A bibliometric study of pool fire related publications

Journal Pre-proof A bibliometric study of pool fire related publications Jiahao Liu, Jie Li, Chuangang Fan PII:

S0950-4230(19)30787-9

DOI:

https://doi.org/10.1016/j.jlp.2019.104030

Reference:

JLPP 104030

To appear in:

Journal of Loss Prevention in the Process Industries

Received Date: 26 September 2019 Revised Date:

1 December 2019

Accepted Date: 5 December 2019

Please cite this article as: Liu, J., Li, J., Fan, C., A bibliometric study of pool fire related publications, Journal of Loss Prevention in the Process Industries (2020), doi: https://doi.org/10.1016/ j.jlp.2019.104030. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

CRediT author statement Jiahao LIU: Conceptualization, Formal analysis, Validation, WritingOriginal draft preparation, Project administration, Funding acquisition Jie LI: Methodology, Software, Visualization, Investigation, Resources, Data curation, Funding acquisition Chuangang FAN: Writing- Review and Editing, Supervision

A bibliometric study of pool fire related publications Jiahao Liua,*, Jie Li a, b,*, Chuangang Fan c a b

College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, China

State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China c

School of Civil Engineering, Central South University, Changsha 410075, China

Abstract: Pool fire is a common form of fire, which is constantly investigated along with the development of fire science and is also comprehensively employed as stable fire sources in examining other fire scenarios such as building and tunnel fires. According to the records in Science Citation Index Expanded database in the Web of Science Core Collection, a total of 1073 articles or reviews related to pool fires have been published from 1966-2019. In order to have a better understanding of knowledge structure of this topic and further identify its development history and currently popular concerns, a bibliometric analysis of pool fire research is conducted by means of visualization software VOSviewer and CiteSpace. This work visually provides a comprehensive overview of pool fire research in terms of annual publication output, source journals, productive countries/regions, authors and their cooperation network, subject terms, and reference co-citation analysis. The analysis provides networks of co-cited references, authors, countries, subject terms, and their respective clusters, indicating their ranking in contributions to the pool fire related publications. The results can be applied to enhance the understanding of pool fire research and support further work in this area. Keyword: Pool fire; bibliometric analysis; knowledge structure; visualization

1. Introduction In industrial process and power plant systems, accidental spill of liquid fuels can pose a severe fire hazard (Karlsson and Quintiere, 1999). Burning of the fuels in a pool is probably the simplest form of combustion applicable to a wide range of industrial fire protection concerns (Babrauskas, 1983). A pool is characterized by a confined body of fuel, and it can form due to the liquid fuel released in a low spot, such as a trench, or can exist as a result of normal storage of fuels in tanks or containers (Gottuk and White, 2016). Thus, a pool fire tends to imply that the fuel is liquid, or according to Drysdale (2011), it refers to “stable liquids tend to burn as pools with uniform horizontal surfaces”. More generally, however, both liquefied gases and melting plastics materials, horizontally placed, conform to the same pattern. Thus, pool fire, more comprehensively, is also defined as a 1

buoyant diffusion flame in which the fuel is configured horizontally (Hamins et al., 1996). Specifically, Joulain (1998) introduced that a pool fire is characterized by the establishment of a diffusion flame on top of a horizontal fuel where the buoyancy force is the controlling transport mechanism. Based on that, many common fire scenarios can be classified as pool fires, which can be physically illustrated by a burning of combustible surface where a diffusion flame is stabilized over the vaporing combustible and sustains the fuel gasification by heat transfer to its surface (Joulain, 1998). In 1957, Blinov and Khudiakov (1957) initially conducted the most extensive single study to investigate the burning rates of hydrocarbon liquid pool fires with diameters ranging from 0.0037 to 22.9 m. Based on the experimental results above, Hottel (1959) performed an in-depth analysis of flame heat transfer phenomena associated with the burning of liquid pool fires, and further indicated thermal radiation heat transfer dominates real fires, not the conduction and convection heat transfer that characterize smaller scale laboratory fires. This explanation of how physical dimension and different heat transfer mode affect pool fire behaviors stimulates the subsequent research interests in pool fires (Alpert, 2002), because the heat feedback from the flame directly determines the mass burning rate and thus the heat release rate, the single most essential parameter in characterizing a pool fire (Babrauskas, 1983). Since then, extensive studies are conducted to validate and modify the theory, seeking to establish more precise heat feedback models to predict the burning rate of pool fires with various scales (Hamins et al., 1996; Joulain, 1998). Many specialized researches on such topic can be found in the literature, such as the report by the National Institute of Standards and Technology (NIST) (Hamins et al., 1999) or the book “An introduction to fire dynamics” by Drysdale (2011). In the meanwhile, other important parameters in pool fires, including flame height, puffing frequency, air entrainment, temperature profile, soot formation and radiation, are also comprehensively investigated, and many empirical, semi-empirical, or theoretical models, have been established to interpret these parameters. Behaviors of pool fires can also be affected by the ambient conditions, such as the absence or presence of an enclosure, ventilation, wind, or ambient pressure. Under such circumstances, the relevant parameters will be influenced or even changed totally, and need further examination, e.g., Hu (2017) indicated that the burning behaviors, heat feedback mechanism, soot production, and radiation property of large scale pool fires subject to wind are still the most significant challenges. Given the importance and considerable quantity of the pool fire research, it is essential to examine its characteristics and the intellectual framework on which it is based. To date, according to the Web of Science (WoS), one of the largest databases of peer-reviewed academic literature, more than 1000 papers related to pool fires have been published, while no study has been conducted to visualize and analyze the overall intellectual structure of the theme. The objective of this study is therefore to evaluate the research on pool fires, seeking to have a structural overview of relevant information. Bibliometric analysis is a technique which makes it possible to provide a macroscopic overview of large amounts of academic literature. It has the potential to 2

introduce a systematic, transparent, and reproducible review process, and thus improve the quality of review (Nicolaisen, 2010). Through a unified analysis of information available in the database, such as titles, authors, affiliations, abstracts, keywords, references, etc., the characteristics and the development of scientific output within a particular research field can be mapped (Jia et al., 2014; Li and Hale, 2016). Furthermore, it scrutinizes the most frequently occurring themes in the publications to identify potential weaknesses and gaps in scientific research into the subject (Dzikowski, 2018). The specific results include number of publications per year, most cited articles, most prolific journals, authors, and nations, etc. The analysis also provides networks of co-authors, co-citation reference, journals, and their respective clusters, revealing their rankings regarding contributions to pool fire research. The bibliometrics is a currently popular method in evaluating and predicting the development tread of science and technology using mathematical, statistical, and other measurement methods (Xu et al., 2017). It has been extensively used in various research topics. In some relatively relevant aspects, Li et al. (2017) used the bibliometric approaches to analyze the “domino effect” in the process industry, revealing the publication trend in this hot topic. Juárez-Orozco et al. (2017) conducted a bibliometric analysis of causes and effects of forest fires in tropical rainforests, aiming to evaluate the main factors that motivate investigations in this field of study. Recently, Amin et al. (2019) performed a bibliometric review of process safety and risk analysis to provide a systematical overview of in this field. Similar methods are also employed to analyze the research in natural hazards, emergency, and disaster management (Barnes et al., 2019; Demiroz and Haase, 2019). From the above, the bibliometric techniques and methods have been successfully applied in the analysis of popular research topics associated with different hazards, while in fire science, such studies are still scarce. In fact, after decades of development, fire science has accumulated a large amount of literature. The pool fire therein is one of the most concerned topics throughout the fire science research, still being a hot topic so far. Obviously, bibliometrics can be used in the pool fire research to reveal its important features. Thus, this paper attempts to use the bibliometric and visualization methods to systematically examine all the pool fire publications included in the Science Citation Index Expanded (SCI-Expanded) database.

