Nitrous Oxide

Mar 12, 2014 - In the combustion process, flame structures are commonly determined by the atomization performance and relative velocity ratio of each ...
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Atomization and Combustion Characteristics of Ethanol/Nitrous Oxide at Various Momentum Flux Ratios Inchul Lee, Min Son, and Jaye Koo* *

School of Aerospace and Mechanical Engineering, Korea Aerospace University, 76, Hanggongdaehang-ro, Deokyang-gu, Goyang-si, Gyeonggi-do, Republic of Korea ABSTRACT: The flame structure of ethanol and nitrous oxide combustion was experimentally studied using a tricoaxial injector at various momentum flux ratios. The objects of the study were to investigate the effects of the additional supplement of nitrous oxide that gas jets inject at the outer annular gap at various injection velocities and to obtain and analyze the flame structure. The fully developed patterns due to the transfer of momentum and viscosity mixing were similar to that of the axial flow when the measurement position was increased from Z/d = 1 to Z/d = 10. The correlation equation, using the momentum flux ratio, was used along with the linear regression method to calculate the breakup length and Sauter mean diameter (SMD). The effects were clearly observed and significant. The inner gas injection caused the SMD to decrease, and the outer gas injection was able to create a boundary layer around the spray jets. As the momentum flux ratio of the inner gas jets increased, the spray angle and flame angle increased. OH radicals extended toward the rear flame region with the increase in momentum flux ratio from the inner gas jets. As the momentum flux of the outer gas jets increased, the boundary of the OH radicals developed and the intensity of the OH radicals generated in the flame region was enhanced. Also, the flame temperature increased as gas was injected from the outer-stage.

1. INTRODUCTION Since the 1960s, toxic propellants with dinitrogen tetroxide (N2O4), hydrazine (N2H4), monomethyl hydrazine (MMH), and unsymmetrical dimethyl hydrazine (UDMH) have been used for propulsion systems. However, for future aerospace applications, due to its capabilities as a green propellant, ethanol (C2H5OH) offers many benefits in both combustion performance and adherence to pollutant regulations. Ethanol and nitrous oxide (N2O) have nontoxic characteristics and do not need self-contained atmospheric protective ensemble (SCAPE) operations. Nitrous oxide is usually stored as a gas and in cryogenic fluids at an ambient pressure of −91 °C. This makes its thermal conditioning handling and management easier. As such, nitrous oxide can realize cryogenic states with greater ease than liquid oxygen or liquid hydrogen.1 Shear coaxial injectors are used in many liquid rocket engines, including space shuttle main engines (SSMEs), Vulcains, Vincis, RL-10s, and J-2s.1 Liquid rocket engines require a high flow injector assembly to supply the excessive propellants. For example, the gas-generator of a unit element injector is limited to a flow rate of 100−200 g/s, and the main chamber is also limited to 500−1000 g/s. In the case of the preburners, the oxidizer gas runs the turbo pump and is injected into the main combustion chamber. In the combustion process, flame structures are commonly determined by the atomization performance and relative velocity ratio of each phase of the propellants. However, in the case of the shear coaxial injector, mixing creates a problem for the combustion system. Shear coaxial injectors have center posts and gas posts installed in the injector head of the liquid rocket, and this translates to design problems and additional manufacturing costs. Shear coaxial injectors have drawbacks in terms of their high flow rate conditions and the fabrication process with respect to the post and sleeve.2 The high velocity gas at the © 2014 American Chemical Society

annular gap affects the shear forces at the center of the liquid column, which undergoes breakup and mixing processes with the gases. The flow characteristics of the gas jets injected by a shear coaxial injector have been the subject of active research due to the fact that the breakup process of the liquid jet is governed by the shear forces of the gas jets. In a combustion system, diffusion flames with reaction kinetics are controlled by the momentum ratio or by turbulent mixing. To understand the mixing phenomenon in the flow field, many studies have been conducted using cold flow tests with hot-wire and pilot tubes and various velocity conditions. Forstall and Shapiro 3 investigated the flow field with focus on the density, temperature, and velocity at various injection velocity ratios. They concluded that it was the most important parameter in changing the flow configuration and velocity distribution. Champagne and Wyganski4 showed that neglecting the total and static pressure had an effect on large errors in the velocity fields. In an effort to study the velocity distributions in the near field of the coaxial jets, Moon5 investigated static pressure, mean velocity, and turbulent intensity in the developing region of nonreactive coaxial jets. He concluded that the effects of pressure gradients could significantly influence the mixing and combustion process and that the turbulence mixing models did not appear to be adequate to predict the flow in the developing region of the coaxial jet. The spray characteristics of the atomized and mixed propellant injected into the combustor vary as a result of the momentum ratio and velocity ratio. Generally, combustion stability and flame structures are Received: November 15, 2013 Revised: March 9, 2014 Published: March 12, 2014 2770

