Flow Control and Design of Environmentally Benign Spray SystemsEPA Grant Number: R829587
Title: Flow Control and Design of Environmentally Benign Spray Systems
Investigators: Plesniak, Michael W. , Frankel, Steven H. , Sojka, Paul E.
Current Investigators: Plesniak, Michael W. , Frankel, Steven H. , Shu, Fangjun , Sojka, Paul E.
Institution: Purdue University
EPA Project Officer: Richards, April
Project Period: January 1, 2002 through December 31, 2004
Project Amount: $350,000
RFA: Technology for a Sustainable Environment (2001) RFA Text | Recipients Lists
Research Category: Sustainability , Pollution Prevention/Sustainable Development
Description:This program is aimed at reducing pollutant emissions in a broad spectrum of spray processes by improving transfer efficiency through flow control. For many spray coating processes the transfer efficiencies are lower than 50%, i.e., half of the material sprayed ends up as "overspray" that is released to the environment. We have previously demonstrated a ligament-controlled effervescent atomization technology to eliminate pollutants at the source by allowing spray formulations with environmentally benign components. However, VOCs cannot be completely removed from all consumer products because some active ingredients lose their efficacy when dissolved in water (e.g., water-based paints are unable to provide the finish quality that some furniture customers demand). More importantly, particulates cannot be removed from paints/coatings because they are the "active" ingredients. As a result, particulate emissions will always remain a concern for coating processes.
Approach:Our objectives are two-fold. (1) The practical goal is to reduce pollutant emissions by improving the droplet transfer efficiency, i.e., reducing overspray and, hence, reducing particulate and solvent effluents. Our hypothesis is that spray turbulence characteristics can be controlled by modifying the dispenser exit orifice geometry. Passive control of local turbulence at the surface to be coated will be achieved by making these geometrical changes to the nozzle exit orifice. Turbulent eddy manipulation at the nozzle will alter the small-scale turbulence at the target and thus increase transfer efficiency. (2) The fundamental goal is to demonstrate that the widely held view of how a design engineer should increase spray transfer efficiency, i.e., make the spray "softer" by decreasing its momentum (equivalent to lowering the gun supply pressure), is incomplete and that the turbulent flow structure near the target plays an equally, if not more important, role.
Our approach consists of a synergistic coupling of complementary experimental studies and numerical simulations. We will control the turbulent structures and evaluate the resulting turbulent eddy sizes and kinetic energy distributions for a number of modified orifices. Non-invasive optical diagnostics, including Particle Imaging Velocimetry (PIV) and Phase Doppler Particle Analyzer (P/DPA), will be used to quantify the structural and statistical characteristics of the complex two-phase spray turbulence. Large Eddy Simulation (LES) of the governing equations will be used to further investigate the details of the droplet-turbulence interactions. These simulations will provide additional insight into the flow physics and transport processes, in particular the relative roles played by spray momentum and local turbulence at the surface to be coated on droplet transfer efficiency. A thorough understanding of the turbulence field and droplet deposition processes will enable design optimization of the spray nozzles and to facilitate model development for industrial computational codes to be used.
Expected Results:Successful completion of this program will provide the impetus for a substantial paradigm shift for spray transfer efficiency?from a view of a process dominated by spray momentum to one where local turbulence at the surface to be coated plays a major role. This work will provide industry with the sound science base necessary to reduce particulate and VOC solvent emissions without losing product efficacy. The proposed work is unique because (1) it will clearly demonstrate the relative importance of spray momentum and local turbulence levels at the surface to be painted on transfer efficiency, and (2) show how spray turbulence characteristics can be tailored to increase the transfer efficiency of the smallest droplets while having little effect on the transfer efficiency of the largest drops. Results in our laboratory show that control can be achieved with nozzle geometries that are easily manufactured using standard techniques. Miniaturization and economical mass production can be achieved by using MEMS technology. Reducing VOC solvent and particulate emissions clearly satisfies the Non-reaction-based Engineering for Pollution Avoidance and Prevention goals, i.e., novel thermal or fluid and/or multiphase/particulate systems resulting insignificantly lower hazardous effluent production, as well as Environmentally Benign Manufacturing.
Successful implementation of this methodology will (1) allow reduction of particulate and VOC solvent emissions via transfer efficiency improvement and (2) significantly enhance our understanding as to which physical mechanism controls transfer efficiency. Paint spray contains hazardous components including lead, chromium, polyisocynates and liquid organic solvents. Substantial exposure can cause nervous-system disorders, skin and eye irritation, and a variety of respiratory problems. Organic compounds used in solvents include hydrocarbons, both aliphatic and aromatic such as toluene, and oxygenatic compounds such as ethers, ketones and esters. Inorganic components such as the yellow and red chrome pigments used for color may be carcinogenic to the lungs, nasal cavity, larynx and stomach, cause ulcers of the skin and mucous membranes and exzematous dermatitis. Implementation of the control methodology by consumer product and paint/coatings manufacturers is expected to eliminate over 34,000 tons of VOC solvent emissions and over 48,000 tons of particulate emissions per year. Because of the widespread use of consumer products and paints/coatings by people of all races, cultures and economic statuses, the project described here is expected to have considerable impact in all areas of the US and the world.