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The Comparison of the Hot-Fire and Cold-Flow Observations of NTO/MMH Impinging Combustion

Tony Yuan1 , Cetera Chen2, and Berlin Huang2 Institute of Aeronautics and Astronautics, National Cheng Kung University,

Tainan, Taiwan, R.O.C.

[Abstract] The combustion phenomena of doublet impingements of nitrogen tetroxide (NTO) and monomethylhydrazine (MMH) were studied in this research. The total flow rates of the propellants were controlled at ~8.00 g/s to simulate the operation of a 5-lbf rocket, and the ratios of the mass flow rates (O/F) of NTO and MMH were varied from 1.0 to 2.4. With a 2-axis translation module, the thermocouple array measured the 2-D temperature distributions in the hot-fire experiments at the 20mm downstream of the impinging point. The results showed that the temperature distribution was strongly affected by O/F and mixing effect. Comparing to the previous PLIF cold-flow studies in the same conditions, the results demonstrated that the cold-flow analyses could adequately predict the position and shape of the high temperature zone in hot-fire situation. In views of the combustion phenomena, an induction period (length) to reach intensive reactions always exists, and it can be correlated to the MMH droplet size distribution or the uniformity of the MMH spray. And, a maximum length of ∼80mm of the intensive reaction zone was shown for the conditions investigated. This may be caused by the diffusion-controlled combustion in the downstream reactions.

I. Introduction he applications of low-thrust nitrogen tetroxide(NTO)/monomethylhydrazine(MMH) bipropellant thrusters are mainly on the reaction control of missiles and satellites for its storable, hypergolic, and superior specific-

impulse characteristics comparing to mono-systems. Besides its hypergolic characteristics, MMH/NTO impinging combustion follows the conventional spray combustion pattern. Droplets had been observed experimentally in the downstream of MMH/NTO impinging burning spray,1 and the auto-ignition and the following combustion process proceed mainly in the gas phase.2 The impingement provides the energy and momentum required for atomization and mixing that take place in the immediate vicinity of the impinging point.

It is obvious that the mixing of the propellants is crucial for the performance of NTO/MMH thrusters. Factors such as momentum flux of the impinging jets, physical properties of fluids, and types of injectors may alter the distribution of liquid mass significantly. Rupe3,4 investigated the mixing of doublet impinging jets and defined the optimum mixing condition from the calculation of the mixing efficiency. He proposed that the momentum and the diameter of the impinging jets must be equal to acquire the optimum uniformity and mixing characteristics. Same conclusions were made in the observations of doublet and split-triplet impingements made by Won. et al.5 Riebling6 extensively reviewed the mixing data in the literature and deduced a correlation of area ratio, density ratio and mixture ratio to the optimum mixing efficiency. In his correlation, mixture ratio is the most significant factor to the mixing efficiency. The mixing mechanism proposed by Ashgriz. et al.7 concluded that the momentum of the impinging jets influences the mixing of the liquids before atomizing, and, when the liquid jets disintegrated into droplets, the turbulent dispersion would improve the mixing in the spray.

Instead of using the conventional mechanical patternator, the mass distributions of sprays were analyzed by Jung et al.8,9 with PLIF technique. Their optical measurements were justified by comparing to that from a mechanical patternator. Besides the high resolution and non-intrusive characteristics of PLIF technique, by ratioing the fluorescent signals and the scattering signals of droplets in sprays, the Sauter Mean Diameter (SMD) distribution was also deduced with careful calibration.10 Similar technique was used to observe the mass distribution in unlike- 1 Associate Professor, Dept. of Aeronautics and Astronautics, National Cheng Kung Univ., #1, Dasyue Rd., Tainan, Taiwan, Republic of China.; yahn@mail.ncku.edu.tw. 2 Graduate Student, Department of Aeronautics and Astronatics, National Cheng Kung Univ.

T

45th AIAA Aerospace Sciences Meeting and Exhibit8 - 11 January 2007, Reno, Nevada

AIAA 2007-781

Copyright © 2007 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

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doublet impinging sprays.11 Both jet’s momentum flux and surface tension showed significant effects on fluid atomization as well as spray uniformity.

