Nanofluids are engineered colloids made of a base fluid and nanoparticles (1-100 nm) Nanofluids have higher thermal conductivity' and single-phase heat transfer coefficients than their base fluids In particular the heat transfer coefficient increases appear to go beyond the mere thermal-conductivity effect, and cannot be predicted by traditional pure-fluid correlations such as Dittus-Boelter's In the nanofluid literature this behavior is generally attributed to thermal dispersion and intensified turbulence, brought about by nanoparticle motion To test the validity of this assumption, we have considered seven slip mechanisms that can produce a relative velocity between the nanoparticles and the base fluid These are inertia, Brownian diffusion, thermophoresis, diffusioplwresis, Magnus effect, fluid drainage, and gravity We concluded that, of these seven, only Brownian diffusion and thermophoresis are important slip mechanisms in nanofluids Based on this finding, we developed a two-component four-equation nonhomogeneous equilibrium model for mass, momentum, and heat transport in nanofluids A nondimensional analysis of the equations suggests that energy transfer by nanoparticle dispersion is negligible, and thus cannot explain the abnormal heat transfer coefficient increases Furthermore, a comparison of the nanoparticle and turbulent eddy time and length scales clearly indicates that the nanoparticles move homogeneously with the fluid in the presence of turbulent eddies so an effect on turbulence intensity is also doubtful Thus, we propose an alternative explanation for the abnormal heat transfer coefficient increases: the nanofluid properties may vary significantly within the boundary layer because of the effect of the temperature gradient and thermophoresis For a heated fluid, these effects can result in a significant decrease of viscosity within the boundary layer, thus leading to heat transfer enhancement A correlation structure that captures these effects is proposed

Convective Transport in Nanofluids

Low thermal conductivity is a primary limitation in the development of energy-efficient heat transfer fluids that are required in many industrial applications. In this paper we propose that an innovative new class of heat transfer fluids can be engineered by suspending metallic nanoparticles in conventional heat transfer fluids. The resulting {open_quotes}nanofluids{close_quotes} are expected to exhibit high thermal conductivities compared to those of currently used heat transfer fluids, and they represent the best hope for enhancement of heat transfer. The results of a theoretical study of the thermal conductivity of nanofluids with copper nanophase materials are presented, the potential benefits of the fluids are estimated, and it is shown that one of the benefits of nanofluids will be dramatic reductions in heat exchanger pumping power.

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Investigation on Convective Heat Transfer and Flow Features of Nanofluids

Turbulent friction and heat transfer behaviors of dispersed fluids (i.e., uttrafine metallic oxide particles suspended in water) in a circular pipe were investigated experimentally. Viscosity measurements were also conducted using a Brookfield rotating viscometer. Two different metallic oxide particles, γ-alumina (Al2O3) and titanium dioxide (TiO2), with mean diameters of 13 and 27 nm, respectively, were used as suspended particles. The Reynolds and Prandtl numbers varied in the ranges l04-I05 and 6.5-12.3, respectively. The viscosities of the dispersed fluids with γ-Al2O3 and TiO2 particles at a 10% volume concentration were approximately 200 and 3 times greater than that of water, respectively. These viscosity results were significantly larger than the predictions from the classical theory of suspension rheology. Darcy friction factors for the dispersed fluids of the volume concentration ranging from 1% to 3% coincided well with Kays' correlation for turbulent flow of a single-phase fluid. The Nusselt n...

Hydrodynamic and heat transfer study of dispersed fluids with submicron metallic oxide particles

Effective thermal conductivity of mixtures of e uids and nanometer-size particles is measured by a steady-state parallel-plate method. The tested e uids contain two types of nanoparticles, Al 2O3 and CuO, dispersed in water, vacuum pump e uid, engine oil, and ethylene glycol. Experimental results show that the thermal conductivities of nanoparticle ‐e uid mixtures are higher than those of the base e uids. Using theoretical models of effective thermal conductivity of a mixture, we have demonstrated that the predicted thermal conductivities of nanoparticle ‐e uid mixtures are much lower than our measured data, indicating the dee ciency in the existing models when used for nanoparticle ‐e uid mixtures. Possible mechanisms contributing to enhancement of the thermal conductivity of the mixtures are discussed. A more comprehensive theory is needed to fully explain the behavior of nanoparticle ‐e uid mixtures. Nomenclature cp = specie c heat k = thermal conductivity L = thickness Pe = Peclet number P q = input power to heater 1 r = radius of particle S = cross-sectional area T = temperature U = velocity of particles relative to that of base e uids ® = ratio of thermal conductivity of particle to that of base liquid ¯ = .® i 1/=.® i 2/ ° = shear rate of e ow Ω = density A = volume fraction of particles in e uids Subscripts

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Thermal Conductivity of Nanoparticle -Fluid Mixture

Thermal conductivity of carbon nanotubes and their polymer nanocomposites: A review

