mixing-chamber

from The Century Dictionary.

• noun A volume or space within which a mingling of substances may occur; specifically, a vessel, stationary or revolving, in which the stone, cement, and sand which together form concrete for building purposes may be intimately compounded.
• noun The chamber, space, or passage in a gas-engine or other internal-combustion motor in which the fuel, in atomized or vaporized form, is intimately mixed with the air which is to support its combustion.