Heat Transfer Problems

ME 341 HEAT TRANSFER FOR ENGINEERING Exam 2

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3) You are the mechanical engineer working in a power plant where at the secondary air-cooled condenser steam flows through pipes whose outer diameter is D1=3 cm and whose walls are at a temperature of 120°C. Circular aluminum fins (k=180 W/m - °C) of outer diameter D2=6 cm and constant thickness t=2 mm are considered to be attached to the pipe, as shown below. The space between the fins is 3 mm, and thus there are 200 fins per meter length of the tube. Heat is transferred to the surrounding air that is at T=25°C, with an average convection heat transfer coefficient of h=60 W/m2 - °C. The fin tip condition may be considered to be adiabatic. Cross-sectional area and the perimeter of the fin may be reasonably approximated using the inner diameter of the fin. a) Determine the percentage increase in heat transfer from the tube per meter of its

length as a result of adding fins. b) Justify the fin outer diameter of D2=6 cm by investigating the impact of

alternative fin outer diameters of D2=4 cm; D2=8 cm; D2=10 cm; and D2=12 cm. Provide an Excel plot of the change in the percentage increase in heat transfer as a function of the fin outer diameter. Comment on your findings.

(35 points)