Alessandro Gomez, Ph.D
Professor of
Mechanical Engineering
Director of the
Yale Center for
Combustion Studies
Yale University
P.O. Box 208286
New Haven, CT 06520-8286
USA
Phone: 203.432.4384
Fax 203.432.7654
alessandro.gomez@yale.edu
YALE UNIVERSITY
FACULTY OF ENGINEERING
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RESEARCH
A common thread through all of our research is striving to have a measure of
control in the technical problems we tackle. To the extent possible we devise
well-defined experiments to sort out cause-and-effect relationships and draw
unambiguous conclusions. Our group applies this
approach to two primary research areas: combustion, and electrospray
fundamentals and applications. Our activity is currently funded by the National
Science Foundation, the Army Research Laboratory, and the Air Force Office of
Scientific Research. We also have some internal seed funds for exploratory projects.
Combustion in Well-Defined and Well-Controlled Systems
Research is focused on:
turbulent flames,
jet fuel
combustion,
soot in diffusion flames, and
microscale combustion.
To achieve
the necessary control in these disparate systems, we rely on the counterflow
flame configuration with which we have had a “love affair” for a long time.
Research on counterflow turbulent flames is finalized at synthesizing key
aspects of turbulent combustion under both nonpremixed and premixed conditions,
which should provide dramatic advantages in concurrent computational modeling.
The project on jet fuel combustion is aimed at developing a flame database on
jet fuels and their surrogates, with the ultimate goal of establishing a
chemical kinetic mechanism that captures essential features of this complex fuel
blend, for subsequent incorporation in CFD codes. Research on soot formation
exploits the unique advantages of the counterflow diffusion flames with suitable
feed stream compositions to probe soot behavior with adequate resolution, even
in high-pressure flames. Research in microcombustion applies the developments in
multiplexed electrosprays to the clean burning of jet fuel in mm-size
combustors. Recently, we also initiated an internally-funded project aimed at
optimizing the performance of cook-stoves - the predominant cooking device in
developing countries.
We
investigate a particular class of sprays, the cone-jet electrospray (ES). The
unique ability of such a system to generate droplets or particles (via either
spray drying or spray pyrolysis) of narrow size distribution over a phenomenally
wide range of sizes can be particularly useful in high tech/high value-added
technologies. A crucial drawback that has hampered ES applications to date is
the low flow rate. Multiplexing the spray source, that is, operating several
electrosprays in parallel, is the only way to address this problem. A first goal
of our research is to implement large scale multiplexing using
microfabrication. The second goal is to apply it in various fields
including: the synthesis of biological micro-(nano-)particles for
controlled/targeted
drug delivery, jet-fuel atomization,
microcombustion and
electric propulsion.
Specific Projects
Highly
turbulent counterflow flames are stabilized as a very useful benchmark of
complexity intermediate between laminar flames and practical systems. By
operating in a turbulent Reynolds number regime of relevance to practical
systems such as gas turbines and internal combustion engines, these flames
retain the interaction of turbulence and chemistry of such environments, but
offer several advantages including: a) the achievement of high Reynolds numbers
without pilot flames, which is particularly advantageous from a modeling
standpoint; b) control of the transition from stable flames to local extinction/reignition
conditions; c) compactness of the domain by comparison with jet flames, with
obvious advantages from both a diagnostic and, especially, a computational
viewpoint (see photographs below); and d) the reduction or, altogether, elimination of soot formation,
thanks to the high strain rates and low residence times of such a system, and
the establishment of conditions of large stoichiometric mixture fraction, as
required for robust flame stabilization. The aim of the current research is to
probe the behavior of nonpremixed and premixed turbulent flames under conditions
ranging from vigorous burning to local extinction. In addition to probing the
phenomenology in all of these regimes, we are developing an experimental data
base for the validation of computanional models pursued in other groups (e.g.,
Professor Pope's, Cornell's University). The program is leveraged with the participation of the group of
Dr.
