ACME is focused on advanced combustion technology via fundamental microgravity research. The primary goal is to improve efficiency and reduce pollutant emission in practical terrestrial combustion. A secondary objective is fire prevention, especially for spacecraft.
Currently, ACME includes five independent experiments (see ACME Experiments below) investigating laminar, gaseous, non-premixed flames.
The ACME design is complete and the engineering hardware is being assembled for integrated testing. On-orbit testing is expected to begin in 2016 and continue for a few years.
B. Ma, S. Cao, D. Giassi, D.P. Stocker, F. Takahashi, B.A.V. Bennett, M.D. Smooke, and M.B. Long, “An experimental and computational study on soot formation in a coflow jet flame under microgravity and normal gravity,” to be presented, 35th International Symposium on Combustion, San Francisco, CA, 3-8 August, 2014.
S. Cao, B. Ma, B.A.V. Bennett, D. Giassi, D.P. Stocker, F. Takahashi, M.B. Long, and M.D. Smooke, “A computational and experimental study of coflow laminar methane/air diffusion flames: Effects of fuel dilution, inlet velocity, and gravity,” to be presented, 35th International Symposium on Combustion, San Francisco, CA, 3-8 August, 2014.
SLICE was conducted on the International Space Station in early 2012 to prepare for ACME’s Coflow Laminar Diffusion Flame (CLD Flame) experiment. CLD Flame is led by Yale professors Marshall Long and Mitch Smooke, whose combined experimental and computational research (including SLICE) is described in the 2012 Yale Engineering magazine’s cover story. Two ACME posters will also be presented at the symposium:
D.P. Stocker, F. Takahashi, J.M. Hickman, and A.C. Suttles, “Advanced Combustion via Microgravity Experiments: Planned International Space Station Research on Gaseous Flames,” Work-in-Progress Poster to be presented, 35th International Symposium on Combustion, San Francisco, CA, 3-8 August, 2014.
Y. Zhang, M. Kim, P.B. Sunderland, J.G. Quintiere, J. De Ris, D.P. Stocker, F. Takahashi, P.V. Ferkul, “Burning Rate Emulator for Microgravity Studies,” Work-in-Progress Poster to be presented, 35th International Symposium on Combustion, San Francisco, CA, 3-8 August, 2014.
Finally, an updated list of ACME Publications and Presentations is now available online.
ACME NUMBERS 3, 5, 8, 11
5 current ACME experiments (see below).
8 universities with which ACME investigators have been affiliated, but where investigator Prof. Felix J. Weinberg of Imperial College London regrettably died in 2012. There are also ACME investigators from the NASA Glenn Research Center, which is the agency’s lead for microgravity combustion research.
11 current investigators for the ACME experiments.
Objective: Fire Safety- To improve our fundamental understanding of materials flammability, such as extinction behavior and the conditions needed for sustained combustion, and to assess the relevance of existing flammability test methods for low and partial-gravity environments.
Flame/Burner: Flat perforated disk fed with gaseous fuel to simulate the burning of solid and liquid fuels, where measurements are made of the thermal feedback upon which the vaporization of such fuels depend.
Principal Investigator: Prof. James G. Quintiere, University of Maryland
Co-Investigator: Prof. Peter B. Sunderland, University of Maryland
Objective: Energy & Environment – To extend the range of flame conditions that can be accurately predicted by computational models, especially for highly dilute and heavily sooting conditions.
Flame/Burner: Coflow flame where the gaseous fuel issues from an inner tube which is centered within a much larger outer tube, where a mixture of oxygen and nitrogen issues from the annulus.
Principal Investigator: Prof. Marshall B. Long, Yale University
Co-Investigator: Prof. Mitchell D. Smooke, Yale University
Objective: Energy & Environment – To gain an improved understanding of ion production in flames and investigate how an electric field can be used to control flames through those ions.
Flame/Burner: Gas-jet flame, where (1) the gaseous fuel issues from a small circular tube into a still atmosphere, and (2) a high-voltage electric field is established between the burner and disk-shaped copper mesh centered a few centimeters “above” the burner. Electric-field tests may also be conducted with the coflow burner used for the CLD Flame experiment.
Principal Investigator: Prof. Derek Dunn-Rankin, University of California – Irvine
Objective: Energy & Environment – To improve our understanding of soot inception and control in order to enable the optimization of oxygen-enriched combustion and the “design” of non-premixed flames that are both robust and soot free.
