HTTA 2018 09 11 Solutions

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Heat T ransfer and Thermal Analysis - A.Y. 2017/18 - Date 11/09/2018 Surname: Name: ID number: Signature: (“matricola” or person code) Part A 1. The surface tension between water and air is 72 mN/m. Calculate (writing the equation you use) the value of the pressure difference between the inside and the outside of a bubble of air in water if the bubble has radius 5 µm. 2. Sketch a second surface mirror, evidencing in the drawing the working principle and stating why for a solar reflector it is better than a first surface mirror. 3. Place the following materials in ascending order of conductivity: rubber, air, iron, stainless steel, copper. 4. State the contributions to the pressure drop for a two-phase flow in a vertical duct having constant section. 5. Draw the temperature profiles T (x) for a counter-current heat exchanger having ˙CH > ˙CC. 6. Write the general expressions of temperature and heat flux as a function of the radius in a hollow cylinder in steady-state conditions and with heat source, before application of any b.c.. 7. Write the cosine (or Lambert) law for a black body, stating the quantity indicated by each of the used symbols. Heat T ransfer and Thermal Analysis - A.Y. 2017/18 - Date 11/09/2018 - Part B INSTRUCTIONS - Read them very carefully! • test duration is 1 h 45’. After 35’ part A will be collected. If you finish part A in a shorter time, you can immediately move to part B; • you must answer to part A questions in the spaces provided after the questions themselves; remember also to fill in all the fields (surname, name, progressive number, ...) on the part A paper; • for part B, you can use only one foolscap paper for the final copy (additional papers will not be collected). On top-left of the first page of the foolscap paper you have to write (in a large…