This paper presents preliminary results of an experimental study of water mist suppression mechanisms in a PMMA boundary layer diffusion flame. It is motivated by the need to optimize the design of water mist systems for deployment in Navy ships. Results of tests to characterize the base case (no suppressant) flame and tests to study the effects of oxygen dilution are presented. These results can be summarized as follows: 1. Temperature profiles in the gas phase show a higher gradient in the leading section, and this is consistent with the higher burning rate measured in this section. 2. A disproportionately higher burning rate is measured in the leading section; therefore, most of the mist should be delivered in this section for optimum effectiveness. 3. The dilution of oxygen tends to destabilize the flame by increasing the quenching distance. With 19.6% oxygen (by volume) the quenching distance was about 15 mm under the current test conditions. Oxygen dilution also led to lower flame temperature, which is due to the decrease in heat generation rate especially in the leading section of the flame where chemistry is important.
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