On the Anomalous Convective Heat Transfer Enhancement in Nanofluids: A Theoretical Answer to the Nanofluids Controversy

2013 ◽  
Vol 135 (5) ◽  
Author(s):  
C. Yang ◽  
W. Li ◽  
Y. Sano ◽  
M. Mochizuki ◽  
A. Nakayama
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Volume Fraction ◽  
High Energy ◽  
Constant Heat Flux ◽  
Anomalous Heat Transfer ◽  
Buongiorno Model ◽  
Energy Equations

A theoretical answer to the controversial issue on the anomalous convective heat transfer in nanofluids has been provided, exploiting the Buongiorno model for convective heat transfer in nanofluids with modifications to fully account for the effects of nanoparticle volume fraction distributions on the continuity, momentum, and energy equations. A set of exact solutions have been obtained for hydrodynamically and thermally fully developed laminar nanofluid flows in channels and tubes, subject to constant heat flux. From the solutions, it has been concluded that the anomalous heat transfer rate, exceeding the rate expected from the increase in thermal conductivity, is possible in such cases as titania–water nanofluids in a channel, alumina–water nanofluids in a tube and also titania–water nanofluids in a tube. Moreover, the maximum Nusselt number based on the bulk mean nanofluid thermal conductivity is captured when the ratio of Brownian and thermophoretic diffusivities is around 0.5, which can be exploited for designing nanoparticles for high-energy carriers.

Convective Heat Transfer for Water-Based Alumina Nanofluids in a Single 1.02-mm Tube

2009 ◽  
Vol 131 (11) ◽  
Author(s):  
W. Y. Lai ◽  
S. Vinod ◽  
P. E. Phelan ◽  
Ravi Prasher
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Convective Heat Transfer ◽  
Effective Thermal Conductivity ◽  
Convective Heat ◽  
Volume Fraction ◽  
Pure Water ◽  
Submicron Particles ◽  
Strong Temperature Dependence ◽  
Water Based

Nanofluids are colloidal solutions, which contain a small volume fraction of suspended submicron particles or fibers in heat transfer liquids such as water or glycol mixtures. Compared with the base fluid, numerous experiments have generally indicated an increase in effective thermal conductivity and a strong temperature dependence of the static effective thermal conductivity. However, in practical applications, a heat conduction mechanism may not be sufficient for cooling high heat dissipation devices such as microelectronics or powerful optical equipment. Thus, thermal performance under convective heat transfer conditions becomes of primary interest. We report here the heat transfer coefficient h in both developing and fully developed regions by using water-based alumina nanofluids. Our experimental test section consists of a single 1.02-mm diameter stainless steel tube, which is electrically heated to provide a constant wall heat flux. Both pressure drop and temperature differences are measured, but mostly here we report our h measurements under laminar flow conditions. An extensive characterization of the nanofluid samples, including pH, electrical conductivity, particle sizing, and zeta potential, is also documented. The measured h values for nanofluids are generally higher than those for pure water. In the developing region, this can be at least partially explained by Pr number effects.

Theoretical Conclusions About the Claims of Anomalous Heat Transfer Enhancement Associated With Nanofluids

2013 ◽  
Author(s):  
Wenhao Li ◽  
Chen Yang ◽  
Akira Nakayama
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Free Convection ◽  
Volume Fraction ◽  
Cold Wall ◽  
Nanoparticle Volume Fraction ◽  
Transfer Enhancement ◽  
Anomalous Heat Transfer ◽  
Buongiorno Model ◽  
Anomalous Heat

A theoretical answer to the controversial issue on the anomalous convective heat transfer in nanofluids has been provided, exploiting the Buongiorno model for convective heat transfer in nanofluids with modifications to fully account for the effects of nanoparticle volume fraction distributions on the continuity, momentum and energy equations. Firstly, a set of exact solutions have been obtained for hydrodynamically and thermally fully developed laminar nanofluid forced convection flows in channels and tubes, subject to constant heat flux. From the solutions, it has been concluded that the anomalous heat transfer rate, exceeding the rate expected from the increase in thermal conductivity, is possible in such cases as titania-water nanofluids in a channel, alumina-water nanofluids in a tube and also titania-water nanofluids in a tube. Moreover, the maximum Nusselt number based on the bulk mean nanofluid thermal conductivity is captured when the ratio of Brownian and thermophoretic diffusivities is around 0.5, which can be exploited for designing nanoparticles for high-energy carriers. Secondly, another set of exact solutions have been obtained for free convection in a vertical channel filled with a nanofluid, exploiting the Buongiorno model with nanoparticle volume fraction modifications. The effects of the bulk mean volume fraction of nanoparticles, the ratio of Brownian and thermophoretic diffusivities and the buoyancy ratio on both velocity and temperature profiles has been investigated in depth for the first time. The volume fraction of nanoparticles increases exponentially towards the cold wall, which makes the velocity and temperature gradients steeper near the hot wall than those near the cold wall. Unlike the case of forced convection, no anomalous heat transfer enhancement has been observed in this case of free convection, so that the Nussult number based on the thermal conductivity of nanofluid stays close to unity as in pure base fluid free convection.

