Performance improvement for ocean thermal energy conversion systems. Final report. [Helical rotor inserts for preventing biofouling

Abstract

Experimental data with rotating helical ribbon inserts tested in an OTEC heat transfer test loop are presented. The objectives were to investigate the heat transfer enhancement achievable with rotating helical inserts mounted in heat transfer tubes, and their effectiveness in preventing biofouling at the inner tube wall surface. The inserts tested were 6 Teflon helix-rotors, and one soft-edge helix-rotor. They were all mounted at their upstream ends in heat transfer tubes and free to rotate due to the liquid flow. For coolant flow rates applicable to OTEC plants (above 1 m/s) the increase of the overall heat transfer coefficient was found to range below 20% for all helix-rotors. The local, brine-side heat transfer coefficient ranged up to ca. 25%. However, further improvements of the soft-edge helix-rotor might result in higher heat transfer enhancement. These helix-rotors were quite effective for biofoulant growth control. Tests conducted with San Francisco Bay water over a 6-week period with Teflon helix-rotors mounted in aluminum and titanium tubes demonstrated that the repetitive sweeping of the helix edge over this tube-side surface effectively prevented the accumulation of marine growth that proliferated in the same tubes in sections without helix-rotors.