//Example 5-14 Fan Selection for Air Cooling of Computer
V = 12*40*40 //volume of computer case [cm^3]
D = 5 //diameter of hole [cm]
deltat = 1 //time required to replace air in computer case completely [s]
eta_fan = 30 //efficiency of fan [%]
rho_air = 1.2 //density of air [kg/m^3]
alpha_2 = 1.10 //kinetic energy correction factor