raccolta1314

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Heat Transfer and Thermal Analysis - A.A. 2013/14 - Date 2014/07/11 Surname: Name: ID number: Signature: Part A (exact answer 1.5 pts, no answer 0 pts, wrong answer −0.5 pts. Part A sufficient if ≥ 16) For a double-pipe heat exchanger in which the hot fluid undergoes a phase change, the energy balance allows to state that the heat transfer rate between the fluid is given by: ⃝ ˙QH→C = ˙MHcPH (THu−THi) ⃝ ˙QH→C =hHu−hHi ⨂ ˙QH→C = ˙MHTH(sHu−sHi) ⃝ ˙QH→C =UHu−UHi 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): ⃝ ˙Q ′′ (r) =−˙U ′′′ /(3λ)r +C1/r2 ⃝none of the other answers is correct ⨂ ˙Q ′′ (r) = ˙U ′′′ /3r−C1λ/r2 ⃝ ˙Q ′′ (r) = ˙U ′′′ /3r The most general form of the conduction equation for homogeneous, isotropic media with conductivity varying with temperature is: ⃝λ∇2T + ˙U ′′′ =ρc∂T/∂τ ⃝∇2T + ˙U ′′′ /(ρc) =−λ∂T/∂τ ⨂∇(λ∇T) + ˙U ′′′ =ρc∂T/∂τ ⃝∇2T = 0 If the ratioGr/Re2 is≈ 1, convection is: ⃝natural ⃝forced ⨂mixed ⃝it depends also on the value of Prandtl number A gray emitter hasε = 0.6,ρ = 0.4,τ = 0. If it is atT = 25◦C and its total irradiation isEi = 100 W, what is its total radiosity? ⨂309 W ⃝269 W ⃝40 W ⃝60 W A small sphere (D1 = 1 cm) is completely enclosed in another much larger one (D2 = 50 cm). Both are gray emitters. The small one is atT1 = 150◦C and hasε1 = 0.8. The large one is atT2 = 75◦C. How much radiative power do they exchange at steady state? ⃝ ˙Q ′′ = 0.3094 W/m2 ⨂ ˙Q = 0.2475 W ⃝the calculation is impossible as data are missing ⃝ ˙Q = 0.2475 W/m2 For an air bubble havingD = 2 mm, immersed in water at 20◦C (surface tension of water 72 mN/m), the pressure difference between the internal air and the external water is approximately: ⨂144 Pa ⃝208 Pa ⃝312 Pa…