Haisheng Chen

A benchmark study on the thermal conductivity of nanofluids

This article reports on the International Nanofluid Property Benchmark Exercise, or INPBE, in which the thermal conductivity of identical samples of colloidally stable dispersions of nanoparticles or “nanofluids,” was measured by over 30 organizations worldwide, using a variety of experimental approaches, including the transient hot wire method, steady-state methods, and optical methods. The nanofluids tested in the exercise were comprised of aqueous and nonaqueous basefluids, metal and metal oxide particles, near-spherical and elongated particles, at low and high particle concentrations. The data analysis reveals that the data from most organizations lie within a relatively narrow band (±10% or less) about the sample average with only few outliers. The thermal conductivity of the nanofluids was found to increase with particle concentration and aspect ratio, as expected from classical theory. There are (small) systematic differences in the absolute values of the nanofluid thermal conductivity among the various experimental approaches; however, such differences tend to disappear when the data are normalized to the measured thermal conductivity of the basefluid. The effective medium theory developed for dispersed particles by Maxwell in 1881 and recently generalized by Nan et al. [J. Appl. Phys. 81, 6692 (1997)], was found to be in good agreement with the experimental data, suggesting that no anomalous enhancement of thermal conductivity was achieved in the nanofluids tested in this exercise.

Rheological behaviour of nanofluids

This work aims at a more fundamental understanding of the rheological behaviour of nanofluids and the interpretation of the discrepancy in the recent literature. Both experiments and theoretical analyses are carried out with the experimental work on ethylene glycol (EG)-based nanofluids containing 0.5–8.0 wt% spherical TiO2 nanoparticles at 20–60 °C and the theoretical analyses on the high shear viscosity, shear thinning behaviour and temperature dependence. The experimental results show that the EG-based nanofluids are Newtonian under the conditions of this work with the shear viscosity as a strong function of temperature and particle concentration. The relative viscosity of the nanofluids is, however, independent of temperature. The theoretical analyses show that the high shear viscosity of nanofluids can be predicted by the Krieger–Dougherty equation if the effective nanoparticle concentration is used. For spherical nanoparticles, an aggregate size of approximately 3 times the primary nanoparticle size gives the best prediction of experimental data of both this work and those from the literature. The shear thinning behaviour of nanofluids depends on the effective particle concentration, the range of shear rate and viscosity of the base liquid. Such non-Newtonian behaviour can be characterized by a characteristic shear rate, which decreases with increasing volume fraction, increasing base liquid viscosity, or increasing aggregate size. These findings explain the reported controversy of the rheological behaviour of nanofluids in the literature. At temperatures not very far from the ambient temperature, the relative high shear viscosity is independent of temperature due to negligible Brownian diffusion in comparison to convection in high shear flows, in agreement with the experimental results. However, the characteristic shear rate can have strong temperature dependence, thus affecting the shear thinning behaviour. The theoretical analyses also lead to a classification of nanofluids into dilute, semi-dilute, semi-concentrated and concentrated nanofluids depending on particle concentration and particle structuring.

Hydrogen production by sorption-enhanced steam reforming of glycerol

Catalytic steam reforming of glycerol for H(2) production has been evaluated experimentally in a continuous flow fixed-bed reactor. The experiments were carried out under atmospheric pressure within a temperature range of 400-700 degrees C. A commercial Ni-based catalyst and a dolomite sorbent were used for the steam reforming reactions and in situ CO(2) removal. The product gases were measured by on-line gas analysers. The results show that H(2) productivity is greatly increased with increasing temperature and the formation of methane by-product becomes negligible above 500 degrees C. The results suggest an optimal temperature of approximately 500 degrees C for the glycerol steam reforming with in situ CO(2) removal using calcined dolomite as the sorbent, at which the CO(2) breakthrough time is longest and the H(2) purity is highest. The shrinking core model and the 1D-diffusion model describe well the CO(2) removal under the conditions of this work.

Design of Hydrophobic Polyoxometalate Hybrid Assemblies Beyond Surfactant Encapsulation

Grafting of C-6, C-16 and C-18 alkyl chains onto the hydrophilic Mn-Anderson clusters (compounds 2-4) has been achieved. Exchange of the tetrabutyl ammonium (TBA) with dimethyldioctadecyl ammonium (DMDOA) results in the formation of new polyoxometalate (POM) assemblies (compounds 5-6), in which the POM cores are covalently functionalized by hydrophilic alkyl-chains and enclosed by surfactant of DMDOABr. As a result, we have been able to design and synthesize POM-containing hydrophobic materials beyond surfactant encapsulation. In solid state, scanning electron and transmission electron microscopy (SEM and TEM) studies of the TBA salts of compounds 3 and 4 show highly ordered, uniform, reproducible assemblies with unique segmented rodlike morphology. SEM and TEM studies of the DMDOA salts of compounds 5 and 6 show that they form spherical and sea urchin 3D objects in different solvent systems. In solution, the physical properties of compound 5 and 6 (combination of surfactant-encapsulated cluster (SEC) and surface-grafted cluster (SGC)) show a liquid-to-gel phase transition in pure chloroform below 0 degrees C, which are much lower than other reported SECs. By utilizing light scattering measurements, the nanoparticle size for compounds 5 and 6 were measured at 5 degrees C and 30 degrees C, respectively. Other physical properties including differential scanning calorimetry have been reported.

