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An Experimental Study on Fuel Atomization in Pressurized Injector

M. H. Ziraksaz

Science and Research Branch, Islamic Azad University, Tehran, Iran,

Experimental evaluation of reverse engineering swirl-pressurized atomizer is the main Scope of this experimental work. Fuel mass flow rate, Spray cone angle, Droplet velocity and SMD are the parameters which are considered in this work. A set of twelve reverse engineering atomizers without any spray defections is contributed. A set of eight original atomizers is used as a well performed sample. More than 350 tests, mostly repeated, were performed to make the evaluation.

Key Words: Swirl Atomizer, Experimental Evaluation, Defections, Fuel Rate, Spray Cone Angle, Droplet Velocity, SMD. Introduction

In liquid propellant rocket engines and cruise missiles several atomizer types are used. Atomizers’ performance affects the missile mission especially in mini turbojet engines installed on cruise missiles. This work focuses on the evaluation of the Reverse Engineering (RE) atomizers. During the process of spray formation in swirl-pressurized atomizers, the injected fuel tries to experience an onion-shaped form. This form is always created in a low pressure, almost about 5 Psig and continues to grow up to 15-20 Psig. The onion nature helps the injected fuel to flow from its root, the atomizer tip, to all around the onion and finally meet the upper point of the onion where the fuel flows over the separated sheets. This is the starting point of the spray. In this case, the spray is not yet formed but this is a quite primary shape of the spray named tulip form. Increasing the fuel pressure, the onion becomes smaller but the tulip grows up until the onion disappears and the tulip shape spray fully develops at about 50 Psig. Neya studied the effects of water injection pressure in a conical spray. The cone angle increases

proportionally with the injection pressure [1]. The most important fact approved by conducted tests is that the spray never appears unless the onion does. This means that onion creation is one of the key parameters to creating and developing the spray in these atomizers. Therefore, onion creation is the starting point of fuel atomization. More orifice diameter, unmatched convergence walls of the swirl chamber, and improper roughness in these parts are the most important reasons preventing the onion creation in the atomizer.

Although onion creation is the key point, tests do not approve its sufficiency. In fact, onion is observed in some cases, but it is neither able to build the tulip nor to fully develop spray and that is because of some problems in nozzle tips, especially in nozzle tip length and its roughness. That is why the fuel spray is not formed completely or partially. In these cases, despite the spray being observed somehow, combustion process cannot completely take place due to improper spray characteristics, especially the drop size and fuel pattern distribution. Such fuel sprays cause high fuel consumption and hot

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spot creation all around the inner layer of the combustion chamber.

At the end of the onion, where the walls separate and spread away, the tulip starts to form. Its body looks like a cup which its walls try to grow up and separate away. This shape is the very first shape of the fuel spray. As the tulip grows, the onion experiences a smaller size until it disappears. In this case, the fuel spray takes its primary shape and pressure increase causes the spray to develop more and find its final shape. Therefore, the tulip is fully developed and deformed to a conical or umbrella shape spray. Crua in a study about fuel spray creation and auto-ignition use some different methods to show the fuel injection effects on fuel spray creation and development [2]. Fuel spray creation is not a criterion for evaluating the atomizer performance. It just shows if the spray creation process takes place correctly. But performance is another matter. During the conducted tests, several fully developed sprays with undesired performance were observed. This means that spray creation is the very first step for evaluating the atomizer performance. In fact, atomizer performance strongly depends on spray characteristics. There are several difficulties to evaluate the atomizers via theoretical methods. The problems highlighted when the atomizer manufactured based on RE method. That is why the main scope of this work is experimental evaluation of RE swirl atomizers.

Atomizer Structure

Figure 1 illustrates the schematic non-dimensional drawing of the studied swirl atomizer. There are several parts but the main ones are: internal fuel passages, Spin valve, complex swirl chamber, and the final orifice. The atomizer performance strongly depends on the proper

dimension, roughness, and burnishing of these parts:

1- Internal Fuel Passages, 2- Spin Valve, 3- Orifice Internal and External

Passages, and 4- Nozzle Tip Length and

Diameter.

Fig.1. The Studied Swirl Atomizer Schematic

Map. Laboratory Conditions

Using the real fuel during the test is impossible because of the safety. Therefore, a replaced liquid is used. To release the best results, the replaced liquid must have the same properties as the original fuel. The liquid which is used in this work is a calibration liquid, MIL-F-7024A-TypeII. The replaced liquid has no difference just burning. The properties of this fuel, JP4 and JP5 are listed in Table 1. Although fuel properties are fixed but the fuel-fuel mixtures have different properties in different mixing ratios.

