Exploring the Design Space with System Simulation to Manage Increasing Thermal Loads on Aircraft Fuel Systems Today s modern military fighter jets are like a flying thermos bottle according to Steve Iden, AFRL Invent Program Manager [1]. Many engineers have been tackled with trying to put the thermal loading constraints of these fighter jets on ice. This places increasing demands on utilizing the fuel system as a heat sink to dissipate thermal loads coming from onboard electronics, oil and hydraulic systems, avionics bay cooling, and weapons modules. Engineers today are looking to simulation to help tackle these design challenges, and they have more power than ever with simulating fuel systems to evaluate feasible system designs with given requirements on thermal power loading, fuel capacity, and tank geometry. With a given set of key performance metrics and limits on a design, the design space can be quickly explored to find optimum metrics such as flight mission duration or required component sizing to meet thermal load requirements. Tackling the Simulation Challenges There are many modern simulation tools that can be used to tackle the design challenges related to handling increasing thermal loads on aircrafts, and often several tools are used in parallel with one feeding boundary conditions to the other. Following the typical V cycle of product development, many tools are used together throughout this process, as shown in Figure 1. Furthermore, aircraft juggernauts like Boeing and Airbus employ many software applications to meet this task [2].
Figure 1: Software used throughout the V cycle of product development. One way to increase efficiency throughout the V cycle is to combine various design tasks into a single software tool. For example, the heat loads generated in an avionics or electronics bay can be cooled by a vapor cycle system (VCS), and the heat can then be rejected to the fuel system as shown Figure 2. These two systems are often modeled in different tools. A system simulation software such as GT-SUITE can integrate the VCS system and fuel system together to account for system interactions, guiding engineers to determine the proper sizing of components such as heat exchangers, fuel tank layout, and so on, to meet a given thermal load. These models often need to run in real time to meet the demands for control development and long mission simulations. Furthermore, the simulation can be carried out early on with concept level trade studies, down to more detailed predictions as CAD data and physical dimensions become available. A common trend in the industry is to co-simulate multiple software packages together. This can be achieved through a common FMI data exchange format or through specialty links established between specific tools. Tools like GT-SUITE support both methods for linking to other CFD, FEA, controls, and inhouse codes to name a few. Simulating Multiple Systems Together One such example entails modeling the thermal management of the fighter jet, by dissipating thermal loads generated in an avionics bay via a vapor cycle system, and ultimately transferring this heat to the fuel system to act as a heat sink. Figure 2 shows a model schematic combining these systems in the GT- SUITE software package. Other software packages such as Simulink by Mathworks can easily be included for co-simulation to provide boundary conditions for a flight mission where conditions like ambient pressure, temperature, and fuel burn throughout the mission can be provided. The model can be run as an early concept trade study or as a more detailed model with geometry based on CAD.
Figure 2: Integrated system model of vapor cycle system extracting heat load from avionics bay, and transporting heat to the fuel system as a heat sink. Simulation Details and Results The simulation accounts for full transient thermal and fluid dynamics to predict results based on aircraft environmental and thermal conditions throughout the flight mission accounting for changes in altitude and fuel burn. In this example, a transient thermal load is generated by the avionics bay, which is then cooled by the vapor cycle system to an acceptable maximum temperature for the electronics. A controller is used to govern the maximum temperature limit by regulating the speed of the VCS compressor. This vapor cycle system considers transient two-phase flow and allows different refrigerants to be studied very easily as part of exploring the design space. The heat is then carried through the refrigerant to the condenser where this heat is exchanged with the fuel side of the condenser and the heat is then recirculated through the fuel tanks. Finally, co-simulation with Simulink is used to provide boundary conditions for fuel burn and ambient conditions for the flight mission. The simulation user can explore very simple effectiveness-based heat exchanger performance when geometry is not available, or model various types of higher-fidelity heat exchangers such as plate-fin and then scale and optimize this geometry to meet the thermal demands. The fuel system considers fuel transfer control strategies and contains all components to model the fluid in the fuel system including pumps and valves and tanks. The tank can be a very simple volume based approach, or go a level deeper to consider the 3D CAD shape for fuel level and heat transfer to the walls. Detailed effects can be included such as variable thermal loading on different sides of the fuel tank, accounting for depleting fuel over the course of the mission and variable heat transfer with altitude.
Through system integration, engineers can get a complete picture of the systems working together. This is illustrated in Figure 3, where the compressor speed in the VCS system is changing all the time to regulate to the electronics temperature limit in the avionics bay. The two-phase solution of the VCS system is robust and stable, even under times of zero refrigerant flow when the compressor speed drops to zero. For the first few minutes of the simulation, the VCS compressor flow is zero, causing the temperature to rise to the avionics bay limit, after which the VCS compressor controller kicks on and regulates the temperature to this limit. After a half hour into the flight, the heat load suddenly drops for a few minutes, causing the VCS compressor flow to go to zero once more. The fuel temperature is dropping for the first hour and a half since the aircraft is exposed to the cold temperature at altitude, and only increases when the aircraft is descending. Furthermore, the spread between the avionics temperature and fuel temperature indicates there is more room to heat up the fuel. This model could be extended to include other heat sources on the aircraft to push the fuel temperature to its limit. Figure 3: Temperature results in different parts of the aircraft over a typical flight mission. There are many inputs to explore in this type of model such as thermal load input, heat exchanger size and performance, fuel tank size, and flight mission conditions. To handle these inputs, a Design of Experiments (DOE) can be set up to run many trade studies. One such example involves optimizing the heat exchanger size to properly transfer the heat from the avionics bay to the fuel system, which is crucial to consider for minimizing the weight of the aircraft.
The trend toward creating a Virtual Integrated Aircraft (VIA) model from tip to tail is of high importance to study and find better designs for current and next generation aircraft. Using system level simulation tools such as GT-SUITE can help resolve challenges and keep the next generation fighter aircraft on ice. [1] http://aviationweek.com/awin/afrl-s-invent-program-tackles-aircraft-system-efficiency [2] http://www.engineering.com/plmerp/articleid/14370/boeing-airbus-and-the-hardship-of-dealingwith-plm-obsolescence-tv-report.aspx