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ME 525: Combustion Lecture 6: Chemical Time Scales, Partial Equilibrium, Catalysis, Example Problems, COSILAB Tutorial and Demonstration. Grading policy clarification Definitions of time scales on the basis of type of reaction Partial Equilibrium Reduced Mechanisms
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ME 525: CombustionLecture 6: Chemical Time Scales, Partial Equilibrium, Catalysis, Example Problems, COSILAB Tutorial and Demonstration • Grading policy clarification • Definitions of time scales on the basis of type of reaction • Partial Equilibrium • Reduced Mechanisms • Catalysis and Heterogeneous Reactions • COSILAB Tutorial and Demonstration
ME 525: CombustionGrading Policy Clarification Course grade will be based on the best score out of scores obtained using the Following two grade weightings: • Two Midterm Examinations: 30% • Final Examination (Comprehensive): 30% • Text Book Home Work Problems: 20%. • Special Project(s): 20%. Homework 15% May require use of software. Project 15% Report due on 4/23/2013 Exam 1 20% Lectures 1-10, 2/19/2013 Exam 2 20% Lectures 1-20, 4/04/2013 Final Exam30% Comprehensive, week of 4/30/13 OR
Time Scale of Unimolecular Reactions • The reaction rate and initial condition is given by: • Integrating:
Time Scale of Bimolecular Reactions Consider a reaction between species A and B producing products C and D:
Time Scale of Ter-molecular Reactions Consider a reaction between species A and B producing products C in the presence of inert M: Review pages 131and 132 in preparation for a Homework to be assigned on January 29th, 2013 and in preparation for Exam. 1.
Partial Equilibrium Consider a mechanism consisting of many reactions between reactant and product species and their intermediates. The faster reactions will finish their work and will be waiting for the slower reactions to catch up and in the process the faster reactions will appear as if they are in steady state. This allows considerably simplified calculations. Review pages 133 and 134 in preparation for a Homework to be assigned on January 29th, 2013
Reduced Chemical Mechanisms Combustion of a typical complex hydrocarbon fuel involves 1000 to 5000 elementary reactions between 100 to 500 species with some species being heterogeneous such as carbon particles, silicon, iron, and other metals and non-metals added by design or by accident. Engineers have to always work with reduced chemical mechanisms dictated by the specific objectives of the combustion calculation. Principles to be followed in the creation of reduced mechanisms involve: (1) always follow conservation laws (mass, momentum and energy can not be created or destroyed), (2) use differences in time scales of reactions to establish steady state including partial or quasi equilibrium for some and unsteady state for others, (3) use chemical pathway diagrams and quantitative reaction pathway diagrams to identify the species that must be retained and the steps that dominate.
Catalysis and Heterogeneous Reactions As shown in Figure 4.2 of Turns, heterogeneous reactions occur by a gas phase molecule colliding on a solid surface that has a surface structure matching the size/charge combination of the colliding shell of the molecule. The molecule will then adhere to the surface and decompose with a trapped and a free radical. The free radical travels on the surface and is typically trapped by another surface feature. These processes couple with the gas phase and have been used in fuel processing as well as exhaust treatment. More recently, the field of combustion synthesis of exotic materials such as carbon nanotube and graphene is emerging. Review Text Book Equations HR.1 –HR.4 Now, CEC, COSILAB, EES demonstrations