Single phase natural convection loop was experimentally and numerically investigated. The rectangular loop (approximately 0.42m high and wide) made of the pyrex glass tube (inner diameter of 0.03m) is oriented in a vertical plane, heated over the lower horizontal branch, cooled on the upper horizontal branch and thermally insulated on both right and left vertical branch. Working fluids are water and four kinds of silicone oils with different viscosity. Water and silicone oils KF96-1, KF96-2 (Shin-Etsu Chemical) with the nearest viscosity of water showed unstable oscillation with chaotic flow reversal. This phenomenon was observed in silicone oils for the first time. Unstable oscillation occurred in the heat flux ranged from 11.0-23.4kW/m
2 in water, 9.65-13.8kW/m
2 in KF96-1 and 15.2kW/m
2- in KF96-2. The stability map for laminar flow is constructed based on one-dimensional model. In comparison between this map and experimental results, the agreement is much improved when heat transfer coefficient and viscosity coefficient are multiplied by some factors (over 10). It was concluded that the eddy viscosity influenced on these factor. Also, another stability map was constructed in Prandtl number vs heat flux. It reveals unstable oscillation occurs in the turbulent flow region.
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