Two-dimensional measurement of hydrocarbon fuel concentration using multiple laser-induced plasma-forming regions.

A two-dimensional measurement of fuel distribution in a gasoline spray flow was performed using multiple laser-induced plasma-forming regions. Multiple plasma-forming regions were generated by a laser sheet with a low breakdown threshold for a two-phase flow. To observe the formation of multiple laser-induced plasma-forming regions, shadowgraphs were imaged using a high-speed camera. Hydrogen and oxygen atomic emissions from the plasma-forming regions were obtained by attaching bandpass filters to the high-speed camera, and a two-dimensional visualization of the fuel distribution in the wide plasma-forming region was obtained by dividing the hydrogen line-filtered image with the oxygen line-filtered image. The result complements a novel method for two-dimensional measurement of instantaneous fuel concentration in the reacting flow by utilizing laser-induced breakdown spectroscopy (LIBS). © 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement

dimensional gas-to-particle phase transition in flames using laser-induced nanoplasmas [13]. Moreover, a line LIBS technique has been developed to measure the 1D fuel-to-air ratio in laminar and turbulent flames [14]. However, no studies have been done to observe the formation of multiple laser-induced breakdowns in a sheet region and apply them to the measurement of an elemental composition distribution.
In our previous work, we developed and tested a miniaturized device for fuel concentration and flame diagnostics, which we named 'LIBS plug' [15,16]. The device was intended to provide a novel feedback control strategy for flame stabilization with simultaneous in situ combustion flow diagnostics in a scramjet engine combustor. The LIBS plug was constructed using two photodiodes and two bandpass filters which pass either the H (656.3 nm) or the O (777 nm) atomic line. Using the LIBS plug, the construction of a calibration curve between the H/O intensity ratio and the equivalence ratio in both a single gas phase flow and a two-phase flow was performed. We also suggested methods of measuring the equivalence ratio inside a gasoline spray flow and performing flame diagnostics with the LIBS plug. In our previous study, we analyzed the laser-induced breakdown characteristics inside a two-phase flow [17], demonstrating that the two-phase flow experiences a drastically reduced breakdown threshold and multiple breakdowns in the laser path when a single laser beam is used. Since the plasma volume exceeded the size of fuel droplets, the signal of the atomic emissions from the plasma represented chemical compositions of both fuel droplets and air. Subsequently, the present research showed that the equivalence ratio at the focal point could be estimated using a laser-induced plasma even inside a two-phase gasoline flow.
This study proposes a measurement method for the two-dimensional hydrocarbon fuel distribution, using characteristics of a laser-induced breakdown in a two-phase flow, and specific lines of LIBS that are required to perform a fuel distribution analysis. To produce multiple plasma-forming regions in a specific region, a laser sheet was constructed using a high-energy pulsed laser. However, since the laser energy was not strong enough to create a full two-dimensional plasma sheet over the region of interest, and the threshold of the breakdown in liquid is significantly lower than gas, the plasma-forming regions were limited only near where the fuel droplets were present. Hence, 30 shots of images were averaged to compensate plasma-absent regions in each shot. The shadowgraph imaging performed by a high-speed camera confirmed the formation of multiple breakdowns. Two bandpass filters were sequentially located in front of the high-speed camera to collect either the H or the O atomic emissions from multiple plasma-forming regions. The attenuation of the plasma emission intensity was observed, which aligns with the results obtained using the conventional LIBS and the LIBS plug. The two-dimensional H/O intensity ratio was obtained by dividing the H-filtered image with the O-filtered image. To estimate the exact fuel-air ratio from the H/O intensity ratio, a calibration curve between two-dimensional H/O intensity ratio was obtained from multiple plasma regions, and the equivalence ratio was established from a uniform droplet stream. In the gasoline spray flow, the increase of the H atomic line emission and the decrease of the O atomic line emission were both observed two-dimensionally, according to an increasing gasoline flow rate. The direct linear relation between the H/O intensity ratio and the equivalence ratio was then applied to the non-uniform spray nozzle flow, to obtain the final distribution of two-dimensional fuel distribution. To validate our result, we compared it to the H/O intensity ratio obtained using the conventional LIBS system at the same position and flow condition.

