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1.
A thermal conductivity model for nanofluids including effect of the temperature-dependent interfacial layer 总被引:1,自引:0,他引:1
Chatcharin Sitprasert Pramote Dechaumphai Varangrat Juntasaro 《Journal of nanoparticle research》2009,11(6):1465-1476
The interfacial layer of nanoparticles has been recently shown to have an effect on the thermal conductivity of nanofluids.
There is, however, still no thermal conductivity model that includes the effects of temperature and nanoparticle size variations
on the thickness and consequently on the thermal conductivity of the interfacial layer. In the present work, the stationary
model developed by Leong et al. (J Nanopart Res 8:245–254, 2006) is initially modified to include the thermal dispersion effect due to the Brownian motion of nanoparticles. This model is
called the ‘Leong et al.’s dynamic model’. However, the Leong et al.’s dynamic model over-predicts the thermal conductivity of nanofluids in the case of the flowing
fluid. This suggests that the enhancement in the thermal conductivity of the flowing nanofluids due to the increase in temperature
does not come from the thermal dispersion effect. It is more likely that the enhancement in heat transfer of the flowing nanofluids
comes from the temperature-dependent interfacial layer effect. Therefore, the Leong et al.’s stationary model is again modified
to include the effect of temperature variation on the thermal conductivity of the interfacial layer for different sizes of
nanoparticles. This present model is then evaluated and compared with the other thermal conductivity models for the turbulent convective heat transfer in nanofluids
along a uniformly heated tube. The results show that the present model is more general than the other models in the sense
that it can predict both the temperature and the volume fraction dependence of the thermal conductivity of nanofluids for
both non-flowing and flowing fluids. Also, it is found to be more accurate than the other models due to the inclusion of the
effect of the temperature-dependent interfacial layer. In conclusion, the present model can accurately predict the changes
in thermal conductivity of nanofluids due to the changes in volume fraction and temperature for various nanoparticle sizes. 相似文献
2.
This study investigates flow boiling heat transfer of aqueous alumina nanofluids in single microchannels with particular focuses
on the critical heat flux (CHF) and the potential dual roles played by nanoparticles, i.e., (i) modification of the heating
surface through particle deposition and (ii) modification of bubble dynamics through particles suspended in the liquid phase.
Low concentrations of nanofluids (0.001–0.1 vol.%) are formulated by the two-step method and the average alumina particle
size is ~25 nm. Two sets of experiments are performed: (a) flow boiling of formed nanofluids in single microchannels where
the effect of heating surface modification by nanoparticle deposition is apparent and (b) bubble formation in a quiescent
pool of alumina nanofluids under adiabatic conditions where the role of suspended nanoparticles in the liquid phase is revealed.
The flow boiling experiments reveal a modest increase in CHF by nanofluids, being higher at higher nanoparticle concentrations
and higher inlet subcoolings. The bubble formation experiments show that suspended nanoparticles in the liquid phase alone
can significantly affect bubble dynamics. Further discussion reveals that both roles are likely co-existent in a typical boiling
system. Properly surface-promoted nanoparticles could minimize particle deposition hence little modification of the heating
surface, but could still contribute to the modification in heat transfer through the second mechanism, which is potentially
promising for microchannel applications. 相似文献
3.
The temperature histories are calculated for spherical nano-sized aluminum particles with no protective oxide shell inserted
in air at 300 K. In calculations, initial particle temperatures varied and the minimum initial temperature leading to the
particle ignition was determined. The particle, initially without any oxide coating, was assumed to react adiabatically forming
a monomolecular oxide coverage; the following oxidation was assumed to be governed by the Cabrera–Mott reaction mechanism.
Convection and radiation heat losses were considered. Convection was accounted for using a transition regime heat transfer
model by Fuchs. Aluminum particles with diameters less than about 68 nm are predicted to be pyrophoric, e.g., ignite without
appreciable initial pre-heating. 相似文献
4.
Federico Bassetti Lucia Ladelli Giuseppe Toscani 《Journal of statistical physics》2011,142(4):686-709
We introduce a class of Kac-like kinetic equations on the real line, with general random collisional rules which, in some
special cases, identify models for granular gases with a background heat bath (Carrillo et al. in Discrete Contin. Dyn. Syst.
