835 PAWAR et al: REVIEW OF HEAT TRANSFER THROUGH HELICAL COILS OF CIRCULAR CROSS SECTION Journal of Scientific & Industrial Research Vol. 70, October 2011, pp. 835-843 *Author for correspondence E-mail: sspawar_ltcoe@yahoo.co.in A critical review of heat transfer through helical coils of circular cross section S S Pawar 1* , Vivek K Sunnapwar 1 and B A Mujawar 2 1 Lokmanya Tilak College of Engineering, Navi Mumbai 400 709, India 2 Shri Pandurang Pratishtan, Karmayogi Engineering College, Tal. Pandharpur, Dist. Solapur 413 304, India Received 17 January 2011; 16 August 2011; accepted 19 August 2011 Heat transfer enhancement techniques [active techniques (electric field, acoustic or surface vibration, etc.) and passive techniques (fluid additives or special surface geometries)] can improve performance of heat exchangers to perform a certain heat transfer duty. Curved tubes have been used as one of the passive heat transfer enhancement techniques and are most widely used tubes in several heat transfer applications. This paper reviews heat transfer through helical coil of circular cross sections in terms of different dimensionless numbers, their validity, and effect of geometry, friction factors, different coil curvature ratios, fluid types, laminar and turbulent flow on heat transfer rate. Keywords : Curved tubes, Friction factor, Helical coil, Laminar flow, Turbulent flow Introduction Heat exchangers are commonly used for heat transfer (HT) between two or more fluids of different temperatures in refrigeration & air-conditioning systems, heat recovery processes, chemical reactors, food & dairy processes, power engineering and other thermal processing plants. Due to their compact structure and high HT coefficient, curved tubes as one of the passive HT enhancement techniques are widely used in various industrial applications. Helical coils of circular cross section have been used in a wide variety of applications due to simplicity in manufacturing. This paper reviews research on HT in helical coil of circular cross section and single phase flow. Review of Experimental Works Due to curvature of tubes, as fluid flows through curved tubes, centrifugal force is generated. A secondary flow induced by centrifugal force has significant ability to enhance HT rate. Kalb & Seader 1 studied entrance region HT to gases flowing in a uniform wall temperature in helical coil of aspect ratio 15 for turbulent to laminar flow [Reynolds (Re) number (He, 1610; air, 2520-12800) and Prandtl (Pr) number (He, 0.67; air, 0.71)] and thus developed a novel gradient method based on measurement of the wall internal and external surface- temperature distributions for design or rating of the entire coil for fully developed HT. Manafzadeh et al 2 examined experimentally mechanism of Nusselt (Nu) number oscillation at the junction of a straight and helical tube. Experiments were carried out for a range of Dean number using distilled water and geometry showed a 50% increase of Nusselt number over that for the helical coil. It was also shown that Nu number enhancement could be achieved in heat exchanger coils for laminar flow by variations in pipe curvature, which exploit thin boundary layers created at the onset of a secondary flow development. Austen & Soliman 3 studied influence of pitch on pressure drop and HT characteristics of helical coil [Two pairs of coils (D/d = 29 and 49) were tested for laminar flow and for h/d = 60, h/D = 2.07, 50 < Re < 7000, 3 < Pr < 6, 300 < Gr < 5800] explored for the condition of uniform input heat flux. Significant pitch effects, noted in friction factor and Nu number results at low Re number, were attained to free convection and diminish as Re number increases. Enhancements in the values of friction factor and Nu number were noted in the region where coil pitch had an influence. Prasad et al 4 performed experiments for determining pressure drop, HT characteristics and performance of a helical coil heat exchange device and the following correlations were recommended for the