Aircraft employ a variety of engines, for example gas turbine engines, which typically include a fuel system for supplying fuel from a fuel source to a combustor where the fuel in combination with compressed air is ignited to produce combustion gasses to drive the engine's turbine system. The fuel system typically includes pumps, valves and regulators so as to assure proper fuel pressure is maintained for delivering ample pressurized fuel supply to the engines. Traditional fuel systems may supply either a constant flow or a varied flow rate of fuel to the engine which in a large part is determined by the pumps that are employed within the fluid delivery circuit. Based upon the system employed, different performance characteristics can be achieved.
One such example of a fuel system employs a gear pump which provides a constant flow rate of fuel at a given pump speed. The constant flow pump delivers the same amount of fuel irrespective of the current demand on the engine. In such scenarios, the fuel pump(s) deliver excessive amounts of fuel to the engine, which results in un-burnt fuel having to be recirculated within the system or to be exhausted to the atmosphere. It is ecologically undesirable to exhaust the un-burnt fuel to the environment so reclaiming the fuel is preferred. This scenario results in energy inefficiencies, increased friction, and increased heat transfer loads. It would be preferable to avoid these problems.
Once such arrangement to accommodate the un-burnt fuel within a fuel pumping circuit would be to redirect the un-burnt fuel to a gear pump and reintroduce it into the fluid delivery circuit, which in turn delivers the fuel to the engine where it may be combusted. Such arrangements sometimes employ additional pumps, valves and devices which increase weight and overall inefficiencies. Further, such systems tend to reintroduce the un-burnt fuel into the fresh fuel supply loop. The un-burnt fuel, however, is in a heightened temperature state because of the friction generated as a result of it already having been pumped once. Thus, if the un-burnt heated fuel is mixed with the ambient temperature fuel that is normally present in the supply line, then the resulting mixed fuel takes on an elevated temperature. Due to the fact that fuel pumps are not 100% efficient, each pass through the pump causes additional energy loss. Additionally, because fuel is sometimes used for tasks such as taking heat out of engine oil through heat exchangers, heating the fuel unnecessarily through the pumping process is undesired as it reduces the capacity of the fuel to extract heat from the engine oil.
In addition, when the un-burnt fuel is recirculated by introducing it to a traditional constant flow pump, such arrangement typically requires larger pumps to handle the increased fuel flow rate. As such when larger pumps are employed, weight and costs are added to the aircraft's fuel system. Thus, it would be helpful to avoid increased pumping requirements, avoid heavy pumps and other components, and avoid introducing the heated un-burnt fuel directly back into the fresh fuel supply loop.
It would be helpful to address the concerns raised herein by providing an improved fluid circuit reducing the weight of the fluid pumping system, and even harnessing the energies from the un-burnt fuel so as to redirect those energies to a more effective purpose with the engine.