energies Article
Characteristics of Soot and Radiation of Post Combustion Simulated Gas from a Gas Generator Hakduck Kim and Juhun Song *
ID
School of Mechanical Engineering, Pusan National University, Busan 46241, Korea;
[email protected] * Correspondence:
[email protected]; Tel.: +82-51-510-7330; Fax: +82-51-512-5236
Received: 31 May 2018; Accepted: 29 June 2018; Published: 2 July 2018
Abstract: It is necessary to install a specific burner system to burn out fuel-rich post combustion gas produced from a gas generator in a rocket engine when the performance of gas generator is separately evaluated in a test facility. Because of the fuel-rich reburning conditions, the burner still emits a significant amount of soot and produces thermal radiation. In this study, a laboratory-scale coflow diffusion burner was developed to examine the effects of fuel composition in combustion products on the soot emission and radiation behavior. The post combustion gas was simulated by adding carbon monoxide and carbon dioxide to two different base fuels: kerosene vapor and ethylene. The radiation flux sensor and laser extinction apparatus were used to measure the radiation intensity and soot emission, respectively, within a flame. The flame length and temperature were measured to examine the combustion behavior of each sooting flame having a strong radiation. Finally, the relationship between soot emission and radiation intensity was proposed based on all the experimental data. Keywords: gas generator; fuel rich; post combustion gas; soot emission; radiation
1. Introduction In Korea, a test facility was developed to separately evaluate the components of rocket engines, such as the turbopump, gas generator, and combustor. In some instances, this test, conducted around the facility, produces large amounts of combustion gas products, including soot. Accordingly, because of the soot emission, radiative heat is produced, creating environmental and safety issues for humans and buildings in the surrounding area. An example of the source of these soot and heat is the reburning system for combustion products from the gas generator, which is required for the separate testing of the gas generator. Because it is operated under fuel-rich conditions to maintain the exit temperature below 1000 ◦ C, the flue gas may contain significant amounts of unburned fuel vapor, partially burned products, such as carbon monoxide (CO), hydrogen (H2 ), and carbon dioxide (CO2 ) [1,2]. Under an insufficient air supply condition, the reburning of these combustion products would still produce large amounts of soot. The soot is formed from aromatic soot precursors to soot aggregates by the complex process of surface growth, coagulation and agglomeration, and oxidation. Fuel types and fuel dilutions are known to change the size and morphology of soot. In an ethylene-based flame, such an effect of the fuel additive is well evaluated and documented in literature [3–6]. A strong soot suppressing effect was reported, particularly when using CO2 as the diluent [7,8]. In addition, the effect of different hydrocarbon components in kerosene fuel was evaluated on soot formation [9]. Previous studies have shown that soot levels in flames are proportional to aromatics concentrations in the fuel. However, it is not clear how each component diluted with pre-vaporized kerosene fuel in real combustion products of a gas generator would affect the soot emission and resultant radiation from the flame during the reburning process. In general, a fire hazard or safety issue is closely related with flame radiation, which is mainly affected by soot emission [10,11]. The emission from soot is known to be the dominant source of radiation in the reburning system of post combustion gas. Thus, if there is no soot in a certain flame, Energies 2018, 11, 1731; doi:10.3390/en11071731
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the total radiation could be less. Some researchers found that the presence of soot in the combustion product of fuel rich mixture greatly affected the explosion behavior [12]. The higher initial temperature increases the explosion pressure significantly due to lesser soot emission and thus radiation heat loss as fuel-air mixture is at rich condition. This effect is more prevalent with methanol blended fuel because of suppression effect of the methanol on the soot formation. However, some studies showed that the presence of soot would not affect radiation [13,14]. This provides the incentive for measuring the radiation intensity and checking any dependence of the radiation on the soot emission in the reburning system fed with combustion products. In this study, a burner system for reburning combustion products was developed to examine the effect of fuel composition on the soot formation and radiation mechanism. Several different flames diluted with combustion products, including kerosene vapor, CO, and CO2 were used during the reburning experiment. Finally, a primary mechanism governing flame radiation was identified through a further analysis of experimental data. 2. Experiment 2.1. Reburning Burner and Vaporizer In this study, a reburning burner system for combustion products was developed to examine the effect of fuel composition on the soot formation and radiation mechanism. Figure 1 presents a schematic of the coaxial diffusion burner and vaporizer apparatus used in this study. The burner was composed of three concentric tubes. The fuel was supplied through the innermost tube, whereas the primary and secondary air were respectively supplied through the second and third tubes. The inner diameter of innermost tube was 7 mm. The diameter of second and third tubes were 18 and 70 mm, respectively. In the latter tubes, 2-mm-diameter alumina beads were packed to a certain height to ensure a laminar flow condition. The flame was stabilized by adjusting the amount of primary and secondary air. The flow rates of gaseous fuel and air were controlled by the mass flow controller. The flow of kerosene in the liquid phase was controlled by liquid chromatography pump and was vaporized to the gas phase in the stainless-steel tube, which is wrapped with a heating tape. This was a similar liquid fuel vaporization technique to that used by several researchers [5,15,16]. The fuel analysis summarized in Table 1 indicates that the kerosene fuel contained 79% n-paraffins, 10% cyclo-paraffins, and 11% aromatics. This fuel is known as JP 10 and is practically used in gas generators operated in test facilities. Table 1. Post combustion simulated gas composition. Composition case 1 case 2 case 3 case 4 case 5 case 6
Kerosene Vapor (%)
100 50 35
C2 H4 (%)
CO (%)
CO2 (%)
100 50 35
50 35
30
50 35
30
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Figure 1. Schematic of coflow burner and vaporizer apparatus. Figure 1. Schematic of coflow burner and vaporizer apparatus.
