www.cet-journal.com Page 1 Chemical Engineering & Technology Received: May 22, 2018; revised: February 11, 2019; accepted: March 19, 2019 This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process, which may lead to differences between this version and the final Version of Record (VOR). This work is currently citable by using the Digital Object Identifier (DOI) given below. The final VoR will be published online in Early View as soon as possible and may be different to this Accepted Article as a result of editing. Readers should obtain the final VoR from the journal website shown below when it is published to ensure accuracy of information. The authors are responsible for the content of this Accepted Article. To be cited as: Chem. Eng. Technol. 10.1002/ceat.201800251 Link to final VoR: https://doi.org/10.1002/ceat.201800251 This article is protected by copyright. All rights reserved. Effect of helical surface disc turbulators on heat transfer and friction factor characteristics in the annuli of double pipe heat exchanger: An experimental study Saurabh Yadav 1 *, Santosh K Sahu 1 1 Discipline of Mechanical Engineering, Indian Institute of Technology Indore, Khandwa Road, Simrol, Madhya Pradesh 453552, India *Correspondence: Saurabh Yadav (E-mail: saurabhyadav1124@gmail.com), Discipline of Mechanical Engineering, Indian Institute of Technology Indore, Khandwa Road, Simrol, Madhya Pradesh 453552, India Abstract The heat transfer and pressure drop characteristics in annuli of double pipe heat exchanger using helical surface disc turbulators are studied through experimental investigation. The effect of helical surface disc turbulator is studied for three pitch ratios, three diameter ratios and Reynolds Number. Water flows in the inner tube and air flows through the annulus. The tests are conducted for air with uniform wall temperature condition. The heat exchanger with least pitch ratio and least diameter ratio is found to exhibit the highest Nusselt number and pressure drop. Also, thermal performance factor is found to be greater than unity for all the cases. Correlations have been developed for Nusselt number, friction factor and thermal performance. Keywords: Turbulators; Heat exchanger; Nusselt number; Thermal Performance factor 1. Introduction Augmentation in heat transfer plays a vital role in heat exchanging equipment. It reduces the size and cost of the heat exchangers [1]. DPHE heat exchangers are widely used for both cooling and heating in various engineering and scientific applications such as heat recovery system for engine cooling, solar air heater, air conditioning, food processing industry and chemical industry [3-4]. In general, two different techniques namely active and passive techniques are employed to enhance heat transfer in various thermal devices [5-6]. The passive techniques using turbulence promoters such as artificial roughness, ribs baffles, fins obstacles, turbulators and swirl flow devices such as conical rings, coiled wire twisted tape, vortex rings and winglets [7-8] have been reported in the previous study. Among all passive techniques turbulators are simple in design, less expensive and increase the heat exchanger reliability [7-8] and are widely used in the annulus of DPHE. Numerous studies have been reported that investigate the heat transfer performance of various fluids by using turbulators. It is expected that turbulators generate higher re-circulation, higher contact surface area between the fluid and the heating wall surface, and hence improves the mixing in the fluid resulting in enhancement in the heat transfer performance [7-9]. Karakaya and Durmus [9] investigated heat transfer using converging, diverging and converging-diverging conical spring