Combustion in swirling flow

C O M B U S T I O N IN SWIRLING FLOW A. MESTRE AND A. BENOIT

O.gice National d'Etudes et de Recherches Aerospatiales, Chatillon, France Combustion in high-speed swirling flow is an alternate process to the high-intensity, lowweight combustion chamber for gas turbines. The increase in combustion intensity due to the centrifugal force field, the more-rapid dilution, and the greater stability of the cooling film on the outer wall, are the main advantages of this system. Further, the high air velocity may give a better pneumatic spray of kerosene injected in liquid phase. A test combustion chamber with premixed air and kerosene injected tangentially into an annular channel of o,uter diameter 100 mm and inner diameter 60 mm is used to check the validity of this concept. Velocities upstream of the flame, which was stabilized by means of a wedge-shaped flameholder, were of the order of 110 to 180 m/sec. Stability limits of the flame have been measured. They correspond to a fuel-air equivalence ratio of 0.65 for the lean limit, and 1.55 for the rich limit. Temperatures of 2300°K were obtained by means of a double-throat pneumatic pyrometer. Combustion is complete at a distance of 125 mm downstream from the flameholder. 1. Introduction The swirling flow that issues from the last rotor of a gas-turbine compressor is usually straightened in the axial direction before entering the combustion chamber. Not only does the flow velocity decrease and the pressure increase, which promotes efficient combustion, but the parallel flow with straight streamlines simplifies the design and definition of the combustion chamber, and leads to easy and reliable laboratory models for research and development of the combustion. It is necessary to note that, in the present state of the art of the combustion chamber, the flow is accelerated through the injection ports of the flame tube, in order to obtain well-defined primary and secondary jets used: to build up the recirculation zones necessary for flame stabilization; and to ensure rapid mixing between air and fuel in the combustion zone, between air and burned gases in the dilution zone. At the combustion-chamber exit, the flow is accelerated and deflected in the turbine guide vane channels and swirling flow is again obtained at the turbine entrance. The last compressor stator and the first turbine inlet-guide vanes have opposite effects, and increase the weight and volume of the gas turbine. If a swirling flow combustor with high flow velocity were used, these two elements could be reduced. An ad-

vantage of this process is the fact that a centrifugal force field tends to accelerate the mixing of two flows having different densities, and thus increases notably the reaction rate in the combustion process. The effect of an acceleration field on the combustion rate has been explored by G. D. Lewis in a paper that was presented at the Thirteenth Symposium on Combustion} However, no steady flow was used, and combustion took place at constant volume in a rotary sealed tube. Combustion research on a continuous swirling flow was undertaken at ONERA, in order to study the effect of a centrifugal force field on the combustion rate of a turbulent flame in a steady premixed air-kerosene flow. Determination of local and average temperatures and of stable combustion conditions was the primary aim of these experiments.

2. Experimental Set-Up 2.1. Combustion Chamber The combustor consists of an annular space between two coaxial cylinders. The outer one has a 100 mm diameter, and a length of 500 mm. I t terminates in a converging nozzle of 68 mm diam. At the exit, the inner tube of 60 mm diam has a length of 600 mm, part of which goes

719

FLAMES tN FURNACES AND COMBUSTORS

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Fro. 1. Swirling combustion test chamber. through the end wall of the eombustor in such a way that the downstream exit section can be correctly adjusted for the desired mass flow and chamber pressure. The swirling flow is obtained by tangential injection (Fig. 1); the inlet flow is a homogeneous air-kerosene mixture vaporized at 600°K. Liquid kerosene is injected through a venturi, where the flow velocity is high enough to obtain a good pneumatic spray. The distance between the injection nozzle and the combustion inlet is of the order of 1 m, and complete vaporization of kerosene and homogeneous keroseneair mixtures is obtained. Flame stabilization is obtained by means of a baffle placed on the outer wall; various types of flameholders were tested: wedge-shaped, straight, or annular flameholders gave the best results. The flame was initiated with a hydrogen-oxygen torch. 2.2. Measurements Qualification of the combustion was the main objective. Temperatures of the order of 2300°K were obtained. Double-throat temperature probes (pneumatic pyrometers) were used. 2 An analog computer (Fig. 2) converts the probe readings into actual temperature values. The principle of this well-known method is the following: A cooled probe having two sonic throats (1 and 2 of sections A1 and A2) is used. The total pressure pi~

