HTTA 2025 08 28 Solutions

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Heat Transfer and Thermal Analysis - A.A. 2024/25 - Date 28/08/2025 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. NOTES: ˆStudents attending the 5 CFU course may have some different questions in part A. ˆFor the exercise queries, in case none of the multiple-choice answers the exercise matches the result you get, write your own result instead of marking an answer (so that in case we did a mistake and your answer is correct you can get the related points). ˆStudents having right to a test reduction should answer only to: all part A questions; part B questions 1 and 2; queries 1, 2 of the first part B exercise; queries 1, 2, 3 of the second part B exercise. Part A 1) For a generic sphere with uniform conductivity, the steady-state, 1D, heat flux profile can be written as: A ˙Q ′′ =− ˙U ′′′ r 3λ − C1λ r2 B* ˙Q ′′ = ˙U ′′′ r 3 − C1λ r2 C ˙Q ′′ = ˙U ′′′ r 2 + C1λ r D ˙Q ′′ = ˙U ′′′ r 2λ − C1λ r 2) For a diffuse, gray, opaque surface: Aε= 1 Bα= 1 C*τ= 0 Dρ= 0 3) For a hydrophilic surface: Athe contact angle is large because the liquid tends to spread as much as possible on the surface B*the contact angle is low because the liquid tends to spread as much as possible on the surface Cthe contact angle is negative because the liquid surface energy is lower than the solid surface energy Dthe contact angle is negative because the liquid surface energy is larger than the solid surface energy 4) The general form of the Laplace-Young equation reads: A∆P= 2σ / R B∆P= 1/RI −1/RII σ C*∆P=σ(1/R I + 1/RII) Dnone of the other answers is correct 5) The Weber number, used for convection with phase change, is defined as? A ρV w2DC σ2 LV B* ρV w2DC σLV C ρLw2D2 C σ3 LV D ρLw2D2…