Degree Type

Dissertation

Date of Award

2019

Degree Name

Doctor of Philosophy

Department

Mechanical Engineering

Major

Mechanical Engineering

First Advisor

James . Michael

Second Advisor

Song Charng . Kong

Abstract

Drop wall interaction and vaporization are two important processes observed during fuel spray

in combustion systems and spray cooling application. In combustors such as gas turbines and

internal combustion engines, multicomponent fuel is injected at the start of the combustion phase

when the in-cylinder pressure and temperature is high. Generally, the operating pressure and

chamber wall temperature in practical combustion systems are in the range of 10 - 150 bar, 25 -350

°C. The change in pressure and wall temperature is dynamic and depends on the combustion

cycles. During the fuel injection event , a spray travels towards the piston crown, and during the

penetration process, the spray disintegrates into primary and secondary droplets. The secondary

droplet vaporizes while penetrating through a hot gaseous environment. The velocity of the droplet

varies in different ranges and is determined by the fuel injection pressure, aerodynamics resistance,

and charge ow conditions. As multicomponent fuel are used, vaporization of secondary droplets

may be controlled by the composition of the fuel. A multicomponent fuel is composed of different

hydrocarbon distillates having various molecular weights. The difference in saturation temperature

and molecular weight can induce thermal and mass diffusion processes which can significantly

influence the vaporization rates of a secondary droplet.

In this dissertation, an experimental investigation on the vaporization of multicomponent and

bicomponent fuel drop is carried out. In the experiment, the air stream temperature range is 100-

500 °C and the ambient pressure is 1 bar. The study is carried out in the convective environment

and is different from prior studies in which mostly drop vaporization was studied in a quiescent

environment. For this investigation, an experimental setup is designed and a new cross fiber

system is developed to suspend a single isolated droplet at the intersection of two quartz fibers

of 20 μm diameter each. This drop suspension system is developed to ensure a spherical shape

throughout the vaporization process of the droplet. Images of the vaporizing droplet were captured and processed to report and compare steady-state vaporization rates of n-heptane, n-dodecane,

their binary mixtures, and multicomponent fuels gasoline and diesel in a convective environment.

N-heptane represents highly volatile lighter component and n-dodecane is a lower volatile heavier

component . The results establish stagewise preferential vaporization in bicomponent fuels with a

higher percentage of n-heptane, and a non- linear vaporization trend for gasoline and diesel. The

vaporization rates reported in this study cover a wide range of combustion stream temperature,

fuels, and can be used to develop better drop vaporization models.

During the spray impingement, drop wall interactions are observed when the secondary droplets

formed after the disintegration impacts the piston crown and the cylinder wall. In most of the

operating cycles, the walls are dry and hot due to variable heat

flux from the in-cylinder charge. The

wall temperature varies as it cools during the intake stroke, and heats up after the combustion phase.

The impact sequences of a drop on these heated surfaces are classified as drop wall interactions

on a dry heated wall. As discussed before, after fuel injection the multicomponent fuel drop

vaporizes and eventually collide on a hot piston crown wall/cylinder liner surfaces in a velocity

range to produce specific sequences. These sequences are the outcome of complex processes in

which several parameters interplay. Some of the established parameters in the literature are

fluid dynamic

properties such as surface tension and viscosity, surface roughness, and thermo-physical

characteristics. For a nearly stationary droplet, literature classifies regimes based on heat transfer

as film evaporation, nucleate boiling, and film boiling. The inception and transition of these heat

transfer regimes may be significantly in

influenced by the wall temperature, droplet momentum, fuel

composition, and ambient pressure. In this dissertation, the in

fluence of fuel volatility is examined

in a Weber number range of 27-700 at 1 bar pressure using single component fuels: n-heptane, n-decane;

their binary mixtures, and multicomponent fuels gasoline and diesel. Using a conventional

backlit experimental setup, images were captured using high-speed CMOS camera and processed

to report the effect of Weber number, fuel volatility on liquid film spread and disintegration, and

secondary droplet characteristics. Regime diagram mapping Weber number, drop impact sequences,

and fuel volatility are presented in the study. Another investigation is carried out to examine the effect of ambient pressure on the drop impact sequences, the inception of the film boiling regime,

and rebound motion of single, bicomponent, and multicomponent fuels. For this purpose, a new

experimental setup is designed to carry high-pressure drop wall interaction studies in the range of

1-30 bar and wall temperatures between 35-350 °C. The designed vessel has four optical windows to

access and record the impact sequences. The fuel is injected in the vessel at room temperature using

a syringe pump and cooling jacket assembly. Results of these investigations establish the significant

in

uence of fuel volatility, and ambient pressure on the transition of impact sequences, inception

of film boiling regime, and important parameters such as liquid film contact time, maximum liquid

film spread, and droplet rebounding velocity. The high-pressure study is a novel study and the

results can be used for the development of future spray impingement models.

Copyright Owner

Abhijeet Chausalkar

Language

en

File Format

application/pdf

File Size

160 pages

Available for download on Thursday, December 03, 2020

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