Phase transition studies in liquid hydrogen fuels for future sustainable aviation

Over the last years, interest in using ”clean” fuels to support a more sustainable aviation industry has been outstripping. The increasing demand for air travel whilst using conventional kerosene fuels has spiked rapidly the emission levels of CO2 and NOx to magnitudes without precedent. To reduce the aviation environmental impact, hydrogen fuels have been identified as a potential and promising solution. A well popular trend today in aviation is to align the future technologies with the ”Net zero by 2050” project plan established by the European Union during the Paris Agreement, which aims, among many, to neutralize the CO2 emissions. By using hydrogen as an aviation fuel, CO2 emissions are expected to be erased whilst NOx levels to be considerably decreased. Nevertheless, hydrogen exhibits several challenges in terms of storage and thermal management, assuming its initial cryogenic condition in liquid state. The aim of this work is to study the phase transition phenomena in hydrogen-fuel system to under??stand the single and multi-phase flow physics behind it, ending with a preliminary design of heat exchangers. The hydrogen phase transition problem in fuel system was simplified to a straight pipe. Initial 2D and 3D numerical single-phase flow simulations were conducted to quantify the change in thermodynamic properties during liquid state. Similarly, a 2D and 3D steady-state and a 3D unsteady numer??ical two-phase flow simulations were performed using the Volume of Fluid method to observe the evolution from liquid to gaseous state. To provide a design guide, a parametric study was performed on the 3D two-phase flow model where the material properties were varied to characterize the effect on the flow transition. Although its computational cost, the most representative method for phase transition in hydrogen was the 3D transient numerical simulation with specific heat coefficient function of temperature. Heat exchangers design proved to work efficiently and converts all the liquid into gas. This work has established the groundwork for a CFD capability to model hydrogen two-phase flows in heat exchangers. While further validation with experimental data is still required, the methods presented herein captured relevant physical phenomena, such as the density decrease, gas volume fraction increase and temperature increase. This piece of work is relevant as research as it sets the stage for further design of heat exchange systems in hydrogen fuel-system.