Combustion Stability Characteristics of Coax-Swirl ...

Combustion Stability Characteristics of Coax-SwirlInjectors for Oxygen/Kerosene Sebastian Soller 1 , Robert Wagner 2 and Hans-Peter Kau 3 Technische Universität München, D-85747 Garching, Germany

Philip Martin 4 and Chris Mäding 5 Astrium Space Transportation GmbH, D-81663 München, Germany

A number of Coaxial Swirl injectors were hot fire tested in a single-element rocket thruster. The experiments were conducted at combustion pressures in the range of 4.0 MPa to 8.5 MPa and mixture ratios between 2.4 and 3.5. The injector performance was assessed by static wall pressure measurement and the analysis of heat transfer to the water cooled combustion chamber wall. Other criteria to describe the behavior of the injectors are the discharge coefficients of fuel and oxidizer orifices as well as the combustion efficiency ηc*. The investigations were completed by dynamic pressure measurements located in the oxidizer feed line as well as in the combustion chamber itself. In some configurations, combustion instability was encountered. The analysis of dynamic pressure measurements confirmed the assumption that the instability matched the first longitudinal mode and its harmonics. Within this paper, four design variations are discussed with respect to their influence on combustion stability.

Nomenclature Latin characters A = area [m², mm²] a = sonic velocity [m/s] cD = discharge coefficient [-] c* = characteristic velocity [m/s] d = diameter [m, mm] f = frequency [Hz] l = length [m,mm] L* = characteristic length [m] = mass flow [kg/s] m& O/F = mixture ratio [-] p = pressure [MPa, bar] Q/A = heat flux density [MW/m²] r = radius [m,mm] V = volume [m³, mm³]

Greek Characters ε = area ratio [-] η = efficiency [-] κ = isentropic coefficient [-] π = natural constant, pressure ratio [-] ρ = density [kg/m³] Abbreviations DAQ = data acquisition FFT = Fast Fourier Transformation GOX = gaseous oxygen LOX = liquid oxygen ORSCC = oxidizer rich staged combustion cycle Indices c e n

= chamber = exit = nozzle

1

Research Assistant, Technische Universität München, D-85747 Garching, AIAA Student Member Research Assistant, Technische Universität München, D-85747 Garching, AIAA Student Member 3 Professor, Technische Universität München, D-85747 Garching, AIAA Assosciate Fellow 4 Research Engineer, Astrium Space Transportation, D-81663 München 5 Research Engineer, Astrium Space Transportation, D-81663 München 2

1 American Institute of Aeronautics and Astronautics

C

I. Introduction

ombustion stability has been a critical issue in the development of rocket engines ever since the first days of rocketry. Elimination of instabilities has been historically one of the largest components of all new liquid rocket engine development programs. This is because there has been little methodology to ensure stability by design and also because of the potentially catastrophic consequences of instabilities. During the development of the F1 rocket engine, more than 100 injector head configurations were tested in above 1000 test in order to ensure the reliable and save operation of that engine1. A standard reference comprising the findings of that program and the related research activities has been issued by NASA and is still a valuable tool in analyzing the stability behavior of liquid rocket engines2. Due to their geometrical boundary conditions, liquid rocket engines are usually sensitive to radial and tangential modes of pressure fluctuation. Countermeasures to avoid combustion instabilities are baffles, which impede the propagation of radial and tangential pressure oscillations near the injector face plate, and damping devices as quaterwave- or Helmholtz resonators, which remove energy by dissipation if properly tuned to the critical instability frequency. Injector elements implemented in modern oxidizer rich staged combustion cycle (ORSCC) engines feature additional design characteristics which enhance combustion stability. A typical representative of this injector type is implemented in the Russian RD-170 engine built by Energomash3. In these elements, the hot oxidizer rich exhaust gas from the precombustor is directed through a central tube. The fuel – kerosene – is injected through tangential orifices into an annular duct, forming a swirling sheet of liquid. In a mixing and prereaction zone recessed from the combustion chamber, the combustion process is initiated and stabilized. Thus, the heat release rate is protected from negative influences of radial or tangential pressure fluctuations. The central oxidizer post is tuned as quarter-wave resonator. An orifice at the entrance into the post provides sufficient acoustic impedance to prevent pressure fluctuations from propagating into the oxidizer manifold. The heritage of Russian gas-liquid coax swirl injector element is documented in Oxidizer several publications dealing with these elements’ ability to control combustion 4, 5 stability . Due to their advantages and the potential benefits of the propellant combination LOX/Kerosene implemented in oxidizer rich staged combustion cycle Oxidizer engines6, research activities in the United States focused on ORSCC engine design in Manifold recent years7. Objective of most programs was to improve existing models to predict the stability margin of the injector design with respect to the influence of the oxidizer post’s acoustic characteristics8. Analytical work was accompanied by experiments in a single-element setup at moderate chamber pressures using kerosene RP-1 as fuel and decomposed hydrogen peroxide as oxidizer9. The design of experiment ensures well defined boundary conditions in the oxidizer post as well as a decoupling of the Fuel Manifold injector from the oxidizer feed system. Another work was published focusing on the Fuel effect of vortices, which are shed by the collar of the fuel sleeve and impinge onto the chamber wall10. The interaction of these vortices and the spray preparation and heat release is discussed as candidate mechanism influencing the development of combustion instabilities. Ongoing research activities have been published in the Combustion Chamber following years11. Fundamental investigations of effects dominated by coupling of the acoustics of injector and feed system have been conducted in France12, 13 and Figure 1. Injector in Russia14, 15. RD-170 engine

II. Experimental Setup and Approach Astrium Space Transportation and the Institute for Flight Propulsion have been conducting several research programs to investigate key technologies of future launcher propulsion systems. Experiments dealt with cooling aspects as well as with injector concepts for ORSCC engines 16-19. A subscale rocket test facility has been set up and numerous experiments conducted in a water-cooled single element combustion chamber. The influence of several design parameters on the performance of the injectors was assessed in spray tests and subsequent hot fire testing. Spray tests as well as combustion experiments were conducted at chamber pressures between pc = 4.0 MPa and pc = 8.5 MPa and at mixture ratios in the range from O/F = 2.4 to O/F = 3.5.


A. Single Element Combustion Chamber The modular design of the combustion chamber is shown in Figure 2. The setup allows for high flexibility with respect to injector configuration and chamber length. The injector head consists of the fuel and oxygen manifold, into which the injector is fit. The coax swirl injectors investigated consist of a oxygen post and a fuel sleeve mounted in the injector head. The oxygen is directed through the central tube of the oxidizer post. The kerosene flows through spirally wound channels on the outer surface of the post, which together with the sleeve form the fuel injection slots. A collar at the exit of the fuel channels enhances the formation of a homogeneous thin film entering the recess area. The energy necessary to initiate combustion is provided by a gas torch igniter, which is mounted perpendicular to the main chamber flow in the so called igniter ring. Up to four water cooled chamber segments can be clamped together with the nozzle segment by tie-rods in order to vary the characteristic length L* and the eigenfrequencies of the model combustor. The cooling system of the chamber can be divided into two flow passages, one providing the cooling water to the cylindrical section of the chamber, the other cooling the nozzle section. In the present studies, the two circuits were connected in series. In this way, testing times of up to 180 seconds with nine operating points investigated in one run were realized. The geometrical characteristics of the chamber setup are given in Table 1.

