Enhancement Of Heat Transfer Coefficient Research Articles

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.

Pool boiling heat transfer characteristics of low-GWP refrigerants in a horizontal tube bundle configuration

Heat transfer enhancement techniques have been adapted on the shell side to improve the overall performance of the flooded evaporators, such as finned tubes. In recent years, it has been demonstrated that the metal foam structure can offer enhanced heat transfer performance under pool boiling conditions. However, there are not much research in the open literature that examine the feasibility of metal foam embedded tubes in horizontal tube bundle configurations. Therefore, this paper proposed a novel metal foam embedded tube (i.e., foam embedded outside the tube) to improve the heat transfer behavior of flooded evaporators. The experiments were performed on a horizontal tube bundle with a staggered arrangement. Moreover, the performance of low global warming potential (GWP) refrigerants (R-1234yf and R-1234ze(E)) is compared against R-134a for both plain tubes and metal foam tubes with porosities of 81%, 75%, and 62%. The results showed that a metal foam tube with a porosity of 62% showed a maximum heat transfer coefficient (HTC) enhancement of 291% compared to the plain tube. As compared with R-134a, the HTCs of R-1234yf and R-1234ze(E) are nearly 10% higher and 5% lower, respectively.

Aluminum Micropillar Surfaces with Hierarchical Micro- and Nanoscale Features for Enhancement of Boiling Heat Transfer Coefficient and Critical Heat Flux.

The rapid progress of electronic devices has necessitated efficient heat dissipation within boiling cooling systems, underscoring the need for improvements in boiling heat transfer coefficient (HTC) and critical heat flux (CHF). While different approaches for micropillar fabrication on copper or silicon substrates have been developed and have shown significant boiling performance improvements, such enhancement approaches on aluminum surfaces are not broadly investigated, despite their industrial applicability. This study introduces a scalable approach to engineering hierarchical micro-nano structures on aluminum surfaces, aiming to simultaneously increase HTC and CHF. One set of samples was produced using a combination of nanosecond laser texturing and chemical etching in hydrochloric acid, while another set underwent an additional laser texturing step. Three distinct micropillar patterns were tested under saturated pool boiling conditions using water at atmospheric pressure. Our findings reveal that microcavities created atop pillars successfully facilitate nucleation and micropillars representing nucleation site areas on a microscale, leading to an enhanced HTC up to 242 kW m-2 K-1. At the same time, the combination of the surrounding hydrophilic porous area enables increased wicking and pillar patterning, defining the vapor-liquid pathways on a macroscale, which leads to an increase in CHF of up to 2609 kW m-2.

Maximizing heat transfer potential with graphene nanofluids along with structured copper surfaces for pool boiling

Increasing the demand and depletion of energy sources confront every technological development with energy conversation and its utilization. Boiling heat transfer is the most applicable heat transfer method in many industries, from electronic chip manufacturing to heavy nuclear power plants. Hence, the augmentation of it is of prime importance for future requirements. The present investigation focused on the augmentation of boiling heat transfer by considering a novel V-pattern staggered copper surface for pool boiling in graphene nanofluid media. The study encompasses a detailed exploration of nanofluid concentrations ranging from 0.1% to 0.40 vol% prepared in equal volumes of dimethylformamide and distilled water. The nanofluid's stability is observed to be quite promising, and the results are presented. The wall superheat was drastically reduced to 48.73%, and the maximum heat transfer coefficient enhancement of 83.47% was achieved for 0.04% concentration of graphene nanofluid with a V-pattern surface compared to a smooth copper surface with water. This is due to the micro-level structured convection patterns promoted by the microstructures. The thermophysical properties of nanofluid and peculiar microstructures enhanced the relative heat transfer coefficient by 63.37%. On the other hand, two popular machine learning models, Random Forest Regression (RFR) and Multi-Layer Perceptron (MLP), are used to predict the boiling heat transfer coefficient. Moreover, it is observed that the RFR model fitted the data well and yielded the lowest mean absolute percentage error within the bounds of computational time.

