Fluid Physics: Fluid and interface physics

Dynamics and stability of fluids

54th ESA Parabolic Flight Campaign

23 May 2011

S. van Vaerenbergh ⁽¹⁾, Q. Gland ⁽¹⁾, R. Naim ⁽¹⁾, J. Marc ⁽²⁾, L. Emanuela ⁽³⁾

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The aim of the QNEM experiment is the experimental investigation of the thermal diffusivity and conductivity of nanofluids. “Nanofluids” are suspensions of nanometre sized particles in conventional liquids. Usually,
particles of metals or metal oxides, of size of a few nm to 200 nm, are dispersed in a base liquid (water, ethylene glycol or slicon oils).
Over the last decades, the nanofluids sparked excitement as well as controversy, and were the subject of a considerable amount of research works and scientific publications. The purpose of this interest lies in their potential to improve significantly the performances of heat transfer devices. In presence of a mere few particles, a significant increase of the effective heat transfer conductivity of the fluid has been reported. Some experimental
studies show outstanding performances, largely underevaluated by theoretical models.
Other studies have reported less surprising performances. Recently, the latest theoretical developmenst have helped to explain, partially, some experimental results. However the experimental data are still relatively few, and are not always consistent. In addition, the physical mechanisms responsible for the thermal properties of nanofluids have truly not yet been clarified. Numerous hypotheses have been considered, viz. micro-convection cells around particles moving in liqui, efficient transport of heat through particles aggregates,
crucial influence of the chemical properties of the suspensio], influence of an ordered layer of molecules from the base fluid around clusters of nanoparticles.
The techniques used to measure the thermal conductivity of liquid systems s are well known. Tthe most used are: the different variants of the hot wire technique, the temperature oscillation techniques, the steady state parallel heated plates and the optical beam deflection technique. Although Intensive efforts have been developed to devise experimental set ups which limit the influence of convection on ground based experiments, It is now clear for a long those experiments can be strongly affected by parasitic convective heat transfer. Such
restrictions make it difficult to obtain precise results, especially for liquids with low viscosity.
In parallel, microgravity experiments were conducted for classical liquids and confirmed that the effects of convection are far from being negligible.
In QNEM, we implement a transient optical technique in microgravity conditions. The thermal diffusivity is determined by observing the response of the liquid sample to a thermal stimulus using interferometric technique. Moreover, the experiment timeline is adapted and take advantage of the sequence of different gravity levels (0 - 1 – 2 g) available in parabolic flights to evaluate precisely the influence of gravity on the results. This aspect is truly significant because it would provide an element of answer in the present debate about nanofluids.

The studied liquid is placed in a parallelepipedic transparent glass cell. Two thermal regulation
modules are placed on the lower and upper parts of the cell.
At the beginning of the experiment, the same temperature is imposed on both sides of the cell to obtain a homogeneous temperature throughout the liquid. Later, the temperature of one side of the cell is modified, the second being maintained constant. This causes diffusion of heat trough the liquid layer, which results in a deformation of the temperature field in the liquid.This effect I observed by direct visualization with a Mach-Zehnder interferometer.
This technique is very interesting because it provides a two dimensional and non intrusive measurement of the temperature of the liquid. Different nanofluid sample, will be studied during one single flight.
The experimental set up is equiped with a ‘liquid management system’ to allow renewing the studied liquid
between the parabolas.

Work in progress is a prospective research. The primary purpose is to obtain reference scientific data of the best quality.
If it is confirmed that the thermal properties of nanofluids are of interest, those fluids could replace traditional liquid in standard industrial heat exchange device.

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Olivier Minster (e-mail: olivier.minster@esa.int)