Jonathan Frank (Sandia National Laboratories) for the implementation of
state-of-the-art laser diagnostic techniques. (See
articles no. 13,14, 20-22 in the publication list for details).
Above: long exposure time photographs of two
turbulent nonpremixed flames at the same engineering Reynolds number, the
pictures are at the same scale showing a much smaller counterflow (top) domain
as compared to that of a jet flame (bottom).
Images were rotated 90°
to ease their incorporation in the page.
Despite
the nearly ubiquitous use of logistic fuels in military applications, efforts to
model their chemical kinetic behavior have been modest. A coordinated
experimental and computational program was initiated a few years ago under AFOSR
sponsorship to develop chemical models that can be used within a larger research
and development program. To be able to predict flame structure, extinction
behavior, flame speed and emissions in systems burning jet fuel is an enormous
task, in view of the number of individual chemical components comprising
practical fuel blends and the composition variability that may occur from
batch-to-batch. The problem remains arduous but it is perhaps manageable once
one identifies a fuel surrogate that captures key features of the practical jet
fuel and is amenable to detailed chemical kinetic modeling. In collaboration
with the computational group of Professor M.D. Smooke at Yale, we initiated a
joint experimental and computational program, designed to test the feasibility
of surrogate formulations of JP-8 to match overall properties and structure of
diffusion flames. Once the surrogate is identified, its kinetic behavior is
modeled and, after additional validation, the model will be reduced to a
sufficiently small subset of critical kinetic reactions to be incorporated in
realistic computational codes. A comprehensive campaign of measurements and
computations is under way in gaseous counterflow diffusion flames doped with
trace amounts (O(1000) ppm) of jet fuel and its surrogates and spanning a broad
range of composition and pressure (up to 40 Bar). The work is leveraged through collaborations with the
group of Professors Ranzi and Faravelli at the University of Milan (Italy) that
has been focusing on the chemical kinetic modeling. (See relevant articles no.
10-12 in the publication list for details).
An experimental system was designed (see
figure below) to
investigate soot formation in counterflow diffusion flames at elevated
pressures, up to 40 atm. One of the main difficulties of studying soot formation
under these conditions is that sooting tendency is exacerbated at high
pressures, with experimental complications that make it challenging to perform
fundamental studies. A counterflow configuration may be advantageous in this
respect for the following reasons: the unparalleled level of control that it
provides on the soot formation process, the suppression of buoyancy
instabilities that typically plague co-flow flames at high pressures, and the
opportunity of modeling the system in subsequent studies as one-dimensional,
which is critical for systems with a very large chemical mechanism as required
by soot modeling. The flame is operated as a reactor in which the soot loading
is “dialed” by varying the composition of the feed streams and the strain rate.
As an additional novelty, by experimenting with a high-diffusivity diluent such
as Helium, the transition to turbulent conditions can be delayed and sufficiently thick flames can be stabilized, despite the
high-pressure conditions, for probing the flame structure with
adequate resolution.
There has been a frenzy of announcements
recently on clean-burning cook-stoves including the establishment of an X-prize
on the topic and of a Global Alliance for Clean Cookstoves. Cook-stoves are
responsible for indoor smoke and the associated acute respiratory infections,
accounting for 22% of all communicable child deaths in developing countries.
Women who cook and the infants and children they care for are particularly
affected.
The fundamental cause of health and environmental problems are the poor mixing
of the fuel and air and short residence time in the combustion chamber. By
keeping combustion intermediates and the oxidizer segregated, pockets of fuel
rich intermediates develop that eventually lead to the formation of soot, also
referred to as particulate or black carbon. The soot, if air mixing and
residence time are inadequate, is not burned off before being released from the
stove. As a result of incomplete combustion, there are large emissions of
non-CO2 green house gases (GHG) and soot per unit of heat released.
