Combination of Visualization Techniques for the Analysis of

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Combination of Visualization Techniques for the Analysis of Evaporating Diesel Sprays R. Payri, F. J. Salvador,* A. García, and A. Gil CMT-Motores Térmicos, Universitat Politècnica de València, Camino de Vera, s/n, 46022 Valencia, Spain ABSTRACT: Evaporating diesel sprays from three different multi-hole nozzles have been studied by means of three different and complementary techniques: Mie scattering, double-pass schlieren, and CH radical chemiluminescence. These three kinds of measurements provided valuable information that has allowed us to shed light on the influence of nozzle geometry on the air− fuel mixing and evaporation processes. For this purpose, three six-hole sac nozzles, with different orifice degrees of conicity have been used. These nozzles had been geometrically and hydraulically characterized in a previous publication, where important nondimensional coefficients describing the nature of the flow have been evaluated. Injection at different pressure conditions has been carried out in an inert environment of gas to study the mixture process, avoiding window fouling by soot deposition from combustion. Studies have been carried out for three different injection pressures and four different gas densities during spray injection, representative of real engine conditions, with changing temperature and pressure in the discharge camera. Liquid-phase penetration and stabilized liquid length for all nozzles and pressure conditions have been determined with the Mie scattering technique. The cylindrical nozzle provided the highest value of the liquid length at a low injection pressure (30 MPa), because of its higher diameter and the absence of cavitation. When the two conical nozzles are compared, no clear differences have been shown between them. Because with this Mie scattering, it is not possible to follow the vapor behavior and considering constraints associated with the geometrical configuration of diesel sprays generated by the multi-hole nozzles, a double-pass schlieren technique has been used for this purpose. The combination of Mie scattering and schlieren allowed for the liquid and vapor phases to be analyzed in terms of penetration. As far as the CH radicals are concerned, these radicals are supposed to appear beyond the position of the stabilized liquid length; however, results show how first CH radical intensity is detected before liquidphase penetration reaches the stabilized value. When CH radical apparition delays from the start of the injection between all of the nozzles and for all injection conditions are compared, in overall terms, conical nozzles present less delay than cylindrical nozzles. This means that the air−fuel mixing process is more effective in the case of conical nozzles. Thus, the analysis of all of the results made it possible to link the nozzle geometry, fuel−air mixture, spray evaporation, and incipient combustion.

1. INTRODUCTION The purpose of the fuel injection process in a diesel engine is to prepare the fuel−air mixture for a clean and efficient combustion. The understanding of this process is important to design new injection systems and optimum injection strategies that may reduce fuel consumption while still complying with the increasingly stringent emission regulation. Fuel injection and its interaction with air within the combustion chamber is a key topic for the understanding of diesel engine behavior. To characterize the spray features in these systems, experimental studies by imaging the scheme inside the combustion chamber have been widely applied, producing accurate information on the fuel spray development in terms of atomization, evaporation, autoignition, and combustion.1−9 Moreover, evaporation and atomization of fuel in the combustion chamber are processes strongly related to the nozzle geometry and cavitation.10−20 Quantitative information can be available by digital image processing, in which accuracy depends upon image characteristics. Most of the experimental techniques are used in combination with others that are complementary. In evaporative conditions, schlieren is normally used in combination with Mie scattering for liquid spray imaging.21,22 These experimental techniques have been employed in the characterization of multi-hole10,12,14,23−27 and single-hole3,28 nozzles. Other optical © 2012 American Chemical Society

techniques are used to directly analyze the combustion process in the chamber.4−6 CH radicals are produced in low-temperature reactions; hence, they are an indicator of pre-reactions in diesel combustion.7 Also, CH radical measurement can be used for autoignition characterization in diesel engines.31,32 Therefore, soot flame visualization and CH/OH chemiluminiscence are considered to be the most important techniques in the literature.29,30 It is the aim of this paper to show the potential of the combination of different imaging techniques addressed to the study of diesel spray development and evaporation. As a practical application, a study aiming at shedding light on the link of nozzle geometry and spray air−fuel mixing and evaporation has been performed. Three different six-hole sac injectors with different orifice degrees of nozzle conicity have been used for this purpose. The nozzles were characterized by means of the Mie scattering technique in ref 10 in terms of stabilized liquid length in real engine conditions. In the present work, results from schlieren and CH chemiluminiescence techniques are presented and analyzed and, in combination with previous Mie Received: May 22, 2012 Revised: July 26, 2012 Published: July 27, 2012 5481

