Explain how the infrared flame detector work?
Infared flame detectors have an imaging sensor that "sees" infared light.

When a flame ignites the various  types of materials burn at different strengths, intensities, and -what we care about- wavelengths.  In the visible  spectrum  we can describe a flame as red or blue which can be simplified flames discrete wavelengths.

The IR sensor can detect the wavelengths  given off outside that visible light spectrum. This helps eliminate sources  like a technicians  flashlight from setting off a pure visible  optical sensor.

An example from Wikipedia shows a distribution  of this emission.

! Fire spectrum