Diesel Blends in a

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The investigation on soot characteristics of gasoline/ diesel blends in a laminar co-flow diffusion flame Fushui Liu, Yongli Gao, Han Wu, Zheng Zhang, Xu He, and Xiangrong Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b04051 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 19, 2018

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Energy & Fuels

The investigation on soot characteristics of gasoline/diesel blends in a laminar co-flow diffusion flame Fushui Liu1,2, Yongli Gao1, Han Wu1*, Zheng Zhang1, Xu He1, Xiangrong Li1 1

School of Mechanical Engineering, Beijing Institute of Technology, Beijing 100081, China 2

Beijing electric vehicle Collaborative Innovation Center, Beijing 100081, China

(Corresponding author: H. Wu*, Tel: 86-10-68918581, Email: [email protected])

Abstract: Gasoline addition in diesel fuel could not only improve the overall energy efficiency, but also could reduce the soot emission on diesel engine. However, the influence of gasoline on the soot forming characteristics has not been revealed fundamentally. A liquid burner system, modified from a Gülder burner, was applied to create strict laminar diffusion flame to carry out the related study. And, optical diagnostics technologies including digital camera imaging and high speed two-dimension line-of-sight attenuation (2D-LOSA), were used to record the natural luminosity flame structure, smoke point, and soot volume fraction. During experiments, the diesel ratio was varied as 0%, 20% and 40% by volume, while fuel flow rate was varied from 6g/h to 9g/h. The results show that diffusion flames of blends can be distributed into typical three parts, soot free region, soot growth region, and soot oxidation region. With the fuel flow rate increasing more soot was generated and the flame height was lifted. With diesel ratio increasing in gasoline/diesel blends, the visible flame height decreased, flame luminosity intensity decreased, smoke point increased, and 2D soot volume fraction value reduced, which indicates that diesel is less likely to

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produce soot than gasoline in the laminar diffusion flame. The soot reducing effect of diesel is regarded because diesel can enhance the cool flame combustion in free soot region due to its more obvious NTC (Negative Temperature Coefficient) effect. It is also reasonable to expound that the soot emission reduces with addition of gasoline in diesel on engines is mostly because of its volatility rather than its chemical characteristics. Keywords: gasoline/diesel blends; soot characteristics; laminar diffusion flame; 2D-LOSA

1. Introduction As known, diesel and gasoline are still the main energy sources for vehicles in long terms [1-3]. Specifically, gasoline with high volatility, easy to vaporize and form mixture, is usually applied to spark-ignition (SI) engines, while diesel fuel which has the feature of low auto-ignition temperature and low volatility is usually used in compression-ignition (CI) engine [4-6]. Compared to the SI engine, the CI engine has the advantages on horse power, thermal efficiency, and reliability, but suffering from emission problem especially the particle emission [7]. However, combining gasoline and diesel as a blended fuel has been regarded as a potential solution to obtain an effect of high efficiency and low emission [8]. Within this scenario, the petroleum refining process may be simplified for automobile industry in future. Recent years, investigations related to the effects of gasoline and diesel blended fuels on the engine performance and emissions have drawn an increasing attention. Zhong et al. [9] found that with 10% of diesel in gasoline fuel could significantly improve the ignitability and expand the HCCI (Homogeneous Charge Compression Ignition) operating limitation. Meanwhile, the harmful emissions of HC and NOx was reduced largely. Turner et al. [10] reported that diesel-gasoline blends, named ‘dieseline’, was able to extend low misfire limit, increase engine stability and


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Energy & Fuels

reduce combustion violence during HCCI combustion. Zhang et al. [11,12] investigated the performance of ‘dieseline’ on PPC (Partially Premixed Compression) combustion mode. They reported that compared to conventional diesel combustion, the smoke and NOx emissions can be reduced by 95% with the help of dieseline. Han et al. [13] tested dieseline in a premixed Low Temperature Combustion (LTC) mode and summarized that longer ignition delay and higher volatility of dieseline could produce a more homogeneous air-fuel mixture and finally led to a simultaneously reduction of NOx and smoke emissions, and an extension of low-emission and high efficiency operational range. These studies showed that gasoline and diesel blended fuels are beneficial for new combustion modes such as HCCI, PPCI and LTC especially on soot emission reduction. Previous works have shown that gasoline and diesel blends could help internal combustion engine to achieve a compromise between soot emission and thermal efficiency. However, the mechanism including physical and chemical effect resulted by gasoline addition on soot formation is still not clear enough. Few fundamental studies on constant volume chamber or optical engines were conducted. Payri et al. [14] found that spray penetration and cone angle under non-evaporative conditions have little differences between the gasoline and diesel fuels. However, Kim et al. [15] observed that gasoline presents significantly shorter liquid penetration length, narrower spray angle, longer lift-off length and lower combustion luminosity than those of diesel. In addition, experiments studies demonstrated that the lower viscosity and surface tension of gasoline could decrease droplet size [16, 17]. The optical results from Zheng et al. [18] proved that the increase of gasoline proportion results the increase of flame lift-off length, usually corresponds to better air-fuel mixing, which decreased the soot production. It is obvious


