January 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
mentation is about 95% of the weight of the sugar fermented] while yeast is obtained a t about 45% yield. Twice as much lactic acid as yeast can be obtained and, in addition, a n appreciable quantity of acetic acid is recovered. A mill of average size producing 100 tons of pulp daily per 300day year could produce 9,000,000 pounds of lactic acid annually. To utilize such large production new uses for lactic acid would be necessary, which might be found in the paper industry or in the field of acrylate plastics, adhesives, and elastomers. ACKNOWLEDGMENT
This work was initiated a t the request of Karl W. Fries of the Rhinelander Faper Company, who had already undertaken some experimental work on fermentation at the time this project was started. The authors are indebted to Dr. Fries for his fine cooperation in supplying most of the sulfite waste liquor and information about the samples. They owe thanks to D. L. Reed and R. S. Hatch for other samples of liquor. Miss E. McCoy of the Department of Agricultural Bacteriology supplied the culture stocks. The work was supported in part by a fellowship established by the Rhinelander Paper Company. LITERATURE CITED
(1) Allgeier, R. H., Peterson, W. H., and Fred, E. B., IND.ENQ. CHEM.,21, 1039 (1929). (2) Anon., Pulp & P a p e r I n d . , 19, No. 6, 18 (1945). (3) Arnold, J. H., J . Am. Chem. SOC.,52, 3937 (1930). (4) Coolidge, A. S., Inter. Crit. Tables, Vol. 3, p. 398 New York. McGraw-Hill Book Co., 1929.
67
(5) Elgin, J. C., Chem. M e t . Eng., 49, No. 5, 110 (1942). (6) Filachione, E. M., and Fisher, C. H., IND.ENG.CHEM.,36, 23 ( 1944). (7) Ibid., 38, 228 (1946). Fred, E. B., Peterson, W. H., and Anderson, J. A., J . B i d . Chem., 48, 385 (1921). Fred, E. B., Peterson, W. H., and Davenport, A,, Ibid., 39, 347 (1919). Friedemann, T. E., and Graeser, J. B., Ibid., 100, 291 (1933). Hatch, R. S.. Paper Trade J.. 122.54 (1946). Marten, E. A., Sherrard, E. C., Peterson, W. H., and Fred, E. B., IND. ENG.CHEM.,19, 1162 (1927). Othmer, D. F., Bergen, W. S., Shlechter, N., and Bruins, P. F., Ibid., 37, 890 (1945). Othmer, D. F., White, R. E., and Trueger, E., I b i d . , 33, 1240 (1941). Pan, S. C., Peterson, W. H., and Johnson, M. J., Ibid., 32, 709 (1940). Partansky, A. M., and Benson, H. K., Paper Trade J . , 102, No. 7, 29 (1936). Saeman, J. F., Locke, E. G., and Dickerman, G. K., F I A T Final Report 499, Joint Intelligence Objectives Agency, Washington, D. C., 1945. Sankey, C. A., and Rosten, M. M., P u l p & Paper M a g . Can., 45, 171 (1944). Shaffer, P. A., and Somogyi, M., J . Biol.Chem., 100, 695 (1937). Wiley, A. J., Johnson, M. J., McCoy, E., and Peterson, W. H., IND. ENQ.CHEM.,33, 606 (1941). Wise, L. E., “Wood Chemistry,” A.C.S. Monograph 97, New York, Reir\hold Publishing Corp., 1944. R E ~ I V ESeptember D 16, 1946. Presented before the Division of Bugar Chemistry at the 110th Meeting of the AMERICAN CHEMICAL SOCIETY, Chicago, Ill. Published with the approval of the director of the Wisconsin Agricultural Experiment Station.
Atomization of Liquids in High Velocitv Gas Streams J N
H. C. LEWIS’, D. G. EDWARDS2, M. J. GOGLIA3, R. I. RICE4, AND L. W. SMITH University of Illinois, Urbana, I l l . Despite the important uses of atomization in the chemical industry, the technical literature contains little quantitative information on the performance of gas-atomizing and liquid spray nozzles. Attention is called to the general usefulness, for purposes of design and control, of the empirical equations recently published by Nukiyama and Tanasawa (11-16). The mathematics of the equations is discussed, and it is shown that drop size distribution data can be expressed in terms of a simple straight-line relation. In the case of gas-atomizing nozzles, analysis of the data in the literature reveals substantial agreement with the equations. In the case of liquid spray nozzles the agreement of the equations is more qualitative. Experiments on gas-atomizing nozzles are described. These corroborate and extend the work of Nukiyama and Tanasawa and give an indication of the effects of gas density, gas viscosity, heat transfer from gas to liquid, and the scale of the apparatus on the performance of gas-atomizing nozzles.
