In an existing aircraft air conditioning system, high temperature and pressure air from the engine core compressor enters the air conditioning packs to reach the desired cabin temperature and pressure. This conditioned cabin air is discharged to the outside ambient. A significant amount of energy is wasted by this process. In order to recover some of this energy, a new air conditioning pack was designed, analyzed and tested. In the new pack, the cabin out-flow air passes through an out-flow turbine which is shaft connected to a compressor. The compressor receives the ambient air and delivers it to a heat exchanger, where it is cooled by out-flowing turbine air and is passed on to the cabin. Experiments were performed to verify the new concept of using the out-flow turbine. In the experiments, an automobile turbo-compressor was used. A series of T-type thermocouples, 10 pressure transducers and five orifice flowmeters were used to measure the temperature, pressure and flowrate at different locations. A 32.5 m3 tank, simulating the aircraft cabin, was connected to the turbo-compressor as well as a pressurized air duct, simulating the in-flow from the other existing packs. The turbine outlet was connected to a low pressure 4.2 m3 tank, simulating altitude condition, and the compressor inlet was connected to another low pressure 2.1 m3 tank, simulating ram air condition (total pressure and temperature). It should be noted that due to the aircraft speed the ram air pressure is higher than the ambient pressure. During each test, three pressure control valves were used to maintain the pressure of the three tanks nearly constant with time.

In the experiments the cabin pressure, ambient pressure and ram pressure were varied parametrically simulating different altitudes. Data were taken at altitudes ranging from 6000 to 9000 m. In the experiments, the simulated in-flow air from the other packs fluctuated by 10%. From the obtained data, it was observed that by placing a flow control valve in the compressor outlet duct the cabin pressure could be controlled within 0.4% while the in-flow air flowrate was varied 10%.

The analytical results as well as experimental data show that nearly 56% of the delivered air to the cabin can be provided by this new pack. This decrease of the extracted engine air by 56% results in a significant reduction of fuel consumption.

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