Thermohydraulics of laminar flow of viscous oil through a circular tube having axial corrugations and fitted with centre-cleared twisted-tape Sujoy Kumar Saha Mechanical Engineering Department, Bengal Engineering and Science University Shibpur, Howrah 711 103, India article info Article history: Received 28 August 2011 Received in revised form 16 December 2011 Accepted 19 December 2011 Available online 5 January 2012 Keywords: Laminar flow Forced convection Corrugations Twisted tapes Heat transfer enhancement Swirl flow abstract The experimental friction factor and Nusselt number data for laminar flow through a circular duct having axial corrugation and fitted with centre-cleared twisted-tape have been presented. Predictive friction factor and Nusselt number correlations have also been presented. The thermohydraulic performance has been evaluated. The major findings of this experimental investigation are that the centre-cleared twisted tapes in combination with axial corrugation perform better than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain amount of centre- clearance. Ó 2011 Elsevier Inc. All rights reserved. 1. Introduction Axially corrugated channels used in heat exchangers are typically sinusoidal channels. Fig. 1 shows the axial corrugation in a circular duct. Fig. 1 is the item 9 (i.e. test section) of the exper- imental rig as shown in Fig. 3. The effect of corrugation angle was investigated by a number of investigators, [1,2]. The corrugation angle ranged from 0° to 90°. Both friction factor and Nusselt num- ber increase monotonically up to a certain value of the corrugation angle. Focke and Knibbe [3] have shown that at corrugation angle 45°, the fluid flow is predominantly along the furrows. Focke et al. [1] suggested similar flow patterns up to corrugation angle 60°. The reason for increase and decrease of the friction factor and Nusselt number is the positive and negative interaction of criss-crossing fluid streams inducing secondary swirl motion, change of flow pat- tern and accelerating or decelerating effect on them. Stasiek et al. [2] investigated the effect of corrugation pitch to channel height ratio. Abdel-Kariem and Fletcher [4] developed friction factor and Nusselt number correlations. For laminar regime, f ¼ 15Re 0:3 h 45  2:5 ð1Þ Nu ¼ 0:777Re 0:444 Pr 0:4 h 45  0:67 ð2Þ Twisted tapes as shown in Fig. 2 cause the flow to spiral along the tube length. Continuous twisted-tape shown in Fig. 2a has been extensively investigated. Variants of twisted-tape that have been evaluated include short sections of twisted tapes at the tube inlet, or periodically spaced along the tube length. Early works on twisted tapes have been reported in [5,6]. Later works have been reported in [7–27]. Fig. 2b shows the layout of a circular duct having full-length centre cleared twisted-tape. The details and method of making cen- tre-cleared twisted-tape will be described in Section 2. It has been observed from the literature review that the combined effect of axial corrugations and centre-cleared twisted-tape inserts has not been studied in the past. The helical fluid flow due to axial corrugations coupled with centre-cleared twisted-tape-generated swirl flow is likely to give larger swirl intensity and vortex in the flow. Also there may be enhanced fluid mixing with increased heat and momentum diffusion. This may increase heat transfer even if it may also give increased pressure drop. In this paper, therefore, the laminar flow experimental heat transfer and pressure drop results of combined effect of axial corrugations and full-length centre- cleared twisted-tape inserts in circular ducts are presented. Friction factor and Nusselt number correlations are presented. Also the per- formance of this combined geometry is evaluated. 2. Experimental set-up, operating procedure and data reduction The pressure drop measurements were taken in a 13 mm ID and 2 m long circular acrylic duct, in which the corrugations were 0894-1777/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved. doi:10.1016/j.expthermflusci.2011.12.008 E-mail address: sujoy_k_saha@hotmail.com Experimental Thermal and Fluid Science 38 (2012) 201–209 Contents lists available at SciVerse ScienceDirect Experimental Thermal and Fluid Science journal homepage: www.elsevier.com/locate/etfs