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2023 Volume 3
Article Contents
Abstract
Introduction
Origin and distribution, germplasm, and characteristics of M. cordata
Chemical resources and biological activities of M. cordata
Total synthesis, semi-synthesis, and structural modification of alkaloids in M. cordata
Total synthesis of major secondary metabolites in M. cordata
Semi-synthesis and structural modification of novel 6-substituted dihydrobenzo[c]phenanthridine alkaloids in M. cordata
Multi-omics analysis of the biosynthesis mechanism of sanguinarine in M. cordata
Discovery of the alkaloid biosynthesis pathway in M. cordata
Integrated analysis of genomics, transcriptomics, metabolomics, and proteomics in M. cordata to elucidate the mechanisms of sanguinarine storage, synthesis, and transport
Metabolic engineering for the biosynthesis of sanguinarine
Breeding research on M. cordata
Molecular breeding of M. cordata
Study on the domestication and DUS testing guidelines of M. cordata
The role of M. cordata in animal production and the mechanism and safety of sanguinarine as a feed antibiotic substitute
Future challenges and prospects
Author contributions
Data availability
Acknowledgments
Conflict of interest
Rights and permissions
References
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ARTICLE   Open Access    

Study on the effect of channel spacing on premixed syngas-air explosion inside parallel narrow channels

  • Jiangsu Key Laboratory of Urban and Industrial Safety, College of Safety Science and Engineering, Nanjing Tech University, Nanjing 210009, China

More Information
  • Experiments of premixed syngas/air explosion inside parallel narrow channels were conducted. The flame propagation, explosion pressure and temperature inside the narrow channel are investigated to find out the effect of narrow channel spacing on the explosion. The experimental results show that the narrow channel spacing has an influence on the flame before and inside the parallel narrow channels. The flame inside the narrow channels accelerates and reaches a peak value at the middle or rear of the channel. The flame front velocity, the maximum explosion Pmax and the maximum explosion flame temperature Tmax decrease as the channel spacing decreases. The channel wall has an effect of heat dissipation on the flame, resulting in the explosion inside the channel weakening. The explosion pressure in rear of the narrow channel is larger than that in the front of the narrow channel.

  • Introduction

    Macleaya cordata (Willd.) R. Br., commonly known as boluohui in China, is a perennial, upright herbaceous plant belonging to the Papaveraceae family (Fig. 1)[1]. Modern pharmacological research has identified that plants of the Macleaya genus are rich in benzylisoquinoline alkaloids (BIAs), which exhibit significant anti-inflammatory and antibacterial properties, regulate intestinal microflora, and promote animal growth[2]. The European Food Safety Authority (EFSA) has approved M. cordata as a safe plant for the manufacture of feed additives. In China, four alkaloids found in M. cordata, including sanguinarine (SAN), chelerythrine (CHE), allocryptopine (ALL), and protopine (PRO) have been successfully developed into veterinary medicines (Fig. 1).

    Figure 1.  Morphological characteristics of M. cordata. The figure shows various parts of M. cordata, commonly known as boluohui. The main image displays a field of mature plants with characteristic upright growth. The top left inset highlights a single leaf with a broad, lobed structure. The top middle inset shows the flower of M. cordata, featuring clustered blooms with delicate petals. The top right inset illustrates the capsule, the fruiting body containing seeds. These images collectively depict the key morphological features of M. cordata, which is valued for its rich content of benzylisoquinoline alkaloids.
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    This review aims to provide a comprehensive overview of the latest advancements in the study of M. cordata. It will cover the plant's resources, chemical composition, pharmacological activities, biosynthesis mechanisms, and breeding strategies. By synthesizing current knowledge, this review seeks to offer a broad perspective on future research and potential applications of M. cordata in various fields.

    Origin and distribution, germplasm, and characteristics of M. cordata

    The Macleaya genus comprises two species: M. cordata, and M. microcarpa. M. cordata is primarily distributed in China and Japan, originating in the regions south of the Yangtze River and north of the Nanling Mountains in China. It extends south to Guangdong, west to Guizhou, and northwest to southern Gansu. M. cordata predominantly grows in hills, low mountain forests, shrubs, or grasses at altitudes between 150 and 830 m. It is notably found in grasslands or shrubs on slopes at elevations ranging from 450 to 1,600 m in provinces such as Jiangsu, Jiangxi, Shanxi, Gansu, and Shaanxi.

    Both M. cordata, and M. microcarpa are upright herbaceous plants with a woody base, producing a milky yellow sap. Their stems are hollow, with the upper parts branching out, and the leaves are broad ovate or nearly circular. They possess large conical inflorescences that can be both terminal and axillary, with a flowering and fruiting period from June to November. Plants of the Macleaya genus are rich in various chemical constituents, including alkaloids, flavonoids, terpenes, and phenylpropanoids, with alkaloids being the predominant components[3]. Benzylisoquinoline alkaloids, such as sanguinarine, are among the most active components in this genus. Sanguinarine is also abundant in other plants like poppy (Papaver somniferum) and bloodroot (Sanguinaria canadensis); however, due to regulatory restrictions and over-exploitation, these plants are no longer viable sources. Consequently, M. cordata has emerged as the primary commercial source of sanguinarine.