T
HE following article gives a n outline of the general principles that govern the atomization of liquids in a Venturi
throat, and describes the application of these principles t o several devices for military use. It was pointed out t h a t the action 1 9
8
4
Present Present Present Present
address, address, address, address,
Georgia School of Technology, Atlanta, Ga. California Reeearch Corporation, Richmond, Calif. Purdue University, West Lafayette, Ind. American Coating Mills, Elkhart, Ind.
in a typical device of this kind is quite complex and often involves simultaneous atomization, heat transfer, and evaporation, as well as a n unsteady-state process. For this reason the development of useful munitions could not wait upon the relatively slow procedure of a study of fundamental mechanisms and had t o be based on the qualitative methods described in the following article. I n the present article a report is made of the initial experiments in a program intended t o throw light on fundamental mechanisms and lead t o improved designs. The primary concern in this paper is with atomization. I n all the experiments reported, evaporation is kept a t a minimum by the use of nonvolatile liquids. I n one of the sets of experiments heat transfer is minimized by using both liquid and gas in the range of atmospheric temperature; and in the other runs the effect of heat transfer is measured only in so far as i t modifies the degree of atomization obtained. Because of widespread use of atomizing nozzles in chemical industry, the results reported here should be of general interest. Atomization is a practical method of obtaining intimate contact between a gas and a liquid, and is frequently employed in operations involving chemical reaction, absorption, evaporation, etc. Familiar examples are oil burners, burners for liquid sulfur, and t h e nozzles used in spray chambers for evaporation, absorption, and air conditioning. Most of the published information on pneumatic and hydraulic spray nozzles has come from five groups of investigators. First, there are articles by physicists and others interested in liquid
INDUSTRIAL AND ENGINEERING CHEMISTRY
68 loo,
m
I
I
I
I
I
Vol. 40, No. 1
I
DIAMETER AS PER CENT OF CHARACTERISTIC DIAMETER Do
Figure 1. Volume Frequency Curves for Typical Nozzle Coefficients
atomization from a theoretical viewpoint. In this category falls the work of Castleman (Z), Kleinschmidt (6),Littaye (8, 9, 10), and Schweitzer (19). Studies of this character are helpful in understanding the qualitative nature of atomization, but as yet they have not progressed far enough t o yield results useful for quantitative calculations on commercial nozzles. A second source of data on nozzles is the information furnished by nozzle manufacturers. Normally a manufacturer tests his nozzles with water only, and the measurements reported in his catalog are usually confined t o the dimensions of the spray pattern and the nozzle capacity in gallons of water per minute at various pressures. Agricultural experiment station bulletins provide the third source of data on spray nozzles. These contain useful information on the characteristics of nozzles suitable for spraying orchards and crops; but unfortunately they do not include many data on drop size distributions, which are of paramount interest t o the chemical engineer. The information in a number of agricultural bulletins is summarized by Crane ( 2 ) . Fourth in the list of sources of data on nozzles is the work done on dissipation of fog in the vicinity of airports by spraying a calcium chloride solution into the air (3, 4,5 ) . Finally, the mechanical engineering literature contains several articles on the atomization of liquid fuels charged t o internal combustion engines. Sauter (17, 18) and Nukiyama and Tanasawa (11-16) give quantitative data on carburetors, and Lee (7) presents quantitative information on Diesel injection systems. EMPIRICAL EQUATIONS OF NUKIYAMA AND TANASAWA
I n view of the general applicability of two empirical equations developed by Nukiyama and Tanasawa, these will be discussed in some detail. The equations are based on the result of several hundred measurements under differing conditions using different
PER CENT OF CHARACTERISTIC DIAMETER
Figure 2.