2.2. Burner Conditions 2.2. Burner Conditions The post combustion gas was simulated by adding carbon monoxide and carbon dioxide to two The post combustion gas was simulated by adding carbon monoxide and carbon dioxide to two different different base base fuels: fuels: kerosene kerosene vapor vapor and and gaseous gaseous ethylene. ethylene. Ethylene Ethylenewas wasselected selectedas as the the first first base base fuel because it produces a large amount of soot, as well as a stable flame condition free of blow‐off fuel because it produces a large amount of soot, as well as a stable flame condition free of blow-off behavior. In addition to this type of flame, flames diluted with a certain amount of CO and CO behavior. In addition to this type of flame, flames diluted with a certain amount of CO and CO22 were were compared in the coaxial reburning burner. The flow rate of each component, such as CO and CO 2 compared in the coaxial reburning burner. The flow rate of each component, such as CO and CO2 were were determined to provide the same heating value, whereas case 1 of the base fuel, listed in Table 1, determined to provide the same heating value, whereas case 1 of the base fuel, listed in Table 1, was a 3/min. This would result in the composition ratio of was a condition where ethylene flowed at 182 cm condition where ethylene flowed at 182 cm3 /min. This would result in the composition ratio of the the combustion products summarized in Table 1. Among cases 1–3, case 3 contained 35% fuel, 35% combustion products summarized in Table 1. Among cases 1–3, case 3 contained 35% fuel, 35% CO, CO, and 30% CO 2, which was consistent with the theoretical composition of a fuel‐rich combustion and 30% CO2 , which was consistent with the theoretical composition of a fuel-rich combustion product 2 dilution was limited below 30% because a dilution over product estimated by Kim et al. [1]. The CO estimated by Kim et al. [1]. The CO2 dilution was limited below 30% because a dilution over this this amount could produce a blow‐off condition for case 3. amount could produce a blow-off condition for case 3. For the additional experiment, kerosene vapor replaced ethylene as the base fuel. The flames For the additional experiment, kerosene vapor replaced ethylene as the base fuel. The flames were diluted with with CO and 2 in the same proportions as in the ethylene flame previously were diluted CO and COCO 2 in the same proportions as in the ethylene flame previously examined. examined. These conditions correspond to cases 4–6 in Table 1. Among all the cases, case 6 of the These conditions correspond to cases 4–6 in Table 1. Among all the cases, case 6 of the flame diluted flame diluted with three components (kerosene vapor, CO, and CO 2) would simulate real fuel‐rich with three components (kerosene vapor, CO, and CO2 ) would simulate real fuel-rich combustion 3/min, combustion products to a high extent. The HPLC pump delivered a kerosene liquid at 0.5 cm products to a high extent. The HPLC pump delivered a kerosene liquid at 0.5 cm3 /min, which was which was an amount comparable to that of the gaseous ethylene fuel in the previous experiment. an amount comparable to that of the gaseous ethylene fuel in the previous experiment. The heated The heated line of the vaporizer from the pump to the burner entrance was maintained at 300 °C to line of the vaporizer from the pump to the burner entrance was maintained at 300 ◦ C to prevent any prevent any condensation or cracking of the kerosene vapor. At the entrance of the vaporizer, the condensation or cracking of the kerosene vapor. At the entrance of the vaporizer, the carrier gas (N2 ) carrier gas (N was supplied2) was supplied at 200 cm at 200 cm3 /min, together3/min, together with the kerosene fuel. The flow rates of the with the kerosene fuel. The flow rates of the primary and 3/min, respectively. primary and secondary air were fixed at 500 and 20,000 cm secondary air were fixed at 500 and 20,000 cm3 /min, respectively. 2.3. Soot and Radiation Measurements 2.3. Soot and Radiation Measurements The laser extinction technique was used to measure the soot volume fraction at different heights The laser extinction technique was used to measure the soot volume fraction at different heights of flame above above the the burner. The same system applied for different stabilized and of the the flame burner. The same system waswas applied for different flamesflames stabilized and diluted diluted with fuel vapor, CO, and CO 2. The soot emission measurement system consisted of a He‐Ne with fuel vapor, CO, and CO2 . The soot emission measurement system consisted of a He-Ne laser with laser with a wavelength of 532 nm and two photodiodes to monitor the incident and attenuated light a wavelength of 532 nm and two photodiodes to monitor the incident and attenuated light signals [5]. signals [5]. Two bandpass were in two front of the two photodiodes. In principle, the Two bandpass filters were filters mounted in mounted front of the photodiodes. In principle, the incident light incident light with a constant intensity (Io) is projected to a soot laden flame. The soot particles could absorb and scatter the incident light so that its intensity (I) is decreased. The ratio of the attenuated
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with a constant intensity (Io ) is projected to a soot laden flame. The soot particles could absorb4 of 13 and Energies 2018, 11, x FOR PEER REVIEW scatter the incident light so that its intensity (I) is decreased. The ratio of the attenuated signal to the incident signal could represent the volume fraction of the soot present in the flame. For a proper signal to the incident signal could represent the volume fraction of the soot present in the flame. For measurement, the intensity of the incident light was adjusted to the optimal value. a proper measurement, the intensity of the incident light was adjusted to the optimal value. A radiometer A radiometer (Newport (Newport model model 70,260) 70,260) was was used used to to measure measure the the axial axial variation variation of of radiation radiation intensity along each of the different flames. It consisted of a probe and power display meter. intensity along each of the different flames. It consisted of a probe and power display meter. Two Two convex optical lenses were used to detect the radiation intensity emitted at a spatial resolution convex optical lenses were used to detect the radiation intensity emitted at a spatial resolution of 5 of 5 mm within flame. The lens the fused silicaand andprobe probecan cantransmit transmit the the radiation radiation signals mm within the the flame. The lens of of the fused silica signals at at wavelengths of 190 nm–6 µm. Both signal of laser and radiation intensity were acquired at the rate of wavelengths of 190 nm–6 μm. Both signal of laser and radiation intensity were acquired at the rate of 11 kHz and time averaged value over 60 s was used for experimental result. The experimental errors kHz and time averaged value over 60 s was used for experimental result. The experimental errors were and shown in in thethe results of laser intensity and radiation intensity in nextin relevant figures. were analyzed analyzed and shown results of laser intensity and radiation intensity next relevant Figure 2 presents the schematic of the laser extinction measurement device for the soot emission and figures. Figure 2 presents the schematic of the laser extinction measurement device for the soot the radiometer device for the radiation intensity. emission and the radiometer device for the radiation intensity.
