PPV Fans (Positive Pressure Ventilation Fans) are used in firefighting to remove smoke from a burning building, so that fire fighters can have a clear view inside the house and injured people do not have to breathe toxic smoke. This can be done by placing a PPV fan in a distance of about two meters in front of a door of the burning building. On another, carefully chosen position in the building, e. g. a window, a door or at the roof an opening has to be created, where the smoke can leave the building. The same volumetric flow rate of gas that is blown into the building by the PPV fan has to leave the building at a chosen opening. Because the gas entering the building is air and the gas leaving the building is a mixture of smoke and air, the smoke concentration in the building can be reduced.
To test the performance of such PPV fans, a test building with a door in the first floor and a window in the 3rd floor has been built. To measure the volumetric flow rate of the smoke and air mixture through the window in the 3rd floor that is leaving the building, a flow meter nozzle was designed. The design process was done using the commercial Navier Stokes solver Star CCM+, where three nozzle designs, such as a nozzle with constant velocity increase, a quarter circle nozzle and a non-curved nozzle were investigated for different volumetric flow rates. Also, a rounding at the window, where the nozzle is placed, was investigated to prevent flow detachment and shock losses at the inlet of the nozzle.
The volumetric flow rate through the nozzle can be calculated, by measuring the pressure at the nozzle wall (before the contraction) and applying Bernoulli’s law, the continuity equation and assuming atmospheric pressure at the free jet flow at the end of the nozzle. The so calculated volumetric flow rate was compared with the actual flow rate, given by the numerical CFD simulations. With these values, the nozzle specific coefficient of discharge for several volumetric flow rates has been calculated and a function fitting was done to get obtain analytical relation between pressure and volumetric flow rate.
The detailed design process of the three nozzles, the numerical results of the CFD studies and the determination of the nozzle specific coefficients of discharge are shown and discussed in detail in this work.