Enhancement Of Heat Transfer Coefficient Research Articles

Experimental evaluation and artificial neural network modeling of heat transfer performance of aerosolized magnesium oxide nanoparticles flow through pipes

This study presents a novel approach to modeling the convective heat transfer coefficient (CHTC) of aerosolized magnesium oxide (MgO) nanoparticles in a circular pipe using artificial neural network (ANN) technique, by leveraging experimental data. The work addresses a gap in existing research on the heat transfer characteristics of nanoaerosols under varying thermal and flow conditions. MgO nanoparticles (30-50 nm) were dispersed in compressed air at volume fractions of 0.005, 0.01, and 0.05 to generate the nanoaerosol. This aerosol was then driven through the pipe at volumetric flow rates between 10 and 50 liters per minute (lpm). The pipe was subjected to controlled heat fluxes of 4546.83 W/m², 9093.66 W/m², and 13640.49 W/m² to evaluate the aerosol heat transfer coefficient (AHTC). Experimental results demonstrated that incorporating MgO nanoparticles significantly enhanced the heat transfer coefficient by up to 1.4 %, 111 %, and 89.7 % at the specified heat flux values, corresponding to increases in the volumetric flow rate from 10 lpm to 50 lpm, respectively. An ANN-based correlation was developed to model the heat transfer coefficient in relation to heat flux, particle volume fraction, and volumetric flow rate. This model accurately predicted the experimental data, achieving a mean absolute percentage error (MAPE) of 9.9 × 10-5, a mean square error (MSE) of 0.038433, and a coefficient of determination (R²) of 0.99. These findings confirm the ANN model's efficacy in predicting the enhancement of the nanoaerosol heat transfer coefficient and provide a robust tool for future thermal management applications involving nanofluids.

Unveiling the fundamentals of flow boiling heat transfer enhancement on structured surfaces.

Micro- and nanostructured surfaces offer the potential to enhance two-phase heat transfer. However, the mechanisms behind these enhancements are not well-understood due to insufficient diagnostic methods, leading to reliance on trial-and-error surface development. We introduce in situ boroscopy to investigate microscale bubble dynamics during flow boiling nucleation and subsequent flow regime development. This method was applied in saturated flow boiling experiments within chemically etched aluminum and copper tubes. Although the surfaces have self-similar surface structures, our findings revealed varied heat transfer coefficient enhancements, with increases of up to 391% on aluminum and 41% on copper. Using boroscopy, we identified key mechanisms of heat transfer enhancement. We further used mercury porosimetry to determine the impact of pore size distribution on thermal performance. The boroscopy technique introduced here not only elucidates the underlying processes of flow boiling heat transfer enhancement but also has potential applications for the study of other two-phase phenomena.

Bio-inspired powder bed fusion 3D printed surfaces for enhanced critical heat flux and boiling heat transfer

The thermal performance and sizing of two-phase heat exchangers employed in various industries can be significantly improved by modifying the surface topology of tubes. The advancement in metal 3D printing technology enables the modification of surfaces to generate distinct patterns that enhance the heat transfer coefficient. This study aligns with these advances by investigating the boiling heat transfer performance of 3D printed surfaces using methanol as the working fluid. The enhanced surfaces studied include four bio-inspired 3D printed surfaces (Mushroom, Cactus, Petal, and Torus), each strategically designed to improve the heat transfer coefficient (HTC) and delay the onset of critical heat flux (CHF). The primary objective of this research is to discuss and analyze the heat transfer mechanism in terms of bubble dynamics, particularly focusing on the interaction between surface morphology and bubble behavior. Experimental results reveal significant enhancements in CHF and HTC for 3D printed surfaces compared to plain surfaces. Among the 3D printed surfaces, the Mushroom surface exhibits the highest enhancement in CHF of 88 % compared to the plain surface. Similarly, the maximum HTC enhancements are 138 %, 86 %, 65 %, and 46 % for the Mushroom, Cactus, Petal, and Torus surfaces, respectively, compared to the plain surface. The methodologies and findings of this study are poised to have a lasting impact on the field, potentially opening new avenues for improving the efficiency and effectiveness of various two-phase heat exchangers.

