Hiroumi Tani

Computational Study of the Rate Coefficients for the Reactions of NO2 with CH3NHNH, CH3NNH2, and CH2NHNH2

The reactions of NO2 with cis-/trans-CH3NHNH, CH3NNH2 and CH2NHNH2 have been studied theoretically by quantum chemical calculations and steady-state unimolecular master equation analysis based on RRKM theory. The barrier heights for the roaming transition states between nitro (RNO2) and nitrite (RONO) isomerization reactions and those for the concerted HONO and HNO2 elimination reactions from RNO2 and RONO, affect the pressure dependences of the product-specific rate coefficients. At ambient temperature and pressure, the dominant product of the reactions of NO2 with cis-/trans-CH3NHNH and CH2NHNH2 would be expected to be HONO with trans-CH3NNH and CH2NNH2, respectively, whereas it is CH3N(NH2)NO2 for CH3NNH2 + NO2. The product-specific rate coefficients for the titled and related reactions on the same potential energy surfaces were proposed for kinetics modeling.

Effects of Injector Geometry on Cryogenic Shear Coaxial Jets at Supercritical Pressures

For shear coaxial injectors, which are widely used in liquid oxygen–gaseous hydrogen rocket engines, the key parameters that characterize injector geometry are the recess length, taper angle, and wall thickness of the liquid oxygen post, which is the inner tube of the coaxial injector. In the present study, the effects of the recessed length and taper angle of the liquid oxygen post on cryogenic coaxial jets at supercritical pressures were experimentally investigated. The large-scale structure and mixing enhancement of cryogenic nitrogen and gaseous helium coaxial jets were evaluated using a high-speed backlighting technique. Instantaneous backlit images revealed that the dark core, which is believed to be dense nitrogen, exhibited sinusoidal forms induced by the presence of the outer helium jets. However, the sinusoidal oscillations were less influenced by the injector geometry. The mixing enhancement of coaxial jets was evaluated with respect to the dark-core length and the jet spread angle. The recesse...

Injector Geometry Effects on Cryogenic Coaxial Jets at Supercritical Pressures

The effects of LOX post recess length on the mixing characteristics of coaxial jets, such as the inner-jet-core length and jet spread angle, were investigated under supercritical pressures. In experiments, cryogenic nitrogen/gaseous helium coaxial jets were visualized using the shadowgraph technique. The LOX post recess enhanced the mixing of the inner nitrogen jet when its length was longer than one LOX post exit diameter. The outer helium jet also expanded as the recess length increased. In addition to the effects of LOX post recess, it is also noted that the inner nitrogen jet had sinusoidal structures only in coaxial injection cases. In numerical simulations, the numerical model for Reynolds-averaged Navier-Stokes (RANS) simulations was in the first place validated by comparing RANS results with Direct Numerical Simulation results of a temporally developing mixing layer at supercritical pressures. Unsteady RANS simulations of coaxial jets at supercritical pressures were conducted in the cases with and without the LOX post recess. As in experiments, the inner jet core in the case with the recess was shorter than that in the case without the recess. These effects of recess were most likely due to the enhancement of the inner mixing layer instability inside the recess region.

Uncertainty Quantification for Destructive Re-Entry Risk Analysis: JAXA Perspective

In order to improve the accuracy of the expected casualty risk prediction for the destructive re-entry, the related uncertainty factors are identified and the uncertainty quantification approach are proposed. Based on our studies and experience, the identified dominant uncertainty factors are the model accuracy, the attitude stability mode, the shape complexity and shape change, and the initial conditions at the entry start and the break-up. High-fidelity numerical simulations play the important role to model complex multi-disciplinary physics and to cover the wide range of environmental conditions with small numbers of model validation data. The real shape and the deformation effects are initialy modeled by the high-fidelity numerical simulations and finally modeled by the reduced empirical physics-based models. Flight data acquisition also plays important the role to quantify the uncertainties of the attitude stability mode, the break-up altitude, and the temperature distributions and to validate the integrated risk analysis model. Some of the key findings obtained by the high-fidelity simulation are discussed. It is also shown that various types parameter dependencies such as Mach number, wall temperature, surface curvature, and the effect of the turbulent flows on the aerodynamic characteristics and the heat flux distributions should be considered in the empirical models.