    Chemical resources and biological activities of M. cordata

    Japanese researchers such as Tani & Takao[4] and Takao[5] isolated alkaloids like sanguinarine, chelerythrine, berberine, and coptisine from M. cordata, initiating the study of its chemical constituents. Subsequently, numerous researchers contributed to the isolation and structural identification of secondary metabolites in the Macleaya genus[6−9]. From 2009 to 2024, Qing et al.[10] developed a new method for the discovery and structural identification of alkaloids in the Macleaya genus using mass spectrometry, identifying over 200 alkaloids. These alkaloids belong to ten major categories: benzylisoquinoline alkaloids, tetrahydroprotoberberine alkaloids, N-methyl-tetrahydroprotoberberine alkaloids, protopine alkaloids, protoberberine alkaloids, 7,8-dihydroprotoberberine alkaloids, aporphine alkaloids, benzophenanthridine alkaloids, dihydrobenzophenanthridine alkaloids, and benzophenanthridine dimers (Fig. 2). Using mass spectrometry-guided techniques, over 20 new compounds were isolated from Macleaya species, established a mass spectrometry database and metabolic profile for alkaloids in Macleaya, and proposed new biosynthetic pathways for berberine and sanguinarine[10−13]. In addition to a large number of alkaloids, Macleaya species also contain small amounts of flavonoids and polyphenols[3]. The rich chemical composition of Macleaya endows it with various biological activities. Modern pharmacological studies have shown that Macleaya possesses antibacterial, anti-inflammatory, insecticidal, antitumor, antifibrotic, hepatoprotective, antiviral, antioxidant, immune-enhancing, gut microbiota-regulating, animal growth-promoting, wastewater-purifying, and soil erosion-preventing properties[13−16].

    Figure 2.  Types of compounds identified in Macleaya species.
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    Total synthesis, semi-synthesis, and structural modification of alkaloids in M. cordata

    Total synthesis of major secondary metabolites in M. cordata

    The primary active secondary metabolites in Macleaya cordata are protopine alkaloids and quaternary benzo[c]phenanthridine alkaloids. To date, there have been no reports in the literature regarding the total chemical synthesis of protopine alkaloids. The total chemical synthesis of quaternary benzo[c]phenanthridine alkaloids is generally achieved through multi-step reactions using conventional chemical reagents[17]. In most reported synthetic routes, the cyclization reactions to form rings B and C constitute the final steps in constructing the tetracyclic system. Key chemical reactions for constructing ring B include Heck coupling to form the C10a-C11 bond[18], Pictet-Spengler reaction (PS reaction) or Bischler-Napieralski reaction (BN reaction) to form the C6-C6a bond[19,20], amidation to form the N5-C6 bond[21], and electrocyclization to form the N5-C5a bond[22]. For ring C, key reactions include enamide-aldehyde cyclization to form the C11-C11a bond[23] and Friedel-Crafts reaction to form the C12-C12a bond[24]. These reactions have been widely applied in the synthesis of benzo[c]phenanthridine derivatives with anticancer activity (Fig. 3).

    Figure 3.  Total synthesis of quaternary benzo[c]phenanthridine alkaloids, the major active components of M. cordata. This figure illustrates the chemical structure of quaternary benzo[c]phenanthridine alkaloids, highlighting the tetracyclic framework composed of rings A, B, C, and D. These alkaloids are notable for their complex ring system and significant biological activities. The positions of the rings are labeled for clarity, facilitating understanding of the synthetic pathways and structural modifications discussed in the study.
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    Semi-synthesis and structural modification of novel 6-substituted dihydrobenzo[c]phenanthridine alkaloids in M. cordata

    In recent years, with the deepening research on the chemical constituents of M. cordata, a series of 6-substituted dihydrobenzo[c]phenanthridine alkaloids have been successively isolated and structurally confirmed (Fig. 4)[6,9,25,26]. Currently, studies on the biological activities of these alkaloids are still in their infancy, and no biosynthetic pathways for 6-substituted dihydrobenzo[c]phenanthridine alkaloids have been reported. Based on the structural types and chemical properties of the C-6 substituents, it is speculated that these alkaloids may be biosynthesized in plants through radical reactions or nucleophilic substitution pathways. As shown in Fig. 4, quaternary benzo[c]phenanthridine alkaloids or dihydrobenzo[c]phenanthridine can be converted to α-amino carbon radicals through single-electron reduction[27,28] or oxidation[29,30]. These radicals can then undergo various types of radical reactions to achieve functionalization at C-6 of the benzo[c]phenanthridine ring structure, resulting in the semi-synthesis of the aforementioned alkaloids[31]. Additionally, the N5=C6 double bond in quaternary benzo[c]phenanthridine alkaloids can accept nucleophilic reagents, leading to the formation of 6-substituted dihydrobenzo[c]phenanthridine alkaloids.

    Figure 4.  Novel 6-substituted dihydrobenzophenanthridine alkaloids in Macleaya and their semi-synthesis.
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    Multi-omics analysis of the biosynthesis mechanism of sanguinarine in M. cordata

    Discovery of the alkaloid biosynthesis pathway in M. cordata

    To date, nearly 300 secondary metabolites, including 204 isoquinoline alkaloids have been identified from M. cordata using untargeted LC-MS metabolomics technology[10]. This breakthrough surpasses the limitations of traditional methods for isolating and identifying secondary metabolites in plants, guiding the targeted analysis of alkaloid metabolism and metabolic profiling in M. cordata. Using untargeted LC-MS metabolomics technology, nearly 2,000 characteristic ions were discovered in M. cordata. Through stable isotope 13C6-labeled tyrosine tracing experiments, 179 13C6-labeled compounds were identified[1]. A database search for the non-labeled counterparts of these 13C6-labeled compounds quickly identified 40 alkaloids and four non-alkaloids, providing evidence for determining the biosynthetic pathways of sanguinarine and chelerythrine in M. cordata.

    Targeted analysis of the metabolic accumulation patterns of intermediates in the biosynthesis of sanguinarine and chelerythrine in different developmental stages and organs of M. cordata, combined with transcriptomics, proteomics, and metabolomics analysis revealed the biosynthetic tissues and gene expression patterns of alkaloid biosynthesis at different growth stages. This study elucidated the biosynthetic pathways of sanguinarine and chelerythrine and discovered the pattern of synthesis by 'root-pod compartmentalized synthesis', where precursor compounds like protopine are primarily synthesized in the roots and then transported to the pods for the synthesis of sanguinarine[1,32]. Laser microdissection with fluorescence detection was used to separate different tissue cells in the roots of M. cordata. Combined with LC-MS targeted analysis of trace alkaloids in each tissue, this approach accurately identified the storage cells of the alkaloids, clarifying the 'synthesis-storage-transport-resynthesis' biosynthetic mechanism of sanguinarine, chelerythrine, and their precursors[33] (Fig. 5).