Table 1 Single element combustion chamber characteristics O/F pc dc

εC L*

εn de

[-] [MPa] [mm] [-] [m] [-] [mm]

2.4 … 3.5 4.0 … 8.5 20.00 2.5 1.10 1.9 17.40

Figure 2. Combustion chamber setup with gas torch igniter During the test campaign, a large variety of injector configuration has been under investigation. Table 2 gives an overview over all tested injectors. A detailed analysis of the injectors that showed good stability behavior has been published before16. This publication focuses on the influence of combustion instability on performance parameters like discharge coefficients, heat flux density and combustion efficiency. Furthermore, the spectral analysis of the elements will be discussed. The injector configurations considered within this work address four design characteristics ensuring the efficient and stable operation of the injectors in the existing single-element setup. The reference configuration is element 205. It consists of a standard sleeve combined with a post which features an orifice at the beginning of the GOX-post. For element 206 this orifice was removed (see Figure 3, top left). This injector configuration was prone to trigger longitudinal combustion instabilities. To asses the influence of the recess length on combustion stability, the two configurations 207 and 208 were tested, featuring a recess length of 7 mm and 0 mm, respectively. Both designs did not significantly improve the stability behavior in the present setup. Element 210 (Figure 3, top right) with increased fuel injection velocity did also show unsatisfactory stability behavior.


Table 2 Survey of element design variations ID

GOX Orifice

Recess

Fuel Area

Recess tapered

GOX Swirl

204

12mm

Yes

nominal

No

No

205

12mm

Yes

nominal

No

No

206

12mm

No

nominal

No

No

207

7mm

No

nominal

No

No

208

0mm

No

nominal

No

No

209

12mm

No

nominal

By 45%

No

210

12mm

No

reduced

No

No

211

12mm

No

reduced

By 62%

No

212

12mm

No

reduced

By 45%

No

301

12mm

No

nominal

No

A

302

12mm

No

nominal

No

C

303

7mm

No

nominal

No

C

304

12mm

No

nominal

No

E

305

12mm

No

nominal

No

F

Three designs proved stable operation in the complete operating range. The first feature which inhibits the development of combustion instabilities is the mentioned orifice at the entrance to the oxidizer post in configuration 205. The second measure to provide combustion stability by the design of the element was a conical constriction applied to the recess. As an example, results obtained for the element configuration 209 will be discussed within this publication. An image illustrating the redesigned fuel sleeve can be seen on the lower right hand side of Figure 3. As a third possibility to improve the element performance, swirl caps were fixed to the oxygen post. Variations of these injectors can bee seen in the bottom left corner of Figure 3. Stability rating was accomplished with the injector types 301 to 303. In order to protect the hardware from damage due to oscillating pressure loads, most configurations were tested only at a constant mixture ratio of 3.2 and chamber pressure levels of 6.0 MPa and 8.0 MPa for five seconds each. If they featured stable behavior, they were characterized in long-run experiments covering the above mentioned full range of operating parameters.

Baseplate Fuel Sleeve

GOX Post

Aref

Ared,1 R

Figure 3. Element configurations under investigation


B. Measurements and Data Treatment Four pressure taps were used to record the axial distribution of static wall pressure within the combustion chamber. The sensors PC0 to PC3 were located at an distance of 7 mm, 104 mm, 234 mm and 394 mm from the face plate. The fuel and oxidizer sided discharge coefficient of the element was calculated with the chamber pressure and distinctive pressures in the manifolds. The required mass flow information was provided by coriolis mass flow meters installed in the test bench’s feed systems. The discharge coefficient on the fuel side was calculated assuming incompressible fluid flow: c D , fuel =

m& f uel

A fuel ⋅ 2 ⋅ ( p fuel − pc )⋅ ρ fuel

(1)

On the oxidizer side, compressibility effects were taken into account: c D ,ox =

m& ox

(2)

2 κ +1 ⎡ ⎤ ⎛ pc ⎞ κ ⎛ pc ⎞ κ ⎥ ⎢ ⎟⎟ − ⎜⎜ ⎟⎟ Aox ⋅ 2 ⋅ ⋅ p ox ⋅ ρ ox ⋅ ⎜⎜ ⎢⎝ p ox ⎠ κ −1 ⎝ p ox ⎠ ⎥⎥ ⎢⎣ ⎦

κ

The thermal loads of the combustion chamber wall were calculated using pressure and temperature information recorded at the inlet, transition and outlet manifolds of the water cooled segments. Mass flow measurement of the cooling fluid was realized by calibrated orifices in the feed line as well as in the dump line of the open cooling system. All the recorded signals were time averaged and used as an input for the computation of combustion efficiency ηc*.This is expressed as ratio of the characteristic velocity c*test obtained in the test and the theoretical value as predicted by a calculation with NASA’s CEA2-code20, 21 assuming complete combustion at the beginning of the contraction:

η c* =

c *test c *theor .

(3)

The characteristic velocity c*test is calculated as follows:

c *test =

ptot ,throat ⋅ Athroat m& ox + m& fuel

(4)

To calculate the total pressure in the throat section, the static wall pressure in the last chamber segment was used as an input for a CEA2-calculation loop. The pressure in the chamber was set to match the measured pressure at the end of the chamber with the “combustion end” value of a finite area combustor calculation. The resulting total pressure in the throat, which accounts for losses due to heat release and the contraction of the flow, was used as input value in equation ( 3 ). The heat losses to the water cooled wall were compensated for by a correction of the enthalpy of the reactants being injected. Additional losses due to friction and boundary layer effects were taken into account by a discharge coefficient of the nozzle obtained from calculations with the TDK-code:

Athroat = cD ,nozzle ⋅ Ageo

(5)

In order to asses the dynamic behavior of the chamber pressure, a piezoelectric pressure measurement probe was flush mounted 7 mm downstream of the injector face plate.A second sensor was fixed in the oxidizer feed line 165 mm upstream of the entrance to the manifold to investigate potential coupling of pressure fluctuations in the chamber with the gas reservoir of the oxygen feed system. The dynamic pressure sensors were sampled on a separate DAQ system at a rate of 15,000 Hz. The signals recorded were processed with a Fast Fourier Transformation implemented in Matlab®. The results were transformed in pressure level values. In order to obtain 5 American Institute of Aeronautics and Astronautics

the correct value of the amplitude as well as the frequency, a rectangular as well as a flat top window filtering have been applied to the signal.

