square cylinder into circular cylinder on heat transfer phenomenon is studied. The lateral sides of the computational domain are kept constant to maintain the blockage as 5 %. A structured non-uniform mesh is used for the computational domain and the Finite Volume Method (FVM) based commercial software Ansys FLUENT is used for numerical simulation. The heat transfer characteristics over the rounded corner square cylinder are analyzed with the isotherm patterns, local Nusselt number (Nulocal), average Nusselt number (Nuavg) at various Pr and various corner radii. It is found that the heat transfer rate of a circular cylinder can be enhanced 14 % by introducing a new cylinder geometry of corner radius, r=0. 51. [ABSTRACT FROM AUTHOR]

• Heat transfer enhancement system including two flexible vortex generators is analyzed by using an immersed boundary method. • Effects of the inclination angle, bending rigidity, and the gap distance on the thermal performance are analyzed in detail. • The thermal performance of the present heat transfer system is enhanced up to 122% as compared to baseline flow without flexible vortex generators. Heat transfer should be considered in the design process of various industrial fields such as electric-electronics, plants and refrigeration and air conditioning, etc. Many researchers have introduced vortex generators to promote mixing of fluids, thereby improving heat transfer performance. Convective heat transfer can be enhanced by using vortex generators where, however, the disadvantage of increased mechanical loss due to increased pressure drop is not avoidable. In order to compensate for the penalty, the present study utilizes the characteristics of self-sustained flapping motions of the flexible vortex generator to improve heat transfer without much increasing the pressure drop. The two flexible vortex generators (FVGs) fixed on the upper and lower channel walls, respectively, are introduced in the present study and they take the form of asymmetric configurations by adjusting the inclination angles. The heat transfer performance is observed to depend on the parametric conditions of FVGs such as the inclination angle, bending rigidity, and the gap distance between them. The present system including the asymmetric FVGs shows the improvement of thermal performance by 122% as compared to the baseline flow when the bending rigidity is 0.06, the initial inclination angle is 75°, and the gap distance is 2.4 times the length of FVGs. The attracting Lagrangian coherent structures (LCS) are visualized by calculating the finite-time Lyapunov exponent (FTLE) field to analyze the effects of the vortex structures near the heated channel walls on the fluid mixing. The net energy loss and average heat transfer are predicted by the three machine learning algorithms, i.e., artificial neural network (ANN), support vector regression (SVR), and random forest (RF), where the inclination angle, bending rigidity, and gap distance of FVGs are selected as input data. The SVR and ANN algorithms show the best performance in predicting the mean energy loss and the heat transfer, respectively, with the R2 value of above 0.99 and 0.95. [ABSTRACT FROM AUTHOR]

An unsteady two-dimensional laminar forced convection heat transfer around a square cylinder with the rounded corner edge is numerically investigated for Pr = 0.01–1000 and non-dimensional corner radius, r = 0.50–0.71 at low Reynolds number (Re = 100). The effect of gradual transformation of a square cylinder into a circular cylinder on heat transfer phenomenon is studied. The FVM (finite volume method) based commercial code Ansys FLUENT is used for numerical simulation. The heat transfer characteristics over the rounded cornered square cylinder are analyzed with the isotherm patterns, local Nusselt number (Nu local ), average Nusselt number (Nu avg ) at various Prandtl numbers and various corner radii. The heat transfer characteristic is predicted by the GEP (gene expression programming) and the GEP generated explicit equation of Nu avg is utilized in PSO (particle swarm optimization) to optimize the corner radii for maximum heat transfer rate. It is found that the heat transfer rate of a circular cylinder can be enhanced 14.17% by introducing a new cylinder geometry of corner radius r = 0.51. [ABSTRACT FROM AUTHOR]

A Lattice Metal Frame (LMF) has advantages such as easy design of topology and shape of structure, and consequently easy controlling of pressure drop and heat transfer. The aim of this study is to review the reported studies on heat and fluid flow in 3D LMFs in different heat transfer areas and to categorize the reported studies focused on the forced convection heat transfer enhancement. The categorization is done based on the studied domain, structure topography, analysis approaches, solid and fluid materials, solution method and the selection of reference temperature and characteristic length for definition of dimensionless numbers such as Reynolds and Nusselt numbers. It is found that for enhancement of heat transfer in channels with cubic, tetrahedral, kagome, wire woven and octet struts were received the highest attentions among different structure. Furthermore, in this study, the values of friction factor, Nusselt number and efficiency index of the studies on single layer LMF defined channel height as characteristic length are also compared and discussed. It is found that X type structure with OA flow arrangement has the highest friction factor as well as Nusselt number while X type with OB flow arrangement has the highest efficiency index (thermos-hydraulic performance). It is suggested to separate the solution approaches into two groups as single layer (such as fin approach) and multilayer (such as volume average) approaches. Unification of characteristic length and reference temperature difference used by researchers will accelerate studies in this field. • A comprehensive classification from different points of views on 3D lattice metal frames is done. • Among various configurations, cubic, tetrahedral, kagome, wire woven and octet truss structures have the highest interest. • Different characteristic lengths and reference temperatures in the studies make difficulties for comparison of structures. • Only 20% of reported studies included pressure drops in their studies showing an important lack in the reported studies. [ABSTRACT FROM AUTHOR]

The design and model of a channel with heat transfer intensifiers in the form of wide protrusions are developed in a detailed manner. The calculated coefficients of heat transfer and friction resistance are compared with experimental data in a wide range of Re numbers and sizes of protrusions. The optimal geometrical characteristics of such channels are determined, and their advantages over channels of other configurations are shown. [ABSTRACT FROM AUTHOR]

Results from an experimental investigation of heat transfer on microstructured surfaces obtained using the deforming cutting method and having different design shapes and sizes are presented. Heat transfer enhancement by a factor of up to 9 as compared with that on a smooth surface is obtained. Principles for constructing physical models of boiling enhancement are given. [ABSTRACT FROM AUTHOR]

Abstract: Numerical investigations pertaining to heat transfer enhancement of a plate-fin heat exchanger using two rows of winglet type vortex generators (VG) have been performed in this work. Five different strategic placements of the VG, namely, common-flow up in series (CFU–CFU), common-flow down in series (CFD–CFD), combined (CFD–CFU), inline rows of winglet (IRW) and staggered rows of winglet (SRW), were considered. Performance parameters in terms of Nusselt number and quality factor were evaluated from the velocity and temperature data obtained from the solutions of full Navier–Stokes and energy equations. The Reynolds number was varied in the range of 250–1580. Results show that amongst the different types of arrangements of the VG, performance of CFU–CFU configuration is best in terms of heat transfer as well as quality factor. [Copyright &y& Elsevier]