By PLIF technique and using the simulants, the cold-flow observation of the mass distributions of doublet and triplet impingements for a 5-lbf MMH/NTO rocket was performed.12 The distributions of local mixture ratio, pseudo flame temperature were deduced, and the estimation of characteristic exhaust velocity was also performed. Although cold-flow studies provided an assessment of the hot-fire phenomena of NTO/MMH, the factors, such as the hypergolic characteristics, rapid heat release to thermal expansion of the hot gas, and the pre-vaporization of NTO, may cause the hot-fire phenomena different from that of the cold-flow observations. Thus, in order to verify the cold-flow predicted results, this work of NTO/MMH hot-fire observations was performed. This research investigated the hot-fire phenomena of doublet impingements of a 5-lbf NTO/MMH rocket operation conditions in atmospheric environment by imaging technique and thermocouple array measurements. The combustion phenomena of the hot-fire experiment and the cold-flow observation were compared and analyzed.

II. Experimental Method A nitrogen-compressing fluid supply system was used to control the propellant flow rates in this study (Figure 1).

The flow system was calibrated to have ±0.001cm3/min accuracy. The injector’s orifices were both 0.3mm ID (L/d ≈5) and oriented to have a 60° impinging angle. By controlling a constant total flow rate of ∼8g/sec to meet the operation specification of a 5-lbf MMH/NTO thruster, doublet impinging combustion experiments were performed. In order to compared the hot-firing results with the previous cold-flow studies12, the mass flow rate ratio (O/F) of NTO and MMH were varied from 1.0 up to 2.4, and in practical, the O/F was usually controlled at ∼1.65, a fuel-rich conditions.13,14

The flame images and the flame temperatures were acquired by CCD camera and C-type thermocouple array, respectively. Two high-resolution pseudo-color CCD cameras (2600×1960pixels) and a snap-shot CCD camera (1504×1000 pixels) were used to observe the front and side view flame images (Figure 2). In the images of each experiment, the intensive reaction zone was defined as the area between the peaks of the absolute values of light intensity gradients. And, the induction length was defined as the minimum distance in z-direction from the impinging point to the reaction zone, while the intensive reaction length were defined as the maximum distance in z-direction of the reaction zone (Figure 3). At the 10mm and 20mm downstream of the impinging point, the front view and side view flame angles were also measured. The flame angle was defined as the angle between the connecting lines of the impinging point to the side edges of the intensive reaction zone. Estimate of the temperatures near these edges were about 1500K.

From the calculation of the adiabatic flame temperature of NTO/MMH, the reaction temperature can be as high as 3000K.12 In the study, C-type thermocouple was used to measure the flame temperature in the reaction zone. Seven thermocouples were lined in an array and the distance between each thermo-couple was 10mm (Figure 2). With a 2-aixs translation module, the thermocouple array measured the 2-D temperature distributions ( yxT , ) in the

hot-fire experiments at the 20mm down stream of the impinging point with a spatial resolution of 5mm×0.1mm.

III. Results and Discussion The local flame temperature depends on the local mixture ratio and the local propellant density. By using the

thermocouple array, the planar temperature distributions of NTO and MMH impinging combustion at 20mm downstream of the impinging point were measured (Figure 4), where the peak temperatures and the cold-flow measured mixing efficiency are listed in Table 1. The peak temperatures increased as NTO supply increased because that the experimental O/Fs were all in the fuel-rich conditions (stochiomatric O/F≈2.5). However, as O/F>1.6, the mixing efficiency decreased because that the NTO jet was too strong to disintegrate properly. The resulted incomplete combustion accounted for the decrease of the peak temperatures.

