Jason M. Meyers

Diode Laser Absorption Sensor Design and Qualification for CO2 Hypersonic Flows

The design of a diode laser absorption spectroscopy sensor to probe freestream conditions of hypervelocity CO2 flow in the Longshot Free-Piston Facility is presented. The Longshot Facility of the von Karman Institute is an important facility for ground test and numerical validation applications as it can produce high Mach number and highReynold’s number conditions for bothN2 andCO2 flows.Currently, freestreamconditions are determined from measurements of reservoir pressure, pitot pressure, and stagnation-point heat transfer to a hemisphere using a thermodynamic model for the expanding test gas. A nonintrusive absorption sensor can provide a direct measurement of freestream gas dynamic variables, which could be used to evaluate the data reduction process. Detailed descriptions of the design, analysis, and development of the diode laser absorption sensor are presented.

Laser Spectroscopic Investigation of Surface-Catalyzed Reactions for Mars Exploration Vehicles

Discrepancies exist as to whether appreciable amounts of CO2 recombine at the surface of Martian entry vehicles. This is a significant concern as overly conservative design measures need to be used to accommodate this uncertainty owing to the the variation in heat-fluxes that arise for different recombination models used. The University of Vermont (UVM) has recently developed an 30kW Inductively Coupled Plasma (ICP) Torch Facility. This facility is designed for investigation of material testing and gas-surface interaction chemistry. Two-photon absorption laser induced fluorescence (TALIF) and diode laser absorption spectroscopy (DLAS) strategies to characterize O, CO and CO2 concentrations near sample material surfaces are presented here.

Near Surface CO2 Detection in an Inductively Coupled Plasma Facility Using Diode Laser Absorption

Uncertainty exists as to whether appreciable amounts of CO2 are formed by surface recombination for Mars entry vehicles. It is well understood that highly catalytic thermal protection system (TPS) material can lead to nearly two times the heat flux to the vehicle surface as a non-catalytic material. Conservative design practice assumes a supercatalytic surface to accommodate this uncertainty. Risk avoidance leads to oversized TPS geometries that add mass to entry vehicle designs that could be otherwise employed. Determining whether significant near surface production of CO2 occurs at trajectory heating conditions would add significant insight. The University of Vermont (UVM) has developed a 30kW Inductively Coupled Plasma (ICP) Facility to provide a pure high temperature plasma environment for aerospace material testing and gas-surface interaction chemistry investigations. The goal of the present work is to develop and test a diode laser absorption spectroscopy (DLAS) sensor to target near sample surface CO2 concentrations. A DFB diode laser at 2744 nm with a tuning range of around 3 nm is chosen as the light source. Current investigations show no appreciable amount of CO2 near a highly catalytic surface (water cooled copper) within the sensors’ detectivity limits. Work is ongoing to further improve these sensitivity limitations.

Surface Catalyzed Reaction Efficiencies in Nitrogen and Oxygen Plasmas from Laser Induced Fluorescence Measurements

Recent efforts to advance laser diagnostics in plasma facilities have resulted in the capability to measure gradients of temperature and species in the reacting zone above a high-temperature material. These measurements allow a direct qualitative estimate of the surface-catalyzed reaction efficiency when the sub-millimeter spatial distribution of the reactive species is compared with measurements over reference materials. Measurements of nitrogen atom and oxygen atom distributions are presented for reference materials and silicon carbide for both nitrogen and oxygen plasmas to elucidate the relative efficiencies for homogeneous surface catalyzed recombination.

Investigations of Surface-Catalyzed Recombination Reactions in the Mars Atmosphere

In the design of a thermal protection system (TPS) for a planetary entry vehicle, accurate modeling of the trajectory aero-heating poses a significant challenge owing to large uncertainties in chemical processes taking place at the surface. Even for surface-catalyzed reactions, which have been investigated extensively, there is no consensus on how they should be modeled; or, in some cases, on which reactions are likely to occur. Current TPS designs for Mars missions rely on a supercatalytic boundary condition, which assumes that all dissociated species recombine to the free stream composition. While this is recognized to be the the most conservative approach and leads to an increased TPS mass, discrepancies in aero-heating measurements in ground test facilities preclude less conservative design options. The present work is aimed at providing more information about surface catalyzed reactions for Mars exploration missions. Measurements of dissociated species above a catalytic surface are obtained using two-photon absorption laser-induced fluorescence (TALIF) implemented in a new 30 kW inductively coupled plasma torch facility.

