Bharadwaj et al. [13] experimentally investigated both pressure drop and heat transfer characteristics for 75-start spirally grooved tube with twisted tape inserts under both laminar and turbulent flow regime. The comparison with the behavior of spirally grooved tubes without twisted tape showed that the heat transfer increase was considerable, especially in the laminar flow regime as compared to the turbulent one: under constant pumping power conditions, spirally grooved tube with twisted tape showed a maximum enhancement over the smooth tube of 600% and of 140% in the laminar and turbulent flow regime respectively.