INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
742
panies \Those films were used in the investigation and in particular to E. I. du Pont de Neniours & Co., Inc., for obtaining x-ray diffraction patterns on the cellulose acetate-butyratp film.
Vol. 46, No. 4
absolute pressure on upstream side of nienibrario, ciii. o f mercury Subscripts f, 0, and p refer to the feed gas, the high pi outlet gas, and the lorn pressure oritlet g a ~ respect,ively, , nrilc,~~ otherwise indicated. Superscript,s and sub R,C , and I) d e r to gas coniponents 8 , B , C, and D T
=
~
NOhlENCL&TURE
= ( a - 1)TV A = area of membrane, sq. cin. b = ( - I ) [ T ( ~ - X;") a e! e = CYTXj4 d = membrane thickness, centimeters D = diffusion constant, sq. cm. per fecond E , = activation energy of permeation, calories per gram-mole F = fraction permeated, N,/,l;f J = T' R' -17 = gas rate of flow, inoles per second P = permeability constant, cc.-cm./sec.-sq. em.-cni. of mercury Pa = permeability constant a t infinite temperature P = pressure on downstream side of membrane, cm. of mercury E = gas constant, calories per gram-mole per O K. R' = PA - P B $9 = solubility coefficient, cc. of gas per cc. of polymer at 1 atmosphere pressure r , i = temperature, E(. r 1 = P A - PC t = time, seconds [ - = PB - Pc 1' = fraction of gas not perineated = I - I;' TTr = T F - pv gas composit,ion, mole fraction y = ( T - PIT' - T L a
+ +
+
I,
LP'1'EIE I'LEtE (:ITEIP
(I) limeroiigen, G. T. van, J . Polu?iici Sei.. 5, 307-32 (1950). (2) Uarrer, R. M., "Diffusion in and LhroiiRh Mids," Londoi!, ('mibridge Press, 1911. (3) Barrer, R. M., Rubber Chrm. & Tech/iol., 21, 133-40 (194%). (4) Henedict, M , , C h e m Eng. Progi., 43, 41 (1947). (5) Brubaker, D. IT., and Kammermeyer, K., Anal. Chcui., 25, 424-6 (1963).
(6) Brubaker, D. W., a,nd Kammei e r , K . , IXD.ICsc. ( ' I I I ~ . , 44, 1465-74 (1352). ( 7 ) I h i d . , 45, 1148-52 (1953). (8) Cohen, K., U . S . Saual M e d . Bull., Suppl. 6 (1948). (9) Graham, T., Phil. Mag., 32, 401 (1866). (10) Ruckins, H. E., and Kamniernieyer, K., C'hern. Eng. P 180-4, 295-8 (1953). (11) Kammermeyer, IC., Proc. Iowu A c a d . Sei..57, t i l (IWJ). (12) Keith, P. C., Chmn. ,?rig., 5 3 , 11% (February 1946). (13) lIitchel1, J. Y., .I. Roy. i n s t . , 2, 101, 307 (1831). '. L., and Hershberger, A,, J l o d e r n Pla. (14) Simril, 1 (June 1950). (15) Steiner, 1 %'. A , , and Weller, S.,C.d . Patent 2,697,907 (AI:{?. 2 7 , 1952).
x =
(16) Teller, S., and Steiner, ITr. A , , Chenz. Eiig. Progr., 46, .jScj~~!10
% =
S.,and Stfiner, !IA\.,, .I. A p p l , P h y ~ . ,21, 2iCJ S3 (1950). RECEIVED lor review September 26, i9:3. .\LCEPTI:D Soreniber 2 Presented before the Division of Industrial and Engineeiing Chern the 124th hIeeting of t h e -41111Hxc.4~C I I I ~ I I C I~LO C X E T Y Chicagn, , Ill.
a
=
B = Y
=
6
=
(1950).