Figure 2. Schematic of laser extinction measurement and radiometer device. Figure 2. Schematic of laser extinction measurement and radiometer device.
The temperature within each flame was measured by using the rapid insertion method. In a The temperature within each flame was measured by using the rapid insertion method. In a sooting sooting flame, the effective diameter and emissivity are affected by the soot deposition on the flame, the effective diameter and emissivity are affected by the soot deposition on the thermocouple thermocouple junction. Therefore, this effect would reduce temperature readings. To minimize this junction. Therefore, this effect would reduce temperature readings. To minimize this effect, a thermocouple effect, a thermocouple was rapidly swept at the measuring location, which is a similar technique was rapidly swept at the measuring location, which is a similar technique widely used in other works widely used in other works to measure the temperature in a sooting flame [17]. The temperature to measure the temperature in a sooting flame [17]. The temperature reading is further corrected by reading is further corrected by considering the radiation and convection cooling with the emissivity considering the radiation and convection cooling with the emissivity reported by Bradley et al. [18]. A fine reported by Bradley et al. [18]. A fine wire R‐type thermocouple was used with a 10 μm wire and wire R-type thermocouple was used with a 10 µm wire and junction diameter of 50 µm. The test was junction diameter of 50 μm. The test was repeated five times to provide error bar of temperature repeated five times to provide error bar of temperature measurement. The radial temperature variation measurement. The radial temperature variation was recorded at a fixed height of 40 mm, where an was recorded at a fixed height of 40 mm, where an extensive amount of soot is formed as shown from the extensive amount of soot is formed as shown from the result of the soot volume fraction result of the soot volume fraction measurement. All signals from the laser photodiode, radiometer, and measurement. All signals from the laser photodiode, radiometer, and thermocouple were displayed thermocouple were displayed and saved as digital values through the National Instruments (NI) data and saved as digital values through the National Instruments (NI) data acquisition system. In acquisition system. In particular, the temperature signal was acquired at a much faster sampling rate. particular, the temperature signal was acquired at a much faster sampling rate. The flame structure and length were observed by a natural emission imaging method with a The flame structure and length were observed by a natural emission imaging method with a high-resolution camera. The flame length was assumed to be vertical distance between burner exit high‐resolution camera. The flame length was assumed to be vertical distance between burner exit and visible yellow flame tip. The soot samples were collected from a 40 mm height after the burner and visible yellow flame tip. The soot samples were collected from a 40 mm height after the burner on the transmission electron microscopy (TEM) grid (used for TEM imaging) and quartz wire for on the transmission electron microscopy (TEM) grid (used for TEM imaging) and quartz wire for elemental analysis. The elemental analyzer (EA1110-Fisons, Thermo Scientific, Waltham, MA, USA) elemental analysis. The elemental analyzer (EA1110‐Fisons, Thermo Scientific, Waltham, MA, USA) was used to analyze samples from the quartz wire. The morphology and microstructure of soot was used to analyze samples from the quartz wire. The morphology and microstructure of soot samples on TEM grids were examined with Scanning Transmission Electron Microscope (STEM) at different magnifications ranging from 7000 to 390,000.
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5 of 13 with Scanning Transmission Electron Microscope (STEM) at different magnifications ranging from 7000 to 390,000. 3. Results and Discussion 3. Results and Discussion 3.1. Combustion Behavior of Post Combustion Simulated Gas Including Gaseous Ethylene Fuel 3.1. Combustion Behavior of Post Combustion Simulated Gas Including Gaseous Ethylene Fuel Ethylene was selected as the first base fuel because it produces a large amount of soot, as well as Ethylene was selected as the first base fuel because it produces a large amount of soot, as well as a a stable flame condition free of blow‐off behavior. In addition to this flame, flames diluted with a stable flame condition free of blow-off behavior. In addition to this flame, flames diluted with a certain certain amount of CO and CO 2 were compared in the coaxial reburning burner. Figure 3 presents amount of CO and CO2 were compared in the coaxial reburning burner. Figure 3 presents flame patterns flame patterns of post combustion simulated gases, including ethylene, which are observed by the of post combustion simulated gases, including ethylene, which are observed by the natural emission natural emission imaging method. As CO was added to ethylene, the flame length was decreased imaging method. As CO was added to ethylene, the flame length was decreased from 60 to 55 mm. from 60 to 55 mm. Further reduction of the flame length to 52 mm was observed in flames diluted Further reduction2of the flame length to 52 mm was observed in flames diluted with CO and CO2 (case 3). with CO and CO (case 3). The flames were observed to widen or radially spread as CO and CO 2 are The flames were observed to widen or radially spread as CO and CO are added. For example, the flame 2 added. For example, the flame width in cases 2 and 3 was 4 mm, which was 1.15 times wider than width in cases 2 and 3 was 4 mm, which was 1.15 times wider than that in case 1 (3.5 mm). This behavior that in case 1 (3.5 mm). This behavior was consistent with the flame‐spreading behavior in the fuel was consistent with the flame-spreading behavior in the fuel with CO2 addition, which was observed by with CO 2 addition, which was observed by Lee et al. [19]. The non‐luminous region (blue color) also Lee et al. [19]. The non-luminous region (blue color) also increased as CO and CO2 were added. increased as CO and CO 2 were added.
Figure 3. Flame patterns of post combustion simulated gases, including ethylene with different Figure 3. Flame patterns of post combustion simulated gases, including ethylene with different dilutions observed by the natural emission imaging method. dilutions observed by the natural emission imaging method.
