Using Cantera from C++#

One of the main reasons to use the Cantera C++ interface directly is to be able to couple it with a higher-level simulation code, where Cantera is used to calculate thermodynamic properties, reaction rates, and transport properties for the host code. This tutorial demonstrates how to initialize Cantera objects from YAML input files and use those objects to calculate properties for mixtures with different thermodynamic states.

A very simple C++ program#

A short C++ program that uses Cantera is shown below. This program reads in a specification of a gas mixture from an input file, and then builds a new object representing the mixture. It then sets the thermodynamic state and composition of the gas mixture, and prints out a summary of its properties.

#include "cantera/core.h"
#include <iostream>

using namespace Cantera;

// The actual code is put into a function that can be called from the main program.
void simple_demo()
{
    // Create a new Solution object
    auto sol = newSolution("h2o2.yaml");
    auto gas = sol->thermo();

    // Set the thermodynamic state by specifying T (500 K) P (2 atm) and the mole
    // fractions. Note that the mole fractions do not need to sum to 1.0 - they will
    // be normalized internally. Also, the values for any unspecified species will be
    // set to zero.
    gas->setState_TPX(500.0, 2.0*OneAtm, "H2O:1.0, H2:8.0, AR:1.0");

    // Print a summary report of the state of the gas.
    std::cout << gas->report() << std::endl;
}

// The main program just calls function simple_demo within a 'try' block, and catches
// CanteraError exceptions that might be thrown.
int main()
{
    try {
        simple_demo();
    } catch (CanteraError& err) {
        std::cout << err.what() << std::endl;
        return 1;
    }
    return 0;
}

Tip

Before you can run this program, it first needs to be compiled. On a Linux system using the GCC compiler, a typical command line for compiling this program might look like this:

g++ combustor.cpp -o combustor -O3 $(pkg-config --cflags --libs cantera)

This example relies on the pkg-config tool to determine the appropriate compiler flags, such as those specifying the Cantera header and library files. For more advanced and flexible methods of compiling programs that use the Cantera C++ library, see Compiling Cantera C++ Applications.

Running this program produces the output below:


  ohmech:

       temperature   500 K
          pressure   2.0265e+05 Pa
           density   0.36118 kg/m^3
  mean mol. weight   7.4093 kg/kmol
   phase of matter   gas

                          1 kg             1 kmol     
                     ---------------   ---------------
          enthalpy       -2.4772e+06       -1.8354e+07  J
   internal energy       -3.0382e+06       -2.2511e+07  J
           entropy             20699        1.5337e+05  J/K
    Gibbs function       -1.2827e+07       -9.5038e+07  J
 heat capacity c_p            3919.1             29038  J/K
 heat capacity c_v              2797             20724  J/K

                      mass frac. Y      mole frac. X     chem. pot. / RT
                     ---------------   ---------------   ---------------
                H2           0.21767               0.8           -15.644
                 H                 0                 0  
                 O                 0                 0  
                O2                 0                 0  
                OH                 0                 0  
               H2O           0.24314               0.1           -82.953
               HO2                 0                 0  
              H2O2                 0                 0  
                AR           0.53919               0.1           -20.503
                N2                 0                 0

As C++ programs go, this one is quite short. It is the Cantera equivalent of the “Hello, World” program most programming textbooks begin with. But it illustrates some important points in writing Cantera C++ programs.

Making Cantera classes and functions available#

Cantera provides several header files designed for use in C++ application programs. These are designed to include those portions of Cantera needed for particular types of calculations. The headers and their functions are:

core.h

Base classes and functions for creating Solution and Interface objects from input files, as well as associated classes accessed through these objects such as ThermoPhase, Kinetics, and Transport. (New in Cantera 3.0).

zerodim.h

Zero-dimensional reactor networks.

onedim.h

One-dimensional reacting flows.

reactionpaths.h

Reaction path diagrams.

thermo.h

Base thermodynamic classes and functions for creating ThermoPhase objects from input files (Superseded by core.h in Cantera 3.0).

kinetics.h

Base kinetics classes and functions for creating Kinetics objects from input files (Superseded by core.h in Cantera 3.0).

transport.h

Base transport property classes and functions for creating Transport objects from input files (Superseded by core.h in Cantera 3.0).

Creating a Solution object from an input file#

Here, newSolution() imports all information held by a YAML input file into a Cantera Solution object, which is accessed by the pointer sol. The thermodynamic information is accessible via sol->thermo(), which itself returns a pointer to a ThermoPhase object.

Class ThermoPhase is the base class for Cantera classes that represent phases of matter. It defines the public interface for all classes that represent phases. For example, it specifies that they all have a method temperature() that returns the current temperature, a method setTemperature(double T) that sets the temperature, a method getChemPotentials(double* mu) that writes the species chemical potentials into array mu, and so on.

