Iron Contamination in High-Enthalpy Test Facilities: OH PLIF Imaging Considerations

Abstract

Thermometry measurements were performed in a scramjet combustor using thermally-assisted laser-induced fluorescence. Experimental data were obtained through laser-induced fluorescence where a laser beam was focused into the combustor and the OH () transition at 283.55 nm was excited. Measurements in the 360 nm spectral region resulted in detection of a strong fluorescence line. Investigation of this phenomenon led to the conclusion that when the OH line is chosen for excitation, another line of a different species that lies very close to is also unintentionally excited. The second species has been identified as iron, Fe I. The assumption is that the erosion of the shock tunnel walls introduced iron in the flow that is excited with the laser when exciting OH. Thus, it is very important to ensure that this is avoided in future. Instead, another OH excitation line should be used for OH excitation line in planar laser-induced fluorescence (PLIF) experiments where iron may potentially also be present in the flow. Alternatively, fluorescence from this iron line can be circumvented by proper filtering of the PLIF. A proposal is made to turn the iron contamination into advantage by using iron present in the non-combusting flow for PLIF.