Mixed-flow Exhaust Silencer With Cooling Air Inlet
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Solution Overview
Problem
Existing noise attenuation systems for gas turbine engines, such as those in aircraft auxiliary power units, face challenges in reducing convective heat transfer from exhaust gases, which leads to elevated temperatures that degrade the structural integrity of silencers, increasing costs and weight due to the need for high-temperature materials and insulation.
Innovation Solution
An exhaust silencer assembly featuring a casing with an acoustic-attenuating porous liner and a cooling air inlet system that directs air to cool the liner and reduce convective heat transfer, thereby minimizing the use of high-temperature materials and insulation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the exhaust silencer is positioned downstream from the exhaust diffuser to attenuate low frequency noise, then noise attenuation performance is improved, but the silencer is exposed to high-temperature exhaust gases that convectively heat the silencer to elevated temperatures above the auto-ignition temperature of fuel, degrading structural integrity
Solution Approach 1:
The exhaust silencer is segmented into an inner region through which exhaust gases flow and an outer region containing the silencer structure. This segmentation allows the exhaust gases to be separated from the silencer structure, enabling noise attenuation while preventing direct thermal exposure to the silencer components.
Solution Approach 2:
A thermal barrier or insulating structure acts as an intermediary between the hot exhaust gases and the silencer. This intermediary layer blocks convective heat transfer to the silencer, maintaining it below auto-ignition temperature while allowing the silencer to perform its noise attenuation function downstream of the exhaust diffuser.
2Strength
If high-temperature materials and insulation layers are used to protect the exhaust silencer from elevated temperatures, then structural integrity is improved, but the cost and weight of the exhaust silencer increase
Solution Approach 1:
By segmenting the silencer into inner and outer regions, the design protects the outer region from direct thermal exposure. This reduces or eliminates the need for heavy high-temperature materials and thick insulation layers, thereby reducing overall weight while maintaining structural integrity at operating temperatures.
Solution Approach 2:
The thermal barrier serves as an intermediary that protects the silencer structure from high temperatures. This allows the use of standard materials with lower temperature ratings, reducing both material cost and weight compared to using high-temperature resistant materials throughout the entire silencer structure.
3Strength
If high-temperature materials and insulation layers are used to protect the exhaust silencer from elevated temperatures, then structural integrity is improved, but the cost of the exhaust silencer increases
Solution Approach 1:
Segmenting the silencer into thermally exposed and protected regions allows the use of cost-effective standard materials in the outer region rather than expensive high-temperature materials, reducing overall manufacturing cost while maintaining adequate structural integrity for the protected components.
Solution Approach 2:
The thermal barrier intermediary enables the use of lower-cost materials for the silencer structure by blocking direct thermal exposure. This reduces material costs and simplifies manufacturing requirements compared to designing the entire silencer to withstand high temperatures.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution effectively attenuates noise while reducing thermal buildup and the need for heavy, costly materials, resulting in a lighter, more cost-effective noise reduction system that complies with aviation noise standards without requiring additional eductor systems.
Implementation Method 1
the exhaust silencer to dissipate acoustic energy of the low-frequency noise as the noise travels downstream from the APU exhaust diffuser
Implementation Method 2
exhaust gases can convectively heat the exhaust silencer to elevated temperatures
Implementation Method 3
a cooling air inlet opening configured to receive cooling air and direct the cooling air to flow through the outer region
Data Source
AI summary
An exhaust silencer assembly comprising a casing having an exhaust gas entrance opening, an exhaust gas exit opening, and a cooling air inlet opening, and an acoustic-attenuating porous liner disposed within an interior chamber of the casing, thereby dividing the interior chamber into an inner region located between the exhaust gas entrance opening and the exhaust gas exit opening, and an outer region located between the acoustic-attenuating porous liner and the casing, where the cooling air inlet opening is located at the outer region.