What is the purpose of the pressure relief door fitted on the PW1127G-JM engines of the A320NEO?
Inside the nacelle cavity of a turbofan engine and in the casing of the engine core, there are different zones or closed compartments used for ventilation/cooling and in the case of the nacelle lip, also for anti-icing (see the first two images below). If a highly-pressurised duct (such as the anti-ice inlet duct shown in the 3rd image) bursts, the pressure builds up to a dangerous level, despite the ventilation exits. To prevent structural damage to these closed compartments, pressure relief doors are installed which are triggered automatically.

 





Edit: my first answer lacked some details, so I rewrote it.