Thermo-hydraulic modeling of fast fluid transients in pipelines for multienergy systems

The pipeline infrastructure for multicarrier energy systems will increasingly transport fluidswhose thermodynamic behavior differs substantially from that of water, including hydrogen,carbon dioxide, liquefied natural gas, and synthetic fuels. This study investigates hydraulic shock in such systems and assesses how the choice of thermodynamic flow formulation affects predicted pressure extrema, waveform evolution, numerical cost, and safety-related conclusions. Four one-dimensional model formulations are compared: a simplified isothermal model solved using the method of characteristics, and isothermal, adiabatic, and non-isothermal compressible models solved using a high-resolution WENO scheme. Thermophysical properties are evaluated using Helmholtz-energy-based equations of state, and pipe-wall elasticity is incorporated into the density-based formulation through an effective wave-speed treatment. The approach is validated against experimental water-hammer data and benchmarked against OLGA for liquid hydrogen. The numerical scenarios are then considered for liquid hydrogen, supercritical, gaseous, and liquid CO2, LNG, and methanol. The results show that isothermal models can substantially underestimate the pressure extrema for cryogenic liquids, liquid CO2 and supercritical CO2; the underestimation of the maximum pressure increase reaches approximately 22–28% in the most sensitive cases. The results also show that, for scenarios close to phase boundaries, the selection of thermodynamic models can influence safety-related conclusions about cavitation or partial vaporization. Heat transfer mainly affects subsequent wave reflections, but its influence is state-dependent and can either increase or decrease the wave speed in CO2 systems. The findings demonstrate that model selection should consider not only compressibility but also thermodynamic process, proximity to phase boundaries, heat exchange, and transient duration.