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Experimental study of fuel evaporation characteristics
Fuel 169 (2016) 33–40
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Experimental study of fuel evaporation characteristics Yingwen Yan a,⇑, Yunpeng Liu a, Yajun Wang a, Jinghua Li a, Wenxiang Cai b a b
Jiangsu Province Key Laboratory of Aerospace Power Systems, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing, China
a r t i c l e
i n f o
Article history: Received 6 September 2015 Received in revised form 10 November 2015 Accepted 1 December 2015 Available online 11 December 2015 Keywords: Optical measurement method Fuel evaporation ratio Physical measurement method Weber number Momentum ratio
a b s t r a c t Fuel evaporation characteristics are very important for the design of combustors. To quantitatively measure the fuel evaporation ratio of jet fuel in air cross flow, optical and physical measurement methods are proposed separately. The optical measurement method combines a high-speed camera and image-processing technologies, where as the physical measurement method applies the centrifugal force of a cyclone separator. The fuel evaporation ratio at different inlet conditions and different measurement cross-sections is analyzed separately for jet fuel in air cross flow. Finally, the experimental results of the physical method are compared with those of the optical method. The experimental results show that (1) the results of the optical measurement method are in good agreement with those of the physical measurement method, and these two methods can be used to quantitatively measure the fuel evaporation ratio at individual test cross-sections; (2) when other inlet parameters are the same, the fuel evaporation ratio at the test cross-section gradually increases with increasing inlet air Weber number; (3) when other inlet parameters are the same, the fuel evaporation ratio at the test cross-section gradually decreases with increasing momentum ratio; and (4) when inlet parameters are the same, the fuel evaporation ratio at the test cross-section increases sharply with increasing distance between the test cross-section and nozzle. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Fuel evaporation and the mixing of fuel and air in a gas turbine combustor are important for designing combustors. Fuel evaporation characteristics can determine the local equivalence ratio in the combustor and affect combustion efficiency, which affects the temperature distribution and combustion products at the combustor outlet. Therefore, the fuel evaporation characteristics significantly influence the combustor and engine [1]. With lean premixed prevaporized (LPP) low-emission combustion technology, the fuel evaporation and mixing of fuel and air are particularly important [2]. At present, the measuring techniques for gas liquid mixing mainly include laser spectrometry, the Schlieren method, the Mie scattering method, high-speed photography, holographic method, laser-induced fluorescence, laser tomography, and infrared absorption [3]. These noninterference optical measurement methods have achieved high resolution in time and space. Many studies have investigated fuel characteristics by using the above optical measurement techniques. Nishida et al. [4] measured the spray ⇑ Corresponding author. Tel.: +86 13770507240; fax: +86 25 84892200x2314. E-mail address: [email protected] (Y. Yan). http://dx.doi.org/10.1016/j.fuel.2015.12.001 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
atomization, liquid/vapor equivalence ratio, and fuel evaporation rate of a diesel nozzle jet spray by using ultraviolet laser absorption and scattering techniques at different conditions. The fuel spray characteristics of an LPP low-pollution combustor were studied by applying particle image velocimetry (PIV) [5] and an imageprocessing algorithm. Moreover, the fuel distribution and Sauter mean diameter (SMD) distribution of the liquid drops were obtained at different inlet conditions. The droplet size distribution for a direct injection nozzle was measured using a laser diffractionbased method [6], the velocity distributions of the droplets and air flow were measured by laser-induced fluorescence PIV, and the concentration distributions of the liquid and vapor phases in the spray were studied using a two-wavelength laser absorption–scattering technique. The fuel concentration of diesel sprays in isothermal conditions was measured using the planar laser-induced fluorescence technique [7]. The excitation wavelength was 355 nm, and the correction and calibration procedures to perform accurate measurements were studied. These procedures included a study of the fluorescence characteristics of fuel as well as the correction of the laser sheet non-homogeneities and losses due to Mie scattering, absorption, and auto-absorption. In order to measure the fuel vapor flux in evaporating liquid fuel spray, an infrared laser extinction measurement technique was used [8]. Kim [9]
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Y. Yan et al. / Fuel 169 (2016) 33–40
applied an exciplex laser-induced fluorescence technique and high-speed natural luminosity cinematography to investigate the evaporating sprays in high-speed direct-injection diesel engines and focused on the measurement method and signal calibration process of the fluorescence. In order to obtain fuel evaporation ratios at different crosssections for the fuel injected directly into the crossflow, the inlet fuel flow mass is measured. If the liquid fuel masses can be measured at different cross-sections, then the fuel evaporation ratios can be obtained based on the fuel mass continuity law. In order to measure the liquid fuel mass at different cross-sections, two kinds of measurement methods are investigated. The first is the optical measurement method: a high-speed camera was used to capture the raw images of liquid fuel-spray characteristics at the measurement cross-section within a certain distance from the nozzle. Then the diameter and distribution of the liquid fuel droplets at the measurement cross-section are obtained by various imageprocessing technologies. Then, the liquid fuel mass at the measurement cross-section can be obtained by using a statistical analysis of the droplet size and number, and finally, the fuel evaporation ratio at the measurement cross-section can be obtained. The second is the physical measurement method, which is used to verify the correctness of the optical measurement method. The cyclone separator is designed such that liquid droplets and vapor can be separated using the influence of centrifugal force within the separator. The separated liquid droplets are then collected; therefore, the liquid fuel mass at the measure cross-section can be obtained by weighting, and the fuel evaporation ratio at the measurement cross-section can also be obtained. Finally, the correctness of the optical measurement method is verified by comparing the physical measurement method with the optical measurement method. 2. Research objectives In order to reduce NOx emissions, LPP combustors with a staged lean combustion technology are universally used. The pilot swirler and primary mixer are designed such that fuel is injected into the combustor from the pilot atomizer and primary injectors separately. The fuel supply to the primary injectors exceeds 80% of the total fuel amount; therefore, the fuel evaporation characteristics in the primary swirler are one of the most important factors in the low-emission performance of an LPP combustor. Usually, the fuel is injected directly between the vanes of the primary swirler in an LPP combustor, and then atomized and evaporated. The fuel vapor mixes with air, and finally this mixture enters the combustor and creates uniform combustion at a lower temperature, which generates fewer pollutants. The fuel atomization and evaporation of the direct injector in the primary swirler vane channel of an LPP low-pollution combustor is similar to that of fuel injected directly into the crossflow. Therefore, the fuel evaporation characteristics of fuel direct injection into the crossflow were investigated in this study. Fig. 1 shows the configuration of the experimental device. Fig. 1 (a) shows the optical measurement system. There is an optical quartz glass observation window on each of the three sides (in addition to the bottom where the nozzle is fixed), which is convenient for the laser input and receiver. Fig. 1(b) shows the physical measurement system. The cyclone separator is installed at the downstream section, which is 20 mm away from the direct injection nozzle. The liquid droplets can be collected by applying the centrifugal force of the cyclone separator. There is a valve at the bottom of the separator; when the experiment is over, the valve is opened and the liquid fuel is collected. The cyclone separator system is designed to effectively separate liquid droplets with diameters greater than 5 lm, and the separation efficiency of the cyclone separator is more than 95%.
