An investigation of transcritical effects for fuel injection and mixing applications

Thesis: S.M., Massachusetts Institute of Technology, Department of Aeronautics and Astronautics, 2016.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 131-134).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.by Georgi Valeriev Subashki.S.M