dx.doi.org/10.1021/ef3008823 | Energy Fuels 2012, 26, 5481−5490

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Table 1. Physical and Chemical Properties of Repsol CEC RF-06-99 Fuel test density at 15 °C viscosity at 40 °C volatility 65% distillated 85% distillated 95% distillated cetane number cetane index calorific value higher calorific value lower calorific value fuel molecular composition

unit

result

uncertainty

kg/m3 mm2/s

843 2.847

±0.2 ±0.42

°C °C °C

294.5 329.2 357.0 51.52 49.6

±3.7 ±3.7 ±3.7 ±2.5 ±0.51

MJ/kg MJ/kg

45.58 42.78 C13H28

Table 3. Schlieren Setup Main Optical Characteristics light output of 20 μs at 1/3 of the maximum light intensity optical fiber bundle with an 80° output cone and diameter of 7 mm condenser system diameter of 1 mm by a short focal length and an iris diaphragm spherical mirror lens L1 diameter of 152 mm focal length of 609 mm distance to the test section of 950 mm optical access sapphire window diameter of 50 mm thickness of 15 mm test section combustion chamber diameter of 46 mm depth of 35 mm beam splitter 50:50 beam splitter 45° 50% incident light to the test section distance to the test section of 620 mm collimating lens lens L2 diameter of 60 mm focal length of 350 mm distance to the beam splitter of 150 mm adaption lens lens L3 distance to L2 of 350 mm camera 12-bit charge-coupled device resolution of 1280 × 1024 (CCD) camera (SensiCam by pixels, Nikkor 60 mm PCO) camera lens

Table 2. Real Hole Nozzle Geometry Characterization by Silicone Methodology nozzle

Di (μm)

Do (μm)

k factor

1 2 3

175 176 175

155 160 175

2 1.6 0

stroboscopic xenon flash lamp system by PerkinsElmer (model MVS 7010)

illumination

Table 4. Engine Test Matrix Conditions injection

Figure 1. Experimental arrangement for the schlieren setup.

scattering results, are used to thoroughly examine evaporation and autoignition processes. As far as the paper structure is concerned, it is organized into four sections. First of all, after this Introduction, in section 2, the experimental facilities and methodology used throughout the investigation are described. Special attention is given to optical arrangement for schlieren setup, CH radical chemiluminiscence setup, and image acquisition and processing. In section 3, a combined analysis of experimental measurements is presented. A review of some significant results on liquidphase penetration and stabilized liquid length previously obtained with Mie scattering and data obtained with CH radicals and schlieren are compared and analyzed to efficiently characterize evaporative sprays. Finally, some remarkable conclusions about the potential of the combination of three complementary measurement techniques are drawn.

chamber condition

acquisition

Pinj (MPa)

ET (ms)

P at TDC (MPa)

T at TDC (K)

density (kg/m3)

Δt (μs)

repetition

30 30 30 30 80 80 80 80 160 160 160 160

2 2 2 2 1 1 1 1 1 1 1 1

5 5 7 7 5 5 7 7 5 5 7 7

950 800 950 800 950 800 950 800 950 800 950 800

18 22 26 30 18 22 26 30 18 22 26 30

30 30 30 30 20 20 20 20 20 20 20 20

3 3 3 3 3 3 3 3 3 3 3 3

2. EXPERIMENTAL SECTION 2.1. Injection System, Fluid Properties, and Nozzles Used in the Experiments. A high-pressure common rail (CR) diesel engine has been used. Maximum injection pressure in the rail was 180 MPa. Fuel employed was Repsol CEC RF-06-99. The fuel properties and uncertainties in their determination are given in Table 1. Three similar nozzles (six-hole sac) are studied in this work. The Bosch flow number [defined as ṁ f/(ΔP)1/2] is the same for the three systems, but different conicity (k factor in Table 2) identifies each nozzle. Additional data defining the geometry of the nozzles can be found in Table 2. 2.2. Hot Spray Test Rig. A two-stroke single-cylinder engine was used for CH chemiluminiscence and schlieren measurements. The engine is motored by an electric engine and equipped with an optical accessible cylinder head, which contains a cylindrical combustion chamber. The piston compression−expansion strokes simulate realistic 5482


