OCR Text |
Show including the thermocouple error [1]. The error for a type B thermocouple is less than 4°F at temperatures above 1400°F [2]. The flames from burners tested in BERL generally have high turbulence levels causing spatial and temporal temperature fluctuations. Because the spatial resolution is larger and temporal response is much slower than the turbulence scales in the furnace, the suction pyrometer can only measure the average temperature in the vicinity of the tip. As with the in-furnace gas sampling, depending on gradients encountered, about 50 to 60 radial points are necessary to obtain a reasonable temperature distribution at a given axial location. One radial scan requires about an hour. Ten minutes is required to move from one axial location to the next. Setup and maintenance time is an additional hour. The total time required to traverse five levels is about seven hours. 3.1.5 Radiometer An ellipsoidal radiometer is used to measure total radiative heat flux to the furnace walls. The radiometer consists of a thin walled orifice, an ellipsoidal mirror and a thermopile (see [1] page 57). Radiation enters the element through the thin walled orifice and an ellipsoidal mirror focuses radiation on a thermopile. This thermopile contains a blackened heat sensitive head that absorbs from 950/0 to 980/0 of the radiation entering the element [1]. The thermopile produces an e.m.f. that is linearly proportional to the energy absorbed by the thermopile. Measurements are performed at the furnace wall. The time required for the radiometer to reach steady state is - 2 minutes. This slow response prevents measurement of rapid changes in flame radiation. Therefore, this radiometer measures only the average flux from the surroundings. Approximately one hour is required to measure the radiative flux at five axial locations. 3.2 Laser-Based Diagnostics This final section is intended to give the potential user who may not be familiar with laser-based diagnostics a brief introduction as to the utility of the diagnostics available at BERL and how long each might be expected to take. 3.2.1 Planar Mie Scattering Planar Mie scattering is a technique where laser light, formed into a sheet, scatters off seed particles revealing the interface between incoming seeded fluid and unseeded fluid already in the furnace. A schematic of the optical configuration is shown in Figure 6. The frequency-doubled beam from an Nd:YAG laser (532 nm) operating at 10 Hz is formed into a sheet using cylindrical lenses to expand the beam in one dimension, forming a sheet approximately .5 mm thick. The Nd:YAG pulse duration of 6 ns effectively freezes the flow. In order to obtain the highest possible contrast, the seeding is started and stopped impulsively. This allows the furnace to completely clear of seed before each ensuing photograph. -10- |