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determined by the momentum flux ratio, which changes the homogeneous droplet distributions and the dynamic process of the liquid phase breakup. The atomization process initiated by the shear coaxial injector is governed by the interaction of the shear forces on the liquid column, which experiences internal instability in the form of turbulent motion. Combustion performances are mainly influenced by atomization characteristicsbreakup lengths, angle, mass flux distributions, and droplet mean diameters. The atomization characteristics of shear coaxial injectors have been widely studied for their applications with liquid rocket engines. Eroglu et al.6 studied the potential core length using a shear coaxial injector and showed an empirical correlation that was derived for the purpose of predicting the liquid core length. They maintained that the Reynolds number and the aerodynamic Weber number were the crucial parameters with respect to atomizing the liquid jet. Farago and Chigier7 showed three breakup modesthe Rayleigh type mode, membrane type mode, and fiber type breakup modein which the Weber number had an aerodynamic effect that ranged from 100 to 500. Rehab et al.8 investigated the near field flow structure of shear coaxial jets with high gas/liquid velocity ratios. They demonstrated that the primary instability mechanism was the jet-preferred mode with high frequency oscillation. Lasheras et al.9 studied near and far field breakup and atomization characteristics. They proposed a simple correlation equation for the liquid core length and described the dependence of shedding frequencies on the momentum ratio. As an effect of the momentum ratio, the liquid and gas velocities were studied in research using a shear coaxial injector. The momentum ratio is very important due to the fact that it can be used to compare fluids of densities and velocities. Sankar et al.10 and Glogowski et al.11 concluded that increasing the gas velocity helped to improve the breakup process. Spray behavior related to combustion has been studied as one of the important characteristics controlling the combustion performance of liquid propellant sprays. A lot of research has made significant contributions to the understanding of the effects of flame and spray behavior. However, combustion characteristics using a tricoaxial injector configured with an inner- and outer-stage have rarely been studied. Previous studies by Oefelein on atomization and combustion characteristics with a shear coaxial injector have looked into the shear coaxial injection process in liquid rocket combustion.12 In his research, Oefelein13 found that surface tension forces emanated a heterogeneous spray and that the diminished intermolecular forces increased a diffusion process prior to atomization and jet vaporization. Another investigation of atomization and combustion with a shear coaxial injector was presented by Mayer et al.14 They observed that the spray, including various droplets, no longer existed at combustion conditions, and they noted that the flame existed in the recirculation zone behind the injector post. They also determined that length and OH radicals chemiluminescence had very crucial roles for flame stabilization, since the performance frequently changed to the velocity ratio or momentum ratio. Generally, the liftoff length increased linearly with gas jet velocities and was based on the balance between the mean propagation speed and mean flow velocity.15 There are many studies that probe the flame structures of gas−gas phases with premixed and nonpremixed conditions.16−19 However, studies on specific flame structures using a liquid−gas phase are rather scanty even though the combustion performance in terms of numerical data and

experimental results for liquid rockets includes the velocity, pressure, and frequency characteristics in a combustor. Also, there are no specific studies on the influence of the momentum flux ratio and different design parameters of injectors on atomization and combustion processes. In order to obtain a better understanding of the gas flow structure of a tricoaxial injector that causes the atomization and combustion performances, measurements of the velocity injected into the inner- and outer-stages were conducted with precision. The aims of the present work using a tricoaxial injector were to investigate the velocity distribution, atomization characteristics, and flame structures with ethanol and nitrous oxide fuel at various momentum flux ratios. The focuses are liftoff length, flame structure, and OH radicals chemiluminescence. Liftoff length and flame structures are observed and correlated with respect to their atomization characteristics.

