A Numerical Investigation of Heat Transfer From Small Horizontal Cylinders at Near-Critical Pressures

A numerical investigation of laminar natural convection heat transfer from small horizontal wires at near-critical pressures has been carried out with carbon dioxide as the test fluid. The parameters varied are: (i) Pressure (P): 7.50–9.60 MPa, (ii) Bulk fluid temperature (T_b): 5–50°C, (iii) Wall temperature (T_w): 5.1–200°C, and (iv) Wire diameter (D): 25.4, 76.2 and 100 μm. The steady-state Navier-Stokes equations (low Mach number asymptotic form) are solved with variable properties. The results of the numerical simulations agree reasonably well with available experimental data. The dependence of heat transfer coefficient (h) on P, T_b, T_w and D were investigated. The results obtained are as follows: (i) For given P and D, h is strongly dependent on T_b and T_w. (ii) The heat transfer coefficient decreases with increasing values of P (P/P_c > 1). (i) For fixed P, T_w and T_b, the dependence of h on D varies from h ∝ D^−0.47 to h ∝ D^−0.29 as D increases. (iii) For a given P, the maximum heat transfer coefficient is obtained for conditions where T_b < T_pc < T_w, where T_pc denotes the pseudocritical temperature. Based on the analysis of the temperature and flow field once can qualitatively show that this peak in h when k, C_p and Pr in the fluid peak close to the heated surface.