Bibliometric Network Analysis of Trends in Cyclone Separator Research: Research Gaps and Future Direction

: Cyclone separators are used extensively in diverse applications and research domains to collect particle-laden ﬂows. Despite the technological advances in this ﬁeld, no bibliometric reports on this topic have been documented. Understanding the state of the art in this ﬁeld is crucial for future research. Using bibliometric mapping techniques, this study examined the quality, quantity, and development of research on cyclone separators. Relevant data were extracted in plain text formats through search queries reﬁned by publication year (2000–2021) and document type (article and review articles). A sample of 487 publications, limited to the Web of Science Core Collection (WoSCC) database was used for the bibliometric analysis. Data analysis was performed using RStudio software package (R Bibliometrix tool). Of the 487 publications that appeared during this period, China had the highest number, followed by the Islamic Republic of Iran, whereas chemical engineering journals dominated the cyclone separator research publications. Collaboration among the researchers was low (MCPR < 0.5000). Furthermore, the pattern of single-author publications was found to outstrip that of the multiple-author publications. The ﬁndings suggest that researchers in various parts of the world, particularly Africa and the Middle East, should route their research efforts towards this ﬁeld, in light of the lack of publications from these regions on this subject. The aim of this study was to serve as a seminal reference for potential technological research directions and collaboration among researchers in this and other related ﬁelds.


Introduction
Particle separation and control of bulk solids or powder, solid-gas particles, fine particulate matter, and solid-liquid particles remain among the developed axes of investigative research in industrial processing and handling applications because of the drastic emissions and environmental threat of pollutants [1]. A plethora of professional research engineers and scientists across different engineering and industrial fields have employed particle separation technologies in their fields. These include mechanical engineers, chemists, metallurgists, pharmacists, environmental engineers, civil engineers, biologists, and plastic and cosmetic scientists in different generic forms over the past 100 years. Furthermore, statutory environmental control requirements of various countries have resulted in significant improvements in the design of equipment for large solid particles, intermediate-sized particles, solid-liquid-gas particulates, and fine particulate matter (PM), such as cyclone The literature presents a growing body of evidence on how cyclone separators have gained attention in various engineering and industrial domains. However, on a broader scale, there remains a lack of all-inclusive research on cyclone separators from these perspectives, especially as publications in this field continue to grow. Owing to the diversity of methodologies and applications of cyclone separator research, only quantitative and qualitative data analysis can be useful for defining and evaluating the output level of publications on cyclone separator performance and applications. Consequently, parametric studies on geometric variables, such as the inlet duct width/height, cyclone diameter, vortex height, cylinder height, and other parameters, and their effects on performance parameters, such as pressure drop [10], collection efficiency [11], particle concentration/distribution [10], separation efficiency, and gas flow fields/pattern [9], are usually investigated. These include experimental and numerical studies [2,5,12].
Studies conducted by other researchers have anticipated improvements in the performance of cyclone separators. These studies were conducted by investigating cyclone separators under different temperature and flow conditions [13], configurations or arrangements [14], and applications [15]. Furthermore, new cyclone separator models, coupled with geometric design variations, have been proposed to improve the performance. Modabberifar et al. [16] proposed three new cyclone separator geometries by increasing the vortex length to improve pressure drop and cyclone collection efficiency. It was concluded that cyclones 1D3Dn and 1D2Dn followed 2D2Dn with a marked collection efficiency difference of approximately 20%. Behrang et al. [17] designed and investigated a multi-helical dust cyclone separator using experimental and computational fluid dynamics (CFD) simulations. A novel cyclone separator has been reported for separating fine particles at low velocities. Jebeli et al. [8] optimized the removal efficiency of particulate matter (PM10) using a newly designed cyclone separator with adjustable vortex finder height and inlet angle.
In addition to experimental investigations, advanced numerical and analytical simulation approaches have been employed to predict and validate the performance of cyclone separators in terms of turbulence effects, gas-particle flows, particle distribution, and particle trajectory. These include the Reynolds stress turbulence model (RSM) for turbulent flow simulations [18], hybrid Euler-Lagrange [19], large-eddy simulations (LES) [20], discrete element method (DEM) [21] for particle trajectory tracking, and three-dimensional computational fluid dynamics (CFD) methods [21]. Others have combined several modeling techniques [21,22]. Izadi et al. [23] combined CFD, multi-gene genetic programming (MGGP), design of experiments (DOE), and ten different algorithms to optimize six geometrical variables of the cyclone separator. The optimized designs increased the collection efficiency and decreased the pressure drop by 5.64% and 3.3%-27.5%, respectively. Park et al. [22] modelled the critical diameter of a cyclone separator using CFD and machine learning techniques and showed a significant response in performance prediction.
The literature presents a growing body of evidence on how cyclone separators have gained attention in various engineering and industrial domains. However, on a broader scale, there remains a lack of all-inclusive research on cyclone separators from these perspectives, especially as publications in this field continue to grow. Owing to the diversity of methodologies and applications of cyclone separator research, only quantitative and qualitative data analysis can be useful for defining and evaluating the output level of publications on cyclone separator performance and applications.
Bibliometric analysis is a comprehensive statistical knowledge system that focuses on publication patterns and acknowledges the academic efforts of authors, publishers, and other related documents [24,25]. Using a bibliometric study would significantly improve trend identification and detect growing collaboration patterns, research activities [26], and the volume of publications on cyclone separators as a baseline for future studies [27]. Therefore, researchers need a more prolific literature review tool to measure, analyze, and understand previous experimental findings. Through bibliometric analysis, the new volume of information, topics researched, content information, contributions of scholars, and trends of articles over time in a particular discipline can be effectively analyzed [25,28]. Moreover, a bibliometric study is a suitable scientific mapping technique to assess empirical contributions, knowledge of research themes, conceptual developments, research hotspots, and authors' contributions [25,28] within a particular study field, as it is gradually becoming burdensome and impracticable to remain abreast with everything being published.
Despite the significant traction in cyclone separator performance research in recent years, no bibliometric mapping analysis has been documented to summarize the existing body of literature in the field prior to this review. The distinct themes, unabridged scope, and most prominent scholars in the field, from the past to the present, have not yet been revealed. Therefore, there is a need for a bibliometric mapping overview that combines quantitative and qualitative analyses to provide a framework for understanding past, present, and future studies on the performance and applications of cyclone separators.

