A thermocouple, with a spherical junction diameter of 0.5 mm, is used for measuring the temperature of hot air flow in a circular duct. The convection heat transfer coefficient of the air flow can be related with the diameter (D) of the sphere and the average air flow velocity (V) as h = 2.2(V /D)0.5, where D, h, and V are in m, W/m2•K, and m/s, respectively. The properties of the thermocouple junction are k = 35 W/m•K, rho = 8500 kg/m3, and cp = 320 J/kg•K. The maximum response time of the thermocouple to record 78 percent of the initially applied temperature difference is 5 s. Determine the minimum air flow velocity that the thermocouple can be used.