Application of high-speed, species-specific chemiluminescence imaging for laminar flame speed and Markstein length measurements in spherically expanding flames
Introduction
Chemiluminescence has been used as an optical diagnostic for combustion processes for centuries and remains a widely popular technique for quantitative measurements of flame characteristics. In contrast to temperature-dependent incandescence, which arises primarily from emission from high-temperature soot in hydrocarbon flames in the infrared and visible regions, chemiluminescent emission from electronically excited species comprises electromagnetic radiation primarily in the visible and ultraviolet region. This emission was first differentiated in the late 1940s [1]. Soon thereafter [2], [3], [4], [5], specific spectral lines in hydrocarbon flame emission spectra were identified and shown to correspond to specific electronic transitions of excited species in the flame, including OH*, CH*, CO2*, and C2*, among others [6], [7], [8]. Since then, chemiluminescence diagnostics have been used for many decades to correlate chemiluminescence with various properties of combusting mixtures, including temperature [9], laminar flame speed [10], [11], [12], equivalence ratio [9], [13], [14], [15], [16], degree of fuel spray penetration [17], heat release rate [9], diffusion rates [18], flame thickness [19], flame front structures [20], [21], and even fire detection for process safety applications [22], [23]. These measurements have been used as validation targets for chemical kinetics mechanisms and inputs for computational fluid dynamics (CFD) calculations, as the nonintrusive nature of a chemiluminescence diagnostic allows for temporally resolved (albeit line-of-sight) measurements of reacting flow systems, laminar or turbulent. Chemiluminescence diagnostics have been applied to a number of premixed or diffusion flame apparatuses, such as Bunsen burners [11], [13], counterflow burners [16], heat flux burners [24], flat flames [25], diffusion jet burners [20], and co-flow nozzle burners [26], as well as shock tubes for ignition delay time measurements [27] and real-size or scaled-down combustion devices with optical access [28], [29], [30]. It suffices to say that chemiluminescence diagnostics abound in the combustion community, and the preceding list of references is quite far from exhaustive.
In the laboratory, spherically expanding flames offer the advantages of well-characterized stretch rate behavior, fewer flow effects than burner-generated flames at elevated pressures, and access to a wide range of stretch rates over the course of a single experiment [31]. However, chemiluminescence techniques have yet to see a widespread application to combustion testing in the premixed, spherically expanding flame configuration, perhaps due in part to the unsteady nature of the spherically expanding flame. In cases of anchored flames in a flow with a slight degree of unsteadiness, such as with a counterflow burner [32], a co-flow nozzle burner [26] or heat flux burner [24], researchers tend to average many images to produce a single image for analysis, or use long (i.e., ~millisecond) camera exposure times. This degree of image integration is not viable for spherically expanding flames.
Chemiluminescence measurement techniques have been applied to spherical flames occasionally in the literature, but with the exception of two publications [12], [21], chemiluminescence has never been used to measure laminar flame speed from spherically expanding flames in the conventional closed flame bomb configuration. Dandy and Vosen [33] captured measurements of OH* emission of nominally spherically expanding methane-air flames using a photomultiplier tube (PMT), but their technique was limited to measurements of intensity per unit flame surface area and therefore lacked spatial resolution. Yoo et al. [34] measured OH* and CH* intensity profiles for nearly spherical hydrogen/methane/air flames using an intensified camera, but they used a steady diffusion flame apparatus, not a spherically expanding flame. Tinaut et al. [9] measured OH* and CH* intensities of methane-air flames in a spherical, constant-volume chamber using PMTs and related their relative intensities to the equivalence ratio, rate of heat release, and adiabatic flame temperature. In an unconventional use of shock tubes, researchers with the group of Hanson at Stanford have recently used high-speed chemiluminescence imaging of OH* emission to measure laminar flame speeds of methane-diluent and propane-diluent flames at high (>750 K) pre-ignition temperatures, showing that laminar flame speed can exhibit a negative temperature dependency regime [35]. Halter et al. [36], [37] developed an optically accessible, spherically expanding flame apparatus for measuring laminar flame speed using the constant-volume pressure rise method and included a chemiluminescence diagnostic for tracking the flame radius and monitoring for flame front instabilities. Halter et al. [36], [37], Marley and Roberts [12], and Marley et al. [21] provide seemingly the only publications regarding laminar flame speed measurement derived from chemiluminescence imaging of spherically expanding flames, but only total chemiluminescence was used, precluding any species-specific analyses.
