日本燃焼学会誌 第 62 巻 202 号（2020 年）338-346 Journal of the Combustion Society of Japan doi: 10.20619/jcombsj.1910 (早期公開) Vol.62 No.202 (2020) 338-346

■ORIGINAL PAPER■

Multi-Schlieren 3D-CT Measurements of Instantaneous Equivalence Ratio Distributions of a Premixed Gas Field Based on Modified

NAZARI, Ahmad Zaid, ISHINO, Yojiro*, KONDO, Harumi, MOTOHIRO, Takanori, YAMADA, Ryoya and ITO, Fumiya

Nagoya Institute of Technology, Gokisocho, Showa-ku, Nagoya 466-8555, Japan

Received 20 November, 2019; Accepted 1 June, 2020 J-STAGE Advance Published Date: 6 November, 2020

Abstract: In quantitative measurements and investigations, "equivalence ratio" and "refractive index" are the most significant parameters in combustion flow fields. Experimental and analytical studies were performed to understand the relation between equivalence ratio and refractive index in flow fields. A "universal relationship" between the refractive index of a premixed gas of fuel and air and its equivalence ratio has been established in this study. Through this relationship, we can calculate the equivalence ratio of any premixed gas (except those of fuels) from its refractive index using a unique coefficient. The uniqueness of the relation coefficient enables us to obtain the equivalence ratio of even a multi-fuel premixed gas from its refractive index. In this study, the novel relationship is applied to a non-scanning 3D-CT (computerized tomography) technique using a multi(20)-directional quantitative schlieren optical system with a flashlight source. Instantaneous equivalence ratio distributions of a dual-fuel premixed gas flow with two jets of ethanol/air and propane/air mixtures are measured by CT-reconstruction. Good agreement was observed between the CT-reconstructed data and the proposed comprehensive relation.

Key Words: Multi-directional quantitative schlieren optical technique, Premixed gas field, Flow visualization, Computerized tomography (CT), 3D measurement, Equivalence ratio distribution, Refractive index, Bi-fuel, Dual-fuel, Multi- fuel

premixing fuel-air technology. For calculating the equivalence 1. Introduction ratio, it is necessary to know the type of fuel and its The global community is currently looking for fuel-efficient concentration ratio. It is complicated to measure the equivalence and environmentally viable alternatives for many traditional ratio in a flow field with multiple fuel types using conventional energy conversion approaches. One of the efficient ways is to use investigation techniques. Hence, the objective of this work is to "dual-fuel vehicles" (vehicles with multifuel engines), which are study the equivalence ratio, which is important for combustion. used in certain regions. For improving life cycle value of the Further, we propose a measurement method for the equivalence vehicles, Yanmar has developed a reliable "dual-fuel engine" [1] ratio of premixed gas with multiple types of fuels of unknown that can use diesel and gas for the marine engine sector. components, based on the modified refractive index of the Evaluating air/fuel mixture and calculating the equivalence ratio premixed gas field. Furthermore, this is applied to the three- in these dual-fuel engines requires certain new techniques that dimensional computerized tomography (3D-CT) technique using can facilitate the complicated treatment of two or more mixed a multi-directional quantitative schlieren optical system to try to fuels. In this study, a universal relationship between a "modified measure the instantaneous 3D equivalence ratio distributions of refractive index nm" of a premixed gas of fuel and air and its the flow field where the premixed gas exists. First, the "3D

"equivalence ratio ϕ" is introduced. modified refractive index nm" distributions data of the target flow The equivalence ratio represents the mixing ratio of various are obtained using 3D-CT reconstruction of the projection’s fuels and oxidizers (air), and is an important parameter for images. Further, the corresponding "equivalence ratio ϕ" is expressing conditions such as proper mixing, excess fuel, and calculated using the proposed "universal relationship" between a excess air for combustion, and particularly, for state-of-the-art modified refractive index and its equivalence ratio. Kawahara et al. [2] developed a heterodyne * Corresponding author. E-mail: [email protected] system with a fiber-optic sensor. This system provides a