Enhanced flow boiling heat transfer in narrow gap with different widths using different-mode-interacting boiling

The concept of different-mode-interacting boiling (DMIB) utilizes the areas of two different thermal conductance materials by expanding the wetted area over the regions of their dry areas. This enhances the critical heat flux (CHF) and heat transfer coefficient (HTC) in a wide channel (e.g., Wch = 80 mm) to provide adequate cooling in previous study. In this study, the effectiveness of DMIB in enhancing the CHF and HTC was investigated when using a channel width (Wch = 10 mm) equal to that of the heating surface, aiming to provide better efficiency for both uniform surface (US) and nonuniform surfaces (NUSs) with material widths (W = 0.5 and 1.0 mm) in narrow gaps. Experiments were performed with a heating surface of 10 mm2 using subcooled deionized water at different temperatures ΔTsub = 2.0, 10, 20, and 30 K, inlet velocities v = 0.1, 0.2, and 0.4 m/s (corresponding to mass fluxes: 100, 200, and 400 kg/m2s), and gap sizes h = 1.0, 2.0, and 5.0 mm. The results revealed that the heat transfer performance of the NUSs was significantly enhanced compared to the US for both Wch. However, both the CHF and HTC values were higher with Wch = 80 mm than with Wch = 10 mm owing to the expansion of coalesced bubbles in the flow direction, sustained nucleate boiling regime, and effective liquid rewetting during the intermittent nucleate boiling-partial dryout near the CHF owing to the DMIB. The maximum CHF and CHF enhancement ratios for Wch = 10 mm were 3.59 MW/m2 and 52.2 %, respectively, whereas for Wch = 80 mm, they were 3.66 MW/m2 and 89 %, respectively. In addition, the HTC and HTC enhancement ratio for Wch = 10 mm were 1261 kW/m2K and a factor of 7, respectively, whereas those for Wch = 80 mm were 2026.9 kW/m2K and a factor of 12, respectively. In contrast, only a minimal difference existed in the heat transfer performance of Wch = 10 mm compared to that at Wch = 80 mm; therefore, Wch = 10 mm can be considered as having a better cooling efficiency per channel width due to the minimal pumping power and significant CHF and HTC enhancement.

Ionic liquid as a cosurfactant for critical heat flux enhancement during boiling with aqueous surfactant solutions

Surfactants are often used to improve boiling heat transfer due to the ease of their implementation. While aqueous surfactant solution significantly increases the heat transfer coefficient (HTC), it is known to deteriorate the critical heat flux (CHF) relative to the base fluid water. The high foamability of most surfactant solutions causes vapor crowding on the heater surface, making surface modification techniques ineffective for CHF enhancement. Here, we address the issue of relative degradation of CHF due to surfactant by using an ionic liquid as a cosurfactant. We employ a unique boiling fluid, where 1-ethyl-3-methylimidazolium chloride ([C2mim][Cl]), a low-foaming ionic liquid, is introduced as a co-surfactant in aqueous sodium dodecyl sulfate (SDS) solution. Motivated by the synergistic increase in foamability of the aqueous surfactant solution due to the addition of ionic liquid, we examine the boiling performance at various combinations of concentrations of the two additives in the solution. The mixture consistently exhibits higher CHF and HTC than an SDS only solution, with the best enhancement observed for a mixture of ≈ 30 ppm SDS and ≈ 750 ppm [C2mim][Cl], resulting in ≈1.6× and ≈1.5× enhancements in CHF and HTC, respectively, compared to the SDS only solution (≈ 30 ppm). We next fine-tune the concentration of cosurfactant to demonstrate an exceptionally high HTC of ≈93 kW/(m2 – ℃) with a CHF value comparable to that of pure water. Enhanced foamability and in-situ improvement in the wettability of the heater surface enhance the HTC and the CHF, respectively. This facile technique for enhancing HTC with water as the base fluid, without compromising on CHF, holds promise for high heat flux thermal management applications.

