Results are reported from recent tests where hydrothermal flames spontaneously ignited in a Supercritical Water Oxidation (SCWO) Test Cell. Hydrothermal flames are generally categorized as flames that occur when appropriate concentrations of fuel and oxidizer are present in supercritical water (SCW); i.e., water at conditions above its critical point (218 atm and 374 C). A co-flow injector was used to inject fuel, comprising an aqueous solution of 30-vol to 50-vol ethanol, and air into a reactor held at constant pressure and filled with supercritical water at approximately 240 atm and 425 C. Hydrothermal flames auto-ignited and quickly stabilized as either laminar or turbulent diffusion flames, depending on the injection velocities and test cell conditions. Two orthogonal views, one of which provided a backlit shadowgraphic image, provided visual observations. Optical emission measurements of the steady state flame were made over a spectral range spanning the ultraviolet (UV) to the near infrared (NIR) using a high-resolution, high-dynamic-range spectrometer. Depending on the fuel air flow ratios varying degrees of sooting were observed and are qualitatively compared using light absorption comparisons from backlit images

Hegde, Uday G.

Hicks, Michael C.

Kojima, Jun J.

NASA Technical Reports Server

Sub Topic: Coal and Biomass Combustion and Gasiﬁcation
10th US National Combustion Meeting
Organized by the Eastern States Section of the Combustion Institute
April 23-26, 2017
College Park, Maryland
Spontaneous Ignition of Hydrothermal Flames
in Supercritical Ethanol/Water Solutions
Michael C. Hicks 1,
∗
, Uday G. Hegde 2, Jun J. Kojima 3
1NASA - Glenn Research Center, Cleveland, Ohio 44135, USA
2Case Western Reserve University, Cleveland, Ohio 44135, USA
3Ohio Aeropsace Institute, Cleveland, Ohio 44135, USA
Corresponding Author e-mail: michael.c.hicks@nasa.gov
Abstract: Results are reported from recent tests where hydrothermal ﬂames spontaneously
ignited in a Supercritical Water Oxidation (SCWO) Test Cell. Hydrothermal ﬂames are gen-
erally categorized as ﬂames that occur when appropriate concentrations of fuel and oxidizer
are present in supercritical water (SCW); i.e., water at conditions above its critical point (218
atm and 374 °C). A co-ﬂow injector was used to inject fuel, comprising an aqueous solution of
30%-vol to 50%-vol ethanol, and air into a reactor held at constant pressure and ﬁlled with su-
percritical water at approximately 240 atm and 425 °C. Hydrothermal ﬂames auto-ignited and
quickly stabilized as either laminar or turbulent diﬀusion ﬂames, depending on the injection
velocities and test cell conditions. Two orthogonal views, one of which provided a backlit shad-
owgraphic image, provided visual observations. Optical emission measurements of the steady
state ﬂame were made over a spectral range spanning the ultraviolet (UV) to the near infrared
(NIR) using a high-resolution, high-dynamic-range spectrometer. Depending on the fuel/air
ﬂow ratios varying degrees of sooting were observed and are qualitatively compared using light
absorption comparisons from backlit images.
Keywords: hydrothermal ﬂame, supercritical water oxidation, jet injection, high pressure
1. Introduction
The possibility of hydrothermal ﬂames was ﬁrst posited by E.U. Franck in 1985 when noting
that because of the high miscibility of hydrocarbons and oxygen in supercritical water (i.e.,
above 218 atm and 374 °C) that “... even the generation of ﬂames in such phases can be
considered.” [1] As such, a “hydrothermal ﬂame” is a classiﬁcation of ﬂames that occur in
conditions when the environment is largely comprised of water at supercritical conditions.
Historically, supercritical water oxidation (SCWO) technologies have depended on maintain-
ing conditions in the SCWO reactor where spontaneous ignition of localized hydrothermal
ﬂames was suppressed and the complete oxidation of hydrocarbon wastes occurs at relatively
low temperatures. It was recognized that these ﬂames, if not properly controlled in reactors
for which these conditions were not designed, would lead to accelerated thermal wear on re-
actor components, would accelerate corrosion, and depending on the reactant stream, would
potentially result in increases in NOx or other unwanted products.
