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Recent studies have suggested that the heat buildup of construction materials may be significantly reduced, without affecting the material color, by the incorporation of scattering particles.The scattering particles were found to alter the near infrared diffuse reflectance of the material, depending on their refractive index, volume percent, and particle size combination. In addition, a theoretical model was developed and correlated well with the experimentally measured values, accurately predicted increases in diffuse reflectance and drops in temperature. Thus, the model may be effectively used to design dark colored polymeric materials with reduced heat buildup properties. [Copyright &y& Elsevier]

Highlights • Five parallel microchannels are interconnected by micro-slots. • Micro-slots can enable high frequency nucleate boiling. • Highly coordinated, rapid and sustainable nucleate boiling are realized. • The HTC and CHF are significantly enhanced on the present design. • The enhancements of HTC and CHF are ∼175% and ∼76%, respectively. Abstract Flow boiling in microchannels using dielectric fluids is one of the most desirable cooling solutions for high power electronics. However, it is challenging to promote the flow boiling performances, particularly critical heat flux (CHF), due to their unfavorable thermophysical properties. Flow boiling in parallel and isolated microchannels has been extensively studied. In this study, five parallel microchannels (W = 200 µm, H = 250 µm, L = 10 mm) are interconnected by 4 × 28 micro-slots (20 µm wide and 250 µm deep) starting from the middle section to the channel outlet. Our visualization study shows that these micro-slots designed as artificial nucleation sites can enable high frequency nucleate boiling by drastically reducing the bubble waiting time and remaining fully activated, simultaneously. More importantly, such rapid switch on–off harmonically coordinated nucleate boiling in the neighboring channels creates a highly desirable periodic rewetting mechanism to substantially delay CHF conditions and enhance heat transfer rate. Flow boiling in this innovative microchannel configuration has been systematically characterized with mass flux ranging from 462 kg/m2∙s to 1617 kg/m2∙s. Compared to plain-wall microchannels with inlet restrictors (IRs), flow boiling heat transfer coefficient (HTC) is enhanced up to ∼172% at a mass flux of 462 kg/m2·s primarily owing to the enhanced latent heat transfer including nucleate boiling and thin film evaporation. The peak value of effective HTC is ~60 kW/m2·K in the fully developed boiling regime. Moreover, CHF is substantially enhanced by ∼76% at a mass flux of 1155 kg/m2·s owing to the rapid and periodic rewetting enabled by these micro-slots. Such drastic enhancements have been achieved without compromising two-phase pressure drop. [ABSTRACT FROM AUTHOR]

Highlights • Experimental study was conducted to systematically investigate the influence of nickel oxide layers formed during a practical application on the condensation heat transfer. • The condensation was significantly enhanced by the coexistence of the hydrophilic nickel oxide and the hydrophobic carbon contents. • The maximum condensation heat transfer performance achieved was on a sample surface containing carbon to nickel oxide contents ratio of 3–1. • Increasing or decreasing the thicknesses of nickel oxide layers other than the optimum case is unfavorable to condensation heat transfer. Abstract The search for durable surfaces offering sustainable high rates of condensation is very essential for many applications. Most of the metal surfaces are subject to oxidization when exposed to water vapor and air, as is the case during the condensation under saturation conditions. Due to the relative stability of nickel and nickel oxide (NiO) among other common metals, they were considered herein as the substrates for condensing saturated water vapor under the atmospheric conditions. The main objective of this study was to investigate the influence of NiO layer(s) that would be formed during practical applications on the condensation performance. To mimic such oxide formation, different thicknesses of NiO layers were deposited by atomic layer deposition (ALD) method on the surface of smooth nickel tubes. The influence of the oxide layers on the condensation rate was then experimentally characterized, and the droplet dynamic was analyzed. Due to the presence of the large amount of hydrophobic carbon contents in the deposited NiO-ALD layers, especially at the initial stages of the ALD deposition process, a suitable wettability contrast degree with the corresponding deposited hydrophilic NiO was established. Thus bi-philic wettable surfaces were achieved. The degree of contrast in the wettability was varied by the number of the deposited NiO-ALD layers. It was found that samples with a higher carbon to NiO ratio exhibited a higher condensation heat transfer performance, reaching a maximum heat flux and heat transfer coefficient (HTC) of about 3.9 and 4.2 times that of the filmwise condensation (FWC) at subcooling temperatures of 11.0 °C and 3.5 °C, respectively. [ABSTRACT FROM AUTHOR]