Fig. 2 shows the configuration of the optical measurement system, which combines a high-speed camera and image-processing technologies to measure the jet evaporation ratio at different cross-sections. The laser plane, which has a thickness of 0.5 mm, and the distance of the cross-section from the direct injection nozzle can be adjusted by moving the laser. The laser illuminates all of the fuel droplets in the laser plane, so that the light is scattered in all directions. The high-speed camera is fixed downstream of the nozzle and receives the scattered light of the droplets through the quartz glass observation window; therefore, raw images of droplets at the test section are obtained. Finally, the mixture of air and fuel vapor is exhausted from the side of the buffer cavity. The area of the buffer cavity is 16 times as large as the area of the test section, and sudden expansion of the area can reduce contamination of the quartz glass observation window.
3. Experimental process and method Fig. 3 shows the optical measurement system for the jet evaporation ratio of the direct injection nozzle. When the physical measurement method is applied, the optical measurement component and buffer cavity are simply replaced by the cyclone separator. The compressed inlet air is electrically preheated to about 350 K, and the total temperature and pressure of the inlet air are measured separately. The air then flows into the optical measurement component. Fuel from the supply system is injected into the crossflow by the direct fuel injector. This forms a liquid column, which is fragmented into ligaments/large droplets at the tip of the jet, where large surface waves develop, and the column experiences the strongest shear from the air. Along the side of the jet column, surface stripping of the droplets also occurs. Large droplets form under the action of air, and this is called primary atomization. These large droplets further breakup into smaller droplets during the moving process, and this is called second atomization. The droplets evaporate during the moving process and transfer into the fuel vapor; therefore, their diameters decrease. The fuel vapor mixes with air and forms a premixed mixture, which flows into the buffer cavity and exhaust pipe, finally discharging into the atmosphere after treatment. The fuel supply system includes a small compressor, fuel tank, valve, pressure gauge, and fuel pipe. The kerosene is stored in the fuel tank, and air with stable pressure is forced into the fuel tank by the small compressor; therefore, the kerosene is forced into the nozzle with stable fuel pressure. In this experiment, the fuel supply system can avoid pulsation of the fuel mass flow when the mass flow is small. 3.1. Optical measurement methods Raw images of droplets in the 0.5-mm-thick laser plane are obtained by the high-speed camera, whose exposure time is 200 ls. The resolution of high speed camera is 3840 2400 pixel, and the test area of cross section is 25 mm 25 mm, so the effective pixel is 2400 2400 pixel, therefore, the minimum resolution diameter of droplet which equals 1 pixel is 10.4 lm. Two hundred photos are captured for every case and can be used for subsequent image processing. _ f represents the fuel mass flow at the In this paper, the symbol M _ represents the liquid fuel mass flow at the test section, nozzle, M _ vapor represents the fuel vapor mass flow at the test section. and M First, the pressure difference between the inside and outside of the nozzle can be measured by pressure sensors; therefore, the fuel _ f of the nozzle can be measured. Raw images of the mass flow M droplets at the test section can be captured using the high-speed camera, as shown in Fig. 4.
Y. Yan et al. / Fuel 169 (2016) 33–40
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(a) Experimental device for optical measurement
(b) Experimental device for physical measurement Fig. 1. Configuration of fuel direct injection into the crossflow.
In order to distinguish different droplet size and fuel distribution from these raw images, an image-processing algorithm is compiled by using VC Language, and various image-processing technologies are applied to these raw images. The noise is inevitable produced during the image recording. In order to improve the quality of raw images (Fig. 4), a gradation processing is firstly applied by using linear transform before extracting droplet information. Secondly the boundaries of droplets are determined by edge detection. Edge detection technology is based on the discontinuity of image information where the image brightness changes sharply. There are many ways to perform edge detection, such as Sobel edge detection algorithm and Roberts algorithm. Roberts algorithm is used for edge detection in this study. When the boundaries of droplets are obtained, smooth disposal is applied to correct defects of edge detection and enhance the visibility of droplets. Then the outline of droplets can be extracted, including diameter and position. Finally, the picture disposed results and date disposed results are obtained. It is noted that during the image-processing there is a circular interference fringe around each drop. After all these postprocessing, the droplet size, fuel distribution and droplet quantity
can be obtained. Color and diameter of circular show the size of droplet in Fig. 5, and the unit is micrometer (lm). The literature [5] shows that the image-processing technologies are reliable. At the same time, in order to validate the imageprocessing methods, a verification experiment was completed. Kerosene was injected into a static atmosphere from a pressure atomizing nozzle (Fig. 6). The droplet diameters from the pressure atomizing nozzle were measured using a Malvern laser particle size analyzer. The distance from the measuring position to the nozzle is 60 mm; all droplets at the line are illuminated by the laser line (as shown in Fig. 6), and the average diameter of these illuminated droplets is obtained. The experimental results for the Malvern laser particle size analyzer are shown in Fig. 7. For the same experimental conditions and measuring position, the image-processing method was applied to measure droplet diameters. Table 1 shows that the error is relatively small and results from the two different measuring methods are similar; therefore, the image-processing method is reliable. Droplet size and distribution at the test section can be obtained after image processing, and the droplet number Ni of different
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Fig. 2. Configuration of the optical measurement system.
Fig. 3. Image of jet fuel evaporation measurement.
diameters Di can be found by statistical analysis, as shown in Fig. 8. Assuming all droplets captured in the test are spherical, the total volume Vi of all droplets at the test section can be obtained according to the diameter Di. Therefore, the liquid fuel volume Vsum at the test section is calculated as
V sum ¼
X
Ni V i ¼
X
Ni
p 6
D3i
:
ð1Þ
Because the laser plane is very thin and the exposure time of the camera is short enough compared to the time for which the droplets pass though the laser plane (texposure = 200 ls), all of the droplets that pass though the laser plane are completely captured _ during the exposure time. Therefore, the liquid fuel mass flow M at the test section can be expressed as the ratio of liquid fuel mass to exposure time. _ is calculated as Therefore, the liquid fuel mass flow M
_ ¼ q V sum =t exposure : M
ð2Þ
According to the law of mass continuity, the fuel vapor mass _ vapor is calculated as flow at the test section M
_ vapor ¼ M _ f M: _ M
ð3Þ
So the fuel evaporation ratio g at the test section is calculated as
g ¼ M_ vapor =M_ f ¼
_ f M _ M : _ Mf
ð4Þ
3.2. Physical separation method In order to verify the correctness of the above optical measurement method, the liquid droplets and vapor can be separated by using the influence of centrifugal force in a cyclone separator. Because the outlet section of the nozzle experimental channel is tangent to the inlet of the cyclone cylinder, the mixture of fuel droplets, fuel vapor and air enters the cyclone cylinder and performs a
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Fig. 6. Photograph of the Malvern laser particle size analyzer.