Record-high heat transfer performance of spray cooling on 3D-printed hierarchical micro/nano-structured surface

Managing high-flux waste heat with controllable device working temperature is becoming challenging and critical for the artificial intelligence, communications, electric vehicles, defense and aerospace sectors. Spray cooling, which combines forced convection with phase-change latent heat of working fluids, is promising for high flux heat dissipation. Most of the previous studies on spray cooling enhancement adopted high spray flow rates to strengthen forced convection for high critical heat flux (CHF), leading to a low heat transfer coefficient (HTC). Micro/nanostructured surfaces can enhance boiling, but bubbles inside the structures tend to form a vapor blanket, which can deteriorate heat transfer. This work demonstrates simultaneous enhancement of CHF and HTC in spray cooling by improving both evaporation and liquid film boiling on three-dimensional (3D) ordered hierarchical micro/nano-structured surface. The hierarchical micro/nano-structured surface is designed to coordinate the transport of spray droplets, capillary liquid films, and boiling bubbles to enhance spray cooling performance. Boiling inversion where superheat decreases with increasing heat flux is observed, leading to an ultra-high HTC due to the simultaneous promotion of bubble nucleation and evaporation. Unprecedented CHF is obtained by overcoming the liquid–vapor counterflow, i.e., synergistically facilitating bubble escape and liquid permeation. A record-breaking heat transfer performance of spray cooling is achieved with a maximum heat flux of 1273 W/cm2 and an HTC of 443.7 kW/(m2 K) over a 1 cm2 heating area.

Surface structures and wettability: Their roles in enhancing nucleate boiling heat transfer of refrigerants

Nucleate boiling is an effective heat transfer mechanism. Previous studies have demonstrated that modifying surface structure or wettability can independently improve nucleate boiling efficiency; however, the combined effects and optimal modification sequence remain underexplored. This research aims to comprehensively analyze the influence of surface structure and wettability on the nucleate boiling heat transfer performance of refrigerants. Different surface structures and wettability were prepared using surface polishing, laser ablation, mechanical cutting, and a combination that involved initially employing laser ablation, followed by the application of chemical etching, and finishing with the treatment using materials possessing low surface energy. The static contact angles on these surfaces were measured via the capillary rise method, followed by pool boiling experiments and bubble behavior visualization. The results show that increasing surface wettability can enhance bubble departure frequency and capillary wicking, thereby improving critical heat flux and heat transfer coefficient. The key to optimizing surface modification to enhance critical heat flux involves constructing alternating high and low nucleation core regions to reduce interactions between bubbles; for enhancing heat transfer coefficient, the key lies in constructing compact nanoscale structures that expand the heat transfer and nucleation area, followed by improving wettability.

Novel 2D Layered MXene Nanofluids for Enhancing the Convective Heat Transfer Performance of Double-Pipe Heat Exchangers.

This paper proposes a new class of novel 2D layered structured materials, such as MXene (MX), for the synthesis of innovative nanofluids as coolants to evaluate the convective heat transfer performance of a double-pipe heat exchanger (DPHE). Convective heat transfer experiments were successfully conducted in lab-scale fabricated DPHE using low-concentration MXene nanofluids by varying the volume concentration of MXene nanoparticles (0.01-0.05 vol %) in different base fluids. The influence of the MXene nanofluids on various convective heat transfer parameters, such as LMTD, Nusselt number, heat transfer coefficient, and heat transfer rate without using any inserts in the DPHE was experimentally investigated. The results of the experiments revealed that the heat transfer coefficient and Nusselt number increase with increasing Reynolds number (Re) and concentration of MXene nanoparticles in the base fluids. Maximum enhancement in heat transfer coefficient (126%) was achieved for methanol-based MXene nanofluids at 0.05 vol %. Moreover, the Nusselt number exhibits a maximum enhancement of ∼50% for methanol- and water-based MXene nanofluids. In contrast, the thermal performance factor was also estimated, and it was observed that water- and methanol- based MXene nanofluids showed higher values than castor oil- and silicone oil-based MXene nanofluids. Finally, the LMTD and heat transfer coefficients were successfully validated using Aspen HYSYS 12.1 software.