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that addition of gasoline is able to enhance the air-fuel mixing through increase the evaporation rate of reduce the droplet size. As known, soot emission is typically generated in rich condition. Thus, the gasoline addition in diesel could affect soot formation due to the physical process. In fact, the chemical properties also play important role in soot forming process, the soot forming level varies largely from fuel to fuel [19-22]. Usually, the oxygenated fuel such as methanol, ethanol and butanol etc. form less soot than traditional diesel and gasoline fuels due to the oxidation effect [23, 24]. Even the isomeric state influences the soot forming obviously, such as for butanol, the soot formation tendency decreasing as tert-butanol, 2-butanol, 1-butanol and iso-butanol [25]. The authors’ previous work also showed that it is aromatic hydrocarbon in gasoline that forms PAHs. And the components distribution is different between gasoline and diesel, which was expected could influence the soot forming. Overall, there are two main factors potentially influence the soot forming for gasoline and diesel blended fuel. And, the effect of gasoline has already been found in the spray combustion either on internal combustion engine or on constant volume chamber. But, it is not easy to evaluate the individual effect separately for the two factors since the spray combustion is a physical-chemical comprehensive combined process. So, a fundamental diffusion flame is necessary to conduct the investigation. Currently, the soot characteristics for gaseous fuels on the diffusion flame have been studied widely, but for liquid fuel especially the liquid fuel with high boiling point is still with challenging [23,26]. A tiny fluctuation on liquid fuel supplying and evaporating rate could result in instability on flame. That is also a reason why lacking fundamental experimental data of detailed soot volume fraction distributions for gasoline/diesel blends.


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Energy & Fuels

In order to separate the fuel spray and atomization process during the combustion, the current work is trying to investigate the combustion characteristics of gasoline/diesel blends on laminar co-flow diffusion flames. In this way, the chemical effect of gasoline addition can be evaluated precisely. A liquid fuel burner with high flame stability was modified based a gaseous fuel Gülder burner by setting the heating system and fuel evaporation and mixing system. To elucidate soot formation processes, a two-dimension line-of-sight attenuation (2D-LOSA) method [27] based on high speed camera and Abel inversion algorithm was used [28]. In addition, optical measurement technology of natural luminosity (NL) method using a DSLR color camera was also applied to record the flame structure and flame height to help to analyze the soot forming. During experiments, the fuel blending ratio and fuel supplying rate were varied.

2. Experimental setup and data processing 2.1 Experimental system The diffusion flames were established over an axisymmetric co-flow burner identical to the one described by Khosousi et al. [29]. The burner used in this experiment is designed and modified based on Gülder burner. The overall system including fuel supply, evaporation and combustion system is displayed in Fig.1 (a). The burner head consists of two concentric steel tubes as shown in Fig.1 (b). The fuel flows through the central tube and the oxidizer is injected through the outer one. The inner diameters are 10.9 mm for the fuel tube and 89 mm for the oxidizer tube respectively. To obtain a uniform and steady exit flow profile, the oxidizer is entered from two points and flows first through a volume filled with 5-mm glass beads and finally through a porous plate. In the system, a CEM (Controlled Evaporate Mixer) was heated up to ensure all the