De
Integrated Volume Distribution Curves for Typical Nozzle Coefficients
liquids in small gas-atomizing nozzles, operated with compressed air. The first equation, which gives the average drop diameter of t h e spray in terms of operating variables and the properties of t h e liquid being sprayed, is as follows:
where
D o = diameter of a single drop with same ratio of
surface to volume as total sum of drops, microns
v = relative velocity between the air stream and the
liquid stream, meters per second Q1/Qa = ratio of volume flow rate of liquid to volume flow rate of air a t uena contractu p = liquid density, grams per cc. p = liquid viscosity, poises u = liquid surface tension in dynes per cm. The equation is not dimensionally consistent but is generally applicable to gas-atomizing nozzles in which the liquid is injected into the gas stream when the density of the liquid is between 0.7 and 1.2 grams per cc., surface tension between 19 and 73 dynes per cm., and viscosity between 0.003 and 0.5 poises, and the velocity of the gas is below the acoustic velocity. It appears, therefore, t h a t the magnitude of the drops is mainly governed by the value of
V
4, .
when the ratio
Qo/Ql
is
I
large, whereas the viscosity is of minor importance under these conditions. But when the ratio QJQ1 is small, the magnitude is mainly governed by the value of
(-$=y'46
the surface tension has only a small influence,
(1000 Q1/Q.)'.6,
and
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1948
69
OF TABLEI. RELATION O F CONSTANTS I N EQUATIONS~ NUKIYAMA A N D TANASAWA (For p 2; V = total volume of sample, cubic microns)
-
P
2 1
e
Do, Microns
a
b
1.50/b' 'I 5)b 110/bi 4080/b 213,00O/b' 14 300 000/V 1,170~000~000/b6
General equation, DO =
.-k. ,f,"?
dn
dn
The second empirical equation expresses the data on distribution of drop sizes in liquid sprays and is as follows:
where x = diameter of an individual drop, microns n = number of drops with diameters between zero and z in the total sample a, b, p , q = constants Nukiyama and Tanasawa found in all of their studies that p was always equal t o 2, and, as'will be shown later, this conclusion is supported by the work of 'other experimenters as well. The value of q, according t o the Japanese authors, is unity in most cases when the velocity and quantity of the air stream are high and sufficient (v = 150-300 meters per second and Qo/Q,>5000). However, it will be shown that, although q is a constant for a given nozzle over wide ranges of operating conditions; it is quite sensitive t o variations in the type and size of nozzle and must be determined experimentally in each case. Constants a and b are determined by the values of p , q, DO,and the total quantity of sample chosen as a basis. The relation of a and b t o p , q, Do, and V , the volume of liquid in the total sample, is given in Table I for the case when p = 2. For details of the mathematical analysis involved, reference should be made t o the original artides of Nukiyama and Tanasawa. The value of q is associated with the degree of uniformity of the drop sizes in a spray. If the mass distribution of the drops is concentrated within a relatively narrow range of sizes, the value of q is high. On the other hand, low values of q represent a wide distribution of drop sizes. The effects involved are illustrated graphically in Figures l and 2. If q = 2, the equation for the distribution of drop sizes is of the same form as Maxwell's equation for the distribution of the velocities of the molecules in a perfect gas. This fact suggests that it may be possible to relate the empirical equation for drop size distribution t o fundamental mechanisms. For some purposes it is of more interest t o know the mass median diameter of a spray than DO. If the drop size distribution fits the empirical equation for dn/dx, p = 2, and l / q is an integer, it follows that the ratio of the mass median diameter t o Dois given by Equation 3 : I
(3)
where Y is determined by trial-and-error solution of the equation
The relation between D, and D Ofor different values of q is shown in Figure 3.