2. Data mining and research methods The WoS Core Collection (WoS CC) from Clarivate Analytics provides a unique feature of citation counts, which allows to quantify the relative importance of articles through the use of an objective measure of influence (Dzikowski, 2018). The pool fire articles are retrieved from the WoS CC, and the search is only limited to the SCI-Expanded which is a well-known database collecting peer-reviewed scientific papers with strict scrutiny criteria. This ensures the high quality and representativeness of the selected papers on pool fires. In this study, the publications are collected

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through the topic research with a retrieval type (“pool fire*”), which means that this term is identified in the Title, Abstract, and Keywords of the publications. The document types are further limited to “article” and “review”. In order to collect all relevant papers, the time span is set as “all year” in the retrieval process, that is, from 1900 to the date of the search (30 June 2019). As a resut, a total of 1073 publications related to pool fire are obtained. In fact, if the same search strategy is performed on another day, the results obtained will be slightly different. This is mainly because the WoS is continuously updated, which leads to minor changes over time (Liu et al., 2013). Every document in WoS includes a set of information of source journal, publication year, title, abstract, keywords, authors and their affiliations and nations, subject categories, and references, which is saved as plain text for the following bibliometric analysis. With decades of development, many bibliometric indicators and methods have been gradually shaped and widely accepted, such as the quantity of publications, cited frequencies, co-citation (Small, 1973), journal impact factor, etc. Nowadays, the astonishing growth of math and computer technologies brings the bibliometric studies into a new age, using the visualization software and techniques (e.g. VOSviewer, CiteSpace, Pajek, Histcite, etc.) to display the overall framework of different disciplines. In this work, the VOSviewer, a freely available computer program that is developed for constructing and viewing bibliometric maps (see www.vosviewer.com) (van Eck and Waltman, 2010), is employed to analyze the relationships among source journals, authors, institutions, countries, and research topic in pool fire publications. It helps to construct maps of authors (institutional and national information included) or journals based on co-citation data, and construct maps of keywords or research terms based on co-occurrence data. For each topic, a two-dimensional map can be obtained, where the distance between two items reflects their similarity or relevance (van Nunen et al., 2018). As for the co-citation analysis, another visualization tool, CiteSpace, is used due to its availability in characterizing and interpreting the structure and dynamics of co-citation network and clusters (Chen, 2006). The co-citation network covers the fundamentals of pool fire research in different periods, and the clusters of reference co-citation can clearly reflect the knowledge structure and evolution of pool fire research.

3. Results and discussion 3.1 Annual publication output A set of 1073 publications are obtained, and their annual distribution and corresponding cumulative results are plotted in Fig. 1. The first article recorded in WoS CC dates from 1966, i.e. “Some recent experiments with pool fires” by Corlett and Fu (1966). As reviewed in the introduction, the pool fire research can be traced back to 1957, while the work by Blinov and Khudiakov (1957) was published in the form of a report and not collected by WoS CC. After the pause for eleven years, the relevant pool fire papers start to be published continuously since 1977, with an exception in 1979. However, in the first few years (1977-1990 inclusive), the number of publications per year are not 4

larger than 5, which can be identified as the “precursors’ phase” according to Price’s law (de Solla Price, 1965), where a small group of researchers begin to pay attention to the pool fire research. As predicted by Price’s law, an increasing number of researchers will be attracted by different aspects of the subject that still remains unclear, rendering a proper exponential growth of publications (de Solla Price, 1965). The pool fire publications conform well to the prediction, exhibiting an exponential increase since 1990 but with oscillations in some years (e.g. 2004-2014), as shown in Fig. 1. In this period (1990-present), the exponential increase in publications implies that the number of researchers increase constantly, and the progression of knowledge opens new fields of interest (Dabi et al., 2016). The number of publications reaches a peak in 2017, with 114 published papers, while it may be not the end of the exponential increase. In fact, after the body of knowledge on pool fires is consolidated, the number of publications should decrease, which reflects the maturity and saturation in this domain (Dabi et al., 2016). Obviously, we cannot jump to this conclusion so far. Judging from the general trend in Fig. 1, the pool fire research may still remain thriving in the next few years.

Fig. 1 Temporal distribution of pool fire publications from 1966-2019 In fact, pool fire, as a typical type of fire phenomenon, is being comprehensively investigated along with the development of fire science. In the early 1970s, Professor Howard Emmons at Harvard University initially introduced the conservation of mass, momentum, and energy, and principle of chemical reaction in the study of building fires, providing the precedent of fire science research. Meanwhile, enabled by ample funding, enthusiasm for problems in interdisciplinary elements and challenged by new complex problem, many new researchers engaged in fire research in the 1970s (Quintiere, 2006). Their efforts brought credibility to the fire research interest for many countries such as USA, UK, Japan, etc. The pool fire research also emerged and generally grew in this period. Then,

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the continuously growing community of fire science research simultaneously drove the rapid development of pool fire research, as shown in Fig. 1.

3.2 Source journal analysis Academic journals are important information carriers for dissemination, communication, and inheritance of scientific achievements (Zou et al., 2018). Journal analysis helps to identify the distribution of core journals in pool fire research. The retrieved results show that 1073 articles are published in 170 journals, covering engineering, materials science, energy & fuels, construction & building technology, thermodynamics, and other research domains. The top-15 prolific journals with more than 20 pool fire articles are provided in Table 1. Fire Safety Journal (FSJ), as a professional journal devoted to research on fire safety science and engineering, is the most prolific one with 127 articles, about 11.8% of the total. It is also notable that except FSJ, another three fire related journals, i.e. Journal of Fire Sciences (JFS, 60, 5.6%), Fire Technology (FT, 52, 4.8%), and Fire and Materials (FAM, 23, 2.1%), are also in the top 15, reflecting the pervasiveness of pool fire research in fire science. The second-ranked journal is Combustion and Flame (CNF, 70, 6.5%) which publishes work from experimental, theoretical, and computational investigations on the fundamentals of combustion phenomena and closely allied matters. Another two combustion related journals, Combustion Science and Technology (CST, 58, 5.4%) and Proceedings of the Combustion Institute (PCI, 51, 4.8%), are ranked the fifth and the eighth, respectively. The third-ranked one is Journal of Loss Prevention in the Process Industries (JLPPI, 61, 5.7%), whose broad scope is process safety. Additionally, the pool fire research is also closely associated with the heat and mass transfer and thermodynamics, and the relevant journals include Applied Thermal Engineering (ATE, 32, 3.0%), International Journal of Heat and Mass Transfer (HMT, 23, 2.1%), and International Journal of Thermal Sciences (IJTS, 22, 2.1%). The rest of the journals, Journal of Hazardous Materials (JHM, 54, 5.0%), Tunnelling and Underground Space Technology (TUST, 29, 2.7%), and Nuclear Engineering and Design (NED, 21, 2.0%) are ranked the sixth, eleventh and fifteenth, respectively. Overall, the core journals in pool fire research are almost relevant to this topic, specializing in addressing different aspects of issues induced by pool fires. Table 1. Top 15 source journals ranked by the quantity of publications, 1966-2019. Rank

