An enhanced hydrocarbon pyrolysis reactor for the production of olefins. The reactor provides enhanced heat transfer, reduced pressure drop, and reduced energy requirements while increasing selectivity and olefin yield. The radiant section tubes of the reactor, according to the invention, are shaped to provide a continuously increasing volume per unit length from inlet to outlet of the reactor. This increase in volume may be achieved by either expanding the effective inside radius of the reactor tube and/or increasing the outside radius of the tube while increasing the inside effective radius. Alternatively, since embodiments of reactors according to the invention have internal fins, the fin size and shape may be modified to provide the required increasing volume per unit length.