George A. Raiche

Temperature dependent quenching of the A 2Σ+ and B 2Π states of NO

Collisional quenching of the v’=0 vibrational levels of the A 2Σ+ and B 2Π states of nitric oxide has been studied over the temperature range 300 to 750 K. The pressure dependence of the time decay of laser‐induced fluorescence, in a slowly flowing heated cell, furnished the quenching cross sections σQ. NO and O2 quench the A state rapidly but with no temperature dependence; σQ=37 and 21 A2, respectively. σQA for H2O drops from 105 A2 at 300 K to 34 A2 at 750 K. σQB for O2 is independent of temperature but σQB for NO drops twofold and for H2O decreases by a factor of 3 over the temperature range studied. This variation among these colliders cannot be explained by a uniform, simple picture of the collision dynamics. Evidence is seen for B→A transfer proceeding through an intermediate state, perhaps a 4Π.

Vibrational energy transfer in OH X 2Πi, v=2 and 1

Using an infrared pump/ultraviolet probe method in a flow discharge cell, vibrational energy transfer in OH X 2Πi has been studied. OH is prepared in v=2 by overtone excitation, and the time evolution of population in v=2 and 1 monitored by laser‐induced fluorescence. Rate constants for vibrational relaxation by the colliders H2O, NH3, CO2, and CH4 were measured. Ratios of rate constants for removal from the two states, k2/k1, range from two to five.

Laser-induced fluorescence temperature measurements in a dc arcjet used for diamond deposition

Laser-induced fluorescence (LIF) observations of CH and C(2) radicals in a dc-arcjet plasma are reported. A hydrogen and methane gas mixture flows through a dc-arc and expands through an orifice in the anode to form a luminous jet; diamond film grows under this jet on a water-cooled substrate. At the substrate position for best diamond growth, laser excitation spectra determine a rotational population distribution of CH(X) and C(2)(a), which yields Boltzmann gas temperatures of 2100 ±200 K. The C(2) Swan-band emission from the same observation volume yields an excited C(2)(d) rotational and vibrational population distribution well described by a 5000 K temperature. The difference between the LIF and emission temperatures indicates that chemiluminescent reactions are the dominant excitation mechanism for the optical emission from the gas jet.

SHOCK LAYER OPTICAL ATTENUATION AND EMISSION SPECTROSCOPY MEASUREMENTS DURING ARC JET TESTING WITH ABLATING MODELS

Measurements of optical attenuation and emission spectra for the radiating bow shock region upstream of several ablating arc jet test models are reported. Attenuation measurements were intended to assess light transmission parallel to the model face at k633 nm by the shock layer formed over the ablating surface; the attenuation is attributed to the presence of particles. As the models ablated, the surface receded, effectively translating the detection line of sight upstream from the model surface. Substantial loss of transmission correlated with the macroscopic ablation and surface heating rates, and persisted upstream of the shock front. Simultaneously, optical emission spectra were measured in the same plane as the attenuation laser. These measurements were intended to determine the extent to which ablation products (particles or vapor) influence the bow shock radiation. The spectra were measured rapidly using miniature fixed-grating spectrometers with fiber optic input. With a field of view of 2 mm and acquisition time less than 100 ms, spatial resolution was retained and time dependent intensity trends were observed. Several components of dissociated air can be identified based on their spectral signatures; the radiative contribution by ablation products appears minor, however.

Surface Heating from Remote Sensing of the Hypervelocity Entry of the NASA GENESIS Sample Return Capsule

An instrumented aircraft and ground-based observing campaign was mounted to measure the radiation from the hypervelocity (11.0 km/s) reentry of the Genesis Sample Return Capsule prior to landing on the Utah Test and Training Range on September 08, 2004. The goal was to validate predictions of surface heating, the physical conditions in the shock layer, and the amount and nature of gaseous and solid ablation products as a function of altitude. This was the first hypervelocity reentry of a NASA spacecraft since the Apollo era. Estimates of anticipated emissions were made. Erroneous pointing instructions prevented us from acquiring spectroscopic data, but staring instruments measured broadband photometric and acoustic information. A surface-averaged brightness temperature was derived as a function of altitude. From this, we conclude that the observed optical emissions were consistent with most of the emitted light originating from a gray body continuum, but with a surface averaged temperature of 570 K less than our estimate from the predicted heat flux. Also, the surface remained warm longer than expected. We surmise that this is on account of conduction into the heat shield material, ablative cooling, and finite-rate wall catalycity. Preparations are underway to observe a second hypervelocity reentry (12.8 km/s) when the Stardust Sample Return Capsule returns to land at U.T.T.R. on January 15, 2006.