Flame/Burner: Spherical flame – which is only possible in microgravity – where the burner gas (fed through a small tube) issues from a porous spherical burner. Tests are planned for both (1) normal flames, where the fuel flows into an oxygen/inert atmosphere, and (2) inverse flames, where an oxygen/inert mixture flows into a fuel atmosphere.
Principal Investigator: Prof. Richard L. Axelbaum, Washington University in St. Louis
Co-Investigators: (1) Prof. Beei-Huan Chao, University of Hawaii; (2) Prof. Peter B. Sunderland, University of Maryland; (3) Dr. David L. Urban, NASA Glenn Research Center
Objective: Energy & Environment – To advance our ability to predict the structure and dynamics, including extinction, of both soot-free and sooty flames.
Flame/Burner: Spherical flame – which is only possible in microgravity – where the gaseous fuel (fed through a small tube) issues from a porous spherical burner into a still atmosphere.
Principal Investigator: Prof. C.K. Law, Princeton University
Co-Investigators: (1) Prof. Stephen D. Tse, Rutgers University; (2) Dr. Kurt R. Sacksteder, NASA Glenn Research Center
ACME BENEFITS & RELEVANCE
Why study combustion?
In the United States, nearly 70% of our electrical energy is generated through the combustion of fossil fuels. For example, in 2012, electricity was generated in the U.S. by burning the following fuels, where the percentages indicate the fraction of the total U.S. electrical generation: coal (37%), natural gas (30%), biomass (1.4%), and petroleum (1%).
Our transportation is heavily reliant on combustion, even for electric vehicles because most of our electricity is generated by combustion.
With combustion, we heat our homes, water, food, etc. and also generate heat for industrial processes.
Our reliance on imported fuel contributes to our national trade deficit and affects our national security.
Combustion is a leading man-made source of greenhouse gases, where carbon dioxide is the most important example.
Combustion is the primary man-made contributor to acid rain.
Soot contributes to global warming and is a health problem.
Given our pervasive use of combustion as an energy source, the U.S. consumes fossil fuels which cost on the order of a trillion dollars annually. Therefore, even small improvements in combustion efficiency would significantly reduce fuel needs and pollutant production.
Why study combustion in microgravity?
Flames are strongly affected by gravity because the high-temperature combustion gases are much less dense than the cooler atmosphere which surrounds the flame. Gravity pulls more forcefully on the denser atmosphere and the hot gases are pushed upward as a result. This gravity-driven upward motion of material that is less dense that the surrounding fluid is referred to as buoyancy. This gravity-driven motion, referred to as buoyant convection, feeds the flame with fresh reactant – normally oxygen (in the air) – and removes the combustion products (e.g., carbon dioxide and water vapor) from the flame vicinity.
Low-momentum flames are dramatically influenced by the effective elimination of buoyant convection, where the resulting effects are often advantageous for analysis.
Spherically symmetric flames can be created enabling one-dimensional analysis. Two of the current ACME experiments take advantage of this feature.
Flicker, which is a buoyancy-driven (i.e., gravity-driven) instability, is eliminated yielding quasi-steady flames.
Length scales are increased in microgravity flames facilitating analysis of the flame structure.
Momentum-dominated flames, which are important for most practical combustion, can be studied at low velocities to simplify analysis.
Microgravity flames tend to have a much stronger sensitivity to their atmosphere and exhibit a much broader range of characteristics than normal-gravity flames because of the near absence of buoyant entrainment.
The long residence times in microgravity flames can lead to strong soot production, but many microgravity flames are soot free.
Microgravity flames are great for studies of limit and stability behavior where chemical kinetics are important. Soot, extinction, and stability limits are being studied in the ACME experiments.
Microgravity is of course the appropriate environment for studies related to spacecraft fire safety.
Why study combustion on the ISS?
Microgravity durations in drop facilities, such as the 2.2 Second Drop Tower and 5.18-second Zero Gravity Research Facility, are (1) too short for soot to achieve quasi-steady conditions, and (2) too short to establish a flame and then vary its flow rate, for example, to investigate stability or extinction limits.
While research aircraft flying in parabolic maneuvers can provide reduced gravity durations of ~20 seconds, low-momentum flames are often dramatically disturbed by the aircraft vibrations. Although the jitter can be avoided if the experiment is floated within the aircraft, that reduces the low-gravity duration to mere seconds.
