Structure & Liftoff In Combustion Experiment(SLICE)

Background

Coflow laminar diffusion flames (as shown schematically on the left) are especially valuable for studies of combustion because of the availability of accurate numerical modeling with that flame configuration.  In particular, excellent agreement can be achieved when the flow conditions are such that the flame detaches and lifts above (i.e., moves downstream of) the nozzle.  A coupled experimental and numerical investigation can enable validation and improvements to combustion modeling. For example, the image to the right is not a photo, but a numerical simulation of a 40% ethylene flame. Enhanced modeling capability is important because it can reduce time and cost in the design of practical combustion devices.  Furthermore, flame attachment to (or detachment from) a burner or condensed-fuel surface is of essential importance in both combustion systems and fire safety.  The flame attachment point controls the stability of the entire trailing diffusion flame.

Microgravity testing allows for greater temporal and spatial scales and a broader range of flame characteristics than can be achieved in normal gravity.  As one example of the NASA-recognized value of such studies, the Coflow Laminar Diffusion Flame (CLD Flame) experiment of Marshall B. Long and Mitchell D. Smooke (both of Yale U.) is currently in development for conduct in the Combustion Integrated Rack (CIR) on the International Space Station, as part of the Advanced Combustion via Microgravity Experiments (ACME) project.

The overall goal of the proposed study is to improve our understanding of the physical and chemical processes controlling diffusion (i.e., non-premixed) flame structure and lifting phenomena (i.e., stabilization) and to provide for rigorous testing of numerical models, including thermal radiation, soot formation, and detailed chemical kinetics.  As part of this aim, an important purpose of the SLICE investigation is to conduct preliminary microgravity studies that will maximize the scientific return of the subsequent CLD Flame experiment and mitigate associated risks.  In other words, SLICE is a precursor to the CLD Flame experiment.

Objectives

1. Identify the lift-off velocity limits in air for flames of methane, ethylene, and selected dilutions of each fuel as a function of the nozzle size.  When possible, also identify the blow-out velocities (i.e., when the flame extinguishes). This will enable refinement of the CLD Flame test matrix where the fuels for that experiment are those specified here.
   
   
2. Characterize the hysteresis of flame lifting and reattachment (i.e., stabilization) with variations in flow conditions as a function of the fuel and nozzle size. This will enable further refinement of the CLD Flame test matrix.
   
   
3. Characterize the structure of the lifted flame as a function of the fuel, nozzle size, and flow conditions. This will enable refinement of the flame diagnostic settings and test matrix for the CLD Flame experiment.  It will also allow for a preliminary assessment of the numerical codes’ ability to predict the flame structure under the broader conditions which are only found in microgravity flames.
   
   
4. Characterize the structure of the flame, and especially its base (i.e., stabilizing region), from attached through lifted conditions as a function of the fuel, nozzle size, and flow conditions. This will enable rigorous testing of numerical models beyond that planned for the CLD Flame experiment, extending capabilities to incorporate burner heat loss.

Approach

The experimental hardware for the Smoke Points In Co-flow Experiment (SPICE), of David L. Urban (NASA Glenn) and Peter B. Sunderland (U. Maryland), which is currently onboard the International Space Station, was built to allow studies of coflow laminar diffusion flames.  The SPICE hardware is within the ISS MSG in the image to the right.  While the SPICE investigation has been specifically focused on a study of soot production and oxidation within flames, the hardware can be used without modification to conduct the SLICE experiment.  In terms of the experimental hardware, the only additional requirements for SLICE are more fuel (i.e., gas bottles), recording media, and minor hardware elements such as new nozzle(s).  The three existing SPICE nozzles are all smaller than the nozzle planned for the CLD Flame experiment.  Of course, the SLICE testing could most benefit the CLD Flame experiment by bracketing and/or including the same nozzle size.  It is also possible that screen(s) would be flown to alter the velocity profile of the coflow.

The SLICE operating procedures will have some differences to the standard SPICE procedures given the differing objectives.  However, those changes are fully within the capabilities of the SPICE hardware, as demonstrated by exploratory testing that has already been conducted on orbit.  As a simple example, the standard SPICE procedure calls for a fixed air velocity and an increase of the fuel flow until the smoke point is reached.  In contrast, SLICE will include testing where the fuel flow is fixed and the air velocity is incrementally increased until the diffusion flame detaches and lifts off from the nozzle.  In all cases, still and video measurements of the flame structure will be made for comparison with detailed numerical computations.  Given that the capture of the lifting processes in normal gravity is extremely difficult, SLICE will provide valuable photographic observations on the transient flame behavior.

The lifted nature of the flames can be discerned from the flame shape in the example images (which are not at the same scale) and the distance from the nozzle tip (which is not visible). While the case(s) on the left may look similar to attached flames, the outward fuel-lean flare of each flame’s base reveals it’s lifted nature.