Question

# A metallic airfoil of elliptical cross section has a mass of 50 kg, surface area of $12 m^2,$ and a specific heat of $0.50 kJ/kg \cdot K.$ The airfoil is subjected to air flow at 1 atm, $25^\circ C,$ and 5 m/s along its 3-m-long side. The average temperature of the airfoil is observed to drop from $160^\circ C$ to $150^\circ C$ within 2 min of cooling. Assuming the surface temperature of the airfoil to be equal to its average temperature and using momentum-heat transfer analogy, determine the average friction coefficient of the airfoil surface.

Solution

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$\textbf{Known:}$

• $m = 85 \hspace{1mm} \text{kg}$
• $A= 12\hspace{1mm} \text{m}^2$
• $C_p= 0,5 \hspace{1mm} \frac{\text{kJ}}{\text{kgK}}$
• $V= 5 \hspace{1mm} \frac{\text{m}}{\text{s}}$
• $L= 3\hspace{1mm} \text{m}$
• $T_1 = 160 \hspace{1mm} \text{\textdegree}{\text{C}}$
• $T_2 = 150 \hspace{1mm} \text{\textdegree}{\text{C}}$
• $t= 2 \hspace{1mm} \text{min} = 120 \hspace{1mm} \text{s}$

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