Fig. 4. Raw image before processing.
experiment is over, the valve is opened, the liquid fuel flows out, and the weight of the liquid fuel M at the test section is measured. At the same time, the pressure difference between the inside and outside of the nozzle can be measured using pressure sensors; _ f of the nozzle can be measured. The therefore, the fuel mass flow M experiment time can also be measured, so the mass of the total fuel Mf can be calculated as
_ f time: Mf ¼ M
ð5Þ
Therefore, according to the law of mass continuity, the mass of fuel vapor at the test section Mvapor can be calculated as
Mvapor ¼ M f M:
ð6Þ
Finally, the fuel evaporation ratio g at the test section is calculated as
g ¼ Mvapor =Mf ¼
Mf M : Mf
ð7Þ
In order to avoid further evaporation of liquid fuel on the cyclone cylinder wall, a cooling water jacket on the outside surface of the cyclone cylinder is designed, a thermocouple is fixed at the cylinder wall to monitor the temperature of the cylinder surface, and the wall temperature is adjusted by controlling the mass flow of the cooling water. Therefore, the wall temperature is kept slightly lower than the fuel evaporation temperature. This prevents evaporation of the liquid fuel and ensures that the fuel vapor does not condense into liquid fuel. 4. Experimental results and analysis
Fig. 5. Droplet size and distribution after image processing.
spiral movement along the cylinder wall. Due to the centrifugal force, the droplets are separated from the mixture attached to the wall, and a liquid fuel film forms. Due to the effect of gravity, liquid fuel flows down and is collected at the bottom of the cylinder. Meanwhile, the fuel vapor and air discharge through the exhaust pipe at the center of the cyclone cylinder. When the
In this paper, there are some dimensionless numbers that need to be explained. The first one is the Weber number (We), which is based on liquid properties, We ¼ qair u2air d0 =r, where qair is the air density, uair is the crossflow velocity at the inlet, d0 is the orifice diameter of the direct fuel injector, and r is the fuel–air interfacial tension. The Weber number represents the ratio of inertial force to surface tension. The second one is momentum flux ratio (q): q ¼ ql v 2jet =qair u2air , where ql is the fuel density, v jet is the jet injection velocity at the orifice of the direct fuel injector, and the momentum flux ratio (q) represents the ratio of jet fuel momentum to crossflow air momentum.
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Fig. 7. Experimental results of the Malvern laser particle size analyzer.
Table 1 Comparison of experimental results. Fuel pressure (MPa)
SMD by using Malvern laser particle size analyzer (lm)
SMD by using imageprocessing method (lm)
Error (%)
0.7 0.8 0.9
69.084 67.079 65.303
72.84 70.3 68.34
5.1 4.5 4.4
flows) and test cross-sections away from the nozzle on the jet fuel evaporation ratio were investigated. The temperature of inlet air was 348 K. Table 2 shows the inlet conditions and experimental results of the optical measurement. The distance between the test crosssection and injector is 20 mm for Cases 1, 2, and 3, but 25 mm for Case 4. By comparing Cases 1 and 2, when the inlet air mass flow is the same (i.e., the We number remains constant), the fuel evaporation ratio at the same test section increases with decreasing fuel mass flow (i.e., decreasing fuel momentum ratio). This is because the ratio of air to fuel per unit volume increases when the inlet mass flow is the same and the fuel mass flow decreases. On the other hand, the atomization ability of the transverse flow impacting the jet fuel is enhanced, the average diameter size of the second atomization decreases, and the heat exchange between the air and fuel per unit volume increases. By comparing Cases 2 and 3, when the air mass flow increases (i.e., an increase in We), and at the same time, the momentum flux ratio (q) remains constant, the inertial force of air increases, the large droplets are easier to break into small droplets, and the small droplets increase the evaporation surface area; therefore, the fuel evaporation ratio increases at the same test section. Comparing Cases 3 and 4, when the inlet parameters, including the We number and momentum flux ratio (q), are the same, but the test section is different, when the test section moves back, the residence time of the droplets increases. As the droplets move, they exchange heat with air and evaporate, and then the droplet Table 2 Evaporation ratio for different cases using the optical measurement method.
Fig. 8. Droplet distribution versus diameter.
Case
Inlet air mass flow (m/s)
We number
Fuel mass flow of inlet (g/s)
Momentum flux ratio q
Fuel evaporation ratio (g) (%)
Distance of test section away from nozzle (mm)
1 2 3 4
61.4 61.4 81.9 82
78.6 78.6 140 143
1.37 0.98 1.37 1.36
43.8 23.3 24.3 23.6
43.77 51.61 54.81 71.95
20.0 20.0 20.0 25.0
4.1. Optical measurement results of fuel evaporation ratio at different conditions In this paper, the fuel evaporation ratio of the single-point direct injector at the test section was measured using the optical measurement method. The nozzle diameter was0.4 mm, and the effects of different inlet conditions (inlet air and inlet fuel mass
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diameters decrease; therefore, the fuel evaporation ratio increases. Meanwhile, the evaporation ratio growth rate increases sharply with increasing distance of the test section from the nozzle. This shows that the distance of the test section from the nozzle is a very important factor for the fuel evaporation ratio. 4.2. Physical measurement results for fuel evaporation ratio at different conditions In order to verify the correctness of the optical measurement method, a cyclone separator was designed. The liquid droplets and vapor were separated using the influence of centrifugal force in the cyclone separator, and then the separated liquid droplets were collected; therefore, the liquid fuel mass at the measured cross-section could be obtained by weighting, and the fuel evaporation ratio at the measured cross-section could be obtained. In this study, the distance of the test section away from the nozzle was 20 mm for the physical measurement method. Table 3 shows the experimental results for the fuel evaporation ratio obtained at different inlet conditions using the cyclone separator. It shows that the fuel evaporation ratio increases with decreasing inlet fuel mass flow when the inlet We number is the same. This is because the number of droplets per unit volume decreases as the inlet fuel mass flow decreases, heat absorption reduces due to the evaporation of droplets, ambient temperature per unit volume rises, the temperature difference between the inside and outside of the droplets increases, and the evaporation of the droplets becomes easier; therefore, the evaporation ratio increases. Moreover, comparing Cases 5 and 8, Cases 6 and 9, or Cases 7 and 10, the inlet fuel mass flow is almost the same, and when the We number of the inlet air increases, the fuel evaporation rate also increases. This is because when the We number increases and the fuel mass flow remains the same, the inertial force of the droplets increases, they become easier to break into small droplets, and the surface area of the droplets increases; therefore, the evaporation ratio increases. Therefore, the change law of the fuel evaporation ratio measured by the physical method is consistent with that obtained by the optical method. 