C. Combustion Chamber and Feed System Acoustics In order to characterize the recorded frequencies of pressure oscillations, the resonance frequencies of the combustion chamber were estimated using a simple model of a gas tube. The frequencies for radial, tangential, longitudinal and combined modes were calculated using the equation

f m ,n =

a eff 2 ⋅π

⎛ im ,n ⎜⎜ ⎝ rc

2

⎞ ⎛ j ⋅π ⎟⎟ + ⎜⎜ ⎠ ⎝ lc

⎞ ⎟⎟ ⎠

2

(6)

In a fist approach, the sonic velocity aeff was assumed to be constant throughout the chamber. Its value was calculated using the CEA2-code. The factors i and j are the counters of the particular modes. The experiments focused on the investigation of longitudinal modes, because they are dominant in the used chamber setup. Due to the geometry of the chamber, radial and tangential modes are located at higher frequencies and longitudinal modes are the first ones to be excited in the chamber. The length of the resonance volume lc was calculated taking into account the admittance of the contraction towards the throat. For the interpretation of the recorded pressure signals several possible effects of the chamber geometry were considered. First, it was expected that the recess area of the injectors may contribute to the longitudinal frequencies with its own length. Therefore, longitudinal frequencies were calculated with and without taking into account the recess for a later comparison with experimental data. The sonic velocity was assumed to be constant at the value of complete combustion. Changes in the sonic velocity due to mixing and combustion within the recess were neglected. As there is a linear relation between frequency and chamber length as well as sonic velocity, the sensitivity towards changes of these parameters can easily be estimated. Table 3 highlights the increase in resonance frequency with the chamber length. Table 3 Influence of injector recess on oscillation frequency O/F [-] 2.88 3.07 3.15 3.36 3.69

f(L1) w/o recess [Hz] 1450 1417 1416 1406 1370

f(L1) w/ recess [Hz] 1408 1377 1375 1366 1331

In a similar way a local refined calculation of the sonic velocity, which takes into account the process of mixing and combustion in the first part of the chamber, will lead to an additional decrease in frequencies. Additionally, the resonant frequencies of the oxidizer post were analyzed as well as the length of the oxidizer feed line between the injector manifold and the oxygen control valve. As the latter is operated at sonic conditions, the corresponding entrance to the feed line can be treated as rigid wall in the acoustic model of the feed system. Depending on the injector configuration, different resonator models are possible. If the injector manifold is treated as open end, the feed line may act as λ/4-resonator. The corresponding resonant frequencies of a tube with the length l can be calculated easily. This approach is justified, if the GOX-post of the injector features no or negligible constrictions, as this applies for the configurations 205 to 210. If injectors with swirl caps are tested, the entrance to the GOX Post can be treated as closed wall. In this case the feed line can be modeled as λ/2-resonator. The frequencies calculated for a effective tube length of 350 mm and different typical oxygen temperatures between 250 K and 280 K are given in Table 4.


Table 4 Resonance frequencies of the oxidizer feed line TGOX n

[K] 1 2 3 4 5 6 7 8

250 215 430 645 860 1076 1291 1506 1721

260 219 439 658 878 1097 1317 1536 1756

270 223 447 670 894 1118 1341 1565 1789

280 227 455 682 910 1138 1365 1593 1821

In some test cases the frequencies recorded in the combustion chamber and the feed system agree quite well with the harmonic frequencies of the oxidizer feed line. Data suggest that a coupling of feed system and combustion chamber has taken place leading to low frequent combustion instabilities. Another topic to be addressed was the influence of the gas torch igniter on the pressure oscillations within the chamber. After the ignition of the combustion chamber, the igniter is shut off and purged with nitrogen to protect it from combustion products of the main chamber. In this configuration it can act as a Helmholtz resonator affecting the processes in the combustion chamber. The resonance frequency of a Helmholtz-resonator of the volume V, which is connected to the combustion chamber by a tube of the length l and the diameter d, can be expressed as

f =

ae ff 2 ⋅π

d ² ⋅π . 4 ⋅V ⋅ (l + 0,85 ⋅ d )

(7)

The calculated frequencies are a function of the sonic velocity aeff of the gas within the igniter. Thus, the results can be expected to show some variance during test as some heat and mass transfer to the igniter’s combustion chamber may occur due to changes in pressure levels within the main chamber. For example, if the chamber pressure is decreased from 8 MPa to 4 MPa during the test, half of the gas within the igniter is emptied towards the chamber. If the chamber pressure rises again in a successive operating point, hot combustion gas is pressed into the igniter. Nevertheless, the relevant frequencies can be expected to range between 130 Hz and 150 Hz. Thus, effects of the gas volume within the igniter can be assumed as negligible for the pressure oscillations within combustion chamber. The sensor placement has great influence on the detectable amplitude of an oscillation. If the sensor is placed at an antinode, the real amplitude can be measured directly, but no amplitude will be resolved at a node. As mentioned before, the sensor PCDYN is mounted in the igniter ring, 7 mm downstream the injector faceplate. If the chamber is assumed to be a λ/2-resonator, pressure antinodes are located at throat and the faceplate. In this case, the sensor PCDYN yields a signal which gives around 93% of the real peak. The dynamic pressure sensor PODYN is located in the GOX feed line near the middle of the distance between the control valve, which is operated at sonic condition and thus represents a pressure antinode, and the entrance to the GOX-post. As a consequence, the measured amplitude depends on the oscillation behavior of the feed line. For a λ/2-resonator the sensor is placed at a pressure node and no amplitude will be detectable. Assuming a quarter wave around 70% of the maximum amplitude will be measured.


III. Experiments and Results The analysis of the experimental data focuses on the configurations which showed combustion instability and compares the corresponding results with data obtained from three configurations which operate stable. The element 206 was tested in the complete operating range between 4.0 MPa and 8.0 MPa and led to combustion instabilities repeatedly. Only two short-time tests with this injector were operated at stable conditions. Four design variations were investigated with respect to their sensitivity towards combustion instabilities: The element 205 with the orifice at the oxidizer post inlet was stable in the complete operating range. The same applies for the injectors 210 with a tapered recess and the elements 302 and 303 with oxidizer swirl caps. The injector 209 with increased fuel injection velocity operated unstable in most of the tests. An overview over the test matrix of each element is given in Table 5. Table 5 Performance parameters of conducted stability investigations Injector

HF?

