Assessment of Biofuels/Jet A-1 Blends to Meet Cold Start and Altitude Relight Requirements

Abstract

The certification of new fuels for aircraft applications requires preliminary extensive investigations for the most critical combustion phases. Especially, cold start and altitude relight of an aircraft gas turbine and thus its operating envelope could potentially be affected by the use of new fuels. To assess such effects, a test rig is designed using two different air-assist pressure atomizers. The ignition envelopes for seven different blends with a minimum of 50% Jet A-1 are compared to that of pure Jet A-1. Variation in combustor aerodynamics is accounted for by considering various pressure drops across the combustion chamber, and the ignition envelope is retrieved by finding the minimum and maximum fuel-air ratios leading to a successful ignition event. Effects of fuel physical properties, pressure and temperature which are critical factors for a successful ignition event are also investigated. To simulate cold start and altitude relight, a heat exchanger is used to cool air and fuel, while a bleed valve mounted between the combustor and a steam ejector is used to regulate the operating pressure. Globally, cold start for temperatures ranging from 10°C to −40°C, and the altitude relight for operating altitudes ranging from 4 572 m to 10 668 m, show that the lean limit for the seven blends of fuels are in some cases as good as, and for the other cases better than the pure Jet A-1. Some discrepancies are noted for altitude relight at 9 161 m and 10 688 m, for some biofuels with a minimum of 50% Jet A-1, suggesting a need of real engine testing before final approval. Apart from these isolate cases, almost all the biofuel blends with a minimum of 50% Jet A-1 are truly “drop-in” fuels and should qualify for aviation use since they do not present any negative impact on the typical engine components used in the test program. Furthermore, ASTM D1655 requirements are also achieved for all test conditions. Biofuel blends with less than 50% Jet A-1 are found to always be better than the biofuel blends with minimum of 50% Jet A-1 and, it is recommended to modify ASTM D1655 to include them as acceptable “drop-in” fuels.