Imelda Terrazas-Salinas

CFD Analysis Framework for Arc-Heated Flowfields, II: Shear Testing in Arc-jets at NASA ARC

A three-dimensional framework has been developed for application of tools of computational fluid dynamics to simulate arc-heated flows expanded in convergent- divergent nozzles. Methods that make up the process for prediction of pressure, shear stress, and heat flux on the face of a blunt wedge and a swept cylinder are presented, along with modeling assumptions. The framework is utilized in simulations of the Interaction Heating Facility at NASA Ames Research Center. Results of numerical simulations are compared against experimental data acquired in test entries from the Mars Science Laboratory program. Recommendations are made for possible improvements to the process.

Comparison of Enthalpy Determination Methods for 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.

Emission Spectroscopic Measurements in the Plenum Region of the NASA IHF Arc Jet Facility

A newly designed segment with optical access was installed in the plenum chamber of the 60 MW Interaction Heating arcjet Facility at NASA Ames Research Center. This special segment has ports located off axis, and the optical fibers can be inserted into these ports. The special segment allows for optical examination of the arc-heated flow as it enters the plenum, and thus assists in determining estimates of the thermodynamic state of the inflow to the convergent section of the nozzle. In the present work, optical emission measurements have been made in VIS-NIR region (wavelengths between 500 nm to 900 nm) for two settings of the arc heater—a 6000 A condition (high condition) with the minimum amount of radial injection of cold air in the plenum, and a 3300 A condition (low condition) with significant amount of cold air injection to reduce the enthalpy of the arc-heated stream. The results presented here were obtained using an Acton SP300i spectrometer coupled to a Princeton Instruments PI-max intensified camera. In addition to the optical emission measurements, computations were performed for the flow in the plenum and radiation along lines of sight corresponding to the optical ports. Along the centerline, i.e., the longest line of sight across the plenum cross-section, there is good agreement between computations and measurements for the high enthalpy condition, although the off-axis radial profiles show some differences. For the low enthalpy condition, there are significant differences between computations and measurements. The current working hypothesis is that the computational model does not capture details of the mixing process in the plenum.

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.

Measurements of Radiation Heat Flux to a Probe Surface in the NASA Ames IHF Arc Jet Facility

An optical probe was applied to measure radiation from inside the arc heater incident on a test sample immersed in the arc-heated stream through spectroscopy and radiometry. Unlike efforts of the past, where the probe line of sight was inclined to the nozzle centerline, the present development focused on having the probe line of sight coincide with the nozzle centerline. A fiber-coupled spectrometer was used to measure the spectral distribution of incident radiation in the wavelength range of 250 to 950 nm. The radiation heat flux in this wavelength range was determined by integration of the measured spectral intensity calibrated to incident irradiance from an integrating sphere. In addition, total radiation measurements were made with thermopile sensors. Due to the flat spectral characteristics of these sensors, the detected wavelength range was only limited by the sapphire windows which typically cut off radiation below 160nm and above ~5μm. Several arc-heater conditions, corresponding to stream bulk enthalpy levels between 8 and 24 MJ/kg, were investigated in the 6-inch diameter nozzle of the Interaction Heating Facility at NASA Ames Research Center. The results are compared to former measurements with the 13-inch diameter nozzle. With the probe placed at a distance of 3 inches from the nozzle exit plane, total radiative heat fluxes were measured to be between 8 and 33 W/cm 2 for the different conditions. In correspondence with the measurements in the 13-inch nozzle, the spectra are dominated by continuum emission. However, with the 6-inch nozzle, the ratios of boundbound to continuum are observed to be higher. The factors between measurements with the 6-inch and the 13-inch nozzle yield the conclusion that for the highest condition, without added cold air, the majority of the measured bound-bound radiation is generated in the cathode region of the arc heater. The measured continuum radiation, however, seems to be generated mainly close to the upstream electrode in the first part of the arc-column. For the condition with significant add air, the radiation heat flux increase is almost the same for bound-bound and continuum radiation. If the arc-column distribution is considered to remain unchanged, this indicates additional continuum radiation to be generated close to the downstream electrode and bound-bound emission in the downstream region to be reduced. These effects are attributed to the added room temperature air at this condition. In the tested configuration with the 6-inch nozzle, the measured radiative heat flux accounts for up to 1.5% of the total heat flux on a hemispheric calorimeter or 2.7% on a flat face. Although stronger than anticipated, these values can still be considered negligible as a heat load.

Bulk Enthalpy Calculations in the Arc Jet Facility at NASA ARC.

§The Arc Jet Facilities at NASA Ames Research Center generate test streams with enthalpies ranging from 5 MJ/kg to 25 MJ/kg. The present work describes a rigorous method, based on equilibrium thermodynamics, for calculating the bulk enthalpy of the flow produced in two of these facilities. The motivation for this work is to determine a dimensionally-correct formula for calculating the bulk enthalpy that is at least as accurate as the conventional formulas that are currently used. Unlike previous methods, the new method accounts for the amount of argon that is present in the flow. Comparisons are made with bulk enthalpies computed from an energy balance method. An analysis of primary facility operating parameters and their associated uncertainties is presented in order to further validate the enthalpy calculations reported herein.