Figure 4 shows a variation of the measured flame temperature with radial distance for the three Figure 4 shows a variation the measured flame temperature with radial distance for the three cases. The error level for three of cases was found to be statistically insignificant. The temperature cases. The error level for three cases was found to be statistically insignificant. The temperature measurement was made at height of 40 mm where the highest amount of soot was produced. Case 1 measurement was made at height of 40 mm where the highest amount of soot was produced. Case 1 of the ethylene flame exhibited the lowest temperature of 1240 °C at the center, whereas the peak of the ethylene flame exhibited theflame lowest temperature of 1240 at thethat center, whereas thein peak temperature was observed at the front. This profile was ◦ C similar was observed the temperature was observed at the flame front. This profile was similar that was observed in the ethylene ethylene flame in Koylu’s experiment [12]. Compared with case 1, CO addition in case 2 increased flame in Koylu’s experiment [12]. Compared with case 1, CO addition in case 2 increased the flame the flame temperature up to 1450 °C at increments of 200 °C. This was because of the thermal effect temperature up to 1450 ◦ C at increments of 200 ◦ C. This was because of the thermal effect of CO of CO addition, which was demonstrated by Law and his coworkers [3]. Carbon dioxide addition in addition, which was demonstrated by Law and his coworkers [3]. Carbon dioxide addition in case 3 case 3 reduced flame temperature back to 1280 °C at the center of the flame, and further decreased to reduced flame temperature back to 1280 ◦ C at the center of the flame, and further decreased a lesser a lesser magnitude at the peak location. Similar temperature reductions with N 2 and COto 2 dilution magnitude the peakby location. temperature withradial N2 and CO2 dilution been have been at reported Shin et Similar al. [20] and Lee et reductions al. [21]. The position where have the peak reported by Shin et al. [20] and Lee et al. [21]. The radial position where the peak temperature was temperature was observed, also shifted outward, which is supported by the radial spreading of the observed, also shifted outward, which supported by A thesimilar radial spreading of the observed flame front observed in flames with isCO 2 dilution. increase in the flame flame front width and a in flames with CO2 dilution. A similar increase in the flame2 addition in Gülder’s experiment [5]. The width and a decrease in the visible flame decrease in the visible flame height were observed with N height weregas observed with Nlocations in Gülder’s experiment [5]. distances The maximum gas temperature maximum temperature also shifted to larger radial than the former ones 2 addition locations also shifted to larger radial distances than the former ones because of the dilution [22]. because of the dilution [22].
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Figure 4. Variation of measured flame temperature with radial distance for three flames. Figure 4. Variation of measured flame temperature with radial distance for three flames. Figure 4. Variation of measured flame temperature with radial distance for three flames.
Figure 5 displays the normalized laser intensity related to soot volume fractions on the Figure 5 5 displays normalized intensity soot fractions volume on fractions on the Figure displays thethe normalized laserlaser intensity relatedrelated to soot to volume the centerline centerline of three different flames. The results show that CO addition by 50% decreased peak soot centerline of three different flames. The results show that CO addition by 50% decreased peak soot of three different flames. The results show that CO addition by 50% decreased peak soot emission emission by half from 0.224 to 0.122. This indicates the dominance of the dilution effect of CO emission by half 0.224 This to 0.122. This the indicates the dominance of the effect dilution effect of CO by half from 0.224from to 0.122. indicates dominance of the dilution of CO addition, addition, which was consistent with the linear reduction of the soot level with CO fraction in the fuel addition, which was consistent with the linear reduction of the soot level with CO fraction in the fuel which was consistent with the linear reduction of the soot level with CO fraction in the fuel observed observed by Law et al. [3]. Specifically, the 50% soot reduction with CO addition by 50% observed observed by Law et al. [3]. Specifically, the 50% soot reduction with CO addition by 50% observed by Law et al. [3]. Specifically, the 50% soot reduction with CO addition by 50% observed in this study in this study was similar to their findings. They found that the reduction in the soot inception limit in this study was similar to their findings. They found that the reduction in the soot inception limit was similar to their findings. They found that the reduction in the soot inception limit with CO with CO addition in the ethylene flame was largely due to the fuel dilution. Further addition of CO2 with CO addition in the ethylene flame was largely due to the fuel dilution. Further addition of CO addition in the ethylene flame was largely due to the fuel dilution. Further addition of CO2 to case 22 to case 2 decremented soot emission down to 0.025. In case 3, thermal and chemical effects may to case 2 decremented soot emission down 0.025. In case 3, chemical thermal and chemical effects may decremented soot emission down to 0.025. In to case 3, thermal and effects may contribute to contribute to the largest reduction in the soot emission in addition to the pure dilution effect [4]. The contribute to the largest reduction in the soot emission in addition to the pure dilution effect [4]. The the largest reduction in the soot emission in addition to the pure dilution effect [4]. The presence of presence of CO2 in the flame would decrease flame temperature by thermal effect and influence presence of CO2 in the flame would decrease flame temperature by thermal effect and influence CO 2 in the flame would decrease flame temperature by thermal effect and influence radical pools such radical pools such as OH and H atoms by chemical effect, both of which could significantly suppress radical pools such as OH and H atoms by chemical effect, both of which could significantly suppress as OH and H atoms by chemical effect, both of which could significantly suppress soot formation soot formation and growth. In all three cases, the peak soot emission was observed at a height of 40 soot formation and growth. In all three cases, the peak soot emission was observed at a height of 40 and growth. In all three cases, the peak soot emission was observed at a height of 40 mm. Figure 6 mm. Figure 6 presents the radiation heat intensity along the centerline of the three different flames. mm. Figure 6 presents the radiation heat intensity along the centerline of the three different flames. presents the radiation heat intensity along the centerline of the three different flames. Both CO Both CO and CO2 addition decreased radiation intensity probably because of the lesser soot presence Both CO and CO and CO2 addition2 addition decreased radiation intensity probably because of the lesser soot presence decreased radiation intensity probably because of the lesser soot presence in the in the corresponding flames. The relationship between the radiation intensity and soot emission is in the corresponding flames. The relationship between the radiation intensity and soot emission is corresponding flames. The relationship between the radiation intensity and soot emission is discussed discussed in detail in the next section. discussed in detail in the next section. in detail in the next section.
