Sheng-Qi Zhou

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.

Measurement of the specific heat capacity of water-based Al2O3 nanofluid

In this letter, we have presented an experimental investigation of the specific heat cp of water-based Al2O3 nanofluid with a differential scanning calorimeter. The result indicates that the specific heat cp of nanofluid decreases gradually as the nanoparticle volume fraction ϕ increases from 0.0% to 21.7%. The relationship between them exhibits good agreement with the prediction from the thermal equilibrium model [Eq. (2)]. The other simple mixing model [Eq. (1)] fails to predict the specific heat cp of nanofluid.

Flow Reversals in Thermally Driven Turbulence

We analyze the reversals of the large-scale flow in Rayleigh-Bénard convection both through particle image velocimetry flow visualization and direct numerical simulations of the underlying Boussinesq equations in a (quasi-) two-dimensional, rectangular geometry of aspect ratio 1. For medium Prandtl number there is a diagonal large-scale convection roll and two smaller secondary rolls in the two remaining corners diagonally opposing each other. These corner-flow rolls play a crucial role for the large-scale wind reversal: They grow in kinetic energy and thus also in size thanks to plume detachments from the boundary layers up to the time that they take over the main, large-scale diagonal flow, thus leading to reversal. The Rayleigh vs Prandtl number space is mapped out. The occurrence of reversals sensitively depends on these parameters.

Origin of the temperature oscillation in turbulent thermal convection.

We report an experimental study of the three-dimensional spatial structure of the low-frequency temperature oscillations in a cylindrical Rayleigh-Bénard convection cell. Through simultaneous multipoint temperature measurements it is found that, contrary to the popular scenario, thermal plumes are emitted neither periodically nor alternately, but randomly and continuously, from the top and bottom plates. We further identify a new flow mode-the sloshing mode of the large-scale circulation (LSC). This sloshing mode, together with the torsional mode of the LSC, are found to be the origin of the oscillation of the temperature field.

Effects of shear rate and temperature on viscosity of alumina polyalphaolefins nanofluids

In this paper, the shear rate and temperature dependencies of viscosity of alumina nanofluids have been investigated experimentally. The alumina nanofluids are suspensions of alumina nanospheres or nanorods in polyalphaolefins (PAO) lubricant. The base fluid PAO has a Newtonian behavior. To the first approximation, nanofluids of volume fractions phi = 1% and 3% nanospheres as well as nanofluid of phi = 1% nanorods can be considered as Newtonian fluids because their viscosity shows very weak shear rate dependence. However, our measurement clearly indicates that these nanofluids demonstrate certain non-Newtonian feature due to the addition of nanoparticles. Moreover, the relative viscosity (the ratio of viscosity of nanofluid to that of PAO) of these nanofluids has been measured to be independent of temperature. Nanofluid of a higher volume fraction phi = 3% nanorods has an apparent non-Newtonian shear thinning viscosity and a strong temperature dependence of its relative viscosity. By reviewing the previous studies, the approximation that the viscosity is Newtonian and the relative viscosity is independent of temperature seem to hold for most nanofluids of low volume fraction phi and low aspect ratio nanoparticles

An experimental investigation of turbulent thermal convection in water-based alumina nanofluid

We report heat transfer and flow dynamics measurements of alumina nanofluid in turbulent convective flow. Under the condition of fixed temperature at the top plate and fixed input heat flux at the bottom plate, it has been found that the convective heat transfer coefficient, h , Nusselt number, N u , and Rayleigh number, R a , all decrease with the increasing volume fraction ϕ of the nanoparticle. In contrast, the velocity of the convective flow showed no significant change within experimental uncertainty and over the range of nanoparticle concentration of the measurement (from 0% to 1.08%). Under the condition of constant nanoparticle concentration ( ϕ = 1.08 % ) , a second set of measurements of the heat transport and flowproperties have been made over a broad range of R a (from 2.6 × 10 8 to 7.7 × 10 9 ). For heat transport, a transition near R a c ≃ 2.5 × 10 9 has been found. For R a > R a c , the measured N u of the nanofluid is roughly the same as that of water in terms of both its magnitude and its scaling relation with R a , which suggests that the nanofluid can be treated as a single phase fluid in this parameter range. For R a R a c , N u becomes smaller than that of the water and the deviation becomes larger with decreasing R a . In the parameter range of R a R a c , the measured instantaneous N u ( t ) shows strong and quasiperiodic fluctuations, which is absent when R a > R a c . This suggests that the significant decrease of the nanofluid N u comparing to that of water may be caused by the mass diffusion of nanoparticles. Furthermore, measurements of the flow velocity of the bulk nanofluid showed no significant difference from that of water for R a either above or below R a c . From estimated thermal boundary layer thickness, we found that the deviations of the nanofluid N u from that of water for R a R a c corresponds to the thickening of the thermal boundary layer at both the top and bottom plates. This thickening of the boundary layer at low input heat flux (or low driving strength of the convective flow) cannot be attributed to possible sedimentation of the nanoparticles.

