Quantity addition in Python is incorrect when molar basis is used.

[peterscherpelz]

Problem description

Quantity addition in Python is incorrect when molar basis is used.

Steps to reproduce

Create two quantities and set their basis to 'molar' before adding them. See script below.

Behavior

Enthalpy is not conserved even when both Quantities are set to constant='HP'. The resulting Quantity is different than if basis is kept as mass.

System information

• Cantera version: 2.6.0
• OS: Ubuntu 20.04
• Python 3.8:

Attachments

import cantera as ct

gas = ct.Solution('gri30.yaml')

# Stream A (air)
A = ct.Quantity(gas, constant='HP')
A.TPX = 300.0, ct.one_atm, 'O2:0.21, N2:0.78, AR:0.01'
A.moles = 1
# Stream B (methane)
B = ct.Quantity(gas, constant='HP')
B.TPX = 1200.0, ct.one_atm, 'CH4:1'
B.moles = 0.42

A.basis = 'molar'
B.basis = 'molar'

print(A.enthalpy + B.enthalpy)
# Gives -8521222

# Compute the mixed state
M = A + B
print(M.enthalpy)
# Gives -5408171

In contrast, when not setting to molar basis, everything works fine:

import cantera as ct

gas = ct.Solution('gri30.yaml')

# Stream A (air)
A = ct.Quantity(gas, constant='HP')
A.TPX = 300.0, ct.one_atm, 'O2:0.21, N2:0.78, AR:0.01'
A.moles = 1
# Stream B (methane)
B = ct.Quantity(gas, constant='HP')
B.TPX = 1200.0, ct.one_atm, 'CH4:1'
B.moles = 0.42

print(A.enthalpy + B.enthalpy)
# Gives -8521222

# Compute the mixed state
M = A + B
print(M.enthalpy)
# Gives -8521222

[peterscherpelz]

While I stumbled across this while using cantera, if my analysis is correct, the problem is fairly straightforward. See https://github.com/Cantera/cantera/blob/main/interfaces/cython/cantera/composite.py#L314 . Note that quantities are fetched from the underlying phase without checking that a mass basis is used - but they're always multiplied by the mass, even though the basis may have been set to molar.

[peterscherpelz]

Thanks @speth for the fix!