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in-cylinder thermodynamic conditions, in which injection−combustion processes can be studied. The device has the possibility to work in inert and reacting atmospheres as described in ref 28. In the first case (the option used for the present work), pure nitrogen is supplied to the engine as an intake gas to avoid combustion. The engine runs in a closed-loop circuit, where intake nitrogen is blown into the engine by a roots compressor. After injection, exhaust gases are cooled and filtered by a system that separates a residual fuel and a possible solid particulate. The resulting clean nitrogen atmosphere goes back into the compressor. Images presented here were acquired with the three multi-hole nozzle sprays injected into a cylindrical chamber with an inner diameter of 46 mm and length of 35 mm. 2.3. Optical Configuration for the Schlieren Technique. Shadowgraph and schlieren33 techniques have been traditionally used to study the spatial distribution of density gradients within a transparent medium. The optical setup of conventional schlieren imaging is similar to that of shadowgraphy, except for the spatial filter (usually a knife edge) used to block part of the light from the test section.34,35 Given the constraints associated with the geometrical configuration of diesel sprays generated by multi-hole injector nozzles, the double-pass schlieren arrangement has been used during the tests, as shown in the sketch of Figure 1. Details and main components forming this test setup are resumed in Table 3. 2.4. CH Chemiluminescence Technique. The wavelength band (λ) spectrum is properly defined by the light intensity emitted by CH radicals. In this work, a filter is used to acquire λ values between 375 and 405 nm to detect CH. Moreover, an intensified charge-coupled device (ICCD) camera (LaVision-Dinamight, 512 × 512 pixels) is needed, because the total amount of light emitted at these frequencies is small. The gain level remained unchanged (99%) to allow for a direct comparison of different experiments. Furthermore, the acquisition process is cut off if extended saturation is detected. Besides that, the exposure time for image acquisition is also constant for all of the tests (20 μs). This setup, on the one side, allows for us to obtain information on the appearance of CH radicals and, on the other side, avoids saturation of the camera sensor. Additionally, the camera trigger is conveniently delayed depending upon the injection pressure (30 μs between photographs for Pinj = 30 MPa and 20 μs for Pinj = 80 and 160 MPa) to obtain correct data about the CH radical trend during the engine cycle. Finally, three repetitions

Figure 2. Image processing methodology used in CH radical chemiluminiscence. Results belong to nozzle 3 with chamber conditions of 7 MPa and 800 K and an injection pressure of 160 MPa.

Figure 3. Example of the schlieren imaging sequence obtained for nozzle 1, with an injection pressure of 160 MPa, T of 950 K, and density of 26 kg/m3. 5483


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Figure 4. Summary of the liquid spray penetration results from previous Mie scattering measurements.

Figure 5. Contour maps of CH radicals represented for all of the nozzles and three different engine conditions. Liquid spray penetration from Mie scattering is represented over each map. 2.5. Experimental Matrix. The main data of tested operating conditions are summarized in Table 4. Three injection pressures (Pinj)

are taken at each time step to ensure that test results are statistically consistent. 5484


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Figure 6. Delay of the CH appearance from the SOI for all nozzles and different injection pressure and chamber conditions. are considered (30, 80, and 160 MPa) under four engine conditions given by pressure and temperature at the top dead center (TDC). Pressure and temperature values are quantified at the piston TDC, where injection started. The pressure at the TDC is measured with a piezoelectric sensor, whereas the temperature at the TDC is estimated through a procedure aiming at quantifying the polytropic coefficient of the engine compression. Both pressure and temperature at the TDC are indirectly governed by controlling the engine inlet nitrogen conditions with a root compressor and a heat exchanger. Energizing time (ET) is selected adequately (2 ms for Pinj = 30 MPa and 1 ms for Pinj = 160 MPa) to obtain stabilized liquid-length conditions. 2.6. Image Processing. 2.6.1. Image Processing for CH Radical Visualization. Purpose-made software is used to obtain and process main images. Each image is divided into different sectors that correspond to each nozzle orifice. The software analyses the radial variation of intensity for each nozzle and calculates the mean value in the complete sector. Figure 2 (upper part, central graph) shows the data obtained from the CH radical visualization in terms of the intensity for a single sector of nozzle 3, at a specific time during the test (Pinj = 160 MPa, chamber at 7 MPa and 800 K). Is it clear that the radial evolution of intensity follows a non-symmetric bell distribution, with maximum values near the chamber wall. This statement can also be established when looking at the photograph (upper part, on the left). If the information of all sectors (sprays) and the three repetitions for a given instant is averaged, the information given in the graph in the upper part of Figure 2 (right) is obtained. The bottom part of Figure 2 uses the complete series of images taken during the test to obtain a comprehensive time analysis of the results. Intensity values are integrated over the radial position, and a single value of intensity is therefore obtained for each nozzle at every time step. This type of analysis allows for the determination