Experimental comparison of flow boiling heat transfer in smooth and microfin tubes using R134a, R1234yf, and R513A

Addressing the urgent need for environmentally sustainable and efficient refrigeration systems, this study investigates the performance of low-GWP refrigerants R1234yf and R513A in comparison to the high-GWP refrigerant R134a, focusing on the heat transfer coefficient (HTC) and pressure drop during flow boiling in smooth and microfin tubes. The experimental setup involves tubes with a 6.14 mm internal diameter and 500 mm length. Data were collected across a broad range of operating conditions, including mass fluxes from 170 to 495 kg/m² s, heat fluxes from 10 to 40 kW/m², saturation temperatures of 15 °C and 25 °C, and vapor qualities from 0.15 to 1.0. In a specific case with a mass flux of 170 kg/m² s, heat flux of 23 kW/m², and a saturation temperature of 15 °C, the results indicate that the microfin tube achieves an HTC enhancement of up to 64 % compared to smooth tubes. However, R134a exhibits a higher HTC than R1234yf and R513A, approximately 5 % and 3 % higher, respectively. In contrast, R134a presents a higher pressure drop than R1234yf by about 8 %. While the pressure drop for R134a is slightly higher than R513A in the smooth tube, the microfin tube shows similar trends but more pronounced differences. This study comprehensively explores microfin tube performance with these refrigerants, offering critical insights for designing advanced refrigeration systems that balance environmental responsibility with energy efficiency. These findings were validated against predicted correlations, showing good agreement.

Augmentation of pool boiling heat transfer using a microstructured aluminum surface fabricated by ultrasonic cavitation modification

Micro/nanostructure is a popular and effective structure for boiling heat transfer enhancement. In this study, ultrasonic cavitation modification is employed firstly to create microstructured aluminum surface for boiling heat transfer. With the impact of ultrasonic cavitation microjet, numerous micro pits and holes are formed on the surface in one step, constituting microstructures on aluminum plate. The saturated and subcooled pool boiling experiments are conducted at atmospheric pressure using ethanol as a working fluid. It is found that boiling hysteresis is apparent on the modified plate (MP) with the porous microstructure layer, which can be eliminated by periodically fluctuating heat flux. Under the stable saturated boiling condition, the maximum heat transfer coefficient is enhanced by 208% and wall superheat is decreased by 17.6 °C compared to the bare aluminum surface. Under subcooled boiling conditions, the boiling hysteresis on MP becomes weaker and the positive effect of microstructures on heat transfer performance is enhanced. The enhanced heat transfer coefficient of MP is mainly attributed to the increase of nucleation site density and further to the facilitation of bubble coalescence and departure, while the increased critical heat flux under subcooled boiling conditions is derived from the delaying occurrence of mushroom-like bubble.

3D-printed triply periodic minimal surface (TPMS) structures as catalyst carriers

The flow and forced convection heat transfer properties of triply periodic minimal surface (TPMS) lattices were investigated both experimentally and numerically. The TPMS structures consist of periodically arranged Gyroid (G) unit cells of 81 % porosity and different unit cell sizes (2–4 mm). This is the first study to evaluate the effect of cell diameter on pressure drop and heat transfer characteristics of solid-gyroid structures prepared by selective laser melting (SLM) technique. Moreover, the range of pore-based Reynolds number covered the laminar and turbulent regimes (Rep=11–1700). The results shown that larger cell diameter provides higher heat transfer coefficients and unit pressure drop. An increase of cell diameter from 2 mm to 4 mm can enhance heat transfer coefficient and unit pressure drop of 39 % and 85 %, respectively. New correlations for flow resistance and heat transfer of solid-gyroid structures were developed. It was confirmed that TPMS lattices can be an alternative to conventional catalytic supports, because they provide comparable thermal efficiency index as foam and packed bed.

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.

Exploring heat transfer augmentation and entropy generation in nanofluid flow induced by vibration: Influence of velocity and rheological properties

This study numerically investigates the heat transfer and entropy generation of nanofluid flow under mechanical vibration, employing approved formulations to model the nanofluid density, specific heat, viscosity, and conductivity. Specifically, the impact of vibration on laminar forced convection thermal flow of both pure water and Al2O3-water nanofluid within a pipe is explored using CFD. Various Reynolds numbers are examined under constant heat flux conditions, with nanofluid properties determined using established correlations. Results indicate that applying Al2O3 nanofluid slurry instead of pure water at low Reynolds numbers reduces entropy generation, proving advantageous. Vibration enhances heat transfer by intensifying fluid agitation and promoting particle dispersion near the wall, resulting in a significantly more uniform temperature distribution along the pipe, approximately 100 times more than steady-state flow. Analysis reveals vibration’s effectiveness in reducing irreversibility, especially at lower Reynolds numbers, with substantial enhancements in heat transfer coefficients, up to approximately fivefold compared to steady-state flow, particularly for nanofluid flows. Optimal conditions for maximizing heat transfer enhancement emphasize nanoparticle size and concentration. Mechanical vibration with different frequencies produces significant improvements in heat transfer compared to amplitude variations, primarily influenced by the Reynolds number. Overall, this study offers valuable insights into the intricate relationship between vibration, fluid dynamics, and heat transfer in nanofluid flows, with practical implications for optimizing thermal management systems across various engineering applications.

