Abstract: Improving the energy efficiency of industrial systems is a major challenge in the energy transition. A significant share of primary energy is dissipated as waste heat, particularly at low and medium temperatures, where recovery solutions remain limited. In this context, supercritical carbon dioxide (sCO2 ) cycles are attracting growing interest because of their compactness, their high efficiency potential, and the favorable properties of the fluid near the critical point. However, their deployment is still hindered by technological barriers and by the difficulty of accurately modeling the real fluid. This thesis investigates an innovative concept of thermally driven compression, called SHREC, developed with the industrial partner CIXTEN. Unlike conventional Brayton cycles, which rely on an electrically driven mechanical compressor, the SHREC device directly converts thermal energy, ideally recovered from waste heat, into compression work. By reducing the need for mechanical compression, it aims to lower auxiliary electricity consumption and improve the overall efficiency of sCO2 cycles. The main scientific difficulty lies in modeling supercritical CO2. Near the critical point, small variations in temperature or density induce strong nonlinear changes in density, heat capacity, compressibility, and transport properties. Under such conditions, ideal-gas assumptions or constant-property approximations become unsuitable. In addition, the SHREC device operates at low Mach number and in complex geometries, which imposes stringent requirements in terms of stability, mass conservation, and thermodynamic consistency. To address these challenges, a multi-scale modeling approach was developed. At the system scale, a zero-dimensional (0D) thermodynamic model, based on a cubic equation of state, was established to describe the behavior of the supercritical fluid. It allows rapid prediction of overall performance and enables cycle analysis. Experimental tests on a reduced-scale prototype were carried out to measure the evolution of pressure, temperature, produced power, coefficient of performance, and exergy destruction. The results show that the main irreversibilities are located in the expander-compressor unit and in the main heat exchangers. At the local scale, a three-dimensional numerical framework based on the Lattice Boltzmann Method was developed and adapted to real-fluid conditions. A compressible low-Mach-number formulation was implemented and coupled with a cubic equation of state to reproduce supercritical thermodynamic effects. Complex geometries were handled using the Immersed Boundary Method. The approach was first validated on supercritical jets from the literature, and then applied to the SHREC device to resolve transient temperature and pressure fields. The comparison between CFD simulations and the 0D model shows that the assumption of instantaneous pressure equilibrium between the chambers is not strictly satisfied. The 0D model does not capture local pressure imbalances, but it reproduces the overall compression and expansion dynamics correctly, whereas the CFD simulations provide a detailed description of the internal flow and heat transfer mechanisms. This work shows that reliable investigation of sCO2 systems requires a consistent combination of real-fluid thermodynamics, reduced-order modeling, numerical simulations, and experimental validation. It contributes to the development of thermally driven compression technologies for waste heat recovery and lays the foundations for compact systems for next-generation energy cycles.

Keywords: Supercritical CO2, Heat transfer, Innovative thermal machine

M. BOIVIN Pierre  CNRS, M2P2 - Directeur de thèse

M. RIBERT Guillaume  INSA Rouen Normandie - Examinateur

Mme RASPO Isabelle  CNRS, M2P2 - Examinatrice

M. MELDI Marcello  ENSAM - Président du Jury

M. FERRASSE Jean-Henry  Aix Marseille Université, M2P2 - Co-encadrant

M. FAVIER Julien  Aix Marseille Université, M2P2 - Co-Directeur de thèse

M. SCHMITT Thomas  CNRS, EM2C -Rapporteur

M. SILVA Gonçalo  Universidade de Évora, Portugal - Rapporteur

Raphaël LOUBÈRE, Rapporteur, DR CNRS, Institut de Mathématiques de Bordeaux 

Timm KRÜGER, Rapporteur, PR, University of Edinburgh                   

Gauthier WISSOCQ, Examinateur, IR, CEA CESTA                                 

Bénédicte CUENOT, Examinatrice, Senior Scientist, CERFACS                     

Vincent MOUREAU, Président du jury, DR CNRS, CORIA                                

Pierre BOIVIN, Directeur de thèse, CR CNRS, M2P2                                 

Song ZHAO, Co-encadrant de thèse, IR CNRS, M2P2