ACME is now under development with a set of five independent experiments which are each focused on advancing combustion technology through fundamental research.  Four of the current ACME experiments are specifically directed at addressing energy and environmental concerns, while the fifth experiment addresses fire prevention, especially for spacecraft.  The overall goals are to improve our understanding of materials flammability, combustion at fuel lean conditions where both optimum performance and low emissions can be achieved, flame stability and extinction limits, soot control and reduction, oxygen-enriched combustion which could enable practical carbon sequestration, and the use of electric fields for combustion control.

With the exception of the Burning Rate Emulator (BRE) experiment discussed immediately below, the general goal of the current ACME experiments is to gain fundamental understanding that can enable improved efficiency and reduced emissions in practical combustion processes on Earth, for example through the development and verification of models for chemical kinetics and transport processes in computational simulations.  In addition to enhanced performance, improved modeling capability can lead to reductions in the time and cost for combustor design.  In summary, microgravity investigations of non-premixed flames could lead to eco-friendly combustion systems providing our nation with green power for the future.

Burning Rate Emulator (BRE) Image of a normal-gravity flame ext- ending from a flat burner facing downward at an angle. In this conceptual test, a liquid fuel is being burned with a porous wicking burner. Unlike the other current ACME experiments, the Burning Rate Emulator (BRE) experiment is focused on fire prevention, especially in spacecraft.  Specifically, BRE’s objective is to improve our fundamental understanding of materials flammability, such as ignition and extinction behavior, and assess the relevance of existing flammability test methods for low and partial-gravity environments.  The burning of solid and liquid fuels will be simulated by using a flat porous burner fed with gaseous fuel.  The fuel flow rate will be controlled based on the measured heat flux (at the burner) and surface temperature, mimicking the dependence of condensed-phase fuel vaporization on thermal feedback.  A small number of gaseous fuels will be used to simulate the burning of fuels such as paper, plastic, and alcohol by matching properties such as the surface temperature and smoke production. BRE Aircraft Rig Video Initial Drop Tower Tests of the BRE Aircraft Rig April 18, 2012:  This video shows the initial drop tower tests of the BRE Aircraft Rig.  The rig has been designed and built to test the Burning Rate Emulator experiment concept, one of five major experiments that are planned for the Advanced Combustion via Microgravity Experiments (ACME) suite of investigations to be conducted on the International Space Station in 2016 and 2017.  BRE will study and characterize ignition and flammability of solid spacecraft materials using a gaseous analog.  In the video, the BRE rig is burning ethanol.  The flame is allowed to burn for a few seconds in 1-g and then it is dropped in a drop tower which provides reduced gravity.  The drop start is clearly evident as the flame shape changes.  The flame leans to one side (perhaps residual air flow from the retraction of the igniter) and flashes a couple times.  There are some particle tracers which seem to indicate the general flow direction from right to left.  The flame goes out at impact.  The rig will next be loaded into a modified Boeing 727-200 aircraft in late April 2012 for extended parabolic flights that will provide longer periods of reduced gravity, up to approximately 20 seconds.

Coflow Laminar Diffusion Flame (CLD Flame)   Image of a lifted flame of 50% propylene in a coflow of air (at ambient pressure) from an ex-ploratory test conducted on the International Space Station in 2009 as part of the Smoke Point In Coflow Experiment (SPICE). Research, especially including that already conducted in microgravity, has revealed that our current predictive ability is significantly lacking for flames at the extremes of fuel dilution, namely for sooty pure-fuel flames and dilute flames that are near extinction.  The general goal of the Coflow Laminar Diffusion Flame (CLD Flame) experiment is to extend the range of flame conditions that can be accurately predicted by developing and experimentally verifying chemical kinetic and soot formation submodels.  The dependence of normal coflow flames on injection velocity and fuel dilution will be carefully examined for flames at both very dilute and highly sooting conditions.  Measurements will be made of the structure of diluted methane and ethylene flames in an air coflow.  Lifted flames will be used as the basis for the research to avoid flame dependence on heat loss to the burner.  The results of this experiment will be directly applicable to practical combustion issues such as turbulent combustion, ignition, flame stability, and more.