1
https://ntrs.nasa.gov/search.jsp?R=20170006847 2019-08-31T06:42:37+00:00Z
Sub Topic: Coal and Biomass Combustion and Gasiﬁcation
Recently, a number of SCWO technologies and advanced reactor concepts have been proposed
where controlled hydrothermal ﬂames are used beneﬁcially. This includes hydrothermal
ﬂames for thermal augmentation to initiate or sustain reactions [2–4] , or as a means of
increasing conversion eﬃciencies for traditionally diﬃcult waste streams [5] , or for new
applications, such as for hydrothermal spallation drilling [6, 7].
The purpose of this work is to explore conditions when a mixture of a proxy waste stream,
(i.e. in this work, ethanol and water) is spontaneously ignited using a co-ﬂow injection
conﬁguration where the fuel/water mixture comprise the core and air comprises the annular
region of a co-ﬂow jet. This co-ﬂow jet is injected into supercritical water at temperatures
of 425 °C and a constant pressure of approximately 240 atm. Characteristics of both laminar
and turbulent hydrothermal ﬂames, their sooting propensity as a function of ﬂow conditions,
and their measured emission spectra are studied.
2. Experimental Setup
2.1. Hardware Description
The SCWO Test Cell (Figure 1) is machined from Inconel 625 with a maximum design
pressure of 340 atm at 538 °C and is typically operated at conditions up to 250 atm at tem-
peratures up to 450 °C. The total liquid test cell volume is 57 cm3 and consists primarily of
the two orthogonal window bores 3.75 cm in diameter and 5.3 cm long. The end of each win-
dow bore is closed with a 4.13 cm diameter 2.54 cm thick sapphire window with the C-axis
perpendicular to the window face.
The SCWO Test Cell (hereinafter “reactor”) is heated by four electric cartridge heaters
rated at 100Watts each and located in four holes symmetrically placed around the center
and in the body of the reactor. There is also an electric heater located on each of the two
inlet lines to pre-heat the test ﬂuid to temperatures just above the bulk ﬂuid temperature
before entering the reactor. The injector, as shown in the inset of Figure 1 (a) shows the
arrangement of the co-ﬂow cross-sectional areas, having a ratio of 16:1 between the annular
and core ﬂow areas.
Two cameras are used for imaging the ﬂame and the injection hydrodynamics and the window
port opposite the color camera is blanked oﬀ with a 2.54 cm thick SS-304 disc through
which four thermowells, each clocked at 90 ° and protruding to varying terminal distances,
are inserted. The four thermowells each accommodate a thermistor (Omega 0.59mm RTD
probes) and are used for determining the local ﬂuid temperature. However, because of the
thin ﬂame zone, the extremely steep temperature gradients, and the thermal inertia of the
thermowells the thermistors are not suitable for providing ﬂame temperatures. In fact the
diﬃculty of directly measuring temperatures of hydrothermal ﬂames was noted in earlier
work [8] and this will be the focus of subsequent work.
Optical emission spectroscopy was performed to measure spectral intensity in ultraviolet
to near-infrared along the burner nozzle center axis at varying heights. Optical emissions
were collected using a Nikon UV-Nikkor lens (105-mm, f/4.5) onto a single-core optical ﬁber
(Ocean Optics, 0.6 mm core diameter). The light was guided into an aberration-reduced
imaging spectrometer (i.e., IsoPlane 160 Princeton Instruments with a focal length 203 mm,
2
Sub Topic: Coal and Biomass Combustion and Gasiﬁcation
co-flow injector port 
(units in mm)
(a) (b)
Figure 1: Schematic showing (a) SCWO Test Cell in a “three window” test conﬁguration comprising
orthogonal view ports with one axis (into paper) used for back-lit shadowgraphy (collimated backlight
projecting a shadow image into a high resolution B/W camera) and the orthogonal axis used for ﬂame
imaging (left side of cell) and the opposing side used for four thermowells protruding into the ﬂame zone (b)
a layout of the operational conﬁguration showing the SCWO Test Cell packed in ceramic insulation (rotated
90° from Fig. 1(a)).