    Figure 5.  Isotope tracing of the biosynthetic pathways of sanguinarine and chelerythrine in Macleaya.
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    Integrated analysis of genomics, transcriptomics, metabolomics, and proteomics in M. cordata to elucidate the mechanisms of sanguinarine storage, synthesis, and transport

    In 2013, Zeng completed an integration of transcriptomic, proteomic, and metabolomic data to elucidate the biosynthesis of alkaloids in M. cordata and M. macrocarpa[32]. This study investigated the potential mechanisms of alkaloid biosynthesis in Macleaya species by analyzing transcriptomic, proteomic, and metabolomic data from 10 different samples collected at various times, tissues, and organs. The assembled and clustered transcriptomic data yielded 69,367 unigenes for M. cordata and 78,255 unigenes for M. macrocarpa. Through a multi-level comparative analysis of key gene expressions controlling enzymes in the alkaloid metabolic pathway, the research identified homologous genes for all functional genes in the sanguinarine biosynthesis pathway. This annotation provided foundational data for cloning functional genes in the sanguinarine pathway of M. cordata.

    In 2017, researchers first mapped the complete genome of M. cordata, making it the first Papaveraceae plant to have its whole genome sequenced[1]. The genomic results indicated that the genome size of M. cordata is 540.5 Mb with a heterozygosity rate of 0.92%, predicting 22,328 protein-coding genes, of which 43.5% are transposable elements. Additionally, 1,355 non-coding RNA (ncRNA) genes were identified in the M. cordata genome, including 216 rRNA, 815 tRNA, 75 miRNA, and 249 snRNA genes. Comparative genomics revealed a significant expansion of the flavoprotein oxidase gene family closely associated with sanguinarine synthesis, providing crucial insights into the evolutionary origin of the sanguinarine biosynthetic pathway in Macleaya. Through co-expression analysis of genes and metabolites, researchers successfully identified 14 genes, including flavoprotein oxidases, methyltransferases, and cytochrome P450 enzymes, involved in the biosynthesis of sanguinarine.

    Metabolic engineering for the biosynthesis of sanguinarine

    Recent advancements in genetic engineering have led to a series of modifications on the berberine bridge enzyme (McBBE), a rate-limiting enzyme in the biosynthesis of sanguinarine in M. cordata. These modifications included codon optimization and the design of N-terminal truncated mutants to enhance expression efficiency in heterologous hosts. By employing CRISPR-Cas9 gene editing technology, these optimized genes were successfully integrated into the genome of Saccharomyces cerevisiae. This integration resulted in a significant increase in the production of the key precursor, chelerythrine, achieving a yield 58 times higher than the original level[34]. This accomplishment not only underscores the tremendous potential of genetic engineering in enhancing the synthesis of secondary metabolites but also provides a crucial foundation for subsequent industrial production. Furthermore, the research team developed innovative methods for producing sanguinarine through a 'plant-microbe' co-fermentation system. This system involved the co-cultivation of engineered yeast strains with the non-medicinal parts of M. cordata — specifically, the leaves — in a specially designed co-fermentation reactor[35]. This novel approach effectively converted precursor substances from the leaves into sanguinarine, maximizing the utilization of the non-medicinal parts of M. cordata and reducing production costs. This co-fermentation system not only offers an efficient and sustainable new pathway for the production of valuable bioactive compounds like sanguinarine but also exemplifies the integration of plant biology and microbial biotechnology.

    In 2022, significant progress was made by Jiachang Lian's team, who successfully achieved the de novo biosynthesis of sanguinarine, reaching a yield of 16.5 mg/L[36]. After comprehensive metabolic engineering modifications, the current best-engineered yeast strain achieved a production titer of 448.64 mg/L for sanguinarine[37]. This achievement highlights the immense potential of modern biotechnology in promoting the effective utilization and industrial production of components from traditional medicinal plants. By leveraging advanced genetic engineering techniques, researchers have been able to enhance the efficiency of secondary metabolite production, opening up new possibilities for the pharmaceutical and biotechnology industries.

    The innovative approaches employed in this research, such as codon optimization, N-terminal truncation, and the use of CRISPR-Cas9 for gene editing, represent significant advancements in the field of synthetic biology. These methods not only improve the expression efficiency of key enzymes but also pave the way for the scalable production of complex bioactive compounds. The successful integration of plant and microbial systems in a co-fermentation setup demonstrates a promising strategy for sustainable and cost-effective production processes. This research not only advances our understanding of biosynthetic pathways but also sets the stage for future developments in the industrial application of traditional medicinal plant components

    Breeding research on M. cordata

    Molecular breeding of M. cordata

    In the fields of plant science and biotechnology, artificial genetic transformation technology has become a key tool for molecular breeding. In 2016, a genetic transformation system for Macleaya cordata based on Agrobacterium tumefaciens was successfully established[38]. Utilizing this system, key genes involved in sanguinarine biosynthesis, such as berberine bridge enzyme MCBBE[39] and MCP6H[40], have been overexpressed, demonstrating the significant potential of genetic engineering in enhancing the production of plant secondary metabolites. Additionally, a hairy root culture system for M. cordata and M. microcarpa mediated by A. rhizogenes has also been successfully developed[41,42]. Recently, a CRISPR/Cas9 gene editing system for M. cordata has been established. By re-editing the sanguinarine biosynthesis pathway, researchers have achieved a significant increase in sanguinarine content in transgenic materials[43]. These advancements highlight the potential of molecular breeding techniques to enhance the yield of valuable secondary metabolites in M. cordata, providing a foundation for future research and industrial applications.

