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Concentrated Solar Power (CSP) is one of the developing alternative energy technology, where mirror or lenses are used to concentrate sunlight from a large area and stored in collector filled with heat transfer fluid (HTF). The stored energy from HTF is then used to produce steam for power generation. Nanoparticle Enhanced Ionic Liquids (NEILs), inclusion of nanoparticles into Ionic Liquids (ILs), are considered as a potential HTF for CSP applications. In this present paper, we have experimentally investigated the density, rheological behavior, and thermal conductivity of NEILs forming with N-butyl-N,N,N-trimetylammonium bis(trifluormethylsulfonyl)imide ([N 4111 ][NTf 2 ]) IL and 1 wt% Al 2 O 3 nanoparticles (spherical and whiskers) and carbon black. The experimental results show that NEILs with whisker-shape nanoparticles show highest thermal conductivity enhancement, which is ∼7%. The rheological behavior of NEILs shows the shear thinning behavior occurs at all the measured temperatures and the shear thinning increases with temperature. The inclusion of nanoparticles into ILs enhances the viscosity compared to the base IL and it is highly dependent on the temperature, where viscosity sharply decreases with the increase of temperature. The possible mechanisms of this enhancement are discussed in the paper. [ABSTRACT FROM AUTHOR]

Abstract: Heat transfer and fluid flow characteristics through a porous medium were investigated using numerical simulations and experiment. For the numerical simulations two models were created: a two-dimensional numerical model and a Fluent™ computational fluid dynamics (CFD) porous media model. The experimental investigation consisted of a flow channel with a porous medium section that was heated from below by a heat source. The results of the numerical models were compared to the experimental data in order to determine the accuracy of the models. The numerical model was then modified to better simulate a matrix heat exchanger. This numerical model then generated temperature profiles that were used to calculate the heat transfer coefficient of the matrix heat exchanger and develop a correlation between the Nusselt number and the Reynolds number. [Copyright &y& Elsevier]

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 • 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]

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]

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: 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]

Continual growth of hydraulic and thermal boundary layers along stream wise direction in conventional straight fin mini-channel heat sink (MCHS) causes gradual deterioration of their thermal performance. To enhance thermal-hydraulic performance by breaking and re-development of the boundary layers, this research aims to introduce a novel water cooled inter-connected counter flow mini-channel sink (ICMCHS). Two inter-connectors (ICs) were positioned transversely between two counter flow mini-channels (CMCs) which segmented the flow domain into three zones (zone 1–3). Secondary flow was generated through the ICs utilizing the pressure difference of the adjacent CMCs resulting in disruption of the hydraulic and thermal boundary layers. To examine the effect of the ICs location and width on the thermal-hydraulic characteristics of the counter flow mini-channel heat sink (CMCHS), the present numerical studies were carried out for nine different cases (case 1–9) by varying ICs width from 1 mm to 1.5 mm and ICs location from 4 mm to 9 mm. A corresponding conventional CMCHS was chosen as the base case in contrast to the newly proposed ICMCHS. Experiments were also carried out for CMCHS to validate numerical results, and excellent agreement was found between measured values and the corresponding numerical results. At the lowest considered Re (Re = 150), a maximum value of Performance Evaluation Criterion (PEC) was achieved to ~1.22 for the highest length of zone 1 and 3 and the lowest ICs width (case 7), whereas at the highest Re (Re = 1044), the maximum PEC value (~1.42) was recorded for the intermediate length of zone 1 and 3 and the highest ICs width (case 6). [ABSTRACT FROM AUTHOR]