HTTA 2018 07 11 Solutions

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Heat T ransfer and Thermal Analysis - A.Y. 2017/18 - Date 11/07/2018 Surname: Name: ID number: Signature: (“matricola” or person code) Part A 1. Write the general conduction equation for a object made by a homogeneous material whose conductivity is varying with the temperature. 2. The 1D temperature profile T (r) in a homogeneous, isotropic, very long full cylinder without heat source, that exchanges uniformly by convection on the external surface, is: 1) parabolic, 2) linear decreasing, 3) linear increasing, 4) flat. 3. Sketch the sequence of flow patterns usually encountered during flow boiling in a duct. 4. Write the finite difference approximations for the first and second derivatives of temperature with respect to a space direction using the basic centered form. 5. Sketch a basic heat pipe, evidencing in the drawing the working principle. 6. Put the following convection situations in ascending order of Richardson number: 1) a high-speed water stream at 20 ◦C exchanges with a flat plate at 35 ◦C, 2) a low-speed air stream at 20 ◦C exchanges with a heater at 750◦C, 3) a iron sphere at 250 ◦C is placed in initially still water at 20 ◦C. 7. Write the definition of the void fraction in local and cross-section averaged terms, stating which quantities are represented by the used symbols. Heat T ransfer and Thermal Analysis - A.Y. 2017/18 - Date 11/07/2018 - 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…