Figure 5. Axial variation of soot volume fractions at the centerline of three different flames. Figure 5. Axial variation of soot volume fractions at the centerline of three different flames. Figure 5. Axial variation of soot volume fractions at the centerline of three different flames.
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Figure 6. Radiation intensity at the centerline of three different flames. Figure 6. Radiation intensity at the centerline of three different flames. Figure 6. Radiation intensity at the centerline of three different flames. 3.2. Combustion Behavior of Post Combustion Simulated Gas Including Liquid Kerosene Fuel 3.2. 3.2. Combustion Behavior of Post Combustion Simulated Gas Including Liquid Kerosene Fuel Combustion Behavior of Post Combustion Simulated Gas Including Liquid Kerosene Fuel In the second test, kerosene vapor replaced ethylene as the base fuel. The dilutions with CO and In the second test, kerosene vapor replaced as the base fuel.fuel. The dilutions COthe and In the second test, kerosene vapor replaced ethylene as the base fuel. The dilutions with CO and in the same proportions as those ethylene in the ethylene‐based Figure 7 with shows CO2 were 2 were the proportions same proportions as inthose in the ethylene‐based fuel. Figure the 7 shows the of COcomparison were in thein same as those the ethylene-based fuel. Figure 7 shows comparison 2CO of flame patterns of post combustion simulated gases, including the kerosene vapor comparison flame patterns of post combustion simulated including the kerosene flame patterns ofof post combustion simulated gases, including thegases, kerosene vapor with differentvapor dilutions, with different dilutions, specifically, in cases 4–6. The flame length was observed to increase from 80 with different dilutions, specifically, in cases 4–6. The flame length was observed to increase from 80 specifically, in cases length wasadded observed to6). increase 80 to 160 flame mm when both CO to 160 mm when 4–6. both The CO flame and CO 2 were (case While from a comparable length was to 160 were mm added when both CO and COa2 comparable were added flame (case 6). While a comparable flame length was andobtained between cases 4 and 5, the flame in case 6 shows a length twice as long as that in case 4. CO (case 6). While length was obtained between cases 4 and 5, 2 obtained between cases 4 and 5, the flame in case 6 shows a length twice as long as that in case 4. theThis indicates that CO flame in case 6 shows a2 addition strongly affected the flame structure than CO does. It is notable to length twice as long as that in case 4. This indicates that CO2 addition strongly This indicates that CO2 addition strongly affected the flame structure than CO does. It is notable to affected the flame structure than CO does. It is notable to mention that a broken wing tip was observed mention that a broken wing tip was observed in each of the flames where soot is greatly emitted. mention that a broken wing tip was observed in each of the flames where soot is greatly emitted. Figure 8 shows the variation of the measured flame temperature with radial distance for the three in each of the flames where soot is greatly emitted. Figure 8 shows the variation of the measured flame Figure 8 shows the variation of the measured flame temperature with radial distance for the three cases. The use of kerosene in case 1 (case 4) lowered the temperature from case 1 of ethylene fuel by temperature with radial distance for the three cases. The use of kerosene in case 1 (case 4) lowered the cases. The use of kerosene in case 1 (case 4) lowered the temperature from case 1 of ethylene fuel by more than 300 °C. Although peak temperatures are similar at certain radial distances, CO 2 addition temperature from case 1 of ethylene fuel by more than 300 ◦ C. Although peak temperatures are similar more than 300 °C. Although peak temperatures are similar at certain radial distances, CO 2 addition appeared to significantly reduce the temperature at the center as compared to that in case 4. The at certain radial distances, CO addition appeared to significantly reduce the temperature at the center appeared to significantly reduce the temperature at the center as compared to that in case 4. The 2 radial position, where the peak temperature is observed, was shifted outward. This was similar to as compared to that in case 4. The radial position, where the peak temperature is observed, was shifted radial position, where the peak temperature is observed, was shifted outward. This was similar to the flame spreading effect of CO 2 addition to the ethylene fuel [17]. the flame spreading effect of CO 2 addition to the ethylene fuel [17]. outward. This was similar to the flame spreading effect of CO addition to the ethylene fuel [17]. 2
Figure 7. Flame patterns of post combustion simulated gases, including kerosene vapor with
Figure 7. Flame patterns of combustion post combustion simulated including kerosene vapor with Figure 7. Flame patterns of post simulated gases,gases, including kerosene vapor with different different dilutions observed by natural emission imaging method. different dilutions observed by natural emission imaging method. dilutions observed by natural emission imaging method.
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Figure 8. Variation of measured flame temperature with radial distance for three flames. Figure 8. Variation of measured flame temperature with radial distance for three flames. Figure 8. Variation of measured flame temperature with radial distance for three flames.
