Droplet Size Determination on a Rotary Atomizer Wheel with TimeShift Technique in Comparison to Analytical Theory
Rotary atomizers are used in spray drying applications. Spray drying is a method of producing a dry powder from a fluid solution or slurry. The feedstock is atomized into droplets that are then dried rapidly into particles with a hot gas. Spray drying is widely used in the chemical and food industry and is the preferred method of drying of many materials such as dairy products, foods, pharmaceuticals, polymers and more. For the design of spray drying processes the droplet size distribution has to be known, to be able to predict the resultant powder size distribution. Based on initial assumptions regarding the flow inside the wheel, the behaviour of the feed within the inserts (radial opening of the wheel) is described with analytical equations and compared to numerical and experimental investigations. The liquid accumulates on the trailing edge of the insert and therefore the film thickness on the exit of the inserts is used to compare all approaches. A time shift measurement technique is used to analyse droplet size and velocities of the resulting spray. In addition it allows to investigate the relationship between the jet thickness and the resulting sauter mean diameter D32 of the droplets. Introduction The droplet size distribution in spray drying processes [1] or flue gas cleaning [2] with rotary atomizer wheels, has a significant impact on the outcome of the process, e.g. the properties of the dried powder. The intended use of the dried powder determines the desired size distribution of the powder particles, morphology and porosity. It is important to be able to predict the droplet size distribution for a given geometry of an atomizer and its operation conditions. In rotary atomizers the liquid is first supplied to the internal atomizer reservoir, flows from there into several inserts (mainly driven by the centrifugal force associated with the atomizer rotation) and then it is ejected into the air. The jets of the ejected liquid then breakup and atomize into drops. Besides the spray drying industry [3, 4, 5] rotary atomizers [6, 7] and ligament based atomization is also used in numerous other applications, e.g. painting purposes [8] The primary atomization process for laminar operating rotary atomizers has already been expressed in terms of non dimensional numbers [9, 10] but such atomizers operate at much lower rotational speeds compared to the atomizers investigated in this study. The most important parameter for the atomization process is the jet thickness at the exit of the inserts, because the initial jet thickness corresponds directly to the resulting droplet diameter D32. Therefore in this study the jet thickness at the exit of the inserts is measured with a high resolution imaging system and compared to numerical simulations. Furthermore, the drop size and velocity is measured with the time shift (TS) technique [11] to gain a better understanding of the atomization process. Additionally, an analytical model is developed which is able to predict the rivulet flow in the insert of the rotary atomizer wheel and provide a basis on which the droplet size after the atomization can be estimated. Material and methods The experimental setup for the two different measurement campaigns conducted is shown in figure 1. The setup consists of the liquid supply system, an atomizer, alternatively, a high resolution imaging system (figure 1(a) or a laser based spray characterization system (figure 1(b) The atomizer used in this investigation is fed by water, operated under normal ambient conditions and is provided by GEA Denmark [2] A spray trap surrounds the atomizer wheel and catches the water exiting the inserts. The rotational speeds (10 500 1/min to 29 000 1/min) and feed rates (100 kg/h to 300 kg/h) are varied in the experiments. The mechanism of the primary atomization is observed using a high resolution imaging system, consisting of a pco.edge 5.5 camera and a CAVILUX HF laser system (see figure 1(a) The camera has a resolution of 2560 pxx 2160 px and a Nikon AF Micro Nikkor lens with 105 mm focal length is used in the experiments. The CAVILUX HF laser is triggered by a tachometer signal of the atomizer and generates light pulses of 1 ms duration at a frequency of 30 Hz. The camera is only capable of running at 50 Hz in global shutter mode, which is why the tachometer signal of the atomizer has to be skipped and only every 6 17 revolution can be captured, due to the high speed of the atomizer. The emitted laser light is deflected by