Surface temperature jump beyond active oxidation of carbon/silicon carbide composites in extreme aerothermal conditions

U LTRAHIGH temperature ceramic (UHTC) materials containing hafnium diboride (HfB2) and zirconium diboride (ZrB2) with a silica former, most commonly silicon carbide (SiC), have been studied extensively over the last decade as materials for leading-edge and control surface components on hypersonic vehicles [1–3]. Such components experience extreme aerothermal heating in chemically aggressive, partially dissociated air environments. Promising aspects of diboride-based UHTC materials include the very high melting points of HfB2 and ZrB2 and their refractory oxides hafnia (HfO2) and zirconia (ZrO2), as well as the high thermal conductivities of HfB2 and ZrB2, which enables efficient heat conduction away from stagnation point regions [4]. Zirconium diboride has some advantages over hafnium diboride as an aerospace material, because it is lighter and less expensive. The oxidation of ZrB2 produces both zirconia and boron oxide (B2O3). Significant oxidation of ZrB2 in atmospheric air begins at about 1050 K. The softening temperature for amorphous B2O3 is in the range of 830–900 K [5]; below about 1500 K, the oxide scale consists of a porousZrO2 network filled with liquidB2O3 that acts as an effective oxygen diffusion barrier [6,7]. However, the vapor pressure of B2O3 increases rapidly with temperature [8], resulting in rapid loss of B2O3 above 1500 K. The residual porous zirconia scale provides little resistance to inward oxygen transport and further oxidation [9,10], making the oxidation resistance of pure ZrB2 insufficient for high-temperature hypersonic vehicle applications. The addition of a silica former to ZrB2 improves its oxidation resistance [11–14]. Compositions containing from 10 to 30% (by volume) SiC have generally been found to be optimal in this regard. The virgin ZrB2-SiC surfaces oxidize through parallel reactions that generate ZrO2,B2O3, and SiO2. LiquidB2O3 mixes with amorphous SiO2 to form a borosilicate glass that seals the ZrO2 scale [15]. With increasing temperature, boron oxide evaporates preferentially from Received 10 August 2011; revision received 13 February 2012; accepted for publication 19 February 2012. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc. The U.S. Government has a royalty-free license to exercise all rights under the copyright claimed herein for Governmental purposes. All other rights are reserved by the copyright owner. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0887-8722/12 and $10.00 in correspondence with the CCC. ∗Senior Research Scientist, Molecular Physics Laboratory, 333 Ravenswood Avenue; jochen.marschall@sri.com. Senior Member AIAA (Corresponding Author). Research Physicist, Molecular Physics Laboratory, 333 Ravenswood Avenue. Member AIAA. Professor, Department ofMaterials Science andEngineering, 223McNutt Hall; billf@mst.edu. Professor, Department ofMaterials Science andEngineering, 223McNutt Hall; ghilmas@mst.edu. Ph.D. Candidate, Aeronautics and Aerospace Department. Chaussee De Waterloo 72; panerai@vki.ac.be. ∗∗Associate Professor, Aeronautics and Aerospace Department, Chaussee De Waterloo 72; olivier.chazot@vki.ac.be. JOURNAL OF THERMOPHYSICS AND HEAT TRANSFER Vol. 26, No. 4, October–December 2012

Temperature Jump Phenomenon During Plasmatron Testing of ZrB₂-SiC Ultrahigh-temperature Ceramics

The oxidation behavior and surface properties of hot-pressed ZrB 2 -SiC ultrahigh-temperature ceramic composites are investigated under aerothermal heating conditions in the high-temperature, low-pressure partially dissociated airstream of the 1.2 MW Plasmatron facility at the von Karman Institute for Fluid Dynamics. Samples are oxidized at different flow enthalpies for exposure times of up to 20 min at surface temperatures ranging from 1250 to 1575°C. The microstructure and composition of the resulting oxide layers are characterized using electron and optical microscopies, x-ray diffraction, and energy-dispersive x-ray analysis. Comparisons are made with samples oxidized under similar temperature and pressure conditions in a furnace test environment in which atomic oxygen concentrations are negligible. Changes in surface optical properties are documented using spectral reflectance measurements, and effective catalytic efficiencies are estimated using computational fluid dynamics calculations together with measured surface temperatures and heat fluxes.

Oxidation of ZrB2-SiC Ultrahigh-Temperature Ceramic Composites in Dissociated Air

Efficient and accurate Hermitian-type multipoint finite difference methods are used to develop a general boundary-layer code for analyzing reacting flows around bodies of revolution. The main motivation is to build a reliable code that can be used for the investigation of the influence of different physico-chemical models for transport properties, chemical kinetics, and finite rate wall catalysis on practically relevant quantities like heat flux and skin friction at the body surface. Special care has been devoted to the correct modeling of diffusion fluxes, an aspect that is often neglected in literature. The exact Stefan-Maxwell equations are used to model the diffusion fluxes and are solved with an efficient iterative technique. Finite rate catalysis is an important aspect of thermal protection system (TPS) materials studies, for which a boundary-layer code is a very useful tool because it allows the computation of the heat flux at a cost that is a fraction of a Navier-Stokes approach. Wall catalyticity effects are taken into account by means of a model that allows one to express a suitable set of wall reactions with the associated reaction-rate probabilities

