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Single Droplet Flash Boiling Characteristics of E-Fuels at Low Pressures: A Numerical Study

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Within the Cluster of Excellence – “The Fuel Science Center,” concerted efforts are being made to develop novel e-fuels based on renewable energy and CO2 as carbon sources. Some of these fuels are highly susceptible to flash boiling at low-pressure conditions. In this study, the flash boiling phenomenon of these e-fuels is numerically investigated at a single droplet level using oxymethylene ethers (OMEx) as a generic example. Bubble growth characteristics of OMEx shows three distinct growth phases: (1) surface tension-controlled, (2) transition, and (3) inertia-controlled phase. While flash boiling happens both internally in the droplet and on the outer surface, the internal flash vaporization is found to be the primary source causing the transition of the metastable liquid into the stable state.
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Fuel Science - From Production to Propulsion
Single Droplet Flash Boiling Characteristics of E-Fuels at Low Pres-
sures: A Numerical Study
Avijit Saha1, Abhishek Y. Deshmukh1, Temistocle Grenga1, Mathias Grunewald2, Yakup
Kaya2, Valeri Kirsch2, Manuel A. Reddemann2, Reinhold Kneer2, Heinz Pitsch1
1Institute for Combustion Technology, RWTH Aachen University, Aachen, Germany
2Institute of Heat and Mass Transfer, RWTH Aachen University, Aachen, Germany
Within the Cluster of Excellence “The Fuel Science Center,concerted efforts are being made
to develop novel e-fuels based on renewable energy and CO2 as carbon sources. Some of
these fuels are highly susceptible to flash boiling at low-pressure conditions. In this study, the
flash boiling phenomenon of these e-fuels is numerically investigated at a single droplet level
using oxymethylene ethers (OMEx) as a generic example. Bubble growth characteristics of
OMEx shows three distinct growth phases: (1) surface tension-controlled, (2) transition, and
(3) inertia-controlled phase. While flash boiling happens both internally in the droplet and on
the outer surface, the internal flash vaporization is found to be the primary source causing the
transition of the metastable liquid into the stable state.
* Corresponding author: a.saha@itv.rwth-aachen.de
Introduction
The Cluster of Excellence The Fuel Sci-
ence Center," aims to develop novel e-fuels
based on renewable energy and CO2 as car-
bon sources for enabling highly efficient and
advanced propulsion systems. The thermo-
physical properties of these fuels are exceed-
ingly different from conventional fuels. For ex-
ample, OMEx is one such class of e-fuel can-
didates, which has emerged as a promising
substitute to the conventional fuels as it pro-
duces almost no soot during combustion [1].
However, the higher vapor pressure of short-
chain OMEx compared to conventional fuels
makes it more susceptible to flash boiling at
low cylinder pressures. Flash boiling is a fairly
complex phenomenon consisting of several
sub-processes such as bubble nucleation,
growth, and droplet burst. It is difficult to ac-
curately quantify the characteristics of flash
boiling fuel sprays unless the physics under-
lying the flash boiling process are explored in
detail at the microscopic single droplet level.
Therefore, in this work, the underlying dynam-
ics of bubble growth are studied in a single
droplet configuration using OMEx as a generic
example.
Numerical approach
A flash boiling model combining internal and
external vaporization processes has been im-
plemented in a Lagrangian Particle Tracking
(LPT) framework of the in-house code CIAO,
which is a structured, high-order, finite-differ-
ence solver. The internal vaporization model
is based on heterogeneous bubble nucleation
due to air as a dissolved gas. The bubble
growth rate, , is predicted using the
Rayleigh-Plesset (RP) equation,
where is the bubble radius, the vapor
pressure, and the liquid pressure, which is
assumed equal to the gas pressure, , due
to the negligible surface tension force be-
tween droplet and gas. is the liquid density,
the surface tension, and the liquid viscos-
ity.  denotes the pressure difference, 
represents the hydrodynamic pressure result-
ing from the bulk liquid motion,
indicates
the pressure due to the surface tension force,
and
represents the viscous force at the liq-
uid-vapor interface. The onset of droplet
bursting is modeled by evaluating the critical
void fraction of the droplet-bubble system.
The external vaporization model is based on
the mass transfer due to heat flux from the in-
ner-core of the droplet as well as from the sur-
rounding gas.
Results
Here, bubble dynamics of dimethyl ether
(DME) single droplet are shown in Figs. 1 and
2 for
  bar. Unlike in previous studies,
Fuel Science - From Production to Propulsion
the thermal diffusion-controlled growth is not
found at low-pressure conditions due to the
earlier occurrence of droplet burst. Surface
tension acts as a rate-limiting factor determin-
ing the bubble growth at the very beginning.
The RP equation reduces to   
 
 in the surface tension-controlled growth
phase due to large
. In the later period of
this growth phase, rapid bubble growth in-
crease leads to a significant increase in bub-
ble radius and interface acceleration (see Fig.
1), which in turn slowly generates the liquid
motion, as shown from increased hydrody-
namic pressure, , in Fig. 2.
Fig 1. Bubble growth characteristics of DME
single droplet with a superheating degree of
27.85 K.
Fig 2. Variation of different pressures acting
on the bubble surface.
The transition growth phase is characterized
by a sharp decrease in
together with an in-
creasing . The bubble radius undergoes a
significant increase in this growth phase,
which is also the reason for the sharp de-
crease in
. At the end of the transition
phase,
becomes insignificant, and  in-
creases to such an extent that   . The
peak value of also occurs in the later pe-
riod of the transition phase depicting moder-
ate liquid motion surrounding the liquid-vapor
interface. The transition phase ends at  
, where the bubble growth reaches its
maximum value. The pressure differential,
, remains balanced by  throughout the
inertia-controlled growth phase. The magni-
tude of pressure force due to viscosity of the
liquid,
remains negligible during the bubble
expansion.
As shown in Fig. 3, the internally vaporized
mass, , is found to dominate over externally
vaporized mass, , indicating internal flash
vaporization is the primary source of vapori-
zation in the superheated regime.
Fig 3. Variation of internally and externally va-
porized fuel vapor mass flow.
Summary and Outlook
Flash boiling of OMEx single droplets was
studied under low cylinder pressure
conditions. Bubble dynamics of a DME
droplet are reported here. It was found that
the vapor bubbles in the DME droplet
undergo three distinct growth phases: (1)
surface tension-controlled, (2) transition, and
(3) inertia-controlled growth phase. The
thermal-diffusion controlled growth phase
was not observed since droplet bursting
occurs earlier under low-pressure conditions.
Also, the internal flash vaporization was found
to be the primary source causing the
transition of metastable liquid into the stable
state.
Acknowledgements
This work was funded by the Deutsche For-
schungsgemeinschaft (DFG, German Re-
search Foundation) under Germany ́s Excel-
lence Strategy Exzellenzcluster 2186 “The
Fuel Science Center” ID: 390919832.
References
[1] Omari, A., Heuser, S., Pischinger, S., Fuel., vol.
209, 232-237, (2017).
ResearchGate has not been able to resolve any citations for this publication.
  • A Omari
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Omari, A., Heuser, S., Pischinger, S., Fuel., vol. 209, 232-237, (2017).