Friction and Nusselt number data have been measured and semi-empirically evaluated for twisted tape generated helical flow in annuli. Results have been obtained for radius ratios of 0.41 and 0.61 and twist ratios of ∞, 5.302, 5.038, and 2.659. The increase in pressure drop and heat transfer rates obtained are comparable to those reported for twisted tape generated swirl flow in tubes. Also, for the same heat transfer rates the pumping power requirements compare favorably with those for empty annuli. The analytical predictions based on the principle of superposition of pressure drops and analogy between heat and momentum transfer have yielded excellent predictions for y = ∞ and 5.302 but somewhat poor agreement at y = 2.659.

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