Cold Gas Flow Visualization of SDR Secondary Combustor Flow Field

Yu-Fei Li*, Guo-Qiang He†, and Pei-Jin Liu‡

College of Astronautics, Northwestern Polytechnical University, Xi’an 710072, China

A gas flow visualization facility has been designed and constructed in order to investigate the mixing processes inside of solid ducted rocket secondary combustor. The facility has the capability of monitoring and varying the flow rate through all three inlets of the combustor model. In this experiment particles were seeded in “fuel” flow, and then flow visualization was achieved by illuminating the cross-sectional planes of interest with a laser sheet. Continuous unsteady images were processed in order to gain available steady image which were recorded with CCD camera. In addition pseudo color was used to get satisfied results. Processed image revealed complex mixing patterns within the secondary combustor. Results came from processed image were compared with data obtained from temperature measurement method. It indicates processed results are credible, and this method can be used to investigate mixing in secondary combustor.

I. Introduction The Northwestern Polytechnical University has undertaken an activity to study the use of a solid ducked rocket

for missile propulsion. A facility has been constructed to study different mixing scenarios for solid ducted rocket through flow visualization in wind tunnel.

A solid ducked rocket has several advantages over solid fuel rocket including increased range, higher speed and higher specific impulse. After launched, the solid ducked rocket missile is boosted by an integrated rocket booster and accelerated to supersonic speed. When the booster solid propellant is completely burned, the port covers that seal the air inlets from the secondary combustor open and the fuel rich solid propellant gas generator ignites. Fuel rich exhaust gases from the gas generator are mixed with incoming air in the secondary combustor and reacts. At last, the reacted gases in secondary combustor are accelerated through a converging/diverging nozzle to provide thrust.

A perfect design of SDR secondary combustor can increase secondary combustion efficiency greatly. So its length can be decreased correspond. The length for efficient secondary combustion is controlled by the mixing of the first flow (fuel) and the secondary flow (air). Shorter the complete mixing length is, shorter the secondary combustor can be. In addition, perfect flow organization can make thermal load on the secondary combustor wall more homogeneous, and decrease the requirements to thermal protect systems.

Many studies have been carried out on the mixing in SDR secondary combustor in the world. Brophy and Hawk have investigated a series of four-inlet geometry models using water tunnel. By illuminating 0.0254cm polystyrene spheres seeded in secondary flow with laser sheet, phenomena of flow near the main injector in secondary combustor were observed. In addition, fuel (methane) distribution images were obtained using Raman spectroscopy in direct-connect hot-fire experiments.1-3

Mayer and Stowe used water tunnel to study the mixing in SDR secondary combustor. To visualize the velocities within the combustor, the water was first seeded with 100-micron spherical polystyrene particles. A laser sheet was created to illuminate the planes of interest. In order to resolve the fine-scale turbulence that is very important for the air/fuel mixing, fine bubbles created by cavitation nozzle were seeded in fuel flow.4,5

Ristori used laser sheet to stimulate the tracer dye (fluoresceine) mixed with water as fuel flow, and investigated the flowfield in secondary combustor. In another performed experiment, cold air was used, and flow velocity field was measured using Laser Doppler Velocimeter.6

* Graduate Research Assistant, College of Astronautics, PO BOX 164. † Professor, College of Astronautics, PO BOX 164. ‡ Associate Professor, College of Astronautics, PO BOX 164.

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25th AIAA Aerodynamic Measurement Technology and Ground Testing Conference5 - 8 June 2006, San Francisco, California

AIAA 2006-2811

Copyright © 2006 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

Generally most studies about mixing in the SDR secondary combustor were carried out with water. Compared with real SDR inner flow field, Reynolds number of water flow experiment is smaller, Reynolds number of cold air flow experiment is bigger. As known, when Reynolds number of water flow experiment exceeds some value, its variation does little effects to experiment results. In most cases water flow experiments can match the requirements. The main advantage of a water tunnel for flow visualization is that the Reynolds numbers achieved at a water velocity of 1m/s in the main part of the combustor had an order of magnitude of 105 and therefore representative of the flow in a typical ducted rocket combustor. However the compressibility of gases will affect experiment results. Experiments performed by Brophy and Hawk have proved it. In addition, numerical computation results obtained from Mayer and Stowe supported it. So it is necessary to study cold air flow experiment.

In this paper, to obtain the image of mixing in SDR secondary combustor, cold flow simulation of secondary combustor inner flowfield using gas was carried out. In this experiment particles were seeded in fuel flow, and then mixing images were captured by illuminating the interest cross-sectional planes with a laser sheet. This method need only low power continuous laser sheet system and CCD camera, which is convenience for SDR studies.

