For efficient cooling applications in power plants, the use of small two-phase natural circulation loops becomes attractive. An experimental study was carried out to examine how thermal hydraulic stability and operation conditions of these devices are affected by nucleation sites. A very smooth glass tube with artificial nucleation sites have been employed as boiling channel. The mass flow rate has been determined as function of heat flux and nucleation site location. Particular for low heat flux levels, the nucleation sites have a strong impact to the stability behavior. The observed flow instabilities have been analyzed with regard to non-linear effects and chaotic behavior.

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