HTTA 2023 02 02 Solutions

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Heat Transfer and Thermal Analysis - A.A. 2021/22 - Date 02/02/2023 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 Students having right to a test reduction should answer only to: all part A questions, part B questions 1 and 3, queries 1, 2, 3 of the first part B exercise, queries 1 and 3 of the second part B exercise. Part A 1) If the fluid dynamics boundary layer is larger than the thermal one, the value of the Prandtl number is: A* larger than 1 B smaller than 1 C equal to 1 D negative 2) In which of the following answers is the Wien law correctly written: A λmax emission/T = const B* λmax emission·T = const C ε = 1/α D ε =α 3) In steady-state conditions, the general heat flux profile ˙Q ′′ (r) in a homogeneous, hollow sphere in which a heat source ˙U ′′′ is present reads as (C1> 0): A ˙Q ′′ (r) =− ˙U ′′′ /(3λ)r +C1/r2 B none of the other answers is correct C* ˙Q ′′ (r) = ˙U ′′′ /3r−C1λ/r2 D ˙Q ′′ (r) = ˙U ′′′ /3r 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 B ∇2T + ˙U ′′′ /(ρc) =−λ∂T/∂τ C* ∇(λ∇T) + ˙U ′′′ = ρc∂T/∂τ D ∇2T = 0 5) A diffuse, opaque, gray emitter hasε = 0.7. If it is at T = 35 ° C and its total irradiation (per unit surface) isEi = 250 W, what is its total radiosity (per unit surface)? A 358 W B 75 W C 283 W D* 433 W 6) A sequence of flow patterns encountered during flow condensation in a pipe is: A* single phase vapour - annular flow - slug/plug flow - bubbly flow - single-phase liquid B single phase vapour - bubbly flow - mist flow - annular flow - single-phase liquid C single phase liquid - bubbly flow - annular flow - mist flow -…