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components evaporate completely. The flow rate of the nitrogen using for dilute the liquid fuel was fixed at 0.3L/min (at 273K, 1atm) with an uncertainty of about 0.5%. The air co-flow flow rate was kept at 167L/min (at 273K, 1atm). All the burner system was surrounded by heating apparatus and heat preservation equipment to keep at a temperature up to about 573K (300 °C) to avoid the condensation of the vaporized fuel inside the burner. This experiment was entirely conducted under atmospheric pressure. The detailed information about the system can be found in previous work [23,26]. 2.2 Smoke point measurement Smoke point, known as the height of the flame for incipient production of visible soot, is a common parameter to evaluate the sooting tendency for co-flow diffusion flames. Studies of fuel smoke points have been conducted as early as 1927 [30]. The SP is defined as the height in millimeters of the highest diffusion-controlled flame produced without smoke [31]. It has been well documented that a fuel's sooting tendency is inversely proportional to its smoke point [32-34]. That is, a lower smoke point indicates a higher sooting tendency. Visible changes of the flame structure were used in this research to determine the smoke point of the three blended fuels by consulting the method in Ref. [32]. As the flow rate increasing, nonsmoking flame will change to smoking [33,34]. Visible transition of the flame shape can be apparent and soot will break away from the flame tip as the form of smoke. Around this area the flow rate of the smoke point was finally confirmed by dichotomy. Using this method, the flow rate was obtained with a resolution less than 0.5g/h. This approach is time-consuming because the exact smoke point is a critical point and the flame often display a gradual transition from non-sooting to sooting. Thus this process was conducted several times to find a reasonable value.


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Energy & Fuels

2.3 Soot volume fraction measurements 2.3.1 The 2D-LOSA setup The soot volume fraction distributions were measured by the 2D-LOSA, which is a method proposed by Snelling et al. [27]. The principle of 2D-LOSA is using arc lamp source to provide near-infrared wavelengths at where the assumption that the soot particle radiation interaction is in the Rayleigh regime is more warranted. And then, after a reflecting mirror and a concave mirror, a parallel light beam was obtained. Along the parallel light path, the first lens, a pin-hole aperture, the second lens, and a bandpass filter were arranged as Fig.2 shown. The two lenses are conjugate and the tested flame was located between the first lens and the concave mirror. Finally, the images were recorded by a high-speed CMOS camera (Phantom V7.3) with 512×512 pixel resolution and a pixel size is 97.2μm. The camera was set at an exposure time of 1/2,000 s and a speed of 300 frame/s. The bandpass filter in front of the camera was selected with a center wave of 675nm and a bandwidth of 30nm. Schlieren effect of the flame caused by the beam steering should not be ignored, so an aperture with diameter of 1mm was chosen to reduce the flame radiation. Two convex lenses were used with diameter of 50.8mm and focal length of 250 mm, respectively. 2.3.2 Operational procedure and data processing Experiments were all conducted in a dark room to avoid the influence of environment light. Several procedures were needed to ensure accuracy. First, shut off the light source and the burner to get a background photo which was shown in Fig.3 a); Then light up the flame only to get the emission photo shown in Fig.3 b); And then turn off the burner and turn on the light source to get the lamp photo shown in Fig.3 c); Finally turn on both the light source and burner


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to get the transmission photo Fig.3 d). The calculated two-dimensional gray transmissivity figure was presented in Fig.3 e). The calculation method was described by:

τ=

(1)

where τ is the transmissivity and it indicates the specific value of the transmitted light intensity after flame and the incident light intensity. The transmissivity, τ, is related to soot absorption coefficient by:

∞

= exp (− ∞ )

(2)

where Kext is the dimensionless soot absorption coefficient. The result of Kext should be inverted from the transmissivity data by mathematical algorithm. Three-point Abel inversion method was used to reconstruct the Kext in this study. Since the inversed soot absorption coefficient was extremely affected by the image noise, the Gaussian spatial filter was used to smooth the transmissivity images before further data processing. Scattering can be ignored and only absorption is considered when assuming that the soot particles are in the Rayleigh scattering regime. Thus the radial soot volume fraction can be obtained using the following expression

=

!" # $%&()

(3)

Where Kext is the soot absorption coefficient achieved by the Abel inversion, λ is the light wavelength captured by the camera, E(m) is the soot absorption equation which is determined by the complex index of refraction of soot. Although several researchers make efforts to resolve this function while the complicated structure still makes it difficult to reach a consensus. In this study, E(m) was assumed to be constant value of 0.26 for simplicity [35]. During experiments, 200 flame figures were captured and averaged in each case. The images