2. EXPERIMENTAL APPARATUS AND PROCEDURES Atomized jets undergo liquid jet breakup, mixing, and a droplet combustion process. As such, the atomization performance, which depends on droplet sizes and distributions, plays an important role in the combustion process. In the case of a shear coaxial injector, the breakup of the liquid column is affected by the shear force of the coaxial gas jets injected at the annular gap. Thus, the fundamental breakup mechanisms of the shear coaxial and tricoaxial injector are the same. Breakup length is governed by the momentum flux ratio, which also has an effect on the liftoff length of the flame and the blowout characteristics. A tricoaxial injector can increase the mass flow rate of the gas phase and control the flame angle. This results in the determination of the spray angle. By obtaining experimental results, helpful information regarding the design of a combustion chamber and injector is provided. The schematic of the experimental setup for the spray test is presented in Figure 1. The setup was composed of a tricoaxial injector, laser diffraction system, CCD camera, liquid reservoir with a capacity of 40 L, air reservoir with a capacity of 1000 L, needle valve, and planoconvex lens with a focal length of 500 mm. The tricoaxial injector was the most important part of this setup, since it injects propellants and properly atomizes the liquid jets. Spray images of the breakup phenomenon were captured with a Photron SA 1.1 high speed CCD camera with a 4 μs exposure time and 1024 × 1024 resolution. For each experiment, a large number of spray images were obtained and used to analyze the breakup of the spray jets. On average, 200 images were averaged for each test condition in order to reduce any experimental uncertainty due to the instability of the spray structure. Volume flow rates with gas jets were measured by multiple nozzles in a

Figure 1. Experimental setup for spray test with cold flow conditions. 2771

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chamber that was designed to use the standard methods provided by ANSI/AMCA 210-07, ANSI/ASHRAE 51-07, and KS B 0062. The experimental conditions of the ambient temperature were 293 ± 5 K, the humidity was 50%, and the atmospheric pressure was 105.2 ± 2.0 kPa. The velocity of the gas was measured using hot-wire anemometry due to the fact that the major effect of the breakup process is the shear forces induced by gases at the annular gap. To carry out this process, a Dantec CTA module 90C10 and 55P11 probe with an accuracy of 0.3% was used at measurement positions of 1, 3, 5, and 10 mm at the exit of the tricoaxial injector. To study atomization and gaseous velocity around the flow fields, water and air were used. The experimental ranges of the water included an injection pressure of 5− 100 kPa, injection velocity of 2.5−10.3, Reynolds number of 2686− 10985, and Weber number of 93−1563. A summary of inner-stage and outer-stage operating conditions is shown in Table 1. The experimental setup was put in place, and a combustion test (Figure 2) using a tricoaxial injector was conducted to analyze the flame shape, temperature distribution, and OH radicals around the spray combustion fields. Temperature distributions were measured using an R-type thermocouple, and the OH radicals distributed in the high temperature region by the chemical reaction were visualized with a band-pass filter (UG11) with a range of 240− 395 nm and a short-pass filter (OD2) with a range of 0−425 nm. Ethanol and nitrous oxide were used as propellants, and the injection pressures of both propellants were controlled with a microneedle valve. The classifications of the gas injection modes are shown in Figure 3, namely the inner-stage injection mode, outer-stage injection mode, and tricoaxial injection mode. The breakup mechanism of the inner-stage injection mode was the same as the that of the previous shear coaxial injector. In the case of the inner-stage injection mode, the inner gas was injected at the exit of din. The outer-stage gas, which did not offer efficient atomization, was injected at the exit of dout. In the case of the tricoaxial injection mode, the air-blast gases were injected at both the din and dout annular gaps. Detailed specifications and a crosssectional view of the tricoaxial injector are illustrated in Figure 4. Droplet sizes were measured using the laser diffraction apparatus, with the Mie scattering technique considering the liquid sphere. Droplet sizes were measured with a refractive value (n) ratio of 1.33169, an extinction value (k) of 1.4680 × 10−8, and an absorption value (α) of 2.9152 × 10−3 cm−1. This value was adapted to the experimental results by Hale et al.,20 as illustrated in Figure 5. The laser diffraction system was used to acquire droplet information. The spray jets injected at the tricoaxial injector were located between the transmitter and the receiver. The distance between them was about 500 mm. The laser beam was moved along the center axis of the tricoaxial injector. Ligaments and nonuniform spherical droplets that were not able to be measured precisely with laser diffraction anemometry could not be detected for the detailed spray jet determination. The laser diffraction system (Helos/VarioKF) specifications included a 5 mW, 633 nm He−Ne laser with a 29 mm beam diameter with a total accuracy of 1.15%, which included repeatability and comparability. The receiver had 31-channel multielement detector rings and three radial centering elements that were able to detect droplet diameters as small as 0.1 μm for fully spherical droplets. For the unsteady breakup process of atomized jets, the laser diffraction instrument was set to sample the diffracted light signal at a maximum acquiring rate of 100 Hz, and it averaged all the data. Measured data were considered with ISO-13320-1, that is, the number

