Heat Transfer Enhancement in Nanofluid Suspensions

2004 ◽  
Author(s):  
Peter Vadasz ◽  
Johnathan J. Vadasz ◽  
Saneshan Govender
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Scale Effects ◽  
Nano Scale ◽  
Sufficient Detail ◽  
Macro Scale ◽  
Wave Effects ◽  
Conduction Theory ◽  
Wave Phenomena ◽  
Liquid Layering

The investigation into possible applications of the thermal wave conduction theory to explain the spectacular enhancement of heat flux by a factor of between 1.4 to 2.5 in nanofluid suspensions is presented. While other possible explanations have been proposed to settle this discrepancy they were not investigated into sufficient detail for providing a definite answer and they all apply at the nano-scale level rather than bridging between the nano-scale effects and the macro-system investigated. The possible mechanisms proposed so far are Brownian motion, liquid layering at the liquid/particle interface, ballistic phonon effects, nanoparticle clustering as well as convection and wave effects. Furthermore, most available methods for measuring thermal conductivity assume and make use explicitly of the Fourier mechanism of heat transfer. If somehow the nano-level heat transfer effects impact profoundly on the resulting heat flux at the macro-level, possibly via wave phenomena, the whole concept behind the measurement device might be flawed. The present paper presents a possible way by which the transitions from nano-scale via the micro-scales towards the macro-scale occur, hence bridging the gap from nano devices to macro systems performance.

The Effect of Thermal Waves on Heat Transfer Enhancement in Nanofluid Suspensions

2008 ◽  
Author(s):  
Johnathan J. Vadasz
Keyword(s):  
Heat Transfer ◽  
Thermal Conductivity ◽  
Heat Conduction ◽  
Heat Transfer Enhancement ◽  
Hyperbolic Heat Conduction ◽  
Thermal Waves ◽  
Transfer Enhancement ◽  
Scale Level ◽  
Macro Scale ◽  
Wave Effects

The spectacular heat transfer enhancement revealed experimentally in nanofluids suspensions is being investigated theoretically at the macro-scale level aiming at explaining the possible mechanisms that lead to such impressive experimental results. In particular, the anticipation that thermal wave effects via hyperbolic heat conduction could have been the source of the excessively improved effective thermal conductivity of the suspension is shown to be impossible.

Analysis of Integral Transform Solutions for Thermally Developing Flow in Microchannels With Isothermal Wall

2011 ◽  
Author(s):  
D. J. N. M. Chalhub ◽  
L. A. Sphaier
Keyword(s):  
Heat Transfer ◽  
Mass Transfer ◽  
Heat And Mass Transfer ◽  
Flow Problem ◽  
Integral Transform ◽  
Scale Effects ◽  
Experimental Studies ◽  
Macro Scale ◽  
Optimum Solution ◽  
Solution Methodology

There is a growing interest for applications of heat and mass transfer in microchannels. Consequently, several numerical and experimental studies related to transport phenomena in microchannels have been carried-out. The flow problem in microchannels is different from the macro-scale problems due to rarefaction effects, surface roughness, viscous dissipation heating as well as other effects. As a result, a number of studies have been proposed for investigating the micro-flow problem and how each of these phenomena affect heat and mass transfer characteristics. Naturally, there is particular focus on how the observed micro-scale phenomena differ from the traditionally known macro-scale effects. In the realm of simulation studies for heat transfer in micro-sized channels, this paper proposes a comparison between hybrid solution strategies for solving steady heat transfer problems within microchannels. The Generalized Integral Transform Technique (GITT) is employed as the main solution methodology; however, different solution approaches are investigated in order to determine advantages and drawbacks of each alternative. The presented results can serve as guidance for choosing an optimum solution methodology for thermally developing heat transfer in microchannels using GITT implementations.

High Heat Flux Absorption Utilizing Porous Materials With Two-Phase Heat Transfer

1997 ◽  
Vol 119 (3) ◽  
pp. 171-179 ◽  
Author(s):  
J. T. Dickey ◽  
G. P. Peterson
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Porous Materials ◽  
Metal Flow ◽  
High Heat ◽  
High Heat Flux ◽  
Two Phase ◽  
Macro Scale ◽  
Flow Through ◽  
Two Phase Heat Transfer

By combining two-phase heat transfer with forced convective flow through a porous material, a new heat transfer scheme emerges with the ability to absorb high heat fluxes without the corresponding temperature increase encountered in single-phase systems. In general, flow-through sintered metals are characterized by high thermal conductivity due to the metallic media, and a fluid flow which on the macro scale can be described as slug flow in nature. These same characteristics are exhibited by liquid metal flow cooling systems. To predict the heat transfer attributes of this two-phase flow process, a semi-analytical model was developed using the conservation equations of mass, momentum, and energy along with the apparent physical properties of the composite material. The results indicate that when a heat flux is applied to one side of the bounding surface and adiabatic conditions exist on the remaining sides, the surface temperature asymptotically approaches the same value regardless of the mass flow rate. In addition to the analytical results, definitions for the convection coefficient and Nusselt number for flow-through porous materials with phase change are presented.

