the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 18 Sep 2024
Impact of space charge on neutralization efficiency of highly charged aerosols: analytical and numerical insights using bipolar ions
Abstract. We investigate the influence of space charge on aerosol discharging behavior, emphasizing its importance in understanding and optimizing aerosol neutralization processes. Our study introduces the concept of space charge-assisted ionization and the distribution of charged aerosols. Specifically, we consider the case of highly charged aerosols introduced into a cylindrical chamber containing a bipolar ion source. In this scenario, space-charge-induced motion is explicitly included in solving the dynamical equation using a computational fluid dynamics (CFD) approach. This method allows us to assess the efficacy of corona-based neutralizers for charged aerosols. Our exploration focuses on the effect of spatial ion heterogeneity induced by space charge. Our results demonstrate that the efficiency of charge neutralizers in mitigating high concentrations of charged aerosols is influenced by various factors, including aerosol charge concentration, magnitude of the charge, and design parameters such as flow rate, ion production rate, and neutralizer geometry. We observe that particles at the periphery of the chamber experience a significantly slower neutralization compared to those flowing along the axis. Consequently, the aerosol system as a whole exits the chamber with a residual charge. This incomplete neutralization, caused by space charge effects, alters the particle charge distribution, potentially affecting size distribution measurements using mobility sizing instruments. These findings underscore the need to bridge theoretical concepts with practical applications in aerosol science and technology, with broad implications for environmental monitoring and industrial processes. Furthermore, our coupled model offers potential for investigating processes involving charged aerosols in the atmosphere, where space charge effects are also significant.
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RC1: 'Comment on ar-2024-23', Anonymous Referee #1, 11 Oct 2024
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general comments:
 - The authors present a computational model to simulate the neutralization of a highly charged aerosol, furthermore they present results for four cases, where one different parameter was varied for every case, to show the influence of the selected parameter.
 - In general the model and the study are not progressed enough to justify a publication, as can be seen from my comments below.
 - Major improvement of readability and language are required, several statements are unclear and some terms are used inaccurately. In general, readability can be improved.
 - I want to suggest to revise introduction, especially the first part and the part describing neutralization (i.e. charging behavior of neutralizers is standardized in ISO 15900) and the motivation. I mention specific suggestions in the specific comments section below.
 - Im order to promote consistent terminology I suggest to use "N_{i}t-product" rather than "Nt product"
 - It seems that your model does not consider critical influences as, e.g., aerosol transport, ion-ion interactions, ion-particle interactions and diffusional mixing of the gas. So it is not clear to me how you can draw general conclusions from this model.
 - The results are very weak for a stand-alone peer-review paper. I'd suggest to move the air-flow profile into supplementary or an appendix, such simulations are very basic. The ion-profile calculation does not seem to represent the reality accurately, as you do not consider several influences like ion-ion interactions and diffusion. Furthermore the difference between the 1 lpm and 5 lpm case seems rather extreme. Also it is not clear to me why there are already ions at the inlet and how the ion concentration does not increase between the inlet and the outlet, if have implemented a time-dependent production rate in your model. As the results in the section 3.2 are building up on the aforementioned, I am very critical about the significance of this study considering this point alone.
 - The point I am most critical about is the misconception of neutralized aerosols. In your results you show that an aerosol, where each particle carries 10 positive charges is brought to a condition where every single particle in a 2D-model is electrically neutralized by exposure to bipolar ions. This is not the case in reality, neutralized aerosols have a net-zero charge, single particles are charged, this is why differential mobility analyzers (DMA) operate with a neutralizer - these devices would not work if particles would be uncharged after a neutralizer.
 - To be published the model and the manuscript require improvements regarding aerosol flows, particle charging and fluid dynamics.
specific comments:
17: I would recommend toning down the statement, that mobility size selection is the most precise and reliable one. Although DMAs are the most common devices for size selection, this might also be because of the convenient products available. Please also mention alternative approaches for particle size selection and analysis for the sake of completeness.
22: A more convenient way to classify aerosol chargers is to separate them into direct and indirect (diffusion) chargers, e.g. UV and X-ray chargers use very different mechanisms for aerosol charging. I.e. UV charging ist mostly used for direct charging of particles, rather then generate ions
24: "ionizer" is not a common term in this context, I suggest to use "charger" or "ion source"
25-26: This statement is either incomplete or inaccurate - please specify of which process the two possibilities are possible, or if you want to keep it general add possible ion-ion interactions.
28-29: please specify the challenge for highly charged aerosols, or remove the sentence#
29-34: This passage is unclear in terminology, please clarify the difference between electrically neutral particle and a neutralized aerosol
35: please complete the list of biplar chargers and please explicitely mention that corona chargers are actually unipolar chargers and the alternating operation is operation mode which allows to release both charge polarities
44: replace "say" by "e.g."
45-47: The description using the Nit-product for the description of the charge distribution and the (size dependent) average charge per particle is based on doi:10.1080/02786828808959180 and the "birth-and-death-model" of doi:10.1016/0004-6981(70)90052-1
50-51: The satement that the mentionen theories demonstrated the size dependent behavior is wrong, please revise wording. Theories tend to describe obeservations of physical processes, so if theories indicate a behavior then because they were developed to do so.
63 - 69: The connection between the single statements is not clear, please revise the consistency of the paragraph.
75-79: This general overview is very nice. Please add details about the aerosol (e.g. concentration, particle-size-distribution, material properties) and how the particles are distributed in the tube.