By using the simulants of MMH and NTO, the cold-flow impinging mixing of the propellants was observed by PLIF technique previously.12 The predicted planar adiabatic flame temperature distributions based on the measured local mixture ratios at various impinging O/F ratios are showed in Figure 4. The white lines in the figures are the constant probability distribution contours of mass, and the area of measured flame temperatures above 1700K was enclosed by cyan line. Although the measured temperatures were about 1000K lower than the calculated temperatures predicted from cold-flow mixing data, the positions and shapes of the high temperature regions of various O/F conditions were reasonably predicted by the cold-flow analyses. The over-prediction of the absolute temperature was due to convective and radiative heat losses which were not included in the flame temperature calculations. For the same reason, the size of high-temperature region at O/F=1.0 was over predicted because that

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the actual energy released outside the droplet concentrated region was low, and the local flamelet was rapidly quenched that cannot be observed in hot-fire experiments.

Comparing to the previously determined spray angles that identifying the spatial distribution of liquid droplets in cold-flow experiments,11 the spatial distribution of the reacting flow (intensive reaction zone) are shown in Figure 5 and 6 . More than 2-fold increase of the angles due to liquid evaporation and thermal expansion of the gases were appeared near O/F=1.4. At high O/F ratios (>1.2), the flame angles did not decrease as predicted in the cold flow experiments because of the buoyancy effect of the hot gas as well as thermal expansion. Besides that, the flame angles measured at 20mm is generally less than the angles measured at 10mm, which indicated weak expansion effect of the reaction zone within this region, that is, the reactions of the pre-mixed propellants within the intensive reaction zone might already cease at or before 20mm down stream of the impinging point, and diffusion-controlled reactions were left for the down stream combustion.

The phenomena near the impinging point of NTO and MMH impingement is shown in Figure 3. It indicated that an induction period (length) always appeared before reaching the intensive reaction zone, the bright flame area in the image. Reddish-brown color of NTO (or NO2) appeared in the induction zone as well as at the outer layer of the flame as predicted in the cold-flow analysis because that NTO possesses lower surface tension (Table 2) and is easier to break up and evaporate. Within the induction length, NTO and MMH were breaking up, evaporating, and reacting. The heat of reaction releasing in the induction length is absorbed by the liquid propellants for vaporization. As the evaporation of NTO and MMH were sufficient, a great amount of heat was released due to gas phase reactions in the intensive reaction zone.

The induction length at various O/F ratios were estimated from the front view flame images and plotted in Figure 7. Since the heat release was from the combustion of gas-phase MMH, the induction length shall affected by the vaporization rate of MMH droplets. However, the results showed that the induction length is direct correlated to the uniformity variation of MMH sprays (see Figure 7 and 8). It could only be explained by that whenever the MMH jet was not effective broken up, most of the large MMH droplets distributed in the spray core with small droplets distributed at the outer region of MMH spray. The small MMH droplets had high evaporation rate to react with NTO vapor to shorten the induction length. As O/F increased, although MMH had more uniform mass and droplet size distributions, the amount of small MMH droplets decreased resulting to slower evaporation rate thus larger induction length. Even though it was impinged by high momentum NTO jet, the MMH spray was poorly distributed for the decreasing of instability at O/F=2.4,12 the induction length was shown shorter than that of O/F=1.2∼2.2. Direct measurement of the droplets size distribution of MMH is desired to verify the above scenario to explain the observed induction length distribution.

The estimated intensive reaction lengths were also plotted in Figure 7 for various O/Fs. A maximum intensive reaction length of ∼80mm existed for the O/F investigated. From the probability distribution of NTO and MMH droplets in Figure 8 and 9, respectively,12 it was clearly shown that when O/F was equal to 1.0, there was relatively concentrated MMH droplets surrounding by more uniform distributed NTO droplets. Due to the high fuel-rich condition, most of the NTO reacted in a short distance and the heat release was used to evaporate MMH droplets resulting to low flame temperatures. Better mixing occurred at O/F =1.2, and the increase of NTO induced more intensive combustion reaction (brighter flame) as well as longer reaction length. At O/F above 1.2, fuel-lean combustion occurred in the NTO droplets concentrated region. The reaction became diffusion-type flame after MMH in the region had consumed. Lacking of effective external mixing mechanism, the intensive reaction sustained till NTO consumed the nearby MMH, thus the length of the intensive reaction zone did not change apparently at higher O/F ratio. At about 80mm downstream of the impinging point, the intensive NTO and near-by MMH reactions almost ceased.

