Change of flow structure and heat transfer performance of photosensitive micellar solutions with light irradiation

Abstract

Heat transfer enhancement in the low Reynolds number condition is increasingly required with the perpetual attempt to integrate and miniaturize electronic devices. The authors have suggested the application of photo-rheological fluid, whose rheological properties change by external photic stimulus, to a coolant of small heat exchangers. High heat transfer state and low pressured drop state can be switched by light irradiation to the fluid according to the demand for heat transfer enhancement. This study investigated how the change of viscoelasticity of photosensitive micellar solutions by light irradiation relates to flow structure and heat transfer performance. Flow visualization and heat transfer measurement were conducted using an aqueous solution of surfactant and counterions (cetyl-trimethyl-ammonium bromide/sodium salicylate/ortho-methoxy cinnamic acid) with different light irradiation time in serpentine channels. The results showed that the decrease of the viscoelasticity with light irradiation caused the decrease in heat transfer rate and pressure drop. In the flow visualization, two types of secondary vortex, i.e., a single large-scaled vortex and a pair of counter-rotating vortices, were observed. As the light irradiation time increased, flow unsteadiness diminished, and the frequency in the appearance of a single vortex and fast flow near wall decreased. The decrease in heat transfer rate and pressure drop with light irradiation was ascribed to the deterioration of fluid mixing by flow unsteadiness, the reduction of momentum and thermal diffusion by the decrease in near-wall velocity gradient, and the decrease in the strength of a pair of vortices.