Exploration of Experimental Techniques to Determine the Condensation Heat Flux in Microchannels and Minichannels

2010 ◽  
Author(s):  
Melanie M. Derby ◽  
Hee Joon Lee ◽  
Rose C. Craft ◽  
Gregory J. Michna ◽  
Yoav Peles ◽  
...  
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Condensation Heat Transfer ◽  
Experimental Techniques ◽  
Heat Flux Sensor ◽  
Transfer Coefficients ◽  
Thermoelectric Coolers ◽  
Heat Transfer Coefficients ◽  
Macro Scale ◽  
Flux Sensor

This study seeks to analyze and explore experimental methods to study condensation heat transfer in micro- and mini-channel. Following, an experimental setup was built and initial results are presented. Several experimental techniques were reviewed, while two, thermoelectric coolers and a copper-heat-flux-sensor were analyzed in detail for condensation heat flux. It was concluded that thermoelectric coolers were not suitable as heat flux sensors for single-phase validation, but the copper-heat-flux-sensor was appropriate to measure heat transfer coefficients at the mini-scale. Condensation heat transfer coefficients were obtained experimentally in seven parallel square mini-channels of diameter 1mm. Existing condensation correlations were applied to these data; micro- and mini-scale correlations captured the appropriate trends, but the macro-scale Shah (1979) correlation fit the data best.

scholarly journals Macro-scale conjugate heat transfer in periodically developed flow through solid structures

2016 ◽  
Vol 804 ◽  
pp. 298-322 ◽  
Author(s):  
G. Buckinx ◽  
M. Baelmans
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Conjugate Heat Transfer ◽  
Weighting Function ◽  
Interfacial Heat Transfer Coefficient ◽  
Spatial Moments ◽  
Volume Average ◽  
Scale Temperature ◽  
Macro Scale ◽  
Spatially Periodic

This paper treats the macro-scale description of the periodically developed conjugate heat transfer regime, in which heat transfer takes place between an incompressible viscous flow and spatially periodic solid structures through a spatially periodic interfacial heat flux. The macro-scale temperature of the fluid and the solid structures are defined through a spatial averaging operator with a specific weighting function. It is shown that a double volume average is necessary in order to have a linearly changing macro-scale temperature in response to a constant macro-scale heat flux. Furthermore, with the aid of a double volume average, the thermal dispersion source, the thermal tortuosity and the interfacial heat transfer coefficient all become spatially constant in the developed regime. That way, these closure terms of the macro-scale temperature equations can be exactly determined from the periodic temperature part on a unit cell of the solid structures without taking the spatial moments of the solid into account. The theoretical derivations of this paper are illustrated for a case study describing the heat transfer between a fluid flow and an array of solid squares with a uniform volumetric heat source.

THE MECHANISM OF RAISING AND QUANTIFICATION OF SPECIFIC HEAT FLUX AT BOILING OF NANOFLUIDS IN FREE CONVECTION CONDITIONS

2017 ◽  
pp. 25-34
Author(s):  
V.N. Moraru
Keyword(s):  
Heat Transfer ◽  
Heat Flux ◽  
Heat Capacity ◽  
Specific Heat ◽  
Chemical Properties ◽  
Heating Surface ◽  
Physical Models ◽  
Superheated Liquid ◽  
Two Phase ◽  
Boiling Process

The results of our work and a number of foreign studies indicate that the sharp increase in the heat transfer parameters (specific heat flux q and heat transfer coefficient _) at the boiling of nanofluids as compared to the base liquid (water) is due not only and not so much to the increase of the thermal conductivity of the nanofluids, but an intensification of the boiling process caused by a change in the state of the heating surface, its topological and chemical properties (porosity, roughness, wettability). The latter leads to a change in the internal characteristics of the boiling process and the average temperature of the superheated liquid layer. This circumstance makes it possible, on the basis of physical models of the liquids boiling and taking into account the parameters of the surface state (temperature, pressure) and properties of the coolant (the density and heat capacity of the liquid, the specific heat of vaporization and the heat capacity of the vapor), and also the internal characteristics of the boiling of liquids, to calculate the value of specific heat flux q. In this paper, the difference in the mechanisms of heat transfer during the boiling of single-phase (water) and two-phase nanofluids has been studied and a quantitative estimate of the q values for the boiling of the nanofluid is carried out based on the internal characteristics of the boiling process. The satisfactory agreement of the calculated values with the experimental data is a confirmation that the key factor in the growth of the heat transfer intensity at the boiling of nanofluids is indeed a change in the nature and microrelief of the heating surface. Bibl. 20, Fig. 9, Tab. 2.

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