Thermodynamic analysis and experimental validation of electric pressure build-up unit for pressurization and fuel gas supply in cryogenic fuel tanks

Cryogenic fuels are increasingly being considered for marine propulsion systems, where reliable tank pressurization and accurate prediction of transient thermodynamic behavior are essential for safe and efficient operation. An electric pressure build-up unit (EPBU) provides a practical solution for pressure control during fuel gas supply (FGS). However, the underlying heat- and mass-transfer mechanisms governing EPBU-assisted pressurization remain insufficiently understood. In this study, a thermal multi-zone model was developed to analyze the thermodynamic behavior of an EPBU-integrated cryogenic fuel tank. The model resolves vapor, interface, and liquid regions and incorporates stage-dependent mass and energy conservation for self-pressurization, EPBU-assisted pressurization, and FGS. Heat transfer correction factors representing vapor–interface and interface–liquid interactions were systematically evaluated as functions of liquid fill level, EPBU power input, and operating stage. Wall heat-flux distributions were quantified using finite element analysis, and the model was validated against experimental data obtained from a liquefied natural gas equipment-embedded FGS system (FGSS). The model reproduced experimental pressure histories with an average relative error below 1% over all tested conditions. Additionally, it identified distinct thermodynamic regimes, including a liquid-dominated pressurization regime at low and high filling levels, a transition regime at intermediate filling levels, and a strongly interface–liquid-dominated regime during FGS. These findings demonstrate that heat transfer correction factors reflect regime-dependent thermodynamic behavior rather than acting as simple fitting parameters. The proposed modeling framework provides physically interpretable insights into EPBU-assisted pressurization and offers practical guidance for pressure control and the design of cryogenic FGSSs.