| Popis: |
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. |
