ME 475/675 Introduction to Combustion Lecture 21

Announcements HW 8, Numerical Solution to Example 6.1 Due Friday, Oct. 17, 2014 (?) College Distinguished Lecture The future of drone technology Saturday, October 18, 2014, 5 pm posters; 6 pm Lecture lecture-series lecture-series

Chapter 6 Coupling Chemical and Thermal Analysis of Reacting systems Four simple reactor systems, p Constant pressure and fixed Mass Time dependent, well mixed 2.Constant-volume fixed-mass Time dependent, well mixed 3.Well-stirred reactor Steady, different inlet and exit conditions 4.Plug-Flow Steady, dependent on location Coupled Energy, species production, and state constraints For plug flow also need momentum since speeds and pressure vary with location

Constant pressure and fixed Mass Reactor

First Law (Energy Conservation)

Change in Molar Concentrations

Constant-Volume Fixed-Mass Reactor

Tabulated Data

Reactor Pressure

Example 6.1 (p. 189) This will be HW In spark-ignition engines, knock occurs when the unburned fuel-air mixture ahead of the flame reacts homogeneously, i.e., it auto-ignites. The rate-of- pressure rise is a key parameter in determining knock intensity and propensity for mechanical damage to the piston-crank assembly. Pressure-versus-time traces for normal and knocking combustion in a spark-ignition engine are illustrated in Fig Note the rapid pressure rise in the case of heavy knock. Figure 6.3 shows schleiren (index-of-refraction gradient) photographs of flame propagation for normal and knocking combustion

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 m 3, 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 C 2 H 6 (Table 5.1) Fuel, air, and products all have equal molecular weights: MW F = MW Ox = MW P = 29 The specific heats of the fuel, air and products are constants and equal: c p,F = c p,Ox = c p,Pr = 1200 J/kgK The enthalpy of formation of the air and products are zero, and that of the fuel is 4*10 7 j/kg The stoichiometric air-fuel ratio is 16.0 and restrict combustion to stoichiometric or lean conditions.

Global and Quasi-global mechanisms Sometimes Want These Units