This paper puts forward modeling thermal and flow phenomena in smooth and internally rifled tubes. The proposed model is a distributed parameter model based on solving balance equations describing the principles of the mass, momentum and energy conservation. The model enables an analysis of transient-state processes. The aim of the calculations is, among others, to find the distribution of the fluid enthalpy, mass flow and pressure in smooth and internally rifled tubes and to determine the heat transfer coefficient. The analysis concerns tubes arranged vertically and operating at supercritical steam parameters. The numerical calculation results will be compared to values obtained from computational fluid dynamics.