High Flash-point Fluid Flow System Aerosol Flammability Study and Combustion Mechanism Analysis

The existence of flammable aerosols creates fire and explosion hazards in the process industry. Due to the operation condition of high pressure circumstances, heat transfer fluids tend to form aerosols when accidental leaking occurs on pipelines or storage vessels. An aerosol system is a complicated reactive system; there are neither systematic flammability data similar to the case with pure gases, nor clearly described ignition-to-combustion process of a droplet-air mixture system.

The flammable regions of three main, widely-used commercial heat transfer fluids: Paratherm NF (P-NF); Dowtherm-600 (D-600); and Plate Heat Exchange Fluid (PHE), were analyzed by electro-spray generation with laser diffraction particle analysis method. The aerosol ignition behavior depends on the droplet size and concentration of the aerosol. From the adjustment of differently applied electro-spray voltages (7-10 kV) and various liquid feeding rates, a flammable condition distribution was obtained by comparison of droplet size and concentration. All of the fundamental study results are to be applied to practical cases with fire hazards analysis, pressurized liquid handling, and mitigation system design once there is a better understanding of aerosols formed by high-flash point materials.

On the other hand, the process of combustion from initial stage to global flame formation was simulated with COMSOL-multi-physics in terms of heat transfer, droplet evaporation, and fluid dynamics of liquid-air interaction. The local temperature change through time, as an indicator of luminous flame appearance, was analyzed to describe the flame development and ignition delay time of aerosols. We have conducted a series of simulation regarding physical formula in description of this combustion process, and will conclude with how temperature distribution influenced the appearance of luminous flames, which was the symbol of successful ignition of aerosol. The mitigation implementing timing and location can be characterized with further understanding of this combustion process. The potential application of the ignition delay will be beneficial to the mitigation timing and detector sensor setting of facilities to prevent aerosol cloud fires.

Finally, the scientific method of aerosol flammability study was discussed for its potential impacts on experimental results. A modeling point of view was introduced, with the analysis of electric field application on fuel droplets, and the related fundamental study of the ignition phenomenon on aerosol system. Existing charges from electrospray is beneficial for the monodispersity and control of aerosols for fundamental study. However, the additional charges accumulated on the droplet surfaces are likely to have impacts on flammability due to the excess energy they applied to the aerosols system and droplet-droplet distraction or turbulences. This is a re-visit of aerosol flammability study method, with a conclusion that charges did have positive impact on droplets? ignition concentration range with a balancing effect on turbulence increase to reduce the ignition chance.