stop set at f/3.88, slit width 20μm and grating of 300 grooves/mm) and was dispersed onto
a back-illuminated imaging CCD camera (i.e., PIXIS 400BR Princeton Instruments, with
a 1340×400 pixel detector and a bit depth of 16 bit). Spectral signals were binned over
a height of 14 pixels and typically accumulated on CCD chip over 45 seconds to increase
signal-to-noise ratio and all measurements were line-of-sight.
2.2. Experiment Procedures
For the tests reported in this work the fuel, comprising aqueous solutions of ethanol from
30%-v to 50%-v, was injected through the core and air was injected through the annulus.
The reactor’s bulk ﬂuid, water, was heated to 400 °C and pressurized to 238 atm at whch
point the core ﬂow, 30%-v C2H6O(aq) , was initiated at 1.0ml/min. Once the core fuel ﬂow
was stabilized the annular air ﬂow was initiated at an initial ﬂow rate between 1.0ml/min
to 3.0ml/min. Following ignition the core and annular ﬂow rates were adjusted to the
targeted steady state test conditions; with ﬂow rates independently set between 0.1ml/min
to 2.0ml/min for each ﬂow stream. It should be noted that the air ﬂow was adjusted
manually with a precision metering valve and was diﬃcult to precisely determine at low ﬂow
rates due to the limitations of the ﬂow meter.
3. Results and Discussion
3.1. Flame Characterization
Two representative hydrothermal ﬂames are presented in Figure 2 along with the backlit
images of the co-ﬂow jets that produced them. These ﬂames are referred to, hereinafter,
simply as Flame A (Figure 2 (a) - right) and Flame B (Figure 2 (b) - right) for convenience.
Both ﬂames have a core fuel jet comprising 30%-v C2H6O(aq) with an annular air co-ﬂow.
3
Sub Topic: Coal and Biomass Combustion and Gasiﬁcation
 1 mm
 10 mm
 20 mm
(a) (b)
Figure 2: Representative co-ﬂow hydrothermal ﬂames with 30%-v C2H6O(aq) injected from the core with
air ﬂow in the annulus; showing (a) a high ﬂow rate air/fuel injection resulting in a partially turbulent jet
and a steady state “brush” ﬂame and (b) a reduced ﬂow ﬂow rate air/fuel injection resulting in a laminar
jet and a weak blue ﬂame (may be diﬃcult to see in print).
(a) (b)
Figure 3: (a) Backlit image of co-ﬂow jet with a heavily sooting ﬂame generated from 50%-v C2H6O(aq)
ﬂowing at 1.0ml/min (core) with air ﬂowing at 1.1 ml/min (annulus) and (b) two backlit images comparing
obscuration of the collimated backlight due to the soot ﬁeld immediately following ﬂame extinction (left)
and 217 s following ﬂame extinction (right).
However, for Flame A, the air ﬂow rate is an order of magnitude higher than for Flame B,
which is evident when comparing the corresponding backlit images of the jet ﬂow in Figure 2.
Under certain conditions hydrothermal ﬂames can be be highly sooting, as illustrated by
the ﬂame presented in Figure 3. This shows a steady state co-ﬂow ﬂame with fuel, 50%-v
C2HO(aq), injected from the core at 1ml/min with an annular air ﬂow rate of 1.1ml/min.
The bulk ﬂuid in the reactor was at 430 °C with a pressure of 230 atm. Soot particles
were uniformly dispersed with little apparent agglomeration or settling. Following ﬂame
extinction the reactor was held at supercritical conditions for an extended period of time
to observe dissolution, which appeared to occur based on qualitative comparisons of the
collimated light’s intensity ﬁeld with time, as seen in Figure 3 (b).