205 206 209 210 302 303

+ + -

pc range [Mpa] 4.2 … 8.4 3.5 … 8.0 3.9 … 8.2 5.5 … 8.1 4.0 … 8.2 6.1 … 8.4

O/F range [-] 3.0 … 3.8 2.8 … 3.3 2.8 … 3.4 3.2 … 3.4 2.4 … 3.4 3.2 … 3.3

ηc* min [%] 93.5 84.7 92.9 85.3 94.2 96.7

ηc* max [%] 95.9 94.0 95.4 93.3 97.0 97.3

A. Element 205 with Orifice in the GOX Post The discussion of the combustion efficiency, pressure profile and heat transfer profile of this element configuration has been conducted in former publications18, 19. The presentation of results will focus on the characteristic frequencies obtained in the chamber and the GOX feed line. Figure 4 shows some examples of oscillation profiles recorded in different operating points. On the left hand side of the figures, the dynamic pressure signal is depicted as it was recorded. The signal of the sensor PODYN is colored red, the signal of PCDYN blue. Some signals show a drift due to the heating up of the sensor. This does not affect the quality of the FFT analysis, which has been computed for the time domain given in the graphs on the left hand side and is plotted on the right hand side. The operating point is noted in the upper right corner of the diagrams. Two graphs are plotted for each signal: The location of the peak’s frequency is given by the solid red and blue line. To calculate the amplitude of the recorded oscillation properly, a flat top window has been applied to the signal prior to FFT analysis. The results are resembled by the surfaces behind the solid lines. The borderline of this area gives the real amplitudes of the oscillations. The frequency spectrum of the pressure oscillations within the chamber features two characteristic peaks. One of them is located around 307 Hz – 337 Hz, the other varies between 468 Hz and 542 Hz. The latter of theses peaks can also be resolved at the sensor in the oxygen feed line. Another peak that can be resolved at both sensors is located between 600 Hz and 700 Hz. However, the amplitude of this peak is negligible compared with the two peaks at lower values. For the signal of PCDYN, another plateau can be detected at around 1500 Hz. This value is well above the characteristic frequency of a longitudinal instability in the chamber: The frequency of the peaks is given in Table 6. For each operating point, the frequency of a L1 instability has been calculated and listed in the third column of the table. In the subsequent columns, the dominant frequencies recorded with the sensor PCDYN and PODYN are listed. Typically, the amplitude of oscillation decreases with increasing frequency. The frequencies which can be seen as peaks in the corresponding figures are marked by bold characters.


TEH-2T3-05

12

2.5

OF = 3.23

8 6

PC = 60.1

2

PCDYN

Amplitude [bar]

PODYN, PCDYN [bar]

10

4 2

1.5

PCDYN

[322.3; 0.63]

1 [542.0; 0.44]

0

[1816.4; 0.09] [732.4; 0.17] [1450.2; 0.14] [542.0; 0.12] [1626.0; 0.12] [717.8; 0.10]

0.5

-2

PODYN -4 14.5

15

15.5

16

16.5

17

17.5

0

18

500

1000

1500

2000

Time [s] 35

2500 3000 Frequency [Hz]

3500

4000

2.5

Amplitude [bar]

PODYN, PCDYN [bar]

15

PCDYN

10 5

PC = 81.7

2

20

1.5

1

[336.9; 0.73]

PODYN

[512.7; 0.62]

0.5

[644.5; 0.30]

0

[3222.7; 0.09] [1567.4; 0.21] [1801.8; 0.15]

[659.2; 0.22]

-5 26

26.5

27

27.5

28

0

28.5

500

1000

1500

2000

Time [s] 20

[2959.0; 0.13] [4423.8; 0.12]


3500

4000

2.5

10

5

PODYN 0

1.5

1 [307.6; 0.59]

0.5

[468.8; 0.54]

[629.9; 0.24]

35.5

36

36.5

37

37.5

38

38.5

0

500

1000

[1523.4; 0.14]

1500

2000

Time [s]


3500

4000

Table 6 Frequencies recorded for element 205 O/F [-] 3.08 3.41 2.97 3.23 3.33 3.52 2.98 3.25 3.67

f(L1) [Hz] 1381 1356 1402 1380 1373 1356 1410 1387 1354

307.6 278.3 307.6 322.3 307.6 307.6 322.3 336.9 307.6

512.7 498.0 468.8 542.0 527.3 542.0 483.4 512.7 512.7

PCDYN [Hz] 659.2 673.8 629.9 732.4 688.. 688.5 629.9 659.2 732.4

1406.3 1420.9 1523.4 1450.2 1523.4 1523.4 776.4 1567.4 1538.1

1713.9 1626.0 1684.8 1669.9 1684.8 1601.8 1743.2

512.7 454.1 468.8 542.0 527.3 542.0 483.4 454.1


PODYN [Hz] 688.5 673.8 688.5 717.8 688.5 688.5 644.5 761.7

4500

5000

Page 1 of 3

Figure 4. Frequency spectra of element 205 in different operating points

PC0 [MPa] 4.12 4.20 6.11 6.01 6.16 6.18 8.11 8.17 8.49

5000

PC = 61.1

2 Amplitude [bar]

PODYN, PCDYN [bar]

15

4500

OF = 2.97

PCDYN

-5 35

5000

OF = 3.25

30 25

4500

1713.9

1757.8

Stable Operation

L1 Instability

0.9

0.9

0.8

0.8

0.7 0.6

Discharge Coefficient c D,O [-]

Discharge Coefficient Fuel cD,Fu [-]

Combustion Efficiency η c* [-]

B. No Orifice in the GOX Post The element configuration 206 without the orifice in the GOX post inlet area repeatedly showed instable behavior. Only in two short-time tests no instability was detected. Typically for longitudinal combustion instabilities, the combustion efficiency is decreased significantly by the large pressure fluctuations. Whereas in stable operation conditions values of ηc* ≈ 94% are reached, combustion efficiency drops to around ηc* ≈ 87% under instable conditions. This can be seen in Figure 5, where the values recorded for stable and unstable operation points are depicted as circles and filled circles, 1 respectively. There s a slight increase in ηc* 0.98 with the mixture ratio growing towards stoichiometrich conditions. 0.96 The fluctuations in the flow field also affect 0.94 the discharge coefficients of the element. Figure 6 shows how the discharge coefficients 0.92 both of the fuel and the oxidizer side are 0.9 decreased by the combustion instability. 0.88 Whereas the static pressure level in the oxidizer and fuel manifold is not affected by the 0.86 pressure fluctuations in the chamber, the 0.84 instability leads to a worse mixing behavior and 0.82 reaction rates of the propellants in the recess zone. This yields an overall lower static wall 0.8 2.6 2.7 2.8 2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 pressure level in the combustion chamber, Mixture Ratio O/F [-] which is resembled as well in the worse combustion efficiency as well as in a lower Figure 5. Combustion efficiency of element 206 under stable than usual discharge coefficients. and unstable operating conditions

Stable Operation

L1 Instability

0.5 0.4 0.3

0.7 0.6 0.5 0.4 Stable Operation

0.3 L1 Instability

0.2 0.1 1.4

0.2

1.6

1.8

2 2.2 2.4 2.6 Reynolds Number Fuel ReFu [-]