(17) \Teller,
ct of t ERNEST C. BEKNII,.IRDT1 Technische Hochschule, Darmstadt, German.y
IXIXG and homogenizing steps are of particular importance in the manufacture and compounding of thermoplastic materials. Other chemical industries have found extensive use for ultrasonic energy in dispersion and mixing operations similar t o those encountered in the plastics industry. This investigation on the effect of ultrasound upon molten thermoplastics was undertaken in order to survey t'he potential importance of ultrasonic energy in the processing o€ these materiale. It \vas the objective of this work to investigate: 1. The effect of ultrasound upon the molecular weight of polymer melts. 2. Heating of polymer melts through absorption of ultrasonic energy. 3. Molecular orientat'ion through ultrasonic energy. 4. Effectiveness of ultrasound in dispersing and homogenizing pigments and fillers in polymer melts. 5 . Effect of ultrasonic energy upon the viscosity of polymer melts. APPARATUS
The apparatus (Figure 1) employed in this study consissed of a piezoelectric ultrasonic generator which could produce frequen1
Present address, Polychemicals Department. E. I. d u Pont de Neinours
&- Co., .4rlington, N. J.
cies of 350, 1000, and 3000 kc. ;it energy levels of from :< 1 0 3 v,-attA per sq. cm. The quaitz elements used had a frecly osoillating area at least 34 mni. in diameter. The circuit diagr,;iiii of t'he generator is shown in Figure 2. The quartz was mounted in the bottom of a constant teniperature oil bat,h which was designed t,o reach temperatures of up t o 300" C. Samples were supported directly above the quartz in a, cylindrical chamber of aluminum with a n aluminum meniblane 20 microns thick in the bottom i o transmit the ultrasonic encrgy S to offer nep;ligil)lc, into t'he sample. This membrane T ~ found resistance t o the passage of the ultrasound waves. Melt films 3 mm. thick ~ ~ formed r c on the mcnibranc by pouring premighed granular Pamples int,o the hot chamber immersed in the thermost,aterl oil bath. The cylindrical chmitwr could be blanketed with inert gas t o prevent oxidation of sa.nipies. The oil bath functioned both as a means of controlling tlir tcniperature of the samples and as an electrical insulator for t,hc high voltage across the quartz. It also trmsniitted the uli radonic energy from the quartz t.0 the sample. A special float (Figure 3) was used in the oil bath to ciotorniine the ultrasonic energy emit,ted from the quartz surface through measurement of the sonic pressure. T o make sound intensity determinations, the cylintii.ical.
INDUSTRIAL AND ENGINEERING CHEMISTRY
April 1954
chamber is removed, and the float is positioned so that it swims directly above the quartz. The longitudinal waves impinge upon the bottom surface of the hollow aluminum body. T h e bottom surface has the shape of an inverted 130" cone in order to prevent the creation of standdg waves through reflection of the sound waves to the quartz. The inverted cone also tends to center the float directly above the quartz. A bulb in the hollow glass stem serves to keep the float In an upright position. Sufficient weights are placed upon the small pan on top of the glass stem to cause the float to immerse to a set point along the glass tube between bulb and weight pan. This point is selected so that the reflecting surface of the aluminum body is a t the same location above the quartz which the sample Rill occupy during irradiation. When the quartz is excited, the pressure of the sound waves will tend to raise the float out of the oil bath. The :dditional weight needed on the pan to restore the float to its original depth of immersion is a direct measure of the energy radiated from the quartz. This measurement of sonic pressure could be duplicated within &2%. Equation 1 relates the sonic pressure to the sound intenclity ( g ) .
where
743
The temperature measurements show that in the three materials tested, ultrasonic energy a t 1000 kc. caused greater absorption than a t 350 kc. I n all three thermoplastic melts a definite drop in energy absorption was noted a t the highest frequency of 3000 kc. One would expect a greater absorption with increased frequency, since a t the higher frequencies a more rapid attrition between the molecular chains would occur. This effect, however, is partially offset by the fact that higher frequencies a t the same energy level bring about smaller amplitudes, and in this way fewer chains are disturbed. It may be that the very low absorption a t 3000 kc. is caused by the fact t h a t the molecules can no longer keep up v i t h the sonic waves, and that the relaxation time of the polymers tested exceeds the period of this frequency.