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Figure 9 displays normalized laser intensity related to soot volume fractions on the centerline Figure 9 displays normalized laser intensity related to soot volume fractions on the centerline of three different flames of post combustion gases, including kerosene vapor with different dilutions, Figure 9 displays normalized laser intensity related to soot volume fractions on the centerline of three of three different flames of post combustion gases, including kerosene vapor with different dilutions, specifically, cases 4–6. Experimental error of laser intensity is a little high as compared to gaseous different flames of post combustion gases, including kerosene vapor with different dilutions, specifically, specifically, cases 4–6. Experimental error of laser intensity is a little high as compared to gaseous fuel cases, indicating possible fluctuation of flame with liquid fuel. Under the combustion of the post cases 4–6. Experimental error of laser intensity is a little high as compared to gaseous fuel cases, indicating fuel cases, indicating possible fluctuation of flame with liquid fuel. Under the combustion of the post gas, including kerosene vapor, CO addition by 50% decreased the peak soot emission by less than possible fluctuation of flame with liquid fuel. Under the combustion of the post gas, including kerosene gas, including kerosene vapor, CO addition by 50% decreased the peak soot emission by less than 20%, which was a negligible effect when compared with the ethylene‐based fuel case. The CO2 vapor, CO addition by 50% decreased the peak soot emission by less than 20%, which was a negligible 20%, which was a negligible effect when compared with the ethylene‐based fuel case. The CO2 addition in case 6 initially produced a low soot emission of up to 80 mm. However, thereafter, the effect when compared with the ethylene-based fuel case. The CO2 addition in case 6 initially produced a addition in case 6 initially produced a low soot emission of up to 80 mm. However, thereafter, the addition suddenly caused the emission of a substantial amount of soot. The final soot emission was low soot emission of up to 80 mm. However, thereafter, the addition suddenly caused the emission of a addition suddenly caused the emission of a substantial amount of soot. The final soot emission was 1.5 times higher than that in the kerosene flame (case 4). This indicates a delayed soot formation, substantial of soot. final soot emission was 1.5 times than delayed that in the kerosene flame 1.5 times amount higher than that The in the kerosene flame (case 4). This higher indicates a soot formation, where the soot gradually accumulated as CO 2 was added with the kerosene vapor. This CO 2 effect (case 4). This indicates a delayed soot formation, where the soot gradually accumulated as CO added 2 was where the soot gradually accumulated as CO 2 was added with the kerosene vapor. This CO 2 effect observed in case 6 was remarkably different from the previous effect observed under the with the kerosene vapor. This CO effect observed in case 6 was remarkably different from the previous 2 observed in case 6 was remarkably different 2 was added to the kerosene vapor in the middle of from the previous effect observed under the ethylene‐based fuel condition (case 3). When CO effect observed under theline, ethylene-based (case Whenincrease CO2 was added to ethylene‐based fuel condition (case 3). When CO 2 was added to the kerosene vapor in the middle of heated fuel delivery the amount fuel of condition vaporized fuel 3). would because of the the kerosene lower vapor in the middle of heated fuel delivery line, the amount of vaporized fuel would increase heated fuel delivery line, the amount of vaporized fuel would increase because of the because lower temperature required for the kerosene vaporization with CO 2 dilution. This enhanced vaporization oftemperature required for the kerosene vaporization with CO the lower temperature required for the kerosene vaporization with CO2 dilution. This enhanced 2 dilution. This enhanced vaporization led to a shift to a fuel‐rich condition (under‐ventilated) and momentum‐controlled diffusion flame vaporization led to a shift to a fuel-rich condition (under-ventilated) and momentum-controlled diffusion led to a shift to a fuel‐rich condition (under‐ventilated) and momentum‐controlled diffusion flame condition [23]. As a result, this could produce a higher soot presence and larger flame length. On the flame condition [23]. As a result, this could produce a higher soot presence and larger flame length. condition [23]. As a result, this could produce a higher soot presence and larger flame length. On the other hand, there would be another mechanism associated with CO 2 effect observed in the case 6. In On the other hand, there would be another mechanism associated with2 effect observed in the case 6. In CO2 effect observed in the case 6. other hand, there would be another mechanism associated with CO this case, kerosene may not be completely vaporized as CO 2 is introduced in the middle of heating is introduced in the middle of heating Inthis case, kerosene may not be completely vaporized as CO this case, kerosene may not be completely vaporized as CO2 2is introduced in the middle of heating line. line. This could produce some amounts of liquid droplets in the fuel vapor which may alter flame line. This could produce some amounts of liquid droplets in the fuel vapor which may alter flame structure and thus soot emission. Because of two contrary mechanisms on the degree of vaporization This could produce some amounts of liquid droplets in the fuel vapor which may alter flame structure structure and thus soot emission. Because of two contrary mechanisms on the degree of vaporization with CO 2 addition, more adequate test is needed to provide supporting data on this controversial and thus soot emission. Because of two contrary mechanisms on the degree of vaporization with CO2 with CO 2 addition, more adequate test is needed to provide supporting data on this controversial issue. more addition, adequate test is needed to provide supporting data on this controversial issue. issue.
Figure 9. Axial variation of soot volume fractions at the centerline of three different flames.
Figure 9. Axial variation of soot volume fractions at the centerline of three different flames. Figure 9. Axial variation of soot volume fractions at the centerline of three different flames.
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The TEM images in Figure 10 compare soot samples between the kerosene and CO kerosene The TEM images in Figure 10 compare soot samples between the kerosene and CO2‐diluted 2 -diluted kerosene flames at two different magnifications. These images were obtained at the same height of flames at two different magnifications. These images were obtained at the same height of 40 mm after the40 mm after the burner. This observation supported the aforementioned delayed soot growth with burner. This observation supported the aforementioned delayed soot growth with CO2 dilution, CO2 dilution, where the degree of aggregation was smaller than that of the pure kerosene flame. This where the degree of aggregation was smaller than that of the pure kerosene flame. This behavior was behavior was similar to the soot growth behavior with ethanol addition, as reported in the work of similar to the soot growth behavior with ethanol addition, as reported in the work of D’Anna et al. [24]. D’Anna et al. [24]. They found that ethanol could diminish not only the surface growth but also the They found that ethanol could diminish not only the surface growth but also the aromatization process of soot formation. greater reduction in the population of with aggregates was in of aromatization soot formation.process A greater reduction in theA population of aggregates was observed N2 dilution observed with N2 dilution in the inverse ethylene diffusion flame [20]. They attributed this behavior the inverse ethylene diffusion flame [20]. They attributed this behavior to the lower fuel concentration as to the lower fuel concentration as N2 was diluted. The result of the composition analysis of the three N2 was diluted. The result of the composition analysis of the three soot samples collected at a 40-mm soot samples collected at a 40‐mm height above the burner is summarized in Table 2. A similar value height above the burner is summarized in Table 2. A similar value of the H/C ratio was observed in all of the H/C ratio was observed in all samples. This indicates the comparable aromatization of all samples. This indicates the comparable aromatization of all tested samples, which is consistent with the tested samples, which is consistent with the structural similarity observed in the high resolution structural similarity observed in the high resolution TEM images (not shown here). TEM images (not shown here).
