Entrainment of Air into Thermal Spill Plumes
Thesis DisciplineFire Engineering
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The design of smoke management systems for buildings such as atria, covered shopping malls and sports arenas require appropriate calculation methods to predict the volume of smoky gases produced in the event of a fire. The volume of smoke must be calculated in order to determine the required fan capacity or ventilator area for a smoke management system. In design, consideration is often given to entrainment of air into a smoke flow from a compartment opening that subsequently spills and rises into an adjacent atrium void. This type of plume is commonly known as a thermal spill plume. There has been much controversy over the validity of various entrainment calculation methods for the spill plume and there are considerable differences in the calculated smoke production rates using these methods. There are also scenarios involving the spill plume where design guidance is very limited. Whilst over-sizing of the required smoke exhaust can be uneconomical, under-sizing can compromise the design objectives.
This work attempts to rigorously characterises thermal spill plume entrainment using new data obtained from an extensive series of 1/10th physical scale modelling experiments, supported by numerical modelling using Computational Fluid Dynamics.
Spill plume behaviour and subsequent entrainment appears to be specifically dependent on the characteristics of the layer flow below spill edge, particularly in terms of the width and the depth of the flow. Plumes generated from narrow, deep layer flows entrain air at a greater rate with respect to height compared to plumes generated from wide, shallow layers. The findings of this work go some way to explain and reconcile differences in entrainment reported between previous studies.
New guidance has been developed for the thermal spill plume in smoke management design, in the form of a range of new simplified design formulae,improvements to analytical calculation methods and an initial assessment of the use of numerical modelling using Computational Fluid Dynamics.