    Study on the domestication and DUS testing guidelines of M. cordata

    Previously, wild M. cordata capsules were the primary source of raw materials for M. cordata-related products. However, the popularity of these products has led to a significant decline in wild resources year by year. In response, domestic research teams have developed a standardized cultivation technology system for M. cordata. This system encompasses various techniques, including seedling raising, field management, pest and disease control, harvesting, and primary processing, facilitating the shift from wild to cultivated varieties. Additionally, comprehensive research on the current state of resources and breeding has led to the establishment of 30 traits which form the M. cordata DUS testing guidelines. As a result of these efforts, the team has preliminarily identified a superior strain, 'Meibo 1', known for its high fruit yield and elevated haematoxylin content.

    The role of M. cordata in animal production and the mechanism and safety of sanguinarine as a feed antibiotic substitute

    Sanguinarine from M. cordata as a bioactive compound was initially used in Europe as feed additives to enhance flavor and stimulate appetite in food animals[44]. Studies have shown that M. cordata extracts are safe, with no adverse effects observed in target animals even at ten times the recommended clinical dose. Pharmacodynamic and clinical data indicate that long-term addition of M. cordata extracts at recommended doses has anti-inflammatory and growth-promoting effects, and total protopine alkaloids are effective in treating Escherichia coli-induced diarrhea in poultry.

    Research on the mechanism of antibiotic substitution revealed that M. cordata extract enhances intestinal health by increasing the abundance of lactic acid bacteria, which inhibit pathogenic microorganisms through competitive exclusion[2]. Sanguinarine inhibits the phosphorylation and ubiquitination of I-κB proteins, reducing the dissociation of p50 and p60, thereby inhibiting the activation of the NF-κB signaling pathway. This results in decreased expression of pro-inflammatory cytokines and reduced inflammation levels in animals[45]. Additionally, sanguinarine was found to enhance protein synthesis in animals by inhibiting tryptophan decarboxylase activity, thereby increasing amino acid utilization[46,47]. Based on the growth-promoting effects of M. cordata extract as an antibiotic substitute, Jianguo Zeng's team has developed a comprehensive feed technology focused on 'intestinal health, anti-inflammatory properties, and growth promotion'. This approach avoids the risk of antibiotic resistance associated with traditional growth-promoting antibiotics while still enhancing animal growth performance. This technology supports the development of antibiotic-free feed solutions and the use of non-antibiotic inputs in animal husbandry. Furthermore, chelerythrine in M. cordata, when used as an animal feed additive, interacts with phospholipids on bacterial membranes, increasing membrane fluidity and impairing respiration by disrupting proton motive force and generating reactive oxygen species, leading to reduced intracellular ATP levels. This dual action of downregulating the mobile colistin resistance gene mcr-1 and associated genes offers a new strategy to circumvent colistin resistance. These findings provide strong technical support for the development of antibiotic-free feeding practices and the creation of alternative growth-promoting solutions in animal production[48].

    M. cordata extract is the first traditional Chinese veterinary medicine product approved for feed use in China. It is widely used to improve animal production performance, enhance animal health, and replace growth-promoting antibiotics in feed. Weaning stress is a major factor causing diarrhea and growth inhibition in piglets. Adding M. cordata extract to their diet can improve small intestine morphology, enhance intestinal barrier function, and regulate intestinal microbiota homeostasis, thereby reducing diarrhea and improving growth performance[49−52]. During pregnancy, progressive oxidative stress and parturition stress can lead to metabolic disorders and increased inflammation in sows. Supplementing their diet with M. cordata extract can reduce inflammation, enhance antioxidant capacity, shorten farrowing duration, increase feed intake and milk production during lactation, and ultimately increase the weaning weight of piglets[53,54]. Environmental stress and pathogenic infections are significant threats to poultry intestinal health. Adding M. cordata extract to poultry feed can improve intestinal mucosal morphology, enhance intestinal barrier function, regulate microbial populations, and improve broiler growth performance[55−58]. High-fat and high-protein feed can cause intestinal damage in fish. Supplementing their diet with M. cordata extract can enhance intestinal antioxidant capacity, alleviate intestinal barrier damage, improve microbiota homeostasis, and thus increase survival and growth rates[59−61]. Ruminants have a unique rumen physiology where the microbiota plays a crucial role in nutrient digestion and metabolism. Studies have shown that adding M. cordata extract to dairy cow diets can regulate rumen microbial populations and reduce methane emissions, alleviate intestinal inflammation in weaned lambs[62], promote growth in meat sheep[63], increase milk yield in dairy goats, reduce somatic cell counts in milk, and improves milk quality[64].

    In addition to its applications in animal production, M. cordata extracts have been developed into various products due to their significant insecticidal and antibacterial properties. In 2021, they were registered as a new botanical pesticide in China and the United States for controlling greenhouse plant diseases such as powdery mildew and leaf spot disease. In the bioenergy sector, the combination of M. cordata extracts with trace elements cobalt and nickel can effectively accelerate the fermentation process of agricultural waste, increasing biogas production. Furthermore, Macleaya alkaloids are used in toothpaste and mouthwash for their sustained antibacterial activity, which helps treat periodontal disease and prevent bad breath[65]. These findings highlight the diverse applications and benefits of M. cordata in improving animal health and productivity, controlling plant diseases, and enhancing bioenergy production (Fig. 6).

