Development of a Nonequilibrium Finite-Rate Ablation Model for Radiating Earth Reentry Flows

Alba, Christopher R.; Greendyke, Robert B.; Marschall, Jochen

doi:10.2514/1.a33303

Cited by 31 publications

(14 citation statements)

References 46 publications

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“…It is important to note that the influence of reaction 8 in the ZA-MacLean-model was also analyzed by Alba et al 14 As discussed in the introduction, Alba used the ZA-MacLean-mobile model in the US3D CFD code to simulate high-enthalpy ablation experiments performed at the University of Queensland. A carbon material sample was resistively heated to high temperature and exposed to short duration hypersonic flow.…”

Section: Via2 Backward Rates In Za Modelmentioning

confidence: 97%

Finite-rate oxidation model for carbon surfaces from molecular beam experiments

Poovathingal

Schwartzentruber

Murray

et al. 2016

46th AIAA Thermophysics Conference

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An oxidation model for carbon surfaces is developed where the gas-surface reaction mechanisms and corresponding rate parameters are based solely on observations from recent molecular beam experiments. In the experiments, a high energy beam of oxygen (93% atoms and 7% molecules) was directed at a high-temperature carbon surface. The measurements revealed that CO was the dominant reaction product and that its formation required a high surface coverage of oxygen atoms. As the carbon sample temperature was increased during the experiment, the surface coverage was reduced and the production of CO diminished. Most importantly, the measured time-of-flight distributions of surface reaction products indicated that CO and CO2 were predominately formed through thermal reaction mechanisms and not impulsive reactive scattering. These observations enabled the formulation of a finite-rate oxidation model including surface-coverage dependence, similar to existing finite-rate models used in computational fluid dynamics (CFD) simulations. However, each reaction mechanism and all rate parameters of the new model are determined individually based on the molecular beam data. The new model is compared to existing models using both zero-dimensional gas-surface simulations and full CFD simulations of hypersonic flow over an ablating surface. The new model predicts similar overall mass loss rates compared to existing models, however, the individual species production rates are completely different. The most notable difference is that the new model (based on molecular beam data) predicts CO as the oxidation reaction product with virtually no CO2 production, whereas existing models predict the exact opposite trend.

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Section: Via2 Backward Rates In Za Modelmentioning

confidence: 97%

Finite-rate oxidation model for carbon surfaces from molecular beam experiments

Poovathingal

Schwartzentruber

Murray

et al. 2016

46th AIAA Thermophysics Conference

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“…The main reason for this trend is that the CO probability is sufficiently high (ranging between 0.7 and 0.9) that recombination reactions, which rely entirely on adsorbed O-atoms, cannot compete with CO production and O desorption. Maclean et al [6], the modifications proposed by Alba et al [10] [11], and also in the previous molecular beam-based model of Poovathingal et al [21].…”

Section: Reactions Involving Atomic Oxygen (O) -High Pressure Conditionsmentioning

confidence: 91%

“…We find that a desorption energy larger than that corresponding to a single bond between C and N is required by the model. While the exact desorption energy cannot be deduced from the experimental data, we choose a desorption energy corresponding to a double bond between C and N in reaction (11). It is noted that, from a physical standpoint, this double-bond desorption energy may be high and not fully consistent with the molecular beam data, which indicates a lower energy barrier for CN formation.…”

Section: Reactions Involving Atomic Nitrogen (N)mentioning

confidence: 99%

“…Candler [7] performed a numerical analysis of the ZA and Park models, in addition to the equilibrium B model, for hypersonic conditions, between 20 and 40 km altitude, and found significant differences between model predictions. Lewis et al [8][9] performed a series of hypersonic ablation experiments in the X-2 facility at the University of Queensland, and CFD simulations comparing the Park and ZA models were performed by Alba et al [10] [11]. In these cases, the surface temperature ranged from 1770 to 2410 K. The predicted ablation products were quite different between the Park and ZA models, and neither of the models was able to predict the experimentally observed products.…”

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confidence: 99%

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Air-Carbon Ablation Model for Hypersonic Flight from Molecular Beam Data