Figure 3. Ratio of Mass Median Diameter Dm to Characteristic Diameter DOas Function of Exponent q
Analysis of experimental data on liquid atomization is easily made by converting Equation 2 (when p = 2 ) into the form (4)
A plot of logto
(% :-):-
against xq should yield a straight line
with slope equal t o b/2.3. If q and b are known, the value of Do can then be calculated immediately with the aid of Table I. I n actual practice, construction of straight-line plots from original data is quite simple, as will be illustrated by analysis of the data of Sauter, Houghton, and Lee. Furthermore, the method has an advantage over other straight-line methods of plotting size distribution data, such as, for example, cumulative log-probability paper. The advantage is that the Nukiyama and Tanasawa plot, once its validity is established and the value of q for a given apparatus ascertained, requires the counting of far fewer drops than cumulative methods. To determine the complete distribution of sizes, an accurate count of the drops in only two size ranges is needed, DATA OF SAUTER, HOUGHTON, AND LEE
Sauter (17, 18) tested the performance of three commercial carburetors and a simple atomizer consisting merely of an air Venturi with means for injecting liquid at the throat. Complete data are reported for the atomizer runs only; hence consideration will be limited t o this unit. Since the average drop diameter of the spray was determined by a photometric method, depending on the effect of average drop size on the degree of absorption of a beam of light passed through the spray, no figures on the distribution of drop sizes are available. A comparison of Sauter's data with the equation of Nukiyama and Tanasawa for DOis given in Tables I1 and 111. Sauter's measurements give average drop diameters roughly one third as large as those predicted by the equation. This discrepancy may
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
70
Vol. 40, No. 1
be explained on the basis of unjustified assumptions involved in Sauter’s calculations. I n the first dace, it is assumed that as the spray travels down the duct leading away from the Venturi atomizer, the drops are distributed uniformly across the crops section of the duct. From observations on Venturi atomizers discharging directly from the divergent section into the air, it is apparent that the spray is more concentrated in a zone near the alis than it is near the outer edge. Secondly, Sauter assumes that the drops passing the beam of light are spherical. Some doubt on this point is raised by high speed photographs shouing that the liquid torn off a t the moment of atomization is in the shape of filament: and nets, which require a finite time to contract into spheres. Reproductions of high speed photographs have been published by Castleman ( 1 ) sand by n’ukiyama and Tanasawa (16). Data on one run with a small air-atomizing nozzle are given by Houghton (3). It is stated that the pressure on the air and on the liquid was 15 pounds per square inch. This indicates that the ratio of the upstream absolute pressure to the downstream absolute pressure was 2 . 1 (the critical ratio), and that therefore the velocity of the high speed air jet was equal to the velocity of
0
X, MICRONS
Figure 4. Drop Size Distribution from Small AirAtomizing Nozzle (3, page 1992)
OF WATERIN A VENTURIATOMIZER, TABLE 11. ATOMIZATIOS DATAOF SAUTER( 1 7 )
R u n No. 1 3 4 5
Air Velocity a t Throat, Ft./Sec.
Do,Microns Obsvd. b y Sauter
Calcd. from Equation 1 6
6
2 7 8 10 9
11 a Calculated from inlet air rate, temperature, and pressure, diameter of throat and tinstream section, and formula for adiabatic a n d reversible expansion of a perfect gas. b Computed b y calculating the first term of the equation and adding a n allowance of 10% for the second term (approximate calculations showed t h a t the second term was about 10% of the first term in all of Sauter’s runs on this unit). OF YAPHTHA IN A VEWTURI ATOMIZER, TABLE 111. ATOMIZATIOR‘ DATAOF SAUTER( 1 7 )
Air Velocity Do,Microns at Throat, Obsvd. b y Calcd. from R u n No. Ft./Sec~a Sauter Equation 1b 1 147 2 240 3 297 4 347 6 393 6 445 7 513 9 565 8 600 a See footnote a , Table XI. b See footnote b , Table 11. Physical properties of naphtha: p = 0.81; e =: 30; p not reported.
Figure 5.
Drop Size Distribution from Spraco Type
Nozzle (3, page 1991)
‘I
January 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
71
fact that in coarse sprays a very small number of large drops have a n overwhelmingly predominant effect in determining DAT.4 O F the average diameter of the spray, and it is difficult experi(From a plot ofDocolumn = 4080/(5.94)5 5 against column 20 microns) 2, q = 1/a; b = 5.94; mentally t o get a truly representative sample of the largest drops. For example, with coarse sprays the presence or abDrop Number of 1000 A n Diameter Microns 2, $1 1 3 Ax Dropsi An Log ( n) sence of one large drop in a sample ,of a thousand drops or so 1 1.00 2 5 390,000 8 19 collected on a microscope slide waved through the spray may 5 1.71 5 340,000 6.43 affect the average diameter of the sample by 1007,. I n the 10 2 16 5 165,000 5.52 15 2 46 5 40 200 4 55 absence of a sure method of getting a truly representative sample 5 11,680 3 77 20 2.72 of the large drops, it seems likely that the average drop diameter 5 4,970 3 20 25 2.92 30 3.10 5 2,160 2 68 determined by a straight line plot of the data is more accurate 85 3.27 5 1,730 2 45 5 1,080 2.13 than the average calculated by a summation of volumes and areas 40 3.42 5 650 1.81 45 3.56 directly from the drop count. 7.5 430 1 36 50 3.68 60 3.92 10 350 0.99 Extensive experiments on the injection of fuel oil into Diesel 70 4.11 10 220 0.65 cylinders have been performed by Lee ( 7 ) .. Spray nozzles of several different designs, with orifice diameters in the range of TABLE v# AToxlZAT1oN B Y HorALow-CoNE SPRAY 0.02-0.03 inch, were tested. A typical series of runs has been DATAOF HOUGHTON (3) analyzed by the method of Nukiyama and Tanasawa, with the Do,Microns Orifice Pressure P , Calcd. from Cnlcd. from result that the data are found to agree quite well with the emDiameter, In. Lb./Sq. In. original data Equation l a pirical equation for dnldz. For the particular series examined, 0 063 50 590 590 Values of Do,calculated from the slopes of the straight q = 1/2. 0.063 100 500 580 0.063 200 430 490 lines in Figure 6, are summarized in column 2 of Table VI.
