Atomization of superheated liquids

在过热雾化过程中，待分解的流体相对于环境被加热到蒸发温度以上。因此，除了通常的压力能之外，另一个能量分量（热）被给出用于流体崩解。利用该方法，可以根据喷嘴几何形状、工艺条件和所施加流体的性质产生具有不同尺寸分布的喷雾。与过冷喷涂相比，必须考虑喷嘴内流体的局部蒸发或由于粘度随温度降低而导致的流体流动增强等附加效应。 在拟议的项目中，产品性能对工艺和材料性能的依赖性将通过无量纲量进行分析和描述。应用项目的重点是研究流变复杂流体的雾化行为以及用这些物质合成颗粒。驱动温度差的方向可以通过改变环境温度来设定，环境温度对所产生的颗粒的形态具有显著影响。将专门设置环境温度，以分析该参数对所产生颗粒的影响。对于计划的研究，将使用聚乙烯吡咯烷酮（PVP）溶液、混悬液（例如玻璃珠和水）、两者的混合物和甘露醇溶液。这些物质的溶解度随温度而增加。因此，与过冷喷雾相比，可以雾化更高的浓度（更高的过饱和度），产生精细结晶的颗粒。将分析合成颗粒的粒度分布和形态。由于对于更高粘度的流体（如PVP溶液），液滴尺寸被增强，因此将使用中空锥形喷嘴，以便即使对于更高粘度的介质也产生细液滴。与迄今为止应用的圆柱形喷嘴相比，对于由中空锥形喷嘴产生的液滴，水滴的中等尺寸较小;因此，对于PVP溶液（在25°C下粘度高达514 mPas）可以预期相同。在具有光学通路的反压室中，将研究雾化相对于较高反压的行为。在SPP 1423中，应用了不同的喷涂工艺（空气雾化、通过溶解CO2进行雾化、使用外部混合喷嘴进行雾化）。这些过程将比较其有效性和差异将进行讨论。

In the process of superheated atomization the fluid which is to be disintegrated is heated up above the evaporation temperature in respect to the ambient. Accordingly, another energy component (heat) is given for the fluid disintegration in addition to the usual pressure energy. With this process it is possible to generate sprays with varying size distributions in dependence of the nozzle geometry, process conditions and the properties of the applied fluid. In comparison to subcooled spraying additional effects like local evaporation of fluid inside the nozzle or the enhancement of fluid flow due to a viscosity reduction with temperature have to be taken into account. In the proposed project the dependency of the product properties on process and material properties will be analyzed and described by dimensionless quantities. The focus of the applied project is set on the investigation of the atomization behavior of rheologically complex fluids and the synthesis of particles with these substances. The direction of the driving temperature difference can be set by varying the ambient temperature which has significant influence on the morphology of the generated particles. The temperature of the ambient will be set specifically in order to analyze the impact of this parameter on the produced particles. For the planned investigations Polyvinylpyrrolidone (PVP) solutions, suspensions (for example glass pearls with water), mixtures of both and solutions of Mannitol will be used. The solubility of those substances increases with temperature. Accordingly higher concentrations (higher supersaturation) in comparison to subcooled spraying can be atomized yielding finely crystallized particles. The synthesized particles will be analyzed regarding size distribution and morphology. Since for higher viscous fluids (like PVP solutions) the droplet size is enhanced hollow cone nozzles will be used in order to generate fine droplets even for higher viscous media. The medium size of the water droplets is smaller for droplets generated by hollow cone nozzles in comparison to those made with a cylindrical nozzle which have been applied thus far; hence the same can be expected for PVP-solutions (viscosity up to 514 mPas at 25°C). In a counter pressure chamber with an optical access the behavior of the atomization versus a higher counter pressure will be investigated. In the SPP 1423 different spraying processes are applied (effervescent atomization, atomization by dissolution of CO2, atomization with external mixing nozzles). These processes will be compared in respect to their effectiveness and differences will be discussed.