2.8

3

3.2 x 10

4

0.1 1

1.5

2

2.5 3 3.5 Reynolds Number Oxidizer ReO [-]

Figure 6. Discharge coefficients of element 206 under stable and unstable operating conditions


4

4.5

5 x 10

6

120

120

8 MPa

100

Heat Flux Density Q/A [MW/m2]


6 MPa

80

60

40

20

0 0

0.05

0.1

0.15

0.2 0.25 Axial Position x/L* [-]

0.3

0.35

0.4

0.45

100

80

60

40

20

0 0

0.05

0.1

0.15

0.2 0.25 Axial Position x/L* [-]

0.3

0.35

0.4

Figure 7. Heat flux density profiles of element 206 under stable and unstable operating conditions Figure 7 shows the axial distribution of the heat flux density to the water cooled wall of the combustion chamber for a chamber pressure of 6.0 MPa (left) and 8.0 MPa (right). The heat flux density is more than doubled in the first and second section of the chamber. In general, it is known that longitudinal combustion instabilities yield higher thermal loads to the combustion chamber wall due to the higher heat transfer coefficients caused by the velocity fluctuations in the chamber2. This effect is superimposed by the fact that the combustion instability disrupts the boundary layer and inhibits the formation of a cooling film on the combustion chamber wall. For that reason, the increase in heat flux density is very pronounced in the first and second segment of the chamber. Table 7 and the Figures 8 and 9 compare the results of the FFT analysis of the dynamic pressure signal recorded in stable and unstable operation conditions. In stable operating conditions, the dynamic chamber pressure signal shows two peaks around 300 Hz and 450 Hz. These peaks are located at the same frequencies for element 205 (see e.g. Figure 4). A look at the amplitude of the peaks reveals a difference: Whereas for the element 205 with the orifice the peaks feature quite similar amplitudes, the peak at 300 Hz is increased significantly if the orifice is removed. This indicates that this specific frequency is linked to the impedance of the oxidizer post. Consequently, a reduction in pressure drop on the oxidizer side increases the amplitude of oscillation. The peak at 450 Hz is not affected by the change in the oxidizer flow path. The spectrum of the unstable test in Figure 9 shows clear peaks at discrete harmonic frequencies typical for combustion instability. These frequencies match the first three modes of longitudinal combustion instabilities, which could be resolved with the DAQ system. For comparison, the frequency L1 has been computed for each operating point and is listed in Table 7. The results are calculated with a constant sonic velocity assuming completed reaction and take into account the length of the recess as part of the cylindrical volume to be considered. The assumption of a constant sonic velocity contributes to the difference between measured and calculated values. The occurrence of combustion instability is clearly stimulated by the removal of the orifice at the oxidizer post. By the increase in diameter, the impedance of the oxidizer post is reduced along with the pressure drop and pressure perturbations are forwarded more easily to the oxygen feed line.


0.45

Table 7 Frequencies recorded for element 206 PC0 [MPa] 3.4 3.58 5.47 5.49 5.51 5.56 5.57 5.89 6.03 6.04 6.99 7.23 7.29 7.44 7.59 8.03 8.37

6

O/F [-] 2.82 3.02 3.22 2.82 3.31 3.33 3.33 3.06 3.17 3.2 3.08 2.84 3.26 3.26 3.28 3.2 3.32

f(L1) [Hz] 1372 1352 1350 1385 1347 1346 1342 1384 1377 1372 1372 1390 1355 1358 1355 1380 1370

1318.4 1333 1318.4 1318.4 1303.7 1303.7 1318.4 113.3 307.6 307.6 1303.7 1333 1333 1318.4 1318.4 307.6 322.3

2622.1 2651.4 2636.7 2636.7 2607.4 2607.4 2636.7 307.6 454.1 454.1 2607.4 2651.4 2666 2636.7 2636.7 454.1 483.4

PCDYN [Hz] 3940.4 2666 3955.1 3955.1 3911.1 3911.1 3955.1 483.4 600.6 1113.3 3895.5 3969.7 3999 3955.1 3955.1 600.6 1010.7

PODYN [Hz] 1318.4 1333 1318.4 1318.4 1303.7

3969.7

1303.7 1069.3 1274.4

1259.8

1215.8

2666

1318.4 439.5 439.5 439.5 1303.7 1333 1333

3984.4 3984.4

1318.4 439.5 439.5

615.2

3779.3 3618.2

4130.9 3823.2

4145.5

3676.8

TEH-3T5-02 2.5

PODYN

OF = 3.17

3 2 1 0 -1 -2

PC = 60.3

2

4 Amplitude [bar]

PODYN, PCDYN [bar]

5

1.5 [307.6; 0.77]

PCDYN

1 [454.1; 0.27]

0.5

[600.6; 0.19]

PCDYN

[1069.3; 0.12]

-3 13.4

13.6

13.8

14

14.2 Time [s]

14.4

14.6

14.8

0

15

14

500

1000

1500

[1259.8; 0.10]

2000


3500

4000

2.5

8

Amplitude [bar]

PODYN, PCDYN [bar]

2 PCDYN

6 4 2 PODYN 0

500

OF = 3.20

12 10

4500

PC = 80.3

[307.6; 2.07]

1.5

1

0.5

[454.1; 0.35] [600.6; 0.25]

-2 -4 18.4

18.6

18.8

19 Time [s]

19.2

19.4

19.6

0

500

1000

1500

2000

Figure 8. Frequency spectra of element 206 in stable operating points



3500

4000

4500

500

TEH-3T5-05

150

25

OF = 3.33 PC = 55.6

20

100 Amplitude [bar]

PODYN, PCDYN [bar]

PCDYN

50

[1303.7; 16.68]

15 PCDYN

10 [2607.4; 6.51]

0

5 [3911.1; 3.32]

PODYN

-50 13.9

14

14.1

14.2

14.3

14.4

14.5

0

14.6

500

1000

1500

2000

Time [s] 200


3500

4000

25

Amplitude [bar]

PODYN

0

15 10

[2636.7; 9.08]

5

19

19.1

19.2

19.3 19.4 Time [s]

19.5

19.6

19.7

19.8

0

[3955.1; 4.99]

