Heat transfer enhancement on the outer surface of spirally indented tubes

Heat transfer enhancement on the outer surface of spirally indented tubes was investigated experimentally, The spirally indented tubes studied were characterized by thin walls with a spiral groove on the outer surface which produced a spiral protrusion of the material on the inner surface. Heat transfer data were obtained from experiments in horizontal spirally, indented tubes (inside diameter = 23.8 mm) placed inside of a smooth outer tube forming an annulus. Heated water flowed inside the tubes, and chilled water in the surrounding annulus. A smooth tube and three spirally indented tubes were tested with indentation depth to tube inside diameter ratios of 0.0075, 0.021 and 0.038. The ratio of indentation pitch to inside tube diameter was fixed at 0.9 in all tests producing a ratio of pitch to indentation depth of 23.7 to 120. The annulus flow was turbulent in all tests with the Reynolds number in the range of 7,000 to 115, 300, Heat transfer results for the outside surface of the spirally indented tubes were compared to that of smooth tubes and to that of spirally indented tube inside surfaces, A correlation equation was developed to predict the outer surface heat transfer coefficient of spirally indented tubes over the parameter range of the data.