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ME 475/675 Introduction to Combustion

ME 475/675 Introduction to Combustion. Lecture 18. Announcements. HW 6 Wednesday, Due 10/8/14 Return midterm Wednesday. Hypothetical Chain reactions (example). Globally: ( and are general atoms) (find and for the proposed mechanism) Proposed intermediate steps

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ME 475/675 Introduction to Combustion

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  1. ME 475/675 Introduction to Combustion Lecture 18

  2. Announcements • HW 6 Wednesday, Due 10/8/14 • Return midterm Wednesday

  3. Hypothetical Chain reactions (example) • Globally: ( and are general atoms) • (find and for the proposed mechanism) • Proposed intermediate steps • Slow creation of free radicals (and ), • Chain-initiating step. creates radicals • Fast consumption of and (neglect reverse because and are small) • Chain-propagating step (consumes and creates radical, net zero) • De-energizationter-molecular reaction is slow • Chain-terminating step (consumes radicals) • Assume and are much greater than and • Number of species ( N = 6 • 5 differential equations, 1 algebraic equation (M)

  4. Production - Consumption equations • Reactants, , • Products: (eventually wan this in terms of and , alone) • (need to eliminate ) • Intermediates , (fast, so becomes algebraic, not differential)

  5. Solve for [A] • Divide by -2 • Use + since ; also simplify knowing and ≫ and • Plug into Production Consumption

  6. Production • We were trying to find and for • So

  7. Example 4.3 page 125 • As mentioned previously, a famous chain mechanism is the Zeldovich, or thermal, mechanism for the formation of nitric oxide from atmospheric nitrogen: • Because the second reaction is much faster than the first, the steady-state approximation can be used to evaluate the N-atom concentration. Furthermore, in high-temperature systems, the NO formation reaction is typically much slower than other reactions involving and . Thus and can be assumed to be in (partial) equilibrium: • Construct a global mechanism • Represented as • i.e. determine , , and . Using the elementary rate coefficients, etc., from the detailed mechanisms.

  8. Example 4.4 page 127 • Consider the shock-heating of air to 2500 K atm 3 atm. Use the results of Example 4.3 to determine: • A. The initial nitric oxide formation rate in ppm/s • B. The amount of nitric oxide form (in ppm) in 0.25 ms. • The rate coefficient, is [reference 10 from book]

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