Source Journal

NP

APY

AC

1

Fire Safety Journal

127

2008.8

14.0

2

Combustion and Flame

70

2005.4

26.6

3

Journal of Loss Prevention in the Process Industries

61

2011.2

8.9

4

Journal of Fire Sciences

60

2009.2

10.4

5

Combustion Science and Technology

58

1968.7

15.6

6

Journal of Hazardous Materials

54

2009.1

19.6

6

7

Fire Technology

52

2011.8

7.9

8

Proceedings of the Combustion Institute

51

2012.7

20.9

9

Fuel

34

2016.1

10.9

10

Applied Thermal Engineering

32

2016.2

9.8

11

Tunnelling and Underground Space Technology

29

2013.4

20.5

12

Fire and Materials

23

2012.0

5.3

13

International Journal of Heat and Mass Transfer

23

2014.5

17.6

14

International Journal of Thermal Sciences

22

2015.8

7.0

15

Nuclear Engineering and Design

21

2006.4

6.3

Note: NP=number of papers, APY=average publication year, AC=average citations per document. As listed in Table 1, the average publication year and citations for different journals are also presented, which denote the mean values of all the articles in a certain journal. Fig. 2 visualizes the corresponding results with distinct colored markings, where only the journals with at least 5 publications are presented. The left figure is the time-based analysis of journal productivity as the average publication year decreases from top to bottom, identical to the legend. The right one is the citation-based results, and the legend represents the average citations per document in a certain journal. The size of the circle represents the quantity of publications. From the left hand graph in Fig 2, the recently rising flourish in pool fire research can be observed mainly for ATE, Fuel, IJTS, and HMT, while the traditional fire science journals, such as FSJ and JFS, involve the pool fires with consistent acceptance, resulting in a relatively centered average publication year of 2008~2012. Notably, the CNF and CST concern the pool fire research with an average publication year before 2005, indicating a lower acceptance of papers on this subject in recent years. The objectivity, accumulativeness and fairness of cited frequencies make it very suitable to measure to academic influences of academic journals as a whole (Aleixandre-Benavent et al., 2018). The right graph in Fig. 2 indicates that the most influential journals in pool fire research are CNF, PCI, and TUST in turn, all with average citations above 20, followed by JHM and HMT with 19.6 and 17.6 average citations, respectively. In contrast, despite the larger quantity of publications, the average citations of FSJ, JFS, CST, JLPPI, and FT are relatively lower. This implies that the influences of conventional fire science journals in pool fire research may be not comparable to heavyweight combustion related journals, such as CNF and PCI.

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Fig. 2 Average publication year and average citations of pool fire journals

3.3 Analysis of productive countries According to the retrieved results, all the pool fire papers come from 48 countries or territories in the world. Table 2 lists the productive countries or regions with at least 15 publications, the total of which are 1141, larger than 1073 retrieved publications. This is mainly due to that an article can be written by several authors from different countries or regions, resulting in the duplication in the count. China is the most productive country in pool fire research with 368 papers, accounting for 34.3% of the total. The USA is ranked the second, publishing 270 papers, accounting for 25.1%, nearly 10 percent lower than China. The rest in Table 2 publish no more than 100 papers, and most of them are Occident, except Australia (Oceania), Japan, India, and South Korea (Asia). Table 2 Countries or territories with at least 15 publications in pool fire research Rank

Countries/regions

Continent

NP

AC

1

Peoples R China

Asia

368

9.8

2

USA

North America

270

19.0

3

France

Europe

82

10.0

4

Japan

Asia

45

9.7

8

5

Canada

North America

43

13.5

6

Australia

Oceania

40

12.8

7

England

Europe

40

15.9

8

India

Asia

40

6.3

9

Germany

Europe

34

5.6

10

Spain

Europe

30

20.9

11

Italy

Europe

28

25.1

12

South Korea

Asia

28

15.3

13

Sweden

Europe

26

15.2

14

Belgium

Europe

19

5.9

15

Scotland

Europe

18

13.1

16

Denmark

Europe

15

8.5

17

Finland

Europe

15

10.3

Note: NP=number of papers, AC=average citations per document. The map visualizing the collaboration between different countries or regions are plotted in Fig. 3 by means of VOSviewer, where it only covers countries or regions with at least ten publications on the topic. In the network, a node is allocated to each co-author of publication, and the size and the color of a node denote the number of publications and the cluster to which the node belongs, respectively (Li et al., 2017). The thickness of links represents the strength of collaboration. Meanwhile, it should be noted that for the sake of clarity, only the largest sub-networks are concerned in the analysis. From Fig. 3, two major clusters can be identified: one gathering around China and USA (green cluster), and one gathering around France (red cluster). As the two most prominent nodes in the networks, China and USA show a strong cooperative relationship in pool fire research, e.g. Refs. (Hu et al., 2013a; Huang et al., 2019). Previous researchers (Zheng et al., 2016) indicate that collaborative countries tend to be geographically correlated, gathering around the most productive countries or territories in terms of publication output. However, it is shown that in green cluster, China, as an Asian country, is collaborating with countries and regions in other continents, such as Australia, North Ireland, England, etc., being centered in the network. Yuasa (1962) defined the countries that accounts for more than 25% of the major scientific achievement of the entire world as the centers of scientific activity. Both China and USA meet this criterion, while the USA appears to be much more centered in the network, exhibiting a more comprehensive cooperation relationship with other countries than China. Additionally, the red cluster gathering around France mainly includes the Occident (Spain, Belgium, Canada, Finland, Germany, etc.) and three Asia countries (South Korea, India, and Japan). Compared with China and USA in green cluster, their research outputs on pool fires are relatively smaller.

