COMBUSTION IN A CENTRIFUGAL-FORCE FIELD

Size: px

Start display at page:

Download "COMBUSTION IN A CENTRIFUGAL-FORCE FIELD"

Darleen Lynch
6 years ago
Views:

1 COMBUSTON N A CENTRFUGAL-FORCE FELD GEORGE D. LEWS Pratt & Whitney Aircraft, Florida Research and Development Center, West Palm Beach, Florida Flame propagation through a fuel-air mixture is governed by heat transfer, chemical reaction and mixing processes. For a specific mixture at given levels of pressure and temperature, the first two processes are substantially fixed, and any change in flame propagation rate must be accomplished through manipulation of the mixing processes. Mixing results primarily from diffusion, from the buoyant motion of the hot products of combustion upward through the colder mixture, and from the generation of turbulence by the rapid expansion of the fuelair mixture as it burns. Except in static (nonflowing) systems, the buoyant effects of hot gas in a gravitational field of 1 g are largely overshadowed by the other forces. Centrifugal and gravitational forces act on mass in a similar manner, however, and it is possible to conduct a combustion process in a centrifugal field which will greatly increase the buoyant force acting on the hot products of combustion. The buoyant force per unit volume can be expressed by the equation F = pc(1 - TcTh). For typical values of normal air density, a temperature ratio of 5 and a centrifugal field of 1000 g, the buoyant force is 9100 newtons per cubic meter (57.9 lbffft3). To investigate the phenomenon experimentally, a centrifuge consisting of a steel pipe 5,1 cm (2 in.) in diameter and 1.26 m (4 ft) long was filled with a homogenous mixture of propane and air at stoichiometric proportions. The pipe was rotated at constant speed about its center (like a propeller), and the mixture was ignited at one end by an electric spark. onization probes mounted at intervals along the pipe sensed the flame-front movement, and in some tests a pressure transducer mounted near the center of rotation was used to measure the pressure in the pipe. The measured flame-propagation rate is plotted below as a function of centrifugal-field strength. The data can be expressed by the equation V = 5.67(P)~176 ~ Pressure measurements made during the combustion process show that the bulk burning rate increases in a manner similar to the flame-propagation rate.. ntroduction Heat transfer, chemical reaction, and mixing govern flame propagation through a fuel-air mixture. The first two processes are fixed under given conditions of pressure and temperature. Diffusion, turbulence, and buoyant forces govern the mixing process. Although buoyant forces are normally the least effective of the three in a dynamic system, they can be increased greatly by burning the mixture in a centrifugal field. To investigate the phenomenon experimentally, a centrifuge consisting of a steel pipe 5.1 cm (2 in.) in diameter and 1.26 m (4 ft) long was 625 filled with a homogenous mixture of propane and air at stoichiometric proportions. The pipe was rotated at constant speed about its center (like a propeller), and the mixture was ignited at one end by an electric spark. The measured flame-propagation rate can be expressed as V = 5.67 (p)0.10e (g)0.a~, (1) where V is the flame speed in msec, P the pressure in atm, and g the centrifugal-field strength in multiples of earth gravitational force. Flame propagation through a fuel-air mixture

2 626 TURBULENT FLAME PROPAGATON TURBULENCE GENERATOR -- l o ~ ~ LOCATON ~ SPARK PLUG LOCATONJ 20 (6 in.) CHARGE FTTNG AND PRESSURE TRANSDUCER LOCATON -! 20.3 cm ~~ 1'8,in', 60.L 58.5 cm 7 o --L (23.0 in.) ONZATON PROBE LOCATONS where F is the buoyant force per unit volume in newtonsm 3 or lbfft 3, C the centrifugal acceleration in msec 2 or ftsec 2, p the density of cold gas in kgm 3 or slugft a, Tc the temperature of cold gas in ~ or ~ and Th the temperature of hot gas in ~ or ~ Thus, the buoyant force acting on a volume of hot gas immersed in a colder gas is directly proportional to the centrifugal-field strength, the temperature ratio, and the cold-gas density. When only the acceleration due to the earth's gravitational field is considered, C in Eq. (2) is 1 g (9.81 msee 2 or 32.2 ftsec2). For gas at 1 atm and nominal values of Tc = 300~ (MO~ and T^ = 1500~ (2700~ the buoyant force on the hot gas is a modest 9.1 newtonsm 3 ( lbft3). f a centrifugal field of 1000 g is applied, however, the buoyant force is increased a thousand times to 9100 newtonsm s (37.9 lbjft3). Thus, buoyancy can become the dominant mixing force. An experimental program was undertaken to evaluate its effect. 14o 15o 16o BURST DSK 17~ HOUSNG LOCATON -~ NDCATES RG PURGE FTTNG LOCATONS. Experimental Equipment A centrifuge consisting of a steel pipe nominally 5.1 cm (2 in.) in diameter and 1.26 m (4 ft) long was filled with homogenous mixture of propane and air at stoichiometric proportions. The pipe was rotated at constant speed about FG. 1. Combustion chamber schematic. is governed by heat transfer, chemical reaction, and mixing processes. For a specific mixture at given levels of pressure and temperature, the first two processes are substantially fixed, and any change in flame-propagation rate must be accomplished through manipulation of the mixing proce~,!ses. Mixing results primarily from diffusion, from ~he buoyant motion of the hot products of eorabsjstion upward through the colder mixture, and from the generation of turbulence by the rapid expansion of the fuel-air mixture as it burns. Except in static (nonflowing) systems, the buoyant effects of hot gas in a gravitational field of 1 g are largely overshadowed by the other forces. Centrifugal and gravitational forces act on mass in a similar manner, however, and it is possible to conduct a combustion process in a centrifugal field which will greatly increase the buoyant force acting on the hot products of combustion. The buoyant force per unit volume can be expressed by A. ORFCE TYPE B. SCREEN TYPE 1.79 cm DA (0.750 in.).75 mm in.} 29 HOLES F = pc (1 -- TcTh), (2) FG. 2. Turbulence generators.

