by increasing the thickness of the boundary layer or aiding in its detachment this of course would reduce the ratio of the effective area of the second constriction to the effective area of the first varying the ratio of the effective areas results in the creation of new pressure profile which given that all other factors are constant produces different grades of carbon black means are provided on the second venturi flow constriction for altering the effective area of the constriction since the venturi constrictions are formed from rigid refractory material altering the ratio of the effective areas of the two constrictions is preferably accomplished by injection of gases through ports in the throat of the second venturi constriction these ports comprise means for injection of gases into the reaction tunnel at the second venturi constriction to control the effective area of the second constriction when the ports are so arranged as to inject the secondary gases radially into the second venturi constriction and a high flow rate of secondary gas is used the effective area of the constriction is increased thereby increasing the ratio of the effective area of the second constriction to the effective area of the first tangential injection of the secondary gases results in a decrease in the effective area of the second venturi constriction and in the ratio of the effective areas disposed within the quench zone z are quench means for injecting a quench liquid preferably water to cool the hot flowing gases and reacting carbon black the.