The forced convective heat transfer in straight and coiled tubes, having smooth and corrugated wall, was experimentally investigated in two ranges of the Reynolds number: a lower one (5 < Re < 13) obtained with Glycerol and a higher one (150 < Re < 1500) obtained with Ethylene Glycol. The aim of the research was to verify the effectiveness of these passive heat transfer enhancement techniques when highly viscous fluids are treated. This issue is particularly crucial in applications in which the thermal processing of high Prandtl number fluids is required, such as in the food, chemical, pharmaceutical and cosmetics industries. In the present note, preliminary results, obtained by considering a given geometrical configuration characterized by a tube diameter of 14 mm, a curvature ratio of 0.06, a corrugation depth of 1 mm and a corrugation pitch of 16 mm, are presented. The main conclusion is that the wall curvature enhances heat transfer at all Re, whereas the wall corrugation enhances heat transfer only in the higher Re range; moreover the wall corrugation is totally ineffective in the low Re range and, if helical coils are present, it also destroys the benefit induced by the wall curvature. The largest increment in heat transfer rates is thus obtained by using smooth helical coils at low Re, and corrugated helical coils at larger Re. The results, although of preliminary nature, suggest interesting applications of the passive heat transfer enhancement technique based on smooth wall coiled tubes in the very low Reynolds number values range, while the combined passive technique based on wall corrugation and curvature represents an interesting solution for Reynolds number values in the range 150–1500.
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