Surface Catalyzed Reaction Efficiencies in Air Plasmas Using Laser Induced Fluorescence Measurements

Recent efforts to advance laser diagnostics in plasma facilities have resulted in the capability to measure gradients of temperature and species in the reacting zone above a hightemperature material. These measurements allow a direct quantitative estimate of the surface-catalyzed reaction efficiency when the spatial distribution of the reactive species is compared with measurements over reference materials. Measurements of temperature, relative nitrogen atom density, relative oxygen atom density, and relative NO molecule density in a chemically reacting boundary layer were performed in air plasmas to elucidate the behavior of catalytic efficiencies.

Flexible TPS Surface Catalysis Testing in a 30 kW ICP Torch Facility

Ground-facility testing of a silicon carbide (SiC) fabric for flexible based thermal protection systems is underway using the 30 kW Inductively Coupled Plasma (ICP) Torch located at the University of Vermont Plasma Test and Diagnostics Laboratory (PTDL). The fabric is exposed to high enthalpy nitrogen and air plasma flows to quantify surfacecatalyzed recombination reactions relevant to earth atmosphere re-entry. The catalytic behaviour is characterized using multiple surface and gas-phase measurements including infrared pyrometry, cold-wall heat flux, and laser induced florescence (LIF). The flexible material catalytic recombination efficiency is assessed by comparing the atomic species gradients for the flexible material with those of a solid surface with the same elemental composition and with reference materials of varying catalytic efficiency. This paper presents an overview of the motivation for the investigation, a description of the experimental configureation, and comparative results on nitrogen recombination for rigid SiC, the flexible material, and the bare coupon holder.

Nitrogen Surface Catalyzed Recombination Efficiency from Two-Photon Laser Induced Fluorescence Measurements

Measurements of temperature and species gradients in the thermal boundary layer adjacent to different surfaces exposed to nitrogen plasma are made to characterize surface-catalyzed recombination. L...

Investigation of Pyrolyzing Ablators Using a Gas Injection Probe

The pyrolysis mechanism of Phenolic Impregnated Carbon Ablator (PICA) makes it a viable candidate for thermal protection systems for spacecraft atmospheric entry. However, a better understanding of the pyrolysis mechanism and of the interaction of the pyrolysis gases with the plasma flow is needed to improve heat-shield designs. The present study extends previous work in the design, development, and testing of a gas-injection probe to simulate pyrolysis in the UVM 30kW Inductively Coupled Plasma (ICP) Torch Facility. A parallel effort in Computational Fluid Dynamics (CFD) modeling of the injection probe during plasma testing is also discussed. Measurements were obtained with the injection probe in the facility in an argon buffered nitrogen condition with carbon dioxide being injected into the flow through a FiberForm plug. Spatially resolved high-resolution emission data were acquired during this test. Spectral signatures of CN, OH, and NH bands were detected and their spatial locations determined using a spectrometer equipped with a CCD camera at the exit plane. At the same time the relative conditions in the facility were used to create a CFD model of the experiment. Comparisons between the measured, spatially resolved, emission spectra and the simulations calculated using a radiative transport model from the CFD results show reasonable agreement.

Planar Two-Photon LIF Measurements of Atomic Species in a High-Temperature Inductively Couple Plasma Environment

This paper details work involving a planar two photon LIF approach probing atomic oxygen in an inductively coupled plasma environment which is used to emulate the chemically reacting boundary layer above a TPS surface during hypersonic flight. Planar measurements of atomic species have little to no documentation of experimental efforts as LIF based measurements of atomic species traditionally utilize point-wise approaches with sensitive PMT detection. Fortunately, recent advances in fast gated CCD camera technology has pushed potential planar detectability limits to that of more common point wise PMT-based approaches. Significant signal to noise levels demonstrated in this report with this newly adopted planar mode have led to encouraging results. Temperature and normalized density trends compare well with theoretical intuition as well as previously measured point-wise stagnation line chemically reacting boundary layer profiles.