Natural Convection From Horizontal Cylinders at Near-Critical Pressures—Part II: Numerical Simulations

A numerical investigation of laminar natural convection heat transfer from small horizontal cylinders at near-critical pressures has been carried out. Carbon dioxide is the test fluid. The parameters varied are: pressure (P), (ii) bulk fluid temperature (T_b), (iii) wall temperature (T_w), and (iv) wire diameter (D). The results of the numerical simulations agree reasonably well with available experimental data. The results obtained are as follows: (i) At both subcritical and supercritical pressures, h is strongly dependent on T_b and T_w. (ii) For T_w < T_sat (for P < P_c) and T_w < T_pc (for P > P_c), the behavior of h as a function of T_w is similar; h increases with increase in T_w. (iii) For P > P_c and large T_w (T_w > T_pc), natural convection heat transfer occurring on the cylinder is similar that observed during film boiling on a cylinder. The heat transfer coefficient decreases as T_w increases. (iv) For subcritical pressures, the dependence of h on D is h ∝ D^−0.5 in the range 25.4 ≤ D ≤ 100 μm. For larger values of D (500–5000 μm), h ∝ D^−0.24. (v) For supercritical pressures, the dependence of h on D is h ∝ D^−0.47 in the range 25.4 ≤ D ≤ 100 μm. For larger values of D (500–5000 μm), h ∝ D^−0.27. (vi) For a given P, the maximum heat transfer coefficient is obtained for conditions where T_b < T_pc and T_w ≥ T_pc. Analysis of the temperature and flow field shows that this peak in h occurs when k, Cp, and Pr in the fluid peak close to the heated surface.