    Figure 6.  This illustration showcases the versatile applications of alkaloids derived from M. cordata. The molecular structures of key alkaloids—chelerythrine, sanguinarine, allocryptopine and protopine—are depicted in the center. These alkaloids are utilized in enhancing digestion and improving feed efficiency in ruminants, boosting immunity and promoting growth in pigs, promoting health and growth in fish, improving health in poultry, serving as natural pesticides for crops, reducing methane emissions from ruminants to lower greenhouse gases, acting as antibacterial ingredients in toothpaste and mouthwash, and exhibiting potential anti-cancer activity in cancer treatment. The alkaloids from M. cordata demonstrate significant potential across various fields, from agriculture to medicine.
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    Future challenges and prospects

    M. cordata, a plant rich in bioactive compounds, holds vast potential for applications in medical and agricultural fields. However, future research faces multiple challenges and opportunities. Firstly, although substantial research has been conducted on the primary alkaloids in M. cordata, their regulatory mechanisms remain unclear. Future studies need to integrate multi-omics data and systems biology approaches to elucidate the molecular mechanisms regulating alkaloid biosynthesis in M. cordata, providing a theoretical foundation for precise metabolic pathway regulation.

    Secondly, the application of molecular breeding and metabolic engineering in M. cordata is still in its infancy. Effectively increasing the yield of specific bioactive compounds through genetic engineering, particularly achieving efficient biosynthesis and accumulation of key alkaloids like sanguinarine, will be a focus of future research. Despite the achievement of de novo synthesis of sanguinarine in yeast, the scalability of this production method remains challenging. It is necessary to assess the economic feasibility of these methods to ensure they can meet industrial demands without compromising quality or sustainability. Additionally, the long-term stability and consistency of genetically engineered strains and their products need to be ensured to avoid potential issues in production and application.

    Lastly, as the medicinal value of M. cordata becomes better understood, ensuring its sustainable use and the conservation of wild resources, while developing more high-value products, will be crucial for future development. Large-scale cultivation and utilization of M. cordata may pose potential environmental impacts, including changes in soil health, biodiversity, and local ecosystems, which are not sufficiently studied and require further investigation and evaluation.

    In summary, while the research and application prospects of M. cordata are promising, they also present a series of scientific and technical challenges. By fostering interdisciplinary collaboration and enhancing the integration of basic and applied research, these challenges can be overcome, enabling the efficient utilization and industrial development of M. cordata resources. Addressing the limitations in current research, particularly in ecological impact and production methods, will provide a more balanced perspective and guide future research toward sustainable and scalable solutions.

    Author contributions

    The authors confirm contribution to the paper as follows: study conception and design: Huang P, Zeng J; draft manuscript preparation: Huang P, Cheng P, Sun M, Liu X, Qing Z, Liu Y, Yang Z, Liu H, Li C, Zeng J. All the authors reviewed the results and approved the final version of the manuscript.

    Data availability

    Data sharing not applicable to this article as no datasets were generated or analyzed during the current study.

    Acknowledgments

    This work was supported by Hunan Provincial Natural Science Foundation of China (2023JJ40367 & 2023JJ30341).

  • The authors declare that they have no conflict of interest.

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  • Cite this article

    Guo P, Xu C, Lu J, Wang Z, Chang X, et al. 2023. Study on the effect of channel spacing on premixed syngas-air explosion inside parallel narrow channels. Emergency Management Science and Technology 3:8 doi: 10.48130/EMST-2023-0008
    Guo P, Xu C, Lu J, Wang Z, Chang X, et al. 2023. Study on the effect of channel spacing on premixed syngas-air explosion inside parallel narrow channels. Emergency Management Science and Technology 3:8 doi: 10.48130/EMST-2023-0008
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Study on the effect of channel spacing on premixed syngas-air explosion inside parallel narrow channels

  • Jiangsu Key Laboratory of Urban and Industrial Safety, College of Safety Science and Engineering, Nanjing Tech University, Nanjing 210009, China

  • Received: 15 May 2023
  • Accepted: 28 June 2023
  • Published online: 27 July 2023
Emergency Management Science and Technology  3,  Article number: 8  (2023)  |  Cite this article

Abstract: Experiments of premixed syngas/air explosion inside parallel narrow channels were conducted. The flame propagation, explosion pressure and temperature inside the narrow channel are investigated to find out the effect of narrow channel spacing on the explosion. The experimental results show that the narrow channel spacing has an influence on the flame before and inside the parallel narrow channels. The flame inside the narrow channels accelerates and reaches a peak value at the middle or rear of the channel. The flame front velocity, the maximum explosion Pmax and the maximum explosion flame temperature Tmax decrease as the channel spacing decreases. The channel wall has an effect of heat dissipation on the flame, resulting in the explosion inside the channel weakening. The explosion pressure in rear of the narrow channel is larger than that in the front of the narrow channel.

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    Introduction

    • Combustible gas is a medium often encountered in the process of petrochemical, coal mining, storage and transportation. This media can burn and explode when it mixes with a certain concentration of oxygen or air and encounters an ignition source. Flame arresters are devices that can stop combustion in equipment and piping systems where flammable gases may be present[1], and the principle of flame arresting is mainly due to heat loss (heat transfer from the flame to the wall) or free radical quenching (reaction of flame radicals with the wall)[2−5]. According to this mechanism, many theoretical and experimental studies have been carried out for the propagation and quenching process of explosive flames in narrow channels.

