Flow control of environmentally benign spray systems


Sponsor
U.S. Environmental Protection Agency

Principal investigators
Prof. Michael W. Plesniak
Prof. Paul E. Sojka
Prof. Steven H. Frankel

Time period
August 2002 - present

Motivation
Paint spray contains hazardous components including lead, chromium, polyisocynates and liquid organic solvents. Substantial exposure to these materials present in overspray can cause nervous system disorders, skin and eye irritation, and a variety of respiratory problems. Inorganic components such as the yellow and red chrome pigments used for color may be carcinogenic to the lungs, and cause other health problems. Overspray also results in substantially increased costs for raw materials. Thus, it is desirable to minimize the amount overspray, i.e. to improve transfer efficiency.

Objective
Our approach for improving transfer efficiency is to modify the spray flow field by controlling the nozzle geometry so that a larger number of droplets deposit on the target. The methodology is inspired by the results of a study by Hicks and Senser which reported two important conclusions: (i) that drop transfer efficiency increases with drop diameter, and (ii) that the transfer efficiency for the smaller drops is influenced by the turbulent flow field near the surface of the target. This discovery of the link between turbulent transport and small drop transfer efficiency provides a clear indication of how transfer efficiency may be improved by modifying the turbulence characteristics near the surface of the target. Such a modification would have a relatively great impact on the trajectory of the small droplets that have a low probability of impinging upon the surface using current spraying technology (sub-30 μm droplets) and a minor impact on the trajectory of larger drops that have a high probability of hitting the surface (large ballistic drops). Our current objectives including: using IO nozzles to control jets, understanding how IO nozzles influence the jet and compare the flow field of IO nozzle jet with that of conventional jet.

Experimental facility
Both qualitative and quantitative methods are used to study the performance of IO nozzles. Flow visualization results are taken using LIF (Laser Induced Fluorescence) technology. PIV (Particle Image Velocimetry) system is used to make detailed full field velocity measurements.

Fig 1: Sketch and photograph of the experimental setup
Experimental setup

Several IO nozzles are manufactured. Those IO nozzles can be push installed to a 5th order polynomial contraction and able to rotate with respect to their axes.

Fig 2: Round nozzle and IO nozzles
Experimental setup

Results

♦ Flow Visualization Results of Modified 4-point Nozzle v.s. Round Nozzle

Fig 3: End view flow visualization (main flow is out of page)
Experimental setup

Fig 4: Side view flow visualization
Experimental setup

Fig 5: Sketches of the IO nozzle and round nozzle jet
Experimental setup

When trying to understand the physical mechanisms underlying behavior of these jet flows we consider both the round jet and IO nozzle cases. First consider the canonical round nozzle jet. The axial velocity profile changes from a top hat at the exit of the nozzle to continuous distribution at downstream. The momentum and continuity equations must be satisfied and thus ambient fluid must be entrained into jet (see Figure 5(a)). On the other hand, in an IO nozzle jet, entrainment occurs at the valleys even before the fluid exits the nozzle (Figure 5(b)), because the jet fluid inside the nozzle has lower pressure than ambient. Thus, we expect more and earlier entrainment in valley planes, i.e. the incursion of small radial jets. Those small incursion jets are not produced steadily; they are generated alternatively each time when the "braid" region passes through. Due to Kelvin-Helmholtz instability in shear layer, streamwise vortex pairs are generated (Figure 5(c)). Because of limited space within the main jet, the small radial jets cannot extend beyond the centerline of the main jet. As they propagate further into the main jet, the vortex pairs are in increasingly close proximity. When they are near enough, the vortex pairs can reorganize. For example, in Figure 5, vortices 2 and 3 initially constitute a pair, but near the center of the main jet, vortex 1 pairs with vortex 2, and vortex 3 pairs with vortex 4. After this reorganization by pairing, a radially-outward "excursion flow" will be generated by the resulting induced velocity field (Figure 5(d)). Each subsequent braid generates another set of vortex pairs (Figure 5(e)). Thus, further downstream, multiple excursion vortex pairs are observed (Figure 5(g)). A 3-D sketch of the development of streamwise vortices is shown in Figure 6.

Fig 6: Sketch of streamwise vortex structures in the IO nozzle jet
Experimental setup

♦ Other IO nozzle jets

In spray painting, shaping air is often used to control the profile of the spray, typically elliptic. By modifying incursion and excursion vortex, specially designed IO nozzles can also control the profile of the spray, making them elliptic.

