ETD

• Heat Exchange
• Heat Partitioning
• PWR Subchannel

This thesis presents a comprehensive investigation into heat partitioning phenomena and the development of a thermal-hydraulics digital twin for an experimental facility designed to simulate Pressurized Water Reactor (PWR) subchannels. The work aims to enhance the understanding of subcooled flow boiling and its impact on thermal-hydraulic performance, while also providing a flexible, high-fidelity digital replica to support predictive analysis, control, and optimization of experimental activities.

The first part of the thesis is dedicated to the analysis of wall heat flux partitioning in subcooled boiling. The total heat flux from the heated wall to the coolant is separated into three distinct contributions: single-phase convection, quenching due to bubble collapse, and evaporation driven by bubble growth and detachment. This framework, widely recognized in boiling heat transfer research, is here evaluated both numerically and experimentally. Experimental data were obtained from a facility replicating the thermal-hydraulic conditions of PWR subchannels, with working fluid properties and operating parameters representative of pressurized water environments. High-speed imaging and data acquisition systems were employed to characterize bubble dynamics, flow regimes, and surface temperature distributions. The data were then compared with state-of-the-art heat partitioning models from the literature, highlighting strengths and limitations, and guiding the development of improved closure relations.

The second part of the thesis focuses on the creation of a digital twin of the experimental facility. This digital twin is constructed using advanced thermal-hydraulic modeling tools, combining one-dimensional system codes with three-dimensional CFD where needed. The twin is designed to replicate the physical facility in real time, enabling synchronization with experimental measurements. The model accounts for geometrical details of the PWR subchannels, material properties, boundary conditions, and operational transients. To ensure reliability, the twin was calibrated and validated against experimental data, demonstrating accurate prediction of key variables such as pressure drop, void fraction, wall temperature, and local heat flux components. The validation process revealed that the digital twin is capable of capturing both steady-state and transient behavior with good fidelity.

Beyond validation, the digital twin was exploited for scenario testing and predictive analysis. By varying boundary conditions and heat flux levels, it was possible to explore operational envelopes that would be challenging or costly to reproduce experimentally. The twin also enabled sensitivity studies, assessing the influence of uncertainties in material properties and closure laws on global and local performance indicators. These capabilities demonstrate the added value of integrating experimental campaigns with digital replicas, creating a hybrid framework for nuclear thermal-hydraulic research.

The combined outcomes of the heat partitioning study and the digital twin development contribute to both fundamental understanding and practical applications. On one hand, the refined analysis of heat flux partitioning advances the modeling of subcooled boiling, with implications for safety margins and design strategies in nuclear reactors. On the other hand, the digital twin provides a powerful platform for training, diagnostics, and predictive maintenance, aligning with current trends in digital engineering for nuclear systems. Together, these achievements support the broader goal of enhancing the efficiency, safety, and resilience of PWR technology.