Figure 2. Experimental test setup for combustion test. of particles in the working laser beam where the optical concentration was below 25%.

3. RESULTS When studying spray combustion, it is useful to analyze the gas flow and atomization characteristics. (For example, a blowout of diffusion flames might emerge with high velocity gas jets.) The gas flow area formed by the tricoaxial injector was classified into the main flow region, transition region, and fully developed region. In the case of the tricoaxial injector, gas jets were injected into the inner and outer post gaps. Accordingly, turbulence strength and mixing characteristics could be controlled by the momentum flux ratio. Downstream of the flow region, axial velocities decreased and the jet spread angle increased to a greater width. The decreasing velocity near the injector exit was caused by the pressure gradients and viscous mixing. The viscosity effects caused a faster decrease in the velocity at the exit of the injector tip. Self-similarity characteristics, which explained the fully developed structure of the gas flow, appeared further downstream. Shear coaxial injectors composed of a center liquid nozzle and outer gas nozzle with an annular gap were usually used to break up the liquid jets. All of these experimental data were used to study spray combustion characteristics with liftoff lengths and flame structures. In order to investigate the flow structure of the tricoaxial injector at various Reynolds numbers, hot-wire anemometry was used. Gas velocity measurements were conducted under ambient fields with injection Reynolds numbers in the 877− 2687 range. The axial velocities for the gas injection of the inner-stage are illustrated in Figure 6. The gas velocities at R/d = 0 and z/d = 1 appeared due to the viscosity and pressure effects. When the injection pressure of the inner-stage increased, the gas velocity was measured at 138.0 m/s at z/d = 1, and the gas velocity was at its lowest at R/d = 0. The core velocity from z/d = 1 to z/d = 3 had a greater effect on pressure rather than viscosity. The viscosity effect made the gas velocities of the center axis increase with the same conditions as the boundary velocity. However, the highest velocities were maintained within z/d = 10. When the velocity profiles at z/ d = 3 and z/d = 5 were viewed, the breakup of the liquid column appeared at z/d = 5 due to the fact that the gas velocities caused the shear forces in the liquid column. For the measurement position of z/d = 10, the profiles of the gas velocity at various injection Reynolds numbers show fully developed shapes. To analyze the flow structures of the tricoaxial injector, the Reynolds numbers of the gas jets at the inner-stage and outer-stage were fixed to 877 and 1918, respectively. One of the critical parameters of the tricoaxial

Table 1. Inner-Stage and Outer-Stage Operating Conditions parameters

inner-stage and outer-stage expt ranges

injection pressure (kPa) injection velocity (m/s) mass flow rate (g/s) momentum flux ratio Weber number Reynolds number

2−40 54.4−166.8 0.7−2.1 0.29−2.72 42.8−476.6 877−2687 2772

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Figure 3. Tricoaxial injector gas injection methods.

Figure 4. Tricoaxial injector cross-sectional view and specifications.

Figure 5. Refractive and extinction curves at various wavelengths.20

outer gas jets were injected at each condition at 877 and 1918, the flow structures of the two peaks with maximum velocities were combined at z/d = 5 due to the viscosity effect. The outerstage gas injection did not affect the breakup process due to the fact that the liquid column was only injected between R/d = −10 and R/d = 10. The shear forces only appeared for 100 m/s at the region of R/d = −35 and R/d = 35. As a result, the innerstage gas injection significantly enhanced the breakup of the liquid column, and the outer-stage gas injection was covered with the spray jet. Also, the outer-stage gas flow decreased the overspray phenomenon and the spray angle during the breakup process. The gas flow injected by the outer-stage appeared in a

injector was the velocities of the gas jets at the inner- and outerstages. The gas injections of the inner- and outer-stages created important effects in terms of atomization and combustion. Figure 7 shows a comparison of the axial velocity distributions of the tricoaxial injector at various injection Reynolds numbers. This was done to investigate the influence and interference of the inner and outer flows. The gas velocities at various injection Reynolds numbers at the inner- and outer-stages also show separated profile peak structures. The gas velocity peak structures are shown at z/d = 1 and z/d = 3. Within z/d = 3, the gas injection of the inner- and outer-stages did not affect the flow combination of each gas flow. When the inner and 2773