Experimental setup
The main purpose of this work is to visualize the two-dimensional fuel concentration in a two-phase spray flow using multiple laser-induced plasma-forming regions. The multiple plasma-forming regions were generated by an Nd:YAG laser and a high-speed camera was used to capture the plasma-forming regions.
The experimental setup is illustrated in Fig. 1(a). The Nd:YAG laser (Surelite III, Continuum) used to generate the laser sheet had the wavelength of 1064 nm and a pulse duration of 5 ns. To produce multiple plasma-forming regions using a single laser beam, the energy of the used laser was set to 968 mJ. Laser-induced plasmas were detected using a high-speed camera (HPV-X2, Shimadzu) with a recording speed of 10 million frames per second, which was positioned vertically from the focal point of the laser sheet. The exposure time of each frame was 50 ns and the interval of the frames was 100 ns. The high-speed camera provided a resolution of 400 x 250 pixels, and a field of view was measured to be 65.1 x 40.7 mm. To collect the H (656.28 nm) and the O (triplet at 777.194 nm, 777.417 nm, and 777.539 nm) atomic lines of the plasmas, the bandpass filters (656FS10-50 and 777FS10-50, Andover) with center wavelengths (CWL) of 656 nm and 777 nm and a full width at half maximum (FWHM) of 10 nm were located in front of the high-speed camera in each procedure. The acquired shadowgraph images verified the multiple laser-induced breakdowns. A continuous wave (CW) laser with a wavelength of 532 nm was used as an illumination source, while neutral-density filters and a bandpass filter (532FS03-50, Andover) centered at 532 nm were used to prevent the appearance of a plasma emission. A bandpassfiltered image and the shadowgraph image of the multiple plasma-forming regions are shown in Figs. 1(b) and 1(c), respectively.
To validate the results obtained using multiple laser-induced breakdowns, the results were compared to those obtained from a conventional LIBS experiment, where a LIBS system was used in the same flow field. To generate a single plasma, a laser energy of 100 mJ was used in combination with a plano-convex lens. The plasma emission was collected using an optical collector and fed to the spectrometer (Andover Mechelle 5000) and ICCD (Andor iStar). For measurements, a gate delay time of 400 ns and a gate width of 2 μs were selected, and the H/O intensity ratio was calculated for each point of the spray flow.

Experimental procedure
The atomization of liquid gasoline was done by a siphon nozzle (Delavan 30609-2). An attached dosing pump (Simdos 10, KNF) and a pulsation damper (FPD 10, KNF) were used to control and provide a continuous fuel flow rate. The air flow rate was controlled by a mass flow controller (MFC, TSC-230, MKP) and the inlet flow temperature and pressure were set to 298.15 K and 101.3 kPa, respectively. Shadowgraph imaging and fuel distribution measurement were done in the siphon nozzle flow while a Bunsen burner flow was used to construct calibration curve. To construct calibration curve between the H/O intensity ratio obtained from multiple plasma regions and the equivalence ratio, a uniform droplet stream was generated with ultrasonic vibrating plate nebulizer and sent to a Bunsen burner with a nozzle diameter of 12 mm. In this case, the laser sheet was irradiated right above the nozzle to avoid intervention of the laboratory environment. The experimental flow conditions are presented in Table 1.
Between the Nd:YAG laser and the flow, three cylindrical lenses were attached to form a planar laser sheet. The height of the laser sheet was 25 mm, and the focus was located 8 mm above the center of the siphon nozzle. To compare the breakdown characteristics of a single plasma-forming region and multiple plasma-forming regions, shadowgraph imaging of a single-point laser-induced plasma was also conducted by replacing the cylindrical lenses with a spherical lens. In the single plasma case, a lower laser energy was used because its plasma continuum emission was so intense that interference occurred during shadowgraph imaging. Five different experimental conditions were set, each having different flow rates of fuel, as presented in Table 1. To see the averaged fuel-to-air ratio distribution over the twodimensional region, 30 shots of images were ensemble-averaged for each flow condition.

