Engine exhaust reverse flow prevention

A heat exchanger duct assembly for an aircraft engine having an engine central longitudinal axis, comprising:

• a heat exchanger duct having a duct wall extending from an air inlet to an air outlet, the air inlet connectable to a source of pressurized air, the air outlet fluidly connectable to an exhaust duct of the aircraft engine;
• an air-cooled heat exchanger disposed between the air inlet and the air outlet;
• a chicane disposed in the heat exchanger duct between the air-cooled heat exchanger and the air outlet, wherein the chicane defines a diverted airflow pathway having a first plate extending from a first position on an inner surface of the duct wall to a first plate distal end and a second plate extending from a second position on the inner surface of the duct wall opposite the first position to a second plate distal end, the second plate distal end extending past the first plate distal end to hinder the exhaust gas in the exhaust duct of the aircraft engine from flowing upstream towards the air-cooled heat exchanger at a shutdown condition of the aircraft engine, and wherein the second plate distal end is disposed closer to the air outlet than the first plate distal end;
• and
• an evacuation valve in the chicane, the evacuation valve selectively openable for venting exhaust gas from the chicane.

A method of operating an aircraft engine, comprising:

• the aircraft engine comprising:
• an exhaust duct receiving a gas flow from the aircraft engine;
• and
• a heat exchanger duct having an air inlet receiving a flow of cooling air, an air outlet fluidly connected to the exhaust duct, and a duct wall extending from the air inlet to the air outlet;
• an air-cooled heat exchanger disposed in the heat exchanger duct between the air inlet and the air outlet;
• a diverted airflow pathway in the heat exchanger duct between the air-cooled heat exchanger and the air outlet, the diverted airflow pathway including a first plate extending inwardly in the heat exchanger duct from a first position on an inner surface of the duct wall to a first plate distal end, and a second plate extending inwardly in the heat exchanger duct from a second position on the inner surface of the duct wall opposite the first position to a second plate distal end, the first plate distal end and the second plate distal end extending past one another in a direction transverse to the duct wall;
• and
• a valve in the duct wall of the heat exchanger duct between the first plate and the second plate, the valve selectively fluidly connecting the heat exchanger duct to an evacuation location, the valve being movable between a closed position during an operating condition of the aircraft engine to fluidly disconnect the heat exchanger duct to the evacuation location, and an open position during a shutdown condition of the aircraft engine to fluidly connect the heat exchanger duct to the evacuation location;
• the method comprising:
• operating the aircraft engine in a powered mode that includes passing a gas through the heat exchanger towards the exhaust duct of the aircraft engine;
• and
• transitioning the aircraft engine from the powered mode into shut down, the transitioning including preventing exhaust gas from flowing from the exhaust duct towards the heat exchanger by navigating the exhaust gas through the diverted airflow pathway downstream of the heat exchanger, and evacuating the exhaust gas via the valve disposed fluidly between the heat exchanger and the exhaust duct.