Numerical evaluation of thermohydraulic parameters for diverse configurations of shell-and-tube heat exchanger

This research aims to optimize the internal design of shell-and-tube heat exchangers by investigating the thermal performance of various configurations. A comparative study of segmental baffles with plain and finned tube setups and helical baffles with different angular orientations is conducted using numerical simulations with the Galerkin finite element analysis. Key thermohydraulic parameters, such as shell-side pressure difference, heat transport coefficient, and heat transport coefficient per unit pressure drop, are assessed with shell-side mass flow rates. The outcomes direct higher shell-side oil mass flow rates correlate with increased pressure drop and enhanced heat transfer coefficients. Helical baffles exhibit superior performance to segmental baffles, offering a more significant heat transport coefficient per unit pressure drop. Within segmental baffles, finned tubes lead to higher pressure drops and heat transport coefficients than plain tubes while maintaining similar heat transport coefficients per unit pressure drop. Furthermore, lower helix angles (Φ = 10.81°) in helical baffles are associated with higher pressure drops and heat transport coefficients, whereas Φ = 25.52° achieves the highest heat transport coefficient per unit pressure drop.

Numerical study on 3D nucleate boiling over an oscillating base plate with high wall superheat and multiple nucleation sites

This paper is on 3D numerical study on nucleate boiling at high wall superheat of 13.5 K, over an oscillating base plate, using a sharp interface dual-grid level set method (SI-DGLSM)-based in-house code. These studies are performed for two nucleation sites, with two configurations of the computational domain. A micro-layer evaporation model has been used to enable 3D simulations on a coarser grid and higher wall superheats. The size of the computational domain is chosen based on the nucleation site density, reported in experiments, and the present numerical results compare well with the published experimental results; for a stationary horizontal plate under atmospheric pressure. For nucleate boiling on an oscillating plate, bubbles departing from both the nucleation sites synchronize and lock-on with the plate oscillation. Enhancement in heat transfer coefficient is observed in presence of base plate oscillations, which is attributed to increase in rate of phase change as compared to increase in rate of convective heat transfer. Non-uniform distribution of nucleation sites on the base plate leads to formation of local convergence zones on the plate, which in turn affects the motion of bubbles as they rise within the computational domain. The present work is significant as it presents 3D simulations for nucleate boiling under the effect of base plate oscillations—as a promising technique for heat transfer enhancement.

Numerical investigation of multiple affecting parameters on mixed convective heat transfer for arrays of buildings

This study aims to explore relationships between wind speed, wind direction, exterior surface roughness, temperature difference, plan area density and convective heat transfer coefficients (CHTCs) for external surfaces of arrays of buildings. The present investigation provides correlations for CHTCs at the windward, leeward, top, left, and right lateral surfaces. Three different plan area densities of 0.25, 0.111 and 0.027 are considered and 249 numerical simulations are conducted to investigate the influential parameters including density (λp), wind angle (θ), wind speed (U10), temperature difference (ΔT), and surface roughness (ε). Findings reveal that higher plan area density results in lower CHTC due to heightened airflow obstruction. Increase of the wind angle amplifies CHTC on leeward and right lateral facades while reduces it on the left lateral side. With increased wind speed, CHTC experiences significant increase. The results also shows that the temperature difference's impact is pronounced at lower air velocities, leading to heightened CHTC. Besides, increased surface roughness enhances heat transfer coefficients. Finally, the present study employs extensive simulations to present correlations for different facades of the buildings in the studied arrays offering insights into CHTC variations in terms of plan area density, wind angle, speed, temperature difference, and roughness height.

