Experimental evidence of temperature gradients in cavitating microflows seeded with thermosensitive nanoprobes

Abstract

Thermosensitive fluorescent nanoparticles seeded in deionized water combined with confocal microscopy enables thermal mapping over three dimensions of the liquid phase flowing through a microchannel interrupted by a microdiaphragm. This experiment reveals the presence of a strong thermal gradient up to ~10(5) K/m only when hydrodynamic cavitation is present. Here hydrodynamic cavitation is the consequence of high shear rates downstream in the diaphragm. This temperature gradient is located in vortical structures associated with eddies in the shear layers. We attribute such overheating to the dissipation involved by the cavitating flow regime. Accordingly, we demonstrate that the microsizes of the device enhance the intensity of the thermal gap.

Hydrodynamic cavitation of binary liquid mixtures in laminar and turbulent flow regimes

“…These optical observations demonstrate here that the two phase flow pattern is now managed by Re values in the throttle rather than by the ratio r/r cav . We shall not develop any discussion about the void fraction that could be estimated from these pictures, as former works have demonstrated that the vapour sheet should not occupy the whole height of the microchannel, even if optical pictures could let it believe [10].…”

“…By considering the flow under test over a microsized volume, it becomes possible to use experimental facilities usually devoted to nanosciences. As an example, we could perform a complete thermal mapping of the liquid phase in a cavitating area [10,11]. That was made possible by the correlated benefits offered on the one hand by thermofluorescent nanoparticles seeded in the flow and on the other hand by a confocal microscop able to detect them.…”

“…That may be the way to address questions of both fundamental or applicative interest, related to the cavitation process. Former experiments with de ionized water have exhibited some thermal effects [10,11]. The intensity of thermal effects depends on the temperature gap DT Å = q v L v / (q l c l ) where q v and q l are the density of the vapour and liquid phase respectively, L v is the latent heat and c l is the specific heat of the liquid.…”

Hydrodynamic Cavitation through “Labs on a Chip”: From Fundamentals to Applications

“…Hydrodynamic cavitation in microchannels has allowed us to make a decisive step towards a better analysis of thermal effects in water [13,14]. The principle of the experiment may be summarised as follows.…”

“…Results recorded with a microdiaphragm of hydraulic diameter d H = 77 lm and height H = 124 lm have been published recently [13,14]. In such sheared flows, cavitation is induced by vortices downstream the constriction.…”