Research Objectives
Against this backdrop, the main objective of this study was to provide a comprehensive overview of the literature in the past two decades in the field of cyclone separator research using bibliometric mapping techniques. The specific objectives were to explicitly focus on contributing countries, sources, influential authors, and thematic evolution in the field. The results of this study serve as a reference for understanding the existing methodologies, tools, applications, and directions of cyclone separator research. Interested target groups can use them to learn about existing tools, methods, and approaches for assessing cyclone separators. Thus, the results would be useful in providing multiple stakeholders with a holistic picture, including researchers, authorities, and practitioners, in identifying key areas that require further exploration in the coming years.
This study contributes to achieving the study objectives by asking the following questions to address this research gap: (1) What are the topic clusters and main themes in cyclone separator research and their level of evolution? (2) What is the focus of the current investigations on the performance of cyclone separators and their applications? (3) Based on the analysis, what is the scope of theoretical and practical research directions towards future research in this field?

Research Organization
The overall structure of this paper can be summarized in five sections, including an introduction. The materials and methods used for data collection and the analytical tools used are discussed in Section 2. Sections 3 and 4 report the key findings and discuss the main findings to understand the research output in the field from the bibliometric analysis. In addition, the research gaps and an outlook on future research opportunities for the application of cyclone separators are presented in this section. Finally, Section 5 summarizes the findings and concludes the study.

Data and Methods
In Figure 2, the main steps taken to analyze the bibliometric dataset are data collection, data transformation and processing, data visualization and analysis, and interpretation using the R bibliometric software package (Biblioshiny version. 3.1) are shown (Figure 2a). Bibliometric protocols are briefly described in this section. interpretation using the R bibliometric software package (Biblioshiny version. 3.1) are shown (Figure 2a). Bibliometric protocols are briefly described in this section.

Database Creation
To satisfy the study objectives, a search strategy (Table 1) was adopted to capture the terms and applications of the cyclone separator, ranging from geometric and performance variables to simulation terms. Keywords, titles, and abstracts of English-indexed documents ('articles and review articles') in the study field were accessed from the Clarivate Analytics Web of Science Core Collection database (WoSCC) on 22 August 2022. The WoSCC database provides a dynamic collection of peer-reviewed publication indices of science and engineering disciplines and other citation indexes with seamless navigation for high-quality bibliometric data and past literature from interdisciplinary or specific fields [41]. The WoSCC database was chosen because of its extensive content of academic information for bibliometric analysis, ease of exporting extensive data in batches, and record package for old citations [42].
To include data related to the cyclone separator domains, the Boolean and filtering functions were applied to the search scope for the keyword: TOPIC: ("cyclone separator"). The time period was refined to include publications from 2000 to 2021. Exclusion criteria involved the following document types: corrections, news items, editorial materials, and letters [43]. To ensure fidelity and eliminate conflicting documents from other research fields, sources belonging to other research fields such as plant sciences, respiratory systems, and government law were excluded because they may not be relevant to the