Emission Spectroscopic Measurements with an Optical Probe in the NASA Ames IHF Arc Jet Facility

An optical probe was designed to measure radiation (from inside the arc heater) incident on a test sample immersed in the arc-heated stream. Currently, only crude estimates are available for this incident radiation. Unlike efforts of the past, where the probe line of sight was inclined to the nozzle centerline, the present development focuses on having the probe line of sight coincide with the nozzle centerline. A fiber-coupled spectrometer was used to measure the spectral distribution of incident radiation in the wavelength range of 225 to 900 nm. The radiation heat flux in this wavelength range was determined by integration of measured emission spectral intensity calibrated to incident irradiance from an integrating sphere. Two arc-heater conditions, corresponding to stream bulk enthalpy levels of 12 and 22 MJ/kg, were investigated in the 13-inch diameter nozzle of the Interaction Heating Facility at NASA Ames Research Center. With the probe placed at a distance of 10 inches from the nozzle exit plane, total radiative heat fluxes were measured to be 3.3 and 8.4 W/sq cm for the 12 and 22 MJ/kg conditions, respectively. About 17% of these radiative fluxes were due to bound-bound radiation from atoms and molecules, while the remaining 83% could be attributed to continua (bound-free and/or free-free). A comparison with spectral simulation based on CFD solutions for the arc-heater flow field and with spectroscopic measurements in the plenum region indicates that more than 95% of the measured radiation is generated in the arc region. The total radiative heat flux from the line radiation could increase by a factor of two through contributions from wavelengths outside the measured range, i.e., from the vacuum ultraviolet (wavelengths less than 225 nm) and the infrared (wavelengths greater than 900 nm). An extrapolation of the continuum radiation to these two wavelength regions was not attempted. In the tested configuration, the measured radiative heat flux accounts for only about 1.4% of the nominal heat flux on a flat face model and therefore is considered negligible. In the 6-inch diameter nozzle, on account of shorter path lengths, the radiation heat flux could be significant. Therefore, future tests in the 6-inch nozzle will have radiometers in addition to the optical probe.

Arc Jet Facility Test Condition Predictions Using the ADSI Code

The Aerothermal Design Space Interpolation (ADSI) tool is used to interpolate databases of previously computed computational fluid dynamic solutions for test articles in a NASA Ames arc jet facility. The arc jet databases are generated using an Navier-Stokes flow solver using previously determined best practices. The arc jet mass flow rates and arc currents used to discretize the database are chosen to span the operating conditions possible in the arc jet, and are based on previous arc jet experimental conditions where possible. The ADSI code is a database interpolation, manipulation, and examination tool that can be used to estimate the stagnation point pressure and heating rate for user-specified values of arc jet mass flow rate and arc current. The interpolation is performed in the other direction (predicting mass flow and current to achieve a desired stagnation point pressure and heating rate). ADSI is also used to generate 2-D response surfaces of stagnation point pressure and heating rate as a function of mass flow rate and arc current (or vice versa). Arc jet test data is used to assess the predictive capability of the ADSI code.

Uncertainty Analysis of Coaxial Thermocouple Calorimeters used in Arc Jets

Recent introduction of Coaxial Thermocouple type calorimeters into the NASA Ames arc jet facilities has inspired an analysis of 2D conduction effects internal to this type of calorimeter. Lateral conduction effects violate the 1D finite slab inverse analysis which is typically used to deduce the heat transfer to such calorimeters. The spherical shaped nose associated with most calorimeters (rather than flat) leads to a bias error that over-estimates the stagnation heating. Non-uniform heating on the face of spherically shaped calorimeters leads to conduction losses to the colder rim of the calorimeter which causes an underestimate of the stagnation heating. These two effects come into play at different times of the calorimeters exposure to the arc jet, so they do not cancel. The spherical body effects come into play in the early stages of exposure, while the non-uniform heating effect becomes most severe at the later stages of exposure. The bias associated with spherical effects can be avoided by rewriting the 1D finite slab inverse analysis code to solve for 1D conduction in spherical coordinates. However, reducing the bias error associated non-uniform heating requires a somewhat ad hoc modification to the 1D finite element inverse analysis.

Evidence of Standing Waves in Arc Jet Nozzle Flow

Waves spawned by the nozzle in the NASA Ames 60 MW Interaction Heating Facility arc jet were experimentally observed in pressure surveys at the exit of the nozzle. The waves have been seen in past CFD simulations, but were away from the region where models were tested (for the existing nozzles). However, a recent test series with a new nozzle extension (229mm exit diameter) revealed that these waves intersect the centerline of the jet in a region where it is desirable to put test articles, and that the waves may be contributing to non-uniform recession behavior seen in Teflon sublimation test articles tested in this new nozzle. It is reasonable to assume the ablation recession of thermal protection models will also be nonuniform due to exposure to these waves. This work shows that ablation response is sensitive to the location of test samples in the free jet relative to the location of the wave interaction, and that the issues with these waves can be avoided by choosing an optimum position for a test article in the free jet. This work describes the experimental observations along with the CFD simulations that have identified the waves emanating from the nozzle, as well as the instrumentation used to detect them. The work describes a recommended solution, derived by CFD analysis, which if implemented, should significantly reduce these flow disturbance and pressure anomalies in future nozzles.