500

1000

1500

2000


3500

4000

4500

Figure 9. Frequency spectra of element 206 in unstable operating points

C. Increased Fuel Injection Velocity For element 210, the fuel injection velocity was increased by a reduction of the outer diameter of the GOX post and a corresponding change in the inner diameter of the recess zone. According to the time-lag-theory, this approach is viable if the change affect the area of heat release such that Rayleigh’s criterion for combustion instabilities is no longer fulfilled. The element was tested at a mixture ratio of O/F = 3.2 at chamber pressures of pc = 6.0 MPa and pc = 8.0 MPa. The expected improvement in 1 stability margin could not be verified. This may 0.98 be due to the fact that the kerosene is impinging 0.96 onto the chamber wall and differences in the velocity are attenuated quickly, so that no 0.94 Stable Operation significant effect on the heat release is generated. 0.92 Furthermore, if combustion instabilities are related to fluctuations in the oxidizer feed system, 0.9 changes in the fuel injection velocity have little 0.88 impact on the instability behavior. L1 Instability 0.86 As can be seen in the Figures 10 and 11, the difference in the values of combustion efficiency 0.84 and discharge coefficients are easy to discern for 0.82 stable and unstable operating conditions. Stable operation is marked by empty squares, the values 0.8 2.6 2.8 3 3.2 3.4 3.6 for cases of combustion instability are marked by Mixture Ratio O/F black filled squares. It is noteworthy that the combustion efficiency calculated for stable Figure 10. Combustion efficiency of element 210 under operating conditions is slightly lower for the stable and unstable operating conditions element 210 in comparison to the configuration 206 (93% vs. 94%, see Figure10). Combustion Efficiency η c* [-]

PODYN, PCDYN [bar]

50

-50 18.9

PC = 74.4

20

100

5000

OF = 3.26

[1318.4; 23.35]

PCDYN

150

4500


5000

0.9

0.8

0.8

0.7

0.7

0.6

Discharge Coefficient c D,O [-]

Discharge Coefficent Fuel cD,Fu [-]

0.9

Stable Operation L1 Instability

0.5 0.4 0.3

0.6 0.5 0.4 Stable Operation

0.3 L1 Instability

0.2

0.2

0.1 5

5.5

6

6.5 7 7.5 8 Reynolds Number Fuel ReFu [-]

8.5

0.1 1

9 x 10

1.5

2

4

2.5 3 3.5 4 Reynolds Number Oxidizer ReO [-]

4.5

5 x 10

6

Figure 11. Discharge coefficients of element 210 under stable and unstable operating conditions The influence of combustion instability on the heat transfer to the chamber wall is given in Figure 12. The increase in heat flux density under instable operating conditions reaches levels similar to those obtained for element 206. The influence of fuel velocity in stable operating conditions can be seen if the Figures 7 and 12 are compared. In case of stable operation, the element with the higher fuel injection velocity yields lower heat flux densities in the area of the last cylindrical segment. High frequency combustion instability leads to higher heat loads throughout the chamber. The occurrence of combustion instabilities also leads to a very characteristic pressure profile in this setup. As can be seen in Figure 13, the static wall pressure profile has a maximum in the first chamber segment in the case of unstable combustion. This can be interpreted as a result of the primary reaction zone relocated from the face plate and recess area to a region further downstream. If the combustion is stable, the primary reaction zone is attached to the element’s recess and an increase of the fuel injection velocity does not alter the pressure profile significantly. 120

120

8 MPa 100

100

Heat Flux Density Q/A [MW/m2 ]

Heat Flux Desnity Q/A [MW/m2]

6 MPa

80

60

40

20

0 0

80

60

40

20

0.05

0.1

0.15 0.2 0.25 0.3 Axial Position x/L* [-]

0.35

0.4

0.45

0 0

0.05

0.1


0.35

0.4

Figure 12. Axial distribution of heat flux density of element 210 under stable and unstable operating conditions


0.45

1

1

8 MPa

6 MPa

0.99 Normalized Pressure p/pc [-]

Normalized Pressure p/pc [-]

0.99 0.98 0.97 0.96 0.95

0.97 0.96 0.95 0.94

0.94 0.93 0

0.98

0.05

0.1


0.35

0.4

0.45

0.93 0

0.05

0.1


0.35

0.4

0.45

Figure 13. Axial pressure profile of element 210 under stable and unstable operating conditions An analysis of the pressure fluctuations recorded during stable operating conditions again shows a peak near 300 Hz, as it is known from the element configurations 205 and 206 in stable operating conditions. It stands out that the amplitude of the peak is much higher than in the other cases, whereas the second peak, which was visible in the Figure 8 and Figure 4 is negligible compared to the first peak (note the different scaling in the plot of the operating point with pc = 8.1MPa and O/F = 3.36 in Figure 14). The higher injection velocity leads to an amplification of the oscillation at 307 Hz Table 8 Frequencies recorded for element 210 PC0 [MPa] 5.5 5.54 6.01 6.01 7.24 7.36 8.03 8.01

O/F [-] 3.18 3.33 3.21 3.17 3.2 3.36 3.23 3.36

f(L1) [Hz] 1395 1384 1417 1420 1401 1387 1421 1409

1318.4 1303.7 293 293 1333 1333 293 307.6

2651.4 2622.1 556.6 439.5 2666 2666 439.5 600.6

PCDYN [Hz] 3984.4 3940.4 1010.7 3999 3999 615.2

PODYN [Hz]

1025.4

1215.8

1318.4 1318.4 278.3 439.5 1333

4013.7

293

439.5


TEH-9T10-02 2.5

PCDYN

5

0

[293.0; 1.76]

1.5 PCDYN

1 0.5

PODYN

[556.6; 0.30] [278.3; 0.18]

-5 13.8

13.9

14

14.1

14.2

40

14.3 14.4 Time [s]

14.5

14.6

14.7

0

14.8

Amplitude [bar]

20

PODYN

0 -10 18.8

1000

1500

2000 2500 3000 Frequency [Hz]

3500

4000

4500

5000

OF = 3.36 PC = 81.1

6

30

10

500

7

PCDYN

PODYN, PCDYN [bar]

OF = 3.21 PC = 60.1

2 Amplitude [bar]

PODYN, PCDYN [bar]

10

5 4 3

[307.6; 3.23]

2 1

18.9

19

19.1

19.2 19.3 Time [s]

19.4

19.5

19.6

0

19.7

[600.6; 0.41]

500

1000

1500

2000 2500 3000 Frequency [Hz]

3500

4000

4500

5000

Figure 14. Frequency spectra of element 210 in stable operating points

TEH-9T10-01

100

25

PCDYN

50 0

13.8

13.9

14

200

10

14.1

14.2 14.3 Time [s]

14.4

14.5

14.6

0

14.7

[2622.1; 6.34] [3940.4; 3.55] [1318.4; 1.64]

500

1000

1500

25

2000 2500 3000 Frequency [Hz]

3500

4000

20

100 50 PODYN

10 5

-50 18.9

0

19.1

19.2

19.3 19.4 Time [s]

19.5

19.6

19.7

19.8

5000

15

0 19

4500 OF = 3.36 PC = 73.6

[1333.0; 22.65]

PCDYN

150 Amplitude [bar]

PODYN, PCDYN [bar]

[1303.7; 15.71]

15

5

PODYN

-50 13.7

OF = 3.33 PC = 55.4

20 Amplitude [bar]

PODYN, PCDYN [bar]

150

[2666.0; 8.66] [3999.0; 4.82]

500

1000

1500

2000 2500 3000 Frequency [Hz]

3500

4000

4500

5000

Figure 15. Frequency spectra of element 210 in unstable operating points

D. Conical Constriction of Recess The element 209, in which the orifice of the GOX post has been replaced by a conical constriction of the recess cross section, showed stable behavior in all tested operating points. The performance parameters combustion efficiency, pressure profile and heat flux density distribution have been discussed in earlier publications 18, 19. Compared with the configuration 206 without orifice, the frequency spectrum shows a distinctive peak at a frequency around 300 Hz at a lower level (see Figure 16). Again, a frequency of around 450 Hz can be resolved but again it is less pronounced than in other configurations. Obviously, the pressure drop generated by the tapered recess provides a more efficient decoupling of pressure perturbations in the combustion chamber from upstream feed lines. Furthermore, the chemical reaction in the recess area is protected from oscillations by the relocated pressure drop.