Highlights • The impact of geometrical surface modifications on the thermal performance of the spray cooling system was studied. • The maximum heat transfer enhancement was 80%, for surface modified with four circular and radial grooves. • A significant effect of nozzle differential pressure has been found on spray cooling thermal performance. Abstract An experimental investigation was conducted to study the impact of geometrical surface modification on the thermal performance of a spray cooling system. All experiments were performed using a closed loop spray cooling system. Deionized water was used as the working fluid. Three different modified surfaces were examined and compared with a plain copper surface under the same operating conditions. The first surface (M1) was modified with four circular grooves each having a width and depth of 0.5 mm and a pitch of 1.5 mm. The second and third surfaces (M2) and (M3) were modified with four circular grooves each overlaid with four and eight radial grooves, respectively. Each radial groove had width and depth of 0.5 mm. All surfaces were tested at three nozzle differential pressures: 80 kPa, 140 kPa, and 185 kPa. The nozzle-to-surface distance, coolant inlet temperature, surface temperature, and chamber pressure were maintained at 10 mm, ∼22 °C, <100 °C, and atmospheric pressure, respectively. The results indicated that the nozzle differential pressure had a significant effect on the spray cooling thermal performance of all surfaces. Furthermore, surface (M3) had the highest heat transfer enhancement ratio at all operating conditions, followed by surfaces (M2), and (M1), where the maximum heat transfer enhancements were 80%, 36.3%, and 28.7%, respectively. Thus, signifying that using surfaces modified with a combination of circular and radial grooves can enhance spray cooling heat transfer performance. [ABSTRACT FROM AUTHOR]

Highlights • Numerically investigated the heat transfer and pressure drop in a three-dimensional surface roughness nuclear fuel rod. • Heat transfer coefficient enhanced by 86% at Re = 4.18 × 10 5 compared to the smooth surface. • Pressure drop increased by 15.75% within Re ranging 4.50 × 10 4 - 1.07 × 10 5 . Abstract The paper focuses on the numerical studies of heat transfer and pressure drop in a simulated nuclear fuel rod with three-dimensional surface roughness. Two-dimensional numerical simulations utilizing SST k - ω turbulent model were performed to evaluate heat transfer and pressure drop on the smooth and rough section of the heater rod. The numerical results were compared with experimental data obtained from a heater element which simulates a single Inconel-Nickel fuel rod for pressurized water reactor (PWR). The length of the rod was 2152.6 mm, and an outer diameter 9.5 mm, of which the outer surface of a 304.8 mm long section of the Inconel fuel rod was modified with three-dimensional (Diamond-shaped blocks) surface roughness. The angle of corrugation for each diamond block was 45°, and the length of each side of the diamond block was 1 mm. The numerically computed local heat transfer coefficient, overall Nusselt number, and pressure drop across the test rod shows good agreement with the corresponding experimental results. For the simulated rough surface, heat transfer coefficient was enhanced by 86% at Re = 4.18 × 10 5 as compared to the smooth surface, and pressure drop was found to increase by 15.75% within Re range of 4.50 × 10 4 - 1.07 × 10 5 . Plausible reason of the heat transfer enhancement of the three-dimensional surface roughness was discussed in the paper. [ABSTRACT FROM AUTHOR]

