Atomization characteristics of alternative aviation biofuels, Jet A-1, and water from a hybrid airblast atomizer

Abstract

Studies on the atomization of sustainable aviation fuels (SAF) from aircraft engine atomizers are essential to replace the present fossil type jet fuel to counter the rise in aviation-caused CO2 emissions in the atmosphere. The characteristics of spray droplets resulting from the breakup of liquid film in atomizers are crucial for the description of primary atomization process and combustion dynamics in aircraft engines. The thesis investigates the atomization of camelina- and jatropha-derived drop-in aviation biofuels from a hybrid airblast atomizer (HAA) used in aircraft jet engines. The main focus of the study is on the evaluation of spray droplet characteristics in the near-region of liquid film breakup. The experiments are carried out in a spray test facility. The images of sprays at different flow conditions are captured using backlighted shadowgraphy technique. The measurements of spray droplet characteristics are obtained using phase Doppler interferometry (PDI) and Spraytec at different spatial locations of the spray below the atomizer exit. In the first part of the study, extensive experiments of liquid atomization from the HAA using water are conducted. The size and velocity characteristics of droplets resulted from the liquid film breakup in the simplex swirl atomizer (central atomizer in the HAA), measured within millimetre distance from the actual location of the liquid film breakup, are analysed. The mean axial velocity of the spray droplets measured at the film breakup point is independent of droplet size, which is different from the correlation characteristics of the spray droplets observed in other regions of the spray. The linear film breakup theory overpredicts Sauter mean diameter (SMD) of the spray measured at the breakup point significantly, and an existing scaling law for the determination of volume median diameter of the spray captures the present experimental trend of the droplet size recorded at the breakup point. The droplet size distribution measured at the breakup point is well described by a Gamma distribution with index parameter n governing the corrugation features of ligaments formed in the film breakup. Further, by using the self-similarity analysis of droplet size and velocity, the demarcation region between the near- and far-region of liquid film breakup in the spray is established. A systematic comparison of spray characteristics in the near- and far-region of the liquid film breakup is reported. In the second part of the study, the atomization characteristics of water and drop-in aviation biofuel sprays from the HAA is carried out. The droplet characteristics in the near-region of liquid film breakup are obtained at a distance 19 mm from the atomizer exit using Spraytec. The present droplet size data compare well with the predictions obtained using previously reported empirical correlation with a modified proportionality constant. The spray characteristics of the aviation biofuel sprays from the HAA are almost same as that of the standard fuel (Jet A-1) spray, which confirms the drop-in behavior of the chosen alternative fuels. By using the present experimental data of HAA spray from six experimental fluids, an empirical correlation for the estimation of nondimensionalized SMD in terms of liquid and gas Weber numbers and Ohnesorge number is proposed