Volumetric Heat Capacity

Dunno. What did you find when you asked Google?

Its the sum of their proportions. See Thermodynamic tables for gases. Different values for constant pressure as well as for constant volumes.

Type "volumetric heat capacity table" into a search engine.

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"What is a search engine", you say?

Web search engine - Wikipedia, the free encyclopedia

I'm appalled at how helpless many people have become.

First of all, do you mean the extensive volumetric heat capacity of an actual open system, or do you mean the intensive volumetric heat capacity of a system equivalent to a supposed real open system, but in the limit of approximation of an ideal gas mixture?

Good luck finding it, because as near as I can tell, the actual values of the Cv change with temperature, since the extensive system is undergoing expansion in being carried from state-to-state.

If you are referring to a steady-state condition within the flue of a furnace at a specific point under defined flow rate conditions at a set temperature and pressure, then there should be too much trouble coming up with an approximate heat content of the entire flue gas volume element, or with the contribution of each individual component of the gas in the mix. You can start with certain approximations, and improve on the model as more data is available.