Over the last decade, numerous investigations have been carried out using engineered colloidal fluids called nanofluids in pool boiling for the enhancement of critical heat flux (CHF) which is ...otherwise limited by the use of base fluids. Though there are contradicting results in CHF enhancement, almost all the researchers identified that there is an enhancement in CHF by the addition of nanoparticles (1–100 nm size) with base fluid. The deposition of nanoparticles on the heater surface during boiling of nanofluids is the major factor for the enhancement of CHF. In fact, the CHF enhancement results are influenced by the various parameters such as size of nanoparticles, method of preparation of nanofluids, and its concentration. This article reviews in detail the various factors for the enhancement of CHF and provides further research direction which helps the readers to understand the pool boiling heat transfer in a better way. Also, the mechanisms of CHF enhancement by exploring various boiling heat transfer theories have been addressed.
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•Influence of nanoparticle material, size and its concentration on CHF.•Role of thermal transport properties on CHF.•Effect of system pressure on CHF enhancement.•Effect of surface roughness, wettability and capillary rise on CHF.•Mechanisms behind the CHF enhancement.
•Thermal conductivity of nanofluids is enhanced by mixing two different nanoparticles.•Critical heat flux for hybrid nanofluids in pool boiling is investigated.•Effect of nanoparticle deposition on ...heat transfer coefficient is explored.
Boiling is a process with maximum heat transfer coefficient (HTC) and heat transfer rate. Researchers are working with techniques such as surface modifications and nanoparticle suspensions in base fluids to enhance the thermal performance. The present work focuses on the enhancement of thermal properties and critical heat flux (CHF) by mixing two different nanoparticles viz., alumina and copper oxide at various concentrations in Deionized (DI) water. Volume concentrations of 0.01 - 0.1% are prepared and the stability of nanofluids is confirmed by Zeta potential. Thermal conductivity of hybrid nanofluids is enhanced by 15.72% for 0.1% concentration at room temperature compared with DI water. CHF of hybrid nanofluids is always higher than single type nanofluid. Maximum enhancement of 49.84% in CHF is obtained for 0.1% concentration. HTC is enhanced by 7.1 and 6.72% for 0.01 and 0.03% volume concentrations than DI water and starts decreasing with increasing volume concentrations.
Purpose
The purpose of this paper is to study the flow and heat transfer characteristics of microchannel heatsinks with ribs, cavities and secondary channels. The influence of length and width of the ...ribs on heat transfer enhancement, secondary flows, flow distribution and temperature distribution are examined at different Reynolds numbers. The effectiveness of each heatsink is evaluated using the performance factor.
Design/methodology/approach
A three-dimensional solid-fluid conjugate heat transfer numerical model is used to study the flow and heat transfer characteristics in microchannels. One symmetrical channel is adopted for the simulation to reduce the computational cost and time. Flow inside the channels is assumed to be single-phase and laminar. The governing equations are solved using finite volume method.
Findings
The numerical results are analyzed in terms of average Nusselt number ratio, average base temperature, friction factor ratio, pressure variation inside the channel, temperature distribution, velocity distribution inside the channel, mass flow rate distribution inside the secondary channels and performance factor of each microchannels. Results indicate that impact of rib width is higher in enhancing the heat transfer when compared with its length but with a penalty on the pressure drop. The combined effects of secondary channels, ribs and cavities helps to lower the temperature of the microchannel heat sink and enhances the heat transfer rate.
Practical implications
The fabrication of microchannels are complex, but recent advancements in the additive manufacturing techniques makes the fabrication of the design considered in this numerical study feasible.
Originality/value
The proposed microchannel heatsink can be used in practical applications to reduce the thermal resistance, and it augments the heat transfer rate when compared with the baseline design.
