Characteristics of Soot and Radiation of Post Combustion ... - MDPI

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Jul 2, 2018 - Keywords: gas generator; fuel rich; post combustion gas; soot ..... In all three cases, the peak soot emission was observed at a height of 40 mm.
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. 

Energies 2018, 11, 1731 Energies 2018, 11, x FOR PEER REVIEW    Energies 2018, 11, x FOR PEER REVIEW   

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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