Hydrogen-Helium Mixtures: Fundamental Measurements, Neutral Droplet Buoyancy, Evaporation, and Boiling

For decades NASA has used helium to pressurize liquid hydrogen propellant tanks to maintain tank pressure and reduce boil-off. This process causes helium gas to dissolve into liquid hydrogen creating a cryogenic mixture with thermodynamic properties that vary from pure liquid hydrogen. This can lead to inefficiencies in fuel storage and instabilities in fluid flow. As NASA plans for longer missions to Mars and beyond, small inefficiencies such as dissolved helium in the liquid propellant become significant. Traditional NASA models are unable to account for this dissolved helium due to a lack of fundamental property measurements necessary for a mixture Equation Of State (EOS). This research presented the first Pressure-Density-Temperature-Composition (PρT-x) measurements of parahydrogen-helium mixtures using a recently retrofitted single-sinker densimeter, magnetic suspension microbalance, and calibrated gas chromatograph. These measurements were used to develop the first multi-phase EOS for parahydrogen-helium mixtures. This new EOS has been implemented into NASA’s Generalized Fluid System Simulation Program (GFSSP) to determine the significance of mixture non-idealities. A model was developed for a simple self-pressurization of a liquid hydrogen propellant tank due to boil-off. The model was run assuming that the liquid propellant was pure liquid hydrogen and then assuming helium dissolved into the liquid using the new heliumhydrogen EOS. The analysis showed that having dissolved helium in the propellant did not have a significant effect on the pressurization rate. The largest contribution to uncertainty in the self-pressurization rate of the propellant tank is due to the large uncertainties in the heat transfer coefficients of liquid hydrogen. The models did show that having dissolved helium in liquid hydrogen slows the rate at which the propellant temperature rises. Throughout the course of this research a critical insight was discovered that the transport of helium in liquid hydrogen is much slower than in other cryogenic liquids. When helium is injected into liquid hydrogen it will take an appreciable amount of time for the pressure to achieve steady state as helium continues to come out of solution. This leads to higher helium concentrations than current solubility models can predict because the current models are only valid for steady state conditions. This phenomenon was seen in my experimental system and in NASA’s Evolvable Cryogenics Project (eCryo). This phenomenon was only seen for liquid hydrogen and did not occur for liquid nitrogen or liquid oxygen. There is concern that dissolved helium could affect pump performance or cause cavitation which would destroy the turbopumps. Additional research should be conducted on the transport properties of helium in liquid hydrogen. It is always recommended to inject helium into the ullage. Additionally, PρT-x measurements on liquid methane-ethane mixtures with dissolved nitrogen were conducted to simulate the conditions of Punga, Kraken, and Ligeio Mare, the hydrocarbon seas of Saturn’s moon Titan. These measurements were crucial to the development of solubility models that will be used to design the Titan Submarine. Effervescence measurements were also conducted on methane-ethane-nitrogen mixtures in a custom test cell to determine under what heat loads nitrogen will come out of solution in the seas of Titan. These measurements will further aid in the design of the ballast systems, scientific instruments, and submarine propellers.