Exhaust Duct Cover Using Bimetal Plates to Block Reverse Flow
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Solution Overview
Problem
Temperature-sensitive components in aircraft engines, such as air-cooled heat exchangers, are susceptible to damage from hot exhaust gases when the engine shuts down, as the exhaust gas can vent outward through the duct and cause temperature-related issues.
Innovation Solution
A cover mechanism using bimetal plates is deployed in the exhaust duct downstream of the air-cooled heat exchanger, which moves between open and closed positions based on temperature, preventing hot exhaust gases from flowing back into the heat exchanger during shutdown.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If the air-cooled heat exchanger outlet is fluidly coupled to the exhaust duct to allow cooling air flow during engine operation, then heat exchange efficiency is improved, but temperature-sensitive components are exposed to hot exhaust gas damage when the engine shuts down
Solution Approach 1:
The cover is designed to dynamically change position based on engine operating conditions. During normal operation, the cover remains open to allow cooling air flow through the heat exchanger. When the engine shuts down and exhaust gas temperature rises, the cover automatically closes to block the exhaust duct, preventing hot gas from damaging the heat exchanger. This dynamic adaptation resolves the contradiction between maintaining heat exchange efficiency and protecting against thermal damage.
Solution Approach 2:
The cover acts as an intermediary element between the exhaust duct and the heat exchanger outlet. It selectively controls the flow path, allowing cooling air to pass through during operation while blocking hot exhaust gas during shutdown. This intermediary mechanism enables the system to achieve both efficient heat exchange during operation and protection against thermal damage during shutdown without requiring separate systems.
2Reliability
If a protective cover is added to block exhaust gas from damaging the heat exchanger, then component protection is improved, but device complexity increases
Solution Approach 1:
The cover mechanism utilizes the natural thermal expansion and contraction of bimetallic strips in response to exhaust gas temperature changes to automatically open and close. During engine operation, the bimetallic strips remain cool and maintain a configuration that keeps the cover open. When the engine shuts down and exhaust gas temperature rises, the bimetallic strips heat up and automatically trigger the cover to close, preventing hot gas from entering the heat exchanger. This self-actuating mechanism provides reliable protection without requiring external power sources, control systems, or complex actuation mechanisms, thereby maintaining system simplicity while ensuring component protection.
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 bimetal plate system effectively blocks hot exhaust gases from damaging temperature-sensitive components by transitioning to a closed position when the engine shuts down, protecting the heat exchanger and its components from melting or burning.
Implementation Method 1
the cover includes supports extending across the air outlet and supporting bimetal plates, the bimetal plates operable to displace from a first configuration corresponding to the open position of the cover to a second configuration corresponding to the closed position of the cover at the predetermined temperature of the cover
Implementation Method 2
each of the bimetal plates includes a first metallic strip fixed to a second metallic strip along planar surfaces thereof, the first metallic strip having a first coefficient of thermal expansion and the second metallic strip having a second coefficient of thermal expansion different than the first coefficient of thermal expansion
Data Source
Figure 1
Figure 2A
Figure 2B
AI summary
An aircraft engine (10) includes a core gas path (28) through which a core gas flow (28a) flows, an exhaust duct (30) receiving the core gas flow (28a), an air-cooled heat exchanger (34) disposed in a heat exchanger duct (36, 38) having an air inlet (34a) providing cooling air to the air-cooled heat exchanger (34) and an air outlet (34b) in fluid communication with the exhaust duct (30), and a cover (40) disposed in the heat exchanger duct (36, 38) downstream of the air-cooled heat exchanger (34). The cover (40) is movable between an open position, in which the cover (40) allows the cooling air to flow through the air outlet (34b) into the exhaust duct (30), and a closed position, in which the cover (40) substantially blocks the air outlet (34b). The cover (40) is operable to move from the open position to the closed position at a predetermined temperature of the cover (40).