Chemical Engineering & Processing: Process Intensifcation 157 (2020) 108153 Available online 23 September 2020 0255-2701/© 2020 Elsevier B.V. All rights reserved. Use of the thermal and hydraulic length for the screening selection of turbulence promoters in tubular heat exchangers Martín Pic´ on-Nú˜ nez *, Jorge Carlos Melo-Gonz´ alez Department of Chemical Engineering, University of Guanajuato, Mexico A R T I C L E INFO Keywords: Turbulence promoters Thermal length Hydraulic length Thermohydraulic Performance comparison Heat transfer enhancement ABSTRACT This paper presents an alternative approach for the selection of the most suitable turbulence promoter for the intensifcation of heat transfer in tubular heat exchangers. The approach uses the thermal and hydraulic lengths that are determined based on the two main design objectives in a heat exchanger, the heat duty and the pressure drop. Exchanger dimension in the form of plots of the exchanger length are produced to identify the device that results in the smaller equipment size within the constraints imposed by the pressure drop. The length plots are produced for various types of turbulence promoters under fxed design specifcations such as heat duty, pressure drop, mass fow rate and tube diameter. The results indicate that from a set of 25 different types of turbulence promoters studied, those that exhibit the largest heat transfer enhancement with the lowest pressure drop are the V-cut twisted tape, the square-cut twisted tape, and the straight tape with center wings. The benefts of the use of heat transfer enhancement on the stream that controls heat transfer on a practical heat exchanger design is analysed on a case study. 1. Introduction Passive heat transfer intensifcation techniques are used to achieve two main goals. On the one hand, they are used to enhance the rate of heat transfer so that in a new design, the resulting heat exchanger has the smallest size subject to the restrictions of the allowed pressure drop, and on the other, they are used in retroft situations where the heat transfer rate of an existing heat exchanger must be increased with the lowest increment in the pressure drop. To achieve any of these goals, many heat transfer enhancement techniques that exhibit different merits in terms of heat transfer and friction performance have been developed over the years [1]. In tubular heat exchangers, the increase of the heat transfer rate can be accomplished in diverse ways, these include the use of external en- ergy to increase the velocity of the fuid; the use of extended surfaces attached to the inner side of the tube to increase the surface area, the modifcation of the inner and outer tube surface or the use of turbulence promoters to increase the local turbulence. The most desirable feature of a turbulence promoter is characterized by the maximization of the heat transfer rate with the minimum increase of pressure drop. The charac- terization of the thermo-hydraulic performance is either experimentally or numerically determined in terms of correlations of the Nusselt number or the Colburn factor as a function of the Reynolds number and of the friction factor as a function of the Reynolds number. Many different geometries and variations within these geometries have been developed over the long history of existence of turbulent promoters. From the geometrical point of view, the insertion of turbu- lence promoters in a tube causes the reduction of the free fow area and the reduction of the hydraulic diameter. This situation gives rise to two hydraulic phenomena, the rise of velocity and the promotion of sec- ondary swirl fow. The increase of fow velocity reduces the thickness of the boundary layer while the swirl fow removes the boundary layer increasing the contact between the surface and the fuid to further improve the removal of heat. The level of intensifcation of heat transfer depends on the type of swirl fow generated, with swirl fow moving around a parallel line to the fow direction resulting in a much higher intensifcation than the swirl fow moving around a line perpendicular to the fow direction. Besides, as the fow of the fuid is blocked due to the presence of the insert, the increased viscous effect results in increased pressure drop. Additionally, the changes of direction created by swirl fow increases the pressure drop as well, with perpendicular swirl fow contributing in larger proportion. Since the effect of a tur- bulence promoter on the boundary layer depends on its geometrical features, the performance of each different type also depends on the range of Reynolds number where it operates. For instance, the main * Corresponding author. E-mail address: picon@ugto.mx (M. Pic´ on-Nú˜ nez). Contents lists available at ScienceDirect Chemical Engineering and Processing - Process Intensifcation journal homepage: www.elsevier.com/locate/cep https://doi.org/10.1016/j.cep.2020.108153 Received 7 June 2020; Received in revised form 17 August 2020; Accepted 15 September 2020