Aerosol Generation Approach and Aerosol Combustion Behavior Study

Aerosol generation approach and aerosol combustion behavior study

Shuai Yuan, Yogesh Koirala, and M. Sam Mannan

Mary Kay O¡¯Connor Process Safety Center

Artie McFerrin Department of Chemical Engineering

Texas A&M University

College Station, Texas 77843-3122, USA

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

ABSTRACT FOR AIChE/GCPS

The flash points of heat transfer fluids used in heat exchangers are normally higher than 200¡æ, which makes people ignore their fire and explosion potential.  In an incident in Milliken Carpet factory on 31 January 1995 that resulted in over $400 million total loss, the cause of this incident was attributed to the leak of heat transfer fluid in the form of aerosol at high pressure.  An ignition source ignited the heat transfer fluid aerosol when the surrounding temperature is below its flash point.  Although Eichhorn introduced the concept of aerosol flammability limits, the values of aerosol flammability limits are still unclear.  Understanding the aerosol formation mechanism and aerosol flammability region helps process designers to implement inherently safer design to minimize the risk.

Electrospray method is used to generate aerosol.  The liquid fluid with sufficient electric conductivity is dispensed by syringe pump through the small nozzles.  The nozzles are aligned in a few kilovolts electric field.  Under the influence of electric field force, the fluid is atomized to small droplets.  The advantages of electrospray method compared with other methods are the droplet size controllability and spray stability.  Various literature has shown that the liquid properties, such as liquid conductivity, surface tension, permittivity and viscosity, play the most important roles in determining droplet size.  In the heat transfer fluid chosen to study the aerosol, the droplet size is controlled by changing the conductivity of fluid by adding additives, adjusting the electric field intensity and shifting the fluid feeding flow rate.

The other aspect of this study is aerosol combustion behavior.  Aerosol combustion can be divided into three groups, 1) single droplet combustion, 2) combustion of droplet clouds and 3) combustion in more practical situations.  This research focuses are the first two aspects, single droplet combustion and combustion of droplet clouds.  The characteristics regarding aerosol combustion is flame propagation speed.  Based on the valuable works on single droplet flame propagation speed done by Ballal et al and Polymeropoulos et al, a theoretical flame propagation speed model concerning combustion of droplet clouds is proposed.  In order to test the validity of the proposed model, a fast camera was used to catch the flame propagation after the ignition of aerosol generated by electrospray.  Through the analysis of the flame propagation video, the aerosol flame propagation speed is determined and used to compare with the theoretical results.

Keywords: Aerosol, Electrospray, Flame propagation speed