Surface functionalization enabling highly effective phase-change heat transfer of R1234yf: A molecular dynamics study

Phase-change heat transfer has been demonstrated to be an effective thermal management approach for high-power electronics and power generators. In this work, we propose a strategy using surface functionalization to enhance the phase-change heat transfer of an environmentally friendly refrigerant, i.e., hydrofluoroolefins (HFO)-based R1234yf. Using molecular dynamics simulations, we find that the evaporation (condensation) heat transfer coefficient (HTC) at self-assembled monolayer (SAM) functionalized gold (Au) surfaces can be increased by ∼7 (∼5) times compared to that at the planar Au surfaces. The improvement in phase-change heat transfer of R1234yf at functionalized Au surfaces arises from the high thermal conductance across functionalized Au/R1234yf interfaces and the strong vibrational couplings between SAM molecules and R1234yf molecules. On the one hand, the introduced SAM layer can increase the interfacial thermal conductance (ITC) between Au and R1234yf owing to the strong bonding between Au and the SAM molecules, as well as the strong vibrational couplings at low-frequency modes (0–6 THz) among Au, SAM, and R1234yf. On the other hand, the thermal energy exchange during the evaporation (condensation) process can be improved by surface functionalization through the vibrational couplings between SAM and R1234yf at high frequency (∼12 and ∼30 THz) regions. This dual vibrational coupling will then allow the SAM layer to work as an intermedia to bridge the vibrations between Au and R1234yf molecules, which in return benefits the phase-change heat transfer. By further characterizing the evaporation and condensation processes, we find that the evaporation rate of R1234yf is increased by ∼50% at most at the wall superheat <70 K when the Au surfaces are functionalized using SAMs. When the wall superheat is above 70 K (i.e., 100 K for the planar Au surface), the evaporation of R1234yf becomes like film evaporation in which clusters of R1234yf molecules move off the surface together, which limits the enhancement (i.e., ∼100% at most) of evaporation HTC at the functionalized Au surfaces. At the same time, the condensation rate at functionalized Au surfaces is found to be increased by 15%–100% compared to that of the planar Au surfaces. By elucidating the mechanisms governing phase-change heat transfer on functionalized surfaces, our results provide an efficient strategy to enhance the cooling efficiency of the HFO refrigerants, which may benefit the design of surfaces with high phase-change heat transfer capabilities.

Experimental and Theoretical Investigation on Heat Transfer Enhancement in Micro Scale Using Helical Connectors.

Microscale electronics have become increasingly more powerful, requiring more efficient cooling systems to manage the higher thermal loads. To meet this need, current research has been focused on overcoming the inefficiencies present in typical thermal management systems due to low Reynolds numbers within microchannels and poor physical properties of the working fluids. For the first time, this research investigated the effects of a connector with helical geometry on the heat transfer coefficient at low Reynolds numbers. The introduction of a helical connector at the inlet of a microchannel has been experimentally tested and results have shown that this approach to flow augmentation has a great potential to increase the heat transfer capabilities of the working fluid, even at low Reynolds numbers. In general, a helical connector can act as a stabilizer or a mixer, based on the characteristics of the connector for the given conditions. When the helical connector acts as a mixer, secondary flows develop that increase the random motion of molecules and possible nanoparticles, leading to an enhancement in the heat transfer coefficient in the microchannel. Otherwise, the heat transfer coefficient decreases. It is widely known that introducing nanoparticles into the working fluids has the potential to increase the thermal conductivity of the base fluid, positively impacting the heat transfer coefficient; however, viscosity also tends to increase, reducing the random motion of molecules and ultimately reducing the heat transfer capabilities of the working fluid. Therefore, optimizing the effects of nanoparticles characteristics while reducing viscous effects is essential. In this study, deionized water and deionized water-diamond nanofluid at 0.1 wt% were tested in a two-microchannel system fitted with a helical connector in between. It was found that the helical connector can make a great heat transfer coefficient enhancement in low Reynolds numbers when characteristics of geometry are optimized for given conditions.