Database Creation
To satisfy the study objectives, a search strategy (Table 1) was adopted to capture the terms and applications of the cyclone separator, ranging from geometric and performance variables to simulation terms. Keywords, titles, and abstracts of English-indexed documents ('articles and review articles') in the study field were accessed from the Clarivate Analytics Web of Science Core Collection database (WoSCC) on 22 August 2022. The WoSCC database provides a dynamic collection of peer-reviewed publication indices of science and engineering disciplines and other citation indexes with seamless navigation for high-quality bibliometric data and past literature from interdisciplinary or specific fields [41]. The WoSCC database was chosen because of its extensive content of academic information for bibliometric analysis, ease of exporting extensive data in batches, and record package for old citations [42].
To include data related to the cyclone separator domains, the Boolean and filtering functions were applied to the search scope for the keyword: TOPIC: ("cyclone separator"). The time period was refined to include publications from 2000 to 2021. Exclusion criteria involved the following document types: corrections, news items, editorial materials, and letters [43]. To ensure fidelity and eliminate conflicting documents from other research fields, sources belonging to other research fields such as plant sciences, respiratory systems, and government law were excluded because they may not be relevant to the performance of the cyclone separator and its applications. A preferred reporting item for systematic reviews and meta-analysis (PRISMA) flow diagram was used to gauge the quality of the bibliometric review (Figure 2b). The search was limited to English language and review articles, resulting in a total of 512 articles ( Figure 2c). Subsequently, the articles were narrowed to those related to cyclone separators. This was conducted by carefully reading the titles and abstracts of the literature. In the event that, the abstracts and titles failed to distinguish between relevant and unrelated studies on cyclone separator research, the full article was read independently, and a decision was made on whether to include the article. After reading the abstracts and removing duplicates, screening of articles based on the inclusion criteria resulted in 487 related articles that were used for bibliometric analysis.

Visualization and Analysis of Data
RStudio software (version 4.1.2) with a biblioshiny package for tabulation, visualization, and systematic mapping analysis of the bibliometric indicators was used in this study [25]. Three plain text datasets (Wos1-Wos3) were downloaded independently into the R environment and merged into a single comma-separated values (CSV) file before uploading to the bibliometric R-package (biblioshiny) for further analysis, as outlined previously [44]. The merging of different files has the potential to duplicate the data. As a result, duplicate data were eliminated using the R codes "Combined-Database = mergedDbSources (Wos1, Wos2, Wos3, remove.duplicated = TRUE)". The combined dataset (cyclone.xlsx) was loaded into R bibliometrix (version 3.1) through biblioshiny (interface for R bibliometrix) for a systematic bibliometric workflow analysis (https://www.bibliometrix.org/Biblioshiny.html, accessed on 22 August 2022). The publication structure analysis workflow was obtained, which included annual article publications, most cited articles, top citations per country, most productive authors, most productive countries, most relevant sources, and most relevant keywords. Visualization of the country collaboration network, co-citations, keyword co-occurrence, authors' collaboration, authors' publications over time, and institutional collaboration were performed.

Publication Trends
The number of publications in the study domain was 487 (481 articles = 98.77% and six review articles = 1.23%) from 70 journals and book sources with 518 keywords plus (ID) and 930 author keywords (DE), as shown in Table 2. This implies that the most frequently used keywords by the authors of the cyclone separator studies numbered 930 and a keyword frequency distribution of 518 was in the journal domain. Of the 965 authors, 25 were reported as authors of single-author documents and 940 as multi-author published documents with 1860 appearances. The ratio of the total number of documents (487) to the total number of authors (965) was 0.505, and the estimated metric reciprocal ratio of the number of authors per publication (965/487) was 1.98. Additionally, the number of multiple co-authorships per publication was 3.82, whereas the total number of multi-authored publications to the number of multi-authored publications (940/454) yielded a collaborative index of 2.07. These data corroborate the productivity, robust research, and multiple collaborative engagements per publication among authors in the field. Furthermore, the mean number of citations per publication is 18.91, which implies that cyclone separator research/articles are cited in journals approximately 19 times.
The publication trend based on the number of articles provides a perspective on the level of attention paid to research on cyclone separators, as illustrated in Figure 3. A polynomial model fitted to the data generated positive correlation values (y = (0.01)x 3 + (−55.88)x 2 + (112)x + (−749); R 2 = 0.88). This indicates that further studies and research publications on this subject will be conducted in the future. These results signify a continuing interest in the use of cyclone separators, and that this study may also have scientific potential. The trend and number of publications grew significantly, as can be seen when grouped into five research periods.

Contributing Countries
A country-by-country analysis was conducted to elucidate the contributions of the most productive countries in the cyclone separator research domain. It is imperative to recognize the countries that have contributed to the development in the field of research on this type of apparatus. Table 3 shows the 20 leading countries in terms of publications during the study period . During the study period, more than 44 countries contributed articles to this research niche. The People's Republic of China, Iran, India, Brazil, Japan, Republic of Korea, Poland, the USA, Canada, and Turkey emerged as the top 10 countries to have contributed the most articles on the topic, accounting for 79.88% of the overall production. China topped the list with 196 publications, corresponding to 40.25% of the overall records, followed by Iran (n = 42, 8.62%), India (n = 30, 6.16%), Brazil (n = 20, 4.11%), and Japan (n = 19, 3.90%). The People's Republic of Korea (n = 19, 3.90%), Poland (n = 18, 3.70%), the USA (n = 16, 3.29%), Canada (n = 15, 3.08%), and Turkey (n = 14, 2.87%) were added chronologically to make the top 10 most productive countries. Among the top 20 countries in cyclone separator research, only Iran is from the Arabian Peninsula. In addition, none of the African countries were among the top 20 countries. Furthermore, by comparing the number of single-and multi-authored publications, single-authored publications were more prevalent in cyclone separator research, as shown in Table 3. Among the five countries cited as having the highest international publication collaboration on this subject, Australia, Belgium, Norway, Belarus, and Sweden have multiple-country publication ratios (MCPR) of 0.556, 0.500, 0.667, 0.800, and 0.750 (MCPR > 0.5000), respectively.