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had less gas velocities than did the gas injection of the innerstage. Further increasing the measurement length to greater than z/d = 50 significantly helped in the formation of fully developed structures, which showed the highest velocity at R/d = 0. The pure liquid jets injected into the ambient field showed a fluctuation in their column wave. The basic mechanism of a liquid jet breakup is the effects of hydrodynamic forces with turbulent energy. However, in the case of the tricoaxial injector, it uses shear forces to break the liquid column. Gas jets with shear forces are a more important parameter in influencing the liquid column. Acting on shear forces produces an instability in the liquid jets that takes the form of irregular ligaments on column surfaces. Spray images at various momentum flux ratios of the inner- and outer-stages are shown in Figure 8. Min can be defined as the inner-stage momentum flux ratio, and Mout is the outer-stage momentum flux ratio. The shear forces acted on the liquid column that was injected at the center of the nozzle. As the momentum flux ratio increased, the liquid column experienced an atomization process. At the momentum ratio of 0.29, there were small effects in terms of the shear forces so that the liquid column partially broke the liquid column, and the deformation of the liquid column appeared within z/d = 10. The appropriate state with no liquid jet ligaments to be burned showed a momentum flux ratio over 1.38. However, as the droplet diameters decreased, the gas injection velocity (i.e., shear forces) also increased, and a flame blowout occurred. The momentum flux ratio is very important in determining the liftoff lengths and to retain flame propagation. Figure 9 illustrates the tricoaxial spray images at various momentum flux ratios. At the momentum flux ratio fixed conditions of the inner-stage with 0.56, the liquid column was deformed by the shear forces caused by the gas jets of the outer-stage. However, there were no significant effects in terms of the shear forces breaking the liquid column for the mistlike droplets. Also, it should be noted that such effects could affect the overspray characteristics, spray angle, and flame angle due to the outer gas jet being enclosed and covered by the spray jet. Figure 9b shows the spray images at an innerstage fixed momentum ratio of 1.38 and outer-stage fixed momentum ratios of 0.29 and 2.45. The overall spray patterns were the same at the momentum flux ratio of the outer-stage at 0.29 and 2.45. The primary effects of the breakup process were governed by the gas jet of the inner-stage. Under these conditions, the gas jets of the outer-stage could not help the liquid column to produce a fine droplet. As the outer gas jets increased, the liftoff length increased. However, the additional gas jet that exceeded the flame propagation speeds caused the blow-off characteristics. The liquid column breakup was governed by the aerodynamic forces related to the shear force at the inner- and outer-stages. The breakup length was measured as the average of 100 photos taken using the shadow graph method. In the experimental case of the inner gas injection, from the measured data of the breakup length of the liquid column versus the momentum flux, the empirical correlation equation that considers the linear interpolation and linear multiple regression methods was expressed with eq 1. The calculated and measured liquid column breakup lengths are shown in Figure 10. In the present study, the momentum flux ratio was used for the nondimensional numbers to define the breakup length of the liquid column. In this equation, the R2 coefficient of determination was 94.1%. As the momentum flux ratio increased, the liquid column breakup length (lb/d)

Figure 6. Axial velocities at various Z/d and Reynolds numbers.

Figure 7. Comparison of axial velocity distributions at various injection Reynolds numbers.

partially developed shape with two peak structures at z/d = 10. However, further down the flow field stream, a fully developed gas flow structure appeared, and a boundary layer enclosed the center of the gas flows. The gas flow injected at the outer-stage 2774

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Figure 8. Spray images at various momentum flux ratios for the inner- and outer-stages.

