Heat transfer and pressure drop in large pitch spirally indented tubes

Heat transfer and pressure drop experiments are performed with water, at atmospheric pressure in the Prandtl number range 3.6–8.8, flowing in spirally indented tubes for heat exchanger applications. Large pitch geometries are studied which minimize pressure drop. The pitch to indentation height ratio is in the range of 23.7–120, which is beyond the range generally considered in previous investigations. Four tubes are tested with the indentation height to diameter ratios of 0.0075, 0.021, 0.038 and 0.0 (plain). The tubes are tested in a counter-current flow configuration that produces results directly applicable to heat exchanger design applications. A data reduction technique is developed that is applicable to an arbitrarily augmented tube, and it is verified with the plain-tube data. Heat transfer and pressure drop results from the spirally indented tubes are compared to extensions of correlations for similar geometries and to limited data available at large pitches.