4.3. Results of comparative analysis of two measurement methods By comparing Cases 1 and 5, Cases 2 and 6, or Cases 3 and 8 (Tables 2 and 3), we see that the inlet parameters of the optical measurement method are similar to those of the physical measurement method; therefore, the evaporation ratios obtained using different measurement methods can be compared, as shown as Table 4. Table 4 shows that the experimental results measured using two methods are similar, and the error is very small (less than 5%). Therefore, the optical measurement method combining a high-speed camera and image-processing technologies is feasible and can be used to measure the fuel evaporation ratio at individual test cross-sections away from the nozzle. Table 3 Evaporation ratio indifferent cases for the physical measurement method. Case
5 6 7 8 9 10
Temperature of inlet (K) 348
We number of inlet air 79
142
Fuel mass flow of inlet (g/s)
Fuel evaporation ratio (g) (%)
1.380 0.990 0.623
45.11 49.7 51.3
1.3939 0.9578 0.5951
56.4 62 67.1
Table 4 Evaporation ratio and error of different measurement methods. Case
Fuel evaporation ratio (g) (%)
Case
Fuel evaporation ratio (g) (%)
Error (%)
1 2 3
43.77 51.61 54.81
5 6 8
45.11 49.7 56.4
3 3.7 2.8
The following analysis is the reasons that result in the error of optical measurement methods. Firstly, the shooting direction and laser plane are not completely vertical. When the angle between the shooting direction and laser plane is not equal to 90 degree, therefore, the sizes of the droplets in the raw images are smaller than the actual size. Secondly, the camera cannot capture the reflected lights of small droplets. Because the exposure time of the high speed camera is very short, the ability of camera to capture the reflected light is weak. At the same time the reflected light of the small droplets is weak. Therefore, the reflected lights of small droplets can not be captured. Finally, the resolution of high speed camera is 3840 2400 pixel, and the test area of the cross section is 25 25 mm, so the effective pixels is 2400 2400 pixel, therefore, minimum resolution diameter of droplet which equals 1 pixel is 10.4 lm. When the droplet is less than 10.4 lm, these droplets can not be distinguished by the high speed camera. The above three reasons causes that the measurement mass of liquid fuel is smaller than the real mass of liquid fuel, and the measurement fuel evaporation ratio is larger than the real fuel evaporation ratio. In order to improve the accuracy of the optical measurement method, some methods can be applied for the next experiments. Firstly, Ensure the shooting direction and laser plane is vertical. When the shooting plane and laser plane is not coincident, the raw image of droplets will distort, so the diameter of droplet will decrease. Secondly, because the reflected light of the small droplets is very weak, the laser intensity should be improved. The reflected light of droplets will become stronger when the laser intensity is improved. Therefore the camera can capture the smaller droplets. Finally, the high speed camera with high resolution should be used for the next experiments. Higher resolution means that the minimum resolution diameter of droplet will decrease, so smaller droplets can be extracted. The following analysis is the reasons that result in the error of physical separation method. Firstly, there is a certain distance between the inlet of cyclone separator and the actual separation point of the liquid droplets. During the moving process, the liquid droplets will evaporate, so the measurement fuel evaporation ratio will increase. Secondly, the liquid fuel will evaporate or the fuel vapor will condense into liquid fuel on the wall of cyclone separator. When the wall temperature of cyclone separator is higher than the fuel boiling temperature, the liquid fuel will quickly evaporate into fuel vapor during flowing process of liquid fuel on the wall of cyclone separator. In contrast, when the wall temperature of cyclone separator is lower than the fuel evaporation temperature, the fuel vapor will condense into liquid fuel, therefore, the wall temperature is very key parameter for the physical method. Thirdly, the liquid droplets whose diameter is smaller than 4.7 lm can not be collected. The centrifugal force of the liquid droplet whose diameter is smaller than 4.7 lm is so small that these droplets will be discharged directly from the outlet. Finally, the accuracy of the weighing scale is ±0.2 g, and the measurement time is about 1 min. The total weight of the liquid fuel is about 20–50 g. So the measurement error of scale is about ±0.4–1%. The above four reasons will lead to the measurement error of physical separation method.
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5. Conclusions
Acknowledgments
In order to measure the fuel evaporation ratio at individual test cross-sections, an optical measurement method was put forward in this paper. First, the raw images of droplets at individual crosssections were captured using a high-speed camera. Then, the diameter and distribution of the droplets could be distinguished using the image-processing technologies, and the droplet mass could be obtained by statistical methods at the test section. Finally, the fuel evaporation ratio at the test section could be calculated. At the same time, a physical measurement method to measure fuel evaporation ratio was designed by applying the centrifugal force of a cyclone separator. Finally, the experimental results of the two measurement methods were compared. The conclusions can be described as follows: First, the experimental results measured by the two methods are very similar: they show that both the optical and physical measurement methods in this paper can be used to measure the fuel evaporation ratio at different test cross-sections away from the nozzle. Second, the fuel evaporation ratio increases with decreasing fuel mass flow when other inlet conditions are the same. Also, the fuel evaporation ratio increases with increasing inlet air velocity when the air temperature and fuel mass flow are the same. Finally, the fuel evaporation ratio increases sharply with increasing distance between the test cross-section and nozzle when the inlet conditions are the same.
