Available online at www.sciencedirect.com Chemical Engineering and Processing 47 (2008) 893–905 Numerical study of turbulent forced convection in coiled flow inverter Monisha Mridha, K.D.P. Nigam ∗ Department of Chemical Engineering, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi 110016, India Received 21 July 2006; received in revised form 8 February 2007; accepted 9 February 2007 Available online 3 March 2007 Abstract A numerical study is done to investigate turbulent forced convection in a new device of coiled flow inverter. The proposed device works on the technique based on flow inversion by changing the direction of centrifugal force in helically coiled tubes thus enabling rotation of the plane of vortex. The objective of the present study is to characterize the flow development and temperature fields in coiled flow inverter (CFI) under turbulent flow for the range of 10,000 < N Re < 30,000. The flow pattern obtained for the curved tubes are in agreement to those observed by the previous investigators [1,2]. However, a slight rotation of contours was observed in case of helical coil with finite pitch. Similar rotation of contours was found even with fluids of different Prandtl number (0.7 ≤ N Pr ≤ 150) in helical coil with constant pitch. The study was also carried out for different fluids (air, water, kerosene and ethylene glycol). The coiled flow inverter shows 4–13% enhancement in the heat transfer as compared to the straight helical coil while relative pressure drop is 2–9%. The gain in heat transfer in coiled flow inverter for turbulent flow condition as compared to the straight tube for same flow rate and boundary conditions is 35–45% while the increase in pressure drop is 29–30%. © 2007 Elsevier B.V. All rights reserved. Keywords: Coiled flow inverter; Computation; Heat transfer; Helical coil; Hydrodynamics; Turbulence 1. Introduction Coiled tubes are commonly used in industries for heating and cooling of process fluids due to their higher heat, mass transfer rate and more transfer area per unit volume of space as compared to straight tube. Dean [3,4] was the first to report the analytical expression for flow fields to investigate the pressure driven lam- inar flow in a curved pipe with circular cross section. Since then a number of studies [5–23] has been carried out experimentally and numerically to demonstrate the performance of the coiled tubes over straight tubes. The extensive reviews of fluid flow and heat transfer in helical pipes were reported by Berger et al. [6] and Shah and Joshi [7]. Though most flows relevant to the process industry are turbulent but the number of investigations are limited. Table 1 shows the various studies carried out under turbulent fluid flow in curved tubes [1,2,8–18,21–23]. Heat transfer in the coiled tubes can be further enhanced by inserting some perturbation in the curved path. Literature shows ∗ Corresponding author. Tel.: +91 11 2659 1020/6178; fax: +91 11 2659 1020. E-mail address: nigamkdp@gmail.com (K.D.P. Nigam). that attempts have been made for enhancement of heat transfer by modifying the regular helical coil. Chaotic coil was produced by alternatively turning the axis of curved tubes with respect to neighboring one in a periodic manner. Table 2 shows works done by various researchers who have achieved enhanced heat transfer by chaotic advection [24–30]. Saxena and Nigam [30] introduced a new concept by inverting the axis of the helical coil to 90 ◦ . Multiple flow inversions were achieved at low flow rates by changing the direction of centrifugal force in helically coiled tubes. Under the conditions of negligible and significant molecular diffusion, a significant narrowing of residence time distribution (RTD) in laminar flow condition (10 < N Re < 200), was observed in this device by Saxena and Nigam [30]. The RTD of a chemical reactor or vessel is a description of the time that different fluid elements spend inside the reactor. They reported that the most effective narrowing of RTD was found for equally spaced 90 ◦ bends down the length of the tube. A 20-fold reduc- tion in dispersion number as compared to a straight helical coil was found for the device having 57 bends. Dispersion number (= D/uL) is a dimensionless group that measures the extent of axial dispersion. If dispersion number is negligible that means the flow tends to become as a plug flow. Hence, the experimental 0255-2701/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cep.2007.02.026