Thermal conductivity and rheology behavior of aqueous nanofluids containing alumina and carbon nanotubes

This paper was presented at the 4th Micro and Nano Flows Conference (MNF2014), which was held at University College, London, UK. The conference was organised by Brunel University and supported by the Italian Union of Thermofluiddynamics, IPEM, the Process Intensification Network, the Institution of Mechanical Engineers, the Heat Transfer Society, HEXAG - the Heat Exchange Action Group, and the Energy Institute, ASME Press, LCN London Centre for Nanotechnology, UCL University College London, UCL Engineering, the International NanoScience Community, www.nanopaprika.eu.In this study, thermal conductivity and rheology behavior of aqueous alumina and multi-walled carbon nanotube (MWCNT) nanofluids were measured and compared with several analytical models. Both thermal conductivity and viscosity of the two nanofluids increase with increasing volume fraction. The experimental thermal conductivity data for the two nanofluids are located near the lower Hashin-Shtrikman bound and far away from the upper Hashin-Shtrikman bound. Therefore there is still enough room for thermal conductivity enhancement. Further conductivity enhancement of the nanofluids can be achieved by manipulating particle or agglomeration distribution and morphology. The structure-property relationship was checked for the nanofluids. Possible agglomeration size and interfacial thermal resistance were obtained and partially validated. Based on the Chen et al. model, a revised model was developed by incorporating the effects of interfacial thermal resistance into the Hamilton-Crosser model. The revised model can accurately reproduce the experimental data based on the agglomeration size extracted from the rheology analysis. In addition, thermal conductivity change of the alumina/water nanofluid with elapsed time was also investigated. The average thermal conductivity decreases with elapsed time. Besides, thermal conductivity measurements were conducted for nanofluid mixtures of alumina/water and MWCNT/water nanofluids

Anomalous size dependent rheological behavior of alumina based nanofluids

This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.Rheological behaviour of Alumina (Al2O3) based nanofluids (NFs) has been studied and found to be exhibit unexpected behaviour. Two base-fluids viz, water and ethylene glycols (EG). Three particle sizes (11, 45 and 150 nm), varying over an order of magnitude, were used to analyze the effect of particle size. The experimental data has shown typical Newtonian behavior for both W based and EG based alumina NFs The viscosity of EG based NFs is found to be anomalously reduced compared to the base fluid. This reduction in viscosity may be due to hygroscopic nature of EG or due to the presence of water in as-received high concentration sample also, as told by some researchers. However, this phenomenon was absent for water based NFs. The inter-related effects of particle size, concentration and mode of dispersion (mono or poly-dispersed) were investigated. To eliminate the effect of size variation, mono dispersed NFs are obtained by centrifuging and re-suspension of parent NFs. Particle migration under shear is attributed to the reduction of viscosity. The increase in bulk viscosity with particle size reduction is attributed to the surface forces acting between the particles and the medium in a suspension

Round-robin test on thermal conductivity measurement of ZnO nanofluids and comparison of experimental results with theoretical bounds

Ethylene glycol (EG)-based zinc oxide (ZnO) nanofluids containing no surfactant have been manufactured by one-step pulsed wire evaporation (PWE) method. Round-robin tests on thermal conductivity measurements of three samples of EG-based ZnO nanofluids have been conducted by five participating labs, four using accurate measurement apparatuses developed in house and one using a commercial device. The results have been compared with several theoretical bounds on the effective thermal conductivity of heterogeneous systems. This study convincingly demonstrates that the large enhancements in the thermal conductivities of EG-based ZnO nanofluids tested are beyond the lower and upper bounds calculated using the models of the Maxwell and Nan et al. with and without the interfacial thermal resistance

Modeling transient absorption and thermal conductivity in a simple nanofluid

Molecular dynamics simulations are used to simulate the thermal properties of a model fluid containing nanoparticles (nanofluid). By modelling transient absorption experiments, we show that they provide a reliable determination of interfacial resistance between the particle and the fluid. The flexibility of molecular simulation allows us to consider separately the effect of confinement, particle mass and Brownian motion on the thermal transfer between fluid and particle. Finally, we show that in the absence of collective effects, the heat conductivity of the nanofluid is well described by the classical Maxwell Garnet equation model

Experimental and theoretical studies of nanofluid thermal conductivity enhancement: a review

Nanofluids, i.e., well-dispersed (metallic) nanoparticles at low- volume fractions in liquids, may enhance the mixture's thermal conductivity, knf, over the base-fluid values. Thus, they are potentially useful for advanced cooling of micro-systems. Focusing mainly on dilute suspensions of well-dispersed spherical nanoparticles in water or ethylene glycol, recent experimental observations, associated measurement techniques, and new theories as well as useful correlations have been reviewed

