A turbocharger including a turbocharger housing, a rotor and a heat shroud. The turbocharger housing includes a turbine housing affixed to a bearing housing. The rotor includes an axial turbine wheel with a hub, and a shaft extending through the bearing housing. The hub defines a back-disk surface that faces the bearing housing, and the bearing-housing defines an outer surface facing the turbine hub. The turbine housing forms a radial scroll, and the heat shroud forms a curved portion turning that radial scroll direction to an axial direction. The heat shroud establishes a back-disk cavity between a bearing-housing turbine-end-wall outer surface, the turbine-wheel back-disk surface, the curved portion of the heat shroud, and a cylindrical outer surface of a turbine-end portion of the shaft. The heat shroud includes flat ribs within the back-disk cavity to impede the circumferential flow of exhaust gas within the back-disk cavity.