Extensive experimental investigations along with high speed visualizations have been performed to assess the flow boiling characteristics in Silicon Nanowire (SiNW) microchannels. Experiments have been also performed in Plainwall microchannels to compare their performances with SiNW configurations. HFE-7100 has been used as the working fluid and experiments are conducted in a forced convection loop at mass flux range of 400–1600 kg/m 2 s. Arrays of microchannel consist 5 (five) parallel straight microchannels with Width, Depth and Length dimension of 220 μm, 250 μm and 10 mm respectively. Flow boiling performances including heat transfer coefficient (HTC), pressure drop, two-phase flow instabilities and critical heat flux (CHF) have been studied in both the Silicon plainwall (smooth inner surface) and Silicon Nanowire (silicon nanostructured inner surface) microchannels. High speed flow visualizations have been performed at up to 70,000 frames per s (fps) to understand the difference in boiling mechanisms between Plainwall and SiNW. SiNW performs significantly enhanced HTC (up to 400% improvement), reduces flow boiling instabilities and pressure drop (up to 70% reduction) compared to Plainwall microchannels. However, little/insignificant effect of nanostructured surface has been observed on CHF. In addition, a major difference in two-phase flow regime development has been observed between the SiNW and Plainwall microchannels during flow visualization. Specifically, SiNWs introduce explosive bubble nucleation, reduce intermittent flow regimes (slug/churn), improve rewetting, maintain thin liquid film and thus, improve system performances. [ABSTRACT FROM AUTHOR]

Flow boiling of dielectric fluids in microchannels is one of the most desirable cooling solutions for high power electronics. However, the flow boiling of dielectric fluids is hindered by their unfavorable thermophysical properties. Specifically, without precooling dielectric fluids, it is challenging to promote critical heat flux (CHF) due to its high vapor density, low surface tension and the resulted superior wettability. In this study, each side wall of a five-parallel silicon microchannel array was structured with an array of microscale reentry cavities and four micronozzles bypassed by an auxiliary channel. The present microchannel configuration aims to significantly enhance CHF of HFE-7100 flow boiling by improving global liquid supply using auxiliary channels and micrononozzles as well as by sustaining liquid film using capillarity induced by reentry cavity array. Equally important, these structures can promote nucleate boiling at low heat flux, generate intense mixing, and promote thin film evaporation at high heat flux, resulting in high flow boiling heat transfer rate. Flow boiling of HFE-7100 in the present microchannel configuration is characterized with mass flux ranging from 231 kg/m 2  s to 1155 kg/m 2  s. The effective two-phase heat transfer coefficients (HTCs) are ranging from 6 kW/m 2  K to 117 kW/m 2  K. Compared to the four-nozzle plain-wall microchannels, for example, the effective HTC and CHF can be substantially enhanced up to 208% and 37%, respectively, without escalating pressure drop at a mass flux of 462 kg/m 2  s. Compared to plain microchannels with inlet restrictors, CHF is considerably enhanced up to 70% with a reduction of pressure drop ∼82% at a mass flux of 1155 kg/m 2  s. Significantly reduced pressure drop is achieved by integrating bypass and the enhanced confined bubble removal. A peak CHF value of 216 W/cm 2 is achieved at mass flux of 2772 kg/m 2  s in the present microchannel configuration with inlet temperature at room temperature. [ABSTRACT FROM AUTHOR]

Flow boiling in parallel microchannels can be dramatically enhanced through inducing self-excited and self-sustained high frequency two-phase oscillations as demonstrated in our previous studies using a two-nozzle microchannel configuration. Two-phase mixing induced by the rapid bubble collapse is shown to be the major enhancement mechanism. However, in the two-nozzle configuration microchannels, the mixing effect is limited to the downstream of the microchannels, meaning that only half length of the entire microchannel is functionalized as designated. In this study, a four-nozzle microchannel configuration is developed with an aim at extending the highly desirable mixing effect to the entire channel. Flow boiling in the four-nozzle configuration microchannels is experimentally studied with deionized water and the mass flux ranging from 120 kg/m 2 s to 600 kg/m 2 s. The onset of nucleate boiling temperature is considerably reduced by ∼14% because of more nucleation sites created by the multiple nozzles. Equally important, the improved microchannel configuration successfully extends the mixing to the entire channel as validated by the enhanced heat transfer rate and visualization study. Compared to the previous two-nozzle configuration, the overall heat transfer coefficient (HTC) is significantly improved primarily owing to the enhanced nucleate boiling. For example, the peak overall HTC of 262 kW/m 2 K is achieved at a mass flux of 150 kg/m 2 s, accounting for ∼83.7% enhancement. Additionally, the peak effective HTC reaches 97.6 kW/m 2 K, accounting for ∼67% enhancement. [ABSTRACT FROM AUTHOR]

