Laminar free convective heat transfer from isothermal vertical slender cylinder

An experimental study of the laminar free convective average heat transfer in air from isothermal vertical slender cylinder having circular cross-section was performed using a transient technique. These results obtained for Prandtl number Pr = 0.71 in the range of Rayleigh number 1.5 × 108 < RaH < 1.1 × 109 and dimensionless height of 0 < H/D < 60 are correlated with the equationNuH=ARaHnwhere A = 0.519 + 0.03454(H/D) + 0.0008772(H/D)2 + 8.855 × 10−6(H/D)3 and n = 0.25–0.00253 (H/D) + 1.152 × 10−5 (H/D)2 agree well with the known numerical data which are approximated with the equation NuH/NuH−FP = f(GrH, H/D) valid in laminar boundary layer region (GrH-crit ⩽ 4 × 109) for fluids of Prandtl number within 0.01 < Pr < 100 and for the transverse curvature parameter ξ=32GrH0.25HD<5. Besides the slenderness criterion determining situation when the vertical isothermal cylinder of circular cross-section cannot be treated as a flat plate is presented in the form GrH0.25DH⩽f(Pr).