Improved Convective Heat Transfer in a Fluid with Artificially Modified Turbulence

Abstract

Although a flowing gas loaded with fine particles can give a considerably higher wall heat transfer coefficient compared with the flow of gas alone, earlier studies have shown that this improvement is less than might be realized due to dampening of turbulence near the heat transfer surface by the particles. This paper reports an investigation into off-setting the effect by adding fine fibres to a suspension of 5–40 μm particles in air flowing in tubes of 1, 2 and 3 in diameter. The results indicate that in most situations a fibre-particle suspension is likely to be superior as a coolant compared with a suspension of particles flowing alone.

The purpose of the fibres is (1) to induce a large-scale vortex motion near the pipe wall where normal turbulent fluid mixing is inadequate; (2) to promote controlled residence time of wall-contact of particles of high thermal conductivity and high thermal capacity and (3) to keep heat transfer surfaces clean with the minimum erosion effect.