HTTA 2016 09 21 Solutions

Pagina 1

Heat T ransfer and Thermal Analysis - A.Y. 2015/16 - Date 2016/09/21 Surname: Name: ID number: Signature: Part A 1. Draw (qualitatively but with physical accuracy, including a clear distinction between curves and straight seg- ments and respect of the Second Principle) the steady-state temperature profiles in a passive flat plate having fixed temperatures on the external surfaces and: A) constant conductivity, B) conductivity increasing with temperature, C) conductivity decreasing with temperature. 2. Put the following materials/substances in correct ascending order of Prandtl number values: air, very viscous mineral oil, water, mercury. 3. Sketch the Nukiyama curve stating the different stages during pool boiling. 4. Write the simplified form of the Laplace-Young equation that is valid for a cylinder and explain the cause of the simplification with respect to the general form. 5. Write the Wien law, stating the meaning of the terms and the measurement units of the constant. 6. Write the expression to determine the net energy transfer by radiation between two gray surfaces enclosing a cavity. 7. Write the finite difference approximations of the first derivative of temperature with respect to space in 1D using 1) the forward form, 2) the backward form. Heat T ransfer and Thermal Analysis - A.Y. 2015/16 - Date 2016/09/21 - Part B Exercise 1 (up to 14 points) In a radiator (cross-flow, both fluids unmixed) a mass flow rate ˙Mw = 2 kg/s of water is flowing within N = 60 tubes (Di = 4 mm, L = 1 m, negligible thickness, no fouling) while the external air (atmospheric pressure, mass flow rate ˙Ma = 2.1 kg/s) is flowing between the flat plates of the radiator, that have a total surface area which is 40 times the total internal surface area of the ducts. The internal convective coefficient (for…