Flow analysis of an air-to-air refueling system

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This air-to-air refueling system contains both simple piping and very complex components such as the nozzle.

In today’s mil/aero industry, systems are getting more complex and the time and cost to design them are getting tighter. These systems and components may be mechanical, electrical, or a combination of both.

Three approaches are often used in the design and analysis process of such systems.

One approach is to do the design, produce a physical prototype, test it, change the design, and then repeat the process, which can be extremely expensive and time-consuming.

Another option is to over-engineer, which will result in a “safe” solution but may be less cost-effective, add weight to an airborne system, and compromise the performance of a system intended to run in a narrow bandwidth.

The third approach, virtual prototyping with CFD analysis, early and throughout the design process, can deliver an optimized system at lower cost (because of fewer physical prototypes) and get the system to market faster.

A prime example of a complex aerospace system is the refueling system. It contains not only piping for distributing the fuel but also complex 3-D components such as the fueling nozzle at the plane-to-plane connection.

What follows is a typical example of how the latter design process might proceed and how CFD tools can be used to optimize a refueling system.

Issues requiring optimization

Consider an engineer who is developing a new refueling system or needs to analyze some problems with an existing system. In the analysis and subsequent changes to the design, it is necessary to ensure that the system can deliver three performance criteria.

• Will the system be able to deliver fuel at an acceptable rate to the receiving aircraft? Basically, will the system’s flow rate meet specification?

• Will the system deliver fuel to the fighter tanks at an even rate? The fighter has tanks in the wings; and if the rates to the tanks are uneven, one wing will become heavier faster and the fighter will become unstable and may break off from the tanker.

• If/when the fighter disengages from the tanker, either by plan or in an emergency breakaway, will the “water hammer” effect on the piping cause excessive pressure surges that may damage the system? The maximum pressures the system can tolerate are known, and the analysis can reveal if the design is well within specification.

What is needed is a CFD solution that will enable the engineer to analyze those effects quickly.

First, the engineer will make changes to the design believed to solve problems in the system. Then, the trial changes will be quickly re-analyzed, with a focus on an optimum design to address all specifications.

Tool choices

Two types of CFD analysis tools are available. One can be used to analyze the piping and could be considered a 1-D analysis (i.e., the fuel only flows in the axial direction of the pipes).

The other can analyze very complex components where the fuel flow is 3-D, such as through the fueling nozzle.

Which CFD tool should be used to analyze this system, which is clearly a combination of 1-D piping and 3-D complex components?

The 1-D CFD tool is much faster than the 3-D CFD but lacks the accuracy when simulating the complex nozzle.

However, if the complete system is analyzed using only the 3-D CFD tool, the accuracy needed may be obtained but the computer execution time will be excessive, defeating the goal of rapid and multiple experimenting with several design approaches. The best approach would be integrating the 1-D and 3-D tools and leveraging the advantages of both.

An illustration of how such an integrated system would work can be achieved via Mentor Graphics’ 1-D system simulator, Flowmaster, and 3-D simulator, FloEFD.

Initially, the refueling system designer would define a range of operating boundary values (such as pressure and flow rates) that may be presented to the nozzle. Typical refueling scenarios need to be understood as well as the complete spectrum of possible conditions the system could deliver under normal and extreme conditions.

The MCAD designer of the nozzle would use the 3-D analysis tool embedded in the PTC, Catia, NX, or SolidWorks MCAD system to run detailed fluid flow analysis on the nozzle.

Because the 3-D analysis is embedded, the designer can perform these analyses directly within the MCAD tool using the same interfaces, an analysis model contrived directly from the MCAD model without external interfaces of data translation, and automatic meshing and convergence.

The nozzle designer would set up a set of analyses based on the boundary value spectrum presented from the system designer. The designer simply specifies the range of conditions, and the 3-D analysis software automatically creates the set of conditions called “design of experiments.”

This could result in 30, 40, or even more model batch runs through the 3-D analysis, which, for a complex component, might have to run overnight. The resulting data of those runs would be automatically condensed into detailed characterization graphs that represent a complete model of the nozzle.

That model is then simply opened in Flowmaster and saved to the relational database of the 1-D system analysis tool. Then, the systems designer can run the flow analysis through the series of refueling scenarios anticipated for the trial design.

Design changes can be made to the system and subsequent analysis runs performed. The model of the nozzle remains valid because it covers the full spectrum of possible operating conditions.

The 1-D analysis with the 3-D-derived nozzle model quickly (in minutes) and accurately creates graphs and numerical data to represent those effects at all nodes throughout the system.

The accuracy of the 3-D simulation of the complex component (nozzle) combined with the speed of the 1-D piping system analysis brings the best of both worlds together.

With the speed of the analysis, the systems designer is able to try several design scenarios and create a refueling system to run in the small bandwidth of optimum performance. The tanker system could be designed to service different classes of fighter under a spectrum of operating conditions.

This same combination of 1-D and 3-D integrated analysis methodology can be used for other aerospace systems such as onboard fuel delivery to the engines, engine cooling, interior environmental (air), etc. It also can be applied to industries such as automotive for cooling systems and exhaust, chemical processing, energy, utilities, etc.

This article was written for Aerospace Engineering by John Isaac, Mentor Graphics, Mechanical Analysis Division.

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