Film flow boiling of a magnetic fluid over a vertical plate exposed to a magnetic field under reduced gravity conditions

In this article, the film flow boiling over a vertical flat plate was studied numerically. The saturated liquid upward flow over a vertical flat plate with a uniform free stream velocity was considered. The Volume of Fluid (VOF) model was used for capturing the liquid-vapor interface. The effects of the wall superheat, the free stream velocity of hydrodynamics, and the heat transfer of the flow film boiling were studied in the absence and presence of a magnetic induction (MI). The results showed increased free stream velocity enhances the heat transfer coefficient (HTC). In addition, applying uniform MI results in a normal force at the liquid-vapor interface. This normal force enhances the local heat transfer coefficient (HTC) during film flow boiling on the heated vertical plate. Also, the percentage of HTC enhancement is higher when the magnetic field strength increases. Moreover, the heat transfer enhancement diminishes as the free stream velocity increases. Furthermore, an increase in the magnetic permeability ratios of the phases contributes to improving the HTC and increases the wall shear stress. The effect of the variation of the acceleration of gravity is also studied, revealing that a decrease in gravity’s acceleration actually slightly enhances HTC.

Transient behavior of liquid film deposition and surface heat transfer upon cryogen-spray cooling coupled with cold air jet

Cryogen spray cooling (CSC) has become a promising auxiliary-cooling technique for laser dermatology, which recently enhanced by cold air jet (CSC-CAJ). However, the CAJ-to-spray interactions could worsen the CSC performance and associated liquid film behavior. In this study, the transient behavior of liquid film deposition and surface heat transfer of CSC-CAJ was investigated. A high-speed camera was used to capture the film intensity through Mie-scattering technique. Fast-response TFTC thermocouple was used to measure the transient surface temperature. The large eddy simulation (LES) was used to extract the vortical structure, while the discrete-phase model (DPM) and Eulerian wall film (EWF) were coupled for the spray process and the transient film modeling. The results showed that the CAJ significantly enhanced CAJ-to-spray mixing and mitigated the film thickness (i.e., 32 % reduction). The CSC-CAJ had asymmetric film coverage due to induced counter-rotating vortex-pairs (CRVP) from the CAJ-to-spray interaction, which laterally clustered the spray. It permitted fast evaporation with dominant CAJ at the spray-periphery while permitting fast evaporation with dominant spray at the spray-core. Consequently, the CSC-CAJ showed 75 % temperature increment at the windward side and 15 % temperature reduction with 60 % heat transfer coefficient enhancement at the impingement point. The experimental film intensity and simulated film thickness were linearly correlated and significantly influenced the surface temperature. The combined results affirmed the significant impact of the CAJ on the transient film deposition, which potentially helps promote advanced cooling technology.

Understanding heat transfer and flow characteristics of additively manufactured pin fin arrays through laser-induced fluorescence and particle image velocimetry

This article investigates the thermal and hydrodynamic performance of three distinct additively manufactured fin array configurations in a cooling channel: cylindrical, optimized, and perforated fin. The optimized fin design was achieved through the application of a genetic algorithm (GA) to enhance heat transfer efficiency. Experimental assessments were conducted to quantify the heat transfer characteristics and pressure drop across these fin configurations where water is the working fluid and the Reynolds number ranges from 1100 to 5100, along with visualization using Laser-Induced Fluorescence (LIF) and Particle Image Velocimetry (PIV) techniques. Results reveal that the optimized fin array, generated through genetic algorithm optimization, demonstrates a notable enhancement in heat transfer coefficient, exhibiting an improvement of up to 44 % when compared to the traditional cylindrical fin configuration. Furthermore, the perforated fined array showcases a 63 % enhancement in heat transfer performance compared to the cylindrical fin array without apparent increase of pressure drop. The utilization of LIF and PIV techniques offers insights into the heat transfer and fluid velocity distributions within the cooling channel. These visualization methods provide a profound understanding of the flow patterns, flow mixing phenomena, and heat exchange associated with the pin fin designs, facilitating a deeper comprehension of the underlying heat transfer mechanisms.

