Investigation on vibration excitation of heat exchanger pipes by sub-cooled boiling on the jacket side

The external transverse and longitudinal flow, pulsating internal flow and hydro-dynamic coupling are regarded as the exciting mechanisms for vibration of pipe bundles in heat exchanger units. The aim of this work is to prove the excitation of vibration of pipes loaded with high heat flow densities by the condensation pressure pulses occurring during sub-cooled boiling. A special single pipe test rig was designed, whose geometric dimensions and heat flow density are comparable with those of a fuel rod in a modern pressurized water reactor. Experimental results from the literature of growth and condensation behaviour of steam bubbles in subcooled liquids were confirmed by test. In a narrow subcooling interval, a synchronisation of the separating and condensing steam bubbles is observed in sub-cooled boiling. The effects of fluid speed, fluid sub-cooling, heating surface load and surface roughness of the heating rod on the frequency and amplitude of the periodic pressure pulses was shown in a reproduceable way. Resonance vibrations of the heating rod, which were found in variations of the test parameters, could be identified as forced vibrations due to the periodic condensation pressure pulses occurring during sub-cooled boiling. (orig.)