Fig 7: End view flow visualizations
Experimental setup

Fig 8: A delta tip nozzle jet can generate 12 pairs of streamwise vortex pairs.
Experimental setup

♦ PIV results for modified 4-point nozzle
The mean and instantaneous velocity fields for the IO nozzle jet on the peak plane near the nozzle (x/D<2) are shown in Figure 9. For comparison purposes, the mean and instantaneous velocity fields of a round nozzle jet are shown in Figure 10. Comparing these mean velocity fields reveals that the IO nozzle jets have greater spreading rates in the peak plane, near the nozzle exit. That is because the excursion streamwise vortex pairs, which are near the peak locations, make the jet fluid intrude into ambient flow providing additional entrainment. This result is consistent with the flow visualization results. Spanwise vortex rings, which are generated due to Kelvin-Helmholtz instability, can be observed in Figure 10(b) at x/D=1.5, and instantaneous velocity is higher in the neck between two vortex rings. For the IO nozzle jet at the peak plane (see Figure 9(b)), no closed loop vortex rings are observed, but there are two "eyes", in which flow directions are the same on both sides of the eyes. That is because the excursion flow associated with the streamwise vortices is superimposed on the spanwise vortex ring. The excursion flow has the same velocity as the main jet flow in the streamwise direction, but the reverse flow in vortex ring is much smaller. Thus, when excursion flow superimposed on the vortex ring, the velocity direction in both side of the "eyes" are same.

Fig 9: Mean and instantaneous velocity fields of the IO nozzle jet at peak plane.
Experimental setup

Fig 10: Mean and instantaneous velocity fields of the round nozzle jet.
Experimental setup

In order to examine the persistence of the IO nozzle flow field, PIV measurements are performed over a larger area. Jet half-widths are calculated at different locations for each nozzle and plotted in Figure 11. The jet half width near the nozzle exit is larger for the IO nozzle in the peak plane and smaller in valley plane, when compared to round nozzle jet. But further downstream, the difference gradually diminishes.

Fig 11: Comparison of jet half width.
Experimental setup

The normal Reynolds stress is a key parameter affecting transport of small spray droplets to the target surface. The Reynolds stress of the IO nozzle jet in both the peak and valley planes is plotted in Figure 12, and compared with that for the conventional round nozzle jet. For statistical convergence, 1,300 realizations are taken for each configuration to compute the normal Reynolds stress. The IO nozzle jet has up to 15% larger normal Reynolds stresses in both the peak and valley plane at the near-nozzle region, but this advantage diminishes with downstream distance.

Fig 12: . Comparison of normal Reynolds stress <u'u'> (a) IO nozzle peak plane (b) IO nozzle valley plane (c) Round nozzle.
Experimental setup

Publications and Presentations
  1. F. Shu, M.W. Plesniak and P.E. Sojka. Indeterminate origin nozzles to control jet structure and evolution. In press in Journal of Turbulence
  2. F. Shu, M.W. Plesniak and P.E. Sojka. Indeterminate origin nozzles to control jets. 11th International Symposium on Flow Visualization, August 9-12, 2004, Notre Dame, IN
  3. F. Shu, M.W. Plesniak and P.E. Sojka. Indeterminate origin nozzles to control jets. Presented at the 56th Meeting of the American Physical Society, Division of Fluid Dynamics, November 2003, Meadowlands, NJ
  4. F. Shu, M.W. Plesniak and P.E. Sojka. Control of an impinging jet emanating from a nozzle of indeterminate origin. Presented at the 55th Meeting of the American Physical Society, Division of Fluid Dynamics, November 2002, Dallas, TX
  5. M.W. Plesniak, F. Shu, P.E. Sojka and S.H. Frankel. Flow control and design of environmentally benign spray systems. Presented at the 2002 Meeting of the American Institute of Chemical Engineers, Technology for a Sustainable Environment session. November 3-8, 2002, Indianapolis, IN
  6. F. Shu, M.W. Plesniak and P.E. Sojka. Structure of impinging jet issuing from a nozzle of indeterminate origin with applications to improving transfer efficiency for spray systems. Presented at the 2002 Painting Technology Workshop (PTW2002). June 2002, Lexington, KY
Related links
U.S. Environmental Protection Agency