Electric-Field Effects on Laminar Diffusion Flames (E-FIELD Flames)   Image of a gas-jet diffusion flame (in air at ambient pressure) from a test conducted in NASA's 2.2 Second Drop Tower. The flame is being forced downward by the electric field between the burner and an electrode mesh, which is at +2 kilovolts and is down-stream of the burner. Electric fields can strongly influence flames because of its effect on the ions present as a result of the combustion reactions.  The direct ion transport and the induced ion wind can modify the flame shape, alter the soot or flammability limits, direct heat transfer, and reduce pollutant emission.   The purpose of the Electric-Field Effects on Laminar Diffusion Flames (E-FIELD Flames) experiment is to gain an improved understanding of flame ion production and investigate how the ions can be used to control non-premixed flames.  Outside reviewers recently concluded that the experiment “… will contribute to our critical understanding to our knowledge of combustion processes in the presence of electric fields.”  The experiment will be conducted with a normal coflow flame (as in the CLD Flame experiment) or perhaps with a simple gas-jet flame, where there is no surrounding coflow.  An electric field will be generated by creating a high voltage (up to 10 kV) differential between the burner and a flat circular mesh suspended above (i.e., downstream of) the burner.  Measurements, as a function of field strength and fuel dilution, will be made of the ion current through the flame and the flame’s response time to electric forcing.

Flame Design   Image of a spherical diffusion flame on a porous burner (which is also visible) at the end of a test conducted in NASA’s 2.2 Second Drop Tower. From the burner, there was 1.51 mg/s of 100% ethylene flowing into air at atmospheric pressure. The primary goal of the Flame Design experiment is 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.  An outside review panel recently declared that Flame Design “… could lead to greatly improved burner designs that are efficient and less polluting than current designs."  Flame Design will investigate the soot inception and extinction limits of spherical microgravity flames, created in the same manner as for the s-Flame experiment.  Tests will be conducted with various concentrations of both the injected fuel (i.e., ethylene or methane) and the oxygen enriched atmosphere in order to determine the role of the flame structure on soot inception.  The effect of the flow direction on soot formation will be assessed with an inverse spherical flame unless such testing is not approved by the Payload Safety Review Panel (PSRP).  If inverse spherical flame testing is not allowed, the plan is to use a coflow burner, conducting both normal and inverse flame tests.  In the case of the inverse flames, the oxygen/inert mixture is injected from a central tube, while the fuel is ejected from a surrounding annulus.  The Flame Design experiment will explore whether the stoichiometric mixture fraction can characterize soot and flammability limits for non-premixed flames like the equivalence ratio serves as an indicator of those limits for premixed flames.

Structure and Response of Spherical Diffusion Flames (s-Flame) Image of a partially-premixed spher-ical flame on a porous burner (which is also visible). The microgravity test was conducted in a NASA drop facility. The gas issuing from the burner was 25% propane, 2% oxygen, 49% argon, and 24% nitrogen. The purpose of the Spherical Flame (s-Flame) experiment is to advance our ability to predict the structure and dynamics, including extinction, of both soot-free and sooty flames.  The spherical flame, which is only possible in microgravity, will be created through use of a porous spherical burner from which a fuel/inert gas mixture will issue into the CIR chamber.  Flames will be ignited at non-steady conditions and allowed to transition naturally toward extinction.  Tests will be conducted with various inert diluents, in both the fuel and chamber atmosphere.  The fuel gases include hydrogen and methane for soot-free flames, and ethylene for sooty flames.  One experiment objective is to identify the extinction limits for both radiative and convective extinction (i.e., at high and low system Damkohler numbers, respectively).  Another objective is to determine the existence, onset, and nature of pulsating instabilities that have been theoretically predicted to occur in such flames with fuel/diluent mixtures that are above a critical Lewis number.