The behaviour of ternary hybrid nanofluid: Graphene oxide, Aluminium oxide, Silicon dioxide in heat transfer rate

The miniaturization in the design of the electronic system became inevitable due to the rapid advancement and development of technology. This has imposed challenges to the thermal management capability as the heat flux density has increased tremendously due to a smaller heat transfer surface. Nanofluids adoption in electronic cooling seems to be an alternative way for better heat dissipation. This research explores the feasibility of ternary hybrid nanofluids GO: Al2O3: SiO2 in water with different volume concentrations and mixture ratios in a serpentine cooling plate. In this research, 0.01% GO + Al2O3: SiO2, 0.006% GO + Al2O3: SiO2, and 0.008% GO + Al2O3: SiO2 in mixture ratios of 10:90 and 20:80 (Al2O3: SiO2) were studied. The result showed that 0.01% GO + Al2O3: SiO2 (10:90) nanofluids displayed the highest enhancement of heat transfer coefficient with 1.1 times higher as compared to the base fluid. This was then followed by 0.008% GO + Al2O3: SiO2 (10:90) and 0.006% GO + Al2O3: SiO2 (10:90) with 1.03 times and 0.87 times higher heat transfer coefficient enhancement consecutively as compared to the base fluid. In term of mixture ratios, GO in 10:90 (Al2O3: SiO2) performed better than 20:80. To assess the feasibility of adoption, the advantage ratio (AR) was conducted to measure both heat transfer enhancement and pressure drop effect. The AR analysis showed that at the lower Reynolds, Re number region, the 0.01% GO + Al2O3: SiO2 (10:90) ternary hybrid nanofluids was proven to be the most feasible due to a higher ratio of heat transfer enhancement over the pressure drop penalty.

CHF Enhancement by γ-Fe2O3 Nanofluids with Pool Boiling Condition on a Downward-Facing Plane

In this study, the heat transfer performance of γ-Fe2O3 nanofluid is investigated. The particle size used in the experiment was about 20 nm. It was found by X-ray diffraction that it was consistent with the characteristic peak and no other impurities. Nanofluids with different concentrations were configured through a two-step method. Since the γ-Fe2O3 nanoparticles are not easily dispersed, the ultrasonic time was relatively long. After a series of experiments and data processing, we could see that nanofluids have the best heat transfer performance at 0.07 g/L. Compared to a reverse-osmosis (R·O) water case, the enhancement of critical heat flux (CHF) was about 34.09%, and the heat transfer coefficient enhancement was about 49.32%. The movement of bubbles during the experiment was recorded and analyzed. Compared with the R·O water case, the bubbles were larger and fewer in the nanofluid case, and what is more, the bubble movement was relatively intense. The heating surface was characterized after the experiment, and it was found that the wettability of the heating surface was changed, and the roughness of the heating surface decreased. Scanning electron microscopy showed that the deposition of the nanoparticles on the heating surface was the main cause of CHF enhancement. When the concentration was 0.08 g/L, CHF decreased, mainly because the excessive deposition of the nanoparticles increased the thermal resistance of the heating surface and led to the deterioration of heat transfer.

Analysis of nanofluid and nano-enhanced phase change material (NEPCM) in a heat exchanger for thermal management of electronic devices

Thermal management of high-power electronic devices is essential to control their enormous heat generation, which is usually the cause of their damage. Scientists have found several solutions for cooling electronic devices, but a counter-current heat exchanger has never been used. In the same context, numerical research on the thermal performance of a proposed new cooling system based on a counterflow heat exchanger combined with a heat sink is conducted in this article. Cu-H2O nanofluid, n-Docosane nano-enhanced phase change material, and water were selected as coolants, and they are compared in terms of temperatures, Nusselt number, heat transfer coefficient, pressure drop, friction factor, and performance enhancement coefficient using a two-phase mixture transient model based on Finite Volume method. Two cases of the proposed system are studied to find the optimal distance between heat exchanger pipes and the hot surface. Results are compared with previous experimental data to demonstrate that the system under study is accurate. They revealed that integrating the heat exchanger with the heat sink using Cu-H2O nf as a coolant maintained the hot surface temperature below 308 K with improved heat transfer by 31.22% compared to water. 0.25 mm is the optimum distance to find the highest thermal performance of the cooling system.