Enhancing critical heat flux (CHF) of flow boiling without escalating pressure drop is highly desirable in thermal management of high power-density electronic devices. Usually, an improved CHF can be achieved by restricting flow or at a higher mass velocity, leading to a higher pressure drop. In this study, compared to the two-nozzle microchannel configuration, the improved microchannel configuration as detailed in the Part (I) of this study can enhance CHF without sacrificing pressure drop. In this part, CHF is experimentally evaluated together with the pressure drop with mass flux ranging from 120 kg/m 2 s to 600 kg/m 2 s. Compared to the two-nozzle configuration, our study shows that CHF can be enhanced up to 32% with a ∼53% reduction of pressure drop at a mass flux of 325 kg/m 2 s. The bubble collapse-removal process is significantly improved because more micronozzles are integrated. The enhanced pumping effect, which is created by rapid bubble collapse processes in the entire main channels, enables a more sustainable liquid supply and hence delays the CHF conditions. Moreover, two-phase flow in terms of pressure drop fluctuations is more stable owing to the effective management of bubble confinement in the entire channel. [ABSTRACT FROM AUTHOR]

Condensation heat transfer performance can be improved by increasing the condensate removal rate. Commonly, this can be achieved by promoting dropwise condensation mode in which super/hydrophobic coatings applied on the entire condenser surface. Herein, alternative mini-scale straight patterns consisted of hydrophobic (β) and less-hydrophobic (α) regions were formed on the condenser tubes. The existence of the two adjacent regions generates wettability gradient which can mitigate condensate and increase its removal rates. A parametric study was conducted to experimentally determine the influence of (β/α) ratios on the heat transfer performance and droplet dynamic under saturation condition near the atmosphere pressure with the presence of non-condensable gases (air). The results reveal that all patterned surfaces exhibited a drastic enhancement in terms of condensation heat transfer coefficient and heat flux compared to those of filmwise condensation. More interestingly, some (β/α) ratios significantly outperformed a surface with a complete dropwise condensation. In addition, an optimum (β/α) ratio of (2/1) exists with β and α-regions widths of 0.6 mm and 0.3 mm, respectively. The heat transfer coefficient of the optimum ratio is peaked at a value of 85 kW/m 2 K at a subcooling of 9 °C, which is 4.8 and 1.8 times that of a complete filmwise and dropwise condensation, respectively. Our study also reveals that the β-regions served mainly as droplet nucleation sites with rapid droplets mobility; whereas the α-regions promoted droplet removal from the neighboring β-regions, and served as drainage paths where condensate can be drained quickly under gravitational force. Furthermore, the existence of both α and β-regions on the condensing surface controls the droplets maximum diameters of the growing droplets on the β-regions. The maximum diameter is approximately 0.56 ± 3% mm, which is 26% the size of the droplets maximum diameter on a full β-region surface. In summary, this wettability-driven mechanism allows droplets to be removed from the condensing surface at higher rates, leading to a substantial enhancement in the condensation heat transfer coefficient. [ABSTRACT FROM AUTHOR]

