Design and implementation of tunable diode laser-based multi-species and enthalpy sensing in extreme environments
Keywords: Mechanical Engineering.

A comparison of scanned-DAS and scanned-WMS was completed within the context of a biomass gasification pilot scale facility, while scanned-DAS was implemented and optimized to increase sensing capabilities in the mixing volume (MV) of the 60 MW Interaction Heating Facility (IHF) at NASA Ames Research Center (ARC). In the biomass gasification study, a demonstration of in situ laser-absorption-based sensing of H2O, CH4, CO2, and CO mole fraction is reported for the product gas line of a biomass gasifier. Field measurements were demonstrated in a pilot scale biomass gasifier at WestBiofuels in Woodland, California. The performance of a prototype sensor was compared for the two sensing strategies, scanned-DAS and scanned-WMS. The lasers used had markedly different wavelength tuning response to injection current, which led to establishing guidelines for laser selection for sensor fabrication. The complications of using normalized WMS for relatively large values of absorbance and its mitigation are discussed. The laser absorption sensor provided measurements with the sub-second time resolution needed for gasifier control and more importantly provided precise measurements of H2O in the gasification products, which can be problematic for the typical gas chromatography sensors used by industry. The IHF at NASA ARC is a critical facility used to study and characterize thermal protection systems (TPS) of reentry spacecraft. The IHF generates an electrical arc to energize room temperature air, which is then forced through a converging-diverging nozzle. Material and model test pieces are then subjected to the stagnation environment of the flow stream. The efforts at ARC focused on characterizing the mixing of add-air in the MV of the IHF. Conditions in the IHF, in the mixing volume (MV) where this study focused its optical measurements, range from 5,000 to 7,000 K and 1 to 9 atm. Path-average line-of-sight measurements of temperature and enthalpy were inferred using an electronic atomic oxygen transition near 777 nm. The scanned-DAS sensor was optimized to capture high add-air and high-pressure conditions that previously had not been measurable. Optimization of the sensor allowed for fully resolved absorbance profile measurements at the maximum pressure test condition in the IHF MV. Enhanced sensor capabilities confirmed uniform flow and flat temperature immediately upstream of the arcjet's converging-diverging nozzle inlet. Less uniform and parabolic temperature profiles were observed immediately downstream of add-air injection. This arcjet facility implements injection of room temperature air, i.e. add-air, to tune the bulk and centerline enthalpy generated at the nozzle exit. Centerline temperatures at an axial location downstream of add-air injection confirmed the need for mixing beyond the MV entrance. Enthalpy measurements ranged from 16 to 27 MJ/kg and are in agreement with the IHF's current enthalpy measurement methods. This confirmed the observed reduction in achievable enthalpy after installation of the MV. Centerline measurements quantified the mixing process for various add-air conditions indicating an opportunity for enthalpy recapture. Axial locations of sufficient mixing were identified and provide a potential for reduction of the MV length and thus improved facility performance. Optimization of this sensor will enable future use by non-experts to provide critical data on the arcjet environment in the IHF, thereby enhancing arcjet test results and leading to greater reliability of spacecraft TPS designs.