This work received funding from National Natural Science Foundation of China (No. 50906040, No. 51306092) and The Fundamental Research Funds for the Central Universities (No. NJ20140023). The authors are grateful to Graduate Student Longfei Dang for providing the Plots used in this paper. References [1] Gan XH. Aero gas turbine engine fuel nozzle technology. Beijing: National Defence Industry Press; 2006. [2] Huang Y. Combustion and combustor. Beijing: Beihang University Press; 2009. [3] Wang L. Combustion experimental diagnostics. Beijing: National Defence Industry Press; 2005. [4] Nishida K, Gao J, Manabe T, et al. Spray and mixture properties of evaporating fuel spray injected by hole-type direct injection diesel injector. Int J Engine Res 2008;9(4):347–60. [5] Yan YW, Dang LF, Deng YH, et al. Experimental study of flow dynamics and fuel spray characteristics in Lean Premixed Prevaporized Combustor. Fuel 2015;144:197–204. [6] Li T, Nishida K, Hiroyasu H. Droplet size distribution and evaporation characteristics of fuel spray by a swirl type atomizer. Fuel 2011;90(7):2367–76. [7] Pastor JV, López JJ, Juliá E, et al. Planar laser-induced fluorescence fuel concentration measurements in isothermal diesel sprays. Opt. Exp. 2002;10 (7):309–23. [8] Drallmeiert JA, Peters JE. An experimental investigation of fuel spray vapor phase characterization. AIAA1990. 90-0462. [9] Kim T. Quantitative 2-D fuel vapor concentration measurements in an evaporating diesel spray using the exciplex fluorescence method. Madison (Wisconsin): University of Wisconsin-Madison; 2001.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Experimental study of fuel evaporation characteristics Yingwen Yan a,⇑, Yunpeng Liu a, Yajun Wang a, Jinghua Li a, Wenxiang Cai b a b
Jiangsu Province Key Laboratory of Aerospace Power Systems, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing, China
a r t i c l e
i n f o
Article history: Received 6 September 2015 Received in revised form 10 November 2015 Accepted 1 December 2015 Available online 11 December 2015 Keywords: Optical measurement method Fuel evaporation ratio Physical measurement method Weber number Momentum ratio
a b s t r a c t Fuel evaporation characteristics are very important for the design of combustors. To quantitatively measure the fuel evaporation ratio of jet fuel in air cross flow, optical and physical measurement methods are proposed separately. The optical measurement method combines a high-speed camera and image-processing technologies, where as the physical measurement method applies the centrifugal force of a cyclone separator. The fuel evaporation ratio at different inlet conditions and different measurement cross-sections is analyzed separately for jet fuel in air cross flow. Finally, the experimental results of the physical method are compared with those of the optical method. The experimental results show that (1) the results of the optical measurement method are in good agreement with those of the physical measurement method, and these two methods can be used to quantitatively measure the fuel evaporation ratio at individual test cross-sections; (2) when other inlet parameters are the same, the fuel evaporation ratio at the test cross-section gradually increases with increasing inlet air Weber number; (3) when other inlet parameters are the same, the fuel evaporation ratio at the test cross-section gradually decreases with increasing momentum ratio; and (4) when inlet parameters are the same, the fuel evaporation ratio at the test cross-section increases sharply with increasing distance between the test cross-section and nozzle. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Fuel evaporation and the mixing of fuel and air in a gas turbine combustor are important for designing combustors. Fuel evaporation characteristics can determine the local equivalence ratio in the combustor and affect combustion efficiency, which affects the temperature distribution and combustion products at the combustor outlet. Therefore, the fuel evaporation characteristics significantly influence the combustor and engine [1]. With lean premixed prevaporized (LPP) low-emission combustion technology, the fuel evaporation and mixing of fuel and air are particularly important [2]. At present, the measuring techniques for gas liquid mixing mainly include laser spectrometry, the Schlieren method, the Mie scattering method, high-speed photography, holographic method, laser-induced fluorescence, laser tomography, and infrared absorption [3]. These noninterference optical measurement methods have achieved high resolution in time and space. Many studies have investigated fuel characteristics by using the above optical measurement techniques. Nishida et al. [4] measured the spray ⇑ Corresponding author. Tel.: +86 13770507240; fax: +86 25 84892200x2314. E-mail address: [email protected] (Y. Yan). http://dx.doi.org/10.1016/j.fuel.2015.12.001 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.
atomization, liquid/vapor equivalence ratio, and fuel evaporation rate of a diesel nozzle jet spray by using ultraviolet laser absorption and scattering techniques at different conditions. The fuel spray characteristics of an LPP low-pollution combustor were studied by applying particle image velocimetry (PIV) [5] and an imageprocessing algorithm. Moreover, the fuel distribution and Sauter mean diameter (SMD) distribution of the liquid drops were obtained at different inlet conditions. The droplet size distribution for a direct injection nozzle was measured using a laser diffractionbased method [6], the velocity distributions of the droplets and air flow were measured by laser-induced fluorescence PIV, and the concentration distributions of the liquid and vapor phases in the spray were studied using a two-wavelength laser absorption–scattering technique. The fuel concentration of diesel sprays in isothermal conditions was measured using the planar laser-induced fluorescence technique [7]. The excitation wavelength was 355 nm, and the correction and calibration procedures to perform accurate measurements were studied. These procedures included a study of the fluorescence characteristics of fuel as well as the correction of the laser sheet non-homogeneities and losses due to Mie scattering, absorption, and auto-absorption. In order to measure the fuel vapor flux in evaporating liquid fuel spray, an infrared laser extinction measurement technique was used [8]. Kim [9]
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applied an exciplex laser-induced fluorescence technique and high-speed natural luminosity cinematography to investigate the evaporating sprays in high-speed direct-injection diesel engines and focused on the measurement method and signal calibration process of the fluorescence. In order to obtain fuel evaporation ratios at different crosssections for the fuel injected directly into the crossflow, the inlet fuel flow mass is measured. If the liquid fuel masses can be measured at different cross-sections, then the fuel evaporation ratios can be obtained based on the fuel mass continuity law. In order to measure the liquid fuel mass at different cross-sections, two kinds of measurement methods are investigated. The first is the optical measurement method: a high-speed camera was used to capture the raw images of liquid fuel-spray characteristics at the measurement cross-section within a certain distance from the nozzle. Then the diameter and distribution of the liquid fuel droplets at the measurement cross-section are obtained by various imageprocessing technologies. Then, the liquid fuel mass at the measurement cross-section can be obtained by using a statistical analysis of the droplet size and number, and finally, the fuel evaporation ratio at the measurement cross-section can be obtained. The second is the physical measurement method, which is used to verify the correctness of the optical measurement method. The cyclone separator is designed such that liquid droplets and vapor can be separated using the influence of centrifugal force within the separator. The separated liquid droplets are then collected; therefore, the liquid fuel mass at the measure cross-section can be obtained by weighting, and the fuel evaporation ratio at the measurement cross-section can also be obtained. Finally, the correctness of the optical measurement method is verified by comparing the physical measurement method with the optical measurement method. 2. Research objectives In order to reduce NOx emissions, LPP combustors with a staged lean combustion technology are universally used. The pilot swirler and primary mixer are designed such that fuel is injected into the combustor from the pilot atomizer and primary injectors separately. The fuel supply to the primary injectors exceeds 80% of the total fuel amount; therefore, the fuel evaporation characteristics in the primary swirler are one of the most important factors in the low-emission performance of an LPP combustor. Usually, the fuel is injected directly between the vanes of the primary swirler in an LPP combustor, and then atomized and evaporated. The fuel vapor mixes with air, and finally this mixture enters the combustor and creates uniform combustion at a lower temperature, which generates fewer pollutants. The fuel atomization and evaporation of the direct injector in the primary swirler vane channel of an LPP low-pollution combustor is similar to that of fuel injected directly into the crossflow. Therefore, the fuel evaporation characteristics of fuel direct injection into the crossflow were investigated in this study. Fig. 1 shows the configuration of the experimental device. Fig. 1 (a) shows the optical measurement system. There is an optical quartz glass observation window on each of the three sides (in addition to the bottom where the nozzle is fixed), which is convenient for the laser input and receiver. Fig. 1(b) shows the physical measurement system. The cyclone separator is installed at the downstream section, which is 20 mm away from the direct injection nozzle. The liquid droplets can be collected by applying the centrifugal force of the cyclone separator. There is a valve at the bottom of the separator; when the experiment is over, the valve is opened and the liquid fuel is collected. The cyclone separator system is designed to effectively separate liquid droplets with diameters greater than 5 lm, and the separation efficiency of the cyclone separator is more than 95%.