Table1- Properties of replaced fuel, JP4 and

JP5

JP4 JP5 MIL-F-7024A-Fuel Type

0.0250.025 0.025 Surface Tension

0.9 1.4 1.12 Kinematic viscosity

0.76 0.81 0.762 Specific Gravity [cSt]

80 80 80 Lab Temp. [F]

26.4 26.4 26.4 Lab Press. [in-Hg]

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Gao determines the Ethanol–Gasoline mixture in various mixing ratios. The results show that the mixture properties are different from Ethanol and Gasoline properties [3]. Test Facilities

The system includes a glass cylindrical tank divided to 8 sectors with thin walls each sector connected to a manometer, liquid collector, pumps and some other devices. The atomizer must be installed vertically on top of the cylinder and the fuel pipe connects the fuel manifold to the atomizer. Fuel flows through the pipes into the atomizer and based on the back pressure, the fuel is atomized while flowing through the atomizer. The collected fuel in each sector, indicate the fuel pattern in relevant pressure. On top of the cylinder and just below the atomizer a mechanical device is installed to measure spray cone angle and its skewness. Fuel flows as a narrow jet when the injection pressure is less than 5Psig. But increasing the pressure makes the fuel to flow as a deformed jet. In fact an onion shaped jet with some spiral motion creates just closed to the atomizer nozzle tip. The onion fully develops in 15 Psig and begins to bloom and a tulip shape jet creates. More increasing the pressure, the tulip spreads a way and the onion begins to experience smaller size. At 30 Psig while tulip tries to find its fully developed shape, the onion disappeared. At 50 Psig the tulip fully develops and fuel spray creates.

Observed Defections and Practical Correction

Several tests and corrections have to take place on the manufactured RE atomizer to identify and illuminate the defections. In this work, twenty sets each 12 atomizers manufactured through RE method and bellow

defections were observed during the tests:

1- Low Cone Angle, 2- Onion Absent, 3- Rich Fuel-Air Mixture, 4- Narrow and Long Spray, 5- Hot Spot over Combustion

Chamber, 6- Light and Heavy Streaks, 7- Rotary Streaks, 8- Voids, 9- Hitting Shroud. 10- Skewness, 11- Spray Bell Ringing, 12- Drooling and 13- Spitting.

Figure 2 shows the accuracy of

the tests clarifying that accuracy proportionally increase with pressure. Conducting several tests and performing experimental corrections, all the problems and the defections highlighted step by step and then illuminated. The corrected atomizers show nice fitted performance as the original ones. The accuracy graph of the tests shows that errors decreases by increasing the fuel pressure. Conducted tests show that the fully developed spray appears at 50 Psig where the error tries to diminish. The most observed problems in atomizers are listed below: Less cone angle than the original atomizer and the standard value,

1. Onion absent and consequently absent of fuel spray,

2. Weak Seal Protection especially in spin valve and nozzle tip,

3. Unacceptable fuel passages depth and length,

4. Unacceptable symmetry center and axis,

5. Notch and coarse existence, 6. Less thread depth and 7. More thread length.

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On the other hand the most critical points are:

1- Orifice interior Roughness (Fig.3),

2- Swirl chamber interior section roughness (Fig.4),

3- Orifice exterior and connected channel interior Roughness (Fig.5),

4- Connected channel interior diameter and length (Fig.3 and 5),

5- Final conical portion length and angle (Fig.3 and 5),

6- Spin vale top roof conical angle (Fig.4 and 6),

7- Swirl passages slope, roughness and depth (Fig.6),

8- Spin valve top roof and swirl chamber interior wall adaptation (Fig.7),

9- Threads acceptable depth and length (Fig.8) and

10- Acceptable unthreaded distance between feeding pipe and exterior wall (Fig.8). Gellalles in a work shows that

some of the above parameters strongly depend on each other [4]. Elkotb shows that swirl atomizer geometry affects the atomizer performance [5]. Therefore changing a parameter changes not only all relevant parameters but also the predicted performance.

Fig.2. The Accuracy of the Tests with Inlet Pressure.

