These conservation principles are at the core of physics and are very important in the real-world.

• In a closed system where there are no mass <—> energy conversions, like in nuclear reactions/decay (which we don’t simulate in GUNNS anyway):
  • the total energy in the system must remain constant.
  • the total mass must remain constant.
    • total mass of each chemical compounds is also constant unless there are chemical reactions (which GUNNS does simulate).

• Depending on your simulation, you may or may not care about it.
  • Coolant loops, for example, are very interested in conservation of energy because the whole point of the loop is to transport energy (heat) from A to B.
  • ECLSS models are interested in conserving mass because their main point is to control the concentration of various compounds.

• GUNNS mostly conserves mass and energy when set up properly.
  • Only one known source of error: mass overflows (discussed later).

• Many ways to deliberately add/remove mass or energy:
  • Mostly at constant boundary conditions, such as:
    • wall temperatures that have no thermal aspect
    • all flows to/from Ground node

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