Achieving better thermal environment for the growth of high quality single crystals has been the focus of numerous investigations. However, a literature review reveals that the topics of research dealt either with turbulent flow in industry size cylindrical hydrothermal autoclaves, or laminar flows in rectangular enclosures. This paper attempts to add new insight to the above-mentioned state of the art. The flow and heat transfer in a lower-half heated, upper-half cooled cylindrical model hydrothermal reactor are studied experimentally and numerically. Results show that for the parameters chosen, the flow in the model reactor is transient. The unsteady jet-like flow in each of the chambers originates from the fluid exchange at the baffle opening. The time-averaged temperature and flow profiles appear to be axially symmetric. The temperature and velocity fluctuations in the near-baffle region are significantly larger than in the rest of the chambers. The total heat flow rate is instrumental on the thermal environment in the upper chamber, while the heating conditions on the lower chamber wall have only minor effects.