José M. Ortiz de Zárate

Thermal conductivity of carbon nanotubes and graphene in epoxy nanofluids and nanocomposites

AbstractWe employed an easy and direct method to measure the thermal conductivity of epoxy in the liquid (nanofluid) and solid (nanocomposite) states using both rodlike and platelet-like carbon-based nanostructures. Comparing the experimental results with the theoretical model, an anomalous enhancement was obtained with multiwall carbon nanotubes, probably due to their layered structure and lowest surface resistance. Puzzling results for functionalized graphene sheet nanocomposites suggest that phonon coupling of the vibrational modes of the graphene and of the polymeric matrix plays a dominant role on the thermal conductivities of the liquid and solid states. PACS: 74.25.fc; 81.05.Qk; 81.07.Pr.

Measurement of the thermal conductivity of nanofluids by the multicurrent hot-wire method

We present experimental results of the thermal conductivity of several nanofluids prepared by dispersing nanoparticles of SiO2 and CuO in water and ethylene glycol at various concentrations up to ≈5% in mass fraction. The measurements have been performed by the multicurrent hot-wire technique. Good agreement, within 2%, is found in recommended and published thermal conductivities of the pure fluids. Our experimental technique allows a very accurate determination of the enhancement in the thermal conductivity of the fluids due to the presence of dispersed nanoparticles. Measured enhancements compare well with some of the values published so far in the literature. We have compared our results with simple theoretical models that predict the thermal conductivity of solid suspensions and found that in some cases observed enhancements are several times larger than the predicted ones.

On the Physical Origin of Long-Ranged Fluctuations in Fluids in Thermal Nonequilibrium States

Thermodynamic fluctuations in systems that are in nonequilibrium steady states are always spatially long ranged, in contrast to fluctuations in thermodynamic equilibrium. In the present paper we consider a fluid subjected to a stationary temperature gradient. Two different physical mechanisms have been identified by which the temperature gradient causes long-ranged fluctuations. One cause is the presence of couplings between fluctuating fields. Secondly, spatial variation of the strength of random forces, resulting from the local version of the fluctuation-dissipation theorem, has also been shown to generate long-ranged fluctuations. We evaluate the contributions to the long-ranged temperature fluctuations due to both mechanisms. While the inhomogeneously correlated Langevin noise does lead to long-ranged fluctuations, in practice, they turn out to be negligible as compared to nonequilibrium temperature fluctuations resulting from the coupling between temperature and velocity fluctuations.

Dynamics of fluctuations in a fluid below the onset of Rayleigh-Bénard convection.

We present experimental data and their theoretical interpretation for the decay rates of temperature fluctuations in a thin layer of a fluid heated from below and confined between parallel horizontal plates. The measurements were made with the mean temperature of the layer corresponding to the critical isochore of sulfur hexafluoride above but near the critical point where fluctuations are exceptionally strong. They cover a wide range of temperature gradients below the onset of Rayleigh-Bénard convection, and span wave numbers on both sides of the critical value for this onset. The decay rates were determined from experimental shadowgraph images of the fluctuations at several camera exposure times. We present a theoretical expression for an exposure-time-dependent structure factor which is needed for the data analysis. As the onset of convection is approached, the data reveal the critical slowing down associated with the bifurcation. Theoretical predictions for the decay rates as a function of the wave number and temperature gradient are presented and compared with the experimental data. Quantitative agreement is obtained if allowance is made for some uncertainty in the small spacing between the plates, and when an empirical estimate is employed for the influence of symmetric deviations from the Oberbeck-Boussinesq approximation which are to be expected in a fluid with its density at the mean temperature located on the critical isochore.

Steady states in membrane distillation: influence of membrane wetting

Non-isothermal transport of water through porous hydrophobic membranes is interpreted on the basis of a modified membrane distillation model. In previous papers, it was suggested that two opposing fluxes can take place through the partially wet membrane. The first is a vapour flux, from hot to cold through dry pores, and the second a liquid flux, from cold to hot through liquid-filled pores. We now present some results that confirm the above hypothesis. Temperature- and/or pressure-driven water transport through two PTFE membranes has been studied. A model is developed which characterizes the temperature- and pressure-driven steady states in accordance with the membrane distillation theory, explains the decrease of the fluxes and the steady-state pressure differences by progressive membrane wetting, and defines and evaluates a new parameter, the percentage of pores filled with liquid water, which describes that membrane wetting.