ThermoPhase objects can represent the intensive state of any single-phase solution of multiple species. The phase may be a bulk, three-dimensional phase (a gas, a liquid, or a solid), or it may be a two-dimensional surface phase, or even a one-dimensional “edge” phase. Many different phase thermodynamic models are implemented as classes derived from ThermoPhase.

The report function#

The ThermoPhase::report() method generates a nicely-formatted report of the properties of a phase, including its composition in both mole (X) and mass (Y) units. For each species present, the non-dimensional chemical potential is also printed. This is handy particularly when doing equilibrium calculations. This function is very useful to see at a glance the state of some phase.

Handling errors#

The entire body of the program is put inside a function that is invoked within a try block in the main program. In this way, exceptions thrown in the function or in any procedure it calls may be caught. In this program, a catch block is defined for exceptions of type CanteraError. Cantera throws exceptions of this type, so it is always a good idea to catch them.

Accessing thermodynamic properties#

In the program below, a gas mixture object is created, and a few thermodynamic properties are computed and printed out:

#include "cantera/core.h"
#include <iostream>

using namespace Cantera;

void thermo_demo(const string& file, const string& phase)
{
    // Create a new Solution object
    auto sol = newSolution(file, phase);
    auto gas = sol->thermo();

    gas->setState_TPX(1500.0, 2.0*OneAtm, "O2:1.0, H2:3.0, AR:1.0");

    // temperature, pressure, and density
    std::cout << gas->temperature() << std::endl;
    std::cout << gas->pressure() << std::endl;
    std::cout << gas->density() << std::endl;

    // molar thermodynamic properties
    std::cout << gas->enthalpy_mole() << std::endl;
    std::cout << gas->entropy_mole() << std::endl;

    // specific (per unit mass) thermodynamic properties
    std::cout << gas->enthalpy_mass() << std::endl;
    std::cout << gas->entropy_mass() << std::endl;

    // chemical potentials of the species
    size_t numSpecies = gas->nSpecies();
    vector<double> mu(numSpecies);
    gas->getChemPotentials(mu.data());
    for (size_t n = 0; n < numSpecies; n++) {
        std::cout << gas->speciesName(n) << " " << mu[n] << std::endl;
    }
}

int main(int argc, char** argv)
{
    try {
        thermo_demo("h2o2.yaml", "ohmech");
    } catch (CanteraError& err) {
        std::cout << err.what() << std::endl;
        return 1;
    }
    return 0;
}

Methods that compute scalar properties take no input parameters. The properties are computed for the state that has been previously set and stored internally within the object. Methods that return molar properties have names that end in _mole, while methods that return properties per unit mass have names that end in _mass.

Methods that compute an array of values have names beginning with get and write the computed values into an array provided as an input argument. For example, the method getChemPotentials(double* mu) writes the species chemical potentials into the output array mu.

All thermodynamic property methods are declared in class ThermoPhase, which is the base class from which all classes that represent any type of phase of matter derive. See the documentation for class ThermoPhase for the full list of available thermodynamic properties.

Computing chemical equilibrium#

In the program below, the ThermoPhase::equilibrate() method is called to set a gas to a state of chemical equilibrium, holding the temperature and pressure fixed.

#include "cantera/core.h"
#include <iostream>

using namespace Cantera;

void equil_demo()
{
    // Create a new Solution object
    auto sol = newSolution("h2o2.yaml");
    auto gas = sol->thermo();

    gas->setState_TPX(1500.0, 2.0*OneAtm, "O2:1.0, H2:3.0, AR:1.0");
    gas->equilibrate("TP");
    std::cout << gas->report() << std::endl;
}

int main()
{
    try {
        equil_demo();
    } catch (CanteraError& err) {
        std::cout << err.what() << std::endl;
        return 1;
    }
    return 0;
}

The program output is:


  ohmech:

       temperature   1500 K
          pressure   2.0265e+05 Pa
           density   0.31683 kg/m^3
  mean mol. weight   19.499 kg/kmol
   phase of matter   gas

                          1 kg             1 kmol     
                     ---------------   ---------------
          enthalpy       -4.1789e+06       -8.1485e+07  J
   internal energy       -4.8186e+06       -9.3957e+07  J
           entropy             11283        2.2001e+05  J/K
    Gibbs function       -2.1104e+07        -4.115e+08  J
 heat capacity c_p              1893             36912  J/K
 heat capacity c_v            1466.6             28597  J/K

                      mass frac. Y      mole frac. X     chem. pot. / RT
                     ---------------   ---------------   ---------------
                H2          0.025847              0.25           -19.295
                 H        3.2181e-07        6.2252e-06           -9.6477
                 O        6.2927e-12        7.6693e-12           -26.377
                O2        1.1747e-11        7.1586e-12           -52.753
                OH        3.0994e-07        3.5535e-07           -36.024
               H2O           0.46195               0.5           -45.672
               HO2        1.2362e-14        7.3034e-15           -62.401
              H2O2         6.904e-13        3.9578e-13           -72.049
                AR           0.51221              0.25           -21.339
                N2                 0                 0