Condensation heat transfer performance can be significantly enhanced by patterning the condenser surface with different wettability regions as shown by numerous studies, including part I of this study. In part I of this study, some patterned surfaces with alternative parallel straight stripes consist of hydrophobic (β) and less-hydrophobic (α) regions at different ratios exhibited higher heat transfer rate than others. In this Part II of the study, our objective is to analyse the droplet dynamics during water vapor condensation on hybrid-wettability patterned horizontal tube surfaces under saturation conditions near the atmospheric pressure. Three major outlines were found in the course of the droplets dynamic investigation. First, the existence of an optimum (β/α) ratio that maximized the condensation heat transfer rate, as demonstrated in part I of a sample carrying β and α-regions widths of 0.6 mm and 0.3 mm, respectively is justified. This is because the optimum ratio exhibits the maximum droplet departure frequency and the minimum droplet area coverage rate among other samples. Second, the reduction in the heat transfer rate resulting from any deviation from the optimum ratio is also identified. We observed that by increasing the α-regions width on the hybrid patterned surface, the condensation was dominated by the filmwise mode, thus reducing the condensation rate. In contrast, decreasing the width of α-regions less than the optimum ratio was found to be unfavourable due to the increase in the bridging droplets observed and discussed herein. Lastly, the undesirable observed bridging phenomenon found to occur on all tested hybrid patterned surfaces, can significantly influence the condensation heat transfer performance. A bridging droplet can be referred to a droplet joined (bridged) by two, three, or four neighboring α-stripes. Increasing these unwanted droplets formation frequency can induce additional thermal resistance which can reduce the condensation rate. The most dominant and frequent bridging droplet type observed herein was found to be for droplets that were bridged by two α-regions, followed by those between three and four α-regions. A quantitative method (i.e. Bridging coverage area rate) was adapted herein to quantify the influence of the velocity, frequency, and size of the three types of bridging droplets on the condensation rate of the hybrid patterned surfaces. [ABSTRACT FROM AUTHOR]

Low heat transfer coefficient (HTC) in air/fin-side is the bottleneck of dry cooling strategies for thermal power plants. Inspired by the phase change heat transfer during the perspiration of mammals, a sweating-boosted air cooling strategy with on-demand water dripping is proposed. The testing samples are featured with macroscale grooves for global liquid delivery, and with nanoscale hydrophilic copper oxide (CuO) wick structures for local liquid spreading. The experiments of sweating-boosted air cooling are conducted in a wind tunnel system. There are three wetting conditions with increasing dripping rates: dry, partially wetted, and flooded conditions. In the partially wetted conditions, the surface temperatures reduce and HTCs increase with increasing dripping rates. For a given dripping rate of water, HTCs are enhanced and surface temperatures are reduced with increasing air velocities. High air velocity and low surface temperature have a trade-off effect on the evaporation process. This effect results in almost constant saturated dripping rates for a given thermal load. A linear relationship between the saturated dripping rates and the thermal loads confirms that the evaporation dominates the heat transfer process of sweating-boosted air cooling. Complete surface wetting is obtained on the designed surfaces, but no obvious effect of groove width on HTCs is observed. Sweating-boosted air cooling can significantly increase air-fin side HTC in air cooled condenser (ACC), and dramatically reduce the water consumption compared to current water evaporative condenser (WEC). This research provides a fundamental understanding on the sweating-boosted effects on the air cooling. [ABSTRACT FROM AUTHOR]

In microchannel flow boiling, bubble nucleation, growth and flow regime development are highly influenced by channel cross-section and physical phenomena underlying this flow boiling mechanism are far from being well-established. Relative effects of different forces acting on wall-liquid and liquid–vapor interface of a confined bubble play an important role in heat transfer performances. Therefore, fundamental investigations are necessary to develop enhanced microchannel heat transfer surfaces. Force analysis of nucleating bubble and bubble dynamics in flow boiling silicon nanowire microchannels have been performed based on theoretical, experimental and visualization studies. The relative effects of different forces on flow regimes, instabilities and heat transfer performances of flow boiling in silicon nanowire microchannels have been identified. Inertia, surface tension, shear, buoyancy, and evaporation momentum forces have significant importance at liquid–vapor interface as discussed earlier by other researchers. However, no comparative study has been done for different surface properties till date. Detail analyses of these forces including contact angle effect, channel dimension effect, heat flux effect and mass flux effect in flow boiling microchannels have been conducted in this study. A comparative study between silicon nanowire and plainwall microchannels has been performed based on force analysis in the flow boiling microchannels. Compared to plainwall microchannels, enhanced surface rewetting and CHF are owing to higher surface tension force at liquid–vapor interface and Capillary dominance resulting from silicon nanowires. Whereas, low Weber number in silicon nanowire helps maintaining uniform and stable thin film and improves heat transfer performances. Moreover, results from these studies are compared with the literatures and great agreements have been observed. [ABSTRACT FROM AUTHOR]

