z, Δz | m | Local position, SC node length (z |

| m | BC and SC outlet and entrance positions, total length |

z | m | Boiling boundary, mixture level resp. supercritical boundary within a BC |

α | - | Void fraction |

α |
| Heat transfer coefficient along a BC wall surface |

Δ | - | Nodal differences |

ε | - | Coefficient controlling the additional friction part |

ε | - | Correction factor with respect to Q |

ε | m | Abs. roughness of tube wall (ε |

| - | Two-phase multiplier |

Φ | - | Angle between upwards and flow direction |

ρ, ρ |
| Density and their partial derivatives with respect to (system) pressure and temperature |

| s | Time constant |

| - | Partial derivative |