Abstract: By using the principle of photodiode receiving the radiant light intensity of flame and converting the optical signal into electrical signal, the change of flame intensity and pulsation frequency of n-heptane oil pool fire with time in confined space was studied. Results: Within a certain period of time, after the n-heptane oil tank fire was ignited, the combustion intensity of the oil tank fire increases rapidly due to the sufficient oxygen in the restricted space for a certain period of time. When it reaches a certain time and intensity, the flame intensity begins to weaken gradually due to the reduction of fuel and insufficient oxygen in the restricted space.It was observed from the experiment that in the early stage of oil tank fire combustion, the flame did not fluctuate. After burning for a period of time, the oxygen supply in the restricted space was insufficient, and the outside world needed to provide oxygen. The flame began to fluctuate, and the flame pulsation frequency increased rapidly, reaching a maximum of about 1.5 Hz, the fitting result of flame pulsation frequency and oil pan diameter is f=1.06D- 0.5. In this paper, the low cost photodiode is used to monitor the flame pulsation frequency of oil pool in restricted space, which is beneficial to the development of fire detection technology in restricted space.

Key words: enclosed space, pool fire, pulsation frequency;flame intensity, fire detection