S = sonic pressure J = sound intensity 0 = velocity of sound CY = angle of incidenw
+illsamples in these experiments were exposed a t the same ultrasonic energy level of 3 watts per sq. rm., and a t each of the three frequencies of 350, 1000, and 3000 kc. Standard molding powder grades of polystyrene, polyethylene, and polyvinyl chloride containing 50% dioctyl phthalate were used in these experiments. These materials were exposed to the ultrasound a t their normal working temperatures: Polystyrene Polyethylene PVC-DOP
270' C. 250' C. 2000 c.
EFFECT O F ULTRASONIC ENERGY ON MOLECULAR WEIGHT
Several studies of the effect of ultrasonic energy upon solutions of high polymers have been carried out. Among these are the work of Schmid (9-12) and of Weissler (IS), who showed that ultrasonic radiation may cause depolymerization of polymers in solution. The effects of such factors as the shape of the macromolecule, molecular weight, concentration, temperature, and viscosity of the solvent upon the mechanism of ultrasonic depolymerization have been investigated by the above authors. It is known that ultrasonic depolymerization decreases in solutions both a t very high and a t very low concentrations. Depolymerization is also reduced a t higher temperatures, where the polymer molecules become more flexible and can follow the sonic vibrations more easily. Accordingly, a depolymerization of polymer melts through ultrasound is not likely to occur. lleasurements carried out in the present study confirmed that ultrasonic energy does not influence the molecular weight of polymer melts. None of the three polymers suffered any changes of molecular weight during a 5-minute exposure to ultrasound under the conditions investigated. The solution viscosities of the irradiated samples u ere not detectably different from those of the untreated standards. TEMPERATURE ELEVATION AND ENERGY ABSORPTION
The temperature of each sample was measured during a 10minute continuous exposure to ultrasonic energy. As it was impossible to calculate accurately the heat losses from the samples to the oil bath, the rises in the temperatures of the samples provide only rough estimates of the relative rates of energy abBorption in the different melts a t the three frequencies.
Figure 1. Cross Section of Apparatus B. D. H. I.
Cylindrical c h a m b e r Cover H i g h voltage t e r m i n a l Insulation
0. Oil l e v e l Q.
S.
Q u a r t z with s u p p o r t Tripod
JY. C e r a m i c t u b
In several of the samples the temperature rise receded after a certain time of exposure, and there were definite indications of a reduction in energy absorption. Several plausible explanations may be advanced for this reduced absorption. The molecular chains may become oriented in such a way as to reduce their resistance to. the ultrasonic oscillations, so t h a t a thixotropic effect is created, which would cause a reduction of the frictional losses. It is also possible that low molecular fractions or plasticizers work themselves into the planes of shear, and thus reduce the friction. A third possibility is that a t the high temperatures chemical changes may occur after a certain time of exposure, and alter the characteristics of the polymer (Table I).
RISE I N TABLE I. TEMPERATURE
CENTER O F S.4MPLES
(Ultrasonic energy level, 3 watts per sq. om.) Temperature Rise Exposure, C. Frequency, 1 3 5 7 7 Material KO. min. min. min. min. min. Polystyrene 350 16 29 53 53 62 1000 35 52 59 62 72 3000 17 33 37 35 35 Polyethylene 360 17 48 52 44 42 1000 37 60 69 59 50 35 3000 12 25 35 37 PVC-DOP 350 6 3 9 8 6 1000 27 22 15 11 8 3000 3 6 11 13 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
744
CO
10 A
Cd
Figure 2.