Figure 9. Spray images at various momentum flux ratios.

depended on the momentum flux ratio of the inner- and outerstages, is shown in Figures 11 and 12. The SMD is presented for a wide range (0.29−2.72) of momentum flux ratios. The overall SMD decreased continuously from 360 to 35 μm as the momentum flux ratio increased. In the case of the inner-stage fixed momentum flux ratio of 0.29, the overall SMD value was distributed from 360 to 35 μm. At the inner-stage fixed momentum flux ratio of 1.11, the SMD was not significantly changed due to the fact that the gas jets of the outer-stage could not affect the shear force at the center of the liquid column, and the gas jet injected at the inner-stage had greater shear forces than the outer-stage. As expected, the effects were clearly observed and significant. The inner gas injection caused the SMD to decrease, and the outer gas injection was able to create a boundary layer around the spray jets. The spray jets were subjected to shear forces that affected their flow motion and

decreased, and the scattered data fit well with the R2 = 100% line. At the momentum flux ratio from 0.29 to 0.57 with the inner-stage gas injection, as the momentum flux ratio of the outer-stage gas injection increased, the breakup length (lb/d) continued to show the same data. lb/d = (9.57 − 0.5180Mout)M in(−0.58 + 0.062Mout)

(1)

The Sauter mean diameter (SMD, D32), which can be defined as the volume/surface area ratio as a particle of interest, was used to investigate the atomization performance. Droplet sizes were measured using a laser diffraction analyzer that had measurement errors for an accuracy of 0.3%, repeatability of 0.5%, and comparability of 1.0%. The combined standard uncertainty, which is the standard uncertainty and expanded uncertainty, was calculated at ±5.85 μm. The SMD, which 2775

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2 was expressed by using the linear interpolation and linear multiple regression methods. In this equation, the R2 coefficient of determination was 97.9%. The calculated and measured SMD is shown in Figure 10. In the present study, the momentum flux ratio was used as the nondimensional number to define the breakup length of the liquid column. In this equation, the R2 coefficient of determination was 97.9%. Unlike the breakup length trends, all of the tricoaxial injector momentum flux ratios and SMD distributions fit well with the R2 = 100% line. SMD = (109.83 − 18.34Mout)M in(−1.16 + 0.28Mout)

(2)

The flame structures were studied by using a tricoaxial injector with ethanol and nitrous oxide. Ethanol was injected at the center of the nozzle, and nitrous oxide was injected at the annular gaps of the inner- and outer-stages. In the case of the shear coaxial injector, as the shear forces increased, the atomization performances increased and the blowoff characteristics of the diffusion flame appeared. The liftoff length was defined as the height of the spray flame at the exit of the tricoaxial injector. Figure 13 shows the flame structure of the inner-stage gas injection with momentum flux ratios of 0.29 and 1.38. The liftoff length appeared at 33 mm, and a large scale vortex flame structure also emerged at the momentum flux ratio of 0.29. Also, under a momentum flux ratio of 0.29, unburned fuel droplets that the SMD distributed to 200 μm were combusted further downstream from the flame. Under this condition, the shear forces of the inner gas jet were insufficient to make fine droplets for combustion. As can be seen in Figure 13, the spray flame at a momentum flux ratio of 1.38 had a tendency to create straightforward structures downstream. However, the spray flame with a momentum flux ratio of 0.29 went toward the upward direction due to the fact that weak shear forces could not break the liquid jets properly and enclose the spray flame. Figure 14 shows that the gas jet of the outerstage was able to change the spray flame angle, liftoff length, and flame color at a fixed inner-stage momentum of 1.38 and outer-stage momentum flux ratios of 0.29 and 1.38. The reason for these results can be explained with the images of the spray flame. From the analysis of the spray flame behavior, it was understood that ethanol and nitrous oxide flames moved

Figure 10. Calculated and measured liquid column breakup lengths at various momentum flux ratios.

Figure 11. SMD distributions at various momentum flux ratios.

