HTTA 2023 09 08 Solutions

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Heat Transfer and Thermal Analysis - A.A. 2022/23 - Date 08/09/2023 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: ˆ For 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). ˆ Students 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 and 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? A 1 B near to 0 C* 0.5 D 0.25 2) A surface is considered hydrophobic if the contact angle with water is A lower than 90° B larger than 60° C* larger than 90° D lower than 60° 3) In which of the following answers are the substances placed in correct ascending order of thermal conductivity? A* air - water - stainless steel - copper - graphene B air - iron - water - copper - diamond C diamond - copper - rubber - water D graphene - iron - water - air 4) The most general form of the conduction equation for homogeneous, isotropic media with conductivity varying with temperature is: A λ∇2T + ˙U ′′′ −ρc∂T/∂τ = 0 B ∇2T + ˙U ′′′ /(ρc) +λ∂T/∂τ = 0 C* ∇(λ∇T) + ˙U ′′′ −ρc∂T/∂τ = 0 D ∇2T−ρcλ∂T/∂τ = 0 5) Considering a surface, having emissivityε and uniform temperatureTW, exchanging by radiation with another surface atTE, which of the…