Spark Ignition Measurements in Jet A: part II
Creators: name: Lee, Julian J.
name: Shepherd, Joseph E.
Publisher: name: California Institute of Technology
Year: 2000

description: An improved system for measuring the ignition energy of liquid fuel was built to perform experiments on aviation kerosene (Jet A). Compared to a previously used system (Shepherd et al. 1998), the present vessel has a more uniform temperature which can be held constant for long periods of time. This ensures thermal equilibrium of the liquid fuel and the vapor inside the vessel. A capacitive spark discharge circuit was used to generate damped sparks and an arrangement of resistors and measurement probes recorded the voltage and current histories during the discharge. This permitted measurement of the energy dissipated in the spark, providing a more reliable, quantitative measure of the ignition spark strength. With this improved system, the ignition energy of Jet A was measured at temperatures from 35C to 50C pressures from 0.300 bar (ambient pressure at 30 kft) to 0.986 bar (ambient pressure near sea level), mass-volume ratios down to 3 kg/m^3, with sparks ranging from 10 mJ to 0.3 J. Special fuel blends with flash points (Tfp) from 29C to 73.5C were also tested. The statistical properties of the ignition threshold energy were investigated using techniques developed for high-explosive testing. 

Ignition energy measurements at 0.585 bar with high mass-volume ratios (also referred to as mass loadings) showed that the trend of the dependence of ignition energy on temperature was similar for tests using the stored capacitive energy and the measured spark energy. The ignition energy was generally lower with the measured spark energy than with the stored spark energy. The present ignition energy system was capable of clearly resolving the difference in ignition energy between low and high mass-volume ratios. The ignition energy vs. temperature curve for 3 kg/m^3 was shifted approximately 5C higher than the curve for high mass-volume ratios of 35 kg/m^3 or 200 kg/m^3. The ignition energy was subsequently found to depend primarily on the fuel-air mass ratio of the mixture, although systematic effects of the vapor composition are also evident. As expected, the ignition energy increased when the initial pressure was raised from 0.585 bar to 0.986 bar, and decreased when the pressure was decreased to 0.3 bar. Finally, tests on special fuels having flash points different from that of commercial Jet A showed that the minimum ignition temperature at a spark energy of about 0.3 J and a pressure of 0.986 bar depends linearly on the flash point of the fuel.
description: Explosion Dynamics Laboratory Report FM 99-7. Prepared for and supported by the National Transportation Safety Board Under Order NTSB12-98-CB-0415 .Julian Lee was partially supported by a fellowship from FCAR of Quebec, Canada. George Chung and Carrie Garner provided conscientious and indispensable technical assistance by performing a large part of the experiments. Douglas Shepherd provide valuable assistance in developing the computer data analysis programs and Pavel Svitek assisted in the construction and design of the experimental setup. The ARCO refinery in Carson, CA, provided the fuel samples that were used in this study. We thank Jim DeJovine, David Tong, Aaron Nagayama, Thinh Pham, David Smith, James McGetrick, and Eugene Zaluzec for their assistance in providing these samples. Suzy Dake assisted in the proofreading and final preparation of this document.