Computation of nonequilibrium high-temperature axisymmetric boundary-layer flows

Channel modeling of a plasma sheath for hypersonic reentry vehicles is important in addressing reentry communication blackout issues. Given that the time-varying property of the transmission medium is caused by physical factors such as large-scale flight conditions variation, especially angle of attack, and small-scale fluid turbulence, a new adaptive multistate Markov channel modeling method is proposed to present the dynamic effects of reentry plasma sheaths on wireless channels. First, a quasi-Gaussian function mathematical model for the electron density of a dynamic plasma sheath is established according to the variation frequency of the dynamic physical factors. Then, the time-varying attenuation is calculated by combining the quasi-static Monte Carlo and uniform hierarchical analytical methods for nonuniform electronic density distribution. Finally, a hidden Markov model is employed to model the channel characteristics of the dynamic plasma sheath, and a reversible-jump Markov chain Monte Carlo algorithm is used to adaptively estimate the channel parameters of the multistate Markov channel. Simulation results show that the first-order statistical properties of the proposed channel are in good agreement with the calculated variable attenuation of radio waves, and this method has the adaptive capacity to estimate the number of multiple states and the channel parameters in each state without a need for manual setting.

Adaptive Multistate Markov Channel Modeling Method for Reentry Dynamic Plasma Sheaths

The present paper deals with the experimental determination of the operating envelope of the Plasmatron facility, designed and built at the von K&m&n Institute for Fluid Dynamics as the test bench of the European Space Agency to perform testing of TPS materials before their use in flight. As such the prime requisite of the facility is its capacity to reach specified values of stagnation pressure and catalytic cold wall stagnation point heat flux. These two variables are measured using a water-cooled Pitot probe and a steady-state heat flux probe. The paper contains the description of these probes and details about their use, as well as an analysis of experimental uncertainty. The results of the envelope exploration are presented and it is shown that the facility reaches and outdoes its basic requirements. The experimental results are also compared to the values predicted by a quasi-one-dimensional nozzle code of a plasma flow in chemical equilibrium. It is shown that the experimental values agree well with the predicted values. Results of measurements at various positions of the plasma jet are also reported. The decrease of heat flux and dynamic pressure with axial distance from the torch is essentially linear. The onedimensional model was shown to fail to predict the effect of mass flow rate, due to strong twodimensional effects in the jet. Perspectives of supersonic operation are examined in view of the pumping capacity. A brief description of the facility is also part of the report.

Experimental and computational determination of the VKI Plasmatron operating envelope

The high temperatures associated with the hypersonic reentry process lead to an increase in the collisions between molecules, which may result in the disruption of the electronic structure, producing free electrons and ions. This production of free electrons and ions creates a plasma or ionized flowfield around the vehicle that is known to degrade the quality of radio-wave signal propagation, leading to a loss of communication or “blackout.” This study involves performing hypersonic computational fluid dynamics simulations in the commercial software CFD++ with a blunt-nosed cone geometry at different flight conditions to predict how and when ground communication can be achieved to aid in the design of an alert transmitter, which is an extension of aircraft collision-avoidance system technology. Computational results show that a small blunt-nosed cone geometry has decreased ionization regions as the cone angle decreases due to a shifting of the reaction zone further downstream. As a result, higher freestre...

Blackout Analysis of Small Cone-Shaped Reentry Vehicles

Blackout Analysis of Small Reentry Vehicles

Determination of temperature profiles in plasma jets with a crossflow tube probe

This paper is dedicated to the aerothermodynamic characterization o f the footprint of the Ultra High Temperature Ceramic winglet payload that will be embarked on the ESA reentry demonstrator, EXPERT (Payload 15). Experiments are carried out in the H-3 Mach 6 wind tunnel facility of the von Karman Institute using scaled d own winglet models on a flat plate for the purpose of identifying the aerodynamic and thermal footprint of the winglet on the flat plate. Infrared thermography and sublimation visualization techniques are used to study the thermal footprint; whereas oil fl ow visualization and schlieren techniques are utilized to study the aerodynamic footprint. The position of the winglet and the flow Reynolds number are varied to study the effect of the winglet height to boundary layer thickness ratio. The main flow topology parameters are examined as a function of the winglet height to boundary thickness ratio. The critical regions in terms of increased heat flux and increased aerodynamic shear stress are identified.

http://enu.kz/repository/2010/AIAA-2010-4963.pdf

Vincent Van der Haegen

Papers

Experimental and computational determination of the VKI Plasmatron operating envelope

Blackout Analysis of Small Cone-Shaped Reentry Vehicles

Blackout Analysis of Small Reentry Vehicles

Determination of temperature profiles in plasma jets with a crossflow tube probe

Footprint Analysis of the EXPERT Vehicle Winglet at Hypersonic Conditions