Numerical investigation on the effect of fuel droplets on premixed laminar flames

This thesis explores the combustion of liquid fuels, a process widely applied in industry, with a particular focus on laminar premixed flames containing fuel droplets. Despite their prevalence, these flames have not been extensively studied due to challenges in measuring key parameters and discrepancies in one-dimensional numerical simulations. The present work seeks to enhance our understanding of droplet-laden flames through numerical methods, and where possible, their comparison with experimental results. The work is comprised of two numerical simulation studies investigating droplet-containing flames, as well as a theoretical approach for extracting additional information from existing gas-phase measurements. The first two studies involve simulations of laminar premixed counterflow droplet-laden flames, building on previous experimental work. The simulations employ a 2D axisymmetric geometry with a refined mesh near the axisymmetric axis to capture the sharp temperature rise. A Lagrangian-Eulerian framework with droplets represented as point sources in a discrete phase model is applied for coupled computation of the liquid and gas phases. In the first part of the study, a parametric investigation is conducted for flames containing monodisperse and polydisperse acetone droplets under conditions comparable to previous experiments. The vaporization of droplets leads to a suppressed flame and richer burnt gas, by reducing the gas-phase temperature at the flame and introducing vaporized fuel to the post-flame area. These effects are amplified in the case of non-stoichiometric mixtures and higher liquid fuel fractions. For monodisperse droplets, initial sizes are found to determine their trajectories and residence times, influencing the locations of vaporization and flame suppression. For polydisperse droplets with a non-uniform size distribution, vaporization and flame impacts are more inhomogeneous and deviate from one-dimensionality. To understand the underlying mechanisms of droplet vaporization in the flame and their impact, an investigation of the key non-dimensional parameters governing ratios of governing times (residence, evaporation, acceleration) is undertaken, and their effects are investigated in a scaling study. The vaporization time, represented by the vaporization Damköhler number, is shown to be responsible for the decrease in gas-phase temperature and the change in species fraction in the burnt gas, as a result of its competition with the reaction time. The inertia of droplets, described by the Stokes number, and the flame speed, influenced by the global equivalence ratio, together determine the drag force on the droplets, their residence, and thus vaporization time in the hot environment. Finally, the liquid fraction plays a crucial role in the overall species and energy balance in the gas phase, significantly affecting the flame thickness and temperature. In a separate part of the work, an investigation is undertaken on the approach to recon- structing density and temperature profiles along the axisymmetric axis in a flame, using measured 2D velocity data. The method demonstrates an error ±2% when verified with simulation results. The sensitivities of the accuracy to resolution and measurement error have also been analyzed, suggesting reasonably low error introduction with common measuring techniques. Overall, the current work extends the understanding of the effects of fuel droplets on a laminar counterflow premixed flame, and contributes a density and temperature extraction method to improve measuring techniques for future experimental studies. The insights gained from this research pave the way for more accurate and insightful studies on liquid fuel combustion.

Name dc:type.qualificationname

Doctor of Philosophy (PhD)

Level dc:type.qualificationlevel

Doctoral

Grantor dc:publisher.institution

University of Cambridge

Year dc:date.issued

2023

Author dc:creator

• Ni, Shiyao

Advisor dc:contributor.advisor

• Hochgreb, Simone

• Counterflow
• Droplet vaporization
• Premixed flame
• Spray combustion
• Two-phase flow

Licence

• https://apollo8-f-pro.lib.cam.ac.uk/bitstreams/f3bd0ea9-9e3c-4d7b-8788-9f753cc89e06/download
• https://www.rioxx.net/licenses/all-rights-reserved/

Language dc:language

eng

Author Identifier

0000-0001-5659-2189

OAI identifier oai:identifier

oai:www.repository.cam.ac.uk:1810/368204