(30c) Numerical Simulation of Cryogenic Liquid Hydrogen Tanks with Multilayer Insulation Exposed to Fire

In the ongoing energy transition, hydrogen has emerged as a promising alternative energy carrier with a reduced environmental impact. Among the possible solutions to store hydrogen onboard vehicles, cryogenic tanks equipped with multilayer insulation (MLI) appear to be one of the most effective in ensuring high volumetric energy density. MLI systems consist of several layers of low-emissivity material (radiative layers), typically aluminum or aluminum-coated polyester, interleaved with low thermal conductivity spacers to avoid direct contact between the radiative layers. In cryogenic tank applications, these are enclosed within the vessel double-walled shell, working under high-vacuum conditions. Among the insulation systems available nowadays, MLI-based ones have the smallest volume requirements and the lowest weight. Thanks to these features, MLI appears as the preferable choice in sectors where space and weight constraints play a crucial role (e.g., in the transportation sector).

The widespread deployment of LH₂ new technologies poses also challenging questions related to the hazardous properties of hydrogen. The accidental loss of integrity of cryogenic LH₂ tanks might lead to extremely dangerous phenomena, such as Boiling Liquid Expanding Vapour Explosions (BLEVE), Fireball, and Rapid Phase Transition (RPT). One potential scenario that could give rise to this situation is exposure to an external heat source such as a fire triggered by a road accident. Real-scale fire test results suggest that the insulation performance of MLI systems may undergo severe degradation when these are subjected to high temperatures, leaving the tank almost unprotected and leading to failure in a relatively short time.

In this framework, the availability of models able to simulate the tank response to fire exposure is crucial to ensure a safe design and support emergency response planning. Several CFD and lumped models originally developed for pressurized and atmospheric tanks were extended to cryogenic vessels. However, these provide results in line with experimental data only if the insulation system's equivalent thermal conductivity is fine-adjusted to higher values with respect to normal operating conditions. These models do not integrate, in fact, the description of MLI thermal degradation as a result of fire exposure, which was demonstrated to play a crucial role in determining the response of the cryogenic liquid hydrogen tank.

On the other hand, MLI heat transfer models currently available in the literature are suitable for normal operative conditions only and do not address the material behavior under fire exposure.

This work presents an innovative lumped model to simulate the thermal response of MLI-insulated cryogenic hydrogen tanks in fire scenarios. In particular, the proposed approach enables the prediction of MLI loss of insulation performances due to fire-induced thermal degradation, overcoming the limitations of currently available models.

The MLI degradation model is based on the well-established layer-by-layer approach and integrated with sub-models to estimate the material deterioration due to thermal load. Several sub-models were defined to account for different MLI materials. In particular, for polyester-based MLIs, the deterioration of each layer is defined by the apparent kinetic of thermal degradation retrieved from Thermo-Gravimetric-Analysis (TGA) tests. For aluminum-based MLI, each radiation layer is assumed to vanish when its temperature reaches the melting point of the material.

The model was applied to several case studies addressing different types of MLI (i.e., both polyester-based and aluminum-based ones) and fire scenarios to assess the effect on the heating rate and pressure build-up of the tank lading. The analysis allowed for the performance comparison of the studied MLIs, providing valuable information to support the emergency management of accidental scenarios involving liquid-hydrogen cryogenic tanks. Moreover, the results obtained can be used to define mitigation measures to protect the integrity of cryogenic tanks equipped with MLI.