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COMBUSTION IN SWIRLING FLOW

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FIQ. 3. Combustion-chamber flow pattern. is measured at section A1, near the entrance section, where the total pressure is that of the external flow. The total pressure p~2 and total temperature Tie of the cooled flow are measured at section 2. The mass-flow balance between these two sections is:

FIG. 5. End view of the flame. computer the function

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which is solved without iteration. I n the analog computer of Fig. 2, the ratio A1/A2 is the amplitier gain. Assuming thermochemical equilibrium, 7 and M are known functions of the equivalence

is a slowly varying function of the isentropic exponent 7. Both M and f ( ~ ) are functions only of the temperature arLd gas composition, and Eq. (1) can be solved by an iterative process. An analog computer with a counterreacting loop gives the final result directly. However, an easier way consists of introducing into the

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FLAMES IN FURNACES AND COMBUSTORS

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mixture, heavier than the final products, is centrifuged and tends to flow to the outer radius. This process can be used to improve penetration of the unburned mixture in the final products zone. The combustion rate can

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3. Combustion Chamber Flow P a t t e r n

2000! The swirling flow (Fig. 3) enters the combustion chamber at nearly tangential velocity Vr and gives a helical flow of axial velocity VA. This mean axial velocity is derived from the mass-flow conservation, by averaging pressure and temperature measurements in a plane normal to the axis of the chamber. When there is no combustion, simplifying assumptions can be made: wall friction is neglected and the momentum of the gaseous core is assumed to be constant throughout the chamber. The tangential velocity VT and axial velocity VA remain constant, and a pure helical flow is obtained. A a important centrifugal force field is created. The corresponding body forces are compensated for by the radial pressure gradient pVT2/R, where p is the mean density and R the mean radius. In the case of reacting gases, the unburned

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COMBUSTION IN SWIRLING FLOW therefore be increased b y placing the flameholder near the outer wall. Based on a mass-conservation equation, an increase of axial velocity is to be expected and, therefore, a more-open helical-flow pattern should occur. Thanks to the inner tube, the low-pressure core of the natural vortex 3 is avoided.

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5.1. Description of Swirling Combustion

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4. Test P r o c e d u r e The ONERA Aerothermodynamic Laboratory has a central compression plant that delivers 10 kg/s of compressed air, at a maximum pressure of 14 arm. The main part of this air is used for these tests iu a kerosene-heated heat exchanger, and only an air mass flow of 0.3 kg/s at a temperature close to 600°K and pressure 2 to 3 arm, delivered by this heat exchanger, is used for the swirling combustion test. The flame is ignited only when the correct mass flow and temperature are obtained. Since no external cooling of the combustion chamber is used, the outer wall heats up progressively, due to convection, and radiates heat. The duration of the test is limited, in order not to exceed a wall temperature of 1300°K. A new test is started only after the wall has cooled

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FIG. 9. Variation of temperature as a function of equivalence ratio.

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The tests show that combustion in a swirling flow is a steady and stable process, with no detectable vibratory effects. Figure 4 shows the combustor during a test with a stoichiometrie mixture. The absence of radiation of the exhaust gases must be considered as a sign of a complete combustion. The fact that combustion takes place along helical streamlines is shown during the test by the local heating of helical hue on the outer wall, probably due to the higher heating of the wall in the wake of the flameholder fixation point. In order to have a detailed view of the flame, the upstream end wall was replaced by a silica window, which allowed direct observation and photography of the flame. Figure 5 shows the outer and inner walls, the tangential air-kerosene mixture inlet, the wedge-shaped flameholder. Flame is initiated mainly on the edge of the flameholder, but also at the border of the incoming jet. The total length of the