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Fig. 3 Countries or territories collaboration network of pool fire research

3.4 Prolific authors and their cooperation network Pool fire is a growing research field, which constantly draws the attention of researchers. The total of 1073 collected publications are contributed by 3945 single authors, and 124 authors have a minimum of 5 documents. Among them, the top-10 prolific ones are listed in Table 3. In order to understand their contributions to pool fire research, we ranked them based on the average citations per document (the left list in Table 3) as well as the total number of documents (the right list in Table 3). Both the two parameters are critical in characterizing the impact of the most prolific authors (Dzikowski, 2018), and they offer different perspectives of their contributions and may indicate collaboration in their production. Table 3 List of prolific authors Note: The left list is ranked by average citation per document; the right list is ranked by quantity of documents

Authors

NP

TC

AC

Authors

NP

TC

AC

Huo R

10

360

36.0

Sun JH

38

582

15.3

Gore JP

7

236

33.7

Chow WK

37

513

13.9

Liu S

7

232

33.1

Hu LH

36

782

21.7

de Ris JL

9

258

28.7

Ji J

27

466

17.3

McGrattan KB

5

134

26.8

Lu SX

23

220

9.6

Baum HR

5

132

26.4

Wang XS

22

222

10.1

Quintiere JG

8

197

24.6

Wang J

20

179

9.0

Fan CG

13

317

24.4

Zhang YM

18

283

15.7

Tieszen SR

6

137

22.8

Vantelon JP

18

207

11.5

Hu LH

36

782

21.7

Liu NA

16

191

11.9

10

Note: NP=number of papers, TC=total citations, AC=average citations per document. When considering contributions in terms of average citations per document, leading the ranking are authors such as Huo R, Gore JP, Liu S, de Ris JL, etc. Huo is a professor at State Key Laboratory of Fire Science (SKLFS), University of Science and Technology of China (USTC, China), who specializes in combustion theory, fire dynamics, prevention and control of building fires. In the collected documents, his works are published in the period of 2000~2011, mainly focusing on the pool fire based “building fires” (e.g. (Chow et al., 2000)) and “tunnel fires” (e.g. (Hu et al., 2006)). It is noted that liquid pool fires, as a convenient and controllable buoyant diffusion fire phenomenon with relatively good reproducibility, has been comprehensively employed as fire sources in various fire scenarios, rather than studying itself solely. Moreover, Professor Huo has a strong collaboration with Hu LH (another professor in SKLFS, as shown in Table 3) for a long period of time, producing a great number of articles. For example, they share a paper with 104 citations together, which addresses the maximum smoke temperature under the ceiling in tunnel fires through a series of full-scale pool fire tests in two hypostatic vehicular tunnels (Hu et al., 2006). In addition, the third-ranked Liu S also comes from the SKLFS, who is Professor Hu’s postgraduate student and mainly focuses on wind-blown pool fire behaviors such as heat feedback mechanisms (Hu et al., 2013b) and morphologic characteristics (Hu et al., 2013a). The second-ranked author is Gore JP, a professor in combustion at Purdue University (USA), who contributes on pool fire research mainly between 1991~2007. His work covers flame radiation, soot formation, as well as numerical simulation on buoyant turbulent pool fires. De Ris JL, from FM Global, USA, is another fire expert who gives a great contribution on pool fires, e.g. the radiation fire modeling proposed by him is based on the pool fires between the optically thin and optically thick limits, providing an available reference for similar studies (De Ris et al., 2000). Moreover, it is interesting to note that the rest of six authors in the left list are either from China (Fan CG and Hu LH) or from the USA (Mcgrattan KB, Baum HR, Quintiere JG, Tieszen SR). This well verifies the influences of China and the USA in pool fire research, as shown in Table 2 and Fig. 3. As for the ranking by the quantity of publications, Sun JH ranks first with 38 publications. Sun is a professor at SKLFS, USTC, China, who specializes in building and industrial fire dynamics and corresponding prevention and control methods. Except Prof. Sun, another seven authors in right list also come from SKLFS, i.e. Hu LH, Ji J, Lu SX, Wang XS, Wang J, Zhang YM, and Liu NA, all as professors. It is worthwhile to note that the SKLFS is established in 1992, and its mission is to explore fire dynamics and develop advanced fire safety technologies. Since then, the SKLFS contributes a lot in fire science, e.g. the great number of pool fire publications in the right list of Table 3. Moreover, the exponential increase in pool fire publications (Fig. 1) since 1990 may be also attributed to the contribution of SKLFS to some extent. Besides, the second-ranked Chow WK is a professor at Hong Kong Polytechnic University (PolyU, China), who engages in pool fire related research (in the 11

collected articles) since 1998 (Chow and Tsui, 1998) and is still being enthusiastic about this field until now (Ng et al., 2019). Vantelon JP is a professor from University of Poitiers (France), publishing 18 papers mainly concentrating on the radiative properties of pool fires and oil spill fires (Bouhafid et al., 1988; Garo et al., 2007).

Fig. 4. Co-authorship network among productive authors. The co-authorship network of distinct contributors is visualized in Fig. 4, where only the authors with at least 5 documents are displayed, and each circle, size of a circle, and arc represents an author, quantity of publications, and co-authorship strengths, respectively. The prolific authors in the right list of Table 3 play important roles in the co-authorship network, and significant visible links are observed among them, especially for the professors at SKLFS, USTC (China). Chow WK has a collaborative relationship with Huo R and Hu LH, but in the early age before 2011, as confirmed by their last co-authored article in (Hu et al., 2011b). In contrast, the clusters gathering around Vantelon JP, De Ris JL, Quintiere JG, and Rangwala AS are relatively smaller, while the higher average citations per document in the left list of Table 3 also reflects their significant influences in pool fire research.

3.5 Subject terms co-occurrence analysis Analyzing the occurrence frequencies of the subject terms in publications can provide insights into main topics and research trends in pool fire research (van Nunen et al., 2018). In order to identify the hot topics in the domain of pool fire research, the information of titles and abstracts of the collected papers are extracted to construct the co-occurrence network. Only the terms that occur in at least ten publications are displayed and those with a general meaning such as “world”, “future”, or

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“increase” are excluded. As a result, a total of 294 terms are identified, and their co-occurrence network is visualized in Fig. 5. Each color represents a unique cluster, while the circle size denotes the occurrence frequencies of a certain term. The closeness of words, as well as lines, indicates the strength of their relationship, which is determined by the times that terms co-occur in the titles and abstracts (Rodrigues et al., 2014). In the map, it can be observed that the subject terms of pool fire studies form five clusters, and the terms in each cluster show great relevance in respect of research topics. In view of the characteristics and status of pool fire research, the five clusters can be reduced as follows: cluster #1 fire risk evaluation (red); cluster #2 pool fire experiments (green); cluster #3 building structure fire (blue); cluster #4 fire suppression (yellow); cluster #5 soot and flame radiation (purple). The top-15 subject terms in each cluster are presented in Table 4.

