CFD Simulation of Charged Aerosol Combustion Under High Electric Field

CFD simulation of charged aerosol combustion under high electric field

Shuai Yuan^1,2, Joseph Sang-II Kwon², and M. Sam Mannan^1,2

¹Mary Kay O¡¯Connor Process Safety Center

²Artie McFerrin Department of Chemical Engineering

Texas A&M University

College Station, Texas 77843-3122, USA

+1(917)690-1208, sy2516@tamu.edu

There is a general misconception that fluids are safe below their flash points. However, an aerosol can be ignited below the fluid¡¯s flash point, due to the large surface area of aerosol which will increase the evaporation rate significantly [1]. The fire hazard associated with combustible liquids has received more attentions in recent years because of several serious incidents, such as Milliken carpet factory fire and Buncefield explosion and fire [2]. To study the flammability of aerosols, it is important to consider several factors such as liquid/gas phase temperature, fuel-oxygen ratio, the aerosol droplet size, velocity and concentration. However, due to the inherent complexity arising from the spray combustion process, there is a lack of systematic framework as well as extensive experimental work on understanding the combustion and flame propagation of the generated aerosol particles.

Within this context, we will first collect experimental data by generating aerosol droplets through an experimental pilot scale electrospray process at Mary Kay O¡¯Conner Process Safety Center at TAMU. The system has a modular design that applies the electrostatic field for dispersing liquid into small aerosol particles using various commercial heat transfer fluids [3]. The measured variables for this electrospray system include the Sauter Mean Diameter of droplets, droplet volumetric concentration and the flame propagation speed, while the manipulated inputs are the liquid flow rate from the nozzles and the applied voltage on the nozzles. Then, we ignite the generated aerosols by a propane flame and record the generated flame structure and its propagation speed by a high-speed camera [4].

In order to better understand the underlying dynamics of the aerosol generation and flame propagation processes, we will also focus on the dynamic modeling of the charged aerosol generation and combustion using open source CFD software, OpenFOAM. Within the simulation, the two-phase reaction flow is modeled through the Eulerian-Lagrangian method. Specifically, the gas phase is described by the Eulerian method, which includes mass conservation, momentum conservation, energy conservation, species conservation, turbulence and combustion, while the aerosol droplets are tracked by the Lagrangian manner considering all the forces acting on each droplet. Specifically, the gas phase and liquid phase are interfaced through the evaporation model for droplets, and the drag force acting on the droplets is coupled with the gas phase momentum conservation equation. The developed CFD models are validated against the experimental results obtained from the in-house electrospray process.

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[2] M. S. Mannan. The Buncefield explosion and fire¨Clessons learned. Process Safety Progress, 30(2):138-142, 2011

[3] W. Deng and A. Gomez. Influence of space charge on the scale-up of multiple electrosprays. J. Aerosol Sci., 38(10):1062-1078, 2007

[4] S. Y. Huang, X. Li and M. S. Mannan. Paratherm-NF aerosol combustion behavior simulation: Ignition delay time, temperature distribution of flame propagation, and heat kernel hypothesis of combustion process analysis. J. Loss Prevention in the Process Industries, 26(6):1415-1422, 2013