IV. Conclusion In this NTO/MMH hot-fire research, the results demonstrated an in-depth relation between the cold-flow and hot-

firing observations of NTO/MMH impinging combustion system. The planar temperature distributions predicted by cold-flow impinging experiments of simulants of NTO/MMH adequately described the shapes and locations of flame zone in hot-firing observations. However, because of the thermal expansion and the strong radiative heat loss, the flame distributed wider with lower temperatures in the hot-fire observations. The observations also showed that an induction period (length) to reach intensive reactions can be correlated to the uniformity of the MMH spray, as well as a maximum length of ∼80mm of the intensive reaction zone for the conditions investigated. Detailed examinations of MMH droplet size distribution and the intensive reaction zone are required to verify the scenarios given in the Discussion to explain the observed phenomena.

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References 1Lecourt, R., and Foucaud, R., “Hypergolic Propellant Burning Spray Visualization by Laser Sheet Method: Application to

Droplet Size and Liquid Concentration Measurements,” ASME, Experimental and Numerical Flow Visualization, Fed-Vol.172, 1993.

2Seamans, T. F., and Vanpee, M., “Development of a Fundamental Model of Hypergolic Ignition in Space-Ambient Engines,” AIAA Journal, Vol. 5, No. 9, 1967, pp. 1616-1624.

3Rupe, J. H., “The Liquid Phase Mixing of A Pair of Impinging Streams,” Jet Propulsion Lab., JPL Progress Rept. 20-195, California Inst. of Technology, Pasadena, CA, 1953.

4Rupe, J. H., “A Correlation Between the Dynamic Properties of a Pair of Impinging Streams and the Uniformity of Mixture Ratio Distribution in the Resulting Spray,” Jet Propulsion Lab., JPL Progress Rept. 20-209, California Inst. of Technology, Pasadena, CA, 1956.

5Won, Y. D., Cho, T. H., and Yoon, W. S., “Effect of Momentum Ratio on the Mixing Performance of Unlike Split Triplet Injectors,” Journal of Propulsion and Power, Vol. 18, No. 4, 2002, pp. 847-854.

6Reibling, R. W., “Criteria for Optimum Propellant Mixing in Impinging-Jet Injector Elements,” Journal of Spacecraft and Rocket, Vol. 4, No. 6, 1967, pp. 817-819.

7Ashgriz, N., Brocklehurst, W., and Tally, D., “Mixing Mechanism in a Pair of Impinging Jets,” Journal of Propulsion and Power, Vol. 17, No. 3, 2001, pp. 736-749.

8Koh, H., Kim, D., and Yoon, Y., “Correction of Attenuation Effects in Dense Sprays Using Planar Imaging Technique,” Proceedings of the 10th International Symposium on Flow Visualization, F0188, Kyoto, Japan, August, 2002.

9Jung, K., Lim, B., Yoon, Y., and Koo, Ja-Ye, “Comparison of Mixing Characteristics of Unlike Triplet Injectors Using Optical Patternator,” Journal of Propulsion and Power, Vol. 21, No. 3, May-June, 2005, pp. 442-449.

10Sankar, S. V., Maher, K.E., Robart, D. M., and Bachalo, W. D., “Rapid Characterization of Fuel Atomizers Using an Optical Patternator,” Journal of Engineering for Gas Turbines and Power, Vol. 121, July, 1999, pp. 409-414.

11Yuan, T. and Chen, C., “Observations of the Spray Phenomena of Unlike-doublet Impinging Jets,” Proceedings of the 11th International Symposium on Flow Visualization, F064, IN, USA, August, 2004.

12Yuan, T., Chen, C., and Huang, B., ” Optical Observation of the Impingements of Nitrogen Tetroxide /Monomethylhydrazine Simulants”, AIAA Journal, Vol. 44, No. 10, 2006, pp. 2259-2266.

