Cantera/Gases

Some scattered notes on Cantera gas objects.

Theory

We can begin with the Gibbs phase rule, which relates the number of phases and components in a system to the number of degrees of freedom - the number of independent thermodynamic parameters that must be set to fix the state of a gas.

More info over at Cantera/Phases

Specifying Thermochemical State

The Basics

For a multicomponent mixture of gas containing N components, the thermochemical state of the gas can be specified by specifying N+1 thermodynamic variables.

The gas state can be specified using the set method, which takes a couple of keywords for specifying which variable you wish to set. The following parameters are mass-specific, meaning they are specified per unit mass:

• Pressure P
• Temperature T
• Specific volume V
• Mass density Rho
• Mass fractions Y
• Mole fractions X
• Enthalpy H
• Entropy S
• Internal energy U

(Note that if composition is not specified, it remains unchanged.)

These keywords can be used with the phase set method,

from Cantera import *

g = GRI30()

g.set(X="H2:1.0, CO2:3.0", T=298.15, P=OneAtm)
g.set(P=OneAtm, Rho=0.10)

Specifying Composition

Composition can be specified either as a string, or as an array. If a specified composition sums to greater than 1, the values for each species are normalized.

Specifying composition as a string is the "human method": it ensures the user can specify which species have which quantities, in an arbitrary order.

Specifying composition as an array is the "machine method": it ensures that scripts can pass composition information quickly and easily to gases, without having to translate back and forth between strings or lose information through string formatting.

Let's make a simple gas from an H2O2 oxidation mechanism:

In [11]: g = importPhase('h2o2.cti')

In [12]: g.set(T=298.15,P=OneAtm)

In [13]: g.nSpecies()
Out[13]: 9

In [14]: g.speciesNames()
Out[14]: ['H2', 'H', 'O', 'O2', 'OH', 'H2O', 'HO2', 'H2O2', 'AR']

String Specification

We can specify composition with a string. Using mole fractions first:

In [15]: g.set(X="H2:2.0, O2:1.0")

Using mass fractions:

In [16]: g.set(Y="H2:1.0, O2:8.0")

Vector Specification

Similarly, we can specify composition with a vector. First, we set up the problem by grabbing some information:

In [26]: iO2 = g.speciesIndex('O2')

In [27]: iH2 = g.speciesIndex('H2')

In [28]: nsp = g.nSpecies()

Now we can set our mole fractions:

In [29]: molefracs = zeros(nsp,); molefracs[iO2] = 1; molefracs[iH2] = 2

In [30]: g.set(X=molefracs)

Alternatively, we can set the mass fractions:

In [29]: massfracs = zeros(nsp,); massfracs[iO2] = 1.0; massfracs[iH2] = 8.0

In [30]: g.set(X=massfracs)

Code

Python Interface

The Python interface implements the set() method as a standalone function, in the following file:

/path/to/cantera/interfaces/python/Cantera/set.py

The set() method is a wrapper function to methods that individually set thermodynamic property, like setTemperature() or setDensity(), or methods that set pairs of properties, like setState_UV() or setState_HP().

These methods, in turn, are defined in the Phase classes, which themselves utilize the C API to specify these thermodynamic properties.