Collection of exercises

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Exercises for the HTTA course - A.Y. 2014-15 (version 2015/04/26) Heat Transfer and Thermal Analysis A.Y. 2014-15 Exercise Lectures 1 Exercises for the HTTA course - A.Y. 2014-15 (version 2015/04/26) 2 Conduction This chapter will be devoted to exercises about conduction, with boundary conditions of fixed tem- perature, fixed fluxes, convective or radiative exchanges. For the latter, the convective coefficient will be assumed as known, and radiation will be considered only in terms of total emissive power. Exercise 1 Topic: Heat transfer and moist air In a room with a window, air is at Tfl 1 = 20 ◦C and ϕfl 1 = 50 %. Calculate its dew point tempe- rature and at which temperature of the external air condensation start happening on the internal side of the window glass. Glass thickness s = 3 mm, glass conductivity kglass = 0.8 W/mK, convection coefficients: internal hfl 1 = 8 W/m 2K, external hfl 2 = 25 W/m 2K. Solution: Dew point temperature T p1, giving condensation on the internal side of the glass, may be read on moist air diagrams or calculated using the saturated (water) vapour tables. Heat flux from the internal air to the outside ambient can be calculated using the Newton equation: ˙Q ′′ =−hfl 1· (Tp1−Tfl 1) =−hfl 1· (Tdew−Tfl 1) but also (using the series of passive layers): ˙Q ′′ =−(Tfl 2−Tfl 1)/(1/hfl 1 +s/kglass + 1/hfl 2) So: Tfl 2 =Tfl 1− ˙Q ′′ · (1/hfl 1 +s/kglass + 1/hfl 2) Notes: If outside of the room it is not raining, it is not possible to have dew on the external side of the glasses, because the minimum temperature in the system is Tfl 2, so to have dew on the external side of the glasses it should be Tfl 2<T R which is impossible. Numerical results: 3 Exercises for the HTTA course - A.Y. 2014-15 (version 2015/04/26) Tp1 = 9.27 ◦C. ˙Q ′′ = - 8· (9.27 - 20) =…