An air heater may be fabricated by coiling Nichrome wire

An air heater may be fabricated by coiling Nichrome wire and passing air in cross flow over the wire. Consider a heater fabricated from wire of diameter D = 1 mm, electrical resistivity Pe = 10-6 Ω ∙ m, thermal conductivity k = 25 W/m ∙ K, and emissivity ε = 0.20. The heater is designed to deliver air at a temperature of Try. = 50°C under flow conditions that provide a convection coefficient of h = 250 W/m2 ∙ K for the wire. The temperature of the housing that encloses the wire and through which the air flows is Tsur = 50°C.

If the maximum allowable temperature of the wire is T max = 1200°C, what is the maximum allowable electric current I? If the maximum available voltage is ∆E = 110 V, what is the corresponding length L of wire that may be used in the heater and the power rating of the heater? Him: In your solution, assume negligible temperature variations within the wire, but after obtaining the desired results, assess the validity of this assumption


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