      Zamashchikov[6,7] found that the heat transfer between the flame and the channel wall may be the main factor determining the change in the shape of the symmetric flame front in the narrow channel by examining the flame propagation of propane-air and hydrogen-air mixtures in narrow channels with different pitches. As the spacing decreases, the extent of the flame in the narrow channel gradually decreases and the peak flame velocity shifts toward the combustion-rich region. Ding et al.[8] studied the flame propagation of premixed acetylene-air explosion under different spacing flat narrow channels. The results showed that the flame morphology shifted from tulip-like to mushroom-like in the channel with larger narrow channel spacing, which was due to the strengthening of wall heat dissipation to inhibit the formation of tulip-like flame. In contrast, the flame morphology in the narrow channels with smaller spacing forms a mushroom shape at first, and then the flame is gradually extinguished in the channel due to the enhanced wall heat dissipation. Borisov et al.[9] simulated the propagation of stoichiometric methane-air mixtures in the channels with different narrow channel spacing. The results show that flame extinction is achieved near the ignition source for small gap widths, while in wide gaps the flame propagates faster in the narrow channels due to the less influence of viscosity and heat loss. And the flame propagation speed was calculated as a function of gap width. Alexeev et al.[10] experimentally investigated propane flame propagation in different parallel narrow gaps. The results showed that in the gap width between 2.5 and 4.4 mm, the effect of heat loss on the normal flame velocity was significant and the effect of gap width on the cell structure was investigated and the flame cell characteristics were obtained for different gap widths. Li et al.[11] studied the flame development process and the variation law of the burst onset distance of ethylene/oxygen mixtures in different narrow gaps and obtained the variation of the burst onset distance with the variation trend of the gap height. The effect of gap height on the detonation onset distance gradually decreases with the increase of initial pressure. Huo et al.[12] obtained the variation law of flame propagation velocity with equivalent ratio for different narrow channel spacing, and the flame propagation velocity is highly sensitive to the change of spacing and is nonlinear. For different combustible gases there generally exists an optimal narrow channel spacing at which the flame propagation velocity is the fastest. The existence of the optimal spacing depends mainly on the flame surface tensile strength, and the optimal spacing size is influenced by the combination of wall heat dissipation and wall shear stress. Su et al.[13] experimentally studied the effect of equivalent ratio on the flame propagation characteristics of premixed propane/air explosion at different narrow channel spacing and observed that there are three flame front forms of flame propagation: smooth, wrinkled and fractured. The decrease of narrow channel spacing makes the flame more prone to crease phenomenon, and the flame propagation velocity increases and then decreases as the narrow channel spacing continues to decrease.

      Some other scholars have conducted related studies on the change of channel spacing on the quenching characteristics of the explosion flame in the narrow channel. Gutkowski et al.[14] investigated the flame quenching characteristics of propane - air in narrow channel narrow channels with different pitches and found that after the chemical equivalent ratio propane-air flame propagation velocity in the narrow channel reached its peak, further increase in the narrow channel width would inhibit the flame propagation and make its flame propagation velocity decrease. Ciccarelli[15] pointed out experimentally that the quenching effect is closely related to the size of the gap. Sun[16] used a combination of experiments and simulations to investigate the firestopping performance of pipe flame arrestors. By analyzing the firestopping narrow channel spacing and thickness, it was concluded that the decrease in narrow channel spacing improves the firestopping performance while causing an increase in the pressure and pressure drop rate at the front end of the pipe. Wen et al.[17] conducted numerical simulations on the dynamic propagation and quenching characteristics of gas deflagration flame with different narrow channel spacing. The results show that the quenching distance is related to the narrow channel spacing, and the smaller the narrow channel spacing is, the quenching distance will be reduced and the flame will be more easily flame-retarded. Hu[18] experimentally studied the quenching characteristics of flame in narrow channel and gave the relationship between quenching length and narrow channel spacing based on the experimental results. Huang[19] used numerical simulation for different narrow channel size flame arresters and found that the narrow channel size is one of the main factors affecting the performance of flame arresters, and the increase of the narrow channel gap will make the flame arresting performance worse, but will promote the airflow in the flame arresters. Song[20] conducted an experimental study on the propagation and extinguishing process of premixed acetylene-air deflagration flame in a flat narrow channel. The results showed that under the same critical flame propagation velocity conditions, the extinguishing length value increased rapidly with the increase of narrow channel height, indicating that the narrow channel height has a significant effect on the propagation and extinguishing of premixed flames. Liu[21] studied the propagation and quenching characteristics of premixed methane-air flames at different narrow channel spacing by numerical simulation method, and found that the larger the narrow channel spacing, the larger the flame speed, the higher the flat narrow channel temperature, and the larger the quenching length of the flame inside the narrow channel; as the narrow channel spacing increased, the rise of the wall temperature was more obvious on the quenching length of the flame. Sun et al.[22] investigated the propagation and quenching of propane flames under different narrow channel sizes using numerical simulation methods. The results showed that the narrow channel size was positively correlated with the quenching length, and the narrow channel size was the main factor affecting the quenching length at higher initial flame velocity.

      Scholars on the effect of narrow channel spacing on combustible gas explosion research focused on a single narrow channel, and mainly the narrow channel spacing on the impact of the explosion flame propagation, mostly focused on the simulation of the investigation, and relatively little experimental research on the explosion characteristics. And with the gradual reduction of the multi-layer narrow channel spacing, the flame morphology and explosion characteristics inside the narrow channel are affected very differently. Therefore, this study uses a laboratory-scale research platform to obtain the changes in flame propagation pattern and explosion characteristics of syngas explosion under different narrow channel spacing.

    Experimental setup and method

      Experimental setup

    • As shown in Fig. 1, the experimental setup mainly is made up of the explosion piping system, gas distribution system, ignition system, data acquisition system, synchronization control system, parallel narrow channel device and the corresponding auxiliary equipment.

      Figure 1. 

      Schematic diagram of the experimental setup.

