Evaporating drops and sprays in turbulent air streams (1969)
AuthorsBrehaut, W. J.show all
The evaporation rates of single drops and whole sprays in turbulent air streams have been investigated using a pilot-scale cocurrent spray drier. The six basic parameters of the turbulent air flow, the mean velocity, the relative intensity, the micro- and integral scales, the energy spectrum and the dissipation spectrum of turbulent energy, were measured using techniques developed for a single hotwire probe without the need for expensive correlating or recording equipment. The evaporation rate of supported evaporating droplets was found to depend strongly on both the intensity and integral scale of turbulence as well as the droplet Reynolds number. An optimum value of the integral scale to droplet diameter ratio was found and is believed to result from resonance between the energy containing eddies and the shedding frequency of the drop. The existence of the optimum value of the ratio was previously suggested for heat-transfer from a cylinder and the present results are believed to be the first showing a similar optimum for held spheres or drops. The optimum ratio explains the seeming discrepancies found in the literature concerning the effect of turbulence intensity and scale on heat-transfer for spheres. Drag coefficient data was obtained for free falling and cocurrently entrained, evaporating water droplets. All drops experienced the same drag as solid spheres moving in steady flow conditions and no separate effect of the scale or intensity of the entraining air was discerned. The standard drag curve by Smith (170) is recommended. Further, the reduction in drag due to the intensive mass-transfer effect of the vapour efflux from the evaporating droplet surface can be accounted for using corrected driving forces (incorporating the Mass-Transfer number B) with the boundary-layer theory. An attempted study of turbulence effects on the evaporation rates of sprays was abandoned when a necessary basic assumption was found to be invalid. A spray momentum and entrainment effect in the nozzle zone is reported and significant drying air temperature and humidity radial profiles were found to exist at even the lowest spraying rates. The uncertain knowledge of droplet trajectories during the spray process means that a sound theoretical basis for spray drier design is still some distance away and experimental studies must be relied on for accurate design data. Many aspects of the spray evaporation process still require investigation, particularly those concerned with the spray momentum-transfer and entrainment effects as well as the turbulence aspect of the system. While a significant turbulence effect has been found for fixed evaporating droplets, the influence of turbulence on the evaporation rates of sprays is still an open question.