HTTA 2022 01 14

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Heat Transfer and Thermal Analysis - A.A. 2020/21 - Date 14/01/2022 VERY IMPORTANT: the answers must be written on the “Results” paper sheet, not on this text. Only the “Results” paper sheet must be uploaded and will be evaluated! Part A Part B - Questions Part B - Exercise 1 Part B - Exercise 2 In an extrusion process, a copper wire of diameter D = 5 mm emerges from the extruder at a velocityVe = 0.2 m/s and temperature Ti = 600 ° C and is cooled by convection heat transfer to air in cross flow over the wire, as well as by radiation to the surroundings. The cooling air has a velocity V = 5 m/s and a temperature T∞ = 25 ° C. Neglecting axial conduction, compute: 1. the mean convective heat transfer coefficient, in W /m2K 2. the temperature of the wire at x =L = 5 m, in ° C, considering a mean convective heat transfer coefficient h = 100 W/m2K and neglecting radiation 3. the temperature of the wire at x =L = 5 m, in ° C, considering a mean convective heat transfer coefficient h = 100 W/m2K and also radiation with the surrounding walls at Tsur = 80 ° C with a mean radiative heat transfer coefficient hr = 50 W/m2K 4. the time required to cool down the copper wire to a temperature Tf = 200 ° C, in s, considering a mean convective heat transfer coefficient h = 100 W/m2K and also radiation with the surrounding walls at Tsur = 80 ° C with a mean radiative heat transfer coefficient hr = 50 W/m2K Thermophysical properties of copper : density, ρ = 8900 kg/m3, specific heat, c = 0.4 kJ/kgK Thermophysical properties of air : thermal conductivity, λ = 3.7 × 10−2 W/mK, thermal diffusivity, α = 4.348 × 10−5 m2/s, Pr = 0.69 Nusselt number correlations: Nu L = (0.037Re4/5 L − 871)Pr 1/3 Nu D = 0.3 + 0.62Re1/2 D Pr 1/3 [ 1 + (0.4/Pr )2/3]−1/4 [ 1 + ( ReD 282000 )5/8]4/5 Nu L = 0.18 ( Pr 0.2 +PrRaL…