of the time when the CH radicals start appearing and also the time instant when the maximum level of intensity is placed. Uncertainties in the determination of both measurements are estimated in ±10%. Moreover, two-dimensional (2D) contour maps of the whole process can be drawn (Figure 2, in the middle) if the measured intensity emitted by CH radicals is simultaneously represented (gray scale: 0, white; 1, black) as a function of the radial position (y axis) and time (x axis). It is worth mentioning that the contour map depicts the mean values of all of the chamber sectors and all of the repetitions. This representation provides a complete understanding of the injection and autoignition processes. Finally, three pictures taken at different times (from 0.4 to 0.5 ms) are directly compared to the results shown in the CH radical intensity graph. The intensity level in the first photograph is much lower, because the combustion process is at the first stage. Subsequent pictures show that the CH concentration is higher (mainly near the combustion chamber walls). 2.6.2. Image Processing for Schlieren Measurements. A sample of the results obtained from schlieren images is show in Figure 3. Those results belong to nozzle 1, for an injection pressure of 160 MPa, discharge pressure of 7 MPa, and temperature and density in the chamber of 950 K and 26 kg/m3, respectively. In this sample, sprays are shown in two different regions: a dark area that apparently seems related to the liquid part of the spray, surrounded by a brighter region that apparently corresponds to the vapor phase. The part of the spray close to the nozzle exit is expected to present a continuous core of liquid fuel21,36,37 or, at least, a droplet number density so high that rays crossing the spray would not be detectable. Moreover, the big difference between the refractive index of liquid fuel (around 1.46) and that of nitrogen (around 1) would cause a deflection of rays too strong to be detected. For these reasons, 5485


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considering that the vaporization process is controlled by mixing,10 an expression for the stabilized liquid length LL can be achieved10

LL =

1/2

( 14 π4 Ca)

K p2

Cmvρa

Doρl1/2

1/2

=

1/2

( 14 π4 Ca)

K p2

Cmv

Deq (1)

