Evaporation of liquid fuel droplets is often studied in the laboratory by using a porous sphere technique in which the fuel is supplied at a rate just sufficient to maintain a completely wetted surface on the sphere.

Consider the use of kerosene at 300 K with a porous sphere of I-mm diameter. At this temperature the kerosene has a saturated vapor density of 0.015 kg/m³ and a latent heat of vaporization of 300kJ/kg. The mass diffusivity for the vapor-air mixture is 10^-5 m²/s. If dry, atmospheric air at V = 15 m/s and T_∞ = 300 K flows over the sphere, what is the minimum mass rate at which kerosene must be supplied to maintain a wetted surface? For this condition, by how much must T_∞ actually exceed Ts to maintain the wetted surface at 300 K?

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Liquid kerosene (300 K) Air V, T r T, = 300 K Wetted sphere, D