Table 9 Frequencies recorded for element 209 PC0 [MPa] 3.92 3.93 6.01 6.06 6.09 6.1 6.13 8.16 8.19 8.23 8.3

O/F [-] 3.04 2.82 3.24 3.24 3.44 3.24 2.93 2.88 3.36 3.21 3.25

f(L1) [Hz] 1374 1398 1367 1368 1350 1369 1395 1408 1365 1379 1376

239 293 307.6 307.6 307.6 307.6 307.6 307.6 322.3 307.6 293

454.1 439.5 454.1 468.8 454.1 454.1 454.1 454.1 483.4 468.8 439.5

PCDYN [Hz] 1201.2 1098.6 1201.2 1479.5 1230.5 3676.8 1098.6 1435.5 1113.3 1289.1 1186.5 1333 1230.5 1289.1 1655.3 1318.4 1142.6 1289.1

1933.6 1684.6 2299.8 3955.1 2504.9 1464.8

2680.7 4218.8

PODYN [Hz] 2124 2710 1860.4 2460.9

2871.1 2724.6

1025.4

2519.5

703.1 688.5 908.2 893.6 1303.7 1040

1889.6

4101.6

4321.3

1040 3720.7 3413.1

TEH-12T14-01

8

2.5

OF = 3.36

PCDYN

PC = 81.9

2

4 Amplitude [bar]

PODYN, PCDYN [bar]

6

2 0 -2

1.5

1

[322.3; 0.47]

PCDYN PODYN

[483.4; 0.20] [893.6; 0.31] [1040.0; 0.25]

0.5

-4

[1289.1; 0.18]

PODYN -6 110.5

111

111.5

112

112.5 113 Time [s]

113.5

114

114.5

0

115

6

1500

2000


3500

4000

4500

5000

OF = 3.04 PC = 39.2

2

PCDYN 2

Amplitude [bar]

PODYN, PCDYN [bar]

1000

2.5

4

0

PODYN -2

-6 124.5

1.5

1 [293.0; 0.38]

0.5

-4

[454.1; 0.14]

125

125.5

126

126.5 127 Time [s]

127.5

128

128.5

0

129

10

[1201.2; 0.10] [2710.0; 0.07]

500

1000

1500

2000


3500

4000

2.5

PCDYN

0

5000

PC = 60.6

2

5

4500

OF = 3.24

PODYN

Amplitude [bar]

PODYN, PCDYN [bar]

500

1.5

1 [307.6; 0.51]

-5

0.5

[468.8; 0.22] [1230.5; 0.19]

-10 140

140.5

141

141.5

142

142.5 Time [s]

143

143.5

144

144.5

145

0

500

1000

1500

Figure 16. Frequency spectra of element 209


2000


3500

4000

4500

5000

Page 3 of 3

Combustion Efficiency η c* [-]

E. Oxygen Swirl Cap With Variation of Recess All elements with swirl caps showed good 1 stability behavior at general high performance 0.99 levels. The swirl caps have been tested extensively 0.98 at 12 mm recess. To gather information about the influence of the recess in this configuration, tests 0.97 with a reduced recess length of 7 mm have been 0.96 performed. In the following figures, the tests with 12 mm recess are marked by triangles. The tests at 0.95 7 mm recess are marked by stars. 0.94 Figure 17 shows that the reduction of the recess 0.93 length does not affect the combustion efficiency notably. 0.92 The axial distribution of the heat flux density, 0.91 which is given in the graphs in Figure 18 for 6 MPa 0.9 on the left and 8 MPa on the right hand side, is 2.2 2.4 2.6 2.8 3 3.2 3.4 3.6 Mixture Ratio O/F [-] altered by the reduction of the recess. With the shorter recess higher heat loads were recorded in Figure 17. Combustion efficiency of the element 302 the third cylindrical segment and in the nozzle. (triangles) and of element 303 s (stars) The application of the oxidizer swirl caps changes the frequencies recorded in the combustion chamber significantly. The results of the FFT analysis are given in Table 10 and Figure 19. The peaks located at around 300 Hz could be no longer resolved. This is an indication that these oscillations are caused by an interaction of the processes in the combustion chamber with the oxidizer feed line, which is suppressed upon the installation of the swirl caps. These caps lead to a comparatively high pressure drop on the oxygen side and inhibit the propagation of pressure oscillations into the feed system. The first peak is located at values between 1170 Hz and 1260 Hz, thereby lying roughly 100 Hz below the frequency of around 1300 Hz recorded for the elements without swirl caps. These frequencies match the values of the first longitudinal mode, if the length of the oxidizer post and the swirl cap are taken into account for calculation. In some tests, a second peak is recorded for the frequency of around 1330 HZ, corresponding to the first longitudinal mode calculated without taking into account the acoustics of the oxidizer post and the recess. As an example, at a chamber pressure of 6.3 MPa and a mixture ratio of 3.28, the frequencies of 1216 Hz and 1362 Hz have been recorded. The values calculated for the first longitudinal mode with and without the oxidizer post are 1215 Hz and 1317 Hz, respectively. A tight match of measured and calculated values was also found at 8.08 MPa and O/F = 3.22: The deviation between recorded values (1230 Hz and 1406 Hz) and the calculated values (1232 Hz and 1395 Hz) is less than 1%. Another peak is located within a plateau at a frequency of about 2400 to 2500 Hz, which is the second harmonic to the primary peak. The reduction of the recess length from 12 mm to 7 mm with the element 303 significantly reduced the peaks of the pressure fluctuation in the combustion chamber (see Figure 20). This agrees well with the experimental experience from other injector development programs, that an increase in recess length may lead to higher oscillation amplitudes inside the combustion chamber.