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Energy & Fuels

after processing were used in the data calculating. 2.4 Flame natural luminosity measurement A digital camera is fully needed to record the flame geometry which can not only determine the flame structure but also get the luminosity information. DSLR color camera (Cannon 60D, 5184 by 3456 pixels, capable of exporting 14-bit lossless raw data files) was used to acquire images of co-flow laminar diffusion flames under various test conditions (flow rates, fuel types). Aperture was kept constant at 5.6 and ISO was kept at 100 to obtain clear flame photos, while two different exposure time were used in this paper. Long exposure time(1/10s) was to observe combustion region of the flame tip while short exposure time(1/1000s) was to keep the signal from being saturated thus flame natural luminosity were measured from the images. Thus, both the natural luminosity flame height and the flame nature luminosity can be determined. Once the luminosity image is obtained, the value of radially integral natural luminosity (RINL) at the centerline can be determined by integrating the pixel value horizontally and the value of spatially integrated natural luminosity (SINL) can be calculated by integrating the pixel value over an image. 2.5 Fuels and experimental conditions Pure No. 92 gasoline (named as G100) and mixtures with No.0 diesel (both from commercial gas station in Beijing, China) [36] were investigated in this study. The properties of these two fuels could be seen from Tab. 1. It is apparent that diesel has more carbons than gasoline while the carbon mass percent is similar. Experimentally, diesel is easier to coke under high temperature and the coking material may block the pipe and destroy the whole facility system. Considering this factor, blends with


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high proportion diesel were not tested. Finally, three fuels with varied diesel blending ratio by volume 0%, 20%, 40%, denoted as D0, D20 and D40 were studied. It should be noted that the fuel was blended and stirred well before experiments. The blended fuels were sit for at least 24 hours to avoid delamination. When the experiments were prepared, the fuel would be added to the fuel accumulator in Fig.1. The fuel flow rates were varied as 6g/h, 7g/h, 8g/h, 9g/h. The details were listed in Table 2. In all the tests the flow rate was controlled with an uncertainty of about 0.1 g/h.

3. Results and discussion 3.1 Natural luminosity characteristics The natural luminosity is formed by both radical chemiluminescence and soot incandescence while the latter is usually much stronger than the former one [37]. Thus it is reasonable to demonstrate that the soot luminosity can be well represented by the broadband luminosity to some extent. The luminous flame here is defined as the visible yellow flame edge [38-41]. As expected, the stoichiometric burning will be located on the surface of flame where the stoichiometric air-fuel mixture formed. Based on the flame structure, the laminar diffusion flame can be distributed into three regions, soot free region, soot growth region, and soot oxidation region, shown as in Fig. 4. As known, soot concentrations are the results of two competitive processes, soot formation and soot oxidation. The soot free region is at the bottom of the flame where the visual flame is weak and in “blue” color. It indicates that the luminosity is dominated by chemical chemiluminescence. Most of soot particle precursor is formed at this region through gas chemical kinetics. The soot growth region is in the middle of the flame where the visual flame presents bright “yellow”.


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Energy & Fuels

Obviously, the soot radiation is strong and soot incandescence is the dominator. Most of soot particles is formed in this region since the soot forming rate is much higher than soot oxidation rate. The primary particles experience nucleating, surface growing and coagulating process. Soot oxidation region is at the top of the flame where the particles contact the oxidizer. So, the soot concentration decreases with height until disappeared. The flame structure and RINL information on centerline under varied fuel flow rate condition for a certain fuel, pure diesel, are showed in Fig. 5a. The visual flame height was defined as the distance from the highest visual flame tip to the exit of fuel tube. It can be seen that as the flow rate reduces the flame height decreases obviously and the peak value of RINL decreases as well. Roughly, the flame height reduced from 43mm to 24mm and the RINL peak reduced from 150a.u. to 120a.u. while the position of RINL peak moves from height above burner (HAB) of 19mm to 13mm when the fuel flow rate decreased from 9g/h to 6g/h. It is reasonable because the transportation of fuel and intermediate chemical radials, at the high fuel flow rate case it takes more time and larger region for air-fuel/radicals mixing and reacting. When the larger region and longer period especially at the place where is suit for soot forming, more soot particles would be produced. The natural flame image and RINL information on centerline under varied fuel blends, D0, D20 and D40, under the same fuel flow rate condition of 8g/h are showed in Fig. 5b. It shows that the diesel addition in gasoline influence the flame structure and RINL significantly. The visible flame height decreases while the maximum RINL value reduces as diesel addition. But, the maximum RINL position of these three fuels is at the equal flame height (about 15mm) which indicates the fuel type has little influence on location distribution of the soot forming region. It is


