(188q) 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.

[1] D. R. Ballal
and A. H. Lefebvre. Ignition and flame quenching of flowing heterogeneous
fuel-air mixtures. Combust. Flame, 35:155-168, 1979

[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