Aghajani, Hamed (2013) Modelling the initial spray characteristics of fire sprinklers. (PhD thesis), Kingston University, .
Abstract
Sprinklers are automatically activated fixed installation suppression devices. They have found extensive applications due to minimum protection they provide for a wide range of applications including residential and warehouses. Modelling sprinkler atomization is a challenging task, due to the stochastic nature in impingement of water jets and the added complexity of sprinkler configuration. In the literature, a spray initiation framework has been developed to address the multidimensional stochastic complexity associated with fire sprinklers. The initial sprinkler spray is completely characterized in terms of the following main parameters: droplet spatial location (radius, elevation angle and azimuthal angle), droplet velocity, droplet diameter and the spatial volume flux. The present thesis aims to improve the prediction of the initial sprinkler spray characteristics through exploring different physics based modelling approaches. The sub-models for film flow and sheet trajectory adopted in the development of the fire sprinkler spray models are reviewed. Three new deterministic approaches for sprinkler atomization have been proposed by employing an existing film submodel and a detailed water sheet trajectory sub-model which has never been used for fire sprinkler applications. The developed methods simulate the orthogonal impingement of water jet to a deflecting disk, with the potential to be adapted for tilted deflectors. A comparative analysis is carried out between the three introduced methods and a reference model in terms of their predictions for droplet median diameter and initial droplet location for a range of ambient temperatures and water injection pressures. The developed methodologies have been further expanded by incorporating random behaviour to the spray formation procedure. The stochastically predicted mean velocity and volume median diameter have been compared against robust experimental data and empirical correlations. The improvements obtained by the developed methodologies are promising. In further steps, a dimensionless formulation for predicting spray characteristics, sheet breakup distance and droplet sizes, in impinging atomizers have been developed. The developed formulation is validated for impingements led the spray to occur in the rim breakup mode. Building on the proposed methodologies, a semi empirical model has been developed capable of predicting the near field spray characteristics such as spatial distribution of droplet sizes, velocities and spray volume flux from local volume fraction measurements. The research outcome would benefit the computation fluid dynamic packages to initialize the spray in a more realistic manner. The study undertaken would lead to more efficient fire suppression and/or water and fire interaction studies. In addition to this, the methodology could reduce the cost of experiments in order to quantify new sprinkler sprays.
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