(a)
(c)
(b)
(d)
Figure 10. TEM images of soot sampled at a 40 mm height after the burner (a) kerosene flame soot Figure 10. TEM images of soot sampled at a 40 mm height after the burner (a) kerosene flame soot (case 4); (b) CO2‐diluted kerosene flame soot (case 6) at magnification of 7000; (c) kerosene flame soot; (case 4); (b) CO2 -diluted kerosene flame soot (case 6) at magnification of 7000; (c) kerosene flame soot; (d) CO2‐diluted kerosene flame soot at higher magnification of 74,000. (d) CO2 -diluted kerosene flame soot at higher magnification of 74,000. Table 2. Composition analysis of soot samples collected at a 40‐mm height above the burner. Table 2. Composition analysis of soot samples collected at a 40-mm height above the burner.
Samples C Case4 (kerosene soot) 92% Samples C Case5 (kerosene/CO soot) 91.9% Case4 (kerosene soot) 92% Case6 (kerosene/CO/CO 2 soot) 92% Case5 (kerosene/CO soot) 91.9% Case6 (kerosene/CO/CO2 soot)
92%
H O H/C 0.6% 3.4% H O 0.0065 H/C 0.6% 3.7% 0.0065 0.6% 3.4% 0.0065 0.6% 3.3% 0.6% 3.7% 0.0065 0.0065 0.6%
3.3%
0.0065
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Figure 11 presents the radiation intensity along the centerline of three different flames. It can be Figure 11 presents the radiation intensity along the centerline of three different flames. It can be observed that CO addition had a negligible influence on the radiation intensity. On the other hand, observed that CO addition had a negligible influence on the radiation intensity. On the other hand, CO2 addition had a different impact on the radiation intensity. There was no radiation above 80 mm CO a different impact on the flames. However, strong radiation intensity. There was no radiation above 80 80 mm 2 addition hadand in the kerosene CO‐diluted kerosene radiations occurred from to in the kerosene and CO-diluted kerosene flames. However, strong radiations occurred from 80 to 160 mm with a slight loss in the peak intensity of CO2‐diluted kerosene flame. This behavior is as 160 mm with a slight loss increase in the peak intensity of CO kerosene flame. This behavior is as 2 -diluted expected because of the in the luminous flame region in the natural emission image in expected the increase in the luminous flame region in the natural image Figure 7 Figure 7 because and the ofhigher soot presence in Figure 9. A similar increase in emission radiative heat in fluxes was and the higher soot presence in Figure 9. A similar increase in radiative heat fluxes was observed in observed in flames with large dimensions [25]. The flame dimension was varied by the fuel flow rate flames with large dimensions [25]. The flame dimension was varied by the fuel flow rate and Reynolds and Reynolds number because of the reduced effect of buoyancy. Overall, based on the integration number because of the reduced effect ofto buoyancy. Overall, based on the integration of the radiation of the radiation curve, CO2 addition the kerosene vapor could significantly increase the total curve, CO addition to the kerosene vapor could significantly increase the total radiation, which is radiation, 2 which is discussed in more detail in the next section. This has a valuable industrial discussed in more detail in the next section. This has a valuable industrial implication considering implication considering that the decreased percentage of CO 2 in the kerosene vapor of combustion that the decreased percentage of CO in the kerosene vapor of combustion products could prevent fire 2 products could prevent fire accidents as a result of the reduced radiation around the gas generator accidents as a result of the reduced radiation around the gas generator test facility. test facility.
Figure 11. Radiation intensity at the centerline of three different flames. Figure 11. Radiation intensity at the centerline of three different flames.
3.3. Relationship between Radiation Intensity and Soot Emission 3.3. Relationship between Radiation Intensity and Soot Emission This section discusses the amount of soot emission that contributes to radiation intensity. In this This section discusses the amount of soot emission that contributes to radiation intensity. In this final test, CO2 gas was varied at 5, 10, 20, and 30% in addition to the CO‐diluted kerosene flame to final test, CO2 gas was varied at 5, 10, 20, and 30% in addition to the CO-diluted kerosene flame change the soot emission and radiation intensity. This test was separately designed to confirm the to change the soot emission and radiation intensity. This test was separately designed to confirm findings of CO2 addition to the kerosene vapor as observed in the previous section. Figure 12 shows the findings of CO2 addition to the kerosene vapor as observed in the previous section. Figure 12 the effect of CO2 addition on radiation intensity. The increase of CO2 in the fuel clearly tended to shows the effect of CO2 addition on radiation intensity. The increase of CO2 in the fuel clearly proportionally increase radiation, which correlates well with the increase in soot emission. This tended to proportionally increase radiation, which correlates well with the increase in soot emission. correlation between soot and radiation was typically expected, but was not observed in the This correlation between soot and radiation was typically expected, but was not observed in the experiments of some researchers [8,9]. Figure 13 presents the relationship between the radiation experiments of some researchers [8,9]. Figure 13 presents the relationship between the radiation intensity and soot emission over all previous data measured from kerosene vapor and intensity and soot emission over all previous data measured from kerosene vapor and ethylene-based ethylene‐based flames. The raw data of the radiation curve was integrated to obtain the total flames. The raw data of the radiation curve was integrated to obtain the total radiation intensity. radiation intensity. This integrated intensity was found to be more appropriate for the correlation This integrated intensity was found to be more appropriate for the correlation with soot emission than with soot emission than with peak intensity. The raw data of the measured peak soot emission was with peak intensity. The raw data of the measured peak soot emission was used for the correlation, used for the correlation, and a linear relationship was observed for the curve fit over the data. This and a linear relationship was observed for the curve fit over the data. This indicates that the soot indicates that the soot surface emission contributes to radiation more strongly compared to gaseous surface emission contributes to radiation more strongly compared to gaseous emissions such as CO2 emissions such as CO2 and H2O. A numerical or experimental study on the relation between soot and H2 O. A numerical or experimental study on the relation between soot emission and radiation emission and radiation were extensively performed for flames with gaseous fuels [26]. However, we were extensively performed for flames with gaseous fuels [26]. However, we tried here to extend this tried here to extend this relationship to the liquid fuel (kerosene) flame condition. The result still relationship to the liquid fuel (kerosene) flame condition. The result still showed a linear relation showed a linear relation between them, which is quite instructive for fire hazard and explosion between them, which is quite instructive for fire hazard and explosion application. application.
