|dc.description.abstract||In space conditioning and in cooling systems for high power density electronics, vapor
compression cycles provide cooling. The working fluid is a refrigerant and oil mixture. A
small amount of oil is needed to lubricate and to seal the sliding parts inside the compressors
but, when mixed with refrigerant and carried through the system and in heat exchangers, the
lubricant in excess penalizes the heat transfer coefficient and increases the flow losses: both
effects are highly undesired yet unavoidable.
Nanolubricants - a lubricant with dispersed nano-size particles - can be a cost-effective
technology to address this problem and for improving the efficiency and performances of vapor
compression cycles. Several researchers postulated that the magnitude of the heat transfer
enhancement due to the presence of nanoparticles is much higher than the gain in the liquid
thermal conductivity and that the nano-scale interactions between the nanoparticles and the
refrigerant/oil liquid layers are responsible for the heat transfer intensification.
This research aims at understanding the mechanisms responsible for heat transfer intensification
during two-phase flow processes when nano-thermal vectors are used. This was achieved
by providing a consistent set of experimental data that document the effects of the nanoparticles
on the two-phase flow heat transfer coefficient and pressure drop, and by developing a
theoretical model that captures the effects of the nanoparticles.
The effects of Al2O3 nanoparticles on the thermophysical properties of a polyolester lubricant
(POE) were measured at different nanoparticles mass concentrations (0%, 10%, and 20%)
and with different surfactants, used as dispersants.
Tests on evaporative two-phase flow of mixtures of refrigerant R410A and Al2O3 nanolubricant
were reported for different nanoparticles mass concentrations (0%, 0.05%, 1%, and
3%), and it was observed that the two-phase flow heat transfer coefficient was either enhanced
or degraded depending on oil and nanoparticle concentrations, and on the mass flux. Interestingly,
pressure drop did not seem to be affected by the presence of nanoparticles.
A simulation tool based on correlations and theoretical models was developed to describe
the nanoparticle distribution and behavior within the liquid film. The model provided a platform
for future investigations into the behavior of high viscosity nanoparticle suspensions.||en_US