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 .