Enhancement of heat transfer coefficient of water desalination in multi-stage flushing using hematite nanofluid

This study is the first step toward the enhancement of the evaporation amount and heat transfer coefficient (HTC) in multi-stage flushing (MSF) desalination processes by incorporating hematite nanoparticles (Fe2O3 NPs) into saltwater. The study involved the application of a laboratory experiment aimed at determining various physical properties of saline water (SW) nanofluid (NF). These properties included stability, thermal conductivity (TC), evaporation volume and rate, boiling temperature (BT) and time, and local HTC in stagnant conditions. The investigation took into consideration the impact of salinity, NPs (with regard to both size and volume fraction (φ)), and pressure. The utilization of stable SW NF resulted in reduced BT and expedited boiling. In the actual MSF process, the preheating of the cooling SW occurs rapidly and at low temperatures prior to its arrival at the brine heater. The determined decline in usage of steam was 2 % in the least favorable scenario. Meanwhile, the measurement of the amount of evaporation was conducted by evaluating the condensation content under ideal circumstances, which indicated a 15 % increase under atmospheric pressure and a rise of 25 % under vacuum circumstances. A significant enhancement of the local HTC, reaching to 134 %, was observed under stagnant conditions when utilizing NPs with a size of 20 nm and φ of 0.05 %. This denotes the initial stage in the estimation of the augmentation in output of water through the utilization of SW NF. The deposition of Fe2O3 NPs with high thermal conductivity is expected to have a significant impact on the mitigating surface thermal resistance caused by inorganic scale in MSF plants.

Experimental analysis of a tri-partite brazed plate gas cooler for CO2 heat pump water heaters

Gas cooler is a crucial component in CO2 systems, and its performance significantly influences the overall efficiency of the CO2-based heat pump system. Brazed plate heat exchangers feature a compact design, reduced refrigerant charge, enhanced heat transfer coefficient, and lower pressure drop, resulting in space savings and improved system performance. This experimental study investigates a water-cooled tri-partite brazed plate gas cooler in a CO2 heat pump. The tri-partite brazed plate gas cooler has a double effect: it allows matching the temperature glide of CO2 with that of water and meeting the requirements for both domestic hot water and space heating simultaneously. The experiments are conducted in only domestic hot water and combined space and domestic hot water operative modes. The effects of different operating parameters on heat duty, supply temperature, temperature approach, and pinch point location are evaluated. The actual conductance of gas coolers is calculated, and a new definition is adopted to determine the ϵ−NTU of gas coolers. Results show that preheating gas cooler (GC3) has a higher heat duty at pressures near the critical pressure. As the inlet pressure increases, the domestic hot water supply temperature increases, the pinch point moves to the cold end of GC3, and the temperature approach decreases. The LMTD method may result in an undersized or oversized CO2 gas cooler by up to ±50% for different pressures. The errors reach their maximum values when the temperature difference is low at the cold end and high at the hot end. The effectiveness ranges from 0.56 to 0.94 and the NTU ranges from 2.64 to 5.41. The gas cooler shows promising performance. The outcomes of this research are anticipated to serve as a valuable reference for enhancing the design and optimization of transcritical CO2 systems.

ENHANCEMENT EVALUATION CRITERIA FOR POOL BOILING ENHANCEMENT STRUCTURES IN ELECTRONICS COOLING: CHF ENHANCEMENT RATIO (ER-CHF) AND ENHANCEMENT INDEX (EI)

Extensive research shows the necessity of efficient cooling systems to enable electronic components to operate at high performance levels for a sustained period. While conventional methods have served the cooling needs so far, rising computational power, energy efficiency, and sustainability requirements call for improved techniques. The literature shows the effectiveness of two-phase systems in cooling electronic components like microprocessors. The literature further describes various enhancement mechanisms to elevate the critical heat flux (CHF) and heat transfer coefficient (HTC) in these systems. While a high CHF is desired, having a high HTC is equally important to keep the operating temperatures below a permissible limit. The present article summarizes enhancement structures found in the literature suitable for electronic cooling to provide this dual enhancement in CHF and HTC. New enhancement evaluation criteria that also consider the surface temperature limit imposed by the electronic components are introduced. The CHF enhancement ratio (ER&lt;sub&gt;CHF&lt;/sub&gt;) represents the ratio of CHF for enhancement structures to the CHF for a plain surface, and the enhancement index (EI) represents the ratio of wall superheat at CHF with the enhanced structures to the wall superheat at its respective CHF condition for a plain surface. It is desirable to have a high value of ER&lt;sub&gt;CHF&lt;/sub&gt; coupled with a low value of EI (lower the better), preferably below 1.0.