TABLE Iv. ATOMIZATION IN
SMALL AIR-ATOMIZING NOZZLE, HOUGHTON (3) A
-
0.086 100 1100 780 0.086 200 690 700 0.128 200 1100 1100 a According to t h e Correlation, D Ois calculated from Equation 1 using t h e following values for the parameters: 8 = 3.72 d P / P ; Q D (gal./min.) = 8Sd d P 7 , where d = orifice diameter (in.),
TABLE
ATohllZAT1oN
IN LEE
DATA
(7)
OF
(Orifice diameter, 0.020 inch; physical properties of oil' p = 0.86, p = 0.33, u = 28) Do,Microns Pressure P , Calcd. from Calcd. from Lb./Sq. In. original data Equation l a 450 76 81 880 68 69 2280 61 58 4160 50 53 5700 52 51
sound. Since 15 pounds per square inch on the liquid line would give a liquid velocity low compared to that of sound, v may be estimated as equal to 336 meters per second. Although it is not definitely stated, it is probable that the liquid spray was water. On this assumption, P = 1, P = 0.01, and = 73. The a D O was calculated from Equation 1 using the following values: 21 = 3.72 4%; Q G (gal./min.) = 8900 d d / P / p ; orifice coeffrrient = 0.6. low liquid pressure and the small drop sizes produced combine t o suggest that the ratio of liquid to air was sufficiently low t o make the second term in Equation 1 negligible. On the basis of these assumptions, Do as calculated, by Equation 1 is 15 microns, whereas analysis of the data by means of the drop size distribution equation shows t h a t the measured value of Do is 20 microns. X 8 8 0 L B S / S Q IN Details of the calculations are presented in Table IV. B y plots of the values in column 5 against z raised to A - 2 2 8 0 LBS./SQ IN. various powers, it was found by trial and error that a 0 - 4160 L B S / S Q I N plot against x1/3 yielded a straight line with a slope of 2.58, as shown in Figure 4. AccordingIy, q = '/a and b = 2.3 X 2.58 = 5.94. From the values of q and b, D o was determined according to the relation in Table I. The additional labor introduced by the trial-and-error determination of q is small when a series of experiments is being made. Values of 2 9 can be calculated once and for all, for integral values of q and values of z corresponding to the divisions on cI, the scale of the microscope employed. Furthermore, the value of q appears t o remain constant over wide ranges of operating conditions for any given type of nozzle. Houghton also gives data on the drop size distribution from three hollow-cone hydraulic spray nozzles. If the method of analysis illustrated in the preceding paragraph is applied to these data, it is discovered t h a t q = as illustrated in Figure 5. Values of Do calculated from the slopes of plots of the straight lines in Figure 5 are listed in column 3 of Table V. 14 16 0 2 4 6 8 IO 12 I n some cases it may be noted that the calculated value I of Do is greater than the largest sizes of drops reX I ported by Houghton. This anomaly is due to the Figure 6 . Atomization of Fuel Oil in Diesel Cylinders (7)
-
72
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 1
it is in the former. The agreement betneen the observed and calculated values shown in the tables simply indicates the constancy of the proportionality in Equation 5 for a given nozzle under different operating conditions. These correlations are admittedly rough and are offered here simply as a means of organizing the data on spray nozzles and to serve as a guide in estimating the effect of variables in design and operation. Since no results were available to show the effect of the nature of the liquid being atomized, the correlations do not include any allowance for the effect of liquid viscosity and surface tension on &,. The effect of liquid viscosity and surface tension on Q L for a given nozzle a t a given pressure is presumably small, since catalog data indicate that the flax is generally proportional to the square root of the head. I n