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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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announcements
Announcements
  • HW 6 Wednesday, Due 10/8/14
  • Return midterm Wednesday
hypothetical chain reactions example
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)
production consumption equations
Production - Consumption equations
  • Reactants, ,
  • Products: (eventually wan this in terms of and , alone)
    • (need to eliminate )
  • Intermediates , (fast, so becomes algebraic, not differential)
solve for a
Solve for [A]
    • Divide by -2
  • Use + since ; also simplify knowing and ≫ and
  • Plug into

Production

Consumption

production
Production
  • We were trying to find and for
    • So
example 4 3 page 125
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.
example 4 4 page 127
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]