(54) NAZARI, Ahmad Zaid et al., Multi-Schlieren 3D-CT Measurements of Instantaneous Equivalence Ratio Distributions 339 of a Premixed Gas Field Based on Modified Refractive Index refractive index (consequently, temperature) measurement Table 1 Gladstone-Dale constant for some gases [18]. technique for unburned gas in a commercially produced spark- ignition (SI) engine. Sharma et al. [3] experimentally determined the refractive index and density for binary liquid mixtures of eucalyptol with hydrocarbons, and from measured values, refractive index deviations at different temperatures were computed. Nita et al. [4] present experimental data of the refractive index for pseudo-binary mixtures of reformate gasoline with ethanol, isopropanol, and n-butanol over the entire composition range and for temperature ranging from 293.15 K to 313.15 K. The refractive index mixture rules are discussed by Heller [5]. The schlieren imaging technique has proven to be particularly well suited in combustion and fluids and has been widely implemented [6]. This technique provides useful qualitative and quantitative schlieren system information on the variations in fluid density, temperature, static pressure, and flows can be depicted in detail. In previous studies [7–17], by employing and developing a non-scanning three-dimensional computerized tomography (3D- CT) technique using a delicate multi-directional quantitative schlieren optical system with a flashlight source, the following have been successfully measured and certain new techniques performed. introduced: We propose a new method for obtaining the equivalence ratio (a) local burning velocity of a turbulent premixed flame [7], distributions from the modified refractive index distributions. (b) light-emission distributions of premixed flames [8], Using this method, it is possible to obtain instantaneous 3D (c) instantaneous 3D density distributions of high-speed distributions of equivalence ratios in the presence of multiple premixed turbulent flames [9–11], types of fuels with unknown components for hydrocarbon fuels (d) 3D density distributions of a steady non-axisymmetric with three or more carbon atoms. premixed flame [10], (e) instantaneous 3D density distributions of spark-ignited 2.2. Derivation of Equivalence Ratio based on premixed spherical flame kernels in laminar and turbulent flows Modified Refractive Index nm [12, 13], The method for obtaining the equivalence ratio from the (f) instantaneous 3D density distributions of spark-ignited flame measured modified refractive index is discussed as follows. kernels in propane/air-fuel jets from direct injection nozzle [14], The gas refractive index n is expressed by equation (1) using

(g) instantaneous 3D density distributions of supersonic microjets the modified refractive index nm as, issuing from circular and square micro nozzles [15], (h) a novel "multi-path integration" technique for image-noise (1) reduction, and (i) a new "inverse process" technique to obtain vertical schlieren It can be expressed by equation (2) using the Gladstone-Dale brightness gradient from horizontal experimental data [16, 17]. relation (ρ: density, K: Gladstone-Dale constant) as,

(2) 2. Methods and Experimental Setup

2.1. Overview of the Proposed Method Therefore, the modified refractive index nm, density, and In this study, we obtained an instantaneous 3D modified Gladstone-Dale constant of a gas have the following relationship, refractive index distributions using the 3D-CT reconstruction method. Using the 20 modified optical path length Oplm images (3) derived from the 20 quantitative schlieren images as projection data, the CT-reconstruction procedure has been successfully In the combustion of hydrocarbon-type fuels CaHbOc, the air fuel

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ratio (AFR) of the premixed gas based on the equivalence ratio ϕ and hydrocarbon components is,

(4)

The density of the premixed gas ρmix can be obtained by the following equation using the mass fraction Xi known from the mass ratio obtained by equation (4).

(5) where,

(6)

(7)

Therefore, the product of the density of the premixed gas and Gladstone-Dale constant K is obtained by the following equation (8).