Measured oscillations of the velocity and temperature fields in turbulent Rayleigh-Bénard convection in a rectangular cell

Temperature and velocity oscillations have been found in a rectangular Rayleigh-Bénard convection cell, in which one large-scale convection roll exists. At Rayleigh number Ra=8.9x10(11) and Prandtl number Pr=4, temperature oscillations can be observed in most parts of the system and the oscillation period remains almost constant, tT=74+/-2 s. Velocity oscillation can only be found in its horizontal component vy (perpendicular to the large-scale circulation plane) near the cell sidewall, its oscillation period is also constant, tv=65+/-2 s, at these positions. Temperature and velocity oscillations have different Ra dependences, which are, respectively, indicated by the Péclect number PeT=0.55Ra0.47 and Pev=0.28Ra0.50. In comparison to the case of a cylindrical cell, we find that velocity oscillations are affected by the system geometry.

Heat conduction in alloy-based superlattices

This work reports experimental studies of the thermal conductivity of alloy-based superlattices. Cross-plane thermal conductivity of a Si/Si/sub 0.71/Ge/sub 0.29/ (50 /spl Aring//10 /spl Aring/) superlattice is measured based on the 3/spl omega/ method. The measured thermal conductivity of this superlattice is 2-3 times smaller than that calculated from the Fourier heat conduction. This reduction in thermal conductivity is smaller than those observed in pure Si/Ge superlattices, possibly due to the smaller mismatch of the material properties between Si and Si/sub 0.71/Ge/sub 0.29/ as compared to between Si and Ge. To extend the 3/spl omega/ method for measuring both the cross-plane and the in-plane thermal conductivity of superlattices, a 2-wire 3/spl omega/ method is developed. Preliminary experimental results are reported for a AlAs/Al/sub 0.62/Ga/sub 0.38/As (455 /spl Aring//410 /spl Aring/) thick layer superlattice based on this method.

Laboratory simulation of the influence of geothermal heating on the interior ocean

This study, using laboratory experiments and scaling analysis, evaluates the influence of geothermal heating on global oceanic circulation. Upon a well-developed large-scale convective flow, an additional heat flux perturbation δF/F is employed. The increments of flow and thermal properties, including eddy diffusivity KT, flow velocity V and bottom temperature Tb, are found to be independent of the applied heat flux F. Together with the scaling analysis of convective flow at different configurations, where the flow is thermally driven in the relatively low or extremely high turbulent thermal convections or the horizontal convection, the variances of flow properties, δKT/KT and δV/V, are found to be close to 0.5% and 0.75% at δF/F=2%. This means that the small heat flux perturbation plays a negligible role in the global convective flow. However, δTb/ΔT is found to be 1.5% at δF/F=2%, which would have a significant effect in the local region. The results might provide a clue to understanding the influence of geothermal heating on global oceanic circulation. It is expected that geothermal heating will contribute less than 1% in turbulent mixing and volume flux to global oceanic circulation, so its influence can be negligible in this situation. However, when it comes to the local environment, the influence of geothermal heating cannot be ignored. For example, temperature increases of about 0.5°C with geothermal heating would have a significant effect on the physical environments within the benthic boundary layer.

Statistics and Scaling of the Velocity Field in Turbulent Thermal Convection

The statistics and scaling properties of the velocity field in turbulent Rayleigh-Benard convection in water has been measured using both laser Doppler velocimetry (LDV) and particle image velocimetry (PIV) techniques. It is found that results from both techniques for the mean velocity and all the statistical quantities examined agree with each other. The measurements reveal that the pdfs for the velocity are non-Gaussian in the cell center but more close to Gaussian near the cell boundaries. In addition, the Reynolds shear stress is found to have different signs near the sidewall and near the plates of the cell, suggesting that different mechanisms are responsible for driving the mean flow at different locations of the cell. Moreover, our results confirm a prediction of a recent model by Grossmann & Lohse, in which flow geometries are classified according to the shape of the container.