3.2. Flame Spectra
Spectral measurements of ﬂame emissions, although generally not quantitative, can provide
useful insights into reactant mixing, reaction speciation, and reaction rates during combus-
tion processes with a relatively simple diagnostic setup. Flame spectral emission measure-
4
Sub Topic: Coal and Biomass Combustion and Gasiﬁcation
(a) (b)
Figure 4: (a) Optical emission spectra from the hydrothermal ﬂame, measured at various axial locations
from x = 1mm to 20mm where inset shows peak of OH* chemiluminescence at x = 10mm and (b) optical
emission spectra from the hydrothermal ﬂame, measured at an axial location of x=15 mm for diﬀerent air
coﬂow volumetric ﬂow rates.
ments, brieﬂy described in Section 2.1, are presented for Flame A and Flame B presented
earlier in Figure 2 (a) and Figure 2 (b).
Emission spectra for Flame A in the wavelength range of 290 nm to 690 nm are shown in
Figure 4 (a) for three heights above the burner tip, i.e., for x = 1mm, 10mm, and 20mm.
Consider ﬁrst the spectrum at x = 1mm which is at the base of the ﬂame where it appears
blue, Figure 2 (a). Broadband emission is observed in the wavelength range of 350 to 540 nm
and is generally attributed to CO2*, which may arise from a recombination of CO with an
O atom or collisional excitation of a CO2 molecule from its ground state. It is likely that
CH* emission is masked by the CO2* emission. The line at 590 nm is due to sodium and is
likely the result of contamination of the air stream. A weak OH* signal (around 310 nm) is
observed as shown in the inset. At the locations x = 10 mm and x = 20 mm the spectrum is
dominated by broadband emissions above 400 nm which is likely due to black body radiation
from soot. This emission is much stronger at x = 20 mm which is in the yellow turbulent
brush of the ﬂame.
Flame B is a blue non-sooty ﬂame (may be diﬃcult to see in print) and its emission spectra is
shown for a single height (x= 10 mm) in Figure 4 (b). This location is in the blue turbulent
brush region of the ﬂame and the CO2* emission bands are observed in the 350 nm to 540 nm
wavelength range. There is no sodium line at 590 nm, which is consistent with the likelihood
of it being a contaminant in the air line since the air ﬂow rate is signiﬁcantly lower for
Flame B compared to Flame A. The emission spectra for Flame A, taken at the same height
of 15 mm above the burner tip is also shown. Comparison of the two spectra highlight the
observation that no OH* emissions or signiﬁcant broadband emissions from soot are observed
for Flame B. The general conclusion is that Flame B is much weaker in intensity compared
to Flame A and, considering the apparent lack of OH* or soot, there may be signiﬁcant
diﬀerences in reaction pathways.
5
Sub Topic: Coal and Biomass Combustion and Gasiﬁcation
4. Conclusion
Results are reported from recent tests where hydrothermal ﬂames spontaneously ignited in a
Supercritical Water Oxidation (SCWO) Test Cell. A co-ﬂow injector was used to inject fuel,
comprising an aqueous solution of 30%-vol to 50%-vol ethanol and air into a reactor held at
constant pressure and ﬁlled with supercritical water at approximately 240 atm and 450 °C.
Optical emission measurements of the steady state ﬂames were made over a spectral range
spanning the ultraviolet (UV) to the near infrared (NIR). Depending on the fuel/air ﬂow
ratios varying degrees of sooting were observed. Future work will provide additional ﬂame
emission measurements to identify dominant reactant species, ﬂame temperature measure-
ments, and data on soot morphology and size distributions of these hydrothermal ﬂames.
Acknowledgements
This work is funded under the Physical and Life Sciences Program managed by Francis P. Chiara-
monte at NASA Headquarters and managed locally by William M. Foster at NASA - Glenn Re-
search Center. The authors would like to acknowledge the signiﬁcant contributions of Daniel J.
Gotti for his engineering support during the hardware design and assembly phase as well as his con-
tinued support during the conduct of these tests. The authors would also like to acknowledge the
contributions of Jay C. Owens for his engineering design of the imaging and data storage system.
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Spontaneous Ignition of Hydrothermal Flames in Supercritical Ethanol Water Solutions

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