3 COMBUSTON N A CENTRFUGAL-FELD FORCE 627 its center (like a propeller), and the mixture was ignited at one end by an electric spark. onization probes mounted at intervals along the pipe sensed the flame-front movement, and in some tests a pressure transducer mounted near the center of rotation was used to measure the pressure in the pipe. A more detailed description follows. A. Combustion Chamber The chamber was'formed by welding caps on a standard piece of Schedule 160 stainless-steel pipe. The resulting chamber is shown schematically in Fig. 1. Threaded 14-ram holes were located at about 5.1 cm (2 in.) intervals along the pipe to accommodate ionization probes, and nominal 18-in. pipe connections were provided at the center and near each end. At one end, a 14-mm thre~ed hole permitted installation of a spark plug, and near the other end a burst disk was mounted on a nominal 12-in. pipe fitting. gnition spark was provided by a capacitor-discharge Mercury Outboard Thunderbolt gnition System firing through a slipring on the shaft. To assure the formation of a turbulent flame early in the combustion process, a turbulence generator was welded into the pipe about 3 cm from the spark plug. Two different turbulence generators were tested as shown in Fig. 2, with no significant difference in flame speed attributable to the generators. r- PNEUMATC 3-,n. CRAN '._ VC% TOR.'... i " SHAFTAND!111- ii. 9 i.ousn~ :----~1tt L~_:=~.:... ->SUPPLY.RA~ -.~i~ +~' A,R f-" i l ~. REMOTE (( ~ ~l " O~ERATEO h- --krt- -~ tl SOLENOD oo ;--~ ~-d 9o degrees P~%. w,~.,,o.,, )[J~ k.-; ',t~s~ BURST DSK ~ d NSTALLED -~ PADDLE TRAVEL [ ~ BRASS BUSHNG Fro. 4. Mixing tank schematic, B. nstrumentation 2.86 cm (1.125 in.) L AC C-43 COMMERCAL SPARK PLUG (WTH BASE ELECTRODE CUT OFF) WELD 0.33 cm (0.130 in.) 0.23 cm DA (0.090in.) ~0.23 cm (0.090 in.) NCONEL WELD ROD WELD ROD~ FG. 3. onization probe schematic. onization probes were used to sense the movement of the flame front in the chamber. The probes were built by modifying commercial spark plugs as shown in Fig. 3. These probes were dual pronged and extended beyond the centerline of the combustion chamber. A positive 90-volt bias was applied across the probes, and they were connected through sliprings and a trigger circuit to an oscillograph. The pressure change in the combustion chamber during burning was also measured in order to calculate the bulk burning rate. A bonded strain-gage pressure transducer was mounted near the center of rotation, and its output was also connected through sliprings to the oscillograph. Data were recorded on the oscillograph at a paper speed of 254 cm (100 in.) per second. C. Combustible Mixture Supply Propane and air were mixed in proper proportions prior to entering the combustion chamber. The system consisted of an air-supply tank, propane-supply tank, mix tank, and associated