24(1):59–81, 2009), and models for wealth redistribution in an agent-based market (Bisi et al. in Commun. Math. Sci. 7:901–916, 2009). Conditions on these collisional rules which guarantee both the existence and uniqueness of equilibrium profiles and their
main properties are found. The characterization of these stationary states is of independent interest, since we show that
they are stationary solutions of different evolution problems, both in the kinetic theory of rarefied gases (Cercignani et al.
in J. Stat. Phys. 105:337–352, 2001; Villani in J. Stat. Phys. 124:781–822, 2006) and in the econophysical context (Bisi et al. in Commun. Math. Sci. 7:901–916, 2009). 相似文献
5.
It has been speculated that the application of nanofluids in real systems could lead to smaller, more compact heat exchangers
and reductions in material cost. However, few studies have been conducted which have carefully measured the thermo-physical
properties and thermal performance of these fluids as well as examine the system-level effects of using these fluids in traditional
cooling systems. In this study, dilute suspensions of 10 nm aluminum oxide nanoparticles in propanol (0.5, 1, and 3 wt%) were
investigated. Changes in density, specific heat, and thermal conductivity with particle concentration were measured and found
to be linear, whereas changes in viscosity were nonlinear and increased sharply with particle loading. Nanofluid heat transfer
performance data were generally commensurate with that measured for the baseline. For the 1 wt% concentration, a small but
significant enhancement in the heat transfer coefficient was recorded for 1800 < Re < 2800, which is attributed to an earlier
transition to turbulent flow. In the case of high particle loading (i.e. 3 wt%), the thermal performance was observed to deteriorate
with respect to the baseline case. Discoloration of the fluid was also observed after being cycled at high flow rates and
increased temperature. 相似文献
6.
Fumio Kokai Naoki Tachi Keita Kobayashi Akira Koshio 《Applied Surface Science》2009,255(24):9622-9625
Three types of single-wall carbon nanohorn (SWNH) aggregates hybridized with carbon nanocapsules (CNCs) containing Fe3C, Co, or Ag were produced by laser vaporization of graphite mixed with Fe, Co, or Ag in Ar gas. Characterization by transmission electron microscopy revealed that although the three hybrid structures had different diameter distributions with average diameters of 96, 90, and 85 nm, respectively, their SWNH layers had similar thicknesses (17-18 nm on average). The diameter difference is explained by the sizes (16-24 nm on average) of the encapsulated CNCs, the formation of which depended on the carbon solubility of the three metals and the precipitation of the graphitic layers. In addition, there was a stronger correlation between the diameters of the hybrids and the thicknesses of the SWNH layers for the three types. We suggest that the formation mechanism of the three structures is based on the assembly of SWNHs around a molten metal-carbon particle with certain ranges of lengths and diameters, respectively. 相似文献
7.
In this article, distilled water and CuO particles with volume fraction of 1%, 2% and 4% are numerically studied. The steady state flow regime is considered laminar with Reynolds number of 100, and nano-particles diameters are assumed 20 nm and 80 nm. The hydraulic diameter and the length of equilateral triangular channel are 8 mm and 1000 mm, respectively. The problem is solved for two different boundary conditions; firstly, constant heat flux for all sides as a validation approach; and secondly, constant heat flux for two sides and constant temperature for one side (hot plate). Convective heat transfer coefficient, Nusselt number, pressure loss through the channel, velocity distribution in cross section and temperature distribution on walls are investigated in detail. The fluid flow is supposed to be one-phase flow. It can be observed that nano-fluid leads to a remarkable enhancement on heat transfer coefficient. Furthermore, CuO particles increase pressure loss through the channel and velocity distribution in fully developed cross section of channel, as well. The computations reveal that the size of nano-particles has no significant influence on heat transfer properties. Besides, the study shows a good agreement between provided outcomes and experimental data available in the literature. 相似文献
8.
Nicholas N. Bwana 《Journal of nanoparticle research》2009,11(8):1905-1915
In this study, the open-circuit photovoltage (V
oc) decay technique was used to investigate the relationship between the electrode film morphology and the open-circuit photovoltage.
Results indicate that dye-sensitized solar cells (DSCs) based on ordered arrays of TiO2 nanostructures (100 nm external diameters and 20–50 nm internal diameters) generally show higher open-circuit photovoltage
(V
oc) values than those based on sintered TiO2 nanoparticles (20–40 nm diameters). In particular, cells based on thick nanotubules (wall thickness ≥ 45 nm in our research)
and on nanorods (100 nm diameters) show particularly high V
oc values, indicating slow recombination kinetics under open-circuit conditions. It can be argued that the nanorods and the
thick nanotubules act like singles crystals and therefore the injected electrons in the inner TiO2 molecules are shielded from holes in the electrolyte under open-circuit conditions. The open-circuit recombination time constant
of electrons accumulated in the TiO2 conduction band is therefore prolonged and resulting in high V
oc values. 相似文献
9.