Figure 12. Comparison between the measured and calculated SMD.

breakup processes in the flow field. To derive an empirical equation at various tricoaxial injector momentum flux ratios, eq

Figure 13. Inner-stage gas injection flame structures at Min = 0.29 and Min = 1.38. 2776

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0.29. This implies that the increase of gas jets at the outer-stage was able to play a significant role in increasing the OH radicals. Subsequently, the OH radical distributions along the boundary layer of the spray flame were governed by the outer gas jets. Liftoff lengths and normalized breakup lengths are shown in Figure 17, where the momentum flux ratio of the inner-stage is fixed to 0.57, 1.38, and 2.19, and the outer-stage varies from 0.29 to 2.72. As depicted in Figure 17, the effect of the outerstage was further studied by analyzing the plotted data at various momentum flux ratios at fixed inner-stage momentum flux ratios. As can be seen in Figure 17, despite increasing the momentum flux ratio of the outer-stage, breakup length did not change at the fixed inner-stage momentum flux ratio. When the inner-stage momentum flux ratio was 1.38 and 2.19, the breakup length ranged from 5 to 7. However, the normalized liftoff length appeared to have similar trends with 45 due to the fact that the outer-stage’s only role was to decrease the flame angle, which increased the flame temperature. In addition, the gas jet injected at the outer-stage induced the blowout characteristics over the momentum flux ratio of 1.6. The liftoff lengths and SMD versus momentum flux ratio of the tricoaxial injector are plotted in Figure 18. In the case of the outer-stage, the flame blowout appeared in all experimental conditions. This is because the gas jets of the outer-stage were not able to break the liquid column for spray combustion that satisfied the droplet sizes below 225 μm. Although the SMD was produced to 255 μm, spray combustion did not emerge, due to the fact that the shear coaxial jets injected at the outer-stage exceeded the flame propagation speed so that the spray flame could not achieve liftoff under those experimental conditions. As can be seen in the data for the inner-stage gas injection, SMD varied from 200 to 50 μm. Under these conditions, liftoff length increased to 43 as the momentum flux ratio increased to 2.72. However, as the momentum flux ratio increased over 2.72, the partial mixture ratio changed to an oxidizer rich condition, and at the high temperature region, the gas jet that had been injected at the inner-stage caused a blowout of the existing spray flame. This also explained why the flame propagation speed exceeded the maintenance speed of the spray flame. The gas velocity of the inner-stage gas jet with a momentum flux ratio of 0.29 was 54.4 m/s, and the liftoff length was 33 mm at the tricoaxial injector tip. From the results of Figures 17 and 18, the gas jet of the outer-stage was not able to change the breakup length. Rather, it only affected the liftoff length with a momentum flux ratio of 1.6. The flame temperature characteristics at various tricoaxial injector momentum flux ratios are shown in Figure 19. To measure the flame temperature, an Rtype thermocouple was used, and flame temperature was

Figure 14. Tricoaxial injection flame structure at fixed Min = 1.38, Mout = 0.29, and Mout = 1.38.

downstream of the injector as the momentum flux ratio of the inner- and outer-stages increased. The additional gas jets decreased the spray flame angle and increased the liftoff length. The outer-stage momentum flux ratio of 1.38 was brighter than the momentum flux ratio of 0.29, which means that it added more oxidizer into the spray flame. A band-pass filter and short-pass filter were used to visualize the OH radicals that the time averaged images calculated using the Abel transformation, and this was used to analyze the cross section of the axis symmetry flame. Generally, OH radicals emerged at the conditions of the initial combustion region in which stable molecules combined separately and very quickly. OH radicals, which existed at the chemical reaction region at a high temperature, appeared through the fuel and oxidizer boundary of the flame front. The characteristics of the OH radical distribution were analyzed and compared with the spray flame images, as shown in Figures 15 and 16. In Figure 15, different OH radical structures are shown. This means that the shear coaxial jets strongly affected the spray flame structures and OH radical distributions. In the case of the momentum flux ratio of 0.29, the OH radicals were concentrated in front of the spray flame region, where the atomization characteristics were poor. The OH radicals with narrow band shapes appeared at the momentum flux ratio of 1.38 along the outer boundary of the spray flame. The shear force effect caused the spray flame angle to decrease, and the OH radicals were distributed further downstream. As can be seen in Figure 16, the strength of the OH radicals at the outer-stage momentum flux ratio of 1.38 emerged to a greater extent than the momentum flux ratio of

Figure 15. OH radical images at various inner-stage gas injections using the Abel transformation at Min = 0.29 and Min = 1.38. 2777

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Figure 16. OH radical images at various inner-stage gas injection momentum flux ratios using the Abel transformation at fixed Min = 1.38 and Mout = 0.29, Mout = 1.38.