      The cross section of explosion pipes is 30 mm × 30 mm, which is made from 304 stainless steel. The explosion pipes include a acceleration pipe with 600 mm in length, a visualization pipe with 450 mm in length and a buffer pipe with 350 mm in length. The visualization pipe is equipped with a 450 mm × 30 mm quartz viewing window to capture the explosion flame. The parallel narrow channel device is installed in the middle of the visualization pipe, which is built by stainless steel plates with 290 mm × 30 mm × 1 mm in three dimensions respectively, as shown in Fig. 2. The gas distribution system mainly consists of gas cylinder, air compressor, mass flow controller and drying tube. The synthesis gas and air are dried through the drying tube and then passed into the pipeline through two mass flow controllers (range 5 and 10 L/min, accuracy ± 1.5%FS, maximum pressure 3 MPa). The data acquisition system mainly consists of a high-speed camera, sensors, a data acquisition board and a computer. The FASTCAM Mini UX100 high-speed camera is employed to capture the dynamic process of explosion flame at a rate of 8000 FPS. The explosion pressure and flame temperature are measured by two piezoresistive sensors (range 0−6 MPa, accuracy ± 0.5%, sampling frequency 200 kHz) and a thermocouple (WRe3-WRe25 type, measuring temperature range 0−2,300 °C), which are flush mounted on the parallel narrow channel. The data during the explosion is logged by the ART-PCIE9770 data acquisition card at a frequency of 200 kHz. The synchronization control system is employed to simultaneously trigger the high-pressure package igniter, the data acquisition card and the high-speed camera. The ignition position is 570 mm far away from the entrance of the narrow channel.

      Figure 2. 

      Parallel narrow channel device.

      Experimental method

    • The premixed syngas (CO : H2 = 1:1) and air with stoichiometric ratio is used in experiments. Experiments are conducted at ambient temperature and pressure. Parallel narrow channels with three intervals D1 = 1.0 mm, D2 = 0.5 mm and D3 = 0.3 mm are employed in experiments to investigate the impact of channel spacing on the explosion characteristic in parallel narrow channels. Each experiment is repeated three times to eliminate error.

    Results and discussion

      Effect of narrow channel spacing on explosion flame propagation

    • The flame propagation images with different narrow channel spacing were obtained using a high-speed camera. Figure 3 shows the flame propagation images at different narrow channel spacing.

      Figure 3. 

      Flame propagation images at different channel spacing for equivalent ratio 1.0.

      The flame before the entrance of parallel narrow channels is different for different narrow channel spacing, which is normal finger-shaped flame at narrow channel spacing 1.0 mm, an upward single-headed finger-shaped flame at spacing 0.5 mm and a flat-headed finger-shaped flame at spacing 0.3 mm. This is because that the cross-sectional area of the parallel plate gradually increases with spacing decreasing, which leads to the explosion reflected wave produced by the parallel narrow channel plates becoming more and more intense. Therefore, the finger shape flame front gradually becomes flat under the influence of the reflected wave.

      The flame in the parallel narrow channels inherits the flame front pattern at the front of the narrow channel entrance. When the flame enters the parallel narrow channels, it shows a finger-shaped flame at different narrow channel spacing. And then the flame shape gradually changed. At the narrow channel spacing 1.0 mm (as shown in Fig. 3a), the flame front gradually evolved from the normal finger shape at the entrance of the narrow channel to a slender single-headed finger shape. After propagating in the narrow channel for a period of time, the slender finger flame front gradually flattens (28.2 ms) and gradually forms a variant tulip flame (28.6 ms). Then the flame front continued to stretch at the exit of the narrow channel, turning into a variant tulip shape with a stretched upper end (29.6 ms).

      At the narrow channel spacing of 0.5 mm, the single-headed finger flame gradually flattens after entering the narrow channel (29.9 ms), and then the upper flame inside the narrow channel stretches into the same upward single-headed finger flame as at the entrance. The same upper-end stretched variant tulip flame shape forms at the exit of the narrow channel at the time of 33.0 ms in Fig. 3b.

      And at the channel spacing of 0.3 mm, the flame gradually changed from a flat-headed finger flame when it first enters the narrow channel to a finger flame stretched downward (34.0 ms), and keeps the finger shape propagating to the exit of the narrow channel.

      As shown in Fig. 3, with the decrease of the narrow channel spacing, the flame following after the flame front within the narrow channel becomes dimmer and may even quench. On one hand, after the flame front passes through, the residual reaction gradually decreases, resulting in the heat generation of reaction decrease. On the other hand, as the narrow channel spacing decreases, the contact area between the flame and the channel walls increases, resulting in enhancing the heat exchange between the flame and the cold wall. As a result, the heat generated by explosion is insufficient to offset the heat absorbed by the cold wall, resulting in flame dimming or quenching inside the narrow channel.

      Variation of flame front position and velocity in the narrow channel

    • Fig. 4 shows the flame front position and velocity change at different narrow channel spacing. It should be noted that the flame front position in Fig. 4 starts at the entrance of the parallel narrow channels and the time starts at the moment of the flame front entering the narrow channel. As the parallel narrow channel spacing decreasing, the time for the flame front passing throughout the narrow channel increases, which increases from 3.8 ms for channel spacing 1.0 mm to 4.9 ms for channel spacing 0.3 mm.

      Figure 4. 

      Flame front position and velocity variation curves for different channel spacing.

      As shown in Fig. 4, after entering into the parallel narrow channels the propagation velocity of flame front increases to a peak value, which is because that combustion reaction in the narrow channels supplies continuous energy to flame acceleration. The maximum velocities of the flame front are 124.6 m/s, 100.2 and 93.8 m/s for the channel spacing of 1.0, 0.5 and 0.3 mm respectively. The maximum velocity decreases as channel spacing decreases. This is because the heat absorbed by channel wall increases as the channel spacing decreases.

      The location of the flame velocity arriving at its peak is about the middle or rear of the channel, due to the reflected pressure by the end of buffer pipe. The duration of flame front accelerating to maximum flame velocity increases with decreasing narrow channel spacing. As shown in Fig. 3a, at the narrow channel spacing of 1.0 mm, the tulip flame is observed to form within the narrow channel, indicating an increase in the propagation speed of the flame front inside the narrow channel[23]. As a result, the flame front velocity for channel spacing 1.0 mm increases again.

