Ines MARZOUGUI's thesis defense (mechanical engineering department)

This numerical work focuses on the intensification of heat exchanges and transfers to determine the energy performance of a double-acting Stirling engine, where unsteady laminar alternating flow prevails, driven by the action of two pistons in phase quadrature.

The study presented revolves around the optimization of the operation of this engine, based on a further optimization of the operating conditions and focusing on the characterization of the regenerator, which is the key element of the Stirling engine.

We performed a numerical simulation (CFD) by solving the Navier-Stokes equations coupled to the energy equation in both the hot and cold sources, and in the porous medium of the porous matrix (regenerator).

The numerical method used for this simulation is of the SIMPLER type relying on a finite element-based finite volume (or control volume) scheme.

In the regenerator (porous medium), flow is described by the Darcy-Brinkman-Forchheimer law.

A parametric study of porosity, material type (three types chosen: stainless steel, Hastelloy X and lanthanum-nickel alloy (LaNi5)), permeability and geometric configuration, was then carried out to qualify and quantify heat transfers leading to a better understanding between Stirling energy maximization and minimization of pressure drop in the porous medium. In fact, these three materials were studied and compared.

The results show that the choice of lanthanum-nickel alloy (LaNi5) following its thermo-physical characteristics, leads to the best energy efficiency of the system.

We then studied three geometric configurations of regenerators, forming a porous matrix with a single regenerator, two subregenerators and three subregenerators. The last case, representing a block made up of three sub-regenerators, with the same thickness and material, but with variable porosity, proves to be the most interesting configuration. This leads to a considerable intensification of heat transfer, enabling better production of mechanical energy, which can be converted into electrical energy to be dissipated in an electrolyzer for the production of green hydrogen. The latter would be used as a fuel to power part of the same internal combustion engine (in a closed loop).

Subsequently, we showed that the direction of axial porosity distribution plays a notable effect in the energy responses of the Stirling engine. Indeed, decreasing porosity in the direction from the cold source to the hot source, presents significant advantages.

Stirling engine energy responses.

Finally, to better approximate the behavior of a real gas in a Stirling engine, we considered a working fluid representing a van der Waals-type gas to conduct detailed, multi-objective investigations but for a number of subregenerators varying from 1 to 5. The results thus obtained, were validated by comparison with existing experimental work for the same system as the one studied in our case.

The various investigations carried out by numerical simulations and relying on the design of experiments method, showed that optimal operation could be achieved with a charge pressure of around 30 bar, a heating temperature of around 630°C and a porous matrix made up of 5 sub-regenerators almost close together.

These results clearly demonstrate the efficiency of such a process, using a Stirling engine for cogeneration via the recovery of waste heat from the exhaust of an internal combustion engine, and the possibility of its integration into clean energy production systems.

Rapporteurs/Referees:

• CASTELIN Cathy, CNRS Research Director at LTEN UMR_C 6607, University of Nantes
• Mohamed Sadok GUELLOUZ, HU Maitre de conférences at ENIB (Bizerte), Université de Carthage

Examiners/Examiners:

• LEGRAND Jack, Professor, GEPEA (UMR CNRS 6144), University of Nantes
• JEMNI Abdelmajid, Professor, ENIM (Monastir), University of Monastir, Tunisia
• ALOUI Fethi, Professor, LAMIH (UMR CNRS 8201), INSA Hauts-de-France, UPHF, Valenciennes
• GHEITH Ramla, HDR Lecturer at the University of Monastir (ENIM), Tunisia

Guests/Invited:

• KEIRSBULCK Laurent, Professor, LAMIH (UMR CNRS 8201), UPHF, Valenciennes
• HACHEM Houda, Maître-Assistant at CRTEn, Borj Cédria, Tunisia (Co-supervisor)

Thesis supervisors/Supervisors:

• Fethi ALOUI, Professor at INSA Hauts-de-France (LAMIH, UMR CNRS (LAMIH, UMR 8201)
• GHEITH Ramla, HDR Lecturer at the University of Monastir (ENIM), Tunisia
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