II. Experimental systems In this experiment, a wind tunnel was used to

visualize the non-reacting flow and air/fuel mixing within 110 mm diameter model secondary combustor. Schematic of the test facility is shown in figure 1. First flow and secondary flow were supported from individual sources considering convenience operation. First flow was simulated with nitrogen come from 0.45m3 tank whose maximum pressure is 4MPa. Secondary flow was simulated with air come from 40m3 tank whose maximum pressure is 1MPa. The amount of gas delivered to the model was controlled by throttling supply tube with globe valve. Differential pressure transducers and flow meters were used to measure the flow through the air and fuel inlets. A personal computer was used to monitor the flow meters and pressure transducers. Figure 1. Schematic of particle scatter experiment

Test model was made of plexiglass, whose configuration is shown in figure 2. The geometry had dual inlets spaced 90° apart circumferentially since this is a favored configuration for air-launched missiles. During flight, the dual inlets would normally be below the missile as well. However in the experiment, the inlets were above the

secondary combustor for convenient deployment. The angle between inlet and test model axis is 45°. Cross section of secondary combustor is round whose inner diameter is 110mm. Cross section of inlets is a 50mm×50mm rectangle. In order to ensure an airflow with an as close as possible homogenous velocity distribution over the injection area, the air inlets were relatively long, while a practical solid ducted rocket would have inlets that hugged the side of the combustor and their ends would face directly forward. As shown in figure 3, main injector of first flow is a seven-hole hemispherical geometry which is a non-chocked injector. Distance from main injector to inlet center point is 120mm. A nozzle was added at the end of the secondary combustor to obtain necessary pressure.

Particles that are 50µm in diameter were seeded in first flow. In order to seed the particles in first flow, part of the first flow entered into particle reservoir in tangential direction, and then particles were rolled up by rotating gas flow and entered into first flow. A water-

Figure 2. Basic geometry.

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cooled Argon Ion laser was used to light particles. The laser has 5W energy and includes 514.5nm, 488.0nm and 476.5nm wavelengths. The original laser entered optical fiber by a coupler, and then became 1mm thick laser sheet through a cylindrical lens system. Using of optical fiber made the deployment of optical path easier.

For image acquisition, a Kodak Megaplus ES310 CCD camera and its associated software was used. The camera has a resolution of 648×482 pixels, 256 gray levels and capture speed up to 85f/s. 448M memories in computer allowed 1456 frame images are captured one time. Camera was placed 40° angle with test model axis, which made sure that camera resolution can be used as completely as possible.

Figure 3. Schematic of fuel injector.

III. Results analysis Under the test condition of air flow rate was 0.237kg/s, nitrogen flow rate was 0.054kg/s and secondary

combustor pressure was 0.36MPa, a series of image were obtained under the capture speed of 85f/s. Reynolds number is 1.46×105, air/fuel momentum ratios is 8.776 in this experiment. Typical images are shown in figure 4.

Signal intensity in captured image is related to residence time of particles in laser sheet and concentration of particles. The velocity distribution in the interest cross section is fairly uniform. Hence the scatter light signal intensity variation in the image is varies only with the concentration of particles.

Figure 4. Experiment image Frame 206-210（from left to right）.

Continuous images show that distribution of particles in the same cross section changed greatly, whose reason may be intensify large scale turbulence oscillation. In order to achieve necessary Reynolds number, gas velocity in secondary combustor of wind tunnel experiment is great, and turbulence intensity is great correspondingly. In order to get available image, image processing must be carried out.

IV. Image process Because obtained images show intensify turbulence oscillation characteristics, image series are averaged to get

available steady image referring time averaging method in turbulence theory. Before image process, two problems need to be solved.

Firstly, because signal intensity is proportional to the concentration of particles and the concentration of particles is proportional to the total number of particles come from particle reservoir. In order to get reasonable time averaging steady image, images to be averaged must be in the time when concentration of particles became steady.

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Figure 6. Averaged gray value of one pixel. Figure 5. Gray value of one pixel.

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Because the total number of particles from particle reservoir to the first flow can not be controlled precisely now, the time when concentration of particles became steady will be obtained from image series. Gray value traces of three pixels, whose coordinates are (360,340), (360, 240) and (360,140), are shown in figure 5. Intensify turbulence oscillation characteristics can be observed from curves. Curves in figure 5 show that concentration of particles became steady after 200 images.

Secondly, how many images are necessary to convergent to steady image through process. Variations of gray value of pixel (360,240) with increasing of averaged frame number are shown in figure 6. Curves with different initial frame, 200, 400 and 600 were compared too. Curves in figure 6 show that more than 50 images can ensure to get steady image. With a conservative consideration, number of averaged images reached 200 in this paper.