where Kp is a constant that includes the dependence upon the spray cone angle, Ca is the contraction coefficient,10,16 Do is the diameter of the orifice of the nozzle, Deq is the equivalent diameter, ρa and ρl are densities of air and fuel, respectively, and Cmv is the value of the fuel mass concentration in the axis at which liquid fuel is totally evaporated because of the entrainment of warm air. Values of Cmv, ρa, and ρl remain constant in eq 1 for a given engine operating condition. Therefore, LL variations can be just explained in terms of Do, Ca (which depends upon the cavitation regime), or the spray cone angle. An example of the liquid spray behavior, summarizing the most important results extracted from ref 10, is shown in Figure 4, where the liquid-length penetration is depicted against the start of energizing (SOE). From the experimental results, it is observed that the cylindrical nozzle (nozzle 3) provided the highest value of LL at a low injection pressure (30 MPa), because of its higher diameter and the absence of cavitation (and thus, Ca ≈ 1), as demonstrated in ref 10. At 80 MPa, the bigger diameter of nozzle 3 is compensated by the decrease in Ca because of the degree of cavitation. On the other hand, when the two conical nozzles are compared, no clear differences are shown. In this case, there are opposite effects affecting the stabilized liquid length; from the point of view of the outlet diameter, it is higher for nozzle 2. However, from the point of view of the contraction coefficient, nozzle 1 shows higher values. Neither nozzle 2 nor nozzle 1 showed evidence of cavitation inside during the experiments mostly because of their convergent configuration.12−16 The uncertainty of the measurement is ±3%, and as seen, differences between nozzles (cylindrical and conical) are higher than the uncertainty of the measurement. As far as high injection pressures are concerned, at an injection pressure of 160 MPa, cavitation is more severe in the cylindrical nozzle and the effect of the diameter is compensated for by the decrease of Ca. Furthermore, an increase in the spray cone angle was observed, as expected with the inception of cavitation.12 For this reason, the liquid length for nozzle 3 decreases, and therefore, all nozzles have quite similar values of liquid length in the tests performed at those injection conditions. 3.2. CH Chemiluminescence Technique Results. Figure 5 shows the spatial and temporal distributions of the intensity of CH radicals for all of the nozzles involved in the study obtained with the ICCD camera by means of the procedure explained in section 2.4, and the position of the maximum intensity at each time step is overlapped (continuous line) in the same graph. Besides that, liquid-phase penetration LL obtained using the Mie scattering technique is also plotted in parallel with the chemiluminescence results to extract all of the useful information from the experiments. Trends found are consistent with previous results of the liquid length.10 Delay between the start of injection (SOI) and CH appearance can be easily processed from the pictures. To obtain a clearer idea about the tendencies shown, values of the

Figure 7. Comparison between nozzles in terms of the delay of appearance of CH radicals from the SOI. Values for nozzles 1 and 2 are relative to nozzle 3.

the liquid part of the sprays should appear in the schlieren images as a dark region. In the vapor phase, the refraction index of fuel spray should be closer to that of the surrounding gas and deflection angles should be smaller, appearing in the image as either brighter regions or areas with high contrast. It may induce to erroneously think that the liquid and vapor phases can be clearly identified by schlieren images, but this assumption cannot be stated, as will be shown later in section 3. Nevertheless, important information can be extracted from schlieren images from the total penetration of the spray point of view and also when comparing schlieren images to CH radical maps, as will be seen later. To analyze the pictures taken with schlieren imaging measurements, an algorithm based on the one-dimensional (1D) log-likelihood ratio test (LRT) detailed in ref 38 has been successfully applied to the particular schlieren images presented in this paper, to determine the real spray boundaries. As previously seen in Figure 3, schlieren images contain three classes of pixels: dark and bright pixels that correspond to the spray and intermediate gray levels that correspond mostly to the background. For this purpose, it was necessary to perform a preliminary study on the histogram to identify the shape of the background distribution and to divide the histogram into two parts with only two classes: a histogram for segmentation of the dark part of the spray and the background and another histogram for segmentation of the bright part of the spray. Then, the LRT algorithms were applied independent to the two parts of the histogram for automatic detection of the two thresholds.

3. RESULTS 3.1. Liquid-Length Penetration. If the turbulent spray theory for a fuel parcel in a quasi-steady spray is assumed and 5486


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Figure 8. Comparison of liquid-length penetration and schlieren results, where a distinction is made between bright and black zones. The injection pressure is 80 MPa. The temperature and pressure in the chamber are 950 K and 7 MPa, respectively.

first apparition delay are plotted in Figure 6 for all of the conditions used in this study. To determine the first appearance of CH, a small trigger level of intensity is used (0.001) in the curve of the intensity as a function of time (see Figure 2, at the bottom), obtained following the procedure described in section 2.6.1. As mentioned there, each point of this curve is the average of the radial intensity integration of all of the sprays (sectors) (six) and the three repetitions. The CH radical apparition delay is related to the characteristic mixing time, and therefore, as seen in ref 10, it depends upon the effective injection velocity and the spray cone angle. Because the conical nozzles have a higher effective injection velocity for any injection condition (quantified in ref 10), they should show the lower delay between SOI and CH apparition in Figure 7, and in fact, they do. Nevertheless, as happened with the liquid length, differences between the conical and cylindrical nozzles should be reduced as the injection pressure increases because of the increase of the cavitation intensity. Cavitation would tend to reduce the difference upon the injection velocity, and also, an increase of the spray cone angle is expected as the cavitation intensity increases.12 As seen in Figure 7, the higher differences, contrary to this reasoning, are found at high injection pressures. This disagreement could be due to the uncertainty in the determination of this measurement. In fact, as mentioned in section 2.6, this uncertainty is quantified in ±10%. As seen in Figure 6, differences between nozzles for the injection pressures of 30 and 80 MPa are of the same magnitude order than the uncertainty, and therefore, these results should be treated with caution. It is known that CH radicals are an indicator of lowtemperature pre-reactions that only may occur once the fuel is

