* Associate Director, † Deputy Director, ‡Group Director, § Division Head, ¶ Project Manager (Engines), # Engineer, 1

PERFORMANCE OF A COAXIAL SWIRL INJECTOR WITH FLOW PATH REVERSAL

*V.Gnanagandhi, †G.K.Kuruvilla,

‡A.K.Ray, §S.Venkateswaran, ¶P.Arunkumar, #M.N.Prakash,

Liquid Propulsion Systems Centre (LPSC),

Indian Space Research Organisation (ISRO), Valiamala, Trivandrum, India-695 547.

email: liblpscisro@sancharnet.in. Liquid Propulsion Systems Centre has developed 22N and 10N bi-propellant thrusters employing hypergolic propellant combination of NTO as oxidizer and MMH as fuel. Coaxial swirl injector is used in these thrusters with oxidizer flowing through the inner swirl passage and fuel through the outer swirl passage. In order to enhance the thruster performance and to make it less sensitive to fabrication deviations, different configurations of injector were studied. In the first phase of investigation, fuel was allowed to flow through the outer swirl passage and oxidizer through the inner swirl passage. The difference between the inner and outer flow cone angles was varied by altering the surface finish of critical inner flow passage only. The performance was compiled by conducting cold flow studies and hot firing tests. In the second phase, injector dimensions were modified for flow path reversal to allow oxidizer flow through the outer passage and fuel flow through the inner passage. In this case also a similar experiments were carried out. Deposition of carbonaceous particles which was observed on the injector face while oxidizer flowed through the inner swirl passage disappeared when oxidizer was admitted through the outer passage.The specific impulse also improved by as much as 10secs and was found to be insensitive to minor variations in the inner flow cone angles. The results are discussed in this report.

Nomenclature As-Swirl Number.

Cd-Coefficient of discharge.

C*-C-Star (Combustion efficiency index.)

Fu-Fuel

HAT- High altitude test.

Isp-Specific Impulse

MIB- Minimum Impulse bit

MMH-Monomethyl hydrazine.

NTO-Nitrogen tetroxide.

Ox-Oxidiser.

SMD-Sauter mean diameter.

UDMH-Unsymmetrical Dimethyl hydrazine.

a-Half spray angle.

Ø-Coefficient of useful cross section.

?P-Pressure drop.

41st AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit10 - 13 July 2005, Tucson, Arizona

AIAA 2005-3744

Copyright © 2005 by Indian Space Research Organisation. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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I. Introduction Coaxial swirl injectors are used in small earth storable bi-propellant thrusters as well as in high thrust cryogenic engines. In developing an advanced liquid rocket engine, injector design is critical to obtaining the dual goals of long life and high energy release efficiency. A coaxial swirl injector delivers a spray pattern consisting of two concentric cones and both cones diverge towards the chamber wall. This concept provides a good film cooling capability. Published literatures on spray characteristic studies 1-4 of coaxial swirl injectors have not reported the performance sensitivity of a thruster to propellant flow path reversal. This is more relevant in the case of earth storable propellant fuels such as MMH and UDMH, which have a good amount of hydrocarbons resulting in carbonaceous particle formations. In the present study a 22N thruster which uses NTO as oxidizer and MMH as fuel is investigated for flow path reversal and performance sensitivity by doing hot HAT firings.

II injector description The main design parameters for a co-axial swirl injector are : outer and inner flow outlet diameters, pressure drops of flow and spray cone angles.

INNER.

OUTER.

Fig-1 shows the spray cone concept.

Two types of injectors were realized. In Type-1 fuel flows through the outer swirl passage and oxidizer through the inner swirl passage. In Type-2, the configuration is reversed. Theoretical flow cone angles are given in Table-1 for Types 1&2. Spray scattering angle is determined using the equation 5

Tan? =222 4)]1(1[

2

dd

sd

AC

AC

??? ?

Where Cd=

}1)1({

1

2

2

?? ??sA

and

As=3

2)1(

???

