Analysis of luminosity temperature measurements of coal-fired MHD plasmas

An optically based temperature measurement technique known as the luminosity probe system was studied and analyzed to determine uncertainty levels in temperatures measured with the system. The system is currently used to determine relative temperatures at the United States Department of Energy's Coal Fired Flow Facility (CFFF) located adjacent to the University of Tennessee Space Institute (UTSI), Tullahoma, Tennessee. The CFFF is an experimental, coal-fired, Magnetohydrodynamics (MHD) power generation development facility.

The uncertainty analysis separates uncertainties into precision and bias errors. Uncertainties in both the luminosity probe temperature determination and measurement spatial resolution are considered. For the temperature determination uncertainties, bias errors are considered to be negligible and only precision errors are evaluated. However, for the spatial resolution uncertainties it is shown that bias errors can be significant. The causes of spatial resolution bias are addressed, resulting in a new probe design to eliminate internal reflection. Bias uncertainties can be eliminated by careful installation of the new probe design.

The luminosity probe response equation, which is based on Planck's radiation law, is expanded into a truncated Taylor series form. This form is then used to derive theoretical estimates of calibration and measured parameter uncertainties, and their effect on the derived temperature. Values for the precision uncertainties in the measured temperature are obtained from test conditions, and their effects are evaluated using the truncated Taylor series form to determine the precision uncertainty in the temperature.

The major result of this investigation is an improved optical emission temperature measurement system based on luminosity, that is well suited to making relative temperature measurements in coal fired MHD plasmas. For such plasmas, the theoretical uncertainty in the measured temperature is about ±91 K or ±3.6% for a temperature of 2600K which was the nominal value used. It is recommended that further studies be made to determine this uncertainty more accurately and also recommended that care be taken in implementing the luminosity probes to avoid bias uncertainties.