The proposed research activity is to enlarge the knowledge of the pool boiling heat transfer mechanism in relevant condition for the Isolation Condenser (IC), at atmospheric conditions, high heat flux and industrial grade of the surface roughness.
In the nuclear framework, the characterization of the power exchanged during all the working phases of this component is fundamental to guarantee enough safety margins avoiding unplanned changes in the temperatures. In particular, the evaluation of the heat transfer, obviously, is mainly correlated to the right prediction of all the heat transfer coefficient (HTC) into the pool during the boiling and inside the tubes (condensation). This experimental research is focused on filling the existing gap knowledge evidenced in the pool boiling for all the possible IC conditions, actually not available in any public research. Available data are focused on other pool boiling conditions or on an overall HT evaluation, but with low accuracy due to the difficulties in the measure of the power exchanged in two-phase. Then, this separate effect test will enhance our version of the thermal-hydraulic transient simulation code RELAP5 (reference code for these phenomena) on the prediction of the IC component.
This will be realized through the realization of a small experiment in our laboratory. After that, it is expected to analyze the experimental results and to obtain a new HTC correlation suitable for the assessment of safety and reliability of passive systems, inserting this in a modified version of the transient simulation code RELAP5.
The experiment will investigate pool boiling on the outer surface of cylindrical heater submerged in a water pool at atmospheric conditions. This activity is important as preparatory work for our possible contribution in a future Horizon Europe project on the increase of the TRL of the IC.
Relevant experiments were conducted in past years concerning the nucleate pool boiling on vertical cylinders submerged in an atmospheric water pool. Such campaigns individuate a wide number of parameters that influence this heat transfer mechanism (e.g., dimensions of the tube, surface roughness and heat flux range). Nevertheless, the actual knowledge of the phenomenon is still inadequate and further investigations are required to improve the understanding of the nucleate pool boiling occurring in an in-pool passive heat exchanger. Such evidence has been highlighted in the benchmark exercise conducted on PERSEO facility, that proved the poor capabilities of the most used STH codes to simulate such passive system. Although specific HTC correlations for pool boiling could enhance predictive capabilities, those equations were not developed for geometries and conditions relevant for IC applications. As matter of fact, simulations capabilities are improved but sensible discrepancies are still observed.
The objective of the proposed research activity is to enlarge the knowledge of the pool boiling heat transfer mechanism in relevant conditions for NPP IC (atmospheric conditions, high heat flux, industrial-grade of surface roughness and so on). After that, it is expected to interpolate the experimental results to obtain a new HTC correlation suitable for the assessment of the safety and reliability of passive systems.
The experiment will investigate pool boiling on the outer surface of cylindrical heaters submerged in a water pool at atmospheric conditions. The experience will be carried out at imposed heat flux, ranging from 1 to 550 kW/m2. It is worth emphasizing that actual available experimental results, involving vertical cylindrical heaters, were obtained for heat flux lower than 160 kW/m2, far from typical values expected in NPP ICs (around 500-600 kW/m2). As found in literature, the value of the heat flux considerably affects the heat transfer. The increase of the heat flux determines, at the beginning, an enhancement of the heat transfer, due to the growth of the liquid agitation. But, a further rise of the heat flux determines degradation of the heat transfer, following the formation of large vapor slugs. It was found that the turning point between these two mechanisms is around 50 kW/m2. However, previous studies are limited to 160 kW/m2 and no information is available for higher heat fluxes in cylindrical geometries. Furthermore, past campaigns investigated the effect of surface roughness. However, such experiments used extremely smooth surfaces (roughness in the range of 15.1-670 nm) that are not representative for industrial applications. Typical values for NPP applications range between 1 and 50 ¿m. The proposed activity will investigate such range, assessing the effect of this parameter on the overall heat exchange. Furthermore, the possibility to adopt different characteristic of the bundle is foreseen for the proposed facility. In this frame, different bundles will be analysed (triangular and square) with various pitch to diameter ratios, investigating the effect of such parameters. Moreover, a wide number of thermocouples are foreseen to evaluate the heat transfer in different azimuthal regions of the tubes (e.g. inner and outer) and at different levels of the bundles, investigating the heat transfer in different locations. These are crucial aspects since they could give useful information on the sizing of the IC tubes. As matter of fact, previous activities investigated the influence of the tube length (non-dimensional length, L/D) on the overall heat transfer of a single heated rod, highlighting a decrease of the heat transfer rate when non-dimensional length exceeds 50, due to the larger bubble slug formation. Such effect could be magnified in a tube bundle, where a single pin is surrounded by other heated rods. This could emphasize the formation of vapor slug, reducing the turning point of L/D=50. Furthermore, the pitch to diameter ratio could play a crucial role in this framework. For this reason, the facility will be an adaptable system that will allow the variation not only of the p/D but also of the L/D.
Finally, the proposed facility could provide useful information before the SIRIO experimental campaign. As matter of fact, SIRIO facility will be devoted in the investigation of the whole system operation, dealing with the verification of the prototypical safety system. For this reason, massive instrumentation is not foreseen on the outside wall of the heat exchanger, since it could alter the operation of the whole system. Thus, the results obtained by the proposed facility could determine a starting point for the SIRIO campaign.