Fig. 2 shows the configuration of the optical measurement system, which combines a high-speed camera and image-processing technologies to measure the jet evaporation ratio at different cross-sections. The laser plane, which has a thickness of 0.5 mm, and the distance of the cross-section from the direct injection nozzle can be adjusted by moving the laser. The laser illuminates all of the fuel droplets in the laser plane, so that the light is scattered in all directions. The high-speed camera is fixed downstream of the nozzle and receives the scattered light of the droplets through the quartz glass observation window; therefore, raw images of droplets at the test section are obtained. Finally, the mixture of air and fuel vapor is exhausted from the side of the buffer cavity. The area of the buffer cavity is 16 times as large as the area of the test section, and sudden expansion of the area can reduce contamination of the quartz glass observation window.
3. Experimental process and method Fig. 3 shows the optical measurement system for the jet evaporation ratio of the direct injection nozzle. When the physical measurement method is applied, the optical measurement component and buffer cavity are simply replaced by the cyclone separator. The compressed inlet air is electrically preheated to about 350 K, and the total temperature and pressure of the inlet air are measured separately. The air then flows into the optical measurement component. Fuel from the supply system is injected into the crossflow by the direct fuel injector. This forms a liquid column, which is fragmented into ligaments/large droplets at the tip of the jet, where large surface waves develop, and the column experiences the strongest shear from the air. Along the side of the jet column, surface stripping of the droplets also occurs. Large droplets form under the action of air, and this is called primary atomization. These large droplets further breakup into smaller droplets during the moving process, and this is called second atomization. The droplets evaporate during the moving process and transfer into the fuel vapor; therefore, their diameters decrease. The fuel vapor mixes with air and forms a premixed mixture, which flows into the buffer cavity and exhaust pipe, finally discharging into the atmosphere after treatment. The fuel supply system includes a small compressor, fuel tank, valve, pressure gauge, and fuel pipe. The kerosene is stored in the fuel tank, and air with stable pressure is forced into the fuel tank by the small compressor; therefore, the kerosene is forced into the nozzle with stable fuel pressure. In this experiment, the fuel supply system can avoid pulsation of the fuel mass flow when the mass flow is small. 3.1. Optical measurement methods Raw images of droplets in the 0.5-mm-thick laser plane are obtained by the high-speed camera, whose exposure time is 200 ls. The resolution of high speed camera is 3840 2400 pixel, and the test area of cross section is 25 mm 25 mm, so the effective pixel is 2400 2400 pixel, therefore, the minimum resolution diameter of droplet which equals 1 pixel is 10.4 lm. Two hundred photos are captured for every case and can be used for subsequent image processing. _ f represents the fuel mass flow at the In this paper, the symbol M _ represents the liquid fuel mass flow at the test section, nozzle, M _ vapor represents the fuel vapor mass flow at the test section. and M First, the pressure difference between the inside and outside of the nozzle can be measured by pressure sensors; therefore, the fuel _ f of the nozzle can be measured. Raw images of the mass flow M droplets at the test section can be captured using the high-speed camera, as shown in Fig. 4.
Y. Yan et al. / Fuel 169 (2016) 33–40
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(a) Experimental device for optical measurement
(b) Experimental device for physical measurement Fig. 1. Configuration of fuel direct injection into the crossflow.
In order to distinguish different droplet size and fuel distribution from these raw images, an image-processing algorithm is compiled by using VC Language, and various image-processing technologies are applied to these raw images. The noise is inevitable produced during the image recording. In order to improve the quality of raw images (Fig. 4), a gradation processing is firstly applied by using linear transform before extracting droplet information. Secondly the boundaries of droplets are determined by edge detection. Edge detection technology is based on the discontinuity of image information where the image brightness changes sharply. There are many ways to perform edge detection, such as Sobel edge detection algorithm and Roberts algorithm. Roberts algorithm is used for edge detection in this study. When the boundaries of droplets are obtained, smooth disposal is applied to correct defects of edge detection and enhance the visibility of droplets. Then the outline of droplets can be extracted, including diameter and position. Finally, the picture disposed results and date disposed results are obtained. It is noted that during the image-processing there is a circular interference fringe around each drop. After all these postprocessing, the droplet size, fuel distribution and droplet quantity
can be obtained. Color and diameter of circular show the size of droplet in Fig. 5, and the unit is micrometer (lm). The literature [5] shows that the image-processing technologies are reliable. At the same time, in order to validate the imageprocessing methods, a verification experiment was completed. Kerosene was injected into a static atmosphere from a pressure atomizing nozzle (Fig. 6). The droplet diameters from the pressure atomizing nozzle were measured using a Malvern laser particle size analyzer. The distance from the measuring position to the nozzle is 60 mm; all droplets at the line are illuminated by the laser line (as shown in Fig. 6), and the average diameter of these illuminated droplets is obtained. The experimental results for the Malvern laser particle size analyzer are shown in Fig. 7. For the same experimental conditions and measuring position, the image-processing method was applied to measure droplet diameters. Table 1 shows that the error is relatively small and results from the two different measuring methods are similar; therefore, the image-processing method is reliable. Droplet size and distribution at the test section can be obtained after image processing, and the droplet number Ni of different
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Fig. 2. Configuration of the optical measurement system.
Fig. 3. Image of jet fuel evaporation measurement.
diameters Di can be found by statistical analysis, as shown in Fig. 8. Assuming all droplets captured in the test are spherical, the total volume Vi of all droplets at the test section can be obtained according to the diameter Di. Therefore, the liquid fuel volume Vsum at the test section is calculated as
V sum ¼
X
Ni V i ¼
X
Ni
p 6
D3i
:
ð1Þ
Because the laser plane is very thin and the exposure time of the camera is short enough compared to the time for which the droplets pass though the laser plane (texposure = 200 ls), all of the droplets that pass though the laser plane are completely captured _ during the exposure time. Therefore, the liquid fuel mass flow M at the test section can be expressed as the ratio of liquid fuel mass to exposure time. _ is calculated as Therefore, the liquid fuel mass flow M
_ ¼ q V sum =t exposure : M
ð2Þ
According to the law of mass continuity, the fuel vapor mass _ vapor is calculated as flow at the test section M
_ vapor ¼ M _ f M: _ M
ð3Þ
So the fuel evaporation ratio g at the test section is calculated as
g ¼ M_ vapor =M_ f ¼
_ f M _ M : _ Mf
ð4Þ
3.2. Physical separation method In order to verify the correctness of the above optical measurement method, the liquid droplets and vapor can be separated by using the influence of centrifugal force in a cyclone separator. Because the outlet section of the nozzle experimental channel is tangent to the inlet of the cyclone cylinder, the mixture of fuel droplets, fuel vapor and air enters the cyclone cylinder and performs a
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Fig. 6. Photograph of the Malvern laser particle size analyzer.
