Improving the Modelling of Dissociating Hydrogen Nozzles

While the design of nozzles for diatomic gases is very well established and covered by published works, the case of a diatomic gas dissociating to monatomic along a nozzle is a novel subject that needs a proper mathematical description. These novel studies are relevant to the definition of nozzles for gas-core Nuclear Thermal Rockets (NTR) that are receiving increased attention for the potential advantages they may deliver versus current generation rockets. The article thus reviews the design of the nozzles of gas-core NTR that use hydrogen as the propellant. Propellant temperatures are expected to reach 9,000-15,000 K. Above 1500 K, hydrogen begins to dissociate at low pressures, and around 3000 K dissociation also occurs at high pressures. At a given temperature, the lower the gas pressure the more molecules dissociate, and H₂ → H + H. The properties of the gas are a function of the mass fractions of diatomic and monatomic hydrogen x _H2 and x _H = 1 − x _H2. Dissociation influences the molecular weight of the gas and its heat capacity, dramatically increasing the specific impulse (ISP) of the reactors by changing the gas properties. In the first section, the article reviews the geometry definition. Despite the Method of Characteristics (MOC) theoretically does not apply to a flow of a real gas of changing properties, the MOC is suggested to supply a first guess of the nozzle of a gas-core NTR. The average content of H and H₂ across the nozzle is used to approximate the dissociating hydrogen as the working fluid. In the second section, the article reviews the geometry verification and refinement. It is suggested to use the Navier-Stokes (N-S) solvers of commercial Computational Fluid Dynamics (CFD) packages, incorporating a reliable nonequilibrium model describing the dissociation process. While more complex opportunities are certainly possible, a single transport and diffusion equation with source and sink term is proposed for the mass fraction of the diatomic hydrogen x _H2. Prototyping and testing are now needed to further develop and confirm the dissociation model of the N-S simulations and further refine the design.