Separation of water and glycols by direct contact membrane distillation

Abstract In the present paper the feasibility of using direct contact membrane distillation for the concentration of glycol from used coolant liquids is analyzed. Membrane distillation experiments were performed with water–ethylene glycol mixtures, using a tangential flow cell, various membranes and different temperatures. The experiments show that the membranes reject almost completely the glycol and effective concentration is achieved. This result is expected, since the vapor pressure of glycol is negligible, compared to that of water, at the temperatures employed in the experiments. On the other hand, the effects of both temperature and concentration polarization were very important, and a specific computer code was developed in order to take them into account. From the computer code, values are obtained for the permeability of the membrane, and for the heat and mass transfer coefficients of the boundary layers. The values obtained agree with those of other literature sources and with the theoretical expectations.

Temperature polarization in non-isothermal mass transport through membranes

The water flux induced by transmembrane temperature gradients through five PTFE membranes has been studied. Two sets of experiments have been carried out. In the first set the temperature difference between the two bulk phases was fixed at 10 K while the mean temperature and the stirring rate were varied independently. The mean temperature values were varied between 30 and 55 °C and the stirring-rate values were varied between 0 and 360 r.p.m. In the second set the mean temperature was fixed at 45 °C and the temperature difference and the stirring rate were varied independently. The temperature difference was varied between 5 and 25 K and the stirring rate was varied as above. The results permit us to obtain a ‘correction factor’ for the temperature polarization in non-isothermal water transport. In the literature this correction factor is supposed to be independent of mean temperature and temperature difference. In the present paper this assumption has been experimentally checked. The experiments state that the correction factor is virtually independent of mean temperature but grows with temperature difference.

Fluctuations in fluids in thermal nonequilibrium states below the convective Rayleigh-Benard instability

Starting from the linearized fluctuating Boussinesq equations we derive an expression for the structure factor of fluids in stationary convection-free thermal nonequilibrium states, taking into account both gravity and finite-size effects. It is demonstrated how the combined effects of gravity and finite size cause the structure factor to go through a maximum value as a function of the wave number q. The appearance of this maximum is associated with a crossover from a q−4 dependence for larger q to a q2 dependence for very small q. The relevance of this theoretical result for the interpretation of light scattering and shadowgraph experiments is elucidated. The relationship with studies on various aspects of the problem by other investigators is discussed. The paper thus provides a unified treatment for dealing with fluctuations in fluid layers subjected to a stationary temperature gradient regardless of the sign of the Rayleigh number R, provided that R is smaller than the critical value Rc associated with the appearance of Rayleigh–Benard convection.

Slowing-down of non-equilibrium concentration fluctuations in confinement

Fluctuations in a fluid are strongly affected by the presence of a macroscopic gradient making them long-ranged and enhancing their amplitude. While small-scale fluctuations exhibit diffusive lifetimes, larger-scale fluctuations live shorter because of gravity, as theoretically and experimentally well-known. We explore here fluctuations of even larger size, comparable to the extent of the system in the direction of the gradient, and find experimental evidence of a dramatic slowing-down in their dynamics. We recover diffusive behaviour for these strongly-confined fluctuations, but with a diffusion coefficient that depends on the solutal Rayleigh number. Results from dynamic shadowgraph experiments are complemented by theoretical calculations and numerical simulations based on fluctuating hydrodynamics, and excellent agreement is found. The study of the dynamics of non-equilibrium fluctuations allows to probe and measure the competition of physical processes such as diffusion, buoyancy and confinement.

Nonequilibrium Concentration Fluctuations in Binary Liquid Systems Induced by the Soret Effect

When a binary liquid system is brought into a stationary thermal nonequilibrium state by the imposition of a temperature gradient, the Soret effect induces long-range concentration fluctuations even in the absence of any convective instability. The physical origin of the nonequilibrium concentration fluctuations is elucidated and it is shown how the intensity of these concentration fluctuations can be derived from the linearized random Boussinesq equations. Relevant experimental information is also discussed.