Fig. 5. Terms co-occurrence network of pool fire It is not difficult to find that the research topics in clusters #1 and #2 are relatively popular due to their higher occurrence frequencies. Due to the enormous hazards of pool fires in industry, it is of great importance to reduce the frequency and severity of pool fire accidents as much as possible. Thus, the fire risk evaluation is necessary to provide guidance on fire safety design in pool fire prone areas. Using risk analysis and numerical simulation methods to explore the possible consequences caused by (pool) fires has become an important research branch. As shown in Table 4, the first term in cluster #1 is “simulation”, which also indicates the principal means in fire risk evaluation. The Fire Dynamics

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Simulator (FDS), a Large Eddy Simulation (LES) code developed by the NIST, is usually employed to conduct the consequence modelling of large-scale pool fires (Ahmadi et al., 2019; Ryder et al., 2004). Similar work is also conducted by combining another Computational Fluid Dynamics (CFD) software FLACS with FDS (Dadashzadeh et al., 2013). Table 4. Top-15 subject terms and their occurrence in each cluster of pool fire publications Cluster #1

Cluster #2

Cluster # 3

Fire risk evaluation

Pool fire experiments

Soot

and

Cluster #4 flame

radiation simulation

106

experimental

Building

Cluster #5 structure

Fire suppression

fire

154

plume

84

compartment

80

water mist

54

144

diffusion

69

tunnel

79

oil

48

44

smoke

76

extinction

46

34

fire test

74

efficiency

43

study performance

80

correlation

flame assessment

80

flame height

128

soot

facility

72

pressure

111

large

eddy

simulation gas

71

burning rate

104

particle

29

ventilation

61

pan

38

accident

66

length

89

soot volume

26

fire source

55

nozzle

37

fraction liquid

65

burning

86

methane

22

ceiling

51

evaporation

36

code

65

thickness

74

radiative

19

degrees c

46

effectiveness

34

18

numerical

45

suppression

33

heat transfer release

65

temperature

65

distribution consequence

64

burner

soot formation

64

mixture

simulation 18

room

42

spray

31

17

oxygen

41

fire

30

fraction cfd

62

mass loss rate

63

probability density

concentration

suppression

function risk

59

angle

60

toluene

16

fds

35

agent

26

tank

55

flame

59

oxidation

15

building

34

technology

24

temperature evaluation

46

speed

51

laser

15

position

34

droplet

24

explosion

46

buoyancy

51

flow field

15

enclosure

32

cone

23

calorimeter

As for the cluster #2, the high-frequency term “experimental study” can be summarized as the main research topic. In fire science, the experimental study is a commonly used method to explore the fundamentals of fire phenomena, and pool fire research is no exception. In the collected articles, this 14

can be traced back to 1966, the first publication on pool fires in WoS (Corlett and Fu, 1966). Since then, a battery of experiments studies on pool fires are conducted to reveal the basic laws of the typical parameters such as flame height, flame temperature, burning rate/mass loss rate, as shown in Table 4. Besides, the influences of external factors on pool fires are also extensively concerned, e.g. ambient pressure and wind, corresponding to the terms “pressure” and “angle”, respectively. Cluster #3 mainly encompasses the studies on soot formation and flame radiation property of a pool fire with the numerical simulation (e.g. LES, mixture fraction model, or probability density function) or optical measurement method (e.g. laser-induced incandescence (Frederickson et al., 2011)). This is more inclined to examine the inherent combustion mechanism, seeking to reveal the process of soot formation and quantify the soot volume fraction as well as radiative heat transfer, which is quite different from the function of numerical simulation in Cluster #1. The role of being a stable fire source for pool fires is crystallized in cluster #4 where the fire tests in buildings or tunnels are usually achieved by pool fires. The effects of oxygen depletion and ventilation conditions on fire development and smoke transportation are of significant concerns. Meanwhile, due to the difficulty of performing full-scale fire tests, FDS software is also frequently used to examine such fire scenarios. Fire suppression, another important part in fire safety design, is shown in Cluster #5, where the term “water mist” has the highest occurrences. Water mist with very small droplets can control or extinguish fire in the ways of cooling flame and fire plume, oxygen displacement by water vapor, and radiant heat attenuation (Kim et al., 1997). Its effectiveness in extinguishing pool fires has been widely verified, specifically referring to the “Water mist fire suppression systems” in SFPE Handbook of fire protection engineering (Mawhinney and Back, 2016). The terms co-occurrence network with a timeline is depicted in Fig. 6, where the color denotes the average publication year of a term, representing the evolution of hot topic in pool fire research over time. The average publication year of a term is calculated as the mean value of the publication years of all publications that have the term in their title or abstract (van Nunen et al., 2018). Terms that are used more towards 2014 are shown in red, and the top-20 more recent terms are provided in Table 5. It can be seen that most of terms belong to cluster #2, i.e. the pool fire tests are research hotspots in current days, as confirmed in Fig. 6. More specifically, two main foci can be identified from the terms in Table 5. The first one is pool fires subject to a wind, as evidenced by the terms “cross flow” and “flame tilt angle”. Wind-blown pool fire behaviors are driven by the coupling of buoyancy and wind, which is much more complex than that in still air, being always a significant challenge (Hu, 2017). As early as 1961, Blinov and Khudyakov (1961) found that the presence of wind will enhance the burning rate of pool fires. Subsequently, some sporadic studies on this topic are carried out to investigate the wind effect on the burning rate of pool fires with various burner sizes, while the achievements are still relatively scarce. It is known that in addition to changing the flame envelope, cross wind also affects the heat feedback mechanisms, as well as fuel-air mixing for this mixed buoyancy and boundary layer 15

diffusion combustion. Its complexity and dilemma in key scientific problems such as heat feedback mechanism, flame tilt angle, flame morphologic parameters, and flame base drag are attracting more and more researchers’ attention very recently, as introduced in the review by Hu (2017).

Fig. 6. Terms analysis of pool fire publications with time trends. The second research topic that can be summarized from Table 5 is pool fire research at reduced pressures, as confirmed by the terms “altitude”, “high altitude”, “low pressure”, “Lhasa”, as well as the unit “kPa”. The background of this research topic is the requirement of fire safety design at high altitude or in a cruising aircraft, where a low-pressure environment with lower air density and oxygen concentration than atmospheric conditions naturally exists, resulting in different fire behaviors. The relevant research is initially conducted by Wieser et al. (1997) in 1997, wherein n-heptane pool fires are tested at four altitudes ranging from 420~3000 m. After that, the similar work is very scarce until July 2009, the time when the plateau fire laboratory was formally established in Lhasa city, Tibet Autonomous Region, China (Liu et al., 2018). Since then, considerable experimental studies on pool fires at reduced pressures are conducted to reveal the fundamental essence of pressure effect on the pool fire phenomena, and to establish prediction models for typical parameters at various pressures. In the meantime, the altitude chambers with different internal volumes are also employed to simulate the low-pressure environments for pool fire tests, e.g. the dimensions of 3(L)×2(W)×2(H) m (Liu et al., 2016), 8.11(L)×4.16(W)×167(H) m (Zhu et al., 2019). Terms in Table 5 show that the pool fire

16

research relying on low-pressure environments is still a hot topic in recent years. Except the terms belonging to cluster #2, there are another two terms included in Table 5, i.e. “domino effect” in cluster #1 and “glass” in cluster #4. The topic of domino effect in the process industry starts to receive attention in risk analysis and safety assessment studies over the last two decades, and a bibliometric analysis on this topic has been accomplished by Li et al. (2017) in 2017. The topic of glass in cluster of building fires mainly originates from the problem of breakage of glass curtain wall when subject to adjacent pool fires in high-rise buildings. Prof. Rush David and Dr. Wang Yu from University of Edinburgh carried out a series of excellent studies in this topic very recently, e.g. (Wang et al., 2019). Table 5. Terms of pool fire research in recent years

