Entrainment of oil droplets at the back of mist filters

由于气流通过部分饱和的介质，先前沉积的油气溶胶从雾过滤器中夹带出来。夹带产生宽液滴光谱（0.1 - 2000 µm）。PI的初步工作提出了两种可能的夹带机制，一种用于亲油介质（后壁油膜中的气泡破裂），另一种用于疏油介质（液滴吹出）。在此基础上，首先开发了测量技术，以确定两个相关尺寸范围（0.5 - 10 µm和170 - 2300 µm）内的夹带速率和夹带液滴尺寸。此外，还开发了成像技术来观察液滴破碎和气泡形成。最后，采用了一种繁琐但有用的离线技术来测量差距（10 - 200 µm）。这些测量方法的组合使得能够记录完整的光谱。这些结果证实，上述尺寸范围应足以解释夹带动力学。工作的另一部分集中在获得相关夹带机制的理解，作为介质润湿性和稳态条件下的操作条件（例如，气溶胶质量速率和过滤速度）的函数。通过结合夹带率测量和成像技术，它是可能的相关夹带和排水过程。令人惊讶的是，从疏油过滤器直接吹出并不显著地有助于整体夹带，除了与吹出间接相关的初始爆裂。然而，对于疏油介质发现了第二种机制，其基于液滴表面上气泡的破碎。在泡沫介质的情况下，泡沫破裂似乎是主要的机制。基于这些发现，为下一个项目周期制定了以下工作计划和策略：首先，需要确认气泡破碎是玻璃纤维介质在稳态下的主要夹带机制这一假设。这将通过在相同的操作条件下将夹带速率与气泡形成速率相关联来完成。它需要更广泛的夹带率测量，以及更多的定量数据的气泡破裂率和由此产生的碎片尺寸谱。第二个目标是了解气泡形成的位置以及过滤器上气泡形成的任何优先点。这将通过使用其他类型的介质以及结构化表面（例如泡沫、网格）来完成。这些试验将通过在单独的装置中对自由液体表面上的气泡形成进行实验来增强，对气泡的尺寸和速率进行更多的控制。需要这些数据来解释来自真实的过滤介质的数据。总的来说，这将导致一个全面的理解夹带机制（S）从玻璃纤维介质，以及夹带率对操作条件的依赖性，并有助于减少夹带的策略。

Entrainment of previously deposited oil aerosol from mist filters occurs as a result of gas flow through the partially saturated media. Entrainment generates a broad droplet spectrum (0.1 - 2000 µm). Preliminary work by the PI suggested two possible entrainment mechanisms, one for oleophilic media (bursting bubbles in the oil film on back wall) and for oleophobic media (droplet blow-off). On basis, firstly, measurement techniques were developed to determine entrainment rates and entrained drop sizes in two relevant size ranges (0.5 - 10 µm and 170 - 2300 µm). Additionally, imaging techniques were developed to observe drop fragmentation and bubble formation. Finally a cumbersome, but useful off-line technique for measuring in the gap (10 - 200 µm) was implemented. A combination of these measurement methods enabled the recording of complete spectra. These results confirmed that the above mentioned size ranges should suffice for interpretation of entrainment kinetics. Another part of the work focused on obtaining an understanding of the relevant entrainment mechanism(s) as function of media wettability and operating conditions (e.g. aerosol mass rate and filtration velocity) under steady state conditions. By combining entrainment rate measurements and imaging techniques it was possible to correlate entrainment and drainage processes. Surprisingly, direct blow-off from oleophobic filters does not contribute noticeably to overall entrainment, except for an initial burst which is related indirectly to blow-off. However, a second mechanism was discovered for oleophobic media which is based on fragmentation of bubbles on the surface of drops. In case of wettable media, bubble bursting seems to be the dominant mechanism. Based on these findings, the following work program and strategy was developed for the next project period: Foremost, the hypothesis needs to be confirmed that fragmentation of bubbles is the predominant entrainment mechanism for glass fiber media in steady state. This will be done by correlating entrainment rates with bubble formation rates under equal operating conditions. It requires more extensive entrainment rate measurements, as well more quantitative data on bubble bursting rates and resulting fragment size spectra. Second objective is to gain insight regarding where bubbles form, as well as any preferential points of bubble formation on the filter. This will be done by using additional types of media, as well as structured surfaces (e.g. foam, grid). These tests will be augmented by experiments of bubble formation on free liquid surfaces in a separate set-up with more control over bubble sizes and rates. These data are needed for the interpretation of data from real filter media. Overall, this should lead to a well-rounded understanding of entrainment mechanism(s) from glass fiber media, as well as the dependence of entrainment rates on operating conditions, and contribute to strategies for the reduction of entrainment.