Heat transfer enhancement and pressure drop by pulsating flow through helically coiled tube: An experimental study

Pulsating flow was applied as an active technique for heat transfer enhancement. Helically coiled tube was employed and convective heat transfer and pressure drop were experimentally investigated. Four helically coiled tubes with the same tube diameter and different coil curvature were fabricated and subjected to uniform wall heat flux. Distilled water was utilized as working fluid and its thermo-physical properties were estimated at mean temperature using available correlations. At first, Darcy friction factor and Nusselt number in steady laminar flow were compared with several correlations in the literature and the most accurate relations were found. Afterward, experiments were extended for pulsating laminar flow over a Womersly number range of 16-38 which corresponds to 2-10 Hz pulsating frequency, Reynolds number of 2000-9500, and Helical number of 350-2300. The results revealed that increment in pressure drop is nearly 3-7% in pulsating flow compared to steady one, while convective heat transfer enhanced up to 39%. Findings indicated that more heat transfer enhancement can be attained at low Reynolds numbers. Improvement in heat transfer rate and thermal performance was evaluated and discussed in detail. Also, a new correlation was proposed for Nusselt number in laminar pulsating flow. Among the four pulsating frequencies were studied in the current work, the optimum pulsating condition for achieving the best thermal performance was Wo similar to 24 (4 Hz).