Droplet collisions and interaction with turbulent flows for powder manufacturing

以下可能是喷雾干燥器中粉末快速增长的关键：-控制液滴和颗粒的碰撞和适当的结果(例如，液滴的合并而不是分离或反弹)，-产生适当的液滴或颗粒浓度“模式”-在空间和时间上增加“聚集”-增加碰撞的可能性，-在更广泛的操作条件下保持这些品质。虽然喷雾干燥是一项古老的技术--包括液滴、半固体颗粒和干燥颗粒相互碰撞和团聚的过程--但几乎没有基本知识可以可靠地回答这个简单的问题：考虑到喷雾干燥塔的这种几何结构，我应该产生什么样的喷雾(S)和气流(S)，才能生产出以下质量(如湿度、平均尺寸、粒度分布、形貌)的粉末？答案必须包括对雾化器位置和流动条件的微小改变如何以及为什么从根本上改变粉末质量的解释。为了给出答案，我们需要比以往更好、更广泛的对基本过程的测量：我们必须产生新的理解和想法，以提高我们计算现实液体(最初的原料通常具有冷天牙膏的稠度)和几何形状在湍流中碰撞的位置、数量和结果(反弹、聚集/团聚、粘连)的能力，并对照基本、简单但具有代表性的流动几何来检查进展。该提案的总体目标是通过提高对湍流中液滴或颗粒碰撞概率以及液滴-液滴结果的理解，来创新喷雾干燥器中的粉末制造。颗粒-颗粒或液滴-颗粒碰撞，重点是通过喷雾干燥或其他类似工艺生产粉末时所使用的液体性质和几何形状。研究假设是，喷雾干燥过程中粉末的增长速度比目前认为的要快得多，这是由双液滴碰撞引发的同时多个液滴碰撞所致。这是因为碰撞持续的时间相对较长，会导致瞬间形状变形(韧带、椎间盘等)。具有相对较大的目标表面积。在液滴对碰撞过程中形成的韧带可以在碰撞事件中与流动湍流相互作用并破裂，而不是聚集成更大的液滴。目前，工业喷雾干燥器的设计或用于预测的液滴碰撞计算模型都没有考虑到这两个事件。我们的方法将是在流动配置中使用光学仪器对液体、喷雾和气体运动进行新颖的时间和空间分辨率测量，以便研究液滴碰撞的微观物理过程，并在模型喷雾干燥器中研究宏观过程。我们将利用这些结果来建立湍流和非球形韧带断裂中液滴碰撞结果的新的“碰撞核”和独特的半经验关联式，并提出控制喷雾干燥器中粉末制造的新方法和新的预测计算模型。

The following may be a key to the faster growth of powders in spray dryers: - control of droplet and particle collisions and appropriate outcomes {e.g. droplet coalescence as opposed to separation or bounce-off}, - generation of appropriate droplet or particle concentration 'patterns' - increased 'clustering' in space and time - to increase probability of collisions, - maintaining these qualities over a wider range of operating conditions. Although spray drying is an old technology - 'drying' including the process of collision and agglomeration of droplets, semidried and dried particles amongst each other - remarkably little fundamental knowledge exists which would reliably answer the simple question: given this geometry of spray drying tower, what kind of spray(s) and air flow(s) should I produce in order to manufacture powder of the following quality (e.g. humidity, mean size, size distribution, morphology)? The answer must include an explanation of how and why small modifications to the location of atomisers and flow conditions radically changes powder quality. To formulate the answer, we need better and more extensive measurements of the 'fundamental' processes than hitherto: and we must generate new understanding and ideas that will advance our ability to calculate the location, number and outcome (bouncing, coalescence/agglomeration, sticking) of collisions in turbulent flows for realistic liquids (initially feedstock frequently has the consistency of toothpaste on a cold day) and geometries - and check the advance against fundamental, simple yet representative flow geometries.The overall aim of the proposal is to innovate powder manufacturing in spray dryers by improving the understanding of the probability of droplet or particle collisions in turbulent flows and of the outcome of droplet-droplet, particle-particle or droplet-particle collisions with emphasis on liquid properties and geometries used in powder production through spray drying or other similar processes. The research hypothesis is that the growth of powders in spray drying processes can occur much faster than currently believed through simultaneous multiple droplet collisions, initiated by binary droplet collisions. This is because collisions have a relatively long duration and result in deformed transient shapes (ligaments, discs, etc.) with relatively large 'target' surface area. The ligaments formed during droplet pair collisions can interact with the flow turbulence during a collision event and break up, instead of coalescing into larger droplets. Both these events are not currently taken into account in the design of industrial spray dryers nor in computational models of droplet collisions used for predictions. Our approach will be to make novel time- and spatially- resolved measurements of the liquid, spray and gas motions using optical instrumentation in flow configurations that allow the study of the microscale physics of droplet collisions and in model spray dryers that allow the study of macroscale processes. We will use these results to establish new 'collision kernels' and unique semi-empirical correlations of droplet collision outcomes in turbulent flows and breakup of non-spherical ligaments and propose novel methods for control of powder manufacturing in, and new computational models for predictions of, spray dryers.