High-enthalpy Environments Research Articles

Kinetic Monte Carlo modeling of the interface heterogeneous catalysis effect based on machine learning

<p indent="0mm">Heterogeneous catalysis in a high-enthalpy environment has a remarkable influence on the aerodynamic heating of hypersonic aircraft, and modeling this multiscale effect is an important prerequisite for the lightweight and low-redundancy design of an aircraft thermal protection system. A mesoscale interface heterogeneous catalysis model applicable to macroscale flow field calculation was established by the Monte Carlo method. By the radial basis function neural network algorithm, a machine learning-based reaction kinetics analytical surrogate model was constructed, by which fast and accurate prediction of the catalytic recombination coefficient was realized. The results showed that the machine learning model predicted a recombination coefficient similar to the kinetic Monte Carlo method while reducing the magnitude of the calculation time to that of the traditional macroscale method. Coupled with the macroscale CFD solver, the proposed model can efficiently predict surface heat flux with a multiscale heterogeneous catalysis effect. Modeling of microscale results by the machine learning method and its application in macroscale simulation not only covers the heterogeneous catalytic kinetics but also improves computational efficiency. This modeling provides a theoretical basis for multiscale modeling of complicated high-temperature effects on hypersonic aircraft surfaces.

Experimental investigation on transpiration cooling for porous ceramic with liquid water

Experiments are conducted in the arc-heated wind tunnel to investigate the transpiration cooling performances for SiC porous ceramic (0.98 g/cm3) with liquid water. It is experimentally found a new phenomenon that the ice beard forms on the specimen in the high enthalpy environment (15000 kJ/kg). Besides, the shape and size of ice beard are related to the uniformity of pores distribution on the surface of porous ceramic. At mainstream with specific enthalpy 5500 kJ/kg, heat flux 1900 kW/m2 and pressure 7800 Pa, the surface temperature of SiC porous ceramic with a coolant liquid water rate of 0.2 g/s just climbs to 360 K from 271 K, while its surface temperature without coolant permeating from the ceramic sharply increases to 1025 K. Therefore, transpiration cooling of SiC porous ceramic with liquid water is found to be effective. This work can provide a relatively useful reference for the design of active thermal protection system in hypersonic vehicles.

Significant Supersonic Modes and the Wall Temperature Effect in Hypersonic Boundary Layers

There has been renewed interest in studying supersonic modes in hypersonic boundary layers. Recent computational results have shown supersonic modes in hot-wall flows, upending the notion that they exist only in cold-wall flows. Furthermore, supersonic modes with larger peak growth rates than the second mode have been encountered in a very blunt cone geometry. Therefore, conditions leading to supersonic modes and their dominant amplification must be thoroughly and systematically investigated. Specifically, the impact of wall temperature in high-enthalpy environments is of immediate interest. This work uses thermochemical nonequilibrium direct numerical simulation (DNS) and linear stability theory (LST) to simulate Mach 10 flow over a 1 mm nose radius axisymmetric cone. Despite LST results indicating no supersonic modes in either the hot- or cold-wall flow, DNS results indicate their presence in both cases, with the cold wall exciting the supersonic mode comparably to the second mode. Further fast Fourier transform analyses suggest that this was a result of the nonlinear interaction between an unstable subsonic mode S, stable supersonic mode F1, and the slow acoustic spectrum. Because the supersonic mode in the cold-wall case had a comparable growth rate to the second mode, the supersonic mode could impact transition unexpectedly if not accounted for.

Aero-thermo-chemical characterization of ultra-high-temperature ceramics for aerospace applications

Ultra-High-Temperature Ceramic (UHTC) materials, because of their high temperature resistance, are suitable as thermal protection systems for re-entry vehicles or components for space propulsion. Massive UHTC materials are characterized by poor thermal shock resistance, which may be overcome using C or SiC fibers in a UHTC matrix (UHTCMC).The University of Naples “Federico II” has a proven experience in the field of material characterization in high-enthalpy environments. A hypersonic arc-jet facility allows performing tests in simulated atmospheric re-entry conditions. The Aerospace Propulsion Laboratory is employed for testing rocket components in a representative combustion environment. Ad-hoc computational models are developed to characterize the flow field in both facilities and perform thermal analysis of solid samples.Current research programs are related to a new-class of UHTCMC materials, for rocket nozzles and thermal protection systems. The activities include design of the prototypes for the test campaign, numerical simulations and materials characterizations.

An experimental investigation on transpiration cooling with phase change under supersonic condition

The extremely high heat loads on hypersonic vehicles make great challenges for the research and development of more effective Thermal Protection Technique (TPT). Transpiration cooling using liquid coolant has been confirmed as a promising TPT, because the phase change of liquid can release a large amount of latent heat. An experimental investigation on transpiration cooling using water as coolant was conducted in the arc-heated wind tunnel of China Academy of Aerospace Aerodynamics (CAAA), with a free stream specific enthalpy, mass flow rate and Mach number of 2700kJ/kg, 645g/s and 4.2, respectively. The specimen used in this experiment, a nose cone, consists of two types of materials. One is sintered porous material in the leading edge region as transpiration cooling matrix, and the other is impermeable alloy with zirconia Thermal Barrier Coating (TBC). The thermodynamic and aerodynamic parameters in coolant chamber are controlled by a quantitative injector of coolant mass flow rate to prevent the phenomena of overcooling and ablation. Surface temperature distributions are captured by an infrared thermal imaging system (ITIS). This paper exhibits and analyzes some interesting experimental phenomena, including ice formation in high enthalpy environment, hysteresis of thermal balance due to TBC, and variations of surface cooling effectiveness. A numerical approach to estimate the water mass flow rate is introduced and the relative error referenced experimental results is analyzed.

