Introduction to Combustion

Fall 2015

Assignment #7

Assigned 26 October 2015

Due 4 November 2015

Homogeneous Systems – Constant Pressure/Constant Volume Reactors

In this assignment, you will use CHEMKIN to explore homogeneous constant-pressure and constant-volume adiabatic combustion processes. You are to use the same chemical mechanism and thermodynamic data that you used for Assignment #6 (GRI-Mech 3.0).

Consider methane-air reactants at a fuel-lean equivalence ratio of  = 0.6, and an initial temperature and pressure of and T0 = 1000 K and p0 = 1 atm. For this exercise, use the closed homogeneous batch reactor model in CHEMKIN. The system is adiabatic (zero heat loss). Run your simulation to an end time of 1.4 s, and specify a time interval for printing of 0.1 s; you can accept the default values for all other parameters. To run a constant-pressure case, specify “Constrain Pressure and Solve Energy Equation.” To run a constant-volume case, specify “Constrain Volume and Solve Energy Equation.”

Start with the constant-pressure case. Run the model for the conditions specified above.

 Attach the CHEMKIN output file. Edit it first, so that it contains only the initial conditions at time t = 0 and the final solution at time t = 1.4 s.

 Attach the CHEMKIN output file from a corresponding equilibrium calculation.

 Generate and attach the following three figures:*

o Mole fractions of CH4, O2, H2O, CO2, CO and H2 versus time

o Mole fractions of O, H, OH, HO2, H2O2 and NO versus time

o Mole fractions of all other species having mole fractions greater than or equal to 10-5 (at any point over the solution time interval) versus time

 Comment on your results. For example:

o What is the t = 1.4 s temperature? What is the equilibrium temperature?

o Does the system reach steady state after 1.4 s?

o Does the t = 1.4 s solution correspond to equilibrium (temperature and species mole fractions)?

o What species are present in the steady-state/equilibrium products (with mole fractions > 10-5)?

o What species show up over the solution time interval (with mole fractions > 10-5) that are neither in the reactants nor in the equilibrium products (radicals, stable intermediates, complex molecules)?

o What is the time sequence of species appearing/disappearing?

Repeat the exercise for the constant-volume case. Attach the same output files and plots as for the constant-pressure case. Comment on the differences that you observe between the constant-pressure and constant-volume systems, and discuss/explain the differences.

*Plot the species mole fractions as functions of time (0 ≤ t ≤1.4 s) using a linear scale for time (x axis) and a logarithmic scale for mole fractions (y axis), and set the y-axis minimum value to 10-5. This can be done by right-clicking on the figure, and selecting Properties. Then work through the pop-up menus.

 

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