Pressure drop and heat transfer augmentation due to coiled wire inserts during laminar flow of oil inside a horizontal tube M.A. Akhavan-Behabadi a , Ravi Kumar b, * , M.R. Salimpour c , R. Azimi a a School of Mechanical Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran b Department of Mechanical & Industrial Engineering, Indian Institute of Technology, Roorkee 247667, India c Department of Mechanical Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran article info Article history: Received 13 August 2008 Received in revised form 13 June 2009 Accepted 14 June 2009 Keywords: Enhancement Heat transfer Fanning friction factor Coiled wire insert Turbulence promoter abstract An experimental investigation has been carried out to study the enhancement in heat transfer coefficient by coiled wire inserts during heating of engine oil inside a horizontal tube. The test-section was a double- pipe counter-flow heat exchanger. The engine oil flowed inside the internal copper tube, while saturated steam, used for heating the oil, flowed in the annulus. First of all, the data were acquired for the heating of engine oil while flowing in the plain tube. Later, seven coiled wires having pitches of 12–69 mm and wire diameters of 2.0 mm and 3.5 mm were put one by one in the oil-side of test-section. The effects of Reynolds number and coiled wire geometry on the heat transfer augmentation and fanning friction factor were studied. Finally, two empirical correlations have been developed for predicting the heat transfer enhancement of these coiled wire inserts. These correlations predict the experimental Nusselt number in an error band of 20 percent. Ó 2009 Elsevier Masson SAS. All rights reserved. 1. Introduction The enhancement of heat transfer has always been a consistent endeavor of designers to reduce the heat exchanger dimensions and consequently the equipment material cost. The use of enhancement techniques to augment inside tubes convective heat transfer coef- ficient has been investigated for many years [1,2]. These techniques are classified into two main categories viz. active techniques and passive techniques. Active techniques are such methods which need external force field to augment heat transfer, for example, tube or fluid vibrations; on the other hand, passive techniques are such ones which need no external energy to enhance heat transfer, such as, tube inserts like twisted tapes, coiled wires, in-tube meshes and brushes and so on [3–6]. The heat transfer can substantially be increased by providing microfin tube or helical wire inserts [7]. It is also envisaged that during heating of liquid the rotational centrif- ugal convection has favorable effect [6,8]. Heat transfer during laminar flow is often envisaged in the engi- neering applications, especially at viscous liquid heating and cooling processes. The insert technology is useful for laminar region and coil wire insert has less pressure drop penalty in comparison to twisted tape insert [9]. However, the fanning friction factor of the tube with the coiled wire inserts increases [10]. In an engine oil heating system the oil-side heat transfer coefficient is very low; hence, the enhancement of oil-side heat transfer coefficient is imperative in order to increase the overall heat transfer coefficient of the system. Chiou [11] investigated the effect of coiled wire inserts during cooling of oil inside the tubes. The enhancement in heat transfer coefficient was observed due to disruption of laminar sub-layer of liquid film and increasing the degree of turbulence. The coiled wires usually do not generate swirling flow. Chiou [11] also affirmed that in the heating mode operation, the swirling flow has favorable centrifugal convection effect, which can substantially increase the heat transfer coefficient between the flow and tube wall. In the cooling mode operation; however, the swirling flow may have an adverse centrifugal convection effect which may even reduce the convection effect. Garcia et al. [12] investigated the heat transfer enhancement due to coiled wire inserts in laminar-transient-turbulent regimes for 80 < Re < 90,000 and 2.8 < Pr < 150 experimentally. They used water and water–propylene glycol mixtures at different tempera- tures to cover a wide range of fluid properties; however, they considered the properties of the fluid to be constant in each test run. * Corresponding author. Tel.: þ91 1332 285740; fax: þ91 1332 285665. E-mail addresses: ravikfme@iitr.ernet.in (R. Kumar), salimpour@cc.iut.ac.ir (M.R. Salimpour). Contents lists available at ScienceDirect International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts 1290-0729/$ – see front matter Ó 2009 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ijthermalsci.2009.06.004 International Journal of Thermal Sciences 49 (2010) 373–379