Chapter 7, Problem 7.42P

Consider a thin, $50\text{mm}\times 50\text{mm}$ fuel cell similar to that of Example 1.5, with air in parallel flow over its surfaces. Very small-diameter wires are stretched across both sides of the fuel cell at a distance $x=x_c$ from the leading edge in order to trip the flow into turbulent conditions. Using an appropriate correlation from Chapter 7, determine the minimum velocity needed to sustain the fuel cell at $T_c=77°\text{C,}$ and the associated location of the wire. The air and large surroundings are at $T_{\infty }=T_{\text{sur}}=27°\text{C}$ and the fuel cell dissipates ${\stackrel{.}{E}}_g=11\text{W}.$ The fuel cell emissivity is $\epsilon =\mathrm{0.85.}$