Experimental investigation and visualization of steam condensation heat transfer on horizontal conical surfaces with microgrooves

ABSTRACT This paper presents a novel conical copper plate with microgrooves structure to enhance heat transfer by removing accumulated condensate in horizontal surface condensation. The experimental investigation focuses on the condensation occurring on horizontal conical plates under atmospheric pressure. Results show that the heat transfer performance of all conical grooved plates surpasses that of smooth plates. Increasing groove number enhances heat transfer coefficient while having negligible influence on pressure drop. In contrast, increasing top diameter leads to higher pressure drop values. The optimal surface heat transfer coefficient strengthening ratio reaches 1.58, providing valuable insights for future studies and practical engineering applications.

Effects of cover plate wettability on pool boiling heat transfer in a narrow gap

An experimental study is conducted to evaluate the effects of cover plate wettability (hydrophobic to hydrophilic) on boiling heat transfer in a narrow horizontal gap (0.64mm to 5mm thick). When the gap thickness is greater than 1mm, the cover plate wettability plays a significant role. A hydrophobic cover plate causes vapor bubbles to move from the heating surface and get attached to the cover plate, maintaining liquid water on the heating surface. When the vapor layer does not reach the heating surface, the nucleate boiling heat transfer coefficient is hardly affected by the gap. However, the critical heat flux (CHF) is reduced relative to the unconfined case as vapor fills the narrow gap. Contrarily, when a hydrophilic cover plate is used with a gap thickness greater than 1mm, a vapor slug forms and hovers over the heating surface with transient lateral movement. Thin liquid film evaporation occurs under the slug, causing an enhanced heat transfer coefficient at low heat flux (q'' < 250kW/m2). As the heat flux increases, the vapor slug movement causes fluctuations in the heater temperature due to partial dryout at the heating surface. Consequently, a rapid decrease in the heat transfer and a reduced CHF are observed relative to the unconfined case. When the gap thickness is reduced to below 1mm, a film boiling-like behavior is observed regardless of the cover plate wettability. However, there is an increase in the heat transfer coefficient at low heat flux (q'' < 250kW/m2).

High-speed visualization and infrared thermography of pool boiling on surfaces having differing dynamic wettability

Wettability is a key surface characteristic that affects boiling behavior. Recent studies have shown that surface wettability as described by a static contact angle offers an incomplete description of this behavior, whereas the dynamic wettability described by advancing and receding contact angles is required for accurate description of ebullition dynamics. The observed influence of dynamic surface wettability on boiling performance has been previously explained based on arguments of single-bubble growth and departure; however, the mechanisms by which these dynamic wettability regimes influence boiling performance at high heat fluxes are still unknown. In this study we experimentally reveal the mechanisms by which ambiphilic surfaces (having receding contact angle <90° and advancing angle >90°) offer better boiling performance compared to hygrophilic and hygrophobic surfaces (for which both receding and advancing contact angles are below or above 90°, respectively). These mechanisms are identified through analysis of surface temperature distributions acquired via high-speed infrared (IR) thermography, complemented with side-view visualizations. The low receding contact angle of ambiphilic and hygrophilic surfaces prevents dewetting during bubble growth, thereby preventing the premature critical heat flux (CHF) that occurs for hygrophobic surfaces. Furthermore, the high advancing contact angle of ambiphilic surfaces acts to significantly improve the nucleation rate per unit surface area, which is identified as the primary mechanism of nucleate boiling heat transfer coefficient enhancement over hygrophilic surfaces. These first-of-their-kind observations of the surface temperature distributions during boiling on surfaces spanning all regimes of dynamic wettability provide new understanding of the avenues available for performance enhancement by controlling the surface dynamic wettability.

