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The complex direct contact condensation phenomenon is investigated in horizontal flow channels both experimentally and numerically with special emphasis on its implications on safety assessment studies. Under certain conditions direct contact condensation can act as the driving force for the water hammer phenomenon with potentially local devastating results, thus posing a threat to the integrity of the affected NPP components. New experimental results of in-depth analysis of the direct contact condensation phenomena obtained in Kaunas at the Lithuanian Energy Institute will be presented. The German system code ATHLET employing for the calculation of the heat transfer coefficient a mechanistic model accounting for two different eddy length scales, combined with the interfacial area transport equation will be assessed against condensation induced water hammer experimental data from the integral thermal-hydraulic experimental facility PMK-2, located at the KFKI Atomic Energy Research Institute in Budapest Hungary.

A hybrid Heat Transfer Coefficient (HTC) model has been developed based on the Surface Renewal Theory and implemented into a system code. The model accounts for both large and small eddies for the removal of heat from the two-phase interface towards the subcooled liquid bulk. The hybrid approach for the calculation of the Surface Renewal Period will increase the applicability region of the individual Surface Renewal Theory (SRT) based HTC for various flow regimes. Validation of the new Hybrid HTC model is performed against experimental data.

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