Flow stability in a natural circulation loop: influences of wall thermal conductivity

In the study of single-phase natural circulation loops, flow stability has been an important subject because of the engineering needs for obtaining stable operating state. As an attempt of proposing method for stability control, this article studied the change of the flow stability due to the variation of the tube wall thermal conductivity. A nonlinear dynamical model was proposed to formulate the wall effects. It is suggested that in a loop made of better thermal conducting material, the flow is also more stable. Experiments were performed in three natural circulation loops; they are in the same shape and are at the same thermal boundary conditions. An unstable Lorenz flow was observed in a glass loop. In a copper loop and a copper–glass loop, both the flows were stable for any experimental cases. The results verified qualitatively the conclusions drawn by the model.