The NIST 20 MW Calorimetry Measurement System – Exhaust Flow Calibration Using Tracer Gas Dilution
Rodney A. Bryant
Exhaust flow measurements have been found to be a significant source of uncertainty for measurements of heat release rate in large-scale fire experiments. Asymmetric or skewed velocity distributions are often present in the exhaust ducts for open calorimetry systems used in large-fire research facilities, therefore making it difficult to measure exhaust flow accurately. Tracer gas dilution is a standard test method for determining volume flow in ducts. It is not sensitive to flow distribution and is derived from measurements independent of most flow monitoring techniques. Therefore, it is well suited for in-line calibrations of flow measurement devices in the exhaust ducts of facilities conducting large-scale fire experiments. For the first time, the method has been applied to calibrate the routine exhaust flow measurements at the National Fire Research Laboratory as a means to reduce the measurement uncertainty associated with open calorimetry systems. Measurement uncertainty for the calibration is estimated at 3 % and accounts for potential error due to incomplete mixing of the tracer. Multi-port sampling of the tracer, which is not part of the existing standard test method, is also demonstrated as a means to reduce the potential for measurement error due to incomplete mixing. Exhaust velocity and mass flow are necessary to compute heat release rate, and both are determined by averaging pitot probes installed in the exhaust ducts. An in-line calibration of the averaging pitot probes was conducted using tracer gas dilution. Measurement uncertainty for calibrated exhaust velocity and calibrated mass flow is estimated at 3 %. The in-line calibration of the exhaust flow measurement is an improvement over the accepted practice of comparing oxygen consumption calorimetry with heat release rate measured at a gas burner to develop a correction for the flow measurement. It is valid for a wide range of flow conditions and decouples measurement error between oxygen consumption calorimetry and fuel consumption calorimetry; therefore improving overall measurement accuracy for heat release rate.
The NIST 20 MW Calorimetry Measurement System – Exhaust Flow Calibration Using Tracer Gas Dilution, Technical Note (NIST TN), National Institute of Standards and Technology, Gaithersburg, MD, [online], https://doi.org/10.6028/NIST.TN.2220, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=934496
(Accessed March 26, 2023)