HTTA 2026 02 18 Solutions

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Heat Transfer and Thermal Analysis - A.A. 2024/25 - Date 18/02/2026 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) Two very long cylindrical shells (with negligible thickness) are co-axial. The smallest one has diameterDS = 0.1 m, the largest one has diameterDL = 0.2 m. Neglecting border effects, which is the view factor from the larger shell to the the smaller one? A1 Bnear to 0 C*0.5 D0.25 2) A surface is considered hydrophilic if the contact angle with water is: A*lower than90 ◦ Blarger than180 ◦ Clower than0 ◦ Dlarger than360 ◦ 3) In which of the following answers are the substances placed in correctdescendingorder of thermal conductivity? Aair - water - stainless steel - aluminium Bair - aluminium - copper - diamond C*diamond - aluminium - iron - water Daluminium - mercury - air - water 4) The most general form of the conduction equation for homogeneous, isotropic media with conductivity varying with temperature is: Aλ∇ 2T+ ˙U ′′′ −ρc ∂T /∂τ= 0 B*∇(λ∇T) + ˙U ′′′ −ρc ∂T /∂τ= 0 C∇ 2T+ ˙U ′′′ /(ρc) +λ ∂T /∂τ= 0 D∇ 2T−ρcλ ∂T /∂τ= 0 5) Considering a surface, having emissivityεand uniform temperatureTW,…