It is known that small quantities of surfactant additives can greatly reduce the friction factors during the flow of a heat transfer medium. The friction factors are reduced because the generation of turbulent vortices is suppressed by the formation of rod-like micelles, and the flow remains laminar in the high Reynolds number range. However, the values of heat-transfer coefficients decrease during flow laminarization; as a result, heat exchangers with a larger heat transfer area are required. The objective of this research is to study the heat transfer enhancement effect of microbubble injection. This paper presents an experimental investigation of the heat transfer and flow characteristics of the two-phase flow of an air-surfactant solution through a horizontal pipe. In the experiment, microbubbles were injected through three types of porous metal. The experimental results are as follows: The size of the bubbles in a bubbly flow decreases with an increase in the filtering accuracy of a porous metal. The flow patterns of the two-phase flow of an air-surfactant solution depend on the size of the microbubbles. The heat-transfer coefficient is promoted by microbubbles injection, but the effect depends on the flow pattern or the size of microbubbles. If the diameter of the microbubbles is approximately 100 μm, there is little enhancement in heat-transfer, but a remarkable enhancement in heat transfer is observed when the diameter of the microbubbles is approximately 400 μm or more.
