CFD Analysis of Near-Field Exhaust Thermodynamics in Hydrogen-Fuelled Jet Engines and Contrail Formation

Motivation

While hydrogen combustion eliminates CO₂ emissions, hydrogen-fuelled engines produce substantially more water vapour per unit of heat released than kerosene counterparts. This raises the question of whether hydrogen aviation could increase contrail formation — with associated radiative forcing — even as carbon emissions fall. Near-field exhaust thermodynamics, where mixing with ambient air and rapid cooling occur, govern whether the exhaust plume crosses the saturation threshold for ice crystal nucleation.

Methodology

A simplified cylindrical nozzle-and-exhaust domain was designed in SALOME, with locally refined hexahedral meshing concentrated at the shear layer between the hot exhaust core and ambient co-flow. Simulations were run using the rhoSimpleFoam solver in OpenFOAM under steady, compressible conditions. Ten inlet configurations were tested, spanning kerosene and hydrogen exhaust compositions at representative thrust-relevant mass flow rates and temperatures.

Post-processing introduced a saturation ratio field computed via the Tetens formula for water vapour pressure as a function of local temperature. A saturation ratio exceeding unity indicates thermodynamic conditions favourable to contrail formation, subject to nucleation kinetics and ambient humidity.

Results

Hydrogen exhaust configurations consistently produced saturation ratios above unity within the near-field mixing zone, driven by the higher H₂O mole fraction in the exhaust stream and the rapid cooling associated with entrainment of cold ambient air. Kerosene cases, by contrast, remained near or below the saturation threshold under equivalent ambient conditions, consistent with their lower water production and different thermodynamic trajectories in the mixing layer.

The spatial extent of supersaturated regions varied with inlet momentum and ambient relative humidity assumptions, but the qualitative conclusion held across all ten configurations: hydrogen exhausts carry a structurally higher contrail formation potential that must be accounted for in lifecycle climate assessments of hydrogen aviation.

Implications

These findings do not imply that hydrogen aviation is net-negative for climate — contrail radiative forcing depends on flight altitude, time of day, and background cirrus — but they establish that contrail physics must be part of the design conversation from the outset. Ongoing work couples these near-field profiles to plume dispersion models at cruise altitude.