The NIST 20 MW Calorimetry Measurement System :

Abstract

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.

Exhaust flow calibration for a large‐scale calorimetry system using tracer gas dilution

“…Additional details of the installation and use of APP at the NFRL are available from previous publications. 15,16,18 _ V e,APP ¼ V e,APP πD 2 4 : ð5Þ…”

“…However, a spatially integrated sample, as for the present case, should be insensitive to small changes in the downstream distribution of the tracer. 18 Preliminary experiments to confirm sufficient mixing were conducted at the 3 MW calorimeter. Figure 6 shows the inlet at the exhaust duct of the 3 MW calorimeter and the single-point injection locations, annotated by "X"s in the far left photo.…”

“…TGD serves as the reference method for determining exhaust volume flow. A detailed analysis of the measurement uncertainty for the method has been described previously 17,18 and the resulting uncertainty budgets for measurements at each calorimeter are summarized in Table 1. On average the estimated measurement uncertainty for volume flow determined by TGD is 3% for the experiments described here.…”

“…Full details of the experimental procedures have been described in previous publications and are summarized here. 17,18 The objectives of…”

“…Flow dampers are installed at various locations (exhaust duct and canopy hood) to isolate a specific calorimeter for use. NIST Technical Notes 2077 and 2220 provide additional details of the system 15,18.…”