Temperature Dependence of Electrical Resistivity – Part II: A New Experimental Set-up to Study Fluid-saturated Rocks

The growing interest in exploiting supercritical geothermal reservoirs calls for the understanding of the physical properties of rocks and the fluids they are interacting with in a high enthalpy environment. Here, we present a new flow-through cell capable for petrophysical measurements of fluid saturated rocks at high pressure and temperature. The permeability and electrical resistivity of a quartz-gabbro from a fossil hydrothermal system on Iceland were determined between 25 – 350 ̊C at a controlled pore pressure of 22.2MPa. The measured resistivities are correlated with fluid resistivity data obtained in an attendant study (see part I of this paper).

Computational Simulation of Arc Heater Flows for Martian Atmosphere

An existing computational fluid dynamics code which simulates a high enthalpy arc heater flow for air, named ARCFLO3, is updated to be able to calculate a high temperature carbonaceous flow. The flowfield is assumed to be in thermochemical equilibrium and the thermodynamic and transport properties for high temperature carbonaceous gas mixture are calculated though the Yos' mixture rule by compiling the most updated set of collision integrals. The radiative transport in arc heater is calculated in a fully coupled manner with flow motion by using a 3-band radiation model developed in the present study. The upgraded code is used to calculate the operational characteristic parameters such as arc voltage, chamber pressure, heater thermal efficiency and mass-averaged enthalpy for existing 60MW arc heater geometry by using CO2 as a test gas. The result shows that by specifying the wind tunnel operating conditions routinely used for air, the similar level of the operational characteristic parameters is obtained, and that a high enthalpy environment during a manned Martian atmospheric entry flight can be produced for the arc heater geometry.

Calculation of High-Enthalpy Aerothermal Environment in an Arcjet Facility

S EGMENTED-CONSTRICTOR type arc-heated wind tunnels are used to test the heat shield materials for spacecraft thermal protection systems. The arc-heated wind tunnel consists of an upstream electrode (anode) chamber, constrictor section, downstream electrode (cathode) chamber, and a diverging–converging nozzle connecting to a test chamber. In the test section, the heat shield materials are exposed to a high-enthalpy flow environment produced by the facility. The high-enthalpy environment is often such that the flowdoes not reach equilibrium condition at the edge of the boundary layer over the tested material. In such a case, we need to calculate the flow properties at the surface of the material using a computational fluid dynamics approach to understand the thermal response of the material [1]. For this purpose, the arcjet freestream conditions must be known accurately. To calculate an arcjet freestream condition, two important physical processes occurring in the arcjet wind tunnel should be accounted for: the heating process in the arc heater region upstream of the nozzle throat and the relaxing process in the expanding nozzle region downstream of the nozzle throat. The ARCFLO3 code has been developed recently to calculate the flowfield in the segmentedconstrictor type of arc heaters [2]. Unlike the arc heater flowfield code named ARCFLO developed in the 1970s [3], which is able to calculate the flow in the constrictor section, this new code calculates the flow from the anode chamber to the nozzle throat [2]. Arcjet freestream conditions can be calculated fully theoretically if a nonequilibrium expanding nozzle calculation is made with the calculated flow properties at the nozzle throat obtained by using the ARCFLO3 code. In addition, because the radial distribution of the flow properties at the nozzle throat is calculated with the ARCFLO3 code, the unified computational method can give the radial flow properties in the arcjet freestream at the test section. We tried to make such a unified calculation very recently for one operating condition in an arcjet facility [1]. However, the question remains as to how well such a computational approach predicts the flow properties in an arcjet freestream. It is the purpose of the present work to test the validity of the unified method. The method is applied to calculate the flowfield in a 0.75-MW arcjet wind-tunnel facility at the Institute of Aerospace Technology of the Japan Aerospace Exploration Agency (IAT/JAXA) in Japan. This facility was chosen for the following reasons: 1) In the recent measurement [4] in the IAT/JAXA arcjet facility, the operational characteristic parameters for a wide range of conditions were obtained. The experimental data offer an opportunity to test the current state of the computational modeling in the proposed method. 2) In our previous work, the ARCFLO3 code was applied for the arc heater flowfield calculation only in a high-power-level arc heater, such as the 20or 60-MW arcjet facility at NASA Ames Research Center [2]. The applicability of the ARCFLO3 code to submegawatt class facilities is unknown.

Radiation transport calculation in high enthalpy environments for two-dimensional axisymmetric geometries

A Monte Carlo method to determine radiative source terms and boundary heat fluxes for generic, twodimensional axisymmetric geometries was developed. For the geometrical discretization, curvilinear structured grids have to be used. Thus, it is possible to use essentially the same grid as for the initial flowfield computation. The spectral behavior of gas radiation was considered as well as thermochemical nonequilibrium conditions of the medium using the NEQAIR code system. The radiation process is simulated by a large number of energycarrying bundles emitted at the appropriate locations, having the appropriate wavelength and direction of emission. These bundles are then traced on their path through the computational domain and their spectral absorption behavior is calculated. Departing from a given flowfield solution for the considered geometry, the radiation simulation method is initiated and yields the radiative source terms in an adequate form for coupling with the energy equations of the flowfield. The iteration is performed in a loosely coupled manner. A first radiation assessment for the case of a trajectory point of the ROSETTA-re-entry is presented.