BIAS IN EMISSION ABSORPTION-MEASUREMENTS OF TEMPERATURE AND ATOMIC DENSITY

Presented here is an analysis of the bias arising from two fundamental difficulties in applying emission absorption methods to large-scale combustion systems: 1) maintaining clean optical windows; and 2) maintaining the optical alignment. The impetus for the study was an anomalously large temperature measurement uncertainty for potassium D-line measurements in a coal-fired magnetohydrodynamic (MHD) flow, while no increased uncertainty was observed for the measured seed atom density. The large uncertainty can be explained by a rapid, oscillatory misalignment. This misalignment biases the measured temperature in the same manner as that for a fouled lamp-side window. Expressions for the biases for fouled optical windows are presented. Expressions are developed for the bias and uncertainty caused by a harmonic misalignment. These expressions provide a consistent explanation of the temperature results. In contrast, the robustness of a two-wavelength emission absorption method for atomic density measurements is demonstrated.