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Improving the State-of-the-Art in Flow Measurements for Large-Scale Oxygen Consumption Calorimetry

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Abstract

The accuracy of the exhaust flow measurement contributes significantly to the uncertainty of calorimetry measurements for large fire testing. Less than ideal flow characteristics such as skewed velocity distributions are typical of these large-scale flows and make it difficult to achieve the desired measurement accuracy. Consensus standards for fire testing recommend either bi-directional probes or orifice plates to determine exhaust flow. Both have limited accuracy in the presence of less than ideal flow conditions. Averaging pitot probes are an off-the-shelf technology widely used to monitor flows for industrial processes. They have been utilized in a system of large fire calorimeters to demonstrate differences of less than 5% between heat release rate measurements by oxygen consumption calorimetry and the theoretical heat output from a gas burner. Differences exceeded 5% for a small set of conditions but were still less than 10%. Both levels of agreement are within the confirmation requirements of the consensus standards and were achieved without a system calibration as recommended by the standards. Including this technology as an alternate method to measure exhaust flow would be an improvement to relevant fire testing standards and to the overall accuracy of calorimetry measurements for large fire testing.

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Notes

  1. Certain commercial entities, equipment, or materials may be identified in this document in order to describe an experimental procedure or concept adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the best available for the purpose.

Abbreviations

B :

Blockage factor for averaging pitot probe

\(D_{eff}\) :

Effective diameter of exhaust duct

\(\left( {\Delta_{\text{c}} H_{fuel} } \right)_{{{\text{O}}_{2} }}\) :

Heat of combustion of hydrocarbon fuel per unit mass oxygen

\(\left( {\Delta_{\text{c}} H_{CO} } \right)_{{{\text{O}}_{2} }}\) :

Heat of combustion of carbon monoxide per unit mass oxygen

\(K_{a}\) :

Flow coefficient for averaging pitot probe

\(\dot{m}\) :

Mass flow

M :

Molar mass

\(P_{amb}\) :

Ambient pressure

\(\Delta P\) :

Differential pressure

\(\dot{Q}_{OC}\) :

Rate of heat release derived from oxygen consumption (OC) calorimetry

\(\dot{Q}_{FC}\) :

Rate of heat release derived from fuel consumption (FC) calorimetry

R :

Universal gas constant

\(Re\) :

Reynolds number

\(s_{i}\) :

Non-dimensional sensitivity coefficient for measurement component

T :

Gas temperature

u :

Standard uncertainty

U :

Expanded uncertainty

\(x_{i}\) :

Measurement component

\(X_{i}\) :

Exhaust stream volume fraction of gas i

\(X_{i}^{o}\) :

Ambient volume fraction of gas i

\(\alpha\) :

Combustion products expansion factor

\(\varepsilon\) :

Estimated measurement error due to skewed velocity distribution

\(\rho\) :

Gas density

\(\sigma\) :

Standard deviation

\(\phi\) :

Oxygen depletion factor

e :

Exhaust duct

eff :

Effective

i :

Gas, probe A or B

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Acknowledgements

The authors are grateful to several members of the NFRL staff for their valuable contributions to this work: Laurean DeLauter and Anthony Chakalis, for installation of the averaging pitot probes; Marco Fernandez, for operations and data collection during confirmation experiments; Brian Story, for scale drawings of the exhaust system; and Christopher Smith, for assisting with the uncertainty analysis for the effective diameter measurement of the exhaust ducts.

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Authors and Affiliations

  1. Fire Research Division, National Institute of Standards and Technology, Gaithersburg, MD, 20899, USA

    Rodney A. Bryant & Matthew F. Bundy

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  1. Rodney A. Bryant
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  2. Matthew F. Bundy
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Correspondence to Rodney A. Bryant.

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Appendix: Accurate Determination Exhaust Duct Diameter

Appendix: Accurate Determination Exhaust Duct Diameter

Accurate determination of the diameter of each exhaust duct is required to determine accurate measurements for volume flow, mass flow and ultimately heat release. The duct diameter is a squared term in the computation of volume or mass flow. Significant error for duct diameter measurements will propagate as a significant increase in flow measurement uncertainty. For example, 0.5% error in the duct measurement will propagate to become 1.0% error in the computation of cross-sectional area, which is used to compute volume or mass flow. Measurements of chord length were conducted at the locations of the averaging pitot probes and at various inclinations to generate an accurate profile of the duct geometry. Using a laser distance meter (Leica DISTO D8) and a digital inclinometer, radial points were projected and marked on the inside surface of the exhaust ducts at increments of 22.5°, Fig. 11.

Figure 11
figure 11

Locations of chord measurements at measurement locations as seen looking upstream and into the flow

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The distribution of chord lengths about the rotational positions, Figs. 12 and 13, confirms that the large ducts are not perfect circles. Therefore, the error in measurement could be substantial if only one chordal measurement were used to represent duct diameter.

Figure 12
figure 12

Distribution of chord lengths at the measurement stations of the averaging pitot probes for the 3 MW and the 10 MW calorimeters

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Figure 13
figure 13

Distribution of chord lengths at the measurement stations of the averaging pitot probes for the 20 MW calorimeter

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Bryant, R.A., Bundy, M.F. Improving the State-of-the-Art in Flow Measurements for Large-Scale Oxygen Consumption Calorimetry. Fire Technol 57, 1457–1478 (2021). https://doi.org/10.1007/s10694-020-01066-x

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