Term

Cluster

Occurrences

Avg. Pub. Year

dimensionless heat release rate

2

11

2017.27

cross flow

2

11

2017.00

decay stage

2

12

2016.75

flame tilt angle

2

11

2016.64

mass burning rate

2

12

2016.17

combustion efficiency

2

14

2016.14

glass

4

13

2015.77

kPa

2

37

2015.76

domino effect

1

12

2015.75

sidewall

2

18

2015.61

altitude

2

24

2015.58

unit area

2

13

2015.23

high altitude

2

18

2015.22

flame length

2

40

2015.18

low pressure

2

23

2015.04

mass loss

2

19

2014.95

lhasa

2

24

2014.92

n-heptane pool fire

2

34

2014.79

open space

2

14

2014.79

fire whirl

2

26

2014.77

3.6 Reference co-citation analysis In the bibliometric analysis, the impact of a publication in relation to the number of citations that publication receives is defined as its quality (Van Leeuwen et al., 2003), which has served as a formal

17

scientific evaluation for decades. Here, the articles with top 10 cited frequencies in the collected 1073 documents are also listed in Table 6 for reference. The cited frequency reflects the number of times the publications on pool fire has been cited by other publications listed in WoS. It generally increases over time, and thus, the earlier publication also has a higher cited frequencies. It is noted that all the documents in Table 6 are published more than 12 years. This result is quite reliable because articles always need a long time to be widely accepted, and ten more year is a must to accumulate sufficient citations (Pilkington and Meredith, 2009). Table 6. The top 10 highly-cited publications in the collected data. Rank 1

TC 270

2

174

3 4

156 155

5

126

6

119

7

113

8

108

9

104

10

102

Title Experimental formulation and kinetic model for JP-8 surrogate mixtures Experiments on the periodic instability of buoyant plumes and pool fires Estimating large pool fire burning rates Advanced gasoline engine development using optical diagnostics and numerical modeling Thermal-radiation hazards from hydrocarbon pool fires

Authors Violi A et al.

Year 2002

Cetegen BM, Ahmed TA Babrauskas V Drake MC, Haworth DC Mudan KS

1993

Comparison of a fractal smoke optics model with light extinction measurements Experimental study of burning rate in hydrocarbon pool fires Transition from momentum to buoyancy-controlled turbulent jet diffusion flames and flame height relationships On the maximum smoke temperature under the ceiling in tunnel fires

Dobbins RA et al. Chatris JM et al. Delichatsios MA

1994

Fire Technology Proceedings of the Combustion Institute Progress in Energy and Combustion Science Atmospheric Environment

2001

Combustion and Flame

1993

Combustion and Flame

Hu LH et al.

2006

Fire in the Brazilian Amazon 2. Biomass, nutrient pools and losses in cattle pastures

Kauffman JB et al.

1998

Tunnelling and Underground Space Technology Oecologia

1983 2007 1984

Journal Combustion Science and Technology Combustion and Flame

In fact, the simple citation counting does not take into account the linking structure of citing works. By contrast, co-citation analysis, defined as the frequency with which two documents are cited together (Small, 1973), is more applicable to the construction of co-citation network. The more similar citations between two articles imply their closer relevance. Through a unified analysis of references in collected articles on pool fires, a co-citation network symbolizing the evolutionary process, research area and hotspots of the publications can be established by means of the CiteSpace software (Chen, 2006), as shown in Fig. 7. In order to obtain a clearer co-citation network, the articles published before 1990 are not included due to the long-time span (1966-1990 inclusive) but fewer publication outputs (a total of 38 articles), and only the references published in the ten years prior to the publication of each article are analyzed. For example, if a pool fire related article is published in 1998, its references within 1988~1998 will be used for the co-citation analysis. As a result, there are 17988 references from 1035 unique articles are extracted from the dataset to construct the co-citation network in Fig. 7. The highly cited references with over 20 cited frequencies is provided in Table 7, where 23 articles are mainly published in CNF (6), JHM (5), PCI (5), and Fuel (2) journals, and the two books on fire dynamics by Drysdale D (2011) and Quintiere JG (2006) also appear in the list. Fig. 7 depicts a total of 15 clusters. In co-citation network, clusters are ranked by their size (i.e. 18

the number of references), and their labels are keywords extracted from the titles of the references by log-likelihood ratio (LLR) algorithm (Dunning, 1993). There are 447 nodes in Fig. 7, and each node represents a reference. The deep orange node implies the reference has a citation burst, reflecting its significant activeness during a certain period. For each cluster, two highly cited references with gray legends are provided, encompassing the information of the authors and publication year. The connection line between two articles symbolizes their co-citation relationship, and its thickness is positively associated with the frequency that the two papers have been co-cited. The evolution of color denotes the shift of clusters over time.

Fig. 7. The reference co-citation network of pool fire publications It is shown that the largest cluster #0 is entitled as “experimental study”, which is also the hot topic in recent years (yellow color). This is also consistent with the observations in Fig. 6, where the pool fire experiments are believed to be the current research focus. As evidenced in Table 7, twelve articles belong to cluster #0, accounting for half of the top cited references. Among them, Hu et al. published four articles pertaining to the effect of cross air flow on the burning characteristics of liquid pool fires, mainly concentrating on their heat feedback mechanisms (Hu et al., 2009; Hu et al., 2011a) and burning rates (Hu et al., 2015; Hu et al., 2013b). Another prior work in 2006 by Woods et al. (2006) also concerns the wind effect on the burning rate of rectangular methanol pool fires. In these works, the tested burner sizes are relatively small, with a maximum of 0.09 m2 burning area, and the tests are conducted in well-design wind tunnels capable of producing stable and uniform air flow field (Hu et al., 2015; Hu et al., 2009; Hu et al., 2013b; Hu et al., 2011a; Woods et al., 2006). Another notable classification in Table 7 is the pool fire research at low pressure environment or 19

at high altitude, which encompasses five articles (Fang et al., 2011; Hu et al., 2013c; Hu et al., 2011c; Li et al., 2009; Tu et al., 2013). As discussed before, the pool fire tests at high altitude date from 2009, Li et al. (2009) firstly published the paper investigating the effect of high-altitude environment (Lhasa) on pool fires, as well as crib fires. However, they only tested two square burner sizes with side length of 0.27 and 0.33 m, which are quite limited. The subsequent work by Hu et al. (2011c) examined the pool fires at higher altitude (Dangxiong, 4250 m, 59.1 kPa), but only a 0.18×0.18 m burner was used. The reason why the work by Fang et al. (2011) has the highest 55 cited frequencies may be attributed to their relatively systematic experimental studies on pool fires at high altitude with a wider range of square burner size (side length from 4 to 33 cm), which covers conduction-, convection-, and radiation-dominated heat feedback mechanisms. The following studies by Tu (2013) and Hu et al. (2013c) mainly concern the burning behaviors of rectangular pool fires dominated by radiation and conduction, respectively. In addition to the two main aspects above, the work by Ditch et al. (2013) also has a citing burst recently. They developed a simple empirical formula based primarily on heat of gasification and smoke point, which can be used to well correlate the mass burning rate data available in the literature. To sum up, cluster #0 mainly includes two recent hot topics, i.e. pool fires subject to a wind or low-pressure environment, corresponding to that in Fig. 6 and Table 5. Table 7. The most cited references with over 20 cited frequencies in the current co-citation network. Rank

TC

Title

Authors

Year

Journal

1

55

Fang J et al.

