Experiment Study On The Stability Of Rayleigh-Bénard Natural Convection In Slurry With PCM Particles

Microemulsion slurry dispersed with phase-change-material (PCM) particles is a kind of functionally thermal fluids. Since it has simultaneously a flowing ability and a large apparent heat capacity in a certain temperature range, more and more researchers'attention has been attracted to its exploitation for industry uses in recent years.This paper aims at understanding its natural convection characteristics with a Rayleigh-Bénard configuration, i.e. a horizontally placed rectangular enclosure cooled from the top and heated at the bottom.Rayleigh-Bénard (RB) natural convection itself is a very classical and complicated nonlinear phenomenon in both physics and heat transfer areas. When the enclosure is filled up with the exploited microemulsion PCM slurry, there will be more complicated phenomena to be revealed. In the paper, two kinds of microemulsion PCM slurries have been selected as the tested fluids. One is dispersed with particles of n-alkane of C18 and the other of n-alkane of C22. Both fluids have the mass concentration of about 30%. Before the experiment of RB convection, the thermal physical properties of each fluid have been measured, which includes the thermal volumetric expansion coefficient, density, viscosity, and thermal conductivity. The specific heat capacity was measured using DSC with both heating and cooling rates of 5℃/min.Since it's still difficult so far to obtain the convective instability features for this kind of fluids, RB convection experiments were carried out. The experimental results indicated that: Firstly, the critical onset Rayleigh number in the slurries natural convection is lower than that of water; secondly, subcritical convection phenomena exist for both slurries; when the average temperature of PCM slurry was close to the peak temperature of melting, the convection began to deteriorate and eventually pass into a state of thermal conduction. The reasons resulting in this phenomenon have been analyzed in detail. At last, the experimental uncertainties and its evaluation have been given in this thesis.Finally, the improvement and some proposals for future study on this subject have been outlined.