(8)

Fig. 1 Relationship between φ and nm. Further, based on equation (3) nm = ρK (i.e. nm = ρmix Kmix), therefore, equation (8) shows the relationship between the equivalence ratio ϕ and modified refractive index nm (= n - 1, n: refractive index) of the premixed gas. The results of this 2.3. Multi-Directional Quantitative Schlieren System relationship are illustrated in Fig. 1 for certain hydrocarbon-type Figure 2(a)–(c) illustrates the concept of the 20-directional fuels. The Gladstone-Dale constant K [18] in Table 1 is used to schlieren camera. The target "two premixed jets/non-uniform calculate the modified refractive index. density field" is observed from a 180º angle using numerous In Fig. 1, except for methane and ethane, it can be observed schlieren optical systems simultaneously. Twenty systems that the relationship between the equivalence ratio and modified capture the multi-directional view in the 20 different angle refractive index can be represented by a single straight line positions from θ = -85.5º to θ = +85.5º at an interval of 9º. Here regardless of the fuel type because they are present in a close angle θ is defined as the horizontal angle from the x-axis. The range and their differences are very small and negligible. We center between two-nozzle exits has been selected as the origin propose the following correlation (equation (9)), which reflects of the xyz-coordinate system. the variation of the results shown in Fig. 1. A single instance of the multi-directional quantitative schlieren camera system is illustrated in Fig. 2(d). This system is (9) composed of two convex achromatic lenses with 50 mm diameter and 300 mm focal length, a flashlight source unit, a vertical Although, the Gladstone-Dale constants in Table 1 differ by more knife-edge (for obtaining horizontal schlieren brightness gradient than 20%. The uncertainty is for the upper limit (Octane C8H18) images), a digital camera (mirrorless single-lens camera, Nikon, +8.9%, the lower limit (Propane C3H8) -11%, and Ethanol J1) and a camera lens (focal length 80 mm, lens diameter 21 +4.6%. Hence, the equivalence ratio can be obtained using a mm). The digital cameras present 8-bit color depth JPEG images universal relationship that does not depend on the fuel type, and this equates to (2^8)^3=16,777,216 total potential RGB color the equivalence ratio can be measured even when multiple fuels values. In the 3D-CT quantitative schlieren measurement with unknown components are mixed together. This method can technique, all JPEG images convert to the portable gray map be applied to gas alarms, which utilize refractive index sensors (PGM) format. It contains 256 monochrome grayscale values that [19–22]. represent different shades of gray from black (0) to white (256). The light unit is a xenon flashtube, full-spectrum white with a

(56) NAZARI, Ahmad Zaid et al., Multi-Schlieren 3D-CT Measurements of Instantaneous Equivalence Ratio Distributions 341 of a Premixed Gas Field Based on Modified Refractive Index

(a) Schlieren camera system (b) Coordinate system

(c) Schematic of 20-direction schlieren camera

(d) Single unit of the quantitative schlieren optical system

(e) Schematic of 3D-CT non-scanning method (f) CT-scanner

Fig. 2 Multi-direction schlieren optical and coordinate systems.

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uniform luminance rectangular area of 0.25 mm × 2 mm and 35 by the schlieren observation as B(X) and Bnj(X) (brightness of

μs duration. The light intensity is adjusted by using a neutral schlieren image in no-jet/no-disturbance condition). Bnj(X) is a density (ND) filter (Fuji 1, exposure adjustment multiple 10). schlieren brightness distribution when there is no refractive index

The stepped ND filter is used for calibrating the cameras by distribution (n*(x, y) = na*). To obtain the modified optical path obtaining images of grayscale in 11 steps without the knife-edge. length Oplm(X(θ)) from B(X) and Bnj(X), both are processed as From a technological point of view, this experimental method depicted in Fig. 3(f)-(h). As indicated in Figs. 3(f) and (g), (CT non-scanning, Fig. 2(e)) has a similarity with a CT-scanner deviation brightness in schlieren image ΔB(X) that used in medical engineering for medical diagnostics (Fig. 2(f)). A CT-scan combines a series of X-ray images obtained (10) from different angles to produce cross-sectional (tomographic) images of specific areas of a scanned object (human body). In the is scaled to d(Oplm(X(θ)))/dX(θ) by the next expression. schlieren 3D-CT measurement technique employs schlieren photography (instead of X-ray images) from 20 directions to (11) reconstruct 3D-density distributions data (cross-sectional density distributions) of a target flow. where α depends on transparent width of the light source image on schlieren stop location (Δs = 0.25 mm × 2 mm (Hor. × Ver.)), 2.4. Image Processing and focal length of the convergent lens (f = 300 mm). Therefore,