Fig3.Orifice interior roughness

Fig4.Swirl chamber interior roughness

Fig5.Orifice exterior and connected chamber

interior roughness

Fig6.Swirl passages slope, roughness

and depth

Fig7.Spin valve top roof (right) and swirl chamber interior wall (left) adaptation

Fig8.Threads acceptable depth and length

(right), Acceptable exterior wall unthreaded distance to feeding pipe (left)

Test Data and Performance Chart

In this work all tested atomizers were fabricated by RE method as a copy of a set of original atomizers. Defections and several existing problems cause that none of them works properly and therefore they performed too bad to be used in motor. That is why several experimental corrections take place to make them performed well. The performance chart of original atomizers is used as a criterion to evaluate the performance of these atomizers. To do so, the

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original atomizers tested first to obtain the performance charts and then the same tests repeats to obtain the RE atomizers performance charts.

A single test in different tries show different but nearly the same results, therefore complete matching of the RE atomizers is impossible. To overcome the matter, original atomizers were tested several times and the results plotted in a single chart to obtain more acceptable regions. The results make a bound. This bound is named the acceptance bound. It means that all results laying in this bound are the correct ones. On the other words RE atomizers can perform well if their test results meet the acceptance bound.

Fuel Mass Flow Rate

Although the original and the RE atomizer charts never matched completely but the physical behavior of them must be the same and the RE charts must be in acceptance bound. In this work two original atomizers tested to obtain the mass flow rate behavior in different inlet pressures. The curves show the upper and lower limits of the acceptance bound. To more clarify the acceptance bound, the results repeats for five original atomizers (fig.9).

Fig9.Mass Flow Rate results of 5 original atomizers.

In this work 12 RE atomizers

were contributed and the same tests performed. To better comparison of the curves, these atomizers were categorized in 3 groups each contains 4 atomizers. The results are shown in

figures 10 to 12. In all the graphs, the acceptance bound is shown by the original atomizers.

Fig10.Mass Flow Rate results of D01, D04, D07 and D08 atomizers.

Fig11.Mass Flow Rate results of D02, D03, D05 and D06 atomizers.

Fig12.Mass Flow Rate results of D09, D10, D11 and D12 atomizers.

The acceptance bound is

highlighted by upper and lower limits. Figure10 shows that the results cover the lower region of the acceptance bound. While fig11 shows that the results of this group cover the central region of the acceptance bound. On the other hand fig12 shows that the third group behave as the same as the first group. The graphs show:

1- All the RE atomizers are in the

acceptance bound,

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2- The first and the third groups behave as the same and

3- The arrangement of the atomizers in a group set installation must be based on the behavior of them in their groups.

Spray Cone Angle

The spray cone angle is the second parameter which is contributed in this study. In this case to evaluate the performance chart of the RE atomizers, the cone angle performance chart of two original atomizers obtains first (Fig.13) and the performance chart of the 3 RE atomizer groups obtain then. To more comparison of the charts, the performance charts of the RE atomizers draw with the upper and lower limits of the acceptance bound. Figures 14 to 16 show the same results as the Mass flow rate charts.

The graphs show that not only the spray cone angle is in the range of 75 to 80 degree but also in most of the cases the curves matched with the original atomizers. However in a group set installation of these 12 RE atomizers, the arrangement is the same as the results obtained from the Mass Flow Rate ones. Fuel Droplet Velocity

The third parameter is the Fuel Droplet Velocity. The procedure is the same as previous parameters. But the velocity is obtained based on the practical relations and the experimental amount of mass flow rate. Although the physical behavior of the experimental results for the original atomizers are the same but they never matched completely and therefore the acceptance bound creates. In this case five original atomizers are considered to obtain the acceptance bound. Figure 17 shows the results while figures 18 to 20 show the results of three groups of the RE atomizers.

Fig13.Spray Cone Angle of two Original atomizers.

Fig14.Spray Cone Angle of D01, D04, D07 and D08 atomizers.

Fig15. Spray Cone Angle of D02, D03, D05 and D06 atomizers.

Fig16. Spray Cone Angle of D09, D10, D11 and D12 atomizers.

The most important notes are: 1- Droplet velocity and mass flow

rate are not proportional necessarily, 2- Uniform velocity causes

uniform fuel spray,

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3- To obtain proper flame pattern, the fuel injection into the combustion chamber must meet special arrangement,

Fig17. Fuel Droplet Velocity of five Original atomizers.

Fig18. Fuel Droplet Velocity of D01, D04, D07 and D08 atomizers.

Fig19. Fuel Droplet Velocity of D02, D03, D05 and D06 atomizers.

Fig20. Fuel Droplet Velocity of D09, D10, D11 and D12 atomizers.

