Droplet wall collisions

Mundo CHR, Sommerfeld M, Tropea C (1995) Droplet-wall collisions experimental studies of the deformation and break-up processes. International Journal of Multiphase Flow 21(2) 151-173. 

The primary mode of heat transfer at the wall is forced convection of the vapor phase. As the liquid does not wet the heating surface during film boiling, heat transfer due to drop-wall collisions is relatively small, resulting in low wall-drop heat transfer (only a few percent of the total heat input). Most of the droplet evaporation occurs because of vapor-drop heat transfer. Just after dryout, the... 

The first form of aerosol modifier is a spray chamber. It is designed to produce turbulent flow in the argon carrier gas and to give time for the larger droplets to coalesce by collision. The result of coalescence, gravity, and turbulence is to deposit the larger droplets onto the walls of the spray chamber, from where the deposited liquid drains away. Since this liquid is all analyte solution, clearly some sample is wasted. Thus when sensitivity of analysis is an issue, it may be necessary to recycle this drained-off liquid back through the nebulizer. 

If the water were to be injected into a cold engine cylinder, the flash steam would immediately condense and there would be no pressure rise. To overcome this problem, the cylinder head and walls are heated and supply additional heat to the wet steam entering the cylinder. The atomised water droplets experience extremely high collision rates with the cylinder walls because of the explosive effect of the flash process. The tiny size of the droplets, coupled with high collision rates ensure rapid absorption of heat allowing them to be quickly converted to steam which is then heated further to superheat. 

Three main processes appear to control the modification and loss (or transport) of analyte aerosol in the spray chamber droplet-droplet collisions resulting in coagulation, evaporation, and impact of larger droplets into the walls of the spray chamber. Aerosol droplets can be lost (impact the walls and flow down the drain) as a result of several processes in the spray chamber [11,20]. Because turbulent gas flows are key to generating aerosols with pneumatic nebulizers, the gas in the spray chamber is also turbulent. Droplets with a variety of diameters... 

As already mentioned, in the present study all the collision interactions between the droplets and particles are disregarded. Although two cases of particle-wall interaction are investigated (a) particles hitting walls are escaped from the computational domain, that is, the trajectories of drop-lets/particles are terminated if striking against the chamber walls, and (b) particles can rebound from the walls with restitution coefficients 0.9 (normal) and 0.5 (tangential). 

To estimate colloidal forces acting between the droplets during the collision, x, z coordinates of the initial (x, Zj) and final (xf, Zf) positions of the mobile droplet before and after the collision are needed. When several pairs of x, z coordinates are plotted on a graph a speeifie seattering pattern will appear. This pattern can be analyzed by eompar-ing the experimental final positions with the ones ealeulated from a theory (2,3), which covers hydrodynamic interactions between the droplets and between the mobile droplet and the wall as well as all external forees aeting on the mobile drop, e.g., those described by DLVO theory. Thus, in the calculations, we assume the existenee of a certain force described by a certain function of the droplet-droplet separation. The final position of the droplet is then calculated and compared with the experimental results. The best match between experimental and theoretical final droplet positions yields the optimum set of parameters or the optimum force-distance profile. 

The methods of destroying aerosols, whether liquid or sohd, are numerous. Some of the more common are illustrated in Figure 13.7. One of the most important from a practical standpoint is the use of a spray (usually water) to wash the aerosol from the gas phase. As already mentioned, for aerosols, almost every collision between aerosol particles, collisions with container walls, or collision with a water droplet will be sticky. For two aerosol particles the result is homocoagulation for the other cases the process is heterocoagulation. In each case the result will be an increase in the size of... 

---