To obtain the modified refractive index distributions, the modified optical path length Oplm is reproduced by transverse-

3D-CT measurement requires the distribution of the "modified integration of d(Oplm(X(θ)))/dX(θ) from schlieren images, as optical path length Oplm" integrated on the ray as projection data. shown in Fig. 3(h). Furthermore, Oplm*(X(θ) = -R) = Oplm*(X(θ)

The modified optical path length distribution Oplm is obtained = +R) = 0. based on the diagram in Fig. 3. Figures 3(b)-(e) show the schlieren observation process from an angle θ for the refractive 2.5. CT-Reconstruction index profile n*(x, y) in Fig. 3(a). Quantities with an asterisk sign The images of modified optical path length Oplm are used as (*) express the actual values and without asterisk represent projections for CT-reconstruction by maximum likelihood- derived values from the CT-reconstruction process. Fig. 3(b)-(h) expectation maximization (ML-EM) [23] an appropriate CT show observations in the direction of θ from x-axis in the algorithm to obtain the 3D reconstruction of modified refractive inclined coordinates which are denoted by X(θ) and Y(θ). The index distributions. ML-EM (distributed back projection) method primary goal is to obtain the modified optical path length Oplm* [7–17] is employed for CT-reconstruction. The CT-reconstruction of deviation modified refractive index (Δnm*), which is shown in procedure is performed in each horizontal plane of z-axis for the Fig. 3(d) with the unit of length (m). Here, the modified reconstruction of deviation modified refractive index refractive index deviation Δnm* is defined as the modified distributions Δnm(x, y) from linear data set of modified optical refractive index deviation Δnm* = nm* - nma* from the modified path length Oplm(X(θ)) (Fig. 3(c)). Further, 2D modified refractive index nma* of the surrounding gas (air) region of refractive index distribution (Fig. 3(b)) is obtained as follows. -4 constant modified refractive index nma* (= 2.726×10 ) on the periphery of the observed range of radius R, as nm* > nma*, Δnm* (12) is non-negative and CT-reconstruction is possible. The modified optical path length of deviation modified refractive index The reconstruction is performed for each cross-section, and

(Oplm*(X(θ))) is called modified optical path length Oplm*. It is the cross-sections are stacked to form the 3D modified refractive automatically obtained from the schlieren observation by index distribution. Therefore, the 2D distributions nm(x, y) are spatially-integration of deviation modified refractive index accumulated in layers to form 3D-CT distribution nm(x, y, z). In

Δnm*(X(θ),Y) along the line of sight. The modified refractive this study, the modified optical path length Oplm(X(θ)) projections index distributions of the target flow field are reconstructed using images of 900(H) × 5(V) pixel (27 mm × 0.15 mm) are used for

2D distributions (image) of modified optical path length Oplm as CT-reconstruction to produce 3D data 900(x) × 900(y) × 5(z) the projection in the CT-reconstruction process. pixel (27 mm × 27 mm × 0.15 mm). The voxel size is 0.03 mm in

In practice, for obtaining modified optical path length Oplm each direction. (Fig. 3(h)), the image processing activity starts from Fig. 3(e) by obtaining two sets of images, "with target" and "without target" 2.6. Experimental Injection Nozzle (without any disturbance in the test section) with a horizontal Figures 4 shows an overview of the premixed gas injection brightness gradient in the x-direction. Two sets of images present device used in this experiment and an outline of its nozzles shape

(58) NAZARI, Ahmad Zaid et al., Multi-Schlieren 3D-CT Measurements of Instantaneous Equivalence Ratio Distributions 343 of a Premixed Gas Field Based on Modified Refractive Index

