OCR Text |
Show on the gas temperature. A temperature profile is obtained by moving the probe to various locations in the gas stream. Since this was the first field implementation of SRS, the first field measurement day was spent investigating the parameters that affect the precision and accuracy of the S R S technique. T o study the effect of the N 2 purge on the Raman-inferred temperature, Raman spectra were recorded with and without purge. The temperatures obtained with the purge on were ~190°C lower than those without the purge. Although the measurement point was -24 c m (-9.5") from the end of the probe (30-cm focal length lens positioned -6 cm inside probe), the purge gas cools the gas stream between the probe and the measurement point and hence influences the temperature measurement. Although the purge gas flow was low, the purge gas can cool the measurement point in this particular furnace because the furnace's gas flow velocity is low (a few m/sec). Consequently subsequent S R S measurements were made without purging. In order to study the effect of the water-cooled probe on the Raman-inferred temperature measurements, Raman spectra were recorded with 30-cm (12") and with 50-cm (20") focal length lenses (without purging). The signal intensities with both lenses were about the same. However, the inferred temperature with a 50-cm focal length lens was -63 °C higher than with the 30-cm focal length lens. W e concluded that because the furnace's gas flow velocity was so low, that the water-cooled probe significantly cooled the nearby gases; therefore, the SRS measurement should be made with the measurement point as far away from the probe as possible. All subsequent measurements were made with the 50-cm focal length lens and without purging. Figure 1 compares the Raman-inferred temperatures as a function of distance from the outside of the furnace wall with the temperatures obtained by a DIAL multi-element thermocouple probe (see below) through this same port, but at a different time during the campaign. It is seen that the Raman-inferred temperatures increase slightly as the distance from the wall increases. 1 500 T CD 2 1400 -; as ® 130 0 -: Q_ E o 120 0 -: r- 1100 -j 1 ooo 4 20 40 60 80 100 120 Distance to Exterior of W all (cm) Figure 1. Raman-inferred (•) and multi-element thermocouple (o) temperatures (in °C) at different distances (in cm) from the exterior of furnace wall of a port on an industrial-scale furnace. The error bars correspond to one standard deviation. Multi-Element Thermocouple Probe The use of multiple thermocouple elements in a probe provides data that gives a measure of the internal consistency of the method. Well-established theory is employed to correct for heat exchange in the flowing gas stream.6 T w o ungrounded, platinum-rhodium Type S thermocouples of diameters 1.59 m m (0.0625") and 3.18 m m (0.125") were housed in platinum sheaths and were mounted on the end of a water-cooled probe that was manually inserted to selected distances into the furnace. Based upon knowledge of the platinum sheath's emissivity, the wall temperature, and the convective heat transfer coefficient, the actual gas temperature can be obtained from the apparent gas |