In an effort to expand the application of species-specific measurements to spherically expanding flames, the authors have developed a high-speed imaging diagnostic method for tracking the growth of spherically expanding flames of diameters up to 12.7 cm (5 in). This work represents the first known effort to measure laminar flame speed and Markstein length from spherically expanding flames using species-specific chemiluminescence, specifically with the objective of measuring multiple flame properties simultaneously from a single flame experiment. From images of spherically expanding flame chemiluminescence, recent investigations have shown that properties like species concentration profile thickness [19] can be measured. This paper addresses the ability of the diagnostic to produce measurements of laminar flame speed and Markstein length.
Laminar flame speed measurements of spherically expanding flames have been conducted traditionally using schlieren imaging, for which a collimated beam of light is passed through the flame, and the gradient of the refractive index caused by the gradient of density across the flame appears as a dark circle in the images. The steep density gradient at the leading edge of the flame makes identification of the flame front quite easy, at least upon visual inspection. Similarly, chemiluminescence images mark the flame front based on the brightness of the spectral emission from excited-state species, allowing the images to be analyzed for laminar flame speed in almost exactly the same manner as schlieren images. However, the accuracy of using the region represented by the bright flame emission as opposed to the region marked by the steep density gradient for flame speed calculations is yet to be experimentally validated. Hence, the intention of this study was to provide imaging data that can be analyzed to produce species-specific measurements of the flame, such as intensity profiles of excited species, while also producing measurements of laminar flame speed, thereby providing multiple data sets simultaneously for the same test. This paper serves as validation of the high-speed chemiluminescence diagnostic experimental approach described previously [38] for the purposes of measuring the laminar flame speed of spherically expanding flames.
Presented first is a description of the experimental facility and testing procedure, along with relevant details of the chemiluminescence imaging system. The data processing techniques are described with a focus on the method of laminar flame speed and Markstein length estimation from high-speed chemiluminescence images. The data processing description also highlights the differences between the image processing method for traditional schlieren imaging and the new chemiluminescence imaging technique. The results of both reactant mixtures (CH4-air and CH4/C2H6-air) are presented in the context of relevant literature data, and laminar flame speed predictions using the AramcoMech 2.0 mechanism [39], [40], [41], [42], [43], [44], [45] are also included for reference. The new chemiluminescence results are then discussed in the context of validation of the experimental technique, followed by further discussion of the additional in-depth information such as excited-state chemical species profiles that chemiluminescence imaging provides beyond commonly used schlieren-based laminar flame speed measurements.
Section snippets
Experimental facility and imaging system
This study was conducted using the Turbulent Flame Speed Vessel (TFSV), an approximately cylindrical stainless-steel chamber rated to withstand initial pressures of up to 10 atmospheres. A full description of the facility is given by Morones et al. [46]. A schematic of the experimental facility and imaging system is shown in Fig. 1. It is similar to the setup employed by Turner et al. [19], [47], [48], Paschal et al. [49], and Parajuli et al. [38], [50]. The chamber possesses approximately 34 L
Data analysis
In this section, the methodologies for processing images of chemiluminescence and extracting measurements of laminar flame speed and Markstein length (burned and unburned) are described. Properties obtained from kinetics predictions, i.e., density ratio and flame thickness, that are involved in the analyses are presented.
Results
Given in this section are the results of the experiments and analyses described above. The laminar flame speed results are given first for the methane-air flames and then for the methane/ethane-air flames. The burned and unburned Markstein length results for both fuel mixtures are presented in the same fashion.
Conclusions
A new high-speed chemiluminescence imaging diagnostic method for spherically expanding flames has been constructed at TAMU. Laminar flame speeds and Markstein lengths of methane-air flames and of 80/20 methane/ethane-air flames were measured from chemiluminescent emission of OH* and of CH*, and from total (broadband) chemiluminescence. The laminar flame speed and Markstein length results and their agreement with literature values for both mixtures investigated indicate that chemiluminescence
CRediT authorship contribution statement
Mattias A. Turner: Methodology, Validation, Formal analysis, Investigation, Data curation, Visualization, Writing – original draft, Writing – review & editing. Tyler T. Paschal: Methodology, Validation, Investigation. Pradeep Parajuli: Investigation. Waruna D. Kulatilaka: Conceptualization, Methodology, Validation, Resources, Writing – review & editing, Supervision, Project administration, Funding acquisition. Eric L. Petersen: Conceptualization, Methodology, Validation, Resources, Writing –
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
This work was supported in part by a grant from the Defense Threat Reduction Agency, grant number HDTRA1-16-1-0031. The support of the United States Department of Energy, National Energy Technology Laboratory through NETL-Penn State University Coalition for Fossil Energy Research (UCFER, contract number DEFE0026825) is gratefully acknowledged.
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