other words, spray nozzles appear to act as orifices in the region of turbulent flow; and it is known that orifice coefficients in this region are insensitive to liquid properties other than density. In computing column 4 of Table V, it was assumed that the liquid sprayed in Houghton's tests was water. I t was also assumed that the liquid flow rates a t various pressures corresponded to the values supplied by the Spray Engineering Company (20) for hollow-cone nozzles Kith orifice diameters identical with those reported by Houghton. E X P E R I M E N T S W I T H VENTURI A T O M I Z E R S
0 0 : 31 MICRONS
,
I
Do -42.2 MICRONS
16'-
The results of tests on three Venturi atomizers with throat diameters of 0.107 inch, 0.500 inch, and 3.34 inches, respectively, are presented in Tables VI1 through XI and Figure 7. In the case of the two smaller atomizers the liquid was injected a t the throat through a small tube coaxial with the Venturi, pointing downstream and terminating just in front of the throat. Liquid was fed to the largc atomizer through spray nozzles pointing radially inward a t the throat. Samples of the spray were secured by waving a microscope slide through the spray cloud; and the relation between the drop diameter observed under the micro-
TABLE l r I I . ATOMIZATIOh' O F
OIL IS VENTURI -4TObfIZER COMPRESSED ~-ITROGEN GAP
USINQ
(Throat diameter. 0.107 inch) 1000 Q L
The empirical equation for Do can be applied to hydraulic spray nozzles as well as to gas-atomizing nozzles, if the action of the nozzle is considered as occurring in two steps. I n the first step, which takes place within the body of the nozzle, the pressure is used to form a sheet of liquid traveling at high velocity. With many types of nozzles this sheet can be seen to extend for a distance of several inches. The velocity a t which the sheet travels may be estimated from Bernoulli's equation. I n the second step, which occurs outside the nozzle, the liquid sheet is broken up, presumably by the atomizing action resulting from the difference in velocity between the liquid and the surrounding air. The empirical equation for D ocan be applied to the second step, if it is assumed that the effective value of Q, is proportional to the diameter of the nozzle orifice and to the square root of the liquid head-that is,
R u n Xo. 1 2 3 4 5 6 7 8
9
10 -~
11 12 13 14
0.14 0.39 0.40 0.42 0.53 0.69 0.80
0.88 0.88 0.98 1.18 1.19
1.28 1.78
Do. Microns Calcd. from Equation 1 20 38 39 40 50 68 80 90 90 103 132 132 147 230
Obsvd. 19 28 25 22 35 47 32 58 60 27 37 45 77 110
a Average conditions of gas a t Venturi throat: density, 1.18 grams per liter; viscosity. 0.017 centipoise; velocity, 700 feet per second. Liquid properties a t operating temperature: p = 1.04, p = 0.09, I = 34; p =
OF OIL IX VENTURI ATONIZER USING TABLE VIII. ATOMIZATION COMPRISSED ETHYLEKE GAS=
(Throat diameter, 0.107 inch)
Do,Microns
(5) k can be determined from experimental data by substitution 6f this expression in Equation 1. Correlations of the data of Houghton and Lee in this manner are shown in Tables V and VI. The average value of the proportionality constant IC for all of Houghton's data on the hollow cone nozzles is 85; for Lee's nozzle, over a wide pressure range, the constant is 8900. This indicates that the effective volume of air engaged in atomization per unit length along the circumference of the orifice is over 100 times as large in the latter case as
Qn
R u n No. 1 2 3 4
5 6
7 8
1000 Q L
Q,
0.12 0 21 0 40 0 67
1.00 1 16 1 18 1 92
Calcd. from Equation 1 20 24 39 65
106 129 132 256
Obsvd. 1.5 10 31 20 32 32 23 80
a Average conditions of gas at Venturi throat: density, 1.13 grams per liter; viscosity, 0.010 centipoise; velocity, 700 feet per second. Liquid properties a t operating temperature: p = 1.04, p = 0.09, u = 34; q = a / r .