Fig. 4. Raw image before processing.
experiment is over, the valve is opened, the liquid fuel flows out, and the weight of the liquid fuel M at the test section is measured. At the same time, the pressure difference between the inside and outside of the nozzle can be measured using pressure sensors; _ f of the nozzle can be measured. The therefore, the fuel mass flow M experiment time can also be measured, so the mass of the total fuel Mf can be calculated as
_ f time: Mf ¼ M
ð5Þ
Therefore, according to the law of mass continuity, the mass of fuel vapor at the test section Mvapor can be calculated as
Mvapor ¼ M f M:
ð6Þ
Finally, the fuel evaporation ratio g at the test section is calculated as
g ¼ Mvapor =Mf ¼
Mf M : Mf
ð7Þ
In order to avoid further evaporation of liquid fuel on the cyclone cylinder wall, a cooling water jacket on the outside surface of the cyclone cylinder is designed, a thermocouple is fixed at the cylinder wall to monitor the temperature of the cylinder surface, and the wall temperature is adjusted by controlling the mass flow of the cooling water. Therefore, the wall temperature is kept slightly lower than the fuel evaporation temperature. This prevents evaporation of the liquid fuel and ensures that the fuel vapor does not condense into liquid fuel. 4. Experimental results and analysis
Fig. 5. Droplet size and distribution after image processing.
spiral movement along the cylinder wall. Due to the centrifugal force, the droplets are separated from the mixture attached to the wall, and a liquid fuel film forms. Due to the effect of gravity, liquid fuel flows down and is collected at the bottom of the cylinder. Meanwhile, the fuel vapor and air discharge through the exhaust pipe at the center of the cyclone cylinder. When the
In this paper, there are some dimensionless numbers that need to be explained. The first one is the Weber number (We), which is based on liquid properties, We ¼ qair u2air d0 =r, where qair is the air density, uair is the crossflow velocity at the inlet, d0 is the orifice diameter of the direct fuel injector, and r is the fuel–air interfacial tension. The Weber number represents the ratio of inertial force to surface tension. The second one is momentum flux ratio (q): q ¼ ql v 2jet =qair u2air , where ql is the fuel density, v jet is the jet injection velocity at the orifice of the direct fuel injector, and the momentum flux ratio (q) represents the ratio of jet fuel momentum to crossflow air momentum.
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Fig. 7. Experimental results of the Malvern laser particle size analyzer.
Table 1 Comparison of experimental results. Fuel pressure (MPa)
SMD by using Malvern laser particle size analyzer (lm)
SMD by using imageprocessing method (lm)
Error (%)
0.7 0.8 0.9
69.084 67.079 65.303
72.84 70.3 68.34
5.1 4.5 4.4
flows) and test cross-sections away from the nozzle on the jet fuel evaporation ratio were investigated. The temperature of inlet air was 348 K. Table 2 shows the inlet conditions and experimental results of the optical measurement. The distance between the test crosssection and injector is 20 mm for Cases 1, 2, and 3, but 25 mm for Case 4. By comparing Cases 1 and 2, when the inlet air mass flow is the same (i.e., the We number remains constant), the fuel evaporation ratio at the same test section increases with decreasing fuel mass flow (i.e., decreasing fuel momentum ratio). This is because the ratio of air to fuel per unit volume increases when the inlet mass flow is the same and the fuel mass flow decreases. On the other hand, the atomization ability of the transverse flow impacting the jet fuel is enhanced, the average diameter size of the second atomization decreases, and the heat exchange between the air and fuel per unit volume increases. By comparing Cases 2 and 3, when the air mass flow increases (i.e., an increase in We), and at the same time, the momentum flux ratio (q) remains constant, the inertial force of air increases, the large droplets are easier to break into small droplets, and the small droplets increase the evaporation surface area; therefore, the fuel evaporation ratio increases at the same test section. Comparing Cases 3 and 4, when the inlet parameters, including the We number and momentum flux ratio (q), are the same, but the test section is different, when the test section moves back, the residence time of the droplets increases. As the droplets move, they exchange heat with air and evaporate, and then the droplet Table 2 Evaporation ratio for different cases using the optical measurement method.
Fig. 8. Droplet distribution versus diameter.
Case
Inlet air mass flow (m/s)
We number
Fuel mass flow of inlet (g/s)
Momentum flux ratio q
Fuel evaporation ratio (g) (%)
Distance of test section away from nozzle (mm)
1 2 3 4
61.4 61.4 81.9 82
78.6 78.6 140 143
1.37 0.98 1.37 1.36
43.8 23.3 24.3 23.6
43.77 51.61 54.81 71.95
20.0 20.0 20.0 25.0
4.1. Optical measurement results of fuel evaporation ratio at different conditions In this paper, the fuel evaporation ratio of the single-point direct injector at the test section was measured using the optical measurement method. The nozzle diameter was0.4 mm, and the effects of different inlet conditions (inlet air and inlet fuel mass
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diameters decrease; therefore, the fuel evaporation ratio increases. Meanwhile, the evaporation ratio growth rate increases sharply with increasing distance of the test section from the nozzle. This shows that the distance of the test section from the nozzle is a very important factor for the fuel evaporation ratio. 4.2. Physical measurement results for fuel evaporation ratio at different conditions In order to verify the correctness of the optical measurement method, a cyclone separator was designed. The liquid droplets and vapor were separated using the influence of centrifugal force in the cyclone separator, and then the separated liquid droplets were collected; therefore, the liquid fuel mass at the measured cross-section could be obtained by weighting, and the fuel evaporation ratio at the measured cross-section could be obtained. In this study, the distance of the test section away from the nozzle was 20 mm for the physical measurement method. Table 3 shows the experimental results for the fuel evaporation ratio obtained at different inlet conditions using the cyclone separator. It shows that the fuel evaporation ratio increases with decreasing inlet fuel mass flow when the inlet We number is the same. This is because the number of droplets per unit volume decreases as the inlet fuel mass flow decreases, heat absorption reduces due to the evaporation of droplets, ambient temperature per unit volume rises, the temperature difference between the inside and outside of the droplets increases, and the evaporation of the droplets becomes easier; therefore, the evaporation ratio increases. Moreover, comparing Cases 5 and 8, Cases 6 and 9, or Cases 7 and 10, the inlet fuel mass flow is almost the same, and when the We number of the inlet air increases, the fuel evaporation rate also increases. This is because when the We number increases and the fuel mass flow remains the same, the inertial force of the droplets increases, they become easier to break into small droplets, and the surface area of the droplets increases; therefore, the evaporation ratio increases. Therefore, the change law of the fuel evaporation ratio measured by the physical method is consistent with that obtained by the optical method. 4.3. Results of comparative analysis of two measurement methods By comparing Cases 1 and 5, Cases 2 and 6, or Cases 3 and 8 (Tables 2 and 3), we see that the inlet parameters of the optical measurement method are similar to those of the physical measurement method; therefore, the evaporation ratios obtained using different measurement methods can be compared, as shown as Table 4. Table 4 shows that the experimental results measured using two methods are similar, and the error is very small (less than 5%). Therefore, the optical measurement method combining a high-speed camera and image-processing technologies is feasible and can be used to measure the fuel evaporation ratio at individual test cross-sections away from the nozzle. Table 3 Evaporation ratio indifferent cases for the physical measurement method. Case
