An investigation of transcritical effects for fuel injection and mixing applications

Abstract

This thesis presents a numerical framework for characterizing transcritical effects on droplet evaporation at ambient conditions typical of modern combustors. The approach combines scaling analysis with a first-principles model to describe droplet evaporation behavior in fluid-independent, non-dimensional terms. The developed model is validated against published experimental data and incorporated in a spray calculation framework. The impact of different fluid-properties and evaporation models on temperature and fuel vapor distributions are assessed as well. The results suggest that Lewis number is the relevant parameter to single droplet evaporation in a non-convective environment. In particular, the data indicates that the transient and quasi-steady evaporation rates vary exponentially with Lewis number. The fluid-independence of the results suggests that a single-component fluid can potentially be used as a modeling surrogate for jet fuel. The first-principles assessment indicates that classical evaporation models are not suitable for transcritical applications due to limitations in fuel-property description and the lack of non-isothermal droplet characterization at near-critical conditions. More specifically, current subcritical models overestimate transient evaporation and underestimate quasi-steady evaporation, with discrepancies up to 70% at trancritical conditions. As a result, the temperature profiles are typically under-predicted and fuel vapor concentrations are over-predicted in standard spray calculations. While the current work focuses on hydrocarbon liquid fuels for large-scale gas turbines, the methodology can be directly applied to other fluid and combustion applications. Future modeling and experimental work is proposed to provide a more complete assessment of transcritical effects in combustor spray calculations.