Circuit Diagram of Generator
TABLE 11. DIELECTRIC COKSTASTSOF POLYVIKYL CIG~RIIXDIOCTYL PHTHhLhTE (Samples exposed t o ultrasound at 1000 kc. and 3 watts per sq. ciii. w e d a t 100 volts. 800 ci'cles)
Ileas-
4 rneaaurements
Perpendicular t o w a r e front Parallel to w a r e front
6.: 4 , ~
5.8 5.6
9.3
6.0 5.6
Of the three frequencies 15 hich ivei e investigated, it appearthat 1000 kc. was the most effective on each of the three poly.. mers. The optimum frequencies should be the ones corresponding to the relaxation times of the niateiials. I n all cases the temperature of the sample climbed mole lapidly in the center than near the edges. There are tx-o causes for this effect: The edges have greater surface contact with the relatively cooler container, and the energy emission of a piezoelectric quartz is higher in the center of the quartz than near its edges ( 3 ) .
Vol. 46,No. 4
sample!: iyerc cut into small rrct angular beams. Thrse beams could theii he laid parallel, arid could be rotated individually about their axes. In this wav it became possible to measure the tiielectric constants parallel and perpendicular t o the direction of t,he sound waves in t h e came sample. This met,liod wcentuates the dielectric constant change, hccausc the constant? are determined paralld aud perpendicular t,o a possible orientation. The initial niea.surement,r, 011 the othrr hand. attempted to compare the dielectric values betwceii a possihly oriented and an unoriented sample. The results shoIy t.hat a distinct orient,ation has taken place, and that the dielectric constant in thc direction perpendicular to the wave fronts has increased (Table 11). Of the dielectric constants measured, even the highest' value of the series parallel t o the xave fronts lies below thc loivert value in the perpendicular direction. From this it can b e concluded that, the dipoles are oriented in the direction of propagation of the sound waves. If it is assumed that the polyvinyl chloride molecules actually behave similarly to Rayleigh disks, and orient themselves parallel t,o the wave fronts, according t'o the molecular structure of polyvinyl chloride the dipoles then actually would orient theinselves perpendicular to the wave fronts. Therefore, the above data are in agreement 11-ith the obqervations in the literature, and demonstrate by means of tlielcctric measurements that mxcromolecular chains: cvcn under highly viscous conditions. C L L ~be oriented by means of an ultr;iwnic' field in planes parallel t o the wave fronts.
MOLECULAR ORIER~TATIOR~ THROUGH ULTR.4SOUND
Orientation effects due to the action of ultrasonic enei'gy had been observed previously. Some of the early studies in t,hie direction had been carried out by Pohlman ( 7 ) , who had noticed that ultrasonic Ivaves have an orienting effect upon suspended particles, and that these line up parallel to the wave frouts, like a Rayleigh disk (8). Lucas (L, 6) had observed the orienting action of ultrasound upon macromolecules of viscous natural oils. For the small samples of polymer: treated in the apparatus of the present study, determinations of refractive index and dielectric const,ant were the two principal methods that, could be considered for the determination of orientation effects. The difficulties of defining the temperature of the sample during ultrasonic radiation precluded the possibility of making measurements directly on the molten samples during their exposure to the ultrasound. For t,hie reason a procedure was adopted whereby the chamber with the sample 1% as removed from the oil bath a t the end of the 2-minute expomre to the ultrasonic energy, and \vas immediately quenched in cold oil. By this method it was hoped to freeze any orientation effects vihich might have been created in the sample. A standard sample was carried through an identical cycle rvithout being esposed to the ultrasound, Polyethylene and polystyrene, beirig nonpolar, were not expected to show dielectric constant differences bctwcen the esposed sample and the standard. Actual dielectric measureineiits confirmed that there was no change. I n t,he case of polyvinyl chloride, hoirever, a slight riae of doubtful significance in dielectric constant w a s noted in three exposed samples coinpared t o three unexposed samples. I n order to check this effect more accurately, the three exposed
Figure 3.
