Experimental determination of heat transfer and friction in helically-finned tubes Gregory J. Zdaniuk a, * , Louay M. Chamra b,1 , Pedro J. Mago b,1 a Ramboll Whitbybird Ltd., 60 Newman Street, London W1T 3DA, United Kingdom b Department of Mechanical Engineering, Mississippi State University, 210 Carpenter Engineering Building, P.O. Box ME, Mississippi State, MS 39762-5925, USA Received 1 September 2006; received in revised form 15 September 2007; accepted 26 September 2007 Abstract Heat transfer coefficients and friction factors were determined experimentally for eight helically-finned tubes and one smooth tube using liquid water at Reynolds numbers ranging from 12,000 to 60,000. The helically-finned tubes tested in this investigation have helix angles between 25° and 48°, number of fin starts between 10 and 45, and fin height-to-diameter ratios between 0.0199 and 0.0327. An uncertainty analysis was completed and plain-tube results were compared to the Blasius and Dittus–Boelter equations with satisfactory agreement. Power-law correlations for Fanning friction and Colburn j-factors were developed using a least-squares regression. The per- formance of the correlations was evaluated with data of other researchers with average prediction errors between 30% and 40%. Published by Elsevier Inc. Keywords: Experimental; Friction; Heat transfer; Helically-finned tube 1. Introduction The use of heat transfer enhancement has become wide- spread during the last 50 years. The goal of heat transfer enhancement is to reduce the size and cost of heat exchan- ger equipment, or to increase the heat duty for a given size heat exchanger. This goal can be achieved in two ways: active and passive enhancement. Of the two, active enhancement is less common because it requires the addi- tion of external power (e.g., an electromagnetic field) to cause a desired flow modification. On the other hand, pas- sive enhancement consists of alteration to the heat transfer surface or incorporation of a device whose presence results in a flow field modification. The most popular surface enhancement is the fin. Webb [1] gives an excellent overview of different enhancement mechanisms available in commercial tubes. One contemporary enhancement geometry is the helical fin shown in Fig. 1. The helical fin is considered to be two-dimensional. Several geometric variables describe a helical fin. Fig. 2 provides a pictorial description of the geo- metric variables. These variables are: the fin height (e), the fin pitch (p), the helix angle (a), number of starts (N s ), and included angle (b). The fin height is the distance measured from the internal wall of the tube to the top of the fin. The fin pitch is the distance between the centers of two fins mea- sured in the axial direction. The helix angle is the angle the fin forms with the tube axis. The number of starts refers to how many fins one can count around the circumference of the tube. Finally, the angle at which the sides of the fin meet is called the included angle. The characteristics of flow inside helically-finned tubes are still not very well understood because experimental data are limited. An extensive literature survey of the topic is given in Zdaniuk [2] and only a small number of studies are referenced here. 0894-1777/$ - see front matter Published by Elsevier Inc. doi:10.1016/j.expthermflusci.2007.09.006 * Corresponding author. Tel.: +44 20 7631 5291; fax: +44 20 7323 4645. E-mail address: gregory.zdaniuk@rambollwhitbybird.com (G.J. Zda- niuk). 1 Tel.: +1 662 325 3261; fax: +1 662 325 7223. www.elsevier.com/locate/etfs Available online at www.sciencedirect.com Experimental Thermal and Fluid Science 32 (2008) 761–775