Contributing Countries
A country-by-country analysis was conducted to elucidate the contributions of the most productive countries in the cyclone separator research domain. It is imperative to recognize the countries that have contributed to the development in the field of research on this type of apparatus. Table 3 shows the 20 leading countries in terms of publications during the study period . During the study period, more than 44 countries contributed articles to this research niche. The People's Republic of China, Iran, India, Brazil, Japan, Republic of Korea, Poland, the USA, Canada, and Turkey emerged as the top 10 countries to have contributed the most articles on the topic, accounting for 79.88% of the overall production. China topped the list with 196 publications, corresponding to 40.25% of the overall records, followed by Iran (n = 42, 8.62%), India (n = 30, 6.16%), Brazil (n = 20, 4.11%), and Japan (n = 19, 3.90%). The People's Republic of Korea (n = 19, 3.90%), Poland (n = 18, 3.70%), the USA (n = 16, 3.29%), Canada (n = 15, 3.08%), and Turkey (n = 14, 2.87%) were added chronologically to make the top 10 most productive countries. Among the top 20 countries in cyclone separator research, only Iran is from the Arabian Peninsula. In addition, none of the African countries were among the top 20 countries. Furthermore, by comparing the number of single-and multi-authored publications, single-authored publications were more prevalent in cyclone separator research, as shown in Table 3. Among the five countries cited as having the highest international publication collaboration on this subject, Australia, Belgium, Norway, Belarus, and Sweden have multiple-country publication ratios (MCPR) of 0.556, 0.500, 0.667, 0.800, and 0.750 (MCPR > 0.5000), respectively.
Regarding rank, the overall number of citations per country has changed slightly. China (n = 2975) and Iran (n = 817) are the two countries that generated the most citations. The total number of publications and citations a nation produces explains its influence in a particular field. Thus, it can be assumed that these countries have significantly affected this niche area. This also reflects the impact of articles on creating changes in practice, discussion, recognition by the scientific community, controversies, and directions for future research on cyclone separators, as the number of citations offers a measure of research quality [45]. African countries were not among the top 20 nations with the highest number of article citations, which is noteworthy. This could be due to the low interest in environmental issues and health implications of pollution on the continent. Research interest in this field has been negatively impacted by this. Furthermore, the lack of technological capability in terms of industrial space on the continent could have impacted research on improving cyclone efficiency. Iran was the only country from the Arabian Peninsula to make the list of the top 10 countries with respect to the number of citations per country. These findings are of great concern, as the application of cyclone separators has shown significant mitigation benefits [7,46] for particulate matter removal, and can be adopted to control the colossal level of industrial emissions and resuspended road dust emissions observed in these countries [47,48].
A stark disparity was observed, with approximately 92.61% of research on cyclone separators emerging from developed countries. By contrast, the number of publications in this field is minimal in low-income countries [49]. This reason may not be distant from the reality that most advanced industries, institutions, research laboratories, and leading research centers in the world are found in developed countries [50,51]. Other contributing factors may include research grant opportunities, research and development (R&D) policies, technical progress factors, exchange program opportunities, and availability of state-of-the-art research facilities and environments in developed countries [50,51]

Research Collaboration and Institutions
Collaborative research involves the coordination and integration of highly interdisciplinary researchers and academic scientists, which forms an essential part of advancing scientific knowledge and increasing efficiency and quality [28]. With the increasing complexities associated with scientific research, collaborative research is gradually gaining incremental significance because of the immense advantages of this research initiative. These advantages may include improved quality research output, research breakthroughs, highprofile journal publications, acquisition of bespoke funding opportunities, and problemsolving techniques [52]. Furthermore, this initiative permits the transfer of technology, promotion of creative thinking, development of young scientists, etc. Collaborative research among scholars increases the number of citations, particularly if an international networking scheme is used to develop research proposals for joint publications [53].
Collaboration between researchers in this niche area spanning different countries is evident in this study. Of the 487 publications extracted from the database of cyclone separators, 451 were contributed by the top 20 leading countries. Single-country publications numbered 362 (74.33%), while 89 (18.27%) were inter-country publications (Table 3). For example, China generated 168 single-country documents and 28 inter-country publications. Iran recorded 32 publications as single-country research articles with ten multiple-country published articles, and India reported 23 single-country publications and seven intercountry articles. Turkey, the United Kingdom, and Spain made the top 20 leading countries with 14, 11, and 5 publications, respectively. However, they did not produce inter-country publications. Collaboration between leading and African countries is relatively scarce, and most collaboration occurs between developed countries.
A collaborative network map of the cyclone separator research is shown in Figure 4.     Research institutions play a significant role in the dissemination and support of scientific knowledge. Therefore, it is vital to acknowledge the institutions that make intensive contributions to cyclone separator research. The authors affiliated with the China University of Petroleum published the most articles in this field (n = 59). This was followed by East China University of Science and Technology (n = 18), Hiroshima University (n = 18), Lanzhou University (n = 14), and Beijing Key Laboratory for Processes, fluid filtration, and separation (n = 13), ranking as the top five. Together, they represent 45.86% of the top 20 institutions in cyclone separator research. At the institutional level, it was observed that no African institution made it into the list of the top 20 institutions. Furthermore, the authors observed a significant dominance of Chinese institutions, corroborating the findings with respect to the bibliometric indices reported in Table 3 and Figure 4, respectively.