2011

Fuel

2

47

Hu LH et al.

2013

Combustion and Flame

Cross air flow

3

45

Tu R et al.

2013

40

Hu LH et al.

2011

Proceedings of the Combustion Institute Combustion and Flame

Low pressure

4

5

37

Influence of low air pressure on combustion characteristics and flame pulsation frequency of pool fires Flame radiation feedback to fuel surface in medium ethanol and heptane pool fires with cross air flow Effects of low air pressure on radiation-controlled rectangular ethanol and n-heptane pool fires A wind tunnel experimental study on burning rate enhancement behavior of gasoline pool fires by cross air flow Introduction to fire dynamics

2011

N/A (book)

N/A

6

37

2004

7

36

Drysdale D Munoz M et al. Li ZH et al.

Large pool fire Low pressure

8

33

Combustion and Flame Proceedings of the Combustion Institute Journal of Hazardous Materials

9

33

10

Burning rate correlation Low pressure

Analysis of the geometric and radiative characteristics of hydrocarbon pool fires Combustion characteristics of n-heptane and wood crib fires at different altitudes

Critical terms Low pressure

Cluster

0

0

0 Cross air flow 0 10

2009

Experimental study on burning rates of square/rectangular gasoline and methanol pool fires under longitudinal air flow in a wind tunnel Pool fires–An empirical correlation

Hu LH et al.

2009

Ditch BD et al.

2013

Combustion and Flame

32

Combustion characteristics of n-heptane at high altitudes

Hu XK et al.

2011

11

32

An investigation of the detailed flame shape and flame length under the ceiling of a channel

Gao ZH et al.

2015

12

30

Predicting the emissive power of hydrocarbon

Munoz

2007

Proceedings of the Combustion Institute Proceedings of the Combustion Institute Journal of

4

0 Cross air flow 0

0

0 Channel fire Large pool

6 4

20

Rank

TC

Title

Authors

pool fires

M et al.

Experimental investigation on influence of different transverse fire locations on maximum smoke temperature under the tunnel ceiling Initial fuel temperature effects on burning rate of pool fire

Ji J et al.

2012

Chen B et al.

2011

Large hydrocarbon fuel pool fires: Physical characteristics and thermal emission variations with height Evolution of heat feedback in medium pool fires with cross air flow and scaling of mass burning flux by a stagnant layer theory solution Burning characteristics of conduction-controlled rectangular hydrocarbon pool fires in a reduced pressure atmosphere at high altitude in Tibet Model of large pool fires

Raj PK

2007

Hu LH et al.

2015

Hu LH et al.

2013

Fay JA

2006

Roh JS et al.

2007

2006

13

26

14

26

15

24

16

24

17

24

18

23

19

23

20

22

21

22

22

21

Critical velocity and burning rate in pool fire during longitudinal ventilation

Woods JAR et al. Quintiere JG Roh JS et al.

23

21

Experimental study of burning rate in hydrocarbon pool fires

Chatris JM et al.

Tunnel fires: experiments on critical velocity and burning rate in pool fire during longitudinal ventilation Effects of transverse air flow on burning rates of rectangular methanol pool fires Fundamentals in fire phenomena

Year

Journal Hazardous Materials International Journal of Heat and Mass Transfer Journal of Hazardous Materials Journal of Hazardous Materials Proceedings of the Combustion Institute Fuel

Critical terms fire

Cluster

Tunnel fire 6 Pool fire tests 0 Large pool fire model 4 Cross air flow 0 Low pressure 0

Journal of Hazardous Materials Journal of Fire Sciences

Large pool fire model 4 Tunnel fire 2

2006

Combustion and Flame N/A (book)

Cross air flow N/A

Tunnelling underground and Space Technology Combustion and Flame

Tunnel fire

0 13

2007

2001

2 Large pool fire

4

Compared with cluster #0, the research work in cluster #4 is older (generally before 2010), while some large-scale pool fire tests are conducted in this period. There are five highly cited references belonging to cluster #4 in Table 7. The earliest paper (Chatris et al., 2001)was published in 2001, in which experiments on gasoline and diesel oil pool fires with diameters of 1.5, 3, and 4 m were carried out. It is worthwhile to note that the effect of ambient wind speed on burning rate was also concerned. Munoz et al. (2004)performed the similar tests but with five concentric circular pools (1.5, 3, 4, 5, and 6 m in diameter) to investigate the geometric (flame height) and radiative characteristics of gasoline and diesel pool fires. Based on the experimental results, they further proposed a mathematical model to give a more accurate prediction of emissive power of large-scale pool fires (Munoz et al., 2007). Efforts to establish the prediction models for typical parameters of large-scale pool fires are also made by Fay (2006) and Raj (2007). The former proposed a two-zone entrainment model of pool fires to depict the fluid flow and flame properties of the fire, and based on that, non-dimensional scaling parameters for extrapolating pool fire visible flame length, flame tilt, surface emissive power, and fuel evaporation rate are developed (Fay, 2006). The latter also developed a prediction model to estimate the overall thermal emission from the fire as a function of its size (Raj, 2007). It can be summarized that the work in cluster #4 mainly concern the more practical large-scale pool fires, seeking to establish applicable models for predicting the possible hazards of pool fires (Chatris et al., 2001; Fay, 2006; Munoz et al., 2004; Munoz et al., 2007; Raj, 2007). It is also interesting to find that the scale of 21

pool fire experiments obviously deceases over a large time span (from 2000-present), as proved by the evolution from cluster #4 to cluster #0. In effect, with the promotion of environmental protection awareness, large-scale pool fire tests are gradually prohibited due to the yields of large amount of toxic gases and particulate matter, and thus, scarce literature on this topic can be found in recent years. Instead, the studies on small-scale pool fires spring up, aiming to interpret the combustion behaviors of pool fires at different environments such as wind, pressure, tunnel, building, etc. Fig. 7 also indicates that clusters #6 and #10 are more recent topics. The critical term “longitudinal centerline” for cluster #6 usually appear in a corridor-like or tunnel fire scenarios. The top cited references in Table 7 presents two papers pertaining to cluster #6, concerning the channel (Gao et al., 2015) and tunnel fires (Ji et al., 2012), respectively. In fact, cluster #6 has a close relationship with cluster #2 (tunnel fire) where another two highly cited references by Roh et al. (2007a; 2007b) studied the effect of longitudinal ventilation on the burning rate of pool fires in a tunnel. With respect to cluster #10 (crude oil pool fire), only Drysdale’s book “An introduction to fire dynamics” (Drysdale, 2011) is listed in Table 7, while Quintiere’s book “Fundamentals in fire phenomena” (Quintiere, 2006) falls into cluster #13 (heat transfer). Both the two books are gems of fire science, providing guidance on various aspects of fire research in different periods. The rest of clusters in Fig. 7 have no corresponding highly-cited reference in Table 7. It is noted that the clusters #3 “water mist”, #5 “liquid pool fire”, #11 “wall fire” mainly include the papers published in the last century. At that time, the increasing number of high-rise buildings result in the urgent needs for fire suppression systems. Water-based fire protection system is believed to be one of the most efficient, economical, and environmental-friendly extinguishing systems, especially the water mist, which is developed and applied in most buildings (Jone and Thomas, 1993). The clusters #1 “radiative heat transfer” and #8 “solid angle number” are closely related, since both of them concentrate on the radiative characteristics of pool fires, which are always the critical problems in estimating the hazards of pool fires. As for the cluster #14 “Viareggio LPG railway accident”, it refers to the accident occurred on June 29th, 2009 in Viareggio, Italy. A tank-car overturned and released its entire content, about 46 t of LPG. The flashing LPG spill caused the formation of a gas cloud and possibly of a boiling pool (Landucci et al., 2011).

