Pre-catalyst engine emissions and detrimental injector deposits have been widely associated with the near-nozzle fluid dynamics during and after the injection events. Although the heating and evaporation of fuel films on the nozzle surface directly affects some of these processes, there are no experimental data for the transient evolution of nozzle surface temperature during typical engine conditions. In order to address this gap in knowledge, we present a non-intrusive approach for the full-cycle time resolved measurement of the surface temperature of production nozzles in an optical engine. A mid-wave infrared high-speed camera was calibrated against controlled conditions, both out of engine and in-engine to account for non-ideal in surface emissivity and optical transmissivity. A custom-modified injector with a thermocouple embedded below the nozzle surface was used to validate the approach under running engine conditions. Calibrated infrared thermography was then applied to characterise the nozzle temperature at 1200 frames per second, during motored and fired engine operation, thus revealing for the first time the effect of transient operating conditions on the temperature of the injector nozzle’s surface.

With aviation's dependence on the high volumetric energy density offered by liquid fuels, Sustainable Aviation Fuels (SAFs) could offer the fastest path toward the decarbonization of aircrafts. However, the chemical properties of SAFs present new challenges, and research is needed to better understand their injection, combustion, and emission processes. While efforts such as the United States National Jet Fuel Combustion Program (NJFCP) that investigated several aspects in detail, certain processes were unfortunately beyond the reach of this program. One of them in particular is about droplet evaporation at relevant pressures and temperatures, and this represents the focus of the present paper. To address this gap, we characterized the evaporation and mixing of spray droplets injected into well-controlled thermodynamic environments at conditions relevant to modern and next-generation aero-engine combustors. We tested three fuels from the NJFCP, namely, an average Jet A fuel (A-2), an alcohol-to-jet fuel containing highly branched dodecane and hexadecane type components (C-1), and a blend made of 40% C-1 and 60% isoparaffins ranging from 9 to 12 carbon atoms (C-4). We also tested a single component normal alkane: n-dodecane, as well as an advanced bioderived cyclo-alkane fuel: bicyclohexyl. The time evolution of fuel droplets was monitored using high-speed long-distance microscopy in a specific configuration that enabled sharp images to be acquired at these extreme conditions. The collected images were processed using a purposely developed and trained machine learning (ML) algorithm to detect and characterize the droplets' evaporation regime. The results revealed different evaporation regimes, such as classical and diffusive. In agreement with previous studies, evaporation regimes appear to be controlled by ambient pressure, temperature, and fuel type. The measurements demonstrate that diffusive evaporation is relevant at high-pressure conditions, such as takeoff combustor pressures for modern commercial aircraft engines. However, classical evaporation mostly controls mixing at lower pressure, such as cruise altitude conditions. The ML analysis emphasized that multiple evaporation regimes co-existed at the same operating condition and no significant relationship was found between droplet size and evaporation regime. The findings of this work constitute a database for validating spray and droplet models that are necessary for implementing lower emissions fuels in aero-engines.