FIRE-SPRINKLER SIMULATIONS USING THE EULERIAN-EULERIAN APPROACH.
The installation of sprinkler systems within occupied enclosures has always been seen as an effective means of reducing fire losses. A recent example of thirteen premises fire bombed in Northern Ireland illustrates this very well. Of these premises eight were totally destroyed whilst four escaped serious damage due to the intervention of the installed sprinkler system. This document describes the Fire Safety Engineering Groups activities in the development of CFD based mathematical models for the simulation of fire suppressant systems using the Eulerian-Eulerian approach.
The use of water sprinkler, spray and mist systems as a means of fire extinguishment and containment has tremendous potential. Areas of application include traditional warehouse and factory environments and the less obvious shopping malls, office buildings, museums, domestic residences, passenger aircraft, military and civil shipping, off-shore oil platforms, etc.
While sprinkler systems have been in use since 1851, many questions relating to their optimal operational configurations and use in novel applications - such as aircraft mist systems - remain unanswered. Parameters such as flow rates, droplet size, throw angle, orifice size, activation times etc need to be carefully considered for each installation, and issues relating to the interaction between compartment ventilation and water sprays remain unresolved. Due of the costs associated with full-scale physical experimentation and the limitations inherent in empirical models, it is unlikely that these issues will be resolved solely by traditional methods.
Using the field modelling approach it is possible to simulate the action of water sprays in a fire compartment. In this case there are now two interacting physical phases, the gas phase involving the general fluid circulation of the hot combustion products and the liquid phase, representing the evaporating water droplets. The numerical procedure is adjusted to take into account these interacting phases. This set of equations including the interphase processes of drag, heat and mass transfer may be solved using the SIMPLEST and IPSA procedures as implemented within the PHOENICS CFD code.
The fire-sprinkler model developed by the Fire Safety Engineering Group is based on the Eulerian volume fraction method. In this model the key assumption is that each 'computational volume' making up the simulated enclosure consists of either air, water or a mixture of both. The model takes into consideration sprinkler/spray parameters such as flow rates, droplet size, throw angle, orifice size and activation times.
The technique has been applied to various building scenarios including office and hospital ward fires.
Comparisons between experimental and numerical gas temperatures indicate that the model is capable of predicting the correct trends in temperature variation.
Reference:
An Extension of the Fire-Field Modelling Technique to include Fire-Sprinkler Interaction: Part 1, The Mathematical Basis. Int J of Heat and Mass Transfer, Vol 36, No.6, pp.1435-1444, 1993. N Hoffmann, and E R Galea.
See publications # 59, 58, 47 - 45, 37, 36, 24, 18.