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proved again by that with the increase of diesel the flame height is decreased but the height of free soot region also increased, seen as in the images. A potential reason for the increase of soot free region height is cool flame combustion. Cool flame occurs preferentially under fuel rich conditions during degenerate branching reactions, and the flame temperatures is approximately 600-1000K, a range commonly associated with the NTC region of hydrocarbon oxidation [42]. It shows an apparent color of faint pale “blue”, due to chemiluminescence of electronically excited formaldehyde [43]. Cool flame can be seen clearly at bottom of the flame (soot free region) in Fig. 4(a). Specifically, two stages combustion generally occur for large molecule hydrocarbons. One is the low-temperature oxidation, i.e. cool flame, and the other one is high-temperature oxidation, i.e. hot flame. The interaction of cool flame and hot flame presents the NTC phenomenon macroscopically, which make the reaction rate more slowly [44]. The stronger NTC tendency means wider initial temperature range of two stage combustion and broader cool flame region for the laminar diffusion flame. During the cool flame region, the large molecules of fuel will generate small molecule intermediate products such as methanol, CO by a series of low-temperature reaction such as dehydrogenation, oxygenation, isomerization and cracking. On one hand, it provides space for fuel diffusing and mixing with oxygen. On the other hand, it promotes the complete combustion of fuel. Reuter et al. [45] found that the larger n-alkanes are substantially more reactive in the low-temperature cool ﬂame regime. Westbrook et al.[46] used kinetic mechanism to study the ignition behavior from n-heptane to n-hexadecane and proved that NTC trend increases as the length of the n-alkane chain increases. Their results showed that longer hydrocarbon usually performs a more typical NTC phenomenon, which is an unavoidable process the tested fuel should experience when flow from down to up in the flame. Though the


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Energy & Fuels

carbon content of gasoline and diesel is nearly the same in this article, diesel has much more long carbon-chain hydrocarbon such as decane, dodecane and hexadecane than gasoline. The carbon numbers of the components for gasoline and diesel range from C4 to C10, and C10 to C22 respectively. An average carbon number of diesel is 14 or 15 [47,48]. Thus, more long carbon-chain hydrocarbon will be introduced into the flame with addition of diesel in gasoline. When the fuel burns near the NTC temperature, the flame can be suppressed by NTC to transit to high temperature flame combustion and stay at cool flame region [49], which is proved by the “blue” color in Fig. 5(c). Reasonably, more fuel burned in the cool flame region definitely will result in a reduced soot forming. It can be seen in Fig. 5(c) clearly that the cool flame region at the flame bottom increases with diesel addition. Therefore, the natural intensity will decrease as diesel adding to the blends. To quantitatively evaluate the overall soot level for tested fuels and under tested conditions, total SINL value applied as seen in Fig. 6. It shows that for the same fuel, the total SINL value increases as the flow rate increases. As to the same fuel flow rate, the total SINL value reduces as the diesel adding to gasoline. One of the direct reason is flame downsizing, the other one is the intensity reducing according to the NTC tendency in Fig. 5(c). However, different to the phenomenon observed on diesel engine, the finding indicates that the soot forming tendency for gasoline/diesel blends reduce with diesel adding. The key reason is the difference between diffusion flame and spray combustion in internal combustion engine. The physical process of fuel atomization and droplet evaporation which influence the air-fuel mixing and further on soot forming significantly that remove the comparison pre-condition. At the same time, cool flame, regarded as to reduce soot forming when diesel added, does not take large proportion in spray


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combustion. 3.2 Smoke point As described in the previous section, smoke point (SP) is an important parameter which reflects the sooting tendency. Fig. 7 presents the SP of the all tested fuels under varied tested conditions. As seen, gasoline has the lowest SP of flame height and fuel mass flow rate, the flame length for gasoline is 52 mm and the flow rate at smoke point is 9.8g/h. On the contrary, D40 has the largest SP of 56.5 mm flame height at fuel flow rate of 10.4g/h. Interesting that as diesel is added to gasoline the SP of the fuel mixture increases indicating a lower sooting tendency, that is gasoline is more likely to smoke. This is quite different from the common knowledge in the engines. But it again proves that the diesel addition could reduce soot producing on diffusion flame. As known, diesel engine produces soot because of the diffusion combustion mode while tradition gasoline engine of port fuel injection dose not because of the premixed combustion mode. However, with rapid development of GDI, this advanced gasoline engine becomes to generate soot inevitably. Even though GDI engine has the soot problem, it is still not as severe as the diesel engine. Studies found that adding gasoline to diesel in a compression-ignition engine will reduces soot emission because the gasoline is easier to evaporate and mix with the air [50]. Once blended fuels are injected to the engine, gasoline will help improving fuel/air mixing and producing more homogeneous mixtures over wider cylinder areas. Though gasoline is more possible to generate soot under diffusion flame combustion condition according to the results in current study, but the improving on fuel evaporation and air-fuel mixing could avoid soot forming concentration region during combustion process for internal combustion engine. In addition, gasoline will decrease the global fuel reactivity in-cylinder because of the high latent heat of


