Loosely-coupled Conjugate Heat Transfer analysis of transient wall condensation

The increasing integration of renewable energy sources into power generation systems has led steam turbines to operate more frequently under off-design conditions, with repeated start-ups and shutdowns introducing thermal transients that can compromise structural integrity. During cold start-up, the interaction between superheated steam and cold walls can trigger film condensation, releasing a large amount of energy and creating a strongly coupled problem between the film dynamics and the thermal response of the solid. While various strategies have been proposed to address the mismatch in characteristic timescales between fluid and solid in Conjugate Heat Transfer (CHT) problems, the limitations of commonly used quasi-steady approaches has been rarely discussed. In light of this, the novelty of this work lies in the development and validation, with both analytical solution and experimental data, of an equilibrium-based phase change model for laminar condensate films and in the demonstration of the inadequacy of widely adopted computational cost-saving strategies in turbomachinery applications, when dealing with thermal transients involving the transport of a laminar condensing film. Finally, the source terms based loosely-coupled Dual Time-Step method is adopted, showing a remarkable ability to predict the non negligible inertia that a condensing laminar film typically exhibits. • Research context: transient cold start-ups of steam turbines. • Proposal and validation of a multiphase model for wall condensation of pure steam. • Monolithic CHT simulation of wall condensation over a vertical plate. • Exploration of multiphysics approaches to reduce computational costs. • Remarkable accuracy by the Dual time-step method against the standard approaches.