Compound convective heat transfer enhancement in helically coiled wall corrugated tubes

The objective of the research was to investigate the passive compound heat transfer enhancement technique achievable by combining the effect of wall curvature and of wall corrugation. To this aim, the forced convective heat transfer in helically coiled smooth and corrugated wall tubes was studied in the Reynolds and Dean number ranges 70 1200 and 12 290 respectively, by adopting Ethylene Glycol as working fluid. Three different curvature ratio values were considered i.e. 0.031, 0.040 and 0.056, while the wall corrugation profile was held constant. For low De values the wall curvature effect prevailed, and the heat transfer enhancement was nearly the same for the corrugated and the smooth helically coiled tubes. For higher De values instead, the wall corrugation brought a further heat transfer enhancement with respect to the wall curvature. This behavior could be explained as a consequence of an early departure from the steady laminar flow regime, as it happens in straight wall corrugated tubes. With this regard a critical Dean number value equal to 120 was identified. The results showed that the effect of the wall curvature alone brings heat transfer enhancement in the range 2 10 over the straight smooth wall tube against pressure drop penalties augmentation in the range 1 1.5, while for the helical coiled wall corrugated tubes an heat transfer augmentation over the straight smooth wall behavior of up to 25 was registered against a maximum friction factor increase of 2.5. The present research highlights that the simultaneous use of the two passive heat transfer enhancement techniques (wall curvature compounded with wall corrugation) is particularly effective if compared to the performance achievable through their separate use.