Soutenance de thèse : Poppy O'Neill

Phase change dispersions as high performance heat transfer fluids

Doctorante : Poppy O'Neill

Laboratoire INSA : CETHIL

Ecole doctorale : ED162 : Mécanique, Energétique, Génie Civil, Acoustique de Lyon

This thesis focuses on the heat transfer, transport, and rheological behaviour of novel two-phase fluids, named phase change dispersions. Phase change dispersions consist of phase change material dispersed into a continuous phase with the aid of surfactants. The optimal formulation procedure for phase change dispersions with high stabilities, low supercooling degrees and high apparent specific heat capacities is discussed and an innovative approach in fine-tuning the thermophysical properties of phase change dispersions with the use of cosurfactants is defined. Two of the developed formulations were then chosen for a heat transfer and rheological behaviour comparison to observe the effect that surfactants have on the transport and heat transfer properties during heating. This was performed using a test-rig to measure the bulk fluid and inner wall temperatures of the phase change dispersions flowing through a cylindrical tube under the constant heat flux boundary condition. The crystallisation heat transfer and rheological behaviour of a phase change dispersion was also examined through calculation of heat balances in a rectangular duct. During melting and crystallisation, an interesting phenomenon was discovered, that the transition from laminar to turbulent with phase change dispersions was much lower than those predicted for Newtonian  fluids. By regression of the experimental results, correlations for the average Nusselt numbers for laminar and turbulent flow are presented, using a modified Reynolds number and a Prandtl number correction factor. A numerical model for the thermal behaviour studies of a phase change dispersion during its cooling in laminar flow through a rectangular duct was developed and is based on the quasi-homogeneous single fluid approach. The evolution of the experimental and theoretical values shows good agreement and the model satisfactorily predicts the behaviour, with variations of less than 5%.