Experimental investigation on heat transfer and frictional characteristics of spirally corrugated tubes in turbulent flow at different Prandtl numbers

Corrugated tubes have been experimentally studied in order to obtain their heat transfer and isothermal friction characteristics. The use of water and ethylene glycol as test fluids has allowed to cover a wide range of turbulent fluid flow conditions: Reynolds number from 2000 to 90 000 and Prandtl number from 2.5 to 100. The paper presents a comprehensive experimental study on a family of 10 corrugated tubes which were manufactured by cold rolling. Artificial roughness is characterised by rib height h/ d ranging from 0.02 to 0.06 and spiral pitch p/ d from 0.6 to 1.2. The results show that a unique dimensionless parameter named severity index ( φ= h 2/ pd) can be used to establish roughness influence on flow. The large amount of experimental data has been correlated in order to obtain easy to use expressions for Fanning friction factors and Nusselt numbers as functions of flow and geometry dimensionless parameters. The real benefits which are offered by corrugated tubes have been assessed by calculating one of the performance evaluation criteria commonly used in the enhanced heat transfer literature. Finally an optimisation study shows the guidelines to choose which roughness geometry offers the best performance for specific flow conditions.