The Throttle Effect – Blower Fan Versus Exhaust Fan

One of the risks connected to fires underground is the throttle effect which may cause unforeseen smoke spread. This paper investigates the throttle effect for a blower fan and an exhaust fan case in a mine drift. The aim of the paper is to perform a parametric study on the throttle effect, varying influencing parameters such as the heat release rate and fan flow velocity. Data from fire experiments in a model-scale mine drift and results from CFD simulations were used during the study. It was found that the differences between the two fan cases were significant both in magnitude and occasionally in direction. For the base cases the throttle effect as well as the backlayering were more severe in the exhaust fan case. When increasing the heat release rate to 116 kW an increasing backlayering resulted, but the throttle effect was found to increase for the exhaust fan case and decrease for the blower fan case. The throttle effect decreased in the blower fan case as the gas density decrease levelled off, but the flow velocity increased even further, causing an increase in the downstream mass flow rate. This finding was confirmed by similar experimental results in model-scale mine drifts. The resulting mass flow rate induced by the fire plume changes was found to be higher than the externally imposed increase of the fan flow velocity. When increasing the distance between the fire and the exhaust fan, the backlayering increased and the throttle effect decreased.