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Energy & Fuels

vaporization and retard combustion phasing because of the low cetane number [36], which will reduce the in-cylinder temperature and soot emission. The results about the soot tendency in this study are not contradictory to those in engines. 3.3 Soot volume fraction All the flame images were captured under stable condition thus flow rates higher than smoke point were not considered when measuring the soot concentration. And smoke point measurement is just to show the qualitative sooting tendency and quantitative data is necessary to further expound the soot distribution characteristics. Fig. 8a-b summarizes the 2-D results of the quantitative soot distribution of different tested conditions. To keep article brief and clear, only one of the fuel mass flow rates, 8g/h, for different tested fuels is illustrated in Fig. 8a, and only one of the tested fuels, D20, at different fuel flow rate conditions is illustrated in Fig. 8b. Red of the color bar means high soot area while blue represents low soot area. The 2-D images show a typical soot distribution of diffusion flames. It can be seen that there is a soot free region at the flame bottom from the burner top to approximately 5-15mm high. At the soot growth region, flame shows high soot concentration because of the high temperature and the lack of oxygen. Soot oxidation region at the top of flame and the flame surface shows low soot concentration because of oxidation. The post-processed 2D flame photos provide strong evidence to the theory in Fig.4. Fig. 8a shows pure gasoline produces the most soot at the same fuel mass flow rate. On the basis of the previous results of the smoke point tendency, diesel is unlikely to smoke at the same diffusion combustion condition. Thus as diesel added to gasoline, the soot region reduces and the peak soot value decreases as well. The results suggest that gasoline will enhance the soot


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production if remains the carbon mass same. Fig. 8b shows that as the flow rate decreases, there is an apparent reduction of the soot area and the red region disappears gradually which means the peak value of soot volume fraction decreases. This could explain that as the fuel mass decreases, the carbon mass introduced into the system will decrease too, which will reduce the production of soot. The maximum value of soot volume fraction is showed in Fig.9. It can be seen clearly that, for the same fuel, the peak soot concentration increases as the fuel flow rate increases. As the percentage of diesel added into fuel increases, the peak soot concentration decreases at the same flow rate. Horizontal and vertical soot volume fraction distribution representing radial and axial flame soot formation are displayed in Fig.10 and Fg.11 respectively. Local radial soot volume fraction profiles are plotted for four different heights above burner in Fig. 10. Only the results of fuel mass flow rate of 8g/h are listed here as a representative. At the lowest height above burner of 15mm in Fig.10a, corresponding to the soot free region in Fig.4, soot of all the three fuels are quite low of about 2.5ppm near the centerline, and reach the peak value at about 25mm radial direction to the centerline, and finally soot vanish at the flame surface at about 42mm radial direction to the centerline. Fig.10b-c shows the soot growth region which is presented in Fig.4b. In Fig.10b at 20mm height above burner, soot volume fraction is quite high at centerline from 6.9ppm of D40 to 9.4ppm of D0, and the peak soot position moves close to the centerline, and soot disappears at 40mm away from centerline. While in Fig.10c at 25mm, soot of D0 and D20 continue to grow, but soot of D40 begins to oxidize and the profile drops off, and soot vanishes at about 32mm away from centerline. In Fig. 10d of 30mm HAB, soot of D20 starts to oxidize and soot of D40 gets


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to entirely oxidation while soot of D0 still lies in the soot growth region. Partial axial distribution of soot volume fractions on the centerline are shown in Fig.11. For comparison purpose, the soot volume concentration is presented as a function of dimensionless heights (height above the burner normalized by the flame lengths). D40 flames tend to produce the lowest quantities of soot among the three fuels. The soot production in the D20 flames are slightly higher than that in the D40 flames, while pure gasoline produces the most soot, as discussed in the former section. It can be seen in Fig.11a that for different fuels at the same fuel flow rate, no soot information is captured from z/hf =0.2~0.4 which falls in the soot free region, and the soot concentration increase at the soot growth region from z/hf =0.4 and reach the maximum value at 0.7, and then soot volume fraction decreases which means soot oxidization dominates gradually in the upper part of flames. As for the same fuel in Fig.11b, all the test conditions experience the similar three regions, and increases as fuel flow rate increases. The soot volume fractions are integrated along the entire radial dimension and defined as +

