Nonlinear Threshold Sooting Index Prediction Method for Surrogate Formulation Emulating Sooting Characteristics: A Case Study Using RP‑3 Jet Fuels

Surrogates that can represent complex
real fuels have been widely
used to model combustion processes in practical applications. To formulate
surrogates that successfully mimic the spray combustion phenomenon
of real fuels, both physical and chemical properties need to be properly
matched between a real fuel and its surrogate. The surrogate formulation
requires a variety of mixing rules to accurately estimate different
properties of a surrogate mixture based on its composition. The present
work provides a methodology to form surrogates for kerosene fuels
used in aero-engine combustors, with a special focus on the emulation
of sooting tendency. A nonlinear regression method is proposed to
predict the threshold sooting index (TSI) for the surrogate mixtures.
Along with other mixing rules for predicting molecular weight, density,
viscosity, H/C ratio, and lower heating value, two surrogates with
three and five components based on a surrogate palette of n-dodecane, n-tetradecane, isooctane, decalin,
and 1,3,5-trimethylbenzene are formed for each of the two kerosene
fuels. The results show that all the surrogates are in good agreement
with their respective target fuels in terms of the target fuel properties.
Some discrepancies in viscosity are observed for the five-component
surrogates because of their inclusions of decalin that has a substantially
higher viscosity compared to other components. As for sooting tendency,
the proposed nonlinear method has a much higher accuracy in predicting
the measured TSI than the conventional linear method. In addition,
the spatially resolved soot volume fraction profiles in diffusion
flames show that the surrogates formed through the nonlinear method
capture the overall flame sooting characteristics of the target real
fuels, further demonstrating the appropriateness of the nonlinear
TSI prediction method for surrogate formulation.