January 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
73
Operation of the small atomizer with 0.107-inch throat was imperfect, in the sense that a considerable fraction of the liquid feed remained unatomized and dribbled off the exit end of the (Throat diameter, 0.107 inch) apparatus. Do,Microns The effect of gas viscosity may be seen by a comparison of the 1000 Q L Calcd. from nitrogen and ethylene runs (Tables VI1 and VIII, respectively). R u n No. Q. Equation 1 Obsvd. 38 At constant gas velocity, gas density, and ratio of liquid to gas, a 12 , 0.09 1 30 14 0.13 2 decrease of about SOT0 in gas viscosity resulted in a n approxi45 15 0.15 3 26 21 0.25 4 mately equal percentage decrease in DO. 48 0.30 5 44 Comparison of the nitrogen and helium runs (Tables VI1 and 0.38 55 0.48 I X , respectively) indicates the effect of gas density. When the 45 0.59 B 51 1.00 9 gas density was reduced to one seventh of its original value in the 2 10 1.10 10 130 nitrogen runs, a t constant gas viscosity and ratio of liquid t o gas, 1.32 11 the magnitude of D owas increased by a factor of about two, dea Average conditions of gas a t Venturi throat: density, 0.169 gram per spite an increase in gas velocity. Over liter; viscosity. 0.019 centipoise; velocity, 1100 feet per second. Liquid 34; q 1/~. properties at operating temperature: p = 1.04, p = 0.09, most of the range of operations the correction due to differences in velocity was relatively small. OF EXHAUST GAS VENTURIATOMIZER ATTACHED TO TABLE X. PERFORMANCE Of the five groups of runs reported TRUCK ENGINE" in Tables VI1 through XI, the one (Throat diameter, 0.5 inch) Do,Microns in which conditions were most nearly Temp. a t Pressure Back Gas Velocity R u n No. Throat, at Throat, Pressure, a t Throat, Calcd. from like the experimental conditions em0 F. Mm. H g Abs. Mm. Hg Ft./Sec. Q. Equation 1 Ohsvd. ployed by Nukiyama and Tanasawa 0.031 11 34 1070 580 52 1 740 0.061 17 33 was the series with nitrogen (Table 630 40 860 2 670 23 37 1050 0.13 580 69 770 3 VII). The nozzles of Kukiyama and 0.14 25 42 1030 540 98 4 750 5 850 700 154 850 0.84 240 62 Tanasawa were operated with com6 800 670 125 430 1.05 340 63 pressed air, the physical properties of p Average conditions of gas a t Venturi throat: density, 0.43 gram per liter; viscosity, 0:033 centiwhich are practically equal t o those q = 1. p = 0.89, p = 0.97, a = 27; poise. Liquid properties a t looo F.: of nitrogen. The scale of their apparatus was about the same as t h a t of the atomizer with 0.107-inch throat; TABLE XI. PERFORMANCE OF EXHAUST Gas VENTURIATOMIZERATTACHEDTO and their experiments included runs AIRPLANE ENGINE" with a liquid having physical prop(Throat diameter, 3.34 inches) I Do,Microns erties almost identical with those of Pressure Back Gas Temp. a t a t Throat, Pressure, Velocity Calcd. the oil used in the runs with nitrogen. Lbs./ a t Throat, 'Oo0 &L from Run Throat, Mm. Hg When a comparison of the nitrogen runs NO. F. Abs. Sq. In. Ft./Sef!. QO Equation 1 Obsvd. with Equation 1 is made, as in Table 1.5 1350 0.236 26 37 661 1 1280 1.5 870 0.461 48 46 2 1350 76 1 VII, it is clear that the values of Do measured average roughly one half the Average conditions of gas a t Venturi t4roat: density, 0.34 gram per liter; viscosity, 0.051 centipoise. Z,iquid properties a t looo F.: p = 1.03,p = 0.063,c = 34; q = 1. values predicted by the equation. Part of this discrepancy may be due to differences in design of the apparatus, in the sampling technique, and in methods of scope and the diameters of spherical drops of equal volume was calculation; but i t seems most probable that the major part of determined by measuring the focal lengths of the spherical lenses the difference is caused by incomplete atomization. formed by the drops on the slides. q, b, and D Owere calculated In the light of information on the effects of gas density and from plots of the drop size distribution data