HTTA 2021 08 26

Pagina 1

Heat Transfer and Thermal Analysis - A.A. 2020/21 - Date 26/08/2021 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 - Exercises Exercise 1 In the N = 50 tubes ( Di = 10 mm, De = 12 mm, stainless steel with conductivity λ = 15 W/mK) of a shell-and-tube counter-current heat exchanger a mass flow rate ˙Mr = 12 kg/s of a refrigerant fluid (having ρr= 1200 kg/m3,cP r = 1400 J/kgK, µr = 2·10−4 Pas,λr = 0.08 W/mK in the liquid phase) is flowing with an inlet temperature Tri = 5 ° C. Shell-side, water is flowing with mass flow rate ˙Mw = 2.5 kg/s and it is cooled from Twi = 80 ° C to Twu = 25 ° C. The shell-side convective coefficient is he = 3000 W/m 2K, while inside the tubes the convective heat transfer coefficient is hi = 2000 W/m 2K. The thermophysical properties of both fluids can be approximated as constant with temperature. Calculate: 1. the refrigerant fluid outlet temperature Tru 2. the total heat transfer coefficient referred to the internal ducts surface Ui knowing that fouling is present within the ducts with a resistance per unit area Rf i = 1·10−4 m2K/W 3. assuming that the refrigerant fluid outlet temperature Tru is 320 K, the logarithmic mean temperature difference 4. the ratio ˙Cmin/ ˙Cmax between the heat capacity rates of the two fluids Exercise 2 Turbine blades mounted to a rotating disc in a gas turbine engine are exposed to a gas stream that is at T∞ = 1200 ° C with a mean convection heat transfer coefficient of h = 250 W/m2K over the blade. The blades are fabricated from Inconel (λ = 20 W/mK) and have a length L. The blade profile has a uniform cross-sectional area of Ac = 6 × 10−4 m2 and a perimeter of P = 110 mm. A proposed…