Narrow Angle Radiometer Instrument Model

Paper from the AFRC 2017 conference titled Narrow Angle Radiometer Instrument Model

Narrow-angle radiometers based on the International Flame Research Foundation design have been; used to measure the incident radiative heat flux in a coal-fired combustion furnace located at the University; of Utah. This method of heat flux measurement uses a probe that limits the field of view, allowing; radiation measurements of relatively precise locations. The probe contains a Wheatstone bridge with; two thermistors, one that is irradiated and one that is not. The measured voltage difference between the; two thermistors is used to determine the heat flux based on calibration curves obtained in a blackbody; furnace. This calibration produces a curve of measured voltage vs. blackbody temperature. The heat; flux is calculated from the blackbody temperature and corrected for the view angle.; Using the concept of an instrument model, we have conducted an extensive analysis of the errors; associated with using this radiometer design to measure incident heat flux. The purpose of an instrument; model is to identify how the measured signal (in this case mV) is converted to the variable of interest; (incident heat flux) and what possible sources of uncertainty might contribute to the calculated heat; flux. We have identified many sources of uncertainty in the construction, operation, and calibration of; the radiometers, including transient ambient temperature effects on the Wheatstone bridge, view angle; variation, lens refractive index variation, blackbody temperature, and target size. We discuss how each; source of uncertainty is managed (determined to be negligible, eliminated, or included in the analysis).; The major contributor to the total uncertainty is the use of the thermistor in the Wheatstone bridge.; There is also unexplained drift in the calibration curves. We present new maximum and minimum; calibration curves that account for all identified uncertainties. The overall uncertainty could be greatly; reduced by modifying the construction of the radiometers for future tests. Specific changes for future; radiometer design are suggested.