Week 7 - Auto ignition using Cantera

Objective:

To understand the variation of auto ignition temperature of a fuel during combustion at varied pressure and temperature.

Auto Ignition Temperature and Auto Ignition Time:

Auto ignition temperature: It refers to the temperature of fuel at which the fuel will auto ignite without external heat source. At elevated temperatures and pressure such conditions are met and hence this property of fuel is leveraged to initiate combustion process. This acts as useful parameter of fuel, as it sets maximum safe level of operations involving that fuel.

Auto Ignition time : The time required by combustion mixture to reach a temperature which is 400K greater than initial temperature is referenced as Auto-Ignition time. During this time no combustion starts, hence it is also termed as ignition delay.

Coding Aspects and Approach:

Numerical simulation to find ignition delay time for various combustion processes is done using `Cantera` library in python. The library is also used to evaluate the information of species involved in chemical reaction at different states and at different times. Modular functions are written which helps in maintaining physical properties like pressure, temperature and species concentration dynamic in nature. Using these functions various properties are monitored for types of reactor types available in `Cantera`

`gri30` mechanism is used to study the effects on fuel `CH_4`.

In part 1 of this study either of pressure and temperature is kept constant to observe effect on ignition delay times.

The constant pressure temperature varies from `950K` to `1450K` at `5 atm ` , and in constant temperature process the pressure varies from`1 atm` to `5 atm` at `1250K`.

Plots of varying parameter (temperature or pressure) vs ignition-delay is also plotted to study the change visually.

In part 2 of this study to understand the formation of various species during intermediate reactions and consumption of reactants during combustion, molar concentrations of certain key species against reaction time were plotted.

The simulation was allowed to run for `10 sec` and the concentrations are observed at different combinations of temperature and pressure.

Plots:

Observations from plots:

• Figure(1) shows the variation of ignition delay at various pressure-temperature combinations.
  
  • Case 1: Constant Pressure varying temperature
    • We can see that with increase in temperature the auto ignition point is reached quickly and hence ignition delay decreases with increase in temperature.
    • We used different Reactor objects like IdealGasReactor, IdealGasConstPressureReactor, ConstPressureReactor to simulate our combustion reaction. We can see the trend is consistent across all the objects. So it can be concluded at constant pressure and with increasing temperature the ignition delay decreases.
  • Case 2: Constant Temperature varying pressure
    • Similar to case 1, it is observed with increase in pressure at a constant temperature, ignition delay decreases.

• Variation of molar concentration of species:
  
  • The combustion reaction for methane can be written as `CH_4 + 2O_2 → CO_2 + 2H_2O`. Also, depending upon the equivalence ratio the fuel and oxidizer reacts.
  • A fuel rich mixture will consume available oxygen(oxidizer) leaving fuel in excess as byproduct. A fuel lean mixture will have oxidizer in excess , hence all the fuel molecules will be burned and heat will be produced. But, since we are are not providing any external heat source in this case, the combustion will only take place when the mixture has attained suitable pressure and temperature conditions.
  • From figure(2) we can observe that
    • The mixture at `500K` and `5 atm` did not attain the conditions required for auto ignition, hence the reactants(fuel and oxidizer) concentration remains unchanged(marked by blue line). The concentration of key product of combustion reaction i.e. `CO_2` is also zero as it has not formed yet.
    • As we increase to the temperature to `1000K ` the combustion starts and formation of intermediate species like OH starts. Simultaneously the concentration of reactants starts to drop(marked by orange line).
    • By the end of reaction the reactants concentration have dropped to almost zero and to conserve mass reactants gets converted to various products, which can be seen by increase in molar concentration of `H_2O`,`OH `and `CO_2`.