Natural pore-forming proteins act as viral helical coats and transmembrane channels, exhibit antibacterial activity and are used in synthetic systems, such as for reversible encapsulation or ...stochastic sensing. These diverse functions are intimately linked to protein structure. The close link between protein structure and protein function makes the design of synthetic mimics a formidable challenge, given that structure formation needs to be carefully controlled on all hierarchy levels, in solution and in the bulk. In fact, with few exceptions, synthetic pore structures capable of assembling into periodically ordered assemblies that are stable in solution and in the solid state have not yet been realized. In the case of dendrimers, covalent and non-covalent coating and assembly of a range of different structures has only yielded closed columns. Here we describe a library of amphiphilic dendritic dipeptides that self-assemble in solution and in bulk through a complex recognition process into helical pores. We find that the molecular recognition and self-assembly process is sufficiently robust to tolerate a range of modifications to the amphiphile structure, while preliminary proton transport measurements establish that the pores are functional. We expect that this class of self-assembling dendrimers will allow the design of a variety of biologically inspired systems with functional properties arising from their porous structure.
Surface Crystallization in a Liquid AuSi Alloy Shpyrko, Oleg G; Streitel, Reinhard; Balagurusamy, Venkatachalapathy S.K ...
Science (American Association for the Advancement of Science),
07/2006, Volume:
313, Issue:
5783
Journal Article
Peer reviewed
Open access
X-ray measurements reveal a crystalline monolayer at the surface of the eutectic liquid Au₈₂Si₁₈, at temperatures above the alloy's melting point. Surface-induced atomic layering, the hallmark of ...liquid metals, is also found below the crystalline monolayer. The layering depth, however, is threefold greater than that of all liquid metals studied to date. The crystallinity of the surface monolayer is notable, considering that AuSi does not form stable bulk crystalline phases at any concentration and temperature and that no crystalline surface phase has been detected thus far in any pure liquid metal or nondilute alloy. These results are discussed in relation to recently suggested models of amorphous alloys.
In this study, reduced graphene oxide (rGO) is synthesized from graphite using modified Hummer and chemical reduction methods. Various characterizations are done using X-ray diffraction, Raman's ...spectra, Fourier transform infrared, scanning electron microscopy and atomic force microscopy. Different concentrations of 0.01, 0.1, and 0.3 g/l of rGO/water nanofluids are then prepared by ultra sonic homogenizer and probe sonicator. Dynamic light scattering technique is used to identify the size of rGO flakes in DI water. The thermal conductivity, viscosity, and surface tension of rGO/water nanofluids reveal their dependency on concentrations and temperature. Due to the improved dispersion stability as evident from zeta potential, the thermal conductivity of 0.01 and 0.1 g/l concentrations exhibits negligible change whereas 0.3 g/l shows a minimal change for a period of five days. The enhancement in thermal conductivity of 0.3 g/l of rGO/water nanofluid at 75 °C is 10%. The rGO/water nanofluids exhibit Newtonian behavior at higher shear rates due to the weakening of intermolecular interactions. The enhancement in surface tension is mainly due to the increase in surface energy by the accumulation of rGO flakes at the liquid–gas interface. Studies on wettability indicate an increase in contact angle with concentrations. Though it is not favorable as it reduces the contact between solid and liquid surface, many research works explain the enhanced boiling heat transfer mainly with porous layer rather than contact angle. Results show that the rGO/water nanofluid can be used as a suitable replacement for the conventional fluids in heat transfer applications.
•Modified Hummer and chemical reduction methods are used to synthesis the rGO.•Zeta potential, sedimentation analysis and pH reveal the rGO has good stability.•0.3 g/l of rGO gives a maximum of 10% enhancement in thermal conductivity.•rGO exhibits Newtonian behavior at higher shear rates.•At higher concentrations, the surface tension is enhanced by Van der Waals forces.
The enhancement in heat transfer characteristics of a copper sintered wick heat pipe with surfactant free CuO nanoparticles dispersed in DI water is experimentally studied. The effect of heat input, ...tilt angle and weight fractions of nanoparticles on the heat pipe thermal resistance, heat transfer coefficient in evaporator and condenser sections, thermal conductivity and thermal efficiency are investigated. The experimental results are evaluated for the vapor temperatures directly measured at the center core of heat pipe. Interestingly, a temperature difference of 5.1°C is observed between the heat pipe surface and the vapor core in the evaporator section. Results showed a reduction in the thermal resistance of 66.1% and enhancement in the heat transfer coefficient and thermal conductivity of 29.4% and 63.5% is respectively, observed for 1.0wt.% of CuO/DI water nanofluid at 45° tilt angle compared with heat pipe kept at horizontal position. Similarly, the thermal efficiency is also improved by 24.9% for the same tilt angle and weight fraction of CuO nanofluid. The use of nanoparticles and tilt angle enhances the operating range and thermal performance of heat pipe when compared with that of the heat pipe with DI water.