Float for 3leasuring Sonic Presstwe
liefract,ive index iiieacurenieiits on polystyrene did not, show any measurable orientation efi'ect brought about by ultrasonic energy. It is very likely that puch an orientation w a p loRt in the short time interval between the exposure to t>heultrasound and the quenching of the Paniple. In the case of polyvinyl chloride it may be possible that the orientation is maintained a little longer, owing t o the gel structure of the material. Seithor the polyvinyl chloride nor t,he polyethylene samples yielded adequate surlaces for reliable ineasuremeiitx of refractive indcs. PIGMEKT DKSPERSION TIIROUGII ULTRASONIC ENERGY
Xathieu-Sigaud and I x v a s e u r ( 6j had succeeded in dispersing agglomerates of precipitated barium sulfate through ultm~onic energy. They studied this cff'ect with the aid of an electxon microscope, and showed that the agglomerates were actutllly separated into the individual crystals. The American f'uint Joz~rnal ( 1 ) has also reported on experiments which indicate that ultrasonics may he used t o aid in the dispersion of pigments in lacquers. It therefore seemed desirable to determine whether
April 1954
INDUSTRIAL AND ENGINEERING CHEMISTRY
745
ultrasound might be effective in the dispersion of pigment>s in the more viscous phstic materials. Two mechanisms appear possible t o bring about a loosening and breaking up of pigment a ggl o m e r a t e s b y ultrasonic energy. First, one might induce oscillations in the individual crystals by exciting them a t their r e s o n a n t f r e q u e n c i e s . The other approach would be to cause oscillations in the medium surrounding the pigment, and thus shear the agglomerates. If the first method were used. and the individual crystals were A. to be excited, very high frequencies would be needed, as t,he Figure 4. Polyethylene-Titanium Dioxide Dispersion (X70) crystals are generally smaller A . Unexposed B. Exposed to ultrasound t,han 0.3 micron. This is a disadvantage, because high frequencies are difficult t'o generate at high energy levels. Another obstacle to this approach would The exposure to ultrasound had no measurable permanent effect upon the viscosity of the samples. be the fact t,hat pigment particles are not of uniform size, and The temporary viscosity changes during exposure of the sameach particle would therefore have to be excited a t its particular reponant frequency. ples to ultrasound, determined from changes in the time required For these reasons it is far more practical to effect pigment disto turn the disk through a given angle of rotat,ion, are: persion by causing oscillations in the medium surrounding the 330 Kc. 1000 Kc. 3000 Kc. pigment agglomerates. These oscillations could be generated Polystyrene, % 10.7 12.7 7.7 Polyethylene, % 5 . 8 10.1 3.3 most efficiently at that frequency which showed the greatest PVC-DOP, % 8.0 11.8 6.7 energy absorption and would cause maximum cavitation in the Because a mokwlar orientation which may be partially replastic melt, F~~this reaSOn these experiments on pigment dissponsible for the observed decrease in viscosity is accompanied persion in plastic melts through ultrasonic energy were confined by a rise in temperature due t o ultrasonic energy absorption, to the frequency of 1000 Itc. which had previously been sholvn to the reduction in viscosity caused by the temperature change must cause the highest energy absorpt.ion. be subtracted from the total viscosity change observed. I t had Samples were prepared by dry blending the granular powder been shown that the temperature rise in the samples wafi not uniof the three polymers LYith 1% titanium dioxide pigment in a form throughout their volume and also changed rapidly with glass bottle. The blending time was kept a t a minimum (2 time. Therefore the viscosity change due t,o the temperature minutes), so that most of the pigment agglomerates which exist rise could not be accurately defined, and a means for separating in the titanium dioxide