Figure 17. Normalized liftoff length and breakup lengths. Figure 19. Flame stabilization characteristics according to SMD and momentum flux.

additional gas injection of the outer-stage helped the mixing process with the existing spray flame via the effect of the innerstage.

4. CONCLUSION A tricoaxial injector was used to experimentally study spray breakup and flame characteristics through a comparison of inner- and outer-stage gas injections. The liquid jet breakup process was widely influenced by the shear force as the droplet mean diameters decreased with an increase in the shear force of the gas jets at the exit of the annular gap. Moreover, the flame structure, liftoff length, and flame angle also changed with shear force variance. In this study, the overall characteristics of the axial gas flow, macroscopic breakup, distributions of droplet mean diameters, and flame structures were investigated using hot-wire anemometry, a spray and flame visualization system, as well as a laser diffraction system. The variances on the flow structures in the axial direction at various gas injection ratios for the inner- and outer-stages were studied throughout the experiments, and the following conclusions were reached. The gas jets injected at the outer-stage showed complete mixing characteristics at Z/d = 10, where a similar phenomenon was observed at Z/d = 5 for the gas injection made at the innerstage. With the increase in Reynolds number for the outer-stage injection of the tricoaxial injector, a phenomenon was observed in which an outer layer was formed by an annular gap along with an outer-stage gas jet around the inner gas jet.

Figure 18. Normalized liftoff lengths at various momentum.

measured 300 mm downstream from the tricoaxial injector, where the thermocouple probe moved 10 mm toward the center axis. Flame temperature generally decreased as the measurement point moved toward the outer boundary. When the inner-stage of the momentum flux ratio increased, at the radial distance of 0 mm, which was the center region, the overall temperature of the spray flame increased from 925 °C to 1055 °C. However, at a fixed inner-stage of the momentum flux ratio, as the gas jets of the outer-stage increased, the flame temperate also increased from 1100 °C to 1250 °C at the center of the spray flame. These results indicate that the 2778

dx.doi.org/10.1021/ef402251s | Energy Fuels 2014, 28, 2770−2779

Energy & Fuels

Article

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Also, with the increase in the momentum flux ratio for the outer-stage injection, the spray angle decreased due to the annular gas jets. The correlation equation, using the momentum flux ratio, was used along with the linear regression method for the calculation of breakup length and SMD. Spray breakup increased as the momentum flux ratio of the innerstage increased. However, the spray jets dispersed to the outer spray field. The flame angle was reduced with the injection of gas from the outer-stage. Also, a fuel rich zone in the rear flame region decreased with the additional supply of gas near the flame boundary. Moreover, OH radicals extended toward the rear flame region with the increase in momentum flux ratio from the inner-stage. As a result of the increasing momentum flux of the outer-stage injection, the boundary of OH radicals evolved, and the intensity of the OH radicals generated in the flame region was enhanced. Also, the flame temperature increased as gas was injected from the outer-stage. The distributions of the axial gas flow due to the different gas injection ratios of the tricoaxial injector were measured, and the correlation between the flow distribution and the liquid breakup was established. In addition, the experimental results of the flame structures and temperature distributions were plotted and presented.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.:+82 10 4732 1316. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korean Government (MEST) (NRF-2012M1A3A3A02033146).

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NOMENCLATURE d = liquid nozzle diameter (mm) dinner = inner gas nozzle diameter (mm) douter = outer gas nozzle diameter (mm) tgas‑1 = inner gas nozzle thickness (mm) lo = liquid nozzle orifice length (mm) lb = liquid column breakup length Rrecess = recess length (mm) θliquid = liquid nozzle inlet angle (deg) θinner = inner gas nozzle inlet angle (deg) θouter = outer gas nozzle inlet angle (deg) M = momentum flux ratio (ρgv2g/ρ1v21) Min = inner-stage momentum flux ratio (ρg-inv2g‑in/ρ1v21) Mout = outer-stage momentum flux ratio (ρg-outv2g‑out/ρ1v21) Pg = air pressure (bar) Rein = inner-stage Reynolds number (ρνind/μ) Reout = outer-stage Reynolds number (ρνoutd/μ) SMD(D32) = average of particle size, Sauter mean diameter (μm) Vin = inner-stage gas velocity Vout = outer-stage gas velocity Vl = liquid velocity

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REFERENCES

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dx.doi.org/10.1021/ef402251s | Energy Fuels 2014, 28, 2770−2779