A study on bubble growth mechanisms and underlying physical phenomena of flow boiling in silicon nanowire (SiNW) microchannels has been performed and compared these results with plainwall microchannels. A new approach in studying bubble dynamics and forces acting on liquid–vapor (L–V) interface of growing bubble has been proposed based on theoretical, experimental and visual studies. Bubble size, liquid film thickness, interfacial properties are measured and L–V interfaces are detected from high speed visualization data and results are analyzed by vision-based approach. Force analysis during instantaneous bubble growth from bubble nucleation to formation of annular flow regime has been performed for both the SiNW and plainwall microchannel configurations. Results from force analysis show the dominance of surface tension at L–V interface of growing bubble which resulted higher heat transfer contact area, lower thermal resistance and higher thin film evaporation. Whereas, inertia force is dominant at L–V interface of fully grown bubble and it helps in bubble removal process and rewetting before flow reversal. Significant differences between SiNW and plainwall microchannels have been observed in bubble growth mechanism, heat transfer mode and forces acting on L–V interface. [ABSTRACT FROM AUTHOR]

Critical heat flux ( CHF ) during flow boiling in silicon microchannels ( H = 250 μm, W = 200 μm, L = 10 mm) using self-excited and self-sustained high frequency two-phase oscillations is studied both experimentally and theoretically. Tests are performed on deionized water over a mass flux range of 200–1350 kg/m 2 s. An enhanced CHF of 1020 W/cm 2 is achieved experimentally at a mass flux of 1350 kg/m 2 s in the present study. Since no existing CHF models and correlations on parallel mini/microchannels considered high frequency two-phase oscillations, hence are not applicable to predict CHF in the present microchannel configuration. Adopting Helmholtz and Rayleigh instability theories and based on experimental study of liquid thin film dry-out phenomena in two-phase oscillations, a semi-theoretical CHF model is proposed. The proposed theoretical predictions show satisfactory agreement with experimental data with a reasonable low mean absolute error ( MAE ) of 25–32%. [ABSTRACT FROM AUTHOR]

Nanoparticle Enhanced Ionic Liquids (NEILs) were synthesized by dispersing aluminum oxide (Al 2 O 3 ) nanoparticles in 1-butyl-3-methylimidazolium bis{(trifluoromethyl)sulfonyl}imide, ([C 4 mim][NTf 2 ]) ionic liquids (ILs). The experimental assessment of NEILs includes investigating the effective thermophysical properties and forced convection heat transfer under laminar and turbulent flow regime. The results show that thermal conductivity and heat capacity enhanced up to ∼11% and ∼49% respectively for 0.9 vol% NEILs. The rheological behavior of NEILs shows non-Newtonian shear thinning behavior with shear viscosity decreasing with increasing shear rate. The viscosity of NEILs shows much higher value compared to the base ILs for a small amount of nanoparticles dispersion and also has a strong temperature dependency. Measured viscosity and thermal conductivity were found to be much higher than predicted by the well-established model for dilute suspensions. The convective heat transfer performance increases with the nanoparticles concentration within the measured nanoparticles vol%; up to ∼27% and ∼40% enhancement in heat transfer coefficient was found in laminar and turbulent flow regime respectively. The possible mechanisms of the enhanced thermal performance are discussed. [ABSTRACT FROM AUTHOR]

