Wednesday, April 1, 2009
Engine Recuperators, Efficiency and Survivability
In their endeavors to produce a V/STOL UAV called the V-STAR (VTOL Swift Tactical Aerial Resource), Frontline Aerospace Inc. wanted to improve the range potential of their prototype UAV. Its use of Rolls-Royce 250-C20 engines required some research into improving range efficiency with that motor. That research brought them to recuperators.
To beat these systems one must plan backward from the objective to the starting point.
Recuperators are described by some skeptics as an attempt to achieve perpetual motion. Recuperators are heat exchangers that capture and exchange exhaust heat to superheat compressor discharge air up to a higher temperature before its introduction into a turbine engine combustor. In practice, compressed air that is heated by exhaust via the recuperator takes less fuel to raise it to the necessary combustor and turbine nozzle temperature to drive the turbine wheel. What is the weight and power price paid for this exchange? That’s the challenge for recuperators in turbo-shaft aircraft; develop a system that effectively exchanges heat at an acceptable weight and power loss penalty.
"Past aircraft recuperator designs have used standard plate and fin designs not unlike your car radiator," explained Frontline’s CEO Ryan Wood. Those designs were about 15-25 percent efficient at extracting exhaust heat and applying it to compressor discharge air. However, the resultant weight penalty was not worth the efficiency gains because what you lost in gross weight was more than the weight of the fuel saved.
Frontline’s MicroFire recuperator is a heat exchanger with better than 70 percent effectiveness and low pressure drop (~4 percent) that raises the thermal efficiency of the Model 250 C20B engine and yields a ~40 percent fuel savings. MicroFire includes the employment of microtubes, microchannels and foil laminates whose heat transfer properties are of a magnitude greater than plate and fin systems of the same dimensions.
Since the Rolls-Royce compressed discharge air gas path uses an exterior plenum from the diffuser to the combustor, its design is ideal for incorporating a recuperator. Frontline’s design taps into the 250-C20’s diffuser section where compressor discharge air is captured and routed through a MicroFire heat exchanger plenum system that raises the ~500° F compressor discharge air up to about ~900° F. This discharge air then only has to be heated with combusted fuel by another 500° F to reach the nominal 1,400° F combustor temperature. Contrast this to the 900° F differential that a conventional engine would characteristically see. Significantly greater specific fuel consumption is required to cover that 900° differential when compared to a recuperator-equipped engine. However, as with most rotorcraft conundrums, that efficiency improvement comes at an engine power performance cost at cruise in the form of exhaust gas back-pressure.
The MicroFire system is designed always to be bypassed for maximum power; it is only when the pilot decides to engage the recuperator system that it begins to save fuel typically at cruise altitude.
The bypass assembly valve is designed to the fail-safe open during electrical failure to allow maximum power available. The internal drag in the microchannel plumbing for the compressor discharge air induces an approximate 1 percent power loss at all times. When engaged, the recuperator exhaust backpressure will induce an approximate 4-5 percent loss of power but at a ~40 percent fuel consumption savings. That’s a huge range performance enhancement that many would consider worth the loss in cruise torque performance as most helicopters cruise at 60-80 percent torque.
It’s easy to see the efficiency value for persistent observation platforms whose loiter missions are constrained by fuel endurance. Further examination sees benefits for current Rolls-Royce 250-C20 equipped platforms providing offshore support, and longer range medical transport. Where fuel is the highest direct operating cost factor in most operators’ ledgers, fuel efficiency becomes a very attractive technology incentive.
Another benefit to this concept is the reduction of infrared signature due to re-absorption of exhaust gas heat in the engine. Analyses show that exhaust temperatures in the neighborhood of 500° F will be the norm for recuperator-equipped engines compared to nearly double that in an unsuppressed engine. This makes IR detection, track, lock, and launch more difficult, and flare and IR jammer effectiveness significantly greater for recuperator-equipped aircraft. Recuperators may make passive IR suppressors obsolete as the technology matures.
In the next three months, Frontline will announce its test stand findings, setting the stage for STCs and retrofits to platforms whose missions stand the greatest benefit from fuel efficiency. There are many other engine platforms and missions that could benefit from the spin-off technology fashioned for the C20 or C30 engines. The whole family of GE CT-7 engines could benefit from long range cruise efficiency enhancements. R&W will report the test stand results as they emerge.