13Schindler, R. C. and Schoenmant, L., “Development of a Five-Pound Thrust Bipropellant Engine,” J. Spacecraft, Vol. 13. No. 7, 1976, pp. 435-442.

14Stechman, R. C. and Smith, J. A., “Monomethyl Hydrazine vs. Hydrazine Fuels: Test Results Using Flight Qualified 100 lbf and 5 lbf Bipropellant Engine Configurations,” AIAA-83-1257, 1983.

Table 1 Experimental conditions and results of NTO/MMH doublet impinging combustion

O/F NTO/MMH mass flow rate (g/s)

NTO/MMH jet velocity (m/s)

Momentum flux ratio （

ffoo vmvm∗∗

/ ） mE

(%)aTpeak (K)

Induction length (mm)

Intensive reaction length (mm)

1.0 3.95/3.95 38.6/63.5 0.61 77 2033 7.8 36.3 1.2 4.41/3.69 43.0/59.3 0.87 91 2439 10.0 79.2 1.4 4.65/3.22 45.4/51.8 1.27 86 2828 - - 1.6 4.77/2.96 46.6/47.6 1.57 79 2554 10.8 74.1 1.8 5.18/2.94 50.5/47.3 1.88 77 2405 15.1 83.3 1.9 5.30/2.73 51.7/43.9 2.29 69 2335 13.6 71.8 2.1 5.45/2.59 53.1/41.6 2.68 65 2554 15.2 70.3 2.5 5.71/2.31 55.7/37.1 3.71 66 2176 8.8 80.9

aThe data of mixing efficiency are quoted from Ref. 12.

Table 2 Physical properties of propellants

Density, ρ(g/cm3)

Viscousity, μ(10-3 N•s / m2)

Surface tension, σ(10-3 N/m)

MMH 0.88 0.85 33.5 NTO 1.45 0.42 26.5 water 1.00 0.89 71.8

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Figure 1. The experimental flow control system

Figure 2. Schematic of flame temperature and image acquisition system

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Figure 3. The front-view flame images of NTO/MMH doublet impingements, where NTO was ejected

from the left hand side of the images (Total propellant flow rate≈8.0g/s).

O/F=1.0

O/F=1.2

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Figure 4. The temperature distribution and the contours of constant probability distribution

(white line) predicted from the cold-flow experiments of NTO/MMH simulants

(The outer contour of constant probability is 24105 −−× mm and the increment

between contours is 23103 −−× mm , Ref. 12). The cyan line enclosed the area with

measured flame temperatures above 1700K.

O/F=1.0 O/F=1.2

O/F=1.6 O/F=2.2

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O/F

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

angl

e

40

60

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120

hot fire image (1 cm)hot fire image (2 cm)cold flow spray (1 cm)

O/F

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

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160

hot fire image (1 cm)hot fire image (2 cm)cold flow spray (1 cm)

Figure 5. The comparison of the front-view hot-fire Figure 6. The comparison of the side-view hot-fire

flame angles and the cold- flow spray flame angles and the clod-flow spray angles.

angles. The cold-flow spray angles were The cold-flow spray angles were adopted

adopted from Ref. 12. from Ref. 12.

O/F

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2 2.4 2.6

indu

ctio

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----- induction length...... intensive rection length

Figure 7. The induction length and intensive reaction length of

NTO/MMH doublet impinging combustion.

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Figure 8. 2-D probability distributions of mass of MMH simulant at 10mm from the impinging

point for doublet impingements simulants (The outer contour of constant probability

is 24105 −−× mm and the increment between contours is 23103 −−× mm , Ref. 12).

Figure 9. 2-D probability distributions of mass of NTO simulant at 10mm from the impinging

point for doublet impingements simulants (The outer contour of constant probability

is 24105 −−× mm and the increment between contours is 23103 −−× mm , Ref. 12).

X axis (cm)-1.0 -0.5 0.0 0.5 1.0

Y ax

is (c

m)

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O/F=1.0 O/F=1.2 O/F=1.8 O/F=2.4

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O/F=1.0 O/F=1.2 O/F=1.8 O/F=2.4