Fig. 3 Process of formation of schlieren brightness and conversion to projections Oplm of CT.

and size. This injection device can inject two different fuels quantitative schlieren imaging is performed. The center between simultaneously by filling the parts shown in red in Fig. 4 with two-nozzle exits has been selected as the origin of the xyz-coordinate premixed gas and rotating the lower motor. A stepping motor system. The ethanol-air premixed gas nozzle outlet center (manufactured by Plexmotion, SSA-PR-42D2, built-in driver) coordinate is (x, y, z) = (0, -8, 0) and propane-air premixed gas was used in this experimental setup. As depicted in Fig. 4(top), nozzle outlet center coordinate is (x, y, z) = (0, +8, 0). the convergent nozzle shape is provided with a constricted flow part with an inlet diameter of 12 mm and outlet diameter of 6 mm 3. Results and Discussion to inject the premixed gas in a laminar flow. The flow rate is adjusted for the airflow 50 l/min and for the propane flow 3 l/min 3.1. Quantitative Schlieren Images to maintain the laminar flow. In this experiment, propane-air and Figure 5 shows quantitative schlieren images of the two ethanol-air premixed gases with an equivalence ratio ϕ = 1 are premixed gas jets obtained simultaneously by the multi(20)- injected simultaneously, and 20-directional simultaneous directional schlieren camera system. The insets in each image are

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Fig. 4 Schematic of injector. Fig. 5 Quantitative multi(20)-schlieren images of stoichiometric (equivalence ratio = 1) two premixed jets in the ambient air (ethanol (almost left) - propane (almost right)).

the values of the shooting angles θ. The images of Fig. 5 are calculated from the universal relationship (equation (9)) between processed by the aforementioned conversion procedure to a modified refractive index and its equivalence ratio. It is projection images of modified optical path length Oplm. Using the observed that the peak value of the equivalence ratio in the modified optical path length Oplm images as projections for potential core of the premixed gas jets is approaching to ϕ = 1 on CT-reconstruction, 3D instantaneous modified refractive index the left and right sides, despite the different fuel types (ethanol, nm distributions of flow targets of two premixed gas jets were propane). The uncertainty value of potential core for ethanol and successfully obtained. propane is +2.4% and -18%, respectively. A good agreement is observed between the CT-reconstructed data and proposed 3.2. CT-Reconstruction Results universal relationship between a modified refractive index of a Figure 6(a) shows a sample of horizontal distributions of premixed gas of fuel and air, and its equivalence ratio. modified refractive index nm for z = 11.1 mm position for stoichiometric (ϕ = 1) premixed jets. 4. Conclusions Figure 6(b) shows the corresponding contour diagram of Fig. 6(a). The modified refractive index is indicated by the brightness An experimental study was performed to obtain the detailed level as shown by the brightness bar charts in the inset. Figure 7 information required for understanding the relationship between shows the radial distribution diagram of the modified refractive the modified refractive index of a premixed gas of fuel and air index nm and equivalence ratio ϕ for x = 0 and z = 11.1 mm. In and its equivalence ratio. Here, a "universal relationship"

Fig. 7, the modified refractive index nm is obtained from between the "modified refractive index nm" of a premixed gas of CT-reconstructed data and corresponding equivalence ratio ϕ is fuel and air and its "equivalence ratio ϕ" is proposed. The results

(60) NAZARI, Ahmad Zaid et al., Multi-Schlieren 3D-CT Measurements of Instantaneous Equivalence Ratio Distributions 345 of a Premixed Gas Field Based on Modified Refractive Index

confirm good agreement between the CT-reconstructed data and the proposed universal relationship. Therefore the equivalence ratio can be obtained by a universal coefficient that does not depend on the fuel type and can be measured when multiple fuels with unknown components are combined. This method can be applied to gas alarms technology. Its application in other fields should be investigated.

Acknowledgments

This research work was partly supported by the JSPS Grants- in-Aid for Scientific Research (C)16K06118. We are grateful for the continuous support of JSPS.

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