One of the challenges in the production of electricity is the cooling water management because the calcium content in circulating cooling water continues to increase with time as pure water evaporates. Thus, the excessive mineral contents in water circulation systems could cause severe fouling in heat transfer equipment. To avoid the catastrophic failure in condensers, the cooling water is discharged after 3 cycles at a rate of 10 million gallons a day in a 1,000-MW thermoelectric power plant. The present study investigated the effect of pulsed spark discharges on the mitigation of mineral fouling in a concentric counterflow heat exchanger. Artificial hard water with calcium carbonate hardness ranging from 250 to 500 ppm was used with velocity varying over a range of 0.1–0.5 m/s and zero blowdown. Fouling resistances decreased by 50–88% for the plasma treated cases compared with the values for no-treatment cases. SEM photographs showed particle with larger sizes for the plasma treated cases comparing to smaller but more organized particles for the no-treatment cases. The different structures of particles were associated with pulsed spark discharge assisted precipitation of calcium carbonate in oversaturated hard water. X-ray diffraction data showed calcite crystal structures for all cases.Copyright © 2010 by ASME

Application of Pulsed Spark Discharge for Mitigation of Mineral Fouling in a Heat Exchanger

Results of experiments conducted at the JPL to evaluate aircraft postcrash fire hazards are presented. The experiments were carried out in a one-third scale simulated aircraft cabin geometry to study pool fire and ventilation flow interactions. It is shown that wind-induced ventilation may significantly affect fire plume orientation, smoke transport, and heat fluxes and thus will affect subsequent fire spread and the immediate survivability of the passengers.

Pool fires in a simulated aircraft cabin interior with ventilation

In vitro flow measurements in ion sputtered hydrocephalus shunts

According to WHO-UNICEF Joint Monitoring Programme on Water Supply and Sanitation (JMP), more than 1 billion human beings, use potentially harmful sources of water Certain viruses and parasites like Giardia and Cryptosporidium are resistant to conventional sterilization methods like chlorination and are easily transported to the point of use Hence, there arises the need for a robust, effective and improved water purification technology for inactivating microorganisms in drinking water Pulsed plasma discharges have proved to be very effective in killing microorganisms in water Experiments were conducted using a high voltage pulsed spark plasma discharge having the following parameters: peak-to-peak voltage 30kV, peak-to-peak current 80A, pulse duration 2µs, frequency 1-50Hz and energy per pulse 17-2J It has already been demonstrated that a rather low energy input of 85J/L and 150J/L of water respectively are required for complete inactivation of Escherichia coli and Bacillus cereus spores of initial concentration 106 Colony Forming Units(CFU)/ml of water The different factors considered by many researchers to be responsible for the microbicidal action of pulsed discharges in water include the production of ultraviolet radiation, active chemical species including ozone, hydrogen peroxide, hydroxyl and superoxide free radicals and shock waves [1] The present study shows that ultraviolet radiation produced by a pulsed spark plasma discharge in water plays a dominant role in the inactivation of E coli in water Disinfection of E coli in water is completely suppressed by adding ∼50mg/L of Benzophenone-9, a non-toxic ingredient in commercial water-soluble sunscreens Further experiments with reactive oxygen species scavengers are being conducted to determine if there is a synergy between ultraviolet radiation and hydrogen peroxide produced in water by pulsed spark plasma discharge

Investigation of the mechanism of inactivation of E. coli in water using a pulsed spark plasma discharge

A dearth of research exists on the effects of smoking on the relationship between sleep and mental illnesses. To examine the association of mental health problems with days of poor sleep based on whether participants smoked. A secondary analysis of the 2015 Behavioral Risk Factor Surveillance System (N = 16,253) was conducted using linear regressions models. The study found that the associations between mental health and sleep quality, measured as days of poor sleep, varied by the respondents’ smoker status among those who reported having less than 20 days of mental health problems. The difference in days of poor sleep between smokers and non-smokers were greatest among those who experienced 10 to 19 days of mental health problems. Of those with 10 to 19 days of mental health problems in the past 30 days, an average of 8.5 days of poor sleep was estimated among smokers, which was 3 more days than that of non-smokers (p = .001). The differences in days of poor sleep were not significantly different between smokers and non-smokers among those who reported 20 or more days of mental health problems. Days of mental health problems were directly related to days of poor sleep. This relationship was moderated by smoking status. Special attention needs to be paid to those who have comparably moderate levels of mental illnesses because non-smoking status can significantly improve sleep quality. Further research is needed to confirm the impact of smoking on the associations of mental illnesses with poor sleep.

https://journals.sagepub.com/doi/pdf/10.1177/21582440221082140

Young I. Cho

Papers

Application of Pulsed Spark Discharge for Mitigation of Mineral Fouling in a Heat Exchanger

Pool fires in a simulated aircraft cabin interior with ventilation

In vitro flow measurements in ion sputtered hydrocephalus shunts

Investigation of the mechanism of inactivation of E. coli in water using a pulsed spark plasma discharge

Moderating Effects of Smoking Status on the Relationships Between Mental Health Problems and Poor Sleep