Sustained TheRmal Accommodation in Flight Engagement
The sponsor for this project, the Air Force Research Lab, has been working in conjunction with Georgia Tech on the More Electric Aircraft initiative. This initiative supports the AIAA Program Committee for Energy Optimized Aircraft Systems. As part of this initiative, AFRL has developed a MATLAB/Simulink modeling and simulation environment for military aircraft thermal and electrical management systems as part of the Integrated Vehicle & Energy Technology (INVENT) program. The STRAFE team's task was specifically focused on the thermal management model, referred to hereafter as the Thermal Tip-to-Tail (T2T) model. The T2T model consists of multiple subsystems, including the Fuel Thermal Management System (FTMS), the Adaptive Power Thermal Management System (APTMS), a Robust Electrical Power System (REPS), and the High-Power Electrical Actuator System (HPEAS). The FTMS tracks the temperatures in the fuel and oil, which can be used as heat sinks for small heat loads. The APTMS contains the Integrated Power Package (IPP), which consists of turbomachinery driving air-cooled and liquid-cooled loops, primarily used for cooling avionics components as well as environmental control in the cockpit. The REPS and HPEAS subsystems deal with power distribution and electrical management, and are less influential on the thermal management system. The T2T model also contains subsytem models for the engine and and the Air Vehicle System (AVS), which includes the mission profile in terms of Mach and altitude.
The STRAFE team was tasked with incorporating a Large Heat Load Device (LHLD) into the existing T2T modeling architecture, and then tuning the design variables for optimal performance of the thermal management systems. The STRAFE team was additionally provided a Simulink model for a Large Heat Load Device, which might represent a RADAR system or some other system generating heat loads on the order of magnitude of 1 MegaWatt. The STRAFE team was tasked with modifying this existing model to allow multiple pulses, and to integrate it with the T2T model to maintain quasi-isothermal constraints for various components within the LHLD model. It was the expectation of both the AFRL and the STRAFE team that thermal closure for the LHLD would not be achievable through the optimization of the T2T system alone, and thus the team also explored cutting edge thermal management technologies currently under development, including jet-impingement models and phase-change materials. The STRAFE team researched companies developing thermal management technologies for military platforms, and then set out to model the best of these within the LHLD Simulink environment with the goal of determining the most feasible technology given the mass, spatial, and thermal constrictions imposed by a tactical aircraft.
In order to approach the problem, the STRAFE team defined three questions to be answered:
1) Can we accommodate additional heat stress on the thermal management system through changes in design variables and controller gains?
2) How much power input is required for the LHLD to be functional, and what types of pulsing missions can it accomplish? What are the constraints?
3) Can new technologies help reduce the additional heat stress on the thermal management system introduced by the LHLD?
For the first question, the STRAFE team focused on the T2T model without the LHLD incorporated. The STRAFE team intended to find how much the current system could be improved with the existing variables and parameters. This study was divided into two separate Designs Of Experiments (DOE). In the first DOE, all of the design variables were held constant to their baseline values, and only the controller gains were changed. In the second DOE, the controller gains remained constant, while the design variables were changed. Design variables included valve positions, shaft speeds for the IPP, and other design choices for the FTMS and APTMS subsystems. For both DOEs, a Latin Hyper Cube sampling was employed because of the sensitivity of the model to failures at the edge of the design space. The space filling DOE helped characterize the full range of the design space evenly.
The second question focuses instead on the model for the LHLD which was presented to the STRAFE team incomplete. Due to its incompleteness, the STRAFE team first improved this model to allow multiple pulses in succession. Other modifications to the LHLD model included user capability to define two separate pulsing missions with different pulse rates occurring in succession, as well as the ability to quantify the power input required for the full mission. Once this model was developed, the STRAFE team used optimization with genetic algorithms to minimize the power input required to meet a specified pulsing mission that included 18 pulses of 5-second duration each over the span of approximately 5 minutes. The constraints on this system were temperature constraints, requiring a few components of the LHLD to remain near isothermal despite efficiencies of approximately 50%.
The third question arose from the common sense that a system with large power levels and high inefficiencies could not be accommodated thermally by the minimal performance of the existing thermal management system. The STRAFE team anticipated that a new thermal architecture would be necessary involving cutting-edge thermal management technologies, some of which are still in testing phases. The STRAFE team performed research to define a few feasible technology options, including jet-impingement devices and/or thermal storage containers filled densely with phase-change materials. The latter two options were modeled within the LHLD environment, and once again genetic algorithms were used to optimize the LHLD and cooling systems. For the jet-impingement model, the goal was to meet the isothermal constraints of the components while minimizing the volume of fluid. The goal for the thermal storage devices was the same, but with the goal of minimizing the number of thermal storage units to reduce the overflow heat from the LHLD components to zero.