      The effect of narrow channel spacing on the explosion pressure

    • The explosion pressure variation curves at the front and rear ends inside the narrow channel under different narrow channel spacing are presented in Fig. 5. There is a duration of pressure starting to rise, which is called trise. The explosion pressure rises at a very fast rate from initial pressure to the maximum explosion pressure (Pmax), and then slowly declines. As shown in Fig. 5c, trise increases as the narrow channel spacing decreasing, which indicates the channel spacing has an effect on the propagation of pressure wave.

      Figure 5. 

      Pmax variation curves before and after with different channel spacing.

      Fig. 5d shows the variation of Pmax at different narrow channel spacing. As the channel spacing decreases, Pmax decreases. And the maximum explosion pressure rise rate (dP/dt)max also increases as the narrow channel spacing increasing, as shown in Fig. 6. When the channel spacing decrease, the heat generated by explosion decreases and the heat absorbed by channel walls increases in the parallel narrow channels. As a result, the explosion in the channel weakens (as shown in Fig. 3), which leads to a lower Pmax.

      Figure 6. 

      Variation of pressure rise rate for different channel spacing.

      Pmax in the back end of the narrow channel is larger than that in the front end, which is especially obvious for the narrow channel spacing 1.0 mm, as well as (dP/dt)max. This is because the wall heat dissipation for channel spacing 1.0 mm is less than that for channel spacing 0.3 mm. As a result, the explosion reaction inside the channel for spacing 1.0 mm is more intense than that for spacing 0.3 mm.

      Effect of narrow channel spacing on explosion flame temperature

    • Fig. 7 shows the temperature and temperature rise rate for different channel spacing. There is a duration of temperature starting to rise, which is about 28 ms. The explosion flame temperature rises rapidly from initial value to the maximum explosion flame temperature (Tmax), and then declines with fluctuation. It is because the flame passes back into the narrow channels from the buffer pipe.

      Figure 7. 

      Variation of temperature and temperature rise rate for different channel spacing.

      Fig. 7b shows the variation of the temperature rise rate at different narrow channel spacing. The temperature rise rate decreases as the narrow channel spacing decreases. And there is another peak due to the flame passing into the narrow channels backwards and forwards from the buffer pipe. As the narrow channel spacing decreasing, the maximum explosion flame temperature of the flame inside the narrow channel also gradually decreases, which is because the explosion weaken.

    Conclusions

    • Experiments of premixed syngas-air explosion inside a parallel narrow channels are conducted. The flame propagation, explosion pressure and temperature inside the narrow channel are investigated to find out the effect of narrow channel spacing on the explosion.

      (1) As the narrow channel spacing decreasing, the flame front shape before the entrance of narrow channel changes from a normal finger shape for channel spacing 1.0 mm to a flattened shape for channel spacing 0.3 mm. In the narrow channel, the flame inherits the flame front pattern at the front of the narrow channel entrance, and then develops. The tulip flame inside the narrow channel is observed for channel spacing 1 mm. While a variant tulip flame appears only at the end of the narrow channel for spacing 0.5 mm. With the decrease of the narrow channel spacing, the flame following after the flame front within the narrow channel becomes dimmer and may even quench, which is because the heat generated by explosion is insufficient to offset the heat absorbed by the cold wall. The flame inside the narrow channels accelerates and reaches a peak value at the middle or rear of the channel. The flame front velocity decreases as the channel spacing decreasing.

      (2) As the narrow channel spacing decreasing, the maximum explosion pressure Pmax and the maximum explosion pressure rise rate (dP/dt)max decrease due to the explosion inside the channel being weakened by the channel walls. Pmax in the back end of the narrow channel is larger than that in the front end, as well as (dP/dt)max.

      (3) The explosion flame temperature inside the narrow channels rises rapidly from initial value to the maximum explosion flame temperature (Tmax), and then declines with fluctuation due to the flame passing back into the narrow channels from the buffer pipe. The explosion flame temperature and explosion flame temperature rise rate decrease with decrease of channel spacing.

      Acknowledgments

      • The authors are grateful for National Natural Science Foundation of China (No. 52074159 and No. 52004132), Key R & D programs (Social Development) in Jiangsu Province (No. BE2020710), Key National Natural Science Foundation of China (No. 51834007), Key Natural Science Foundation in Jiangsu Province (No. 18KJA620003) and Jiangsu Project Plan for Outstanding Talents Team in Six Research Fields (TD-XNYQC-002).

      Conflict of interest

      • The authors declare that they have no conflict of interest. Zhirong Wang is the Editorial Board member of Emergency Management Science and Technology who was blinded from reviewing or making decisions on the manuscript. The article was subject to the journal's standard procedures, with peer-review handled independently of this Editorial Board members and his research groups.

      Rights and permissions
      • Copyright: © 2023 by the author(s). Published by Maximum Academic Press on behalf of Nanjing Tech University. This article is an open access article distributed under Creative Commons Attribution License (CC BY 4.0), visit https://creativecommons.org/licenses/by/4.0/.
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  • About this article
    Cite this article
    Guo P, Xu C, Lu J, Wang Z, Chang X, et al. 2023. Study on the effect of channel spacing on premixed syngas-air explosion inside parallel narrow channels. Emergency Management Science and Technology 3:8 doi: 10.48130/EMST-2023-0008
    Guo P, Xu C, Lu J, Wang Z, Chang X, et al. 2023. Study on the effect of channel spacing on premixed syngas-air explosion inside parallel narrow channels. Emergency Management Science and Technology 3:8 doi: 10.48130/EMST-2023-0008
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  • Introduction
  • Experimental setup and method

  • Experimental setup

  • Experimental method

  • Results and discussion

  • Effect of narrow channel spacing on explosion flame propagation

  • Variation of flame front position and velocity in the narrow channel

  • The effect of narrow channel spacing on the explosion pressure

  • Effect of narrow channel spacing on explosion flame temperature

  • Conclusions
  • Acknowledgments
  • Conflict of interest
  • Rights and permissions
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