"炉内气体" can be translated into English as "furnace gas" or "gas inside the furnace", depending on the context.
"Furnace gas" is a more concise and commonly used term in technical and industrial contexts, referring to the gases present within a furnace during various processes (e.g., combustion, heating, or chemical reactions).
"Gas inside the furnace" is a more literal and descriptive translation, which might be used in situations where clarity or emphasis on the location is needed.
In most technical or engineering discussions, "furnace gas" would be the preferred choice.