Numerical investigation of natural convection heat transfer from vertical cylinder with annular fins

Natural convection heat transfer from a vertical cylinder with annular fins has been studied numerically by varying the Rayleigh number (Ra) in both laminar (104 ≤ Ra ≤ 108) and turbulent (1010 ≤ Ra ≤ 1012) regimes. The computations were carried out by varying the fin to tube diameter ratio (D/d), fin spacing to tube diameter ratio (S/d) in the range of 2-5 and 0.126-5.840 respectively. In the present study, numerical simulations of full Navier-Stokes equation along with the energy equation have been conducted for a vertical cylinder with annular fins of constant thickness using the algebraic multi-grid solver of FLUENT 15. Optimization study of the conjugate heat transfer characteristics has been carried out to find the best fin spacing for maximum heat transfer for the turbulent flow. With the addition of fins to the heated isothermal tube surface, heat transfer goes on increasing for laminar flow and turbulent flow heat transfer first increases and gets a maximum value then starts to decrease. The optimum fin spacing for maximum heat transfer for the cases of turbulent flow varies between S/d = 0.28 to 0.31 (7.0 mm-7.7 mm) for the turbulent regime. The effect of parameters like D/d, S/d and Ra on Nu are analyzed, and correlations for average Nusselt number has been developed for both laminar and turbulent regimes.