Fig21.SMD results of 5 original atomizers.

Fig22.SMD results of D01, D04, D07 and D08 atomizers.

Fig23. SMD results of D02, D03, D05 and D06 atomizers.

Fig24. SMD results of D09, D10, D11 and D12 atomizers.

Fuel Droplet (SMD)

The SMD data outsources from other experimental studies on this Atomizer. The SMD data is obtain from five original atomizers and the test repeats for 3 groups of RE atomizers. Study shows that the

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acceptance bound is very thin because of very nice matching of the original and RE atomizers. Just physical behavior matching of the original and RE atomizers expected in this work. The results show that not only a single RE atomizer shows singular behavior in repeated tests but also individual RE atomizers matched completely with together. Suggestions

During this practical evaluation several problems appear and make the evaluation difficult. Not only Mechanical measurements bring errors but also fog formation all around the atomizers makes low visual field. To better data collection, below notes suggested:

1- Using a width enough glass chamber as a test rige [6],

2- Using a high frequency digital camera to record the events,

3- Using a converter hardware to online data transfer and

4- Graphical analyzing of the recording file, frame by frame.

The advantages of this method are:

1- Possibility of using air jet all around the inner side of the glass chamber to avoid fuel deposition on the walls,

2- Data collection without physical installation of mechanical devices close to the spray,

3- High accuracy and operator errors illumination,

4- Providing various environmental conditions,

5- Providing group tests, 6- Full observation of various

parameters affects the fuel spray characters,

7- Providing wall-spray interaction investigation,

8- Providing sprays interaction investigation in group set installation and

9- Optimization of fuel fog formation in group set installation.

Conclusion

In this work one can find that: 1- RE atomizers contains several

problems and never can performs properly without corrections and evaluation,

2- Most of the problems are fabrication problems,

3- The original atomizer performance charts are used as a criterion,

4- The original atomizers never show the same results in repeated tests, therefore a bound of results creates. This bound is acceptance bound.

5- The RE atomizer tests laying within the bound of acceptance, makes the RE atomizer to be accepted.

6- The Re atomizer mass flow rate tests show that the set of twelve atomizers can be categorized in three groups each include 4 atomizer acting as the same. It means that in group installation, these groups must be installed in specific locations to provide spray pattern properly.

7- All corrected atomizers meet the acceptance bound, whether in lower region or in central region of the acceptance bound.

8- The spray cone angle tests show the same results as the mass flow rate.

9- This practical work shows that the cone angle is between 75 and 80 degree, as expected.

10- The tests show that the droplet velocity and the mass flow rate are not proportional necessarily,

11- To create a uniform fuel spray, velocity profile must be uniform somehow,

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12- Each atomizer must be installed in proper location because of proper flame pattern and in combustion chamber [7],

13- The SMD tests show that the acceptance bound is so much thin to be a bound. In fact the original and the RE atomizers SMD results matches completely.

References

1- K.Neya and S. Sato, Effect of Ambient Air Pressure on the Spray Characteristics of Swirl Atomizers, Ship Res. Inst. Tokyo, paper27, 1968.

2- Cyril Crua, Sergei Sazhin,

David Kennaird, Morgan Heikal, The initial stage of fuel spray penetration , Fuel 82 (2003) 875-885,2003.

3- Jian Gao, Deming Jiang,

Zuohua Huang, Spray Properties of Alternative Fuels: A Comparative Analysis of Ethanol–Gasoline Blends and Gasoline, Science Direct, Fuel 86 (2007) 1645-1650, 2007.

4- A.G. Gellalles, Effect of

Orifice Length-Diameter Ratio on Spray Characteristics, NACA TN-352, 1930: Effect of Orifice Leigth-Diameter Ration on Fuel Sprays for Compression-Ignition Engines, NACA Report 402, 1931.

5- M.M.Elkotb, N.M. Rafat, and

M.A.Hanna, The Influence of Swirl Atomizer Geometry on the Atomization Performance, First International Conference on Liquid Atomization and Spray Systems, pp.109-115,1978.

6- E. Delacourt, B. Desmet, B.

Besson, Characterization of Very

High Pressure Diesel Sprays Using Digital Imaging Techniques, Fuel 84 (2005) 859-867,2005.

7- H. Clare, J. A. Gardiner, and

M. C. Neale, Study of Fuel Injection in Air Breathing Combustion Chambers, in Experimental Methods in Combustion Research, pp. 5-20, Pergamon, London, 1964.