724

FLAMES IN FURNACES AND COMBUSTORS

FIG. 11. Actual annular combustion chamber: guide vanes, injectors, and flameholder (outer wall removed). flame seems to have an angular extent of 3rr/2 radians. 5.2. Temperature Measurements During combustion, a pressure of the order of 2 to 3 arm is sufficient to choke the outlet throat. However, the unknown tangential velocity component does not allow the usual temperature determination, by means of the mass-conservation equation, at the throat since the actual sonic section is not known. Local temperatures were measured by means of the double-throat probe described above. I t was necessary first to direct the probe accurately by means of total pressure measurements. The effect of probe angle with axial direction is shown on Fig. 6 for an equivalence ratio of 4=0.969. The temperature increases as a function of the angle of the probe with the eombustor axis. The fact that the temperature measurements are not too sensitive to the angle of the probe facilitates experimentation. Temperature variations during the test, and the possibility of attaining a steady final temperature, are shown in Fig. 7, which gives the temperatures delivered by the analog computer of Fig. 2. A pseudo-equilibrium temperature seems to be attained after 20 to 25 sec. The length of the lag is due to heating of the walls, the response time of the probe being much shorter. Figure 8 shows the radial temperature distribution at a location 125 mm downstream from the flameholder. Each experimental point corresponds

to the local maximum temperature recorded after a 25-see test. The profile of the maximum temperature in the core of the flow seems flat. The variation in temperature as a function of the air-fuel ratio is represented on Fig. 9, for an exit section of 1130 mm 2. The temperature probe is placed at the mean radius of the annular channel, 125 mm downstream from the flameholder. During the test, the air-fuel ratio slowly increases or decreases, and the temperatures are measured under transient conditions. Their values differ, therefore, from the pseudo-equilibrium values. The curve of Fig. 9 is graduated in mean values of the velocity V upstream of the combustion zone. This velocity is obtained from the tangential velocity VT and axial velocity VA derived from the mass-conservation equation. By displacing the center body parallel to its axis, the exit section varies and the velocity of the combustion changes. Six different positions of the center body were used, corresponding to outlet sections in the range 1130 to 2000 mm 2. For each value of the throat section, a diagram, such as that of Fig. 9, was plotted, using some 50 experimental points, with practically no scattering. These various curves are shown in Fig. 10. Velocity V~ for a given throat section depends slightly on the equivalence fuel-air ratio; these values were reduced to the value corresponding to stoiehiometric mixture (~= 1). Figure 10 shows that the following flamestability limits are obtained: Velocity= 110 m/see, 0.65_<¢_< 1.55, Velocity= 180 m/see, 0 . 8 0 < ~ < 1.35.

6. Conclusion and Future Developments The experimental results obtained for the swirling flow combustion chamber show that steady and efficient combustion can be achieved in a high-velocity flow, even though only velocities of the order of 110 to 180 m/see were obtained in the small test combustor. It is well known that stability limits are widened when the size of the eombustor, i.e., the size of the flameholder, is increased. Stable combustion at higher velocities may be expected in larger swirling combustion chambers. Such a device avoids the use of straighteners at the last compressor exit, and decreases the deflection required for the turbine inlet guide vanes. The centrifugal force field gives a higher combustion intensity, and with high-speed helical flow: the burned gases will be mixed

COMBUSTION IN SWIRLING FLOW rapidly with secondary air, due to the buoyant forces acting on the hot combustion products; the cooling film will be stabilized on the outer wall; and simple pneumatic iniection nozzles, similar to those of the reheat combustion chambers, can be used. I a order to study these last three points, a new elaborate experimental set-up was built (Fig. 11 ). I t represents a reduction of an actual armular combustion chamber, where the exit conditions from the last compressor stage are simulated by means of guide vanes. Kerosene is injected in front of the flameholder, and an annular wall separates the combustion products from the dilution air.

725

Acknowledgment This work was sponsored by the Delegation Generale a 1~ Recherche Scientifique & Technique. REFERENCES 1. L~wIs, G. D.: Thirteenth Symposium (International) on Combustion, p. 625, The Combustion Institute, 1971. 2. GATh~RO%D.: Mesures 37, 95 (1972). 3. TANASAWA, Y. AND NAKAMURA, K.: On the Vortex Combustor as Applied to the Gas Turbine, Tokyo Joint JSME-ASME Meeting, Oct., 1971; Paper JSME-15.