Combustion-related research in Purdue's School of Mechanical Engineering
Professor Paul E. Sojka’s combustion related research is focused on the formation and evolution of super-critical sprays (jets). That work is motivated by recent advancements in gas turbine technologies that have made flight speeds of up to Mach 7 possible. At these flight speeds the ambient air surrounding the engine is no longer a sufficient heat sink so an additional source is needed to remove the extra heat generated due to the viscous effects of the high speed air interacting with engine blades and vanes, as well as with airframe control surfaces and the myriad electronics that are part of a typical flight system. The on-board fuel is being explored as a possible candidate to serve as a heat sink.
By adding heat to the on-board fuel is it quite possible that the temperature and pressure will rise above the critical point. As a result, the fuel nozzle must be designed to accommodate supercritical fuel (SCF) injection. Work is progressing along two lines.
In the first case, a unique high-pressure experimental facility was used to determine how ambient pressure influences the spreading angle of swirling liquid jets. The most recent experimental results extend the pioneering work of DeCorso and Kemeny (1957), as shown in the figure below. Key conclusions drawn from this data are:
- A reduction in cone angle with increased ambient pressures was as ambient pressure reached 7.1 MPa.
- At all levels of Pamb tested, the cone angle was reported to be significantly less than the inviscid pressure swirl prediction.
- At ambient pressures below ~4.0 MPa the spray width was found to be approximately proportional to Pamb-0.48 (Eqn 5.1); above this pressure, the cone angle was observed to remain constant within the experimental uncertainty.
The practical implication of this work is that designers of high-Mach gas turbine engines can now estimate the fuel distribution produced by swirling injectors.
In the second case, schlieren photography (figure below left) was used to map concentration fields in super-critical jets produced by swirling flows (figure below right). Carbon dioxide was used as a surrogate to typical aviation fuels to remove the hazard of explosion or fire. Typical experimental data are provided in the bottom figure.
The following conclusions were drawn from this, and other, data:
- Initial and final spreading angles were for each injector were found to be insensitive to injectant/environment density ratio or mass flowrate;
- As expected, spreading angles increased with injector swirl number;
- All jets demonstrated self-similar mean concentration profiles;
- The distribution of mass widened significantly with an increase in swirl number.
- The spread of mass in the jets (width of concentration profiles at half the maximum concentration) was found to vary linearly with axial distance.
Our current efforts focus on methods to prevent vortex breakdown, and the use of actual aerospace propellants at realistic temperatures and pressures.
Steve Son's research is focused on a wide range of fundamental combustion areas involving energetic materials (propellants, explosives, and pyrotechnics). His group applies and develops state-of-the-art dynamic experimentation, as well as fabricates novel reactive materials. We develop or apply the most advanced diagnostics often applied in harsh reacting environments that are typical for energetic materials to develop improved understanding and characterization. An example of this is applying high speed OH planar laser induced fluorescence (PLIF) to solid propellants to visualize (for the first time) microscale and transient flames in composite solid propellants (collaboration with R. Lucht). This is an image where the effect of catalysts can be seen on the flame structures (top is no catalyst, next is micron catalyst, followed by nanoscale catalysts and bottom is encapsulated nanocatalysts).
We also seek to develop tailored disruptive energetic materials with unique properties. An example of this is fabricating propellant with aluminum-lithium alloy instead of aluminum. This decreases hydrochloric acid in the products, improves theoretical performance, and promises to reduce two-phase flow losses in rocket motors. The images below show baseline aluminized propellant (left) and Li-Al based propellant (right) using backlighting and high-speed microscopic imaging. These images show microexplosions that produce much smaller condensed phase products. Ultimately our research aims to improve the performance, safety, or toxicity of energetic materials. We also have interest in energy topics, including coal combustion and hydrogen storage.