Experimental investigations were carried out regarding natural convection heat transfer of Nanoparticle Enhanced Ionic Liquids (NEILs) in rectangular enclosures of two different sizes with dimensions length × width × height, 50 × 50 × 50 mm and 50 × 50 × 75 mm in heated from below. The NEILs were synthesized by dispersing different wt% (0.5, 1.0, and 2.5) of aluminum oxide (Al 2 O 3 ) nanoparticles of two different particle shapes (spherical and whiskers) into N-butyl-N-methylpyrrolidinium bis{(trifluoromethyl)sulfonyl} imide, ([C 4 mpyrr][NTf 2 ]) ionic liquid (IL). Heat transfer related thermophysical properties, i.e. density, viscosity, thermal conductivity, and heat capacity of base IL and NEILs were also measured and reported. The experimental measurement shows enhanced density, thermal conductivity, viscosity, and heat capacity of NEILs compared to the base IL and they increase with the nanoparticle concentration. However natural convection heat transfer coefficient was observed to deteriorate for the NEILs compared to the base IL irrespective of the shapes of the particles and aspect ratio of the enclosure and the deterioration increases with the increase of nanoparticle concentration. Interestingly spherical Al 2 O 3 NEILs was observed to affect more adversely compared to the whiskers Al 2 O 3 NEILs. The observed degradation of the heat transfer performance of the NEILs could not fully be explained by the change of thermophysical properties, which indicates that other factors may play significant roles in this phenomenon and the possible reasons of the degradation is discussed in this paper. [ABSTRACT FROM AUTHOR]

Abstract: In Part II of this study, we report that pressure drop was reduced by approximately 48% and critical heat flux (CHF) was increased by approximately 300% in SiNW microchannels compared to these in smooth wall microchannels. The hydraulic characteristics of the single annular flow were systematically investigated to reveal the mechanisms responsible for the reduced pressure drop and enhanced CHF. In the single annular regime, the liquid and vapor flows were nearly fully separated during the entire flow boiling process (i.e., from the onset of nucleate boiling to the CHF conditions). Moreover, the entrainment droplets were reduced by flattening the profile of the liquid–vapor interfaces using the high capillary pressure generated by SiNWs. These two factors, i.e., flow separation and reduced entrainment droplets, lead to a dramatic reduction of frictional pressure drop. The separation of liquid and vapor flows as well as the improved global and local liquid supply result in a significant CHF enhancement without using inlet restrictors (IR). Reynolds number based the vapor flow at the exit ranged from 0.1 to 2100. [Copyright &y& Elsevier]

Abstract: Flow boiling with deionized water in silicon (Si) microchannels was drastically enhanced in a single annular flow boiling regime enabled by superhydrophilic Si nanowire inner walls. Part I of this study focuses on characterizing enhanced flow boiling heat transfer. Part II focuses on revealing mechanisms in governing pressure drop and critical heat flux (CHF). Compared to flow boiling in plain-wall microchannels without using inlet restrictors (IRs), the average heat transfer coefficient (HTC) and CHF were enhanced by up to 326% and 317% at a mass flux of 389kg/m2 s, respectively. Additionally, compared with flow boiling in microchannels with IRs, HTC of flow boiling in the single annular flow was enhanced by up to 248%; while CHF in the new flow boiling regime was 6.4–25.8% lower. The maximum HTC reached 125.4kW/m2 K at a mass flux of 404kg/m2 s near the exits of microchannels. The significantly promoted nucleate boiling, induced liquid film renewal, and enhanced thin-film evaporation in the self-stabilized and single flow boiling regime are the primary reasons behind the significant heat transfer enhancements during flow boiling. [Copyright &y& Elsevier]

Abstract: Experimental study of flow boiling heat transfer in a microchannel array consisting of main channels connected to two auxiliary channels (each) was conducted. A microbubble-excited actuation mechanism, powered by high frequency vapor bubble growth and collapse, was established to create and sustain strong mixing in the microchannels. It was shown to significantly enhance flow boiling heat transfer in microchannels. Experimental studies were conducted at mass fluxes ranged from 150 to 480kg/m2 s with de-ionized (DI) water as the working fluids. Compared with microchannels with inlet restrictors (IRs), the average two-phase heat transfer coefficient was improved by up to 149%. More importantly, a 71–90% reduction in pressure drop at moderate mass fluxes ranged from 400 to 1400kg/m2 s was observed. Heat flux up to 552W/cm2 at a mass flux of 480kg/m2 s was demonstrated. Flow and heat transfer mechanisms were studied and discussed. [Copyright &y& Elsevier]