completely mixed with warm air and vaporized. In this sense, Figure 5 shows that the appearance of CH radicals is positioned next to the liquid spray. CH radicals should come into sight beyond the position of the stabilized liquid length LL. However, the intensity of CH radicals is observed before liquid-phase penetration reaches the LL stabilized value. This behavior can be explained because fuel evaporation starts at the boundary of the spray, where the fuel concentration dramatically decreases as we move in a perpendicular way from the spray axis to the periphery of the spray.36,37 This result was also observed by Flynn et al.,39 who reported chemiluminescence emission around the liquid spray. Therefore, it seems reasonable that the CH intensity is detected before the stabilization of the liquid length. Figure 7 represents the non-dimensional time (taking nozzle 3 as a reference) of the appearance of pre-reactions (CH radicals) for all of the nozzles (nozzles 1−3) and different operating conditions. Most of the values obtained are lower than 1, which means that cylindrical nozzle (nozzle 3) needs higher times for evaporation and pre-reactions. On the contrary, conical nozzle 2 provides evidence of the lowest relative values for the two parameters plotted in the graph. 3.3. Schlieren Results. Figure 8 represents the penetration of the liquid phase previously analyzed by the Mie scattering technique compared to the penetration obtained through the different colored pixels obtained with schlieren imaging. Squares in Figure 8 represent the dark zone (black pixels) associated with the dense part of the spray, while triangles correspond to the bright zone (white pixels, vapor phase). This information is given for all of the nozzles (nozzles 1−3) and 5487


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penetration of schlieren images [named in Figure 8 as schlieren results (white zone)]. In fact, if one observes the temporal evolution of the white zone, which can be considered as a global or total penetration from the schlieren images, one can see that it can be considered as the vapor penetration once the points representing the liquid-length penetration and those representing the white zone of schlieren start diverging from each other. Obviously, liquid penetration stabilizes at what is so-called the liquid length, and vapor penetrates until the combustion chamber wall is reached. Finally, the potential of the three experimental techniques involved in the research is shown in Figure 9, where over the CH contour maps has been added the liquid-phase penetration (Mie scattering) and total penetration (schlieren) for all of the nozzles and 160 and 7 MPa of injection and back pressures, respectively. The temperature in the chamber was 950 K, and the density, according to Table 4, was 26 kg/m3. It is worth noting that total schlieren penetration marks the axial limit of chemiluminiscence, and therefore, as stated in the previous paragraph, it can be considered as the maximum vapor penetration, showing the results also as a continuation of those characterizing the liquid penetration obtained by means of the Mie scattering technique.

4. CONCLUSION The potential of the combination of three different and complementary techniques (Mie scattering, double-pass schlieren, and CH radical chemiluminescence) has been used to study the evaporation process of diesel sprays over different multi-hole nozzles. For this purpose, three six-hole sac nozzles, with different orifice degrees of conicity have been used. These nozzles had been geometrically and hydraulically characterized in a previous publication, where important non-dimensional coefficients describing the nature of the flow have been evaluated. Injection at different pressure conditions has been carried out in an inert environment of gas to study the mixture process, avoiding window fouling by soot deposition from combustion. Studies have been carried out for three different injection pressures and four different gas densities during spray injection, representative of real engine conditions, with changing temperature and pressure in the discharge camera. The main conclusions can be summarized in the following points: (1) From liquid-length results obtained with Mie scattering, it was observed that the cylindrical nozzle (nozzle 3) provided the highest value of LL at a low injection pressure (30 MPa), because of its higher diameter and the absence of cavitation (and thus, Ca ≈ 1), as demonstrated in ref 10. On the other hand, when comparing the two conical nozzles, no clear differences have been shown between them. (2) Air−fuel mixing and evaporation processes strongly influence prereactions and, therefore, CH radical chemiluminescence. Although it is supposed that CH radicals appear beyond the position of the stabilized liquid length, the first CH radical intensity is detected before liquid-phase penetration reaches the stabilized value. The explanation can be found taking into account that fuel vaporization begins first in the lateral boundary of the spray, as also observed by Flynn et al.39 (3) When the CH radical apparition delay is compared from the start of the injection between all of the nozzles and for all injection conditions, in overall terms, conical nozzles present less delay than the cylindrical nozzle. This means that the air− fuel mixing process is more effective in the case of conical nozzles, which are mainly due to their higher injection velocity,

