Unstable operation of a main fan in an underground coal mine ventilation system causes transient states of air and gas flow. Such disturbances can have a wide range and a considerable influence on the mine ventilation system and even cause life and health risk for miners. The paper presents the observation of air parameters during unstable operation of the fan-network system at "Siemianowice" hard coal mine. After closing of the mine, which became a pumping station with a reduced ventilation network, a new smaller axial fan has been selected. During measurements of the fan performance unexpected oscillations of the flow and pressure have been observed. Disturbances of flow appeared during increase of resistance by means of dampers in flow appeared during increase of resistance by means of dampers in the channel of the fan, what caused operation of the network on the left part of the fan performance curve. The pulsation of air parameters was of regular oscillation nature, and although their period and amplitude were stable, yet the parameters returned spontaneously to the stable state after a short time. The phenomenon appered in a random way. The parameters oscillation occurred not only for the fan itself but also propagated into workings of ventilation network. To observe the phenomenon measuring instruments - particulary barometers and anemometers - have been installed in the fan station and flow. Additionally a computer simulation of the unstable operation of a fan-network system was made using for this purpose a lumped constants model and compressible flow model.
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