Sultana Ferdous Marios A. Ioannidis Dale Henneke 《Journal of nanoparticle research》2011,13(12):6579-6589
The pendant drop technique was used to characterize the adsorption behavior of n-dodecane-1-thiol and n-hexane-1-thiol-capped gold nanoparticles at the hexane–water interface. The adsorption process was studied by analyzing the
dynamic interfacial tension versus nanoparticle concentration, both at early times and at later stages (i.e., immediately
after the interface between the fluids is made and once equilibrium has been established). A series of gold colloids were
made using nanoparticles ranging in size from 1.60 to 2.85 nm dissolved in hexane for the interfacial tension analysis. Following
free diffusion of nanoparticles from the bulk hexane phase, adsorption leads to ordering and rearrangement of the nanoparticles
at the interface and formation of a dense monolayer. With increasing interfacial coverage, the diffusion-controlled adsorption
for the nanoparticles at the interface was found to change to an interaction-controlled assembly and the presence of an adsorption
barrier was experimentally verified. At the same bulk concentration, different sizes of n-dodecane-1-thiol nanoparticles showed different absorption behavior at the interface, in agreement with the findings of Kutuzov
et al. (Phys Chem Chem Phys 9:6351–6358, 2007). The experiments additionally demonstrated the important role played by the capping agent. At the same concentration, gold
nanoparticles stabilized by n-hexane-1-thiol exhibited greater surface activity than gold nanoparticles of the same size stabilized by n-dodecane-1-thiol. These findings contribute to the design of useful supra-colloidal structures by the self-assembly of alkane-thiol-capped
gold nanoparticles at liquid–liquid interfaces. 相似文献
10.
Mareike Mathieu Steffen Franzka Jürgen Koch Boris N. Chichkov Nils Hartmann 《Applied Physics A: Materials Science & Processing》2010,101(3):461-466
Femtosecond laser patterning of alkanethiol monolayers on gold-coated silicon substrates at λ=800 nm, τ<30 fs and ambient conditions has been investigated. Single-pulse processing allows one to selectively remove the organic
coating. Subsequently, pattern transfer into the gold film via wet etching in ferri-/ferrocyanide solution is achieved. As
demonstrated, burr-free patterning can be carried out over an extremely wide range of laser pulse fluences from above 2 J/cm2 down to 0.5 J/cm2. Moreover, at low fluences, sub-wavelength processing down to λ/5 is feasible. In particular, at a 1/e laser spot diameter of about 1 μm, holes with diameters of 160 nm and step edges below
80 nm are fabricated. These results emphasize the prospects of organic monolayers as high-resolution resists in rapid nonlinear
femtosecond laser processing. 相似文献
11.
M. T. Hamed Mosavian S. Zeinali Heris S. Gh. Etemad M. Nasr Esfahany 《Journal of nanoparticle research》2010,12(7):2611-2619
In this investigation, laminar flow heat transfer enhancement in circular tube utilizing different nanofluids including Al2O3 (20 nm), CuO (50 nm), and Cu (25 nm) nanoparticles in water was studied. Constant wall temperature was used as thermal boundary
condition. The results indicate enhancement of heat transfer with increasing nanoparticle concentrations, but an optimum concentration
for each nanofluid suspension can be found. Based on the experimental results, metallic nanoparticles show better enhancement
of heat transfer coefficient in comparison with oxide particles. The promotions of heat transfer due to utilizing nanoparticles
are higher than the theoretical correlation prediction. 相似文献
12.
R. Russo C. Granata P. Walke A. Vettoliere E. Esposito M. Russo 《Journal of nanoparticle research》2011,13(11):5661-5668
Integrated magnetic sensors based on niobium dc SQUID (Superconducting Quantum Interference Device) for nanoparticle characterizations
are presented. The SQUIDs consists of two Dayem bridges of 90 nm × 250 nm and loop area of 4, 1, and 0.55 μm2. The devices are realized by using an e-beam lithography nano-fabrication process which can directly pattern the devices
in an electron-positive resist and then transferred to a 20 nm single niobium layer by a lift-off post-process. The SQUIDs
were designed to have a hysteretic current–voltage characteristic in order to work as a magnetic flux-current transducer.