Space-based testing is often necessary to achieve microgravity conditions of sufficient duration and quality for combustion research.
Cool-flame extinction during n-alkane droplet combustion in microgravity (Vedha Nayagam, Daniel L. Dietrich, Michael C. Hicks, Forman A. Williams)
ABSTRACTRecent droplet-combustion experiments onboard the International Space Station (ISS) have revealed that large n-alkane droplets, following radiative extinction of the visible flame, can continue to burn quasi steadily in a low-temperature regime, characterized by negative-temperature-coefficient (NTC) chemistry. In this study we report experimental observations of n-heptane, n-octane, and n-decane droplets of varying initial size burning in oxygen/nitrogen, oxygen/nitrogen/carbon dioxide, and oxygen/nitrogen/helium environments at pressures from 0.5 to 1.0 atm, with oxygen concentrations from 14% to 25% by volume. These large n-alkane droplets exhibited radiative extinction of the hot flame, followed by quasi-steady low-temperature burning, which terminated with diffusive extinction accompanied by the formation of a vapor cloud, while small droplets did not exhibit radiative extinction but instead burned to completion or disruptively extinguished. Results for droplet burning rates in both the hot flame and cool-flame regimes, as well as droplet extinction diameters at the end of each stage, are presented. The cool-flame extinction diameters for all three n-alkanes are shown to follow a similar trend as functions of the oxygen concentration, predicted here from a simplified theoretical model that is based on the reaction-rate parameters for the oxygen molecule addition to the alkyl radical and for ketohydroperoxide decomposition.
2015 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Asymptotic Analysis of Quasi-Steady Heptane Droplet Combustion Supported by Cool-Flame Chemistry (Kalyanasundaram Seshadri, Norbert Peters, Forman Williams, Vedha Nayagam)
ABSTRACTA skeletal chemical-kinetic mechanism for heptane cool flames is simplified to the maximum extent possible by introduction of steady-state approximations for intermediaries, following procedures employed previously. A pair of ordinary differential equations in mixture-fraction space is thereby obtained, describing the quasi-steady structures of the temperature and heptylketohydroperoxide (KET) fields. Application of activation-energy asymptotics for the partial-burning regime to this pair of equations is shown to provide convenient expressions for flame structures and extinction. With the mixture-fraction co-ordinate related to radius, these results are used to address droplet-combustion experiments that have been performed in the International Space Station (ISS). Droplet diameters at extinction are predicted as functions of the oxygen concentration in the atmosphere and are compared with experiment. While the results are encouraging, there are noticeable differences that point to deficiencies in the analysis resulting from oversimplifications. Further investigation therefore is recommended.
Combustion Theory and Modelling Journal
Experimental Observations of Cool-Flame Supported Binary-Droplet Arrays Combustion in Microgravity (Daniel L. Dietrich, Vedha Nayagam, Forman A. Williams)
ABSTRACTRecent droplet combustion experiments conducted on board the International Space Station (ISS) showed that large, isolated n-alkane droplets can burn quasi-steadily following radiative extinction, supported by cool-flame chemistry in the Negative Temperature Coefficient (NTC) region. In this study we report preliminary experimental results from the ongoing binary-droplet arrays experiments. For the first time, it is shown that quasi-steady combustion of cool flames can be supported by binary-droplet arrays. Under some conditions binary droplets support cool flames when a single droplet of similar size burns to completion with hot flame. The large, merged hot flame of the binary-array leads to the necessary thermal and species fields that can transition to cool flame combustion following radiative extinction, unlike the single droplet in that ambient environment. These observations may have important implications with regard to spray combustion.
9th U. S. National Combustion Meeting
Organized by the Central States Section of the Combustion Institute
May 17-20, 2015
Project Manager: J. Mark Hickman, NASA Glenn Research Center, 216-977-7105, firstname.lastname@example.org
Deputy Project Manager: Andrew C. Suttles, NASA Glenn Research Center, 216-433-8328, email@example.com
Project Scientist: Dennis Stocker, NASA Glenn Research Center, 216-433-2166, Dennis.P.Stocker@nasa.gov
Deputy Project Scientist: Dr. Fumiaki Takahashi, Case Western Reserve University, 216-433-3778, firstname.lastname@example.org
Engineering Team: ZIN Technologies, Inc.