Thermohydraulics of laminar flow through a circular tube having integral helical rib roughness and fitted with centre-cleared twisted-tape Suvanjan Bhattacharyya, Sujoy Kumar Saha ⇑ Mechanical Engineering Department, Bengal Engineering and Science University, Shibpur,Howrah 711 103, India a r t i c l e i n f o Article history: Received 30 January 2012 Received in revised form 10 May 2012 Accepted 10 May 2012 Available online 22 May 2012 Keywords: Laminar flow Forced convection Integral helical rib roughness Twisted tapes Heat transfer enhancement Swirl flow a b s t r a c t The experimental friction factor and Nusselt number data for laminar flow through a circular duct having integral helical rib roughness and fitted with centre-cleared twisted-tape have been presented. Predictive friction factor and Nusselt number correlations have also been presented. The thermohydraulic perfor- mance has been evaluated. The major findings of this experimental investigation are that the centre- cleared twisted tapes in combination with integral helical rib roughness perform significantly better than the individual enhancement technique acting alone for laminar flow through a circular duct up to a certain amount of twisted-tape centre-clearance. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Laminar flow is encountered in many industrial applications. In case of laminar flow, there is major thermal resistance in the bulk flow in addition to the dominant thermal resistance in the thin boundary layer adjacent to the flow. Twisted-tape inserts are, therefore,used to mix the gross flow effectively in laminar flow to reduce the thermal resistance in the core flow through the chan- nel. Integral helical ribs are also turbulators. Helical rib as shown in Fig. 1 is made as integral surface to the duct wall. The dimension- less geometric parameters that influence the heat transfer and fric- tion characteristics are the rib height and rib helix angle. Integral rib-roughness has been used for the enhancement of tube-side heat transfer coefficient in low-flow automotive radiators and in this case,the flow Reynolds number is small and may be <2000. Farrel et al. [1] tested one fully-ribbed and two broken ribbed flat radiator tube. They obtained friction factors for 200 < Re < 11, 000. However, the heat transfer coefficients were obtained only for tur- bulent flow with Re > 2000. The broken-ribbed tube with the high- est e/D h yielded the highest heat transfer coefficient as well as the highest friction factor.Olsson and Sunden [2] tested two ribbed radiator tubes with airflow. The air heat transfer data were taken with constant wall temperature and the data provide the axially averaged heat transfer coefficient over the tube length. The enhanced tubes showed higher friction factors than the smooth tube in both laminar and turbulent regions. However,as the Rey- nolds number decreased in the strictly laminar region, the friction factors tended to converge and approach the smooth tube value. Also, the laminar–turbulent transition Reynolds number decreased as the friction factor increased. Similar to the friction behaviour, the Colburn j factors also tended to converge at low Reynolds num- bers, and approached the smooth tube value. However, in contrast to the friction factors, the j factors did not show a clear laminar– turbulent transition. Olsson and Sunden [3] investigated the effect of rib configurations for the multiple V-ribbed channel. Twisted tapes as shown in Figs. 2a and 2b cause the flow to spir- al along the tube length. Continuous twisted-tape shown in Fig. 2a has been extensively investigated. Varieties of twisted-tape have been evaluated.They include short sections oftwisted tapes at the tube inlet,or periodically spaced along the tube length. Early works on twisted tapes have been reported in [4,5]. Later works have been reported in [6–28]. Fig. 2b shows the layout of a circular duct having full-length centre cleared twisted-tape. The details and method of making centre-cleared twisted-tape will be described in Section 2. Saha and Dutta [6] have observed that,for regularly spaced twisted-tape elements,thermohydraulic performance oftwisted tapes with multiple twists in the tape module is not much different from that with single twist in the tape module. Twisted tapes with gradually decreasing pitch perform worse than their uniform-pitch counterparts. Patil [7] has worked with varying width twisted-tape inserts for which both friction factor and Nusselt number are lower than those with full-width twisted tapes. Saha et al. [8,9] have 0894-1777/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved. http://dx.doi.org/10.1016/j.expthermflusci.2012.05.002 ⇑ Corresponding author. Tel.: +91 9830493430; fax: +91 3326682916. E-mail address: sujoy_k_saha@hotmail.com (S.K. Saha). Experimental Thermal and Fluid Science 42 (2012) 154–162 Contents lists available at SciVerse ScienceDirect Experimental Thermal and Fluid Science j o u r n a l homepage: w w w . e l s e v i e r . c o m / l o c a t e / e t f s