Proper interpretation and overall accuracy of laminar flame speed measurements of single- and multi-component liquid fuels

Low-vapor pressure liquid fuels, particularly kerosene-based fuels, are notoriously difficult to use in precision laboratory-scale flame experiments. This difficulty could result in several sources of uncertainty when preparing fuel-air mixtures for laminar flame speed experiments in constant-volume vessels. To accurately measure the experimental uncertainties in a spherical, laminar flame, n-decane, a component of several popular kerosene-based surrogate fuels was utilized in a methodical study to elucidate and minimize the primary sources of error and to determine a realistic, overall measurement uncertainty. This careful study allowed for isolated analysis of the overall behavior of the fuel (such as whether or not the fuel is condensing) and the accuracy of the instrumentation used. The results show that for the single-component liquid fuel, equivalence ratio could be accurately measured to within phi=+/- 0.03 and flame speed to within 2.79 cm/s. One of the primary sources of discrepancy and confusion when presenting and comparing laminar flame speed data of complex, liquid fuel mixtures is the identification of the average fuel molecule. This seemingly trivial detail is actually a very important property of the fuel because it leads to the determination of phi, which is commonly used as the independent variable of laminar flame speed plots. However, when reported at all, the uncertainty in the average molecular weight of these fuels is on the order of 15%. Because of this large uncertainty, phi is shown to not be the most useful parameter for comparing different data sets. Rather, fuel mole fraction, X-FUEL, is much more useful as it describes the overall amount of fuel in the mixture, and plotting laminar flame speeds as a function of X-FUEL results in better agreement amongst different data sets from the literature for Jet-A. (C) 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

Low-vapor pressure liquid fuels, particularly kerosene-based fuels, present challenges in precision laboratory experiments. A study using n-decane showed accurate measurement of equivalence ratio and flame speed for single-component liquid fuels. Identifying the average fuel molecule is crucial for comparing laminar flame speed data of complex fuel mixtures, and using fuel mole fraction X-FUEL is more useful in achieving agreement among different data sets.