Heat transfer coefficient evaluated for a designed jacketed vessel for parameter optimization in coal water slurry preparation using dual impellers

ABSTRACT This study examines the heat transfer coefficient during coal water slurry preparation in a jacketed vessel, focusing on key parameters influencing heat transfer. A dual impeller system is utilized to enhance mixing and heat transmission capacity. Using the Ant Colony Optimization (ACO) method, the equipment is designed, comprising a cylindrical flat-bottom tank with an impeller diameter of 0.079 m, a height of 0.3 m, and four uniformly positioned detachable flat baffles around the vessel’s perimeter. Experiments are conducted using the L27 orthogonal array at three different levels, varying impeller speed, clearance height to tank diameter ratio, slurry height to tank diameter ratio, and distance between the two impellers. Two categories of dual impeller experiments, propeller and Rushton, are examined. Statistical analysis indicates that impeller speed and Z/T ratio significantly influence heat transfer coefficient, while C/T ratio and distance between impellers have a lesser impact. In both propeller and Rushton experiments, impeller speed and Z/T ratio have the most substantial effect on heat transfer coefficient, contributing 62.907% and 59.47%, respectively, in propeller and Rushton experiments. These findings underscore the importance of impeller speed and Z/T ratio in enhancing heat transfer coefficient during coal water slurry preparation in a jacketed vessel.

Effect of twisted rib on the hydrothermal performance and entropy generation behavior in minichannel heat sinks cooled by biologically synthesized Ag/water nanofluid

This numerical research study focuses on impacts of using twisted rib geometry on the hydrothermal performance and entropy generation characteristics of biologically synthesized Ag/water nanofluid flowing in heat sink ‌minichannels. To this end, twelve different twisted rib geometries (Case 1 to 12) were designed where the double-row ribs were twisted at the bottom and top portions with twist angles between − 45° and + 45°. To elucidate the cooling performance comprehensively, four different Reynolds numbers (Re = 500, 1000, 1500, and 2000) and four different nanoparticle concentrations (φ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\varphi \\hspace{0.17em}$$\\end{document}= 0%, 0.1%, 0.5%, and 1%) were included in the numerical computations. Results revealed that using twisted ribs significantly smoothen the fluid flow and reduces pressure drop remarkably depending on twist angles, nevertheless, it deteriorates the cooling performance. In Case 12, the convective heat transfer coefficient reduces by up to 15.8% at Re = 2000 and φ\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$ \\varphi$$\\end{document}=1%, compared to the base case. However, pumping power requirement is decreased by 43.9% in this case. Nanoparticle incorporation contributes to enhancement of convective heat transfer coefficient by up to 13.5% at the lowest Re. Frictional entropy generation considerably decreases (up to 31%) when twisted ribs are utilized. However, thermal entropy generation can increase up to 36%, due to ineffective cooling with twisted ribs. This work can be regarded as a first study in the field concerning the effects of top and bottom twist angles on hydrothermal performance and entropy generation in nanofluid-cooled mini-channel heat sinks.

An experimental study on flow boiling heat transfer in porous-ribbed micro-channels

Experiments were carried out to investigate flow boiling heat transfer characteristics in a micro-channel heat sink with porous ribs at three rib-heights (i.e. Hrib = 200 μm, 400 μm, and 600 μm), three mass fluxes (i. e. 200 kg/m2s, 300 kg/m2s, and 400 kg/m2s), and a range of wall heat fluxes (30–1300 kW/m2). The heat transfer coefficients and temperature uniformity in porous-ribbed micro-channels under different conditions were analyzed and compared to the smooth micro-channels (i.e. smooth flat surface and smooth-ribbed surfaces). The flow regimes in both porous and smooth micro-channels were visualized under typical flow boiling conditions. The results showed that micro-channels with higher rib heights experience an earlier transition to the critical heat flux (CHF) region due to the narrower flow path. Compared to the smooth micro-channel, the configuration of porous coating significantly enhances the heat transfer in the micro-channel without ribs or with lower rib height (Hrib = 200 μm) due to the strong hydrophilicity, good liquid storage, and rewetting ability on porous surfaces. At G = 400 kg/m2s, the porous-flat surface achieves a 67.9 % enhancement of heat transfer coefficient in the micro-channel compared to the smooth-flat micro-channel. However, with a higher rib height (Hrib = 400 μm and 600 μm), the thermal resistance is increased which hinders the heat conduction along the rib-height direction. A significant superheat is observed at the bottom of porous surface, thereby deteriorating the heat transfer performance in the porous ribbed micro-channel.