This study aims to advance understanding and visualization of the passive flow inside the core channels of a dual-channel plenum-to-plenum test facility designed as representative geometry of nuclear prismatic modular reactors core. Simultaneous experimental measurements of local surface and coolant fluid temperatures and flow fields in addition to local heat transfer coefficients were provided by integrating sophisticated flow dynamics and heat transfer techniques at different axial locations (Z/L=0.04,0.28,0.41,0.59,0.77,0.96) along the facility heated channel. Results emphasize the significant dependency of thermal and flow fields, of naturally driven gas, on heating intensity and configuration. Interestingly, although the entire length of the channel is subjected to heating, an inflection in surface and gas temperature in addition to changes in velocity distribution were observed through the top section of the heated channel. This highlights the significance of the complexity of the facility which causes severe interactions, and potential recirculation and back mixing, between rising hot gas with the cold gas in the upper plenum. This is validated by the negative heat flux measurements collected at the top section of the channel indicating heat reversal where heat is transferred from air to the channel. Moreover, densitometric Froude number for different heating profiles were estimated and found to range from 0.66 to 0.75 indicating the existence of a jet flow at the hot channel outlet. The obtained results provide valuable benchmark data for validating further investigations by computational fluid dynamics (CFD) and to perform further investigations of thermal hydraulics in prismatic block reactors.