Forced convective heat transfer of nanofluids

Forced convective heat transfer is experimentally investigated using aqueous and ethylene glycol-based spherical titania nanofluids, and aqueous-based titanate nanotubes, carbon nanotubes and nano-diamond nanofluids. These nanofluids are formulated from dry nanoparticles and pure base liquids to eliminate complications due to unknown solution chemistry. All the formulated nanofluids show a higher effective thermal conductivity than that predicted by the conventional theories. Except for the ethylene glycol-based titania nanofluids, all other nanofluids are found to be non-Newtonian. For aqueous-based titania and carbon and titanate nanotube nanofluids, the convective heat transfer coefficient enhancement exceeds, by a large margin, the extent of the thermal conduction enhancement. However, deterioration of the convective heat transfer is observed for ethylene glycol-based titania nanofluids at low Reynolds numbers and aqueous-based nano-diamond nanofluids. Possible mechanisms for the observed controversy are discussed from both microscopic and macroscopic viewpoints. The competing effects of particle migration on the thermal boundary layer thickness and that on the effective thermal conductivity are suggested to be responsible for the experimental observations.

Thermogravimetric kinetics of crude glycerol

The pyrolysis of the crude glycerol from a biodiesel production plant was investigated by thermogravimetry coupled with Fourier transform infrared spectroscopy. The main gaseous products are discussed, and the thermogravimetric kinetics derived. There were four distinct phases in the pyrolysis process of the crude glycerol. The presence of water and methanol in the crude glycerol and responsible for the first decomposition phase, were shown to catalyse glycerol decomposition (second phase). Unlike the pure compound, crude glycerol decomposition below 500 K leaves behind a large mass fraction of pyrolysis residues (ca. 15%), which eventually partially eliminate in two phases upon reaching significantly higher temperatures (700 and 970 K, respectively). An improved iterative Coats-Redfern method was used to evaluate non-isothermal kinetic parameters in each phase. The latter were then utilised to model the decomposition behaviour in non-isothermal conditions. The power law model (first order) predicted accurately the main (second) and third phases in the pyrolysis of the crude glycerol. Differences of 10-30 kJ/mol in activation energies between crude and pure glycerol in their main decomposition phase corroborated the catalytic effect of water and methanol in the crude pyrolysis. The 3-D diffusion model more accurately reproduced the fourth (last) phase, whereas the short initial decomposition phase was poorly simulated despite correlation coefficients ca. 0.95-0.96. The kinetics of the 3rd and 4th decomposition phases, attributed to fatty acid methyl esters cracking and pyrolysis tarry residues, were sensitive to the heating rate.

Steam reforming of crude glycerol with in situ CO2 sorption

Steam reforming of the crude glycerol by-product of a biodiesel production plant has been evaluated experimentally at atmospheric pressure, with and without in situ CO(2) sorption, in a continuous flow fixed-bed reactor between 400 degrees C and 700 degrees C. The process outputs were compared to those using pure glycerol. Thermodynamic equilibrium calculations were used to assess the effect on the steam reforming process of the main crude impurities (methanol and four fatty acid methyl esters). The crude glycerol and steam conversions and the H(2) purity reached 100%, 11% and 68%, respectively at 600 degrees C. No CH(4) was found at and above 600 degrees C. Steam reforming of crude glycerol with in situ CO(2) removal is shown to be an effective means of achieving hydrogen purity above 88% in pre-CO(2) breakthrough conditions.

Heat Transfer and Rheological Behaviour of Nanofluids – A Review

Nanofluids refer to dilute liquid suspensions of nanoparticles. Over the past decade, such materials generated lots of excitement mainly because a number of researchers reported drastic thermal conductivity enhancement with very small particle loadings. This also sparked hot debates on the underlying physics governing the experimentally observed phenomena. This paper gives an updated review on the topic. It is not intended to be exhaustive but meant to cover the main aspects associated with nanofluids with a specific focus on heat transfer applications. The review covers transport properties of nanofluids in particular thermal conductivity and shear viscosity, and heat transfer of nanofluids under convective and boiling conditions. No new physics appears to be behind the experimentally observed thermal conductivity enhancement as the vast majority of the experimental data fall within the range predicted by the conventional effective medium theory in combination with information of nanoparticle structuring. There seems to be no new physics either in terms of the experimentally observed increase in the shear viscosity of nanofluids as almost all the experimental data can be quantitatively interpreted by the conventional rheological and colloidal theories. There is no sufficient quantitative information, however, to infer the dominant mechanisms for heat transfer enhancement under convective and boiling conditions, where many controversies remain and require further research.

Relationship between the thermal conductivity and shear viscosity of nanofluids

Nanofluids are dilute liquid suspensions of nanoparticles. Nanoparticles and liquid media, in such fluids, mix and interact at the nanoscale. Interactions between nanoparticles in nanofluids can lead to structuring of the particles. This paper discusses how the nanoparticle structuring affects the thermal conductivity and viscosity of nanofluids and how the two transport properties are related through the nanoparticle structuring. It is shown that the experimentally measured thermal conductivity enhancement and the viscosity increase due to the presence of nanoparticles can be interpreted by the aggregation of nanoparticles. It is also shown that modification of the conventional form of the effective medium theory by taking into account nanoparticle structuring information from the rheological analyses gives good agreement with experimentally measured thermal conductivity.

Stability of nanofluids in quiescent and shear flow fields

An experimental study was conducted to investigate the structural stability of ethylene glycol-based titanium dioxide nanoparticle suspensions (nanofluids) prepared by two-step method. The effects of particle concentration, fluid temperature, shear rate and shear duration were examined. Particle size and thermal conductivity measurements in quiescent state indicated the existence of aggregates and that they were stable in temperatures up to 60°C. Shear stability tests suggested that the structure of nanoparticle aggregates was stable in a shear interval of 500-3000 s-1 measured over a temperature range of 20-60°C. These findings show directions to resolve controversies surrounding the underlying mechanisms of thermal conduction and convective heat transfer of nanofluids.