In most previous research on internal laminar and transitional forced convection, most measurements (temperatures and pressure drops) have been conducted on the walls of tubes to prevent internal disturbances in the flow field. Together with fundamental analytical equations, which used many simplifying assumptions, solutions have been developed describing the flow field, heat transfer coefficients and friction factors. CFD analyses have revealed much more high-resolution detail that was challenging to obtain experimentally. Therefore, the purpose of this paper was to generate CFD data and use this data complementary to data obtained from experimental measurements. All this data made it possible to identify and quantify several important flow phenomena that have not been identified before. This was done by comparing experimental data with CFD data for water flowing from a square-edged inlet through a 4 mm circular tube at different Reynolds numbers and heat fluxes. The most important phenomena that were identified for the first time were (1) the wall temperature distribution for the simultaneously developing hydrodynamic and thermal boundary layers, (2) a tube recirculation region at the tube inlet, (3) a non-linear fluid temperature gradient for developing flow, (4) and the necessity for a new definition of "fully developed flow".