ID 52

Modelling of internal nozzle flow in high pressure water mist injector for fire suppression applications

 

Abstract:

Water mist atomisers for fire suppression applications are a rather recent technology, able to generate drops with volumetric mean diameters typically between 30 and 300 micrometers, with injection flow rates at least one order of magnitude less than conventional sprinkler systems. Water mist atomisers require high injection pressure and small discharge orifice making their injection systems more crucially dependent on their geometry and operating conditions. A liquid lamella is formed in the nozzle discharge hole, due to the swirl motion conferred to the fluid by the injector inclined passages. The jet at the exit section of the atomizer has a conical shape and its interaction with the external gas induces instabilities on the liquid surface that grow until the jet breaks-up into ligaments and droplets. One of the most important parameter that affects the spray formation is the lamella thickness at the nozzle exit. In the present work, the flow development inside the injector and at the nozzle exit proximity is predicted by implementing 3D Large Eddy Simulations based on the Volume-of-Fluid methodology. The flow is assumed to be incompressible under isothermal non-reacting conditions. The effect of internal nozzle geometry on the lamella characteristics and its interaction with the surrounding air is investigated, for different operating conditions, by modifying independently the inclined swirling channels and the size of the conical swirl chamber. The injector behaviour is mapped and the results can be used to improve semi-empirical correlations available in the open scientific/technical literature.