Heat transfer and flow analyses of microchannel heat sink with twisted blade-like fins

To enhance heat transfer coefficient, decrease pressure penalty, and improve wall temperature uniformity, a twisted blade-like fin with low-drag airfoil section is proposed in this study. The flow and heat transfer performances of microchannels with the fins are numerically investigated at Re = 50–700. Results show that the novel fins improve the heat transfer (Nu/Nu 0) significantly with a small pressure penalty (f/f 0), and thus show an excellent comprehensive thermal performance (TP). The ranges of Nu/Nu 0, f/f 0 and TP are 1.16–4.76, 1.51–2.36, and 1.01–3.58, respectively. The TP of TAMCHS reaches 3.58, exhibiting an increase of 46.7, 37.7, 22.6, and 16.6% compared to four models in previous study. The reduced pressure penalty is attributed to the streamlined configuration, and the improved heat transfer coefficient is attributed to the normalwise secondary flow stimulated by the twisted fins, which enhances heat transfer in the upper portion of the channel. Compared to a smooth microchannel heat sink (SMCHS), the major advantages lie in the significant reduction of sidewall temperatures and notable improvement in temperature uniformity on the top and bottom walls. The average and maximum wall temperature are reduced by 48.1 and 49.0 °C at most, respectively, and the greatest decline of wall temperature gradient is 55.0 °C.

Graphene oxide – adipic acid nanocomposites for thermal energy storage: Assessment of thermophysical properties and energy storage performance

Adipic acid is one of the organic phase change materials with a melting temperature greater than 150 °C and a freezing point greater than 140 °C, making the same a suitable thermal energy storage medium for near-atmospheric pressure steam generation. This work attempts to alleviate the rate limitation posed by the lower thermal conductivity of adipic acid by adding graphene oxide (GO) nanostructures at lower concentrations such that the energy storage capacity is not severely affected. The resultant GO – adipic acid composites (0–2 wt% GO) were characterized in terms of latent heat for phase change, specific heat and thermal conductivity. The composite containing 1 wt% GO exhibited a 16 % enhancement in thermal conductivity with a 3.4 % reduction in latent heat and 3.6 % reduction in specific heat. The impact of GO addition on the solidification rate was remarkable, with a 44 % enhancement in overall heat transfer coefficient during the initial solidification phase, when solidification was carried out in thermal contact with a constant temperature heat transfer fluid (oil bath). Thus, the present work clearly demonstrates the intensification of heat transfer during the discharge cycle of a thermal energy system with 1 wt% GO-Adipic acid composite as the thermal energy storage medium.

Empirical investigation of nanofluid performance in a microchannel heat sink for various attributes in low Reynolds number range

Efficient cooling techniques are imperative and present a significant challenge in the electronics industry due to the increasing miniaturization of devices. Elevated temperatures on the surface of components pose a particular concern within the low Reynolds numbers range due to back axial conduction. The current research endeavours to provide an experimental assessment of cooling efficacy, heat transfer coefficient, thermohydraulic performance and loss in concentration utilizing alumina nanofluids with 1–4 % (w/w) concentration in a microchannel heat sink at low Reynolds numbers (10 ≤ Re ≤ 50). The findings indicate notably, the introduction of nanofluids resulted in a 51.54 % reduction in back axial conduction in the microchannel heat sink compared to base fluid, consequently yielding surface temperatures up to 29 % lower in the microchannel heat sink and a substantial enhancement in heat transfer coefficient and Nusselt number up to 43.34 % and 35.85 %, respectively at Reynold number 50. The optimal thermohydraulic performance was noted to be 1.17 at a 3 % (w/w) nanoparticles concentration and Reynold number 50. Correlations have been devised to predict the Nusselt number and friction factor for the microchannel heat sink based on the corresponding nanofluid concentration. The increase in heat transfer coefficient was elucidated by considering thermophoresis and Brownian motion of nanoparticles in the base fluid. In conclusion, this study affirms the potential of nanofluids in efficiently cooling electronic devices, showcasing superior heat transfer performance at low Reynolds number.