Contactless Space Debris Detumbling: A Database Approach Based on Computational Fluid Dynamics

This paper aimed to investigate the feasibility of a contactless method to detumble space debris by using thruster plume impingement during proximity operation. To detumble the rotational debris, t...

A Numerical Study on Hypergolic Combustion of Hydrazine Sprays in Nitrogen Tetroxide Streams

ABSTRACT Unsteady simulations of hydrazine (N2H4) sprays in nitrogen tetroxide (NTO, NO2-N2O4) streams were conducted to explore the hypergolic combustion in bipropellant thrusters. The Navier–Stokes equations were solved using a detailed chemical kinetics mechanism and dispersed droplets were modeled through direct numerical simulations. Auto-ignition occurred when the sum of the heat transfer from the ambient gas and the heat release from hydrogen abstraction reactions exceeded the latent heat of the droplets. Although the evaporation of the droplets was enhanced as the droplet size decreased, the ignition delay time increased due to the lower temperatures of the mixtures of the N2H4 vapor and nitrogen tetroxide. After the flames reached a steady state, a double flame structure appeared, comprised of outer diffusion and inner decomposition flames. The inner decomposition flame and N2H4 vapor flow exhibited a sinusoidal behavior at a certain droplet size. This behavior was initiated by the locally expanded decomposition gases and developed by the supply of N2H4 droplets to the decomposition gases at relatively high temperatures. In cases of larger and smaller droplet sizes, the sinusoidal behavior was not significant due to less evaporation of the N2H4 droplets and a lower temperature of the N2H4 vapor, respectively. The sinusoidal behavior of the decomposition flames enhanced the mixing and reactions of the fuel components (i.e., N2H4, NH3, and H2). The present study demonstrated a large impact of droplet size on flame dynamics, suggesting that a fine spray is not always better for hypergolic propellant combustion to consume the fuel components quickly.

Interface-Tracking Simulations of Droplet Vaporization and Burning of Hypergolic Propellants

The vaporization and burning of the N2H4 and NTO droplets were simulated with the interface-tracking method to accurately explore the auto-ignition processes and the flame structures. The N2H4 vapor plume developed behind the N2H4 droplet and reacted with the ambient NO2 gas through the hydrogen abstraction reactions. Thus, the N2H4 vapor and NO2 gas mixtures behind the droplet were preheated and reached the auto-ignition at a few ms. The auto-ignition occurred in the multiple points almost at the same time. After the ignition, the premixed flame developed around the droplet. Thus, the vaporization of the liquid N2H4 near the surface became significant. Then, the double flame structures which comprise the inner decomposition flame and oxidation flame appeared around the droplet. The NTO droplet was not auto-ignited in the computational time of the present study because little N2O4 vapor near a saturated temperature decomposed to NO2 gas which is necessary for the hydrogen abstraction reactions. When the ignition was forced, the double flames developed. The outer decomposition flame propagated to the boundaries of the computational domain, while the inner oxidation flame appeared near the droplet. Except for the propagation of the decomposition flame, the NTO droplet combustion was similar to that of the industrial fuels.

Contamination control for the space infrared observatory SPICA

The contamination control for the next-generation space infrared observatory SPICA is presented. The optical performance of instruments on space observatories are often degraded by particulate and/or molecular contamination. Therefore, the contamination control has a potential to produce a significant risk, and it should be investigated in the risk mitigation phase of the SPICA development. The requirements from contamination- sensitive components onborad SPICA, the telescope assembly and focal plane instruments, are summarized. Possible contamination sources inside and outside the SPICA spacecraft were investigated. Based on impact on the SPICA system design, the following contamination sources were extensively studied through simulation and measurement; (1) outgassing from the payload module surrounding the telescope mirror and focal plane instruments, (2) contamination due to the thruster plume, and (3) environmental contamination during the integration, storage and verification phases. Although the outgas from the payload module and the thruster plume were estimated to produce only a negligible influence, the environmental contamination was suggested to affect significantly the telescope and focal plane instruments. Reasonable countermeasures to reduce the environmental contamination were proposed, some of which were confirmed to be actually effective.