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ME 475/675 Introduction to Combustion

ME 475/675 Introduction to Combustion. Lecture 23. Announcements. HW 8, Numerical Solution to Example 6.1 Due Monday, Oct. 20, 2014 College Distinguished Lecture The future of drone technology 1% (= 1 HW) for signing in before and after the lecture Saturday, October 18, 2014,

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ME 475/675 Introduction to Combustion

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  1. ME 475/675 Introduction to Combustion Lecture 23

  2. Announcements • HW 8, Numerical Solution to Example 6.1 • Due Monday, Oct. 20, 2014 • College Distinguished Lecture • The future of drone technology • 1% (= 1 HW) for signing in before and after the lecture • Saturday, October 18, 2014, • 5 pm posters; • 6 pm Lecture • https://www.unr.edu/nevada-today/news/2014/college-of-engineering-distinguished-lecture-series

  3. Example 6.1 • Create a simple constant-volume model of the autoignition process and determine the temperature and the fuel and product concentration histories. Also determine the dP/dt as a function of time. Assume initial conditions corresponding to compression of a fuel-air mixture from 300 K and 1 atm to top-dead-center for a compression ratio of 10:1. The initial volume before compression is 3.68*10-4 m3, which corresponds to an engine with both a bore and a stroke of 75 mm. Use ethane as fuel. Assume: • One-step global kinetics using the rate parameters for ethane C2H6 (Table 5.1) • Fuel, air, and products all have equal molecular weights: MWF=MWOx= MWP= 29 • The specific heats of the fuel, air and products are constants and equal: • cp,F= cp,Ox= cp,Pr= 1200 J/kgK • The enthalpy of formation of the air and products are zero, and that of the fuel is • 4*107 J/kg • The stoichiometric air-fuel ratio is 16.0 and restrict combustion to stoichiometric or lean conditions, . • Constant Pressure, or constant Volume?

  4. Last Lecture • Species Production • 1st Law • Now need • Initial Conditions: • At t = 0, , and , • State Equation • Pressure Rate of change (affects detonation)

  5. Numerical Solution (Excel) • Starting point • http://wolfweb.unr.edu/homepage/greiner/teaching/MECH.475.675.Combustion/Ex6.1.start.xlsx • Results, , • Low temperature, slow start • Temperature and pressure spike and rapid reaction at t = 0.00306 sec = 3.06 ms • Extinction when both fuel and oxidizer are consumed • Realistic results, but not realistic reaction model, which should be multi-stepped

  6. Results, 40,000 steps • As before the reaction is “delayed” • Initial [Fuel] is smaller than before • Reaction occurs at t = 0.0035 sec = 3.5 ms (0.5 ms later than ) • Due to excess air, the mixture takes longer to heat up an react • As reaction is taking place, temporal variations are so great that the numerical solution becomes unstable • Smaller time step may be more stable, but it runs out of steps (this is difficult to do with Excel).

  7. Results, • As before the reaction is “delayed” • Initial [Fuel] is smaller than before • Reaction occurs at t = 0.0035 sec = 3.5 ms (0.5 ms later than ) • Due to excess air, the mixture takes longer to heat up an react • As reaction is taking place, variations are so great that the numerical solution becomes unstable • Could use smaller time step, but this is difficult to do with Excel.

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