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Figure 12. Effect of CO2 addition on radiation intensity. Figure 12. Effect of CO2 addition on radiation intensity. Figure 12. Effect of CO2 addition on radiation intensity.
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Figure 13. Relationship between radiation intensity and soot emission over all previous data Figure 13. Relationship Relationship between between radiation intensity and emission soot emission all previous data measured from both kerosene and ethylene based‐flames. Figure 13. radiation intensity and soot over allover previous data measured measured from both kerosene and ethylene based‐flames. from both kerosene and ethylene based-flames.
4. Conclusions 4. Conclusions 4. Conclusions In this study, the presence of the fuel vapor, partially burned products, and inert gas in post In this study, the presence of the fuel vapor, partially burned products, and inert gas in post combustion gas was simulated by adding carbon monoxide and carbon dioxide to two different base In this study, the presence of the fuel vapor, partially burned products, and inert gas in post combustion gas was simulated by adding carbon monoxide and carbon dioxide to two different base fuels: kerosene vapor and ethylene. The effect of such combustion products on the soot formation combustion gas was simulated by adding carbon monoxide and carbon dioxide to two different base fuels: kerosene vapor and ethylene. The effect of such combustion products on the soot formation and radiation presented in the The reburning burner. The soot formation radiation are two fuels: kerosenewas vapor and ethylene. effect of such combustion products and on the soot formation and radiation was presented in the reburning burner. The soot formation and radiation are two important parameters closely related to the environmental and fire hazards around the test facility of and radiation was presented in the reburning burner. The soot formation and radiation are two important parameters closely related to the environmental and fire hazards around the test facility of the gas generator. important parameters closely related to the environmental and fire hazards around the test facility of the gas generator. The results show that when the post combustion simulated gas, including gaseous ethylene fuel, the gas generator. The results show that when the post combustion simulated gas, including gaseous ethylene fuel, is combusted, flame that length was with CO and CO2gas, addition. Moreover, flame The resultsa show when thedecreased post combustion simulated including gaseousthe ethylene is combusted, a flame length was decreased with CO 2 addition. As CO and CO and CO2 addition. Moreover, the flame temperature increased with CO and decreased with CO 2 were added, the fuel, is combusted, a flame length was decreased with CO and CO2 addition. Moreover, the flame temperature increased with CO and decreased with CO 2 addition. As CO and CO2 were added, the radiation intensity was with significantly accompanied by the As drastic reduction in the soot temperature increased CO and decreased, decreased with CO2 addition. CO and CO2 were added, radiation intensity was significantly decreased, accompanied by the drastic reduction in the soot emission. Under the was combustion of the post combustion gas, kerosene, CO2 the radiation intensity significantly decreased, accompanied byincluding the drasticliquid reduction in the soot emission. Under the combustion of the post combustion gas, including liquid kerosene, CO2 addition produced larger flame lengths and higher soot emissions. This was remarkably different emission. Under the combustion of the post combustion gas, including liquid kerosene, CO2 addition addition produced larger flame lengths and higher soot emissions. This was remarkably different from soot suppressive effect of CO typically observed with gaseous fuel (ethylene) in other studies produced larger flame lengths and2higher soot emissions. This was remarkably different from soot from soot suppressive effect of CO 2 typically observed with gaseous fuel (ethylene) in other studies and in this effect study. ofThis be attributed the shift to fuel a fuel‐rich condition (under‐ventilated) suppressive CO2may typically observed to with gaseous (ethylene) in other studies and in this and in this study. This may be attributed to the shift to a fuel‐rich condition (under‐ventilated) because of the lower temperature required for the kerosene vaporization with CO 2 dilution. Because study. This may be attributed to the shift to a fuel-rich condition (under-ventilated) because of the because of the lower temperature required for the kerosene vaporization with CO 2 dilution. Because of the high soot emission, CO2 addition could significantly increase the total radiation, and it of the high soot emission, CO 2 addition could significantly increase the total radiation, and it becomes a potential source of fire hazard. Finally, a linear relationship between the soot emission becomes a potential source of fire hazard. Finally, a linear relationship between the soot emission
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lower temperature required for the kerosene vaporization with CO2 dilution. Because of the high soot emission, CO2 addition could significantly increase the total radiation, and it becomes a potential source of fire hazard. Finally, a linear relationship between the soot emission and radiation intensity was observed in all experimental data, which indicates that the surface emission from soot is more responsible for radiation than from CO2 and H2 O vapor gaseous emissions. This relationship also supports the explosion behavior of various fuel blends occurring at higher equivalence ratio, which is controlled by soot emission and radiation loss. Author Contributions: Experiment and formal analysis, H.K.; Writing-Review & Editing, J.S. Funding: This research received no external funding. Acknowledgments: The authors thank the Korean Government (MEST) for the financial support through the National Research Foundation of Korea (NRF) grant (NRF-2017R1D1A1B03029138). This work was also supported by the Human Resources Development program (No. 20184030202060) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy. Conflicts of Interest: The authors declare no conflicts of interest.
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