4 628 TURBULENT FLAME PROPAGATON data near the center of the pipe where increasing pressure affected the flame speed. Tests were conducted both with the rig stationary and at various rotational speeds up to 1600 rpm. Pressures before ignition were either 1 or 2 atm absolute, and the fuel-air ratio was in all tests. The experimental apparatus is shown mounted on the test stand in Fig. 5. V. Discussion Fro. 5. Combustion rig. plumbing. The tank was evacuated, propane gas was admitted to a specific pressure, and then air was admitted to form a stoichiometric mixture. The mix tank contained a paddle mechanism which was externally actuated through 90 degrees of travel by a pneumatic valve actuator as shown schematically in Fig. 4. Mixing was accomplished by cycling the pneumatic valve actuator 15 to 20 times.. Test Procedure To conduct a test, the combustion chamber was evacuated and held under vacuum for several minutes to remove all gaseous products of combustion and condensed water from the prior test. The vacuum pump was then valved off, and a stoichiometric fuel-air mixture was admitted from the premix tank until the desired combustion chamber pressure was reached. The admission valve was closed, disconnected from the vacuum and fuel supply line, and capped. At this point, personnel retired to the control room and the chamber was accelerated to the desired rotational speed. t was held at constant speed for about 1 min to allow turbulence generated by the rotational acceleration to decay before the recording equipment was activated and the spark was fired. Measurements were taken between probe stations 3 and 7 in order to: (1) obtain data after the turbulent flame was fully developed, (2) obtain measurements over as long a distance as possible to average out local variations, and (3) avoid The measured flame-propagation rate is presented in Fig. 6 as a function of the average centrifugal acceleration between stations 3 and 7. t is apparent from Fig. 6 that normal processes control the flame-propagation rate until the centrifugal-field strength approaches 50 g, at which point centrifugal forces appear to become dominant. Data are presented for initial pressures of 1 and 2 arm over a range of centrifugal field strengths from 1 to 850 g. The initial pressure level had a negligible effect on the flamepropagation rate, but strongly affected the ignition process. At centrifugal field strengths above 150 g, ignition was marginal at 1-arm initial pressure. n many tests no ignition was obtained until the rotational speed was decreased, and in four other cases abnormally low flame rates were measured. At 2-arm initial pressure, ignition was reliable, and data scatter was reduced. This finding is consistent with the increased vulnerability of flame to interruption by "turbulent stretching" at lower pressures. Because the combustion occurred in a closed container, the volume increase resulting from 90 C) SNGLE HOLE TURBULENCE GEN - 1 atm 80 [] MULTHOLE TURBULENCE GEN - 1 arm._~ 0 MULTHOLE TURBULENCE GEN - 2 atm 70 i S ~ O lo ~ o CENTRFUGAL FORCE - g WEAK GNTON HERE 1000 FG. 6. Centrifugal field mcresses flame propagation rate.

COMBUSTON N A CENTRFUGAL-FELD FORCE 629 the combustion caused a bulk movement of gas from the ignition end of the tube toward the end containing the unburned mixture.

5 COMBUSTON N A CENTRFUGAL-FELD FORCE 629 the combustion caused a bulk movement of gas from the ignition end of the tube toward the end containing the unburned mixture. Thus, the measured flame speed during the early part of the burning period is affected by four main factors: (1) the laminar flame speed, (2) the bulk movement of both hot and cold gases away from the ignition source, (3) the turbulence and mixing caused by the expansion of the burning mixture, and (4) the stirring caused by the buoyancy of the hot gas in the centrifugal field. Of all of these, only the last is substantially affected by rotational speed, and the difference between observed flame-propagation rates at different centrifugal field strengths, therefore, must be due to the buoyancy effect. Two turbulence generators were tested, one with a single hole in the center and one with many holes distributed over the surface, with no detectable difference in results. However, the possibility still existed that the measured high flame-propagation rates were the result of tendrils of flame streaking up the tube and triggering the ionization probes, rather than being the result of an increase in bulk burning E E '~ 140 er ~: loo r i PRESSURE BEFORE GNTON C) atm 0 2 arm J 0 o t~ V = 5.67 (p)0.106 (g) WHERE P S N ATMOSPHERES > g S N msec298 msec 2 -, ~ OBSERVED FLAME SPEED - rnsec FG. 8. Observed vs calculated flame speed. rate. This possibility was increased by the fact that coriolis forces would drive the hot gas toward the side of the tube where the ionization probes were mounted. Therefore, a pressure transducer was mounted near the center of rotation, and the change in pressure rise, then, became a good indicator of the bulk rate of burning of the mixture. Since the rate of pressure rise depends on the initial pressure, the pressure rise data have been normalized by dividing by the initial pressure. These data are presented in Fig. 7, and clearly show that the bulk burning rate increases with increased centrifugal-field strength over substantially the same range as the flame-propagation rate. A regression analysis of the data showed that the flame propagation rate can be expressed by Eq. (1). V = 5.67 (p)0.10~ (g)0.~. Figure 8 shows a plot of the observed and calculated values. Over the range between 50 and 850 g, where centrifugal force appears to strongly affect the combustion process, the equation represents the data with an average deviation of 5.8 per cent. V. Conclusion CENTRFUGAL FELD STRENGTH - g FG. 7. Pressure rise rate. Applying a centrifugal field to a burning fuel-air mixture can substantially increase the rate of combustion.

Flow in a shock tube

Flow in a shock tube April 30, 05 Summary In the lab the shock Mach number as well as the Mach number downstream the moving shock are determined for different pressure ratios between the high and low pressure