Superheated fuel injection modeling: an engineering approach

A liquid fuel undergoes a sudden decrease in pressure during an injection process. If the pressure outside the injector is below the saturation pressure of the fuel, superheated conditions are reached and the fuel undergoes a thermodynamic instability. Flash evaporation might take place inside or outside the injector nozzle, thus the liquid jet behaves following different mechanisms. Flashing conditions greatly influences atomization and vaporization processes as well as the mixture formation and combustion. This work presents a homogeneous one-dimensional model for the prediction of flash evaporation in superheated liquid fuel injections, able to deal with both internal and external flashing, useful for initializing 3D spray calculations in CFD codes. In this model a Volume Translated PengeRobinson Equation of State (VTPREOS) is used to calculate thermodynamic properties of hydrocarbon fuels. A Homogeneous Relaxation Model (HRM) is used to predict the evaporation rate during internal flashing which may lead to effervescent atomization outside the nozzle. Standard thermodynamic analysis of the CJ-point is used instead for the evaluation of the evaporation rate outside the nozzle, when an unbroken meta-stable liquid jet is predicted under superheated conditions. Two different sets of numerical simulations have been carried out and the results have been compared versus experimental data in literature.