Active Control of Fuel Position in Opposed-Flow Strand Burner Experiments

ABSTRACT

This paper describes a new active method of controlling the position of the burning surface of a solid fuel strand in an opposed-flow burner experiment. This method is designed to avoid biases to the combustion process introduced by retaining wires and to avoid experimental delays caused by wire failures. Regression rates are presented for hydroxyl-terminated polybutadiene fuel strands burning in various oxidizer streams. The active control method uses a control loop composed of a diode laser, a photodiode, and a stepper motor. For comparison, tests were also performed using the commonly used passive control method of spring-loading the fuel against a retaining wire. This active control method can only be used at high values of oxidizer mass flux; at low mass flux, soot accumulation obstructs the laser beam. In passive control tests, the retaining wire conducts heat from the flame into the fuel and increases regression rate. The presence of the wire also modifies the extinction limits of the flame. A model was developed for regression rate as a function of oxidizer mass flux, oxidizer composition, and retaining wire diameter. This model can be used to correct future regression rate data for the bias introduced by retaining wires.

KEYWORDS:

• solid fuel
• HTPB
• opposed-flow burner
• shadowgraphy
• ramjet
• hybrid rocket

Acknowledgement

The authors thank Drs. Matthew Finn and Albert Epshteyn for their work in preparing HTPB strands and Dr. Rohit Jacob for his contribution to the LabVIEW control application.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplemental data

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00102202.2022.2114796

Notes

¹ An ancillary benefit of this control method is a reduction in time needed to set up an experiment. Wire-based tests required about ten minutes to set up. Active control tests required about five minutes..

Additional information

Funding

The authors thank the Office of Naval Research for funding this work, both directly through the Energetic Materials program managed by Dr. Chad Stoltz and indirectly through the Naval Research Laboratory Base Program. C.M.G. was supported through a National Research Council postdoctoral associateship award administered by the National Academies of Sciences, Engineering, and Medicine.