Prata

Minton²,

Schwartzentruber³

2020

Preprint

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Recent molecular beam experiments of high velocity O, N, and O2 impacting carbon material at high temperature produced detailed surface chemistry data relevant for carbon ablation processes. New data on O and N reactions with carbon has been published using a continuous molecular beam with lower velocity (2000 m/s) and approximately 500 times higher beam flux than previous pulsed-beam experiments. This data is interpreted to construct a new air-carbon ablation model for use in modeling carbon heat shield ablation. The new model comprises 20 reaction mechanisms describing reactions between impinging O, N, and O2 species with carbon and producing scattered products including desorbed O and N, O2 and N2 formed by surface-catalyzed recombination, as well as CO, CO2, and CN. The new model includes surface-coverage dependent reactions and exhibits non-Arrhenius reaction probability in agreement with experimental observations. All reaction mechanisms and rate coefficients are described in detail and each is supported by experimental evidence or theory. The model predicts pressure effects and is tested for a wide range of temperatures and pressures relevant to hypersonic flight. Model results are shown to agree well with available data and are shown to have significant differences compared to other models from the literature.

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“…The test condition used in this work is representative of a point in the trajectory at an altitude of 50 km, 5 s after peak heating, encountered during the re-entry of the Hayabusa sample return capsule [84,85]. This is one of the well established test conditions in X2 and was used in several Earth-entry ablation experiments previously [67,[86][87][88]. During the actual experiments, it was difficult to experimentally measure the test flow parameters as the wedge model occupied most of the core flow coming out of the X2 nozzle.…”

Section: Earth-entry Flow Freestream Characterisationmentioning

confidence: 99%

Ablation and radiation in hypervelocity earth-entry flows

Ravichandran¹

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Atmospheric entry of spacecraft is an interesting and challenging problem in aerospace engineering as it involves a highly non-equilibrium aerothermodynamic shock layer environment. Re-entry of spacecraft into Earth's atmosphere occurs at very high entry speeds ranging from 8-12 km · s −1 , giving rise to a bow shock wave in front of the blunt body vehicle. The associated shock layer is characterised by a very rapid increase in pressure and temperature. The kinetic energy of the hypervelocity flow is converted into thermal and chemical energy, which is partially dissipated in the form of convective and radiative heat transfer. The gases traversing the shock layer undergo thermochemical changes such as dissociation, ionisation and recombination. To safeguard the re-entry vehicle from such a harsh thermal environment, a thermal protection system (TPS) is employed on the surface of the vehicle. The TPS material, when exposed to the re-entry heating, may undergo ablation due to the combined effect of heat flux and shear from the shock layer flow. The excited gas species in the shock and boundary layer react with the atomic/molecular species ablated from the solid TPS material. However, due to the uncertainties associated with the total heat flux estimation, large safety factors are currently being used in the TPS design, particularly for the afterbody region, compromising the payload mass and vehicle safety.The main objectives of this thesis were to experimentally simulate ablation product interactions with an expanding re-entry flowfield, to characterise the radiation of the entrained ablation products using ultra-violet emission spectroscopy targeting CN radicals, and to compare the ablating flowfields with non-ablating ones. The experiments were performed in the X2 expansion tube by using a stainless steel wedge model designed with a provision to mount a graphite ablation source on its compression face. The test flow condition, representative of a point in the Hayabusa capsule re-entry trajectory, was generated in X2 with a freestream velocity of 8.6 km · s −1 and a temperature of 2500 K, corresponding to an enthalpy of 38 MJ · kg −1 . As the test times available in X2 are limited, the ablating material cannot reach such wall temperatures by aero heating alone, and hence ablation in these experiments was created by electrically preheating the graphite strip to high wall temperatures representative of re-entry conditions. The wall temperatures realised in this work ranges from 1000-3000 K, which were measured by nonintrusive filtered image thermography using a dual-wavelength signal ratio technique. The hot graphite strip, upon exposure to the re-entry flow, ablates and mixes with the flow, and passes through the expansion fan and further into the afterbody region.The flowfield and interaction processes were optically diagnosed using a high frame rate video camera, two-dimensional filtered imaging and UV emission spectroscopy. CN emission measurements were made at various locations in the flowfield such as the fore...

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Development of a Nonequilibrium Finite-Rate Ablation Model for Radiating Earth Reentry Flows

Cited by 31 publications

References 46 publications

Finite-rate oxidation model for carbon surfaces from molecular beam experiments

Finite-rate oxidation model for carbon surfaces from molecular beam experiments

Air-Carbon Ablation Model for Hypersonic Flight from Molecular Beam Data

Ablation and radiation in hypervelocity earth-entry flows

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