The presence of an integrated niobium coil, tightly coupled to the SQUID, allows us to easily excite the SQUID and to flux
bias the SQUID at its optimal working point. Current–voltage characteristics, critical current as a function of the external
magnetic field and switching current distributions were performed at liquid helium temperature. A critical current modulation
of about 20% and a current-magnetic flux transfer coefficient (responsivity) of 30 μA/Φ0 have been obtained, resulting in a magnetic flux resolution better than 1 mΦ0. The authors performed preliminary measurements with and without iron oxide nanoparticles on the SQUID loop in order to show
the device sensitivity in view of nano-magnetism applications. It was showed that the presence of magnetic nanoparticles can
be easily detected and the magnetic relaxation curve measured. 相似文献
13.
The development of liquid fuelled microcombustors faces many challenges, one of which being high asymmetric heat flux across the combustion chamber. Typically, thin walls provide little resistance to convective heat transfer and therefore, allowing high heat loss rates. Insulating the walls results in high wall temperatures, which increases the likelihood that radiation plays an important role. Both of these effects have the potential to induce asymmetries and strong temperature gradients in the gas flow, relative to the more uniform environment of a conventional combustor. This investigation uses planar laser-induced fluorescence (PLIF) to reveal the spatial vapor distribution around a monodisperse acetone droplet stream that is exposed to asymmetric radiant heating. Droplets with diameters from 117 to 222 μm flow past a single-sided array of radiant heating elements to provide the asymmetric heating. A frequency-quadrupled Nd:YAG laser provides a 266 nm light sheet to excite the acetone vapor around the droplets, which are exposed to different experimental conditions by varying parameters such as the droplet diameter and temperature of the radiating elements for a fixed exposure time of the droplets in the heated region. A CCD camera captures the fluorescence of the excited acetone vapor molecules over a broadband wavelength range between 350 and 550 nm, to give the radial and axial vapor concentration around each droplet. After processing the PLIF images, we obtain contour plots of the spatial acetone vapor concentration around the droplets which depict asymmetric vapor distribution. The potential impacts on vaporization, combustion and pollutant formation are discussed. 相似文献
14.
《Journal of magnetism and magnetic materials》2005,293(1):75-79
Iron and cobalt nanoparticle fluids have been prepared by inert-gas condensation into an oil/surfactant mixture. Superparamagnetic iron fluids (mean particle size=11.6±0.4 nm) and ferromagnetic cobalt fluids (mean particle size=51.6±3.4 nm) produced by this technique are promising candidates for magnetic targeting and hyperthermia applications. 相似文献
15.
Yongzhen Yang Xuguang Liu Xingmei Guo Hairong Wen Bingshe Xu 《Journal of nanoparticle research》2011,13(5):1979-1986
Nano-carbon materials were synthesized by the catalytic decomposition of acetylene at 420 °C using iron supported on sodium
chloride as catalyst. The catalysts contain about 0.3, 1.6, 3.3, and 5.2 wt% iron. The samples were examined by scanning and
transmission electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The results show that nano onion-like
fullerenes (NOLFs) surrounding an Fe3C core were obtained using the catalyst containing 0.3 wt% iron. These had a structure of stacked graphitic fragments, with
diameters in the range 15–50 nm. When the product was further heat treated under vacuum at 1,100 °C, NOLFs with a clear concentric
graphitic layer structure were obtained. The growth mechanism of NOLFs encapsulating metallic cores is suggested to be in
accordance with a vapor–solid growth model. 相似文献
16.
Optical properties of spherical gold particles with diameters of 150–650 nm (mesoparticles) are studied by reflectance spectroscopy.
Particles are fabricated by laser-induced transfer of metallic droplets onto metal and dielectric substrates. Contributions
of higher multipoles (beyond the quadrupole) in the scattering spectra of individual spherical particles are experimentally
observed. These observations are performed for particles in a homogeneous environment and for particles located in air on
a metal surface. Good agreement between calculations on the basis of Mie theory and experimental results obtained in homogeneous
environment is demonstrated. Multipole resonance features in the experimental reflection spectra of particles located on a
gold substrate, in the wavelength range of 500–1000 nm, are discussed and theoretically analyzed on the basis of finite-difference
time-domain simulations. High-resolution Raman images of mesoparticle pairs at different polarizations of light are also presented. 相似文献
17.
