A comprehensive description of the adopted computational model is provided in the authors' previous study [10]. An axisymmetric solution domain was utilized for the computations. Four configurations of the vertically oriented storage tubes with *L/D* of 4, 8, 12, and 16 are investigated. The storage fluids that are used in this study are supercritical CO_{2} (Pr = 1.1) and liquid toluene (Pr = 12.41). The initial temperature of the storage fluids is assumed to be different in carbon dioxide and toluene computations, namely, 350 K and 250 K, respectively. A constant temperature difference of Δ*T* = 100 K is considered between the initial temperature of the storage fluids and the outer surface of the tube wall. The governing equations of the problem are continuity, momentum, and energy. Rayleigh number values for different computations are summarized in Table 1. Previous studies on natural convection in vertical cylinders show that turbulent flow is established for $Ra>1011\u223c1013$ [11,12]; therefore, the buoyancy-driven flow in the storage tubes of this study is initially in the turbulent regime. The standard two-equation *k*–*ε* model is utilized to model turbulence.