Far from Equilibrium-The Flashback Oscillator Leonard J. S o k b e r g- , Michele M. Boucher, Dawn M. Crane, and Stacey S. Pazar Simmons College. Boston. MA 02115 In the course of our studies of model systems that demonstrate far-from-equilibrium self-organizing behavior, we have previously described the "gas pendulum" ( I ) and the "continuous-flow bottle" (2). We report here another model svstem. easilv demonstrated. that exemplifies various farfrom-equilibrium phenomena. This system, which we call the "flashback oscillator". makes a ~articularlvdramatic lecture demonstration. We have nreviouslv outlined the ~ r i n c i ~concepts al that form the framework'for interpretink fnr-?n)m-e(1uilihrium behavior (1.21.This hacker~,undincludes the followi~iacon(1) a controlvariableexists by which the experimenter can drive the
system increasingly far from equilibrium; (2) at a critical value of the control variable, a bifurcation occurs, and the system switches to a qualitatively different mode of
behavior; (3) the bifurcation is accompanied by a breaking of spatial andlor
temporal symmetry. Snatial svmmetrv-breaking is exemnlified hv the wave patterns observed in the winfree reagent (4) & with thegas pendulum ( I ) , and temporal symmetry-breaking is typified by the flashback oscillator reported here as well as by a wide variety of oscillating reactions.
of 0.33-mm-diameter nlatinum wire and then windine one end uf the twisred wire pair nruund a 3-mm glass rod tu form n fi~ur-turn helix. This helix is then srretrhed ner~rndicularly to the helix axis, as shown in Figure 2. This'priparation of the pilot wire seems to eive a good combination of active s u r f k ~ a r t a a n dthermalhass. lithe pilot wirr is toolarge, it does not -aet sufficiently hot; it it is too small, rhe wire may melt.' The entire apparatus should he set up in a draft-free location. The torch is mounted so that the nozzle bore vents vertically, and the pilot wire is mounted so that the end two coils of the helix are directly in line with the hore of the burner nozzle. With the OX-5 nozzle (the largest hore), the pilot wire should he about 5 cm above thenozzle outlet. With the smaller nozzles, the pilot may have to he closer to the nozzle. The torch should he heated up before trying any of these experiments in order to prevent water from condensing inside. Let the torch burn with a medium-size gasloxygen flame (about 1100 mL/min fuel gas, 400 mLImin oxygen: 5em flame) for 3 to 5 min. For an initial trial of the oscillator, the oxygen low-pressure regulator should be set to about 10 psi, with the flow off. The fuel gas should then he lit and adjusted to give a flame
The Flashback Oscillator The flashback oscillator capitalizes on a well-known annoyance associated with glass blowing. If the fuel-oxidizer mixture feeding a gas-oxygen torch is made increasingly rich in oxygen, an explosion limit is reached and a small explosion occurs in which the volume of gas inside the torch detonates. This flashback can he viewed as occurring when the reaction rate in the flame becomes uncontrollahlv fast and consumes the combustible mixture faster than it can be re~lenishedhv the flow rate to the torch nozzle. The flashhack oscil1ato"r is arranged so that the mixture re-ignites when the flow has replenished the combustible mixture following an explosion. Figure 1 shows the setup for the flashback oscillator. We employ a Veriflo (formerly National) Model 3A Blowpipe torch with the accessory. oxygen tips OX-l through OX-5. .. For qu:intitiltivc studici, w r introduce the fuel gas and the oxygen through Gilmonr Instruments F-IZtiOshielded tlowmet&; for demonstration purposes, the flowmeters are not needed. We have used both natural gas and pure methane as fuel eases. Because of the oossibilitv of variations in the composition of natural gas supplies in different localities, we eive demonstration Darameters for both natural -eas and for pure methane; the difference appears in the oxygen flow rates required to demonstrate a particular mode of behavior. The pilot wire is made by twisting together two 5-cm strands
twisted P t wire
. -f
Figure 1. Experimental setup for the flashback oscillator. For demonstration in a drafty location, the pilot wire may be replaced by a gas pilot flame supplied from a second gas jet. This arrangement is chemically less interesting but makes it easier to get susta~nedoscillation. Thegas pilot can be aBunsen burner or simply a vertical piece of metal tubing. With a gas pilot, the torch should be mounted so that its gas stream vents horizontally across the pilot flame. The outlet of the pilot should be about 3 cm below the level of the torch outlet or else the detonations will blow out the pilot flame. I
Figure 2. Details of pilot wire construction Volume 64
Number 12 December 1987
1043
that extends upwrlrd about I cm beyond the pilot wirc. The stand holdina the piht wire shuuld be adiucted to oositim the outer coils of the helix directly in the center of the flame; the wire will glow a t medium red heat. Then, the oxygen needle valve should be carefully opened. As the' oxygennow is slowly increased, the flame will become shorter and the platinum wire will glow more brightly, approaching yellow heat. At a critical value of t h e oxygen flow, the flame will flash backinto the burner with a sharp report. The pilot wire will begin to cool down, but as the gasloxygen flow replenishes the combustible mixture in the vicinity of the pilot, the wire will begin to heat up again. When the wire reaches yellow heat, the mixture will re-ignite, detonate, and the process will repeat. This svstem displays a rich varietv of phenomena, depending on the n&lc:;ize, relative andabsniute flow ratesof the two gnses.and proximity of the pilut to the nuzzle. These phenomena include:
This system can be studied over a wide variety of conditions determined by the choices for the control variables or "constraints" (2, 5). Rather than giving an exhaustive account of the system's behavior here, we will focus on those operating conditions that give convenient illustrations of various far-from-equilibrium phenomena. The experiments described here are all performed using the largest nozzle (OX-5) with the pilot wire 5 em above the nozzle.
(1) a steady-state luminous flame (for example, with no O2 flow
The quantitative behavior of this system is quite sensitive to the exact geometry of the setup We have done experiments w'th two seemingly identical Mode 3A Blowplpe torches, a 'vmlage' 1950's one and a brand new one. Thegas flow ratescorrespondingtoa given oscillatory behavior vary as much as 20% between the two torches. We have also varied the length and type (latex,PVC, aluminum)of the gas supply tubing and the settingsOF the low pressure regulators, all of which affectthe relaxation times of the gas supplies. We have found that these factors also influence the quantitative system behavior. Thus, the exact flow rates required for the reader to duplicate these results may differ considerably from the values given here.
when the torch is first lit); 01a steady-+tawnonlumin
