Multicomponent fuel drop vaporization and wall interactions
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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.