3.7 Limitations and further discussion This study only collects pool fire related publications in WoS CC, and is limited to “article” and “review”. This includes only a part of the total data available. Other databases such as Scopus and Google Scholar may be also helpful to understand this topic. Moreover, some journals are indexed as “article” by SCI-Expanded since 2000, e.g. PCI, which is indexed as “conference” paper, i.e. Symposium (International) on Combustion, before that. Therefore, caution should be exercised when generalizing results. More importantly, the current study does not allow the capture of context and

22

intention for scholarly citation, which is an intrinsic drawback of the bibliometric approach. The bibliometric analysis cannot extensively reflect the complex nature of citing behavior. It is also important to be aware of that the number of publications or citations is only an approximate estimation of the scientific relevance of a subject since there are so many possible confounding factors which will affect them. This work can be supplemented with an extended analysis, which can include more critical information on the content of the papers published by scholars on pool fire research.

4. Concluding remarks and future outlook This work performs a bibliometric analysis to investigate and reveal the development of pool fire research from 1966-2019. Based on the 1073 publications collected from SCI-Expanded database in WoS CC, a unified analysis of annual publication outputs, source journals, countries, authors, subject terms, and co-cited references is conducted by means of VOSviewer and CiteSpace software. From the visual networks, some valuable information can be obtained. The earliest article related to pool fires in the collected sample is published in 1966, while the relevant publications begin to have an exponential increase since 1990. Up to now, this increasing tendency still remains unchanged. FSJ collects a maximum of 127 articles on pool fires, followed by CNF and JLPPI, while CNF has the highest 26.6 average citations per document, demonstrating its influences in the domain of pool fire research. China and USA are two most productive countries with 368 and 270 publications, respectively, and they belong to the same cluster and show a strong collaborative relationship in the network. As for the authors and their cooperation, Prof. Huo R at USTC (China) and Prof. Gore JP at Purdue University (USA) have the most prominent average citations per document, and all the top-10 influential authors come from China and USA. It is also found that eight of the top-10 prolific authors are professors at USTC. From the terms analysis, two recent research topics can be identified, i.e. the pool fires subject to cross air flow and low-pressure environment, and the reference co-citation analysis tends to produce the similar results. This work provides guidance for scholars to understand the field of pool fire research from a macro perspective. At the same time, some future challenges or research directions can also be identified. For pool fire itself, the typical parameters including the burning rate, air entrainment, flame morphology characteristics, and heat feedback mechanisms have been investigated extensively in past decades. However, most of them are based on the small- or medium-scale pool fire tests, which are significantly different from the practical large-scale fire scenarios in such aspects as the flame turbulent structure, soot and radiation intensity. This work also indicates that the number of large-scale pool fire tests continuously decreases due to the environmental considerations, decreasing funding supports, or any other reasons, especially in the last decade. Bench-scale fire tests may help to understand the essence of the combustion, e.g. Johansson et al. (2018) recently revealed the chemical 23

mechanisms for soot formation within a flame, while there is still a long way to use this theory to predict the radiative heat flux of an industrial fire. To date, the research on large-scale pool fires are still scarce regardless of experiments, numerical simulation, or theoretical models, which will be still the future challenge. The existing models for pool fires are established mainly based on the tests in still air and normal environment parameters, which are relatively mature. However, the presence of wind will deform the flame morphology and thus change the heat feedback mode, air entrainment as well as soot formation. This complex interaction enhances the turbulence intensity, resulting in the flame fluctuation in both the vertical and the horizontal direction. This makes the accurate and steady measurements of typical parameters (e.g. flame temperature distribution, soot concentration, etc.) to be a big challenge (Hu, 2017). Although some efforts have been made to experimentally investigate the wind-blown pool fires and more scholars are involved in this topic, there is still so much work to do, especially for large-scale fire tests or developing numerical simulation models for that. With respect to another hot topic, i.e. pool fires at sub-atmospheric pressures, this is also a promising research direction. As claimed before, this topic is only concerned in the last decade with the increasing fire safety demands in aviation industry and at high-altitude area. At the present stage, this topic is mainly investigated through the comparative experiments in a high-altitude city, Lhasa and a sea-level city, Hefei. The deceasing pressure means the smaller air density, which definitely affects the air entrainment of pool fires, and thus change flame behaviors, heat feedback, soot formation, etc. This has been proven repeatedly, and relevant modified models have been established by taking the pressure term into consideration. However, most of them are empirical or semi-empirical models, lacking the basic interpretation of mechanisms for the pressure effect on the pool fire behaviors. Moreover, the pool fire research at sub-atmospheric pressure is also restricted by the difficulty of shipping experimental facilities to different altitudes and the size of the available altitude chamber (Liu et al., 2016). Large-scale pool fire tests at lower pressures are also being an attractive topic in the future. Furthermore, pool fire is comprehensively employed as the fire source in examining other fire scenarios, such as the fires in buildings, tunnels, or even fire suppression tests. As concluded from Fig. 7 and Table 7, the tunnel fire is also the recently hot topic. It attracted considerable attention in past two decades. The popularity of this topic is mainly attributed to the widespread construction of long-span tunnels and their incident fire safety issues, as well as the frequent occurrence of catastrophic tunnel fires. In previous work, the pool fires are usually used to investigate the fire development and smoke movement in both reduced- and full-scale tunnels. Moreover, considering possible multiple vehicle fires during accidents in tunnels, the burning behaviors of two or more adjacent pool fires are also investigated recently (Ji et al., 2016), and the effects of fire size, number, spacing and array pattern on the mass loss rate, flame height, flame merging, heat feedback, etc. will be continuously studied for the foreseeable future. It is believed that tunnel fire research will remain 24

hot since the knowledge base must increase apace with the growing number of new tunnel being constructed worldwide (Ingason, 2016). Based on current results, the next step for this research is to conduct a qualitative analysis to bring a more in-depth discussion on pool fires. Moreover, a bibliometric review concentrating on other type of fire such as tunnel fire, building fire, or fire suppression would also be a promising avenue for future work.

Acknowledgements This work is financially supported by Shanghai Sailing Program (Grant No. 18YF1409600), and the National Natural Science Foundation of China (No. 51909152, 51904185 and 51874042). The authors deeply appreciate the supports.

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Highlights
A total of 1073 papers related to pool fires are published from 1966-2019.
Key information of pool fire related publications was visualized and analyzed.
Pool fires subject to wind and low-pressure environment are recently hot topics.