were not broken up, The polymer a possible VisCOBity change due t o temperature rise was not availblended with pigment was placed in the chamber in the heated able. It was not possible to draw conclusions on the effect of oil bath where it melted; subsequently it exposed to the ultrasound upon viscosity, independent of temperature, although ultrasound at 1000 kc. for 2 minutes. the observations of the orientation by ultrasound might lead one I n all cases photomicrographs of t,he samples exposed to the to expect a CorresPonding thixotropic effect. ult,rasonic energy exhibited an improvement in the dispersion over the unexposed samples (Figure 4). However, because CONCLUSIOK S the energy distribution over the area of the sample is by no means even, owing to the nonuniform radiation from the quartz surface, This work has led to the following conclusions: 1. Ultrasonic energy has no depolymerizing effect upon the it is extremely difficult t o assign numbers to these changes. molten thermoplastics in the ranges of temperature frequency and energy investigated. EFFECT OF ULTRASOYIC EVERGY ov vrscoswy OF POLYMER &MELTS 2. Ultrasonic energy absorption in thermoplastic melts causes a rapid rise in temperature. iibsorption is dependent upon the It has been shown that ultrasonic energy may cause molecular frequency of the sound waves. I n the three samples evaluated, orientation of polymer melt8 in a plane parallel to the wave a frequency near 1000 kc. produced the greatest effect. fronts. Therefore, if a viscoiity change takes place, it will prob3. Ultrasonic energy causes molecular orientation in a plane ably be found in such a plane. -4viscometer t o carry out such perpendicular t o the direction of the sound waves. measurements, and to demonstrate a lowering of viscosity dur4. Ultrasonic energy could be used to aid in the homogenizaing exposure of the polymer melt to ultrasound, was constructed. tion and in the dispersion of colorants and fillers in plastic melts. It consisted of a thin round disk resting on and wetted by the The fact that ultrasound does not degrade the molecular strucsurface of the sample. This disk could be turned about its axis ture of plastic melts suggests that it may be considered for a by a known torque exerting a given shear stress. The shear rate number of possible future applications in plastics processing. was determined by measuring the time required to turn the disk I n particular, ultrasound offers a means for rapid and local internal through a given angle of rotation. heating of polar or nonpolar plastics. Thus it could provide a A temporary viscosity change was noted during exposure of the new means for welding and heat-st=aling these materials, and may sample to ultrasound, but the viqcosity returned again to its aid in other forming operations. The fact that ultrasound may original value, after the ultrasonic irradiation was stopped. cause molecular orientation without mechanical flow or dcforma-
INDUSTRIAL AND ENGINEERING CHEMISTRY
746
Vol. 46, No, 4
(5) Lucas, R., J . phus. radium, 10, 151 (1939). (6) Mathien-Sigaud, A , and Levasseur, G., Compt. Tend., 227, 196
tion should also prove to be of practical value in forming and molding processes. The aim of this ivork was a qualitative survey of the behavior of polymer melts in ultrasonic fields. T o define the effects of ultrasonic energy upon polymers quantitatively, new and morc refined apparatus will be needed.
(1948).
(7) Pohlman,R., 2 , p h y s , ,107,497 (1937). ( 8 ) Rayleigh, Lord, “The Theory of Sound,” Xew P o r k , Dover
Publications, 1945.
(9) Schmid, G., Phys. Z., 41, 326 (1940). (10) Schmid, G., and Beuttenmuller, E., 2. Elelztrochenz., 49, 3% 11943’1. - - ~ , \
LITERATURE CITED
Ani. Paint J . Contention Daily, p. 20 (Xov, 5 , 1949). Bergmann, L., “Der Ultraschall und seine Anwendung in Wissenschaft und Technik,” 5th ed., Stuttgart, Germany. 6 . Hirzel Verlag, 1949. Krause, R., 2. angew Phys., 2, 370 (19501 Lucas, R., Conapt. rend., 206, 827 (1938).
