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.

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.