Influence of thermal boundary conditions on the dynamic behaviour of a rectangular single-phase natural circulation loop

Natural circulation of distilled water and FC43 has been experimentally investigated in a rectangular loopcharacterized by internal diameter of 30 mm and total length of 4.1 m. The aim of the present study is toanalyse the influence of thermal boundary conditions on the flow regimes inside the pipes and on thestability of the system. The new aspect of the present research is the possibility of tuning the heat sinktemperature in a range between 20 C and +30 C by means of a cryostat. This kind of analysis could beuseful for the design of systems characterized by a wide range of environment temperatures, as for examplefor aerospatial applications. The other parameters investigated were the heat flux transferred to thefluid, which varied between 0.1 kW and 2.5 kW, and the thermo-physical properties of the working fluid.The system showed both stable and unstable behaviour. In particular, in the case of FC43 the loop wasmore unstable and it was characterized by higher velocities and frequencies compared to the case of distilledwater.It was found that the stability threshold could be crossed by varying only the heat sink temperature, demonstrating the importance of this boundary condition on the dynamics of the system. Different flow regimes and fluid velocities were observed. In the case of steady-state flow, Vijayan’s correlation (Vijayan et al., 2000) was tested and found to give good agreement with experimental data. Linear stability analysis was made following the Vijayan’s model. In particular, the effect of heat sink temperature was considered in the dimensionless Stanton number based on the overall heat transfer coefficient at the heat sink. Finally, Ultrasound Pulsed Doppler Velocimeter (UPDV) was used on a natural circulation loop for the first time, and gave a preliminary validation of the traditional fluid velocity measurement method based on the frequency analysis.