Chapter 1, Problem 1.21P

An electric resistance heater is embedded in a long cylinder of diameter 30 mm. When water with a temperature of $25°\text{C}$ and velocity of 1 m/s flows crosswise over the cylinder, the power per unit length required to maintain the surface at a uniform temperature of $90°\text{C}$ is 28 kW/m. When air, also at $25°\text{C,}$ but with a velocity of 10 m/s is flowing, the power per unit length required to maintain the same surface temperature is 400 W/m. Calculate and compare the convection coefficients for the flows of water and air.