In this paper, it was investigated experimentally that the effect of different kinds of working fluid on the thermal performance of evaporator with capillary wick consisted by multilayered sintered copper mesh under different electric field strengths at the operating pressure of 1.01 × 105 Pa R141b and R123 were used as the working fluids. The electric field strength in this study was in the range of 0kV/m–1600 kV/m, respectively. The experimental results showed that the applied electric field strength has significant effect on heat transfer characteristic. The heat transfer enhancement effects increased with the increase of the electric field. Under the applied electric field strength, the maximum heat transfer enhancement factors could reach as high as 1.5 and 1.32 for the two kinds of working fluids in the experiment. 相似文献
18.
Cumulative and continuous laser vaporization synthesis of single wall carbon nanotubes and nanohorns
A. A. Puretzky D. J. Styers-Barnett C. M. Rouleau H. Hu B. Zhao I. N. Ivanov D. B. Geohegan 《Applied Physics A: Materials Science & Processing》2008,93(4):849-855
The conditions for the scaled synthesis of single wall carbon nanotubes (SWNTs) and single wall carbon nanohorns (SWNHs) by
laser vaporization at high temperatures are investigated and compared using in situ diagnostics. An industrial Nd:YAG laser
(600 W, 1–500 Hz repetition rate) with tunable pulse widths (0.5–50 ms) is utilized to explore conditions for high-yield production.
High-speed videography (50000 frames/s) of the laser plume and pyrometry of the target surface are correlated with ex situ
high resolution transmission electron microscopy analysis of the products for pure carbon targets and carbon/catalyst targets
to understand the effects of the processing conditions on the resulting nanostructures. Carbon is shown to self-assemble into
single-wall nanohorn structures at rates of ∼1 nm/ms, which is comparable to the catalyst-assisted SWNT growth rates. Two
regimes of laser ablation, cumulative ablation by multiple pulses and continuous ablation by individual pulses, were explored. Cumulative ablation with spatially overlapping 0.5-ms pulses is favorable for
the high yield and production rate of SWNTs at ∼6 g/h while continuous ablation by individual long laser pulses (∼20 ms) at
high temperatures results in the highest yield of SWNHs at ∼10 g/h. Adjustment of the laser pulse width is shown to control
SWNH morphology. 相似文献
19.
In experiments of games, players frequently make choices which are regarded as irrational in game theory. In papers of Khrennikov
(Information Dynamics in Cognitive, Psychological and Anomalous Phenomena. Fundamental Theories of Physics, Kluwer Academic,
Norwell, 2004; Fuzzy Sets Syst. 155:4–17, 2005; Biosystems 84:225–241, 2006; Found. Phys. 35(10):1655–1693, 2005; in QP-PQ Quantum Probability and White Noise Analysis, vol. XXIV, pp. 105–117, 2009), it was pointed out that statistics collected in such the experiments have “quantum-like” properties, which can not be explained
in classical probability theory. In this paper, we design a simple quantum-like model describing a decision-making process
in a two-players game and try to explain a mechanism of the irrational behavior of players. Finally we discuss a mathematical
frame of non-Kolmogorovian system in terms of liftings (Accardi and Ohya, in Appl. Math. Optim. 39:33–59, 1999). 相似文献
20.
Peter Nyman 《International Journal of Theoretical Physics》2010,49(1):1-9
In this paper we continue to study so-called “inverse Born’s rule problem”: to construct a representation of probabilistic
data of any origin by a complex probability amplitude which matches Born’s rule. The corresponding algorithm—quantum-like
representation algorithm (QLRA)—was recently proposed by A. Khrennikov (Found. Phys. 35(10):1655–1693, 2005; Physica E 29:226–236, 2005; Dokl. Akad. Nauk 404(1):33–36, 2005; J. Math. Phys. 46(6):062111–062124, 2005; Europhys. Lett. 69(5):678–684, 2005). Formally QLRA depends on the order of conditioning. For two observables (of any origin, e.g., physical or biological) a and b, b|a- and a|b conditional probabilities produce two representations, say in Hilbert spaces H
b|a
and H
a|b
. In this paper we prove that under “natural assumptions” (which hold, e.g., for quantum observables represented by operators
with nondegenerate spectra) these two representations are unitary equivalent. This result proves the consistency of QLRA. 相似文献