Coeff. of useful cross section –ø is determined from the geometry of swirler. Swirl number-As and Coefficient of discharge-Cd are derived from it.

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Spray cone Angles.(2? ) Type

Inner Outer

1 71(Ox) 101(Fu)

2 85(Fu) 101(Ox)

Table-1

Theoretical prediction of Cone angles. In both types, two hardware each were realized (A&B), one (A) with flow passage having a good surface finish using reaming process and another (B) without reaming so that inner cone angle could be different. Thrust chamber is silicide coated and is electron beam welded with the injector6.

III Cold flow tests

Pressure drop evaluation is carried out by using de-mineralized water at rated flow rate. Spray angle is measured4 using digital camera and the values are given in Table-2.

Spray Cone Angles (2? )

Injector Type

Inner Outer Remarks

1A 68 (Ox) 110 (Fu) Ream finish of inner flow path.

1B 63 (Ox) 110 (Fu) No reaming of inner flow path..

2A 77 (Fu) 112 (Ox) Ream finish as done in

1A.

2B 70 (Fu) 112 (Ox) No ream finish.

Table 2

Measured Spray Cone Angles As seen from Tables 1 & 2 there is a max. difference of about 10% in the spray angles computed and measured for Types-1A and 2A. In each type of injector, the inner flow spray angle is varied by controlling the surface finish of the inner flow path as explained in Table2.

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IV Hot test HAT firing was carried out as per the sequence given in Table-3.

Test

Run No. Detail No. of Firing/ Pulses

1.Continuous

1a 100 sec reference 1

1b 400 sec+ 400 sec (hot restart of inj. flange.) 1

1c 1000 sec 1

2.Pulse

2a 10ms on/120sec off 10

2b 16ms on/120sec off 10

2c 32ms on/120sec off 10

2d 64ms on/120sec off 10

Table 3

Hot Test Sequence Nominal continuous firing MR was kept at 1.65±0.05. All the above mentioned tests were carried out at the beginning of life and end of life injection pressures of 16.5 bar(a) and 11.5 bar(a) respectively with measurement of thrust, chamber pressure, propellant flow rates, vacuum level and thruster injector-chamber interface temperature .and injector flange temperature. Throat temperature measurement was carried out using optical pyrometer. Also the injector inner face was inspected for any particle deposition using endoscope at the end of continuous sequence and pulse mode firings.

V. Results and discussion

In continuous mode operation, Isp realized and thruster temperatures are compared and given in Table-4.

Type ISP sec

Injector Temp (Deg.c.)

Throat Temp. (Deg.c)

1A 280 140 800

2A 293 100 950

Table 4

Continuous mode performance comparison (at 16.5 bar )

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Isp for Type 2 injector is more by over 10 sec compared with Type-1. This is due to wider inner spray and less difference in spray cone angles between inner and outer flow for Type 2 as given in Tables 1& 2 and also higher ?P for this Type is more by 20% compared with Type-1. Higher ?P and wider spray angle results in smaller SMD7 and this effect combined with less difference in spray cone angles result in intense mixing of oxidizer and fuel and better combustion efficiency. It can also be inferred from the throat temperature rise that C* efficiency is more for Type-2. The measured throat temp for Type-2 is only 950°c which is much below the permissible operating temperature of 1260°c for silicide coated columbium nozzle. Injector flange temperature for Type-2 is less than Type-1 by 40°c. The inner flow pattern is a fully developed conical spray for Type 1and Type-2. Outer flow pattern for Type-1 is having a waviness in conical spray indicating transition phase from tulip to fully developed regime where as for type 2 injector, a fully developed conical pattern is achieved by higher ?P. Because of this, film cooling at chamber injector interface is better for Type 2 resulting in lower injector flange temperature. A plot of injector flange temperature for 1000 sec is given in Fig-2. Similar trends are seen when the tests were carried out at 11.5 bar as given in Table-5 and Fig.-3.

