Air at 20 degree C flows over a 4m long and 3m wide surface

Air at 20 degree C flows over a 4-m long and 3-m wide surface of a plate whose temperature is 80 degree C with a velocity of 5 m/s. The length of the surface for which the flow remains laminar is 1.5 m 1.8 m 2.0 m 2.8 m 4.0 m (For air, use k = 0.02735 W/m-K, Pr = 0.7228, v = 1.798 times 10^-5m^2/s)

Solution

Re = VL/neu

Taking critical Reynolds number over flat plate, Re = 5*10⁵ we get

5*10⁵ = 5*L / (1.798*10^-5)

L = 1.798 m ~ 1.8 m

Hence, option b is correct.