Quenching Distance of Premixed Jet-A/Air Mixtures
Creators: name: Gupta, Shatakshi
Publisher: name: Purdue University Graduate School
Year: 2024
Subjects: classificationCode: 400199
subject: Aerospace engineering not elsewhere classified
classificationCode: 400201
subject: Automotive combustion and fuel engineering

description: Quenching distance is a fundamental property of hydrocarbon fuel-air mixtures and is a crucial parameter guiding process and equipment design for fire hazard mitigation. Many industrial equipment such as flame arrestors and burners rely on the fundamental principle of flame quenching, i.e., a premixed flame cannot pass through confined spaces below a critical width, given by the Quenching Distance (QD) of the fuel-air mixture. Through the efforts spanning over more than a century, QD is found to depend on various parameters such as temperature, pressure, fuel-air equivalence ratio, and the characteristics of hydrocarbons comprising the fuel. Many investigations on flame quenching behavior have focused on simple fuels such as Hydrogen, Methane, and hydrocarbons upto n-Decane. However, there is a lack of quenching distance data on aviation fuels like Jet-A likely due to the fact that QD property of these fuels is less relevant in practical combustor applications. But in this era of miniaturization, there are several upcoming technologies that will utilize jet fuels or kerosene in confined spaces. For example, a recently proposed Printed Circuit Heat Exchanger (PCHE) is being considered for jet engine performance enhancement by cooling down the compressor discharge air using fuel prior to injection. The cooled air can be used to improve turbine cooling allowing for improvement of the thermal efficiency of the jet engine. However, a major cause of concern during the PCHE operation is the accidental internal fuel leakage from high pressure fuel microchannels into the surrounding air microchannels. Under the severe operating conditions of a jet engine (T &gt;800K, P &gt;10bar), the leaking fuel upon mixing with air pose ignition and sustained combustion risks. This must be evaluated against the competing phenomenon of flame arrestment, since the channel sizes in PCHEs are very small (in the order of a few hundred micrometers). Thus, it becomes imperative to measure the quenching distance of jet fuels to design the microscale passages, predict and mitigate fire hazards to ensure safe operation. In the present work, the quenching distance of homogeneous, quiescent Jet-A/air mixtures at 473K, 1atm under various equivalence ratios (lean to rich) have been studied. For this purpose, experiments were setup using the ASTM Standard Method that involves using flanged electrodes to measure the parallel-plate QD of quiescent, pre-vaporized fuel-air mixtures under various conditions. Validation tests were carried out with Methanol/air mixtures at 373K, 1atm for different equivalence ratios. For tests with Jet-A/air mixtures, the QD variation with equivalence ratio follows similar trends as that of n-Decane/air. On further analyzing the QD variation with equivalence ratio, we see that the QD minimizes on fuel rich conditions with increasing molecular weight of the fuel which is consistent with the trend shown in literature. The flame propagation behavior shows considerable differences on the lean and the rich sides. Moreover, the quenching distance of quiescent Methanol/air and Jet-A/air mixtures have been estimated using three different models taken from literature. Model parameters were calculated using Chemkin Pro simulations of the premixed flames at the similar initial conditions as the experiments. On comparing the experiment data with model predictions, we observe that the models agree well with experiment data for Methanol/air mixtures, whereas they fail to capture the QD variation with equivalence ratio for Jet-A/air mixtures. The disagreement may arise because of the high molecular weight of Jet-A that causes the Lewis number to be non-unity unlike Methanol/air mixtures. Therefore, an empirical power law relation has been developed for estimating the QD of hydrocarbon/air mixtures to the incorporate the Lewis number effect. The model agrees well with Jet-A/air QD data from experiments over the entire equivalence ratios. This will help to further our understanding of the complex fuel combustion and flame quenching for better risk mitigation.