Figure-2 Injector flange temperature for 1000 sec (at 16.5 bar)

Type Isp (sec) Injector Temp. (o.C)

Throat Temp. ( o.C)

1A 277 135 770

2A 289 80 900

Table-5

Continuous mode performance comparison (at 11.5 bar)

22N THRUSTER COMPARISON BETWEEN TYPE-1 & TYPE-2

1000 SECS - TEMPERATURE PLOT (at 16.5 bar).

TYPE-1

TYPE-2

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Figure-3 Injector flange temperature for 1000 sec (at 11.5 bar)

For evaluating the spray angle sensitivity on Isp, each type of thruster was tested for different inner spray cone angles and the results are given in Table-6.

Measured spray angle (2? ) Type

Inner

Isp Sec

1A 68(Ox) 280

1B 63(Ox) 274

2A 77(Fu) 293

2B 70(Fu) 292

Table 6

Isp sensitivity on spray cone angle It can be seen that Isp sensitivity for Type-2 is much less compared with that of Type-1. This is due to less disparity between inner and outer spray angle for Type-2 which makes it less sensitive to minor variations in spray cone angles caused by deviations in fabrication process. In pulse mode firing, MIB comparison is given in Table-7.

22N THRUSTER COMPARISON BETWEEN TYPE-1 & TYPE-2

1000 SECS - TEMPERATURE PLOT (at 11.5 bar).

TYPE-1

TYPE-2

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MIB in mNsec Pulse width

(ms) Type-1 Type-2

10 83 141

16 165 221

32 520 637

64 1200 1332

Table 7

Pulse mode Performance Comparison Pulse mode MIB for Type 2 is substantially better than Type-1 indicating improved combustion efficiency in pulse mode also. A comparison of pulse shapes at 64ms and 16ms pulse widths are given in Fig.4&5.

Figure 4 Pulse shape plot for 64ms

Figure 5

Pulse shape plot for 10ms

22N THRUSTER COMPARISON BETWEEN TYPE-1 & TYPE-2

10ms PULSE SHAPE.( at 16.5 bar)

Type-2

Type-1

22N THRUSTER COMPARISON BETWEEN TYPE-1 & TYPE-2

64ms PULSE SHAPE ( at 16.5 bar).

TYPE-1

TYPE-2

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Observations on injector face Carbonaceous particles were seen deposited on the injector bottom face at the end of continuous sequence of tests for Type-1 whereas no such particles were observed for Type-2 injector when viewed through endoscope. At the end of pulse mode of firings no particles were observed for both Types. In Type-2 injector where oxidizer envelops fuel, diffusion flame is established8 at the oxidizer fuel vapour interface. Even if carbonaceous particles are formed in the fuel vapour due to various reasons, as it moves outwardly towards the oxidizer vapour region, upon crossing the flame, these particles get oxidized leaving no chances for particle formation and settling on the injector face. On the other hand, if fuel is enveloping oxidizer like in Type-1, there are chances of non-oxidized carbonaceous particles getting formed and settled down on the injector face. Even minor variations in the fuel outer flow angle could trigger this, which can affect the performance over a period of time as it can migrate into the flow passages after a number of restarts. This phenomenon is not very significant in pulse mode where outer flow diverging conical pattern is not fully developed, hence re-circulation and settling of un-burnt particles do not take place.

VI Conclusion A study of coaxial swirl injector configuration where-in oxidizer flows through the outer swirl passage and fuel flows through the inner swirl passage was seen to have the following advantages. ? ? Non-formation of carbonaceous particles on injector face . ? ? Better performance in continuous mode as well as in pulse mode firings. ? ? Less sensitivity to fabrication tolerances ? ? Lower injector temperature.

Acknowledgments

The authors gratefully acknowledge the unceasing support given by Mr. N.Vedachalam, Director, LPSC, to the spacecraft propulsion area. Authors wish to thank Mr. N.G.Unni, for the encouragement given during the study. Also thanks to Messrs. V.K.Devaraj, M. Vinodha kumar, K.Umar, T.John Tharakan and David Dasan for their support during the realization and experiments.

References

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