scholarly journals Convective Heat Transfer of Al2O3 and CuO Nanofluids Using Various Mixtures of Water-Ethylene Glycol as Base Fluids

2017 ◽  
Vol 7 (2) ◽  
pp. 1496-1503
Author(s):  
K. Boukerma ◽  
M. Kadja
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Ethylene Glycol ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Thermal Performance ◽  
Volume Fraction ◽  
Numerical Study ◽  
Constant Heat Flux ◽  
Volume Fractions

In this work, a numerical study has been performed on the convective heat transfer of Al2O3/Water-Ethylene Glycol (EG) and CuO/(W-EG) nanofluids flowing through a circular tube with circumferentially non-uniform heating (constant heat flux) under the laminar flow condition. We focus on the study of the effect of EG-water mixtures as base fluids with mass concentration ranging from 0% up to 100% ethylene glycol on forced convection. The effect on the flow and the convective heat transfer behavior of nanoparticle types, their volume fractions (φ=1-5%) and Reynolds number are also investigated. The results obtained show that the highest values of the average heat transfer coefficient is observed between 40% and 50% of EG concentration. The average Nusselt number increases with the increase in EG concentration in the base fluid, and the increase in the Reynolds number and volume fraction. For concentrations of EG above 60%, and for all volume fractions, the increase of thermal performance of nanofluids became inversely proportional to the increase of Reynolds number. In addition, CuO/(W-EG) nanofluids show the best thermal performance compared with Al2O3/ (W-EG) nanofluids.

Experimental Model of Temperature-Driven Nanofluid

2006 ◽  
Vol 129 (6) ◽  
pp. 697-704 ◽  
Author(s):  
A. G. Agwu Nnanna
Keyword(s):  
Heat Transfer ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Small Volume ◽  
Volume Fraction ◽  
Convective Heat Transfer Coefficient ◽  
Isothermal Condition ◽  
Carrier Fluid ◽  
Small Volume Fraction ◽  
The Impact

This paper presents a systematic experimental method of studying the heat transfer behavior of buoyancy-driven nanofluids. The presence of nanoparticles in buoyancy-driven flows affects the thermophysical properties of the fluid and consequently alters the rate of heat transfer. The focus of this paper is to estimate the range of volume fractions that results in maximum thermal enhancement and the impact of volume fraction on Nusselt number. The test cell for the nanofluid is a two-dimensional rectangular enclosure with differentially heated vertical walls and adiabatic horizontal walls filled with 27 nm Al2O3–H2O nanofluid. Simulations were performed to measure the transient and steady-state thermal response of nanofluid to imposed isothermal condition. The volume fraction is varied between 0% and 8%. It is observed that the trend of the temporal and spatial evolution of temperature profile for the nanofluid mimics that of the carrier fluid. Hence, the behaviors of both fluids are similar. Results shows that for small volume fraction, 0.2⩽ϕ⩽2% the presence of the nanoparticles does not impede the free convective heat transfer, rather it augments the rate of heat transfer. However, for large volume fraction ϕ>2%, the convective heat transfer coefficient declines due to reduction in the Rayleigh number caused by increase in kinematic viscosity. Also, an empirical correlation for Nuϕ as a function of ϕ and Ra has been developed, and it is observed that the nanoparticle enhances heat transfer rate even at a small volume fraction.

