Newest data challenges assumptions and norms
By AARON W. SKIBA AND TRAVIS R. SIPPEL|December 2017
The Propellants and Combustion Technical Committee works to advance the knowledge and effective use of propellants and combustion systems for military, civil and commercial aerospace systems.
Collaborative efforts between the University of Michigan and the Air Force Research Laboratory at Wright-Patterson Air Force Base in Ohio in 2017 significantly advanced understanding of the effects that extreme turbulence levels have on the structure of premixed flames in gas turbine combustors. Planar laser-induced fluorescence imaging of several chemical species associated with the preheat and reaction layers of premixed flames was employed in January, February and May to elucidate the effects that engine relevant turbulence intensities have on the structure of these layers. Turbulent premixed combustion theories have long suggested that when the turbulent Karlovitz number (KaT, defined as the ratio of a characteristic chemical time scale to the smallest turbulent time scale) exceeds a value of 1 and 100 for the preheat and reaction layers, they become broad in comparison to their laminar counterparts.
The significance of these theories is that they are commonly invoked when selecting turbulent combustion models to, say, design a gas turbine engine. However, the results of the 2017 experiments indicate that the classical notions that preheat and reaction layers will broaden or extinguish once KaT exceeds 1 and 100, respectively, are incorrect. In fact, reaction layers of flames with values of KaT as large as 530 were found to be continuous and their average thicknesses approximately the same as that of a laminar flame. It is believed that this work will assist modelers with the selection of the most accurate and robust tools for simulating highly turbulent combustion phenomena, such as those found in modern gas turbine engines.
Also through an ongoing effort, a team led by Travis Sippel and James Michael from Iowa State University and Eric Miklaszewski of the Naval Surface Warfare Center, Crane Division, in 2017 conducted experiments focused on enabling electromagnetic control of energetic materials. In research led by ISU funded by AFOSR Space Propulsion and Power, experiments were conducted to specifically address lack of throttleability in solid rocket motors — and inherent mission flexibility — afforded by their liquid or hybrid counterparts. This was accomplished by the use of energetic material dopants that allow electromagnetic control of combustion. Using an alkali metal doping technique, ISU researchers have developed a class of composite solid propellants in which microwave energy can be efficiently deposited to the propellant flame front. The technique exploits the high flame temperature of a propellant in order to cause alkali metal dopants (e.g., sodium) to shed electrons, producing a flame environment that will more favorably absorb microwave energy. Results in 2017 demonstrated that doping enhances flame coupling with low-strength microwave fields and increases burning rate enhancement efficiency. An increase in burning rate of more than 60 percent has been demonstrated. Studies also conducted in early 2017 demonstrated that with higher field strength, doping may not be required at all and at some field strengths, high-temperature propellant flames enable microwave energy deposition directly to aluminum oxide combustion features.
The concept of using microwave energy to control combustion is also being considered in other areas of energetics. In August 2017, researchers at NSWC Crane, in collaboration with ISU explored the use of microwave energy to dynamically control and enhance the photoemission of pyrotechnic compositions. Many pyrotechnic formulations already contain alkali metals and are attractive candidates for microwave enhancement. The goal of these efforts is to produce a next generation of “smart” pyrotechnics whose brightness and/or color emission can be enhanced and controlled on command.