• Two series of circular patterns are formed on the surfaces of copper tubes at different diameters, and gap distances. • The condensation heat transfer rate on hybrid surfaces can be much higher than complete DWC surfaces. • The optimal pattern of the present study with hydrophilic circles is at a diameter of 1.5 mm and gap of 1.5 mm. • The best heat transfer rate achieved in this study is 79% higher compared to that of the complete DWC. Condensation surfaces using a wettability contrast mechanism is one of the effective methods to enhance heat transfer rate. The pattern design plays a critical role in realizing various contrast degrees of wettability. In this study, water vapor condensation of two series of hybrid circular-patterned designs were developed and experimentally characterized under the atmospheric pressure with the presence of non-condensable gasses (air). The outer surfaces of copper tubes in the horizontal orientation were used as the condensation surface with the designed patterns. The condensation heat transfer rates were compared to that of the complete dropwise condensation (DWC) and complete filmwise condensation (FWC), respectively. In the first series, the pattern design consists of hydrophobic (β) circular patterns formed on a less-hydrophobic (γ) background. The diameter of the patterns was varied to find an optimum configuration, which was observed with the pattern diameter and pattern distance at 1.5 mm and 0.5 mm, respectively. Significantly enhanced condensation rate was obtained compared to FWC. In the second series, the design consists of γ-patterns contrasting with a β-background, which is an inverse design of the first series in terms of wetting conditions. A parametric study was conducted to examine the influence of the pattern dimensions on the condensation. In the second series pattern designs, significant enhancements compared with both FWC and DWC were achieved. The optimal pattern was found to be at a diameter of 1.5 mm and gap of 1.5 mm, leading to a 79% higher heat transfer rate compared to that of the complete DWC. [ABSTRACT FROM AUTHOR]

• In this study, experimental investigations were performed to quantify the effect of different configurations of inlet restrictors on the heat transfer performance and pressure drop in a flow boiling microchannel heat sink. • Multiple IRs performs best in CHF enhancement at low mass flux, and the 1IR case performs best at high mass flux. • For lower mass flow rates, cases with multiple-opening inlet restrictors generally work better (e.g., higher CHF) than cases with the single-opening inlet restrictor. • The optimum configuration was obtained by maximizing the heat transfer performance (e.g., in terms of CHF enhancement) and minimizing the pressure drop penalties. Flow boiling instabilities inherent in microchannels / microgaps remain to be a serious issue in two-phase high-heat-flux cooling applications, as they cause significant oscillations in flow rate, temperature, pressure, reduction in heat transfer coefficient (HTC), and eventually early occurrence of critical heat flux (CHF). This study experimentally investigated the effects of various configurations of inlet restrictors (IRs) on the thermal hydraulic performance of flow boiling in a microchannel heat sink, which has a single rectangular microchannel with an aspect ratio of 13.12 and a hydraulic diameter of the 708 µm. The experiments were carried out for the microchannel with three designs of inlet restrictors: one-slot opening (1IR), three-slot openings (3IR), and five-slot openings (5IR), for mass fluxes of 32.68, 81.29 and 144 kg/m2 s, respectively. The effects of the various IRs on the CHF, average HTC and pressure drops of the microchannel heat sink were analyzed. The results showed that all test cases with IRs improved the CHF performance of the flow boiling microchannel heat sink, where the 5IR case works best at low mass flux and the 1IR case works best at high mass flux. The results also illustrated that the IRs reduce the HTC at low mass flux, but improve the HTC at high mass flux and heat flux. IRs always exhibit higher pressure drop penalties. This study also revealed the optimum design for microchannel with IRs, which depends on the operational parameters (e.g., mass flux and heat flux) of the microchannel heat sink. [ABSTRACT FROM AUTHOR]