Experimental Convective Heat Transfer With Nanofluids

2008 ◽  
Author(s):  
S. Kabelac ◽  
K. B. Anoop
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Transfer Coefficient ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Turbulent Regime ◽  
Heat Transfer Characteristics ◽  
Forced Convective Heat Transfer ◽  
Transfer Characteristics

Nanofluids are colloidal suspensions with nano-sized particles (<100nm) dispersed in a base fluid. From literature it is seen that these fluids exhibit better heat transfer characteristics. In our present work, thermal conductivity and the forced convective heat transfer coefficient of an alumina-water nanofluid is investigated. Thermal conductivity is measured by a steady state method using a Guarded Hot Plate apparatus customized for liquids. Forced convective heat transfer characteristics are evaluated with help of a test loop under constant heat flux condition. Controlled experiments under turbulent flow regime are carried out using two particle concentrations (0.5vol% and 1vol %). Experimental results show that, thermal conductivity of nanofluids increases with concentration, but the heat transfer coefficient in the turbulent regime does not exhibit any remarkable increase above measurement uncertainty.

Investigations on Transient Natural Convection in Boron Nitride-Mineral Oil Nanofluid Systems

2012 ◽  
Author(s):  
Shijo Thomas ◽  
C. B. Sobhan ◽  
Jaime Taha-Tijerina ◽  
T. N. Narayanan ◽  
P. M. Ajayan
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Transfer Coefficient ◽  
Natural Convection ◽  
Boron Nitride ◽  
Transfer Coefficient ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Convective Heat Transfer Coefficient ◽  
Mineral Oil

Nanofluids are suspensions or colloids produced by dispersing nanoparticles in base fluids like water, oil or organic fluids, so as to improve their thermo-physical properties. Investigations reported in recent times have shown that the addition of nanoparticles significantly influence the thermophysical properties, such as the thermal conductivity, viscosity, specific heat and density of base fluids. The convective heat transfer coefficient also has shown anomalous variations, compared to those encountered in the base fluids. By careful selection of the parameters such as the concentration and the particle size, it has been possible to produce nanofluids with various properties engineered depending on the requirement. A mineral oil–boron nitride nanofluid system, where an increased thermal conductivity and a reduced electrical conductivity has been observed, is investigated in the present work to evaluate its heat transfer performance under natural convection. The modified mineral oil is produced by chemically dispersing boron nitride nanoparticles utilizing a one step method to obtain a stable suspension. The mineral oil based nanofluid is investigated under transient free convection heat transfer, by observing the temperature-time response of a lumped parameter system. The experimental study is used to estimate the time-dependent convective heat transfer coefficient. Comparisons are made with the base fluid, so that the enhancement in the heat transfer coefficient under natural convection situation can be estimated.

Numerical Study on the Surface Convective Heat Transfer of Mass Concrete Structure

2015 ◽  
Vol 723 ◽  
pp. 992-995
Author(s):  
Biao Li ◽  
Fu Guo Tong ◽  
Chang Liu ◽  
Nian Nian Xi
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Flow Velocity ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Air Flow ◽  
Convective Heat Transfer Coefficient ◽  
Concrete Surface ◽  
Mass Concrete ◽  
Air Flow Velocity

The surface convective heat transfer of mass concrete is an important element of concrete structure temperature effect analysis. Based on coupled Thermal Fluid governing differential equation and finite element method, the paper calculated and analyzed the dependence of the concrete surface convective heat transfer on the air flow velocity and the concrete thermal conductivity coefficient. Results show that the surface convective heat transfer coefficient of concrete is a quadratic polynomial function of the air flow velocity, but influenced much less by the air flow velocity when temperature gradient is dominating in heat transfer. The concrete surface convective heat transfer coefficient increases linearly with the thermal conductivity of concrete increases.

A Kriging-Simulated Annealing Meta-Algorithm for Optimization of Convective Heat Transfer in a Grooved Channel

2011 ◽  
Author(s):  
Sertac Cadirci ◽  
Alpay Akguc ◽  
Hasan Gunes
Keyword(s):  
Heat Transfer ◽  
Simulated Annealing ◽  
Design Optimization ◽  
Convective Heat Transfer ◽  
Convective Heat ◽  
Transfer Problem ◽  
Geometry Optimization ◽  
Constant Heat Flux ◽  
Design Tool ◽  
Cfd Simulations

As computational fluid dynamics (CFD) simulations still take a lot of time in industry for design optimization analyses, we propose to reduce the number of required CFD simulations considerably using a new meta-algorithm. The algorithm consists of employing kriging in conjunction with simulated annealing (SA) as a design tool. The method enables effective and fast design optimization for complex fluid flow systems. In this paper, it is applied to a convective heat transfer problem in a channel with periodic heat sources with constant heat flux. We show that the geometry optimization of two dimensional thermo-fluids problems in complex geometries is made more effective and faster using kriging-simulated annealing meta-algorithm.

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