5 6 7 8 9 10
Temperature of inlet (K) 348
We number of inlet air 79
142
Fuel mass flow of inlet (g/s)
Fuel evaporation ratio (g) (%)
1.380 0.990 0.623
45.11 49.7 51.3
1.3939 0.9578 0.5951
56.4 62 67.1
Table 4 Evaporation ratio and error of different measurement methods. Case
Fuel evaporation ratio (g) (%)
Case
Fuel evaporation ratio (g) (%)
Error (%)
1 2 3
43.77 51.61 54.81
5 6 8
45.11 49.7 56.4
3 3.7 2.8
The following analysis is the reasons that result in the error of optical measurement methods. Firstly, the shooting direction and laser plane are not completely vertical. When the angle between the shooting direction and laser plane is not equal to 90 degree, therefore, the sizes of the droplets in the raw images are smaller than the actual size. Secondly, the camera cannot capture the reflected lights of small droplets. Because the exposure time of the high speed camera is very short, the ability of camera to capture the reflected light is weak. At the same time the reflected light of the small droplets is weak. Therefore, the reflected lights of small droplets can not be captured. Finally, the resolution of high speed camera is 3840 2400 pixel, and the test area of the cross section is 25 25 mm, so the effective pixels is 2400 2400 pixel, therefore, minimum resolution diameter of droplet which equals 1 pixel is 10.4 lm. When the droplet is less than 10.4 lm, these droplets can not be distinguished by the high speed camera. The above three reasons causes that the measurement mass of liquid fuel is smaller than the real mass of liquid fuel, and the measurement fuel evaporation ratio is larger than the real fuel evaporation ratio. In order to improve the accuracy of the optical measurement method, some methods can be applied for the next experiments. Firstly, Ensure the shooting direction and laser plane is vertical. When the shooting plane and laser plane is not coincident, the raw image of droplets will distort, so the diameter of droplet will decrease. Secondly, because the reflected light of the small droplets is very weak, the laser intensity should be improved. The reflected light of droplets will become stronger when the laser intensity is improved. Therefore the camera can capture the smaller droplets. Finally, the high speed camera with high resolution should be used for the next experiments. Higher resolution means that the minimum resolution diameter of droplet will decrease, so smaller droplets can be extracted. The following analysis is the reasons that result in the error of physical separation method. Firstly, there is a certain distance between the inlet of cyclone separator and the actual separation point of the liquid droplets. During the moving process, the liquid droplets will evaporate, so the measurement fuel evaporation ratio will increase. Secondly, the liquid fuel will evaporate or the fuel vapor will condense into liquid fuel on the wall of cyclone separator. When the wall temperature of cyclone separator is higher than the fuel boiling temperature, the liquid fuel will quickly evaporate into fuel vapor during flowing process of liquid fuel on the wall of cyclone separator. In contrast, when the wall temperature of cyclone separator is lower than the fuel evaporation temperature, the fuel vapor will condense into liquid fuel, therefore, the wall temperature is very key parameter for the physical method. Thirdly, the liquid droplets whose diameter is smaller than 4.7 lm can not be collected. The centrifugal force of the liquid droplet whose diameter is smaller than 4.7 lm is so small that these droplets will be discharged directly from the outlet. Finally, the accuracy of the weighing scale is ±0.2 g, and the measurement time is about 1 min. The total weight of the liquid fuel is about 20–50 g. So the measurement error of scale is about ±0.4–1%. The above four reasons will lead to the measurement error of physical separation method.
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5. Conclusions
Acknowledgments
In order to measure the fuel evaporation ratio at individual test cross-sections, an optical measurement method was put forward in this paper. First, the raw images of droplets at individual crosssections were captured using a high-speed camera. Then, the diameter and distribution of the droplets could be distinguished using the image-processing technologies, and the droplet mass could be obtained by statistical methods at the test section. Finally, the fuel evaporation ratio at the test section could be calculated. At the same time, a physical measurement method to measure fuel evaporation ratio was designed by applying the centrifugal force of a cyclone separator. Finally, the experimental results of the two measurement methods were compared. The conclusions can be described as follows: First, the experimental results measured by the two methods are very similar: they show that both the optical and physical measurement methods in this paper can be used to measure the fuel evaporation ratio at different test cross-sections away from the nozzle. Second, the fuel evaporation ratio increases with decreasing fuel mass flow when other inlet conditions are the same. Also, the fuel evaporation ratio increases with increasing inlet air velocity when the air temperature and fuel mass flow are the same. Finally, the fuel evaporation ratio increases sharply with increasing distance between the test cross-section and nozzle when the inlet conditions are the same.
This work received funding from National Natural Science Foundation of China (No. 50906040, No. 51306092) and The Fundamental Research Funds for the Central Universities (No. NJ20140023). The authors are grateful to Graduate Student Longfei Dang for providing the Plots used in this paper. References [1] Gan XH. Aero gas turbine engine fuel nozzle technology. Beijing: National Defence Industry Press; 2006. [2] Huang Y. Combustion and combustor. Beijing: Beihang University Press; 2009. [3] Wang L. Combustion experimental diagnostics. Beijing: National Defence Industry Press; 2005. [4] Nishida K, Gao J, Manabe T, et al. Spray and mixture properties of evaporating fuel spray injected by hole-type direct injection diesel injector. Int J Engine Res 2008;9(4):347–60. [5] Yan YW, Dang LF, Deng YH, et al. Experimental study of flow dynamics and fuel spray characteristics in Lean Premixed Prevaporized Combustor. Fuel 2015;144:197–204. [6] Li T, Nishida K, Hiroyasu H. Droplet size distribution and evaporation characteristics of fuel spray by a swirl type atomizer. Fuel 2011;90(7):2367–76. [7] Pastor JV, López JJ, Juliá E, et al. Planar laser-induced fluorescence fuel concentration measurements in isothermal diesel sprays. Opt. Exp. 2002;10 (7):309–23. [8] Drallmeiert JA, Peters JE. An experimental investigation of fuel spray vapor phase characterization. AIAA1990. 90-0462. [9] Kim T. Quantitative 2-D fuel vapor concentration measurements in an evaporating diesel spray using the exciplex fluorescence method. Madison (Wisconsin): University of Wisconsin-Madison; 2001.