Keywords Analysis
The frequency of words in a research domain serves as a source of information on the patterns and development of knowledge structures in a particular research niche [54]. Additionally, keyword analysis is important in scientific publications as a tool for bibliographic indexing, identifying hot topics, and enabling authors to expand their publication outlook to other related concepts [55]. This is one of the main reasons editors of journals require authors to submit five to seven keywords per manuscript for peer review purposes.
A semantic analysis of keywords related to the most relevant themes of cyclone separator research was conducted. A total of 930 author keywords (DE) and 518 keywords plus (ID) were identified in 487 articles (Supplementary Table S1). Authentic indicators of the subject areas were assumed to be the authors' keywords. From the table, 'cyclone separator' was the most frequently used keyword, with 95 occurrences (10%), which indicates that this word alone is used as a termed concept in the literature. The three most frequently used keywords were 'pressure drop' (92 occurrences), 'cyclone' (84 occurrences), and 'separation efficiency' (67 occurrences). Furthermore, 'CFD' (n = 64), 'collection efficiency' (n = 60), 'computational fluid dynamics' (n = 57), 'flow-field' (n = 33), 'numericalsimulation' (n = 27 and 'collection' (n = 23) followed chronologically in the top 10 keywords. In contrast to the most relevant author keywords, performance, efficiency, computational fluid dynamics (CFD), pressure drop, flow, simulation, collection efficiency,

Keywords Analysis
The frequency of words in a research domain serves as a source of information on the patterns and development of knowledge structures in a particular research niche [54]. Additionally, keyword analysis is important in scientific publications as a tool for bibliographic indexing, identifying hot topics, and enabling authors to expand their publication outlook to other related concepts [55]. This is one of the main reasons editors of journals require authors to submit five to seven keywords per manuscript for peer review purposes.
A semantic analysis of keywords related to the most relevant themes of cyclone separator research was conducted. A total of 930 author keywords (DE) and 518 keywords plus (ID) were identified in 487 articles (Supplementary Table S1). Authentic indicators of the subject areas were assumed to be the authors' keywords. From the table, 'cyclone separator' was the most frequently used keyword, with 95 occurrences (10%), which indicates that this word alone is used as a termed concept in the literature. The three most frequently used keywords were 'pressure drop' (92 occurrences), 'cyclone' (84 occurrences), and 'separation efficiency' (67 occurrences). Furthermore, 'CFD' (n = 64), 'collection efficiency' (n = 60), 'computational fluid dynamics' (n = 57), 'flow-field' (n = 33), 'numerical-simulation' (n = 27 and 'collection' (n = 23) followed chronologically in the top 10 keywords. In contrast to the most relevant author keywords, performance, efficiency, computational fluid dynamics (CFD), pressure drop, flow, simulation, collection efficiency, design, gas, and numerical simulations were used. An important finding from the analysis is that there was no unanimity in the conceptualization of cyclone separators, and that a lack of standardized meaning compels authors to use the terms cyclone separator (n = 95), cyclone (n = 84), cyclone separators (n = 16), and cyclones (n = 14) interchangeably.
Furthermore, through a timeline view analysis, the authors explored trends in cyclone separator research to understand their evolution over time. Different timeline intervals were evaluated in the ranges 2000-2007, 2008-2013, 2014-2020, and 2021-2021 ( Figure 6). Between 2000 and 2007, keywords such as particle separation, CFD, cyclone, cyclone separator, efficiency, pressure drop, and separation efficiency were the most frequently used for cyclone separator articles. 'Pressure drop' appeared most frequently, 185 times in total; 'separation efficiency' appeared 70 times, ranking second; 'cyclone separator' appeared 68 times, ranking third; and 'cyclone' also appeared more frequently. From 2008 to 2013, terms such as collection efficiency and numerical simulation emerged in succession. A pressure drop was recorded 112 times during this period. In subsequent years, the discrete phase model, computational fluid dynamics, and RSM were mentioned more frequently during 2014-2020. However, the word cyclone became a hot topic in 2018. Cyclone separators became a hot research topic in 2020, with 258 appearances. In recent years (2021), through the analysis of hot issue keywords and the time evolution process, it has been found that numerical simulation, cyclone, and pressure drop have become a hot research issue, and there is a greater inclination to optimize the performance of cyclone separators through numerical simulation. Significantly, increasing attention has been paid to studying the cyclone separator performance criteria through variations in geometric parameters during this period.  Figure 6). Between 2000 and 2007, keywords such as particle separation, CFD, cyclone, cyclone separator, efficiency, pressure drop, and separation efficiency were the most frequently used for cyclone separator articles. 'Pressure drop' appeared most frequently, 185 times in total; 'separation efficiency' appeared 70 times, ranking second; 'cyclone separator' appeared 68 times, ranking third; and 'cyclone' also appeared more frequently. From 2008 to 2013, terms such as collection efficiency and numerical simulation emerged in succession. A pressure drop was recorded 112 times during this period. In subsequent years, the discrete phase model, computational fluid dynamics, and RSM were mentioned more frequently during 2014-2020. However, the word cyclone became a hot topic in 2018. Cyclone separators became a hot research topic in 2020, with 258 appearances. In recent years (2021), through the analysis of hot issue keywords and the time evolution process, it has been found that numerical simulation, cyclone, and pressure drop have become a hot research issue, and there is a greater inclination to optimize the performance of cyclone separators through numerical simulation. Significantly, increasing attention has been paid to studying the cyclone separator performance criteria through variations in geometric parameters during this period. Furthermore, this period saw the development of different methods for evaluating cyclone separators by researchers in the field, such as introducing different models, numerical simulations, and many more, depending on the application of cyclone separators [7,56,57]. The detection of fluid flow patterns and characteristics inside the cyclone separator serves as the focal point of research, as keywords such as numerical simulation, computational fluid dynamics (CFD), and discrete phase models are frequently mentioned as techniques for evaluating cyclone separator performance. This could be why the flow pat- Furthermore, this period saw the development of different methods for evaluating cyclone separators by researchers in the field, such as introducing different models, numerical simulations, and many more, depending on the application of cyclone separators [7,56,57]. The detection of fluid flow patterns and characteristics inside the cyclone separator serves as the focal point of research, as keywords such as numerical simulation, computational fluid dynamics (CFD), and discrete phase models are frequently mentioned as techniques for evaluating cyclone separator performance. This could be why the flow pattern and performance of cyclone separators have become hot research topics and have appeared in most publications. Consequently, increasing attention has been paid to numerical simulations and other computational fluid dynamics (CFD) techniques to the study flow behavior and performance using different geometric design models. The outcome of future research on this topic will revolve around a combination of computational fluid dynamics and numerical simulation algorithms to evaluate the performance of cyclone separators.

