It has been widely reported in literature that heat transfer rates in helical coils are higher as compared to those in straight tubes. Due to the compact structure and high heat transfer coefficient, helical coil heat exchangers find extensive use in industrial applications such as power generation, nuclear industry, process plants, heat recovery systems, refrigeration, food industry, etc. (Berger et al. 1983; Abdalla 1994; Rao 1994). Due to the extensive use of helical coils in these applications, knowledge about the pressure drop, flow patterns, and heat transfer characteristics are very important. A double pipe helical coil is advantageous because the secondary flow which is responsible for heat transfer increase in helical tubes will be present in both inner and annulus tubes.

Literature Survey

Huttl and Friedrich [1] used direct numerical simulation for turbulent flow in straight, curved and helically coiled pipes to determine the effects of curvature and torsion on the flow patterns. They showed that turbulent fluctuations are reduced in curved pipes compared to the straight pipes. Li et al. [2] numerically studied the turbulent convective heat transfer in the entrance region of a curved pipe with a uniform wall temperature. Lin and Ebadian [3] numerically studied the effects of inlet turbulence intensity on the development of the turbulent flow and heat transfer in helically coiled pipes. Roger & Mayhew [4] studied heat transfer to fluid flowing inside a helical pipe which was heated by steam. Mori and Nakayama [5] studied heat transfer under constant wall temperature boundary condition for the same helical coils and observed that the Nusselt number is remarkably affected by a secondary flow due to curvature. CFD study of helically coiled double pipe heat exchangers for laminar flow situations were carried out by Rennie and Raghavan [6, 7]. Goering et al. [8] have studied fully developed laminar convective heat transfer in curved pipes to investigate the dual influence of curvature and buoyancy. Dennis and Ng [9] numerically studied laminar flow through a curved tube using a finite difference method with emphasis on two versus four vortex flow conditions. Kao [10] studied the torsion effect on fully developed flow in a helical pipe using a series expansion method to solve the governing differential equations. Kalb and Seader [11] numerically studied the heat transfer in helical coils in the case of uniform heat flux using an orthogonal toroidal coordinate system. Fully developed laminar flow and heat transfer was studied numerically by Zapryanov et al. [12] using a method of fractional steps for a wide range of Dean (10–۷۰۰۰) and Prandtl (0.005–۲۰۰۰) numbers. The effect of pitch on heat transfer and pressure drop was studied by Austen and Soliman [13] for the case of uniform wall heat flux. Most of the studies in literature are carried out using constant heat flux or constant wall temperature boundary conditions. In this paper the author has considered actual conjugate heat transfer. Also the optimization of double pipe helical coil is considered in this paper which has not been reported in earlier research works.