HTTA 2026 01 27 Solutions

Pagina 1

Heat Transfer and Thermal Analysis - A.A. 2024/25 - Date 27/01/2026 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) In which of the following equations is the Rayleigh number correctly written? ARa Dc = g σ(Tf −T w)D 3c µf af BRa Dc = g λLV (Tf −T w)D 3c νf af CRa Dc = g kP (Tf +T w)D 3c 2µ f af D*Ra Dc = g kP (Tf −T w)D 3c νf af 2) Which of the following statements is always true for a diffuse, gray, transparent surface: Aα <0.5 Bα= 1 C*α <1 Dα= 0.5 3) For a hydrophobic surface: A*the contact angle is larger then 90° Bthe contact angle is larger then 180° Cthe contact angle is lower then 90° Dthe contact angle is lower than 10° 4) The 1D,T=T(r), Poisson equation in spherical coordinates can be written as: A* 1 r2 ∂ ∂r r2 ∂T ∂r ! =− ˙U ′′′ λ B 1 r ∂ ∂r r ∂T ∂r ! =− ˙U ′′′ λ C 1 r3 ∂ ∂r r ∂T ∂r ! =− ˙U ′′′ λ D 1 r ∂ ∂r r2 ∂T ∂r ! = 0 5) The correct sequence of regimes during pool boiling is: Amixed convection - nucleate boiling - jets and columns - radiation boiling Bforced convection - film boiling - plugs and slugs - bubble boiling Cfree convection - film boiling - stratified boiling - nucleate boiling…