HTTA 2017 07 19 Solutions

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Heat T ransfer and Thermal Analysis - A.Y. 2016/17 - Date 19/07/2017 Surname: Name: ID number: Signature: (“matricola” or person code) Part A 1. Write the general equation for conduction for a medium having conductivity which varies with temperature. 2. Draw (qualitatively but with physical accuracy) the steady-state temperature profiles in a passive flat plate made of 3 layers A, B, C and having fixed temperatures on the external surfaces, knowing that the thickness is the same for all layers and the conductivity is very high for layer A, very low for layer B and medium for layer C. 3. Write the Kirchhoff law for real opaque emitters, declaring the quantities represented by the used symbols. 4. Write the Richardson number and the criterion in which it is used to distinguish between forced, natural and mixed convection. 5. Write the Laplace-Young equation, declaring the quantities represented by the used symbols, for a generic liquid-gas interface. 6. Put the following materials/substances in correct ascending order of conductivity: water, copper, diamond, air, stainless steel. 7. Write the sequence of flow patterns commonly gone through by the fluid during flow boiling. Heat T ransfer and Thermal Analysis - A.Y. 2016/17 - Date 19/07/2017 - Part B Exercise 1 (up to 15 points) In a heat exchanger a mass flow rate ˙Mr = 8 kg/s of a refrigerant fluid ( ρr= 1200 kg/m 3, cP r = 1400 J/kgK, µr = 2·10−4 Pas, λr = 0.1 W/mK in the liquid phase) is flowing with an inlet temperature Tri = 15 ◦C within N = 160 tubes (Di = 20 mm, L = 3 m, negligible thickness, no fouling). Condensing water is flowing between the ducts and the external shell (perfectly adiabatic) of the heat exchanger. Water enters the heat exchanger as saturated vapour at Tw = 100 ◦C and exits as saturated liquid (latent heat…