Ultrasonically-enhanced convective heat transfer: Evidence of a relationship with the thermal boundary layer

Abstract

The influence of ultrasound on convective heat transfer depending on the flow regime is investigated in this study, using an experimental setup consisting of a rectangular channel with a heating plate on one side and an ultrasonic transducer opposite it on the other. In order to understand the physical phenomena taking place, this work was based on two approaches. The first one focuses on hydrodynamic analysis using Particle Image Velocimetry, focusing specifically on the influence of ultrasound on velocity fields. The second approach aims to determine the convective heat transfer coefficient and Nusselt number, by measuring fluid and heating plate temperatures. Experimental results show that 25 kHz ultrasound allows heat transfer to be enhanced over the Reynolds number range studied, from a laminar regime (Re = 890) to a turbulent regime (Re = 14500), as the acoustic cavitation induce disturbances within the thermal boundary layer. The heat transfer enhancement factor decreases as the Reynolds number increases, until an asymptote is reached in the turbulent regime. For 2 MHz ultrasound, the acoustic streaming generated allows to improve convective effects within the flow, consequently enhancing heat transfer. The heat transfer enhancement factor decreases as the Reynolds number increases up to 7500, at which point 2 MHz ultrasound no longer induces heat transfer enhancement. Finally, ultrasonically induced heat transfer enhancement has been analyzed in terms of the initial thermal boundary layer thickness for silent conditions. The overall results of this study demonstrate the existence of a strong relationship between heat transfer enhancement and initial thermal boundary layer thickness. The thicker the silent initial thermal boundary layer, the greater the heat transfer enhancement.