"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, especially in industrial or technical contexts, referring to the gases present within a furnace during operation.
"Gas inside the furnace" provides a more literal and descriptive translation, which might be used in situations where clarity about the location of the gas is particularly important.