Validation of a new methodological approach for the selection of wire-coil inserts in thermal equipment

Abstract

The use of wire-coils is especially relevant at low Reynolds numbers (below the critical number to turbulent flow in smooth tubes) according to its inherent positive features such as the advance of transition onset and, if they present suitable geometric characteristics, the establishment of an extended transitional flow in a critical Reynolds number interval [ReCL − ReCT ], with a predictable friction coefficient and Nusselt number. This paper presents the experimental validation of a new methodology based on the evaluation of a non-dimensional geometry-based parameter: the TSP (Transition Shape Parameter) that allows to predict the friction coefficient evolution with wire-coil inserts and enables to compute the extension of the transitional flow region. The close relationship between hydraulic and thermal performance of wire-coil inserts makes this methodology a valuable tool for selecting the most appropriate wire-coil geometry for a given tubular heat exchanger. It is observed that to promote an increase in heat transfer, the value of the ReCL of the wire coil must be less than the operating Reynolds number range of the equipment. Thus, the ReCL − ReCT interval of the insert should fall into this range. In order to validate the methodological approach, an application to harp-type solar thermal collectors with typical Reynolds number range [40–6000] is presented. Four representative wire-coils, with a wide geometrical range characterized by TSP values of 759, 196, 35.3 and 3.1 (exhibiting significant differentiated behaviours in their friction factor curves and critical Reynolds numbers) were inserted inside the risers of a modified solar collector and experimentally tested at laboratory conditions. Static temperature at different locations at the absorber plate, and pressure drop were measured to obtain friction factor and Nusselt number inside riser covering the laminar, transitional and low-turbulent regions. For a general application with friction factor constraints the most suitable wire-coil geometry is the TSPW02 = 196 with a range of critical Reynolds number of ReCL = 663 and ReCT = 2286 and Nuw02/Nus = 2.21 for Re = [300 − 3000] with f w02/fs = 3, 82. However, for the case study presented (a harp-type solar collector) it is feasible to insert the third wire-coil geometry TSPW03 = 35.3 due to its early transition, with a range of critical Reynolds number of ReCL = 364 and ReCT = 2324, and Nuw/Nus = 1.35 for Re = 300 with a high friction factor augmentation f w03/fs = 18.84. This geometry also promotes the highest absorber temperature reduction. The greatest temperature reduction is observed in the range of Reynolds numbers [700–2000], reaching approximately 6 ◦C, which represents approximately 15 %