Performance Effects of Dissociated Hydrogen in Nuclear Thermal Propulsion Engines

Nuclear thermal propulsion (NTP) research considers hydrogen dissociation as negligible to design and analysis of propulsion engines. This study reintroduces chemically reacting flow to NTP engine analysis for investigation of the dissociation effect on engine performance. A first-principles approach observes the basic chemical mechanism and reaction within the high-speed, high-temperature NTP flow to baseline expected atomic hydrogen levels and validate equilibrium. A surface reaction study looks into hydrogen absorption through dissociation and its effect on the boundary layer, and bulk flow. For total performance, the resulting dissociated flow is analyzed through nozzle expansion and performance metrics calculated.All historic reactor inlet conditions can be assumed to be at equilibrium for atomic hydrogen values at, or below, 1% and remain at equilibrium until approximately 1600 K. Beyond this temperature, all reactors operated in non-equilibrium at worst conditions of sonic velocity, with one reactor operating as low as 51% of equilibrium but converged toward equilibrium at the outlet. All historic NTP reactors correlated with equilibrium at reactor outlet within 82%. Hydrogen dissociation is present in all historic NTP reactors, but all atomic hydrogen levels were below 0.5% molar fraction of total concentration when considering bulk flow temperatures and any speed up to the sonic speed. Historic reactors demonstrate that similar NTP systems must consider non-equilibrium near the outlet above 1600 K as well as if the flow is disrupted from trace amounts below 1600 K. Highest core exit temperatures contains the highest dissociated atomic hydrogen while highest operating pressures cause the reactor to closely follow equilibrium. Reactor inlets that contain less than 1% molar fraction atomic hydrogen can be assumed to reach equilibrium by the reactor outlet. NTP reactors contain a notable crossflow velocity near the reactor outlet due to the absorption of hydrogen into the NTP cladding material. Tungsten is selected as the cladding of choice due to concerning indications of high hydrogen penetration rates in molybdenum-tungsten (Mo-W) alloy. Zirconium carbide (ZrC) was rejected due to its historic issues with mass loss and carbon chemical reaction but proves to be a promising future candidate. The effects of the boundary layer were in questions as crossflow velocity reached 80 mm/s into NTP cladding resembling a form of wall suction. The absorption of hydrogen has a small effect on the thermal boundary layer. This demonstrated that the fluid mechanics of NTP reactors must consider the surface reaction of dissociated hydrogen flow in design and optimization of NTP engines. A follow-on computational fluid dynamics (CFD) looked closely at the boundary layer effects, assuming wall transpiration of hydrogen and provide the most accurate velocity and mass flow rates exiting the reactor and entering the chamber. A 2-D axisymmetric computational fluid dynamics (CFD) model of the KIWI 4BE nuclear reactor provided the highest quality of chamber conditions for NTP. Atomic hydrogen was found to double over simple 1-D assumptions assuming equilibrium, equilibrium conditions, and assuming centerline temperature. For the KIWI 4BE model, 0.7% molar fraction of atomic hydrogen entered the NTP “Chamber” accompanied by a significant velocity decrement of 7% less than non-chemically reacting assumptions. The thermal profile changes appear relatively insignificant while the “1/7th” turbulent velocity profile was not accurate and needs further update. Analysis of zirconium carbide indicated that hydrogen absorption maybe greatly suppressed but further operational conditions are needed for testing. Further computations were completed in 1-D rocket nozzle expansion to fully understand the performance effect. Calculation of the changing atomic hydrogen level requires a chemical analysis throughout nozzle expansion. NASA Chemical Equilibrium Analysis Application provides the tools to complete this analysis. The specific impulse is found for the chemically reacting flow and nonchemically reacting flow cases and compared. The specific impulse demonstrates the superior efficiency of the KIWI B4E engine over liquid engines. Despite reduced velocity and increased atomic hydrogen, the chemically reacting flow model produced a slight increase in specific impulse of 0.9% over the non-chemically reacting flow model. Modeling of NTP engines will underestimate the engine performance if chemically reacting flow is not considered.