β = 2π , (*)** , and the results are plotted for the entire flame heights in Fig.12. It can be seen that under the same fuel flow rate, soot production increase with diesel proportion. Peak integrated soot volume fractions decrease with diesel ratio, and in all three fuels analyzed in this work, the D40 produces significantly lower soot volume fractions, suggesting that the flames are less easy to smoke. For the same fuel, the peak β value increases as the fuel flow rate increases. Noting that the peak β appears at approximately half the flame height under all the test conditions, independent of fuel type and fuel flow rate. Also the peak integrated soot volume fractions which is labeled as βmax of all the test conditions are presented in Fig. 13. It can be seen that for the same fuel the βmax increases


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nearly linearly as the flow rate increases. While for the same fuel flow rate the βmax decreases as the proportion of diesel increases. It should be noted that the curve tendency of βmax is similar to the fv max curve in Fig. 9.

4. Conclusions In order to deeply realize the soot characteristics of gasoline/diesel blended fuel, the impact of blended ratio and fuel flow rate on combustion and soot forming ware investigated based on a laminar co-flow diffusion flame system. By application of several visualization measurement methods such as high speed 2D-LOSA, DSLR color camera imaging, the flame images were recorded and the soot behavior was analyzed systematically. The conclusions are as follows: 1) All the flames of gasoline/diesel blends show a typical diffusion flame structure, and can be separated into three parts in term of soot distribution along the axial direction: soot free region where flame displays “blue”, soot growth region and soot oxidation region at which flame natural luminosity presents “yellow”. 2) For a certain type fuel, as the fuel flow rate reduces, the visual flame height decreases obviously and the total SINL value declines. While for the same fuel flow rate condition, as diesel proportion increases, the visible flame height reduces and the total SINL value brings down suggesting that diesel has lower sooting tendency than gasoline. 3) The smoke point of pure gasoline is the highest among all the tested fuels implying that gasoline has a higher sooting tendency than diesel. The quantitative maximum soot concentration and the peak integrated soot concentration reduce as diesel added to blends. 4) The soot results of natural luminosity, smoke point and 2D soot distribution prove that gasoline produce more soot than diesel under the same co-flow laminar diffusion combustion


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condition. 5) The soot reducing effect of diesel in gasoline/diesel blends is because diesel can enhance the cool flame combustion due to the NTC. It is reasonable to illustrate that the reason gasoline can reduce the particle matter emission on diesel engine mainly because of the physical process improvement on air-fuel mixing but not the chemical characteristics.

Acknowledgements This study is based upon work supported by National Natural Science Foundation of China (No. 91741124). Any findings, opinions, and conclusions presented in this paper are the point of the author(s) and do not necessarily reflect the views of the funded organization.

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List of tables with caption 1. Nomenclature 2. Tab. 1 Main properties of gasoline and diesel fuels 3. Tab. 2 Test conditions

Nomenclature CI compression-ignition SI spark-ignition

τ Kext

GDI HCCI

E(m)

PPC

gasoline direct injection homogeneous charge compression ignition partially premixed compression

RINL

LTC

low temperature combustion

SINL

NTC 2D-LOSA

negative temperature coefficient two-dimension line-of-sight attenuation natural luminosity digital single lens reflex polycyclic aromatic hydrocarbons controlled evaporate mixer

HAB SP

NL DSLR PAHs CEM

λ

z hf β

transmissivity (-) dimensionless soot absorption coefficient (-) light wavelength (nm) soot absorption equation (-) radially integral natural luminosity (-) spatially integrated natural luminosity (-) height above burner (mm) smoke point (mm) soot volume fraction (ppm) position above burner (mm) flame height (mm) integrated soot volume fraction (ppm·m3)


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Tab. 1 Main properties of gasoline and diesel fuels

Parameters Molecular formula

Gasoline（92#） C4-C12

Diesel（0#） C14-C19

Composition (C,H,O) (mass %)

86.4, 13.5,

Diesel Blends in a

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