200-400 images were averaged, and figure 7a was obtained. Processed image shows that interference of background light is intensify. In figure 7b background was gotten off by the way of subtracting the first frame image from averaged image, when there were no particles entering in to the first flow. To obtain better knowledge from processed image, figure 7b was changed into pseudo color image figure 8.

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Figure 7. Averaged image of Frame 200-400 (a. Origin; b. Background cleared).

From figure 8, it can be observed that fuel is rich in the upper region of secondary combustor cross section. On the contrary, there is a fuel poor hole in the lower portion of secondary combustor cross section. In addition, two sides of the cross section are middle fuel concentration region where should be a high temperature region in real SDR. Above the round cross section, a high pixel intensity region exists, whose reason is reflection of scatter light from secondary combustor wall. Another should be mentioned is the edge of cross section being high pixel intensity, whose reason is adhesion of particles on secondary combustor inner wall and increasing of scatter light intensity as a result. Because the test model was made of ready-made plexiglass tube which possesses stripes formed by manufacture technology, there are radical stripes in cross section.

V. Verify of results In order to verify the processed results,

method of temperature measurement was used to investigate mixing in secondary combustor. Detail of the method is replacing the nitrogen tank with a solid propellant gas generator that can generate hot gas as shown in figure 9. At the interest cross section, a six-probe

Figure 8. Pseudo color visualization.

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temperature rake was placed. By measuring the difference of temperature, mixing level can be evaluated. Higher temperature is, higher fuel concentration is.

The solid propellant gas generator can produce 1448K gas flow. In order to guarantee gas generator work normally, the chamber pressure of solid propellant gas generator should keep a high value. On the contrary, because SDR main injector need work as non-chocked condition, the pressure at the upstream of main injector should be same as in secondary combustor and the flow should be subsonic. So a over expanded nozzle was installed on the outlet of solid propellant gas generator, supersonic gas flow jetted from gas generator flew through a normal shock and became subsonic. In order to resist high temperature, this test model was made of iron.

Figure 9. Schematic of temperature measurement arrangement.

Secondary combustor Reynolds number in two experiments is at the same order. Pressure in secondary combustor is 0.37MPa.

Gray values on the center line of the round cross section were plotted in figure 10. At the same time, temperature values were plotted in figure 10 too for comparison. Obviously all two curves can be divided into two parts, high fuel concentration part and low fuel concentration part. In one part, fuel concentration is higher but concentration gradient is lower. On the contrary, in another part, fuel concentration is lower but concentration gradient is higher. The trends of two curves are identical. However difference between two curves appeared near the wall. Gray values come from processed image will increase abruptly near the wall, whose reason may be adhesion of particles on secondary combustor inner wall. Temperature value come from

temperature measurement method will decrease near the wall, whose reason may be cold boundary layer caused by cold wall.

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VI. Conclusion The performance and initial results from the solid ducted rocket secondary combustor flow visualization facility

indicate that it will be extremely valuable in contribution to the SDR research. The information provided by the various seeding and laser lighting techniques yielded insight into possible combustor characteristics such as thermal distribution characteristics.

The seeding of the fuel inlets with particles produced an easily visualized flow field. Images revealed complex mixing patterns within the secondary combustor which were recorded with CCD camera. Results of the paper clearly demonstrate the feasibility of the method for visualizing flow field.

Results from the temperature measurement method provide valuable information about the mixing properties of solid ducted rocket while revealing the validity of using flow visualization to gain insight into SDR secondary combustor.

References 1 Brophy, C. and Hawk, C., “Mixing and Combustion Studies of Four-Inlet Side Dump Combustors,” AIAA-96-2765, 1996. 2 Brophy, C. and Hawk, C., “A Flow Visualization Facility for Ducted Rocket Engine Mixing Studies”, AIAA-95-2934, 1995.

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3 Brophy, C. and Hawk, C., “An investigation of Four-Inlet Ducted Rocket Engine Flameholding Characteristics”, AIAA-97-2846, 1997.

4 Mayer, A.E.H.J. and Stowe, R.A., “Experimental Study into mixing in a solid fuel ducted rocket combustion chamber”, AIAA-2000-3346, 2000.

5 Stowe, R.A., De Champlain, A., Mayer, A.E.H.J. and Niemeijer, S.F., “Modeling and Flow Visualization of Mixing in a Ducted Rocket Combustor”, AIAA-98-3768, 1998.

6 Ristori, A., Heid, G., Cochet, A. and Lavergne, G., “Experimental and Numerical Study of Turbulent Flow inside a Research SDR Combustor”, AIAA-99-2814, 1999.

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