Multiple laser-induced plasma-forming regions
The shadowgraph images of the formation of multiple plasma-forming regions are presented in Fig. 2, where Fig. 2(a) shows images for a laser focused on a single point in air, Fig. 2(b) a laser focused on a single point in the spray flow, Fig. 2(c) a planar laser sheet focused in air, and Fig. 2(d) a planar laser sheet focused in the spray flow. In the images of Fig. 2, the laser beam is propagated from the right. In air, the laser beam focused on a single point produced a single plasma, forming a strong shock wave, whereas, the one in the gasoline spray flow produced several plasma-forming regions. Our previous research has already described the formation of multiple breakdowns in a two-phase flow [17]. The breakdown threshold for a two-phase flow decreased as the droplet size increased, and multiple plasma-forming regions were created along the laser beam path in front of its focal point. Similar results were obtained using a two-phase spray flow by Kawahara et al., who suggested that the droplet lens effect may explain the displacement of a laser-induced plasma [18]. To produce shadowgraph images for a planar laser sheet (Figs. 2(c) and 2(d)), the focal length of the last lens of the sheet optics was 300 mm and the energy of the laser beam was set to 968 mJ, significantly higher than the energy required for a single breakdown. Under these conditions, the region of the laser sheet, in which the laser energy density exceeds the breakdown threshold, becomes widely distributed. In Fig. 2(d), the formation of hundreds of plasmas can be seen together with their propagation and combination. However, in the case of air without droplets, this sheet laser beam cannot cause air breakdown because the breakdown threshold in air is considerably higher than it is in the gasoline spray flow. The regions of plasma sheet formation with respect to certain gasoline flow rates are shown in Fig. 3. On average, multiple plasma were formed with a height of 20.0 mm and a width of 17.2 mm. The focal line of the laser sheet was located right above the center of the siphon nozzle, but most of the plasma-forming regions were in front of the focal line. When a laser beam induces a plasma, a large amount of energy is absorbed by the medium, which leads to an immense reduction in energy [19]. For this reason, it is assumed that, after multiple plasma-forming regions were generated, the laser irradiance beyond the breakdown threshold from the front side of the focal line loses its energy. As seen in Fig. 3, an increase in the gasoline flow rate of the spray is followed by the overall plasma region being slightly shifted forward, toward the direction of the incoming laser. This indicates that higher gasoline flow rates cause the breakdown to occur closer to the laser. Our previous study presented an analysis of the relation between the breakdown threshold and the droplet size [17], which demonstrated that the breakdown threshold decreases with an increase in the droplet size. This phenomenon was also observed in this experiment, with the droplet size increasing as the gasoline flow rate increased, resulting in the decrease of the breakdown threshold of the total flow. As the breakdown threshold decreased, the location of the breakdown region shifted in the direction of the incoming laser.    [7]. This result flow. The inten urally due to a divide the two he laser energy e obtained by d n in Fig. 6(c). O the laser shee e H/O intensi urve in Fig. 6

Conclusion
Multiple plasma-forming regions were generated in a two-phase flow and used to measure the two-dimensional fuel distribution in a gasoline spray flow. Multiple plasma-forming regions were obtained by enlarging the area of the laser sheet having the irradiance above the breakdown threshold of the liquid, which is substantially lower than the air. To determine the distribution and characteristics of the multiple plasma-forming regions, a high-speed camera was used for the shadowgraph imaging which confirmed the plasma formation. The collected bandpass-filtered images of the multiple plasma-forming regions led to detection of the H and O atomic line emissions. The distribution of H/O intensity in a spray nozzle flow at each flow rate was confirmed by dividing the data from the H-filtered image by that of the O-filtered image, and then converted into the equivalence ratio, using the calibration curve acquired from the uniform droplet stream. The results were validated by comparison with those of the conventional LIBS system applied to the same flow condition. The H/O intensity ratio obtained from both the filtered plasma sheet imaging and LIBS at the same location was similar in scale and maintained the observed spreading behavior of the fuel in the spray flow. Thus, the present technique advances the instantaneous measurement of the two-dimensional distribution of fuel in the combustion chamber of liquid-fueled engines.