We developed a simple cook-stove combustion technology that is well suited for
biomass fuels and that ultimately may result in a very affordable device, at a
cost on the order of 10-20 USD. Cook-stoves are typically designed as a
variation of the rocket-stove design, in which air is drawn through a single
opening at the bottom of the stove by the chimney effect. The new concept is
based on a modification of the entrainment geometry based on the establishment
of a whirl, in which air is introduced tangentially in the combustion chamber
and the biomass fuel is positioned strategically with respect to the air inlets.
This in turn creates some recirculation and a distribution of pressure that is
conducive to better mixing between fuel and oxidizer, more uniform burning and a
reduction of pollutant emission. Thus, strategically positioned slits along the
outer walls of the cook-stove serve the purpose of providing a passive
entrainment without the need of a potentially unaffordable auxiliary fan.
The first-generation stove was designed by positioned two metal half pipes off
center, and connecting them rigidly, as depicted in the image below (left and
left-center). The only challenge is to provide a convenient feed for the biomass
to enable sustainable burning during cooking, as depicted in one realization in
the image below (right-center). Quantitative verification using a retrofitted
smoke-detector showed that the Yale design yielded roughly 50% less soot. Field
testing to optimize the design further was initiated in Bangladesh in the Spring
2011 in collaboration with BRAC.
 
Schematic of the
first generation stove (left); top view of the prototype, including a kao-wool
insulation layer (center-left); schematic of a second generation including
chutes to feed the fuel (center-right); photo of field testing in Bangladesh
(right).
The electrospray (ES) has had a dramatic
technological impact in the field of analytical chemistry, leading to the
awarding of the 2002 Chemistry Nobel Prize to John B. Fenn, for his pioneering
work at Yale in ES mass spectrometry. To address the low flow rate limitation of
a single cone-jet ES, we multiplexed the spray source and operated several ESs
in parallel. Doing so compactly, that is, with a high number of sources per unit
area, is indispensable for dramatically increasing the throughput and reducing
the cost per ES source. We recently demonstrated the successful operation of
systems with an unprecedented packing density greater than 104 sources/cm2 (see
article no. 15 in the publication list.) The devices, microfabricated at ARL,
are designed and operated by implementing three criteria: a) the extractor
electrode configuration should be used to localize the electric field; b) the
viscous pressure drop acting on the liquid should dominate with respect to the
electrostatic pulling force by the electric field; and c) the electric field
“driving” the droplets between the extractor electrode and the collector should
be sufficiently intense to avoid reversal of the droplet motion near the
extractor (satellite trapping). Reliable guidelines are now established for the
successful design and operation of such systems from first principles, that is,
based only on knowledge of the suitability of a liquid for ES dispersion and of
critical liquid properties, such as surface tension, viscosity and electric
conductivity. As a result, the design of these devices for a given application
is achievable without costly trial-and-error microfabrication prototyping. For
further details, see the relevant articles no. 3 and 9 in the publication list,
and article no. 17 for a variant on the theme with digital control of individual
nozzles in the multiplexed system. Current efforts are focused on the mitigation
of space-charge effects by interfacing the multiplexed electrospray with a
corona discharge of opposite polarity. The goal is to increase even more the
dispersable liquid flux, with applications to jet fuel atomization.
A miniaturized combustor must rely on a good design of the fuel atomizer, which
should yield small, rapidly evaporating droplets, to generate the fuel vapor
promptly, mix it with the oxidizer and subsequently burn it with the attending
heat release. To achieve this goal, we relied on the use of multiplexed
electrosprays (link to blurb below) and a catalytic reactor for fuel conversion
consisting of pack of catalyst impregnated meshes (Microliths®). Fuel dispersion
was achieved by microfabricating the fuel distributor in Si using deep reactive
ion etching. Tests performed using JP-8 as the liquid fuel in a fully ceramic
combustor yielded a volumetric heat release rate as large as 270 MW/m3, a value
that is of the same order as that of conventional gas turbines. The small
overall combustor volume, at only 0.22 cm3, suggests that the large volumetric
energy density was achieved despite the device large surface-to-volume ratio and
attending heat losses. The fuel was fully oxidized, with CO/CO2 ratios well
under 1% over a range of equivalence ratios. Ancillary experiments in an
optically accessible combustor, enabling the measurements of droplet size and
velocity, revealed that the spreading of the electrospray by Coulombic repulsion
is the phenomenon controlling the volume of the mixing/evaporation chamber.