120

120

8 MPa

100


Heat Flux Density Q/A [MW/m2 ]

6 MPa

80

60

40

20

0 0

0.05

0.1


0.35

0.4

0.45

100

80

60

40

20

0 0

0.05

0.1


0.35

0.4

0.45

Figure 18. Heat transfer for different recess lengths in elements 302 & 303 (7 mm: stars, 12 mm: triangles) Table 10 PC0 O/F [MPa] [-] Element 302 4.05 2.96 4.07 3.19 6.09 3.25 6.25 2.98 6.3 3.38 6.39 2.48 7.79 2.9 8.04 3.35 8.08 3.22 Element 303 6.13 3.2 6.19 3.25 8.4 3.3 8.43 3.34

Frequencies recorded for element 302 and 303

L1 [Hz]

PCDYN [Hz]

PODYN [Hz]

1235 1216 1223 1246 1214 1295 1259 1222 1232

1171.9 1186.5 1245.1 1215.8 1215.8 1113.3 1186.5 1259.8 1230.5

1333 1333 2475.6 1362.3 1362.3 1538.1 1538.1 1420.9 1406.3

1494.1 1479.5

1655.3 2358.4

2563.5 2563.5

1523.4 1567.4 2607.4 2680.7 2534.2 1552.7

2417 2431.6

2592.8 2607.4

3823.2

4057.6

2446.3

2592.8

1040 3720.7 1245.1 1201.2 1201.2 1098.6 747.1 1259.8 600.6

1236 1234 1235 1238

1215.8 1230.5 1215.8 1215.8

2358.4 2329.1 2373 2387.7

2534.2 2475.6 2519.5 2578.1

2710 2695.3 3632.8 2724.6

2973.6 2929.7 3779.3 3618.2

1127.9 1098.6 1142.6 1113.3


3735.4

4453.1

3837.9 1142.6

4072.3

1230.5

3471.7 3955.1

TEH-5T6-06

10

2.5

OF = 3.25

[1245.1; 2.37]

8 PODYN, PCDYN [bar]

Amplitude [bar]

4 2 0 -2 -4

PCDYN

1.5

1 PODYN

0.5 [2475.6; 0.35]

[1245.1; 0.31]

-6 -8 18

19

20

21 Time [s]

22

23

0

24

15

500

1000

1500

2000


3500

4000

7

4500

5000

OF = 3.35

6

10 PODYN, PCDYN [bar]

PC = 60.9

2

6

PC = 80.4

Amplitude [bar]

5 5

0

-5

4 [1259.8; 3.45]

3 2 1

-10 33

34

35

36

37

38

39

0

40

[1259.8; 0.48]

500

1000

[1420.9; 0.44]

1500

2000

Time [s] 8

[2534.2; 0.40]


3500

4000

2.5

Amplitude [bar]

PODYN, PCDYN [bar]

PC = 62.5

2

2 0 -2 -4

1.5 [1215.8; 1.15]

1

0.5 [1201.2; 0.18]

-6

[2417.0; 0.19]

[1362.3; 0.32] [1523.4; 0.25]

-8 47

48

49

50 Time [s]

51

52

0

53

5000

OF = 2.98

6 4

4500

500

1000

1500

2000

[3896.5; 0.13]

[2592.8; 0.15]


3500

4000

4500

5000

Page 1 of 3

Figure 19. Frequency spectra of element 302 TEH-8T9-02

20

2.5

15

OF = 3.20 PC = 61.3

2 Amplitude [bar]

PCDYN

10

5

0

1.5 PCDYN

1

[1215.8; 0.74]

0.5 PODYN

-5 13.8

13.9

14

14.1

[2358.4; 0.33] [1127.9; 0.22]

14.2

14.3 Time [s]

14.4

14.5

14.6

14.7

0

14.8

80

500

1000

[2534.2; 0.19]

1500

2000


3500

4000

2.5

60

Amplitude [bar]

40 30 20 10

PC = 84.0

2

50

PODYN

1.5

1

0.5

[1215.8; 0.94] [2373.0; 0.47] [1142.6; 0.30]

0 -10 18.9

[2519.5; 0.19]

19

19.1

19.2

19.3 19.4 Time [s]

19.5

19.6

19.7

19.8

5000

OF = 3.30

70 PCDYN

4500

0

500

1000

1500

Figure 20. Frequency spectra of element 303 20 American Institute of Aeronautics and Astronautics

2000


[3632.8; 0.19] [3779.3; 0.14]

3500

4000

4500

5000

IV. Conclusion Several coaxial swirl injector designs for oxidizer rich staged combustion cycle engines have been hot fire tested at chamber pressures between 4.0 MPa and 8.5 MPa and mixture ratios from 2.4 to 3.5. A stability rating was accomplished by dynamic pressure measurements in the combustion chamber and the oxidizer feed line. Some element configurations developed high frequency combustion instabilities in longitudinal modes leading to a loss of combustion efficiency and an increase in thermal load to the water cooled chamber wall. The measured pressure oscillations were subjected to a spectral analysis and correlated with the acoustic characteristics of the combustion chamber and the oxidizer feed system. For the calculation of the resonance frequencies of the combustion chamber the sonic velocity of the gas was assumed to be constant and the recess of the injector was added to the cylindrical length of the chamber. A analysis of the recorded frequencies suggests that the recess of the injector element in its baseline configuration can be treated as a part of the combustion chamber as far as longitudinal modes are concerned. In the present setup, the influence of the igniter system on the acoustics of the chamber is negligible. A sample calculation confirmed that the gas torch igniter cannot act as a Helmholtz resonator and therefore needs not to be considered in instability studies. Four design variations were investigated with respect to their stability behavior: The baseline design with an orifice at the injector GOX post inlet was operated stable in all tests. The element configuration without any device providing a defined pressure drop which prevents pressure perturbations in the combustion chamber from traveling upward to the oxidizer feed line, was unstable in the complete operating range. Several Design variations were investigated with respect to their capability to regain combustion stability. An increased fuel injection velocity did alter the recorded frequencies in stable operating conditions but did not provide satisfactory stability. Combustion stability could be regained by tapering of the recess. The frequency map recorded for this configuration showed an improvement in oscillation amplitudes compared with the configuration with orifice in the GOX post. The application of oxidizer swirl caps also proved to be a reliable measure in order to avoid combustion instabilities. The caps act as a closed wall for the resonance volume of the combustion chamber and minimize interactions with the oxygen feed system. Furthermore, higher decoupling is achieved with the increased pressure drop generated by the swirl caps. With this configuration, the calculation of longitudinal resonance frequencies yielded best comparison to excperiments, if the length of the GOX post and the attached swirl cap was treated as part of the combustion chamber. A variation of the recess length proved that an increase in recess length leads to higher pressure fluctuations in the combustion chamber. The analysis of the feed line system revealed the fact that oscillations in the feed line can couple to chamber pressure oscillations and thereby favor the development of low frequent combustion instabilities. A detailed analysis of the interaction between the two components is being continued. It will focus on aspects like the impedance of the injector configurations and characteristic time scales.

Acknowledgments The experiments were carried out during the research program TEHORA 3, supported by the German national space transportation research program. The contributions of Stefan Köglmeier and Stephan Fischl are kindly acknowledged.

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