M.Sc Thesis
M.Sc Student Gilboa Erez Quenching of Turbolent Flames by Sprays Department of Mechanical Engineering Mr. Alexander Yarin

Abstract

This research project deals with fire quenching and in particular with the investigation of the position of a water jet relative to the fire source. It turns out that the parameters such as the position of the water jet source, its velocity, its mass flux and angle of flow have far reaching ramifications on the ability to extinguish a blaze, and the quenching time.

In the realm of published literature there are a number of articles that deal with sprinklers. Some of them are financed by commercial companies in order to test and authorize newer versions of sprinklers, whereas others deal with testing the different effects of flow parameters on extinguishing differing strengths of fires.

At the first stage, the survey of the research literature in the thesis provides information that deals with a technical explanation of the sprinkler capability and on the commercial parameters for which they are used. Following this, additional information such as radius of the water droplet, mass transfer coefficient and a suitable Sherwood number relevant to this, drag, heat transfer and mass coefficients are provided. These parameters are used, in order to estimate dimensionless groups that are needed for the numerical computations of the model.

The thesis deals with finding the flow field of a fireball, at this stage, without droplets.  This flow field was generated by a distributed vortex in parallel flow. The border line of the vortices pair is calculated as well.

Next, the gas flow velocity components are calculated at each point of the field, at which droplets extinguish the flame. Calculations like these encompass integral formulae. These are not efficient and are too time consuming, and hence the components of the gas velocity are calculated via interpolation.

The research thesis models the process of extinguishing the fireball by means of the absorption of heat by evaporating water droplets moving within it. The assumption is that the fireball is composed of a fuel/ oxygen mixture that was ignited, be it deliberate or of natural causes.

The gas temperature within the fireball changes as a result of a number of physical processes which we treated by the model. The additional tools required for a numerical code, are obtaining the gas temperature, kinematics and dynamic calculations of droplets and assembling equations for evaporation, size and temperature of the droplets within the fireball.

On the basis of all these tools, a numerical code was assembled, simple but accurate that has become a real valuable working tool and which can be used as a source of information for finding the optimum position of a sprinkler. More over; this model may be used in order to save much time and effort in building more complicated models that simulate the extinguishing of fires with the aid of automatic systems.