Applications of CFD Analysis in Arc-Jet Testing of RCC Plug Repairs

Computational simulations are used as an integral part of arc-jet testing from the planning stages of the arc-jet experiments through post-test analysis for increasingly complex test configurations. This paper reports two applications of such analysis: an analysis for reinforced carbon-carbon plug repair tests conducted in a NASA Ames arc-jet facility, and a feasibility study of full-scale Shuttle wing leading edge plug repair tests in the facility, where four possible configurations are investigated. For the plug tests, arc-jet flows over wedge models, with and without plugs mounted, and plugs differing in step height and diameters, are simulated. For the feasibility study, arc-jet flows of four test configurations are simulated, and based on the predicted test environments of these configurations, one is recommended for future arc-jet tests of full-scale plug repair. The present analyses comprise computational simulations of the nonequilibrium flowfield in the facility nozzle and test box as well as the flowfield over the models. These examples further demonstrate the value of computational simulations in planning and analysis of arc-jet tests.

Measurements of Gas Temperature in the Arc-heater of a Large Scale Arcjet Facility using Tunable Diode Laser Absorption

Diode laser absorption measurements of atomic nitrogen and oxygen are made in the arc- heater of the NASA Ames IHF (60 MW) arcjet facility. Temperature of the gas in the arc- heater is inferred from the measured mole fraction of electronically excited O and N atoms assuming thermal equilibrium. These results are the first absorption-based temperature measurements inside the arc-heater, where the temperature range is 5000-9000K and the pressure range is 1.5-6 bar. Rapid scanning of the laser wavelength across the absorption feature provides time-resolved measurements of the number density for specific electronic states of the atoms. The agreement of the temperature inferred from redundant measurements on atomic nitrogen and atomic oxygen suggest the equilibrium assumption is valid. The laser is scanned in wavelength across the absorption feature at a rate of 500Hz, and a 100 scan average provides a time-resolution sufficient to examine the variation of temperature with changes in the gas flow and/or electrical power input. These results illustrate the potential of the diode laser sensors for facility performance monitoring.

Laser‐induced fluorescence and dissociation of acetylene in flames

Laser‐induced fluorescence and photoinduced dissociation of acetylene is observed in both room temperature cells and low‐pressure flames. Acetylene is excited via the A–X electronic transition between 210 and 230 nm. The fluorescence exhibits long vibronic progressions due to the different equilibrium geometries; acetylene is trans‐planar in the A state and linear in the X state. Intense fluorescence from electronically excited carbon radicals (C*2) is also observed upon resonant excitation of C2H2 at room and flame temperatures. In addition, a non‐resonant laser‐induced production of C*2 is observed in the flame. The effects of C2H2 pressure (i.e., electronic quenching) and laser fluence on fluorescence are examined.

Comparison of Enthalpy Determination Methods for an Arc-Jet Facility

Four experimental methods of determining the enthalpy of the flow in an arc-jet facility that is, the heat balance method, the sonic throat method, the heat transfer method, and the emission-spectroscopic method, are compared with a computational fluid dynamics (CFD) solution. The comparison is made for the Interaction Heating Facility of NASA Ames Research Center for one operating condition. The mass-averaged enthalpy values determined by the heat-balance method and the sonic throat method are 28.7 and 28.8 MJ/kg, respectively. The lower bound of the centerline enthalpy value determined by the heat transfer rate method is 30.5 MJ/kg. The spectrometric method resulted in the centerline enthalpy value of 40.6 MJ/kg. The CFD solution yields the centerline and the average enthalpy values at the nozzle throat of 41.0 and 27.0 MJ/kg, respectively.

Test Engineering for Arc Jet Testing of Thermal Protection Systems: Design, Analysis, and Validation Methodologies

*† ‡ An integrated process of defining objectives and conducting arc jet tests in support of thermal protection systems (TPS) development at NASA Ames Research Center has been developed. Computational simulations of the nonequilibrium arc jet flows, validated by detailed measurements of flow field parameters using a variety of laser-spectroscopic and conventional techniques, can be used to characterize test conditions with high fidelity. Using simulations, test programs can be designed for a range of facility conditions that correlate critical aspects of the targeted flight conditions to the expected aerothermal, thermochemical, or thermostructural response mechanisms of a TPS material or subsystem. Basic research initiatives have supported the development of these simulation capabilities that enhance our efforts in meeting program-level requirements of NASA missions. Examples of our research efforts and mission-specific test programs will be described in the context of our testing methodology.