Droplet evaporation occurs in the thin (Peclet number dependent) thermal layer
preceding the catalytic section of the combustor. The design has the potential
of being scaled either up or down, depending on power needs. For more
information see article no. 5 in the publication list.
Synthesis of Drug/Polymer Micro-(Nano-)particles for Controlled/Targeted Release
(NSF sponsored)
Particle synthesis, especially at the nanonscale, is a burgeoning field with a
broad range of applications. The “holy grail” of research in this field is the
ability to generate particles with controlled and sometimes narrow size
distributions at adequate flow rates, as required by the application. The
electrospray, multiplexed by several orders of magnitude, may provide a valuable
path to this synthesis via either spray drying or pyrolysis. We have been
working on a well-controlled method to generate active agent/polymer micro-(nano-)particles
of different morphologies for controlled/targeted drug release using the
electrospray drying route. By judiciously selecting polymer molecular weight,
concentration, and solution flow rate, we can control not only the size but also
the morphology of the resulting polymer relics. We can generate either
spherical, monodisperse particles or tailed and elongated particles at the
microscale, as well as monodisperse nanoparticles. Recent work showed that
multiplexed electrosprays (MES) offer the possibility of coating these variously
shaped particles with emulsifiers that stabilize particles in solution or that
may facilitate further functionalization for targeted drug delivery. This
single-step process offers distinct advantages with respect to the commonly used
solvent-evaporation, including: enhanced encapsulation efficiency (>94%
encapsulation efficiency) of hydrophilic and hydrophobic agents, scale-up
potential, tight control over particle size and excellent reproducibility. In
addition, the ability of the spray drying to control the compactness of the
polymer matrix via the competition between diffusion-evaporation processes on
flight provides a secondary control over the release of drug from the generated
particles. We demonstrated the process with PLGA particles encapsulating
amphiphilic agents such as doxorubicin (DOX) or Rhodamine B. The work is
conducted in collaboration with the group of Professor T. Fahmy at Yale.
For more information see articles no. 16 and 24 in the publication list.
Electric Propulsion using Multiplexed Electrosprays of Ionic Liquids (AFOSR
sponsored)
The primary objective of our research program is
to develop a single propellant electrospray microthruster able to operate both
in colloidal and purely ionic regime: in the first the propellant is dispersed
as nanodroplets, whereas in the second each electrospray (ES) emits ions. The
approach exploits the ability of the electrospray to generate particles of
varying charge-to-mass ratio and is particularly promising to preserve high
propulsion efficiency throught a broad range of thrust and specific impulse.
Since the thrust per emitter, especially at the highest specific impulse, is
minuscule, it becomes crucial to multiplex the electrospray by orders of
magnitude. Silicon microfabrication is practically the only viable choice to
pursue our goal. At the ultralow flow rates per emitter, a new design of the
multiplexed device, is required in order to limit the evaporation of propellant
from the tip of the nozzle, thereby improving the system stability at the
smallest flow rates. Furthermore, a new design of the electrodes is needed to
avoid the erosion of the MES device upon impaction of ions/nanodroplest at
speeds on the order of Km/s and to further boost performances by
postaccelerating the ions/droplets via electrostatic lenses. Microfabrication is
currently pursued at the Center for Functional Nanomaterials at Brookhaven
National Laboratories. The research is conducted in collaboration with the group
of Professor J. F. de la Mora at Yale and Alameda Applied Sciences, Corp. For
more information see article no. 25 in the publication list.
Other applications in different development stages include:
• Chip cooling, under ARL sponsorship (see article no. 23 in the publication
list);
• Synthesis of ceramic nanoparticles via spray pyrolysis (internally funded);
and
• Control of deposit nanostructure for solar cell applications (internally
funded).
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