The Mathematical Modelling Of The Interaction Between Sprinkler Sprays And The Thermally Buoyant Layers Of Gases From Fires

In recent years many doubts have been expressed about the simultaneous use of sprinkler sprays and smoke ventilation. In order to examine these smoke control arrangements, a mathematical model to describe the interaction between sprinkler sprays and the thermally buoyant layers of fire gases has been developed; and incorporated into a computer program. The model is three dimensional and can take account of the parameters of the building; the siting of sprinkler heads; and the spray envelopes. The building assumes a corridor geometry and that the fire source is remote from the location of interest. The corridor exit can be described by a deep down stand; a doorway; or a wide opening with ventilation. Smoke flow can be defined in three ways: by using simplified methods available in the literature or a standardised procedure developed for the model. This development allows for a realistic buoyancy profile to be described over the height and length of the corridor. Physical properties and transport phenomena of the smoke are characterised by using the maximum temperature and depth of the layer as input variables. The smoke and clear air below, is divided into a matrix of finite control volumes. The size of the control volumes can be selected by the user of the program. Known spray characteristics of several sprinklers, at various operating pressures, are used as input data for the droplet emission calculations. The projection of droplets has been examined sequentially around the circumference of the strike plate in discrete discharge sections. This approach allows the continuous cooling of the smoke layer to be calculated. Algorithms to describe the heat, mass and momentum transfer of droplets, over a wide range of Reynolds numbers, were developed following a review of the literature. The mathematical model can examine the effects of many variables on the transfer of heat from the buoyant layer. The available results are a) thermal and physical histories of individual drops within each spray envelope. b) the effect of sprinklers on the buoyant layer as it passes through individual spray envelopes. c) the consequence of sprinkler operation on the egress of smoke from the building. The results are presented in a series of data files, which can be represented in a graphical manner. Analysis of the results allows for a quantitative study of the interaction between arrays of sprinklers and the buoyant layer and the removal of smoke from the building.