Ambient air at \(20^{\circ} \mathrm{C}\) flows over a 30 -cm-diameter hot
spherical object with a velocity of \(4.2 \mathrm{~m} / \mathrm{s}\). If the
average surface temperature of the object is \(200^{\circ} \mathrm{C}\), the
average convection heat transfer coefficient during this process is
(a) \(8.6 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\)
(b) \(15.7 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\)
(c) \(18.6 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\)
(d) \(21.0 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\)
(e) \(32.4 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K}\)
(For air, use $k=0.2514 \mathrm{~W} / \mathrm{m} \cdot \mathrm{K},
\operatorname{Pr}=0.7309, \nu=1.516 \times\( \)\left.10^{-5} \mathrm{~m}^{2} /
\mathrm{s}, \mu_{\infty}=1.825 \times 10^{-5} \mathrm{~kg} / \mathrm{m} \cdot
\mathrm{s}, \mu_{s}=2.577 \times 10^{-5} \mathrm{~kg} / \mathrm{m} \cdot
\mathrm{s}\right)$