Figure 9. Comparison of CH radicals, liquid-length penetration, and vapor penetration for all nozzles. The injection pressure is 180 MPa. The temperature and pressure in the chamber are 950 K and 7 MPa, respectively.

injection and back pressures of 80 and 7 MPa, respectively. The temperature in the chamber was 950 K. As clearly noted, apart from the very beginning of the injection, liquid-phase penetration results do not match with the temporal evolution of the darker zone of schlieren results. The same comparison has been performed for all injection conditions and nozzles, and the comparison was always conclusive. Although at first sight, it seems intuitive that the schlieren technique could make it possible to distinguish between liquid and vapor phases, a previous comparison demonstrates that that distinction cannot be made. Therefore, to discriminate between the liquid and vapor phases, it is necessary to use other techniques. This discrimination, indeed, can be performed if we compare liquid-length results obtained by Mie scattering and total 5488


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ueff = effective velocity at the outlet orifice, defined as ueff = Cvuth uth = theoretical velocity, obtained from Bernoulli equation, defined as uth = (2ΔP/ρl)1/2

as demonstrated in ref 10 Nevertheless, differences found should be reduced with the increase of the injection pressure according to the increase of cavitation in nozzle 3. This last result was not observed in our research. (4) As far as the schlieren measurements are concerned, because of the constraints associated with the geometrical configuration of diesel sprays generated by the multi-hole nozzles, a double-pass schlieren arrangement has been used. An “off-axis” configuration was tested that allowed for the distinction of two different regions: a dark area apparently related to the liquid part of the spray and a brighter zone that apparently corresponded to the vapor phase. (a) The image processing of schlieren images and the comparison to liquidlength penetration results obtained by Mie scattering have demonstrated that such distinction cannot be made. (b) Schlieren penetration marks the axial limit of CH chemiluminiscence, and therefore, it can be considered as the maximum vapor penetration. (c) To discriminate between liquid and vapor phases, other complementary techniques, such as Mie scattering, have to be used, in addition to schlieren measurements.

■

Greek Symbols

■

ΔP = pressure drop, defined as ΔP = Pinj − Pback ρa = ambient density ρl = fuel density

REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +34-963879659. Fax: +34-963877659. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was partly sponsored by “Vicerrectorado de Investigación, Desarrollo e Innovación” of the “Universitat Politècnica de Valencia” in the frame of Project “Estudio del Flujo en Toberas de Inyección Diesel Mediante Técnicas LES (LESFLOWGRID)”, Reference 2837, and “Ministerio de Ciencia e Innovación” in the frame of Project “Estudio Teór ico-Experimental sobre la Influencia del Tipo de Combustible en los Procesos de Atomización y Evaporación del Chorro Diesel (PROFUEL)”, Reference TRA2011-26293. This support is gratefully acknowledged by the authors.

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NOMENCLATURE Aeff = effective area of the nozzle Ao = geometrical area of the nozzle Ca = contraction coefficient, defined as Ca = Aeff/Ao Cv = velocity coefficient, defined as Cv = ueff/uth Deff = outlet effective diameter of a nozzle orifice Deq = equivalent diameter of a nozzle orifice, defined as Deq = Do(ρl/ρa)1/2 Di = inlet diameter Do = outlet diameter ET = energizing time k factor = nozzle conicity, defined as k factor = 100(Di − Do/L) Kp = constant in LL analysis, including the effect of the cone angle L = nozzle length LL = liquid length Pback = back pressure Pinj = injection pressure SOE = start of energizing SOI = start of injection T = temperature in the engine injection chamber TDC = top dead center 5489


Energy & Fuels

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