ACME Status

February 2013 – Preparations are underway to conduct evaluation tests for the Burning Rate Emulator (BRE) experiment in NASA Glenn’s Zero Gravity Research Facility using a prototype burner developed by the University of Maryland investigators, Profs. Jim Quintiere and Peter Sunderland.  This follows forty exploratory tests that were conducted with a similar gas-fueled burner in November 2012 in NASA Glenn’s 2.2 Second Drop Tower.  In each drop facility, apparent weightlessness is momentarily achieved by letting the self-contained experiment freely fall down a vertical shaft.  The November tests were conducted in ambient air and sometimes revealed lifting phenomena and possible tip quenching when the fuel was methane or diluted methane.  Meanwhile, the ethylene flames remained robust throughout the 2.2-second test duration.  The “cup” burner used in the November tests was equipped with a heater, where its use had a significant effect on the ethylene flame as can be seen in the sample images below.

October 2012 – Detailed design is underway, where ACME passed an interim design review in June.  A successful Science Concept Review was held for the Burning Rate Emulator (BRE) experiment in August, where the external reviewers concluded that BRE “may offer critical guidance in flammability assessment in space vehicles.”  The Requirements Definition Review for BRE and the Critical Design Review for ACME are planned for June and November 2013, respectively.  The extra reviews are necessary for BRE because it wasn’t originally an ACME experiment and its design isn’t yet fully specified.  It is currently expected that ACME will begin testing on ISS in 2016.

April 2012 – Tests were recently completed on the International Space Station for the Structure & Liftoff In Combustion Experiment (SLICE) which is a precursor to ACME’s Coflow Laminar Diffusion Flame (CLD Flame) experiment.  The SLICE results will enable refinement of the CLD Flame test matrix and operating procedures so as to maximize its scientific outcome.  You can learn more about SLICE at its Facebook page.

January 2012 – A fifth experiment was added to the ACME project, Burning Rate Emulator (BRE), where the investigators are Profs. J.G. Quintiere and P.B. Sunderland of the University of Maryland.  BRE’s objective is to improve our fundamental understanding of materials flammability and assess the relevance of existing flammability test methods for low and partial-gravity environments.  The burning of solid and liquid fuels will be simulated using a flat porous burner, where the flow rate of gaseous fuel will be controlled based on the thermal feedback to the burner.

January 2011 - The Advanced Combustion via Microgravity Experiments (ACME) Preliminary Design Review (PDR) was held on January 28, 2011.  The Project team demonstrated that the preliminary design meets all system requirements with acceptable risk and within cost and schedule constraints.  The review board has recommended that the project proceed with detailed design.

May 2010 - The Advanced Combustion via Microgravity Experiments (ACME) Requirements Definition Review (RDR) was held for two days, May 10-11, 2010  The Science Requirements Document (SRD) was signed by all parties except for one PI who had to leave early before the signature page was prepared.  

August 2009 -  The ACME project is conducting drop tower testing at the Glenn Research Center’s 2.2 second drop tower with the ACME E-Fields rig.  The drop tower tests are focusing on the high voltage field effects on flames, these tests were conducted during the month of July 2009 by the project scientist and summer intern.

Objective

Modular apparatus designed for gaseous fuel investigations to study:

• combustion structure and stability near flammability limits,
• soot inception, surface growth, and oxidation processes,
• emission reduction through nitrogen exchange,
• combustion stability enhancements via an electric field.
  

Relevance/Impact

• Verified computational models that will enable the design of high efficiency, low emission combustors operating at near-limit conditions.
• Reduced design costs due to improved capabilities to numerically simulate combustion processes.
• Efficient soot control strategies for industrial applications.
  

Development Approach

• Flight design leverages off previous flight design heritage.
• Multi-user, re-usable apparatus minimizing up-mass/volume, costs, and crew involvement.

Four flame designs to be studied by ACME