Effective Thermal Conductivity of Carbon Nanotube-Based Cryogenic Nanofluids

Nanofluids consist of nanometer-sized particles or fibers in colloidal suspension within a host fluid. They have been studied extensively since their creation due to their often times anomalous and unique thermal transport characteristics. They have also proven to be quite valuable in terms of the scientific knowledge gained from their study and their nearly unlimited industrial and commercial applications. This research has expanded the science of nanofluids into a previously unexplored field, that of cryogenic nanofluids. Cryogenic nanofluids are similar to traditional nanofluids in that they utilize nanometer-sized inclusion particles; however, they use cryogenic fluids as their host liquids. Cryogenic nanofluids are of great interest due to the fact that they combine the extreme temperatures inherent to cryogenics with the customizable thermal transport properties of nanofluids, thus creating the potential for next generation cryogenic fluids with enhanced thermophysical properties. This research demonstrates that by combining liquid oxygen (LOX) with Multi-Walled Carbon Nanotube (MWCNT) inclusion particles, effective thermal conductivity enhancements of greater than 30% are possible with nanoparticle volume fractions below 0.1%. Three distinct cryogenic nanofluids were created for the purposes of this research, each of which varied by inclusion particle type. The MWCNT\u27s used in this research varied in a number of physical characteristics, the most obvious of which are length and diameter. Lengths vary from 0.5 to 90 microns and diameters from 8 to 40 nanometers. The effective thermal conductivity of the various cryogenic nanofluids created for this research were experimentally determined by a custom made Transient Hot Wire (THW) system, and compared to each other and to more traditional nanofluids as they vary by type and particle volume fraction. This work also details the extensive theoretical, experimental, and numerical aspects of this research, including a rather detailed literature review of many of the salient sciences involved in the study of cryogenic nanofluids. Finally, a selection of the leading theories, models, and predictive equations is presented along with a review of some of the potential future work in the newly budding field of cryogenic nanofluids

Evaluation Of Thermophysical Properties, Friction Factor And Heat Transfer Of Alumina Nanofluid Flow In Tubes

Various thermophysical properties, fluid flow parameter and heat transfer characteristics were measured for nanofluid with 6% volume concentration of solid Al2O3 nanoparticles in water. Thermal conductivity measurements showed that there is a definite enhancement in thermal conductivity of the nanofluid compared to that of water. At 7°C, the enhancement was 16% which decreased to 6.96% at 50°C. The viscosity measurements of the 6% volume concentration Al2O3/water nanofluid showed that its viscosity is higher by a factor of 1.25 to 10.24 than the viscosity of water. Also the measurements of the viscosity of different volume concentration of Al2O3/water nanofluid showed that, the viscosity decreases as the volume concentration decreases. The plot between the shear stress and strain rate for the 6% volume concentration Al2O3/water nanofluid showed that it is a Newtonian fluid for the range of strain rate between 6-122 s−1. Several readings of viscosity were taken by subjecting the nanofluid to heating and cooling cycle. It was found that above 62.65°C, the 6% volume concentration Al2O3/water nanofluid experiences an irrecoverable increase in viscosity and when cooled from beyond this temperature, a hysteresis effect on the viscosity is seen. The friction factor results for laminar flow for the 6% volume concentration Al2O3/water nanofluid showed that it matches the value given by the Hagen-Poiseulle equation (f = 64/Re). The transition from laminar flow to turbulent was found to occur at a Reynolds number of approximately 1500. The convective heat transfer results were in agreement with that proposed by the Lienhard correlation (Lienhard and Lienhard, 2008). For fully developed laminar flow, the Nusselt number under constant heat flux condition was found to be within ±7% of 4.36. In the laminar flow regime, the Nusselt numbers for thermally developing flow were within ±10% of the value calculated from the Lienhard correlation

Heat transfer properties of aqueous carbon nanotubes nanofluids in coaxial heat exchanger under laminar regime

International audienceThe thermal performance of water-based multi-wall carbon nanotubes nanofluids are measured in a coaxial heat exchanger under laminar regime within the range of Reynolds numbers 500-2500. The convective heat transfer properties with constant wall temperature are evaluated for four different multi-wall carbon nanotubes based nanofluids at low concentration of 0.05% in weight (0.026% in volume). The measurements of thermal and rheological properties of the nanofluids with operating temperature were investigated experimentally. The effects of the aspect ratio of carbon nanotubes, the type of base fluid and surfactant on viscosity, thermal conductivity and laminar convective heat transfer were studied. Based on the experimental results, we reported the shear-thinning behaviour of nanofluids, the nanofluid viscosity being dependant on the base fluid type in the Newtonian region. We also showed that the enhancement of the thermal conductivity and the average convective heat transfer of nanofluids increased with the aspect ratio of nanotubes and decreased when the thermal conductivity of the base fluid increases. This enhancement attains at least 10% in comparison to base fluid even with the low content of nanotubes used