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Scientific research: Cold Cut Cobra outperforms alternative systems

New research, once again, shows that the coldcut™ cobra is outstanding for fighting fires in buildings compared to other systems on the market. The effectiveness of the very small droplets is greatly enhanced by the stirring effect due to the high speed in the water jet and a dimensioned water flow where almost all of the used water vaporizes and causing no water damages.

This summary presents the simulation results of the coldcut™ cobra when used for firefighting in conventional structures by the aid of computerized simulations and experimental data. Full information in the SP Report CFD simulations of the Cutting extinguisher.1

The simulations were done using Fire Dynamics Simulator which is software for modelling fires. As input for the spray model, data was used from earlier research focused on droplet sizes and velocities in the spray.2 In the study, different room types were used to analyze differences that appear with different geometries, obstructions, ventilation aspects etc. For validation, real scale fire tests were conducted for the living room scenario. The result from each scenario demonstrated that the Cobra was outstanding in gas cooling, stirring effects and effectiveness of used water, compared to other systems that were simulated.

Findings of this study confirm that the Cobra can be used for combating fires in confinements using less water than traditional methods with an outstanding result. This is done by reducing both the gas temperature and relative concentration of oxygen in the room instead of cooling the surface of the fuels as traditional methods do. Other systems that were evaluated was a 40 bar system with 100 liters/minute (fog nail) and a 100 bar system with a flow rate of 40 liters/minute (“alternative cutting extinguisher”). The Cobra delivers 58 liters/minute at approximately 300 bars pressure.

The exact understanding of the jet behavior has been lacking until recently when the drop size distribution of the spray from the cutting extinguisher was measured experimentally using laser diagnostics. Knowledge of droplet size distribution allows for a more analytic analysis of the effect of the cutting extinguisher, as well as for numerical simulations.Some of the results from the study are shown in figures below (living room scenario). The results are summarized in the following bullet points:

The first illustration shows the isolated gas cooling effects. The second illustration demonstrates the residual water on walls and floors, which only create collateral damage from water run-off (consider that the systems deliver 100, 58 and 40 liters, respectively).

 

• Gas cooling effect

The high velocity of the Cobra jet induces mixing in the confinement (stirring effects), enhancing the interaction of droplets with hot combustion products and promoting the vaporization of the injected water. Furthermore, the induced momentum to the gases in the room together with the vaporization of the injected water reduces the overall gas temperature inside the room way faster and more efficient than any of the other systems.

• Inerting effect

The result shows that the mean water concentration in the room is as high as 35% for the Cobra, 27% for the “alternative Cobra”, and as low as 15% for the fog nail, for the simulated conditions. The high concentration (35%) is very important for the inerting effects in the room and was maintained for a well ventilated scenario.

• Accumulated water on walls and floor

Water damages due to firefighting water is a current subject for discussions both in Sweden and on an international level. The simulations shows that the firefighting water from the Cobra vaporize up to 90 % causing no water damages. The result are similar for the “alternative cutting extinguisher”. The results for the fog nail shows that almost all used water was accumulated on walls and floor, causing costly damages on the building.

• Important learnings from this study

Generally speaking, suppressing fires in the way that the Cobra does reduces the possibility of fire spread and flash over in the compartment and at the same time provides a safe working environment for the firefighters. This combination is unique on the market for firefighting systems.

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CFD simulations of the Cutting Extinguisher, Svensson, R.,Lindström, J., Ochoterena, R., Försth, M., SP Report 2014:43, SP Technical
Research Institute of Sweden

2 Spray characterization of the cutting extinguisher, Försth, M., Ochoterena, R., Lindström, J., SP Arbetsrapport 2012:14, SP Technical
Research Institute of Sweden. Research done by SP Technical Research Institute of Sweden, sponsored by VINNOVA.

CFD Simulations of the Cutting Extinguisher final_report.pdf

Scientific research: Cold Cut Cobra outperforms alternative systems.pdf

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