J
(11) Schmid, G., Beuttenmuller, E., and Rief, A,, 1Cunsfsto.fTechnik, 13, 65 (1943). (12) Schmid, G., and Rommel, O., Z. phgs. Chem., 185A, 97 (1939). (13) iveissler,A , , J . ~ ~phye,, ~ 21, l 171 (1950). . RECEIVED ior review July 3 , lS53. ACCEPTED.January 6 , 1954 AbEti,art of a thesis prcsented at t h e Teclinische Ilochacliille, Darmstadt. Gernia:i:;,
h i a y 1862.
tudies o
hie
PHILIP F. I W R Z Rattelle Memorial I n s t i t u t e , Columbus I , Ohio
T
HIS paper presents the results of an investigation of the stabi1it)y limits of laminar flames on Bunsen burners shielded to exclude ambient atmospheric air. Two met,hods of excluding ambient air were used: Smithells tubes (8) and an annular shield of constantly flowing inert gas. The effect of the size and configuration of the Smithells tube is discussed. Kith unshielded burners it is possible to observe only three stability limits for laminar flames at any given rate of air input: lean blowoff, lean flash bacli, and rich flash back. In the present x-ork shielding devices were used in order t o study the rich blowoff limit also. This is not possible if ambient air is permitted to circulat,e around the burner port. -4mbient air will stabilize all rich flames, even those containing no primary air, by diffusing into the primary air-fuel mixture or into the fuel and stabilizing the flame on the burner port. The shielding devices were retained in the lean blowoff and flash-back studies, that all the data might be taken under similar conditions. The stability limits of various single fuels, including hydrocarbons and hydrogen sulfide, were observed as a prelude to experiments with binary fuel mixtures. Following this, the behavior of binary hydrocarbon mistures wat! studied on burners with both types of shielding devices. The behavior of mixed hydrocarbon fuels is usually predictable at the lean blowoff and the flash-back limits, but at the rich blowoff limits small inhibiting effects are frequently encountered. The experimental data for binary mixtures can often be correlated Kith an empirical combust,ion equation which permits the prediction of the behavior of fuel mixtures, provided the behavior of the components is known. Likewise. if the behavior of a series of mixtures is known, the stability limits of the individual components can be deduced.
ii witer bath held a t room tempcrature maintains the mixing chambers a t a uniform temperature and also acts as a sump for the water which is circulated through bhe burner-tube jacket, by means of a jet pump. Figure 2 shows several cross sections of the burner used with the Smithells tubes. Air enters through a 1/8-inch tangenti:ii hole into the vortex-generating chamber a t the bottom, from which it pasees through a converging nozzle into a cylindrical throat, which is i/( inch in diameter. The change in cross section accelerates the velocity about fourfold. The fuel in injected radially inward from four l/ls-inch circular ports spaced symmetrically around the periphery of the throat. Thus, mixing of fuel and air is extremely rapid and uniform. When required, an inert gas for quenching the flame after flash back can be injected tangentially in the same plane as the air-injection port’. The fuel-air mixture passes from the vortex miser through :I flow-straightening chamber, consisting of a pipe nipple filled witti lengths of l(,t-inch copper tubing, so that the mixture delivered tci the burner is no longer sn-irling but is essentislly in streamlined flow. h brass burner tube 1.75 em. in inside diameter was used. It was slightly more than 50 tube diameters in length, thus erisuring streamline f l o ~at the burner port. The burner tube is cooled with mater as shown. A special outlet for cooling watrr was installed above the main outlet and as near the burner lip as possible to keep the burner rini adequately cooled. The Smithells tube was concentric with the burnrr t’ubc anti was held in place by a ring of Sauereisen cemrnt, Jyhich also mado an effective seal to prevent leakage of air from the atmospherc: into the flame a t the baAe of the Smithells tube.
DESCRIPTION OF APPARATUS
Figure 1 shows a schematic layout of the Bunsen burner and its auxiliary equipment. A11 gas-flow rates are metered by means of critical-flow orifices’ Compressed air from commercial cylinders was used to ensure control over the delivery pressure at all times. The two fuels were mixed in a tee-chamber in the flow system. Each leg of the chamber contains a fixed, four-slot vane which causes the gases to swirl, ensuring rapid and complete mixing. As shown in Figure 1, provision is made for mixing three fuels progressively, if desired.
Figure 1. Schematic Layout of Bunsen Burner and Auxiliaries for Flame-Stability Studies