Optical and thermo‐physical properties of the nanofluids play a major role in the absorption of solar energy. In the present study, photo‐thermal energy conversion of low concentration ...Al2O3/Deionised water (DI) water and CuO/DI water nanofluids in solar thermal collector is experimentally investigated. Properties of 50,100,150, and 200 ppm concentrations of nanofluids are reported. The absorbance results in the visible range indicate that CuO nanofluid of 200 ppm concentration is nearly three times higher compared to Al2O3 nanofluid. The extinction coefficient, optical energy band gap, and photoluminescence obtained from the absorbance data are also reported. Surfactant free nanofluids are used, and the thermal conductivity measurements show a negligible enhancement for both the nanofluids. Maximum receiver temperatures of 89 and 72°C are found with CuO and Al2O3 nanofluids, respectively, for the maximum concentration. A maximum receiver efficiency of 34.89% is obtained for CuO nanofluids.
Purpose
This study aims to minimize the pressure drop across wavy microchannels using secondary branches without compromising its capacity to transfer the heat. The impact of secondary flows on the ...pressure drop and heat transfer capabilities at different Reynolds numbers are investigated numerically for different wavy microchannels. Finally, different channels are evaluated using performance evaluation criteria to determine their effectiveness.
Design/methodology/approach
To investigate the flow and heat transfer capabilities in wavy microchannels having secondary branches, a 3D conjugate heat transfer model based on finite volume method is used. In conventional wavy microchannel, secondary branches are introduced at crest and trough locations. For the numerical simulation, a single symmetrical channel is used to minimize computational time and resources and the flow within the channels remains single-phase and laminar.
Findings
The findings indicate that the suggested secondary channels notably improve heat transfer and decrease pressure drop within the channels. At lower flow rates, the secondary channels demonstrate superior performance in terms of heat transfer. However, the performance declines as the flow rate increased. With the same amplitude and wavelength, the introduction of secondary channels reduces the pressure drop compared with conventional wavy channels. Due to the presence of secondary channels, the flow splits from the main channel, and part of the core flow gets diverted into the secondary channel as the flow takes the path of minimum resistance. Due to this flow split, the core velocity is reduced. An increase in flow area helps in reducing pressure drop.
Practical implications
Many complex and intricate microchannels are proposed by the researchers to augment heat dissipation. There are challenges in the fabrication of microchannels, such as surface finish and achieving the required dimensions. However, due to the recent developments in metal additive manufacturing and microfabrication techniques, the complex shapes proposed in this paper are feasible to fabricate.
Originality/value
Wavy channels are widely used in heat transfer and micro-fluidics applications. The proposed wavy microchannels with secondary channels are different when compared to conventional wavy channels and can be used practically to solve thermal challenges. They help achieve a lower pressure drop in wavy microchannels without compromising heat transfer performance.
Convective heat transfer and friction factor studies are experimentally carried out in a smooth and five helically corrugated tubes of different heights and pitches of corrugation with spiraled rod ...inserts. The experiment is conducted under turbulent flow (
Re
= 4800–8900) and constant wall heat flux conditions. Deionized (DI) water and titanium dioxide (TiO
2
)/DI water nanofluids are used as working fluids. The average size of TiO
2
nanoparticles is 32 nm. Two volume concentrations of nanofluids (0.25 and 0.5%) are used in this study. The combined effects of nanofluids, inserts and corrugation in tubes on Nusselt number and friction factor are investigated. The results indicate that (i) the addition of TiO
2
nanoparticles in DI water upsurges the heat transfer rate, which increases with nanofluids volume concentrations; (ii) use of inserts and corrugation in tubes enhances the heat transfer rate further; (iii) among the corrugated tubes, the tube having highest corrugation height (
h
c
= 1 mm) and lowest pitch (
p
c
= 8 mm) with spiraled rod insert of smaller pitch (
p
i
= 30 mm) shows the maximum thermal performance factor of 1.56 for 0.5% volume concentration of nanofluids.
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