Enhancement of heat transfer from a heat source to a flowing fluid within a tube is a challenging problem with many practical applications. In this paper, experimental investigation of a low-frequency (20-33. kHz), high-intensity (500-1000. W) ultrasonic field as a potential heat-transfer " process intensifier" is undertaken. Heat-transfer enhancement data collected in a miniaturized furnace tube over a range of flow conditions and ultrasonic process parameters indicate that sonication provides significant augmentation only under near-static (e.g., stagnant) and low-Reynolds number flow conditions. With increasing flow velocity, cavitational and acoustic-streaming fields associated with ultrasound are rapidly diminished in importance, hence playing no role in bulk fluid heat transfer (unless input power levels or frequencies are suitably increased). However, the relevance to some locations, such as those under porous deposits in water-wall tubes of boilers near the goose-neck portion, can spur further study to exploit the impact of ultrasonic heat-transfer enhancement. The critical parameter that determines the efficacy of ultrasonic enhancement of heat transfer appears to be the ratio of the characteristic ultrasonic field velocity (sum of cavitational and acoustic streaming velocities) to the prevailing flow velocity. © 2012 Elsevier B.V.