APU Exhaust Muffler Design Improvements Through Conjugate Heat Transfer CFD Analysis

An Auxiliary Power Unit (APU) is an additional gas turbine engine located in the tail cone section of an aircraft which can be operated while the aircraft is on the ground or in flight. It is used to generate electricity for the aircraft’s electrical systems and to provide air to the environmental control units (ECU’s) when the main engines are not operating or there is a desire to unload the main engines, such as in an engine out situation. The APU is also used for main engine starting. An APU typically has an exhaust system that vents out of the rear of the tail cone. When the APU is in operation, the exhaust emits a very loud noise which, if not muffled, could be an irritant to the members of the ground crew. To reduce the impact of this exhaust noise, the APU is commonly fitted with a muffler. The muffler is placed around the APU’s exhaust pipe so that all of the APU’s exhaust is channeled through the muffler. The muffler is designed and constructed to substantially reduce the intensity of the noise emitted by the APU exhaust. The muffler is made of metal and has a tendency to get very hot during operation of the APU because of the high temperature of the exhaust gasses generated by the APU. It has been observed that the temperatures on the outer skin of the muffler commonly reach above 1,000 degrees Fahrenheit. If the muffler is not insulated, this heat will radiate outward from the muffler to the tail cone. Modern aircraft tail cones are commonly made from composite materials to help keep the overall weight of the aircraft low. Such materials cannot tolerate the high temperatures radiating from the muffler and if exposed to such temperatures for any length of time, may experience some form of failure. Accordingly, aircraft manufactures commonly mandate that the heat radiating from the muffler not exceed a predetermined limit. So it is essential to design a muffler which not only attenuates the noise levels of the APU exhaust, but also need to be insulated with a low conductive insulated blanket around it. A conjugate heat transfer CFD analysis was performed on a new APU exhaust system to optimize the exhaust muffler blanket design. Several different blanket material types and configurations were analyzed to minimize the heat transfer through the muffler. The CFD analysis included many geometric details including the exhaust eductor, to better model the exhaust gas profile, the muffler baffles and the tail cone structure. Muffler blanket outer surface temperature values and tail cone skin temperatures were monitored to determine the optimum design. This paper discusses the CFD model and analysis and discusses the results. It explains the different design iterations needed to arrive at acceptable muffler blanket outer surface temperatures and tail cone skin temperatures.