Compact & Efficient Combustors

Direct-connect Combustor Studies

The backpressure requirement is usually hard to meet  and the greater the backpressure, the shorter the test time will be with a given plenum pressure. Also, with a full flow path, the combustor analysis may be biased due to the upstream inlet conditions.

To avoid these issues, high-speed combustion research is often conducted via direct-connect types. This configuration directly connects the convergent-divergent (C/D) nozzle of the wind tunnel to the inlet of the combustor, isolating the inlet effect. The top left figure presents a typical flow path of a supersonic cavity-stabilized combustor.

The resulting flows can be diagnosed with various techniques, including flame imaging, wall pressure measurements, and laser absorption spectroscopy (LAS).

Efficient & Effective Flameholders: Wire Inserts

Two common practices of supersonic flameholders are cavities and struts. Cavity flameholders induce a recirculating flow inside, increasing the flow residence time with minimal losses. On the other hand, strut flameholders produce strong shocks to elevate the pressure/temperature of the flow and create vorticity to enhance mixing at a cost of total pressure. 

To combine the benefits of the two, mesh wires are inserted in the flow path of a cavity-stabilized combustor. This attempts to create a distributed deceleration region for enhanced mixing and recirculating zones for longer residence times.

CH* Chemiluminescence Result

The mesh insert combustors were tested at an equivalence ratio of 0.2.

The blank configuration (top) required an air throttling downstream to force flow choking for additional flow deceleration.

The mesh-coarse (middle) and mesh-fine (bottom) cases showed well-stabilized flames near the cavity even without the air throttle.

Combustion Efficiency

The combustion efficiency, defined as a fraction of CO2 over the sum of CO and CO2, is estimated. The respective mole fractions were measured via the tunable diode laser absorption spectroscopy (TDLAS) method.

Compared to the blank insert (black), the mesh insert cases exhibited a comparable combustion efficiency at half the equivalence ratio. At a comparable equivalence ratio (at around 0.2), the mesh inserts exceeded the blank case by nearly 40% in combustion efficiency.

This shows the hybrid method (cavity + intrusive structure) can significantly improve the combustor performance.

Referee Combustor M1

Propulsion systems often prefer liquid fuels due to their superior energy densities. Since liquid fuels must be vaporized for the combustion processes, improving the liquid vaporization, i.e., spray atomization, is critical for any liquid-fueled combustors.

To deliver a reference performance, DEVCOM Army Research Laboratory (ARL) developed a referee combustor M1 for comprehensive liquid-fuel combustor studies.

X-ray Phase Contrast Imaging

Spray atomization, or liquid break-up, is a chaotic process in nature. Consequently, an imaging technique must deliver high-speed and high-resolution images of liquid particles to allow such studies.

One of the options is to employ X-ray phase contrast imaging (PCI), which provides strong contrast between the gas phase and the liquid phase at greater frame rates than conventional CMOS cameras.

Machine Learning for Denoising & Velocity Prediction

Several machine learning techniques were employed for PCI images.

• A self-learned denoising algorithm.

• Boundary detection algorithm for assessing the particle size.

• Unsupervised velocity prediction algorithm.

Particle Distribution Characterization

The PCI images along with the ML techniques allowed simultaneous sampling of several quantitative parameters of spray flows:

• Size distribution along the longitudinal direction from which the spray break-up length is inferred.

• Size distribution along the radial direction, which indicates the atomizer performance.

• Shape distribution of particles.

• Velocity distribution of particles.