Aerospace Engineering - Heat Transfer and Thermal Analysis

Multiple choice test

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 diameterD=5 mm emerges from the extruder at a velocityV e= 0:2 m=s and temperatureT i= 600°C and is cooled by convection heat transfer to air in cross ow over the wire, as well as by radiation to the surroundings. The cooling air has a velocityV=5 m=s and a temperatureT 1= 25°C. Neglecting axial conduction, compute: 1.the mean convective heat transfer coecient, in W=m2 K 2.the temperature of the wire atx=L=5 m, in°C, considering a mean convective heat transfer coecienth =100 W=m2 K and neglecting radiation 3.the temperature of the wire atx=L=5 m, in°C, considering a mean convective heat transfer coecienth =100 W=m2 K and also radiation with the surrounding walls atT sur= 80°C with a mean radiative heat transfer coecienth r= 50 W=m2 K 4.the time required to cool down the copper wire to a temperatureT f= 200°C, in s, considering a mean convective heat transfer coecienth =100 W=m2 K and also radiation with the surrounding walls at Tsur= 80°C with a mean radiative heat transfer coecienth r= 50 W=m2 K Thermophysical properties of copper: density,=8900 kg=m3 , specic heat,c=0:4 kJ=kgK Thermophysical properties of air: thermal conductivity,=3:710 2 W=mK, thermal diusivity, =4:34810 5 m2 =s,Pr = 0:69 Nusselt number correlations: N u L= (0 :037Re4 =5 L 871)P r1 =3N u D= 0 :3 +0 :62Re1 =2 DP r1 =3 1 + (0:4=P r)2 =3 1=4" 1 + ReD282000  5=8# 4=5N u L= 0 :18 P r0 :2 +P rRa L 0:29N u D= 2 +0 :589Ra1 =4 D 1 + (0:469=P r)9 =16 4=9