Exam 2018 06 21 Solution

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Department of Energy Politecnico di Milano Author E. Colombo Pag.1 of 13 Date 26/06/2018 Milan, 21st June 2018 Exam – Advanced Thermodynamic and Thermoeconomics Session of 21-06-2018 Exercise 1. (8pt) Consider a process made by the following sequence: 1. an adiabatic combustion chamber where a complete and isobaric combustion of acetylene with excess air takes place. 2. after combustion, flue gases enter a counter-flow heat exchanger and are cooled at constant pressure heating a flow of water that enter as liquid and exit as superheated steam The main hypotheses and data are below described: - Acetylene , C2H2 has LHV=1256460 kJ/kmol and exch=1269310 kJ/Kmole; - Air and Acetylene enter the combustion chamber @ 353 K and environmental pressure. - The adiabatic flame temperature results 2250 K and is associated to the given excess air. - Flue gasses in the heat exchanger are cooled down @ 673 K, - Water enter the heat exchanger @ 323 K and 500 kPa (h=209.3 kJ/kg, s=0.7027 kJ/kg-K) - The superheated steam exits then @ 873 K and 500 kPa (h=3701 kJ/kg, s=8.3513 kJ/kg-K). - Environmental temperature and Pressure are assumed to be T0=298 K and p0=101.3 kPa, - For products P and reactants R useful data are below given: ΔhR (353 K) ΔhP (2250 K) ΔhP (673 K) exph,R (353 K) exph,P (2250 K) exph,P (673 K) kJ/kmol kJ/kmol kJ/kmol kJ/kmol kJ/kmol kJ/kmol C2H2 3101 260 N2 1600 65205 11121 132 45905 3944 CO2 106640 16450 76029 5995 H2O 86162 13164 61636 4697 O2 1622 68771 11647 134 48453 4164 a. (2pt) Write the analytical formulation of the energy balance for the combustion process and derive the analytical expression for the excess air λ. Then calculate it. Do you expect any change in the excess air with the variation of the pressure within the combustion chamber? Please justify your…