Numerical Investigation of Laminar Forced Convection in a Concentric Curved Annular Pipe

Abstract

Three-dimensional numerical simulation is performed to investigate the characteristics of performance evaluation criteria through a concentric curved annular pipe. Four types of curvature angles (45○-bend annular pipe, 90○-bend annular pipe, 120○-bend annular pipe and U-bend (180○-bend) pipe). Finite Volume Method (FVM) is used to solve the governing equations of steady-state fully-developed laminar flow. Thermal conditions of the outer pipe surface insulated (adiabatic) and the inner pipe at constant temperature (isothermal). The SIMPLE algorithm is employed to the pressure – velocity coupling scheme. In this research, the attention has been paid to the effect of curvature angles, aspect ratio, curvature ratio and Dean number on the flow development and characteristics of convection heat transfer. The scope of numerical simulations is conducted for curvature angles, aspect ratio, curvature ratio and Dean number in the ranges of 45○, 90○, 120○ and 180○, 0.1 – 0.75, 0.1, 0.2 and 0.5 and 32 – 632, respectively. Features of axial velocity, temperature field and secondary flow streamlines along the concentric curved annulus flow direction are observed. Additionally, the circumferential friction factor and average Nusselt number are obtained at different axial locations. The numerical results show that Dean number (or can say Reynolds number) and aspect ratio have significant influences on the fluid flow and heat transfer characteristics. Also, it was found that the enhancement of performance evaluation criteria for the concentric curved annular pipe is higher than that of straight annular pipe attributed to the formation of secondary flows in a cross sectional direction, i.e., a pair of counter-rotating vortices also known as Dean's vortices, and the impact of inner pipe wall boundary. The normalized average Nusselt number and performance evaluation criteria increase with increasing Dean number and curvature ratio for all curvature angles of concentric curved annular pipe. Moreover, the normalized average Nusselt number and performance evaluation criteria increase with decreasing the aspect ratio. On the other hand, the normalized friction factor-Reynolds number product decreases with increasing aspect ratio because the annular gap between surfaces of the inner and outer pipes (the boundaries of annulus) increases with decreasing aspect ratio. I In addition, it is determined that the maximum values of Performance Evaluation Criteria up to 1.42, 1.98, 2.051 and 2.14 for 45○-bend, 90○-bend, 120○-bend and U-bend annular pipe angles, respectively, at Dean number = 632.5, aspect ratio = 0.1 and the curvature ratio = 0.5 compared with that of the straight annular pipe.