How can we tell that this is really a state of chemical equilibrium? Well, by applying the equation of reaction equilibrium to formation reactions from the elements, it is straightforward to show that

\[ \mu_k = \sum_m \lambda_m a_{km} \]

where \(\mu_k\) is the chemical potential of species \(k\), \(a_{km}\) is the number of atoms of element \(m\) in species \(k\), and \(\lambda_m\) is the chemical potential of the elemental species per atom (the so-called “element potential”). In other words, the chemical potential of each species in an equilibrium state is a linear sum of contributions from each atom. We can see that this is true in the output above—the chemical potential of H2 is exactly twice that of H, the chemical potential for OH is the sum of the values for H and O, the value for H2O2 is twice as large as the value for OH, and so on.

Computing reaction rates and transport properties#

The following program demonstrates the typical method for accessing the following object types from a Solution object:

#include "cantera/core.h"
#include "cantera/kinetics/Reaction.h"
#include <iostream>

using namespace Cantera;

void simple_demo2()
{
    auto sol = newSolution("gri30.yaml", "gri30");
    auto gas = sol->thermo();

    // Access the Kinetics object. Based on the phase definition in the input file,
    // this will reference a BulkKinetics object.
    auto kin = sol->kinetics();

    // Set an "interesting" state where we will observe non-zero reaction rates.
    gas->setState_TPX(500.0, 2.0*OneAtm, "CH4:1.0, O2:1.0, N2:3.76");
    gas->equilibrate("HP");
    gas->setState_TP(gas->temperature() - 100, gas->pressure());

    // Get the net reaction rates.
    vector<double> wdot(kin->nReactions());
    kin->getNetRatesOfProgress(wdot.data());

    writelog("Net reaction rates for reactions involving CO2\n");
    size_t kCO2 = gas->speciesIndex("CO2");
    for (size_t i = 0; i < kin->nReactions(); i++) {
        if (kin->reactantStoichCoeff(kCO2, i) || kin->productStoichCoeff(kCO2, i)) {
            auto rxn = kin->reaction(i);
            writelog("{:3d}  {:30s}  {: .8e}\n", i, rxn->equation(), wdot[i]);
        }
    }
    writelog("\n");

    // Access the Transport object. Based on the phase definition in the input file,
    // this will reference a MixTransport object.
    auto trans = sol->transport();
    writelog("T        viscosity     thermal conductivity\n");
    writelog("------   -----------   --------------------\n");
    for (size_t n = 0; n < 5; n++) {
        double T = 300 + 100 * n;
        gas->setState_TP(T, gas->pressure());
        writelog("{:.1f}    {:.4e}    {:.4e}\n",
                 T, trans->viscosity(), trans->thermalConductivity());
    }
}

int main()
{
    try {
        simple_demo2();
    } catch (std::exception& err) {
        std::cout << err.what() << std::endl;
        return 1;
    }
    return 0;
}

This program produces the output below:

Net reaction rates for reactions involving CO2
 11  CO + O (+M) <=> CO2 (+M)         3.54150687e-08
 13  HCO + O <=> CO2 + H              1.95679990e-11
 29  CH2CO + O <=> CH2 + CO2          3.45366954e-17
 30  CO + O2 <=> CO2 + O              2.70102741e-13
 98  CO + OH <=> CO2 + H              6.46935827e-03
119  CO + HO2 <=> CO2 + OH            1.86807592e-10
131  CH + CO2 <=> CO + HCO            9.41365868e-14
151  CH2(S) + CO2 <=> CH2 + CO2       3.11161343e-12
152  CH2(S) + CO2 <=> CH2O + CO       2.85339294e-11
225  NCO + O2 <=> CO2 + NO            3.74127381e-19
228  NCO + NO <=> CO2 + N2            6.25672710e-14
261  HNCO + O <=> CO2 + NH            6.84524918e-13
267  HNCO + OH <=> CO2 + NH2          7.78871222e-10
279  CO2 + NH <=> CO + HNO           -3.30333709e-09
281  NCO + NO2 <=> CO2 + N2O          2.14286657e-20
282  CO2 + N <=> CO + NO              6.42658345e-10
289  CH2 + O2 => CO2 + 2 H            1.51032305e-18
304  CH2CHO + O => CH2 + CO2 + H      1.00331721e-19

T        viscosity     thermal conductivity
------   -----------   --------------------
300.0    1.6701e-05    4.2143e-02
400.0    2.0896e-05    5.2797e-02
500.0    2.4704e-05    6.2827e-02
600.0    2.8230e-05    7.2625e-02
700.0    3.1536e-05    8.2311e-02

Examples of additional C++ functionality#

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