85-87: the description of variables is incomplete
93-101: I suggest to revise this passage and especially reconsider the statement regarding the "neutralization conundrum" - it rather seems that a model not considering space charge is incomplete. First, the behavior was already described over 30 years ago, as you even indicate in your sources. Second, the case of a absent space charge is physically impossible - a system with a non-zero net-charge will always bring a space charge (the electromagentic force has an infinite range, although it decays quadratically with the distance). Third you even state in Appendix "A": "[...] space-charge-induced drift is the guarantor of complete neutralization [...] and not merely the symmetry of the ions." - this does not sound like a conundrum, more like a understood physical system.
101: The importance of the section "Ion and charge aerosol dynamics" is not clear at this point, also it is unclear if you consider space charges in your model from here on or not. Furthermore you did not inculde the explenation of how you are considering aerosol dynamics (or even transport) in your model
Figure 1: Please extend the caption in general and describe the symbols in the figure.
115-116: Please revise this statement, the simplification to 2D does not require forced flow. Please reconsider the use of the term "convection"
123: Please specify the boundary conditions
123: Why do you use a turbulent model? The flow at conditions in table 1 is laminar
124: I assume you meant "cell" where you wrote "grid"Â
134: Why did you choose these flow rates? Why do you expert differences at this conditions?
142 - 145: This statement is not clear. I interpret that you meant most likely, that what is shown in fig. 3 are the four flows after the neutralizer, which have been exposed to aerosols at 10E4 #/ccm and a ion production rate of 10E6 #/(ccm s), but for the shown flow the aerosols are not present any more and no ions are produced, also diffusional mixing does not seem to happen. I do not understand why this graphs are of importance, also I am not completely sure if I understood it correctly. Also I am wondering why you only show positive ions and no effects of ion-ion interaction are visible, while you were talking about bipolar charging and various interactions when explaining your model.
151-222: I only give general comment on thisÂ
technical corrections:
 - There is a major misconception of neutralized aerosols. It is not necessary for an neutralized aerosol, that every single particle is electrically neutral. See ISO 15900.Citation: https://doi.org/10.5194/ar-2024-23-RC1 -
RC2: 'Comment on ar-2024-23', Anonymous Referee #2, 12 Oct 2024
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The paper reports a numerical simulation of neutralisation of charged aerosol using bipolar ions, a subject that clearly fall within the scope of this journal and which tackles a subject of interest for its audience.
The research question behind the paper derives from the need to support current understandings of the aerosol neutralisers for high concentration aerosols, and the authors aims to present their model as a tool to explore the basic discharging mechanisms of neutralisers as well as the the way how spatial heterogeneity of ions affect the neutralisation rate.
The paper title is accurate, the related works seems reported in an adequate way, the paper structure is adequate and the style is appropriate, although it may benefit a revision to become more fluent.
Despite the interesting topic, in my opinion the paper shows several critical issues that undermine its validity and discourage its publication at this stage. In particular:
1. The methodologies proposed here lack for adequate details of the numerical modelling and are based on assumptions that cannot be accounted for easily. Among that:i) a planar 2D simulation as the one used here can be used to approximate the behaviour of a channel, but is not appropriate to describe a cylinder, for which axial-symmetric schemes should be used;
ii) the boundary conditions in the domain are not clear: what is the potential at the walls? What about the inlet and outlet sections?;
iii) what kind of numerical modelling is used to solve the fluid dynamic field, the aerosol motion and the electric field equations?
iv) what kind of turbulence model is used and why this is needed?
v) how the ions interacts among them and with the walls?
The paper presents a severe lack of clarity in the methodology section and this limits the readibility and the credibility of the results. The methodology has to be carefully revised and the results updated consequently.Â
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2. The simulated conditions seem not representative of realistic neutralisers or the assumptions behind them are not clear:i) with the simulated diameter and length of the cylindric neutraliser, how it is possible to maintain a constant and spatially uniform production of the ions? Can they be achieved for whatever pressure, temperature, humidity and composition of the gases? Should the ion source positioning respect to the aerosol flow be realistically neglected? Why this scheme can be considered a general one? If the simulation is considering "idealised" neutralization conditions, how its results can be extended to the many possible applications cited in the Conclusion?Â
ii) the aerosol size is not specified, so that the assumed number of charge of 10e charge cannot be compared with its Pauthenier’s limit: are 10e a high or a small fractions of the ions a particle can have? Besides, the aerosol size influences its dynamics in the neutraliser due to the different Brownian diffusivity and the relative effects on thermal and electrophoretic motions. Are the particles considered massless?
The authors must carefully revise the methodology section and provide more details and explanations on these points.
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3. The effect of aerosol concentration is critical, but is treated here in a rather simplified way while, according to the ambitions of the work, this should be one its main topic. What is the contribution of this model in understanding for what conditions the "classical Nt product concept does not hold" [lines 203-204]? I suggest extending the discussion on this topic, after revision of the aforementioned points 1 and 2.
4. Most of the other findings of the model are intuitive and not new: for example it is clear from the basic physics that higher flow rates means lower neutralization time, higher ions concentration fasten the neutralization.
5. The conclusions of the paper seem not sustained by the methodologies and the result, especially the last paragraph that extend the validity of the paper findings without having any substantial basis.Â
Citation: https://doi.org/10.5194/ar-2024-23-RC2
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