Numerical studies of a double-pipe helical heat exchanger Timothy J. Rennie, Vijaya G.S. Raghavan * Department of Bioresource Engineering, McGill University, 21 111 Lakeshore Rd., Sainte Anne-de-Bellevue, QC, Canada H9X 3V9 Accepted 28 October 2005 Available online 9 December 2005 Abstract A double-pipe helical heat exchanger was numerically modeled for laminar fluid flow and heat transfer characteristics under dif- ferent fluid flow rates and tube sizes. Two different tube diameters were used. The overall heat transfer coefficients were calculated for both parallel flow and counterflow. Validation of the simulations was conducted by comparing the Nusselt numbers in the inner tube with those found in literature; the results fell within the range found in the literature. The greatest thermal resistance was found in the annular region. The annulus Nusselt number was correlated with a modified Dean number, and showed a strong linear relationship. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Double-pipe; Heat exchanger; Numerical; Helical 1. Introduction Helically coiled tubes can be found in many applica- tions including food processing, nuclear reactors, com- pact heat exchangers, heat recovery systems, chemical processing, low value heat exchange, and medical equip- ment [1,5,13,15]. The pipe curvature causes centrifugal forces to act on the flowing fluid, resulting in a second- ary flow pattern perpendicular to the main axial flow. This secondary flow pattern generally consists of two vortices, which move fluid from the inner wall of the tube across the center of the tube to the outer wall. Upon reaching the outer wall it travels back to the inner wall following the wall. The secondary flow increases heat transfer rates as it moves fluid across the tempera- ture gradient. Thus, there is an additional convective heat transfer mechanism, perpendicular to the axial flow, which does not exist in straight tube heat exchang- ers (except when produced by buoyancy forces). The majority of the work involving helically coiled heat exchangers has focused on either constant wall temper- ature or constant wall heat flux boundary conditions [14]. However, there is a third common boundary condi- tion that is often not explored, and that is for liquid-to- liquid heat exchangers (the tube wall separates the two fluids), where neither the heat flux, nor the wall temper- ature, is constant. There are a few references that discuss designing shell-in-tube helical heat exchangers [7,12]. However, the calculation procedures are not based on data from helically coiled tubes. For example, Hara- burda [7] assumes that a bank of tubes can be used as an approximation for helical coils in the calculation of heat transfer coefficients. Designing a double-pipe helical heat exchanger, such as in Fig. 1, requires heat transfer coefficients for the both sides of the tube, the flow rate in the helical tube and the flow rate in the annulus, along with desired inlet and outlet temperatures. Some information exists on the heat transfer in a curved annulus, however this is also limited to constant wall temperature gradient in the axial direction [9], making it difficult to predict the heat transfer coefficients on the inner wall of the annulus. 1359-4311/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.applthermaleng.2005.10.030 * Corresponding author. Tel.: +1 514 398 8731; fax: +1 514 398 8387. E-mail address: vijaya.raghavan@mcgill.ca (V.G.S. Raghavan). www.elsevier.com/locate/apthermeng Applied Thermal Engineering 26 (2006) 1266–1273