Contribution of Journals
The 487 publications analyzed were dispersed across 70 journals.

Most Influential Authors
In total, 965 authors contributed to the total number of publications during the study period. This section analyzes the most prolific authors in terms of the number of publications, output, citations, and production over time. Research achieves its purpose only when it is comprehensively shared and when an author's ideas fall within the scope of the

Most Influential Authors
In total, 965 authors contributed to the total number of publications during the study period. This section analyzes the most prolific authors in terms of the number of publications, output, citations, and production over time. Research achieves its purpose only when it is comprehensively shared and when an author's ideas fall within the scope of the audience/stakeholder's interest based on its findings and well-defined production results. However, in recent years, the reader's interest has been aroused by the number of views or downloads, and the number of citations an article receives. The 20 most distinguished authors are listed in Table 4. Wang J led the list with 16 publications since 2005, followed by Elsayed K and Sun G with 14 articles. Yoshida H and Fukui K authored 12 and 11 articles in the fourth and fifth positions, respectively. Furthermore, among the top 20 authors, seven authors published ten or more articles. This was followed by Wang J (n = 16), Elsayed K (n = 14), and Wei Y (n = 10) ( Table 4). It is worth noting that out of the 20 top authors, 16 had 100 or more citations. Author citation analysis is another important parameter. The citation index of the authors shows that Elsayed K has the most citations, with 654 citations, followed by Zhao B with 465 citations, and Wang B in the third position with 306 citations. In terms of the number of citations and publications offered, Elsayed K was the most influential author in the field of cyclone separator research (h-index: 12, g-index: 14), despite being published only in 2010. To understand the research frontiers in a field, the authors delineated another significant index that measures the number of citations and productivity over time. The authors' productivity over time was based on the total number of articles and yearly citations generated (see Supplementary Figure S1 for details). The blue nodes (legend) reflect the number of articles published in a calendar year, whereas the thread connectors (shown in red) linking the nodes signify the consistency of an author's research output on a topic over a specific time span. The node size indicates the number of papers published in relation to a particular year. The depth of the node color reflects the number of citations; the darker the node color, the more citations there are per year.
From the results, in the past 11 years between 2010 and 2021, Elsayed K, Sun G, and Zhao B have been consistent. The authors also deduced that the 16 leading authors from the list were published in 2020. In combination, the h-index, total number of citations, and total number of publications showed that Chinese authors dominated the list. On the other hand, authors from other parts of the world, such as Africa and the Arabian Peninsula, did not make the list of the top 20 most productive authors.

Most Cited Documents
Research citations are progressively utilized as indicators to monitor research performance and within many aspects of the research structure [58]. They are often regarded as potential indicators that reflect a combination of high-quality and impactful research results [58]. Furthermore, a scientific publication's impact on a topic indicates the number of citations generated by that publication [59]. For example, the higher the number of citations, the more significant is the impact of research output on the subject, which can provide constructive feedback for a long time. Therefore, it takes time for publications to attract more citations.
This section analyzes the most cited global publications on cyclone separators. The top 20 most-cited studies on cyclone separators are presented in Table 5. The total number of citations is related to the total number of citations received within the same collection, or from another document in the same collection. By contrast, the total number of citations per year refers to the total number of publication citations divided by the number of years in which the document was cited. The degree to which others cite a document in a particular field reflects its impact on research terrain. Research articles may serve as a source of significant methodology, great foundation, and valuable discussion for empirically driven research by other researchers in the future [60]. The most cited publication or article was placed first and the least cited was placed last.
An  climate, large-scale industries [64], and engineering research concerns. For example, Du et al. [63] developed a test system to evaluate the flow, separation, and load characteristic performance of three commonly used cyclone separators in China. This could also be attributed to the country's experienced and young researchers, overwhelming government support, regional collaborative networks, concern for commercial enterprises, and other institutional sponsored support [26,40].
Analysis of the country collaboration map revealed a significant number of publications resulting from collaborative research between other regions of the world, which agrees with the findings of Ojemaye et al. [50]. Furthermore, it is encouraging that most publications from countries and journal sources receive more attention regarding the number of citations and referrals. However, less international collaboration (MCPR > 0.5000) has been observed among researchers in this field. This shows that the knowledge exchange across geographic boundaries is limited. Countries specializing in cyclone separators and their applications can exchange ideas and/or collaborate.
Bibliometric data regarding the most productive journal publishing cyclone separator research revealed some exciting results. Powder Technology and Chemical Engineering journals remain the top journals publishing cyclone separator research. The differing quality of journal publications on cyclone separator research indicates its level of scrutiny and physical significance. This shows that the research niche has matured across different research horizons. Data analysis in terms of productivity showed that Elsayed K was the most influential author to have contributed to this field.
The number of citations generated by Cortés et al. (2007) can be attributed to the critical development of an algebraic model for predicting the tangential velocity and pressure drop inside an inverse-flow cyclone separator. Furthermore, the fact that their review presented a classical approach in determining the cyclone separator 'natural' length, which led to secondary flows and instability discovery, may account for the significant number of citations this publication accrued. This multifaceted bibliometric review of the literature presents some gaps as substantial space for the development of cyclone separator research, which should receive additional attention from researchers in the future.
It is vital to mention that cyclone separator research has received massive attention in multi-disciplinary, high-impact factor, chemical, and environmental engineering journals dedicated to publishing and disseminating high-quality and novel methodological research. The large number of articles on cyclone separator research in high-impact journals confirms the attention given to this field. This indicates that premier journals are receptive to publishing research in cyclone separators. Furthermore, these premier journals provide a forum for researchers to retrieve, read, and cite a significant amount of global information. This is also important for cyclone separator researchers as a reliable reference source for future research.

Limitations
This study has a few methodological limitations. First, the results of this study stem from the information retrieved from the WoSCC database, and only documents in the English language were selected. The omission of documents from the study written in other languages might have led to bias in the analysis of the results. Therefore, future studies should consider the adding non-English publications, meeting proceedings, media, and other bibliometric databases to make room for articles that might significantly impact the bibliometric outcomes of this subject matter. A blend of different bibliometric analysis software is also recommended to examine citation correlations and calculate TCLS, total global citation score (TGCS) and other bibliometric metrics. Second, this study considered datasets from 2000 to 2021 (early published papers of relevance), and backdated datasets should be considered in future studies on this subject matter. Despite these limitations, this study's distinct value and significance is that no bibliometric analysis has been conducted in the literature. The authors hope that this study will provide directions for researchers in the field to identify hotspots and other application avenues to enhance collaboration in cyclone separator research and solve the challenges that confront us.

Suggestions for Future Research
The following section presents several potential scientific directions for future research on cyclone separators. The present study shows that, in addition to particle recovery, cyclone separator performance can be improved and used for other applications, including road dust particle recovery.

Evaluation of the Performance of the Cyclone Separator Using Road Dust Samples
Research evaluating the cyclone separator performance in relation to road dust particles remains in its infancy. Few studies related to the sampling of road dust in cyclone separator research include the performance evaluation of the cyclone separator for a highvolume sampling of aerosols using the guiding vane angle at 1000 L/min sampling flow rate of road dust [7], or the direct sampling and characterization of resuspended road dust to determine the composition of urban PM10 [46] collected at 0.5, above the road surface. In recent times, road dust particles due to vehicular-induced resuspension and other traffic-related emissions [65] have become an urgent concern, as an estimated 90% of the global population is reported to be exposed to PM2.5 (less than 2.5 µm) in 2019 [66], and this was found to exceed the World Health Organization's (WHO) recommended air quality guidelines (AQG) of 10 µg m −3 .
In addition, road dust is a potential habitat and reservoir for viral particles from roads, pathogenic microbes, and other infectious diseases [33], the roles of which in the spread of epidemiology and viral particles have been scarcely explored. Yilbas et al. [67] reported on human saliva cloaking faster (0.05 s) and infusion with environmental dust particles, which has a high propensity for the spread of viruses such as SARS-CoV-2 in dusty environmental settings. On the other hand, Magnano et al. [68] also demonstrated that percutaneous metals in road dust powder could penetrate damaged human skin at an accelerated rate and diffuse further into the bloodstream. An appropriate application of cyclone separator models to sample and collect worrisome fractions of road dust is worth exhaustive investigation in light of the colossal human risk exposures of resuspended road dust. Road dust trapping technologies must address the shortcomings of the existing particulate matter trapping technologies, including the low separation efficiency of fine particles. The authors proposed that novel gas cyclone separators should be developed to refine and match different sizes of road dust particle trapping. The removal efficiency and flow characteristics of road dust can first be investigated using numerical simulation methods, and the separation efficiency of different road dust particles can then be experimentally and numerically studied. A recent study by Wang et al. [57] demonstrated excellent applicability for the separation of fine particles using a gas cyclone-liquid jet separator. This is currently being investigated by our research group by using cyclone separators. Collaborative research on cyclone separators that consider the harmful emitted particles is essential. From an environmental perspective, human activities using new technologies create hazardous gases, toxic chemicals, and dangerous particles [69]. Unfortunately, these emitted particles penetrate the deepest regions of the human lung and have adverse consequences [70]. Therefore, these observations are why improving the efficiency of cyclone separator collection and other performance parameters have been the main objectives among researchers in the field to separate particles with diameters of 0.1 µm or more [71].

The Performance of Cyclone Separators under Varying Particle Properties
Most researchers in this field have focused on improving the separation performance of cyclone separators by evaluating the effects of particle agglomeration and dust concentration on their overall separation efficiency. Paiva et al. [72] developed a more realistic model to improve the collection efficiency of the sub-micro particles. The model numerically optimized the particles and achieved higher collection efficiency. They concluded that particle agglomeration inside a turbulent cyclone flow seems to justify the higher than predicted collection efficiencies observed for smaller particles in a cyclone separator.
The influence of the attrition and agglomeration of dust particles on the separation efficiency of a Stairmand cyclone was experimentally investigated in ref. [73], and the agglomeration of fine particles was observed at the initial stages. Ahuja et al. [74] and Kim et al. [75] reported that the collection efficiency of fine particles increases substantially with humidity. However, few studies have investigated the effects of dust particle properties, such as particle humidity, particle temperature, and classification, through experimental and numerical simulations.
Cyclone separators have been used in various methods and applications for dust and particle recoveries. However, none of these methods completely solve the problem of road dust sampling. Road dust and its PM10 particles (with diameters less than 10 µm) are difficult to distinguish, and their classification and dust loading concentrations are complex [76]. Furthermore, road dust resuspension is influenced by relative humidity, wind, and vehicle speed [76,77]. The advantages of cyclone separators include the development of adjustable models and improved performance to allow total suspended particle sampling. However, it has been established that as the dust concentration increases, the overall separation efficiency of the cyclone separators increases [78]. The improvement in performance with increased dust loading varies with cyclone separator geometry and inlet velocity [79]. As such, in recovering road dust using cyclone separators, it is important to thoroughly investigate the impact of increased particle humidity on particle size distribution, collection efficiency, and grade efficiency.

Conclusions
In this study, 487 publications indexed in the WoSCC database from 2000 to 2021 were retrieved. A bibliometric analysis was conducted to track the publication progress path and research hotspots and provide research directions for cyclone separator research. The most productive authors, influential publications, institutional affiliations, journals, and keywords were identified and discussed. In addition, the most-cited documents over time and keyword dynamics were analyzed to understand the historical evolution of the most recent development. Since 2000, articles on cyclone separator research have seen a spurt at a significant rate, and it is envisaged that the number of publications will increase in the coming years. Visualizations of publication trends, collaboration networks of publications, journal networks, country and institution networks, and keywords of focus topics were provided. Thus, bibliometric analysis can be used to gain insight into the development of cyclone separators. The results can contribute to the understanding of the overall structure and progression of cyclone separators and help prospective researchers identify impactful scholars and institutions for international collaboration. Countries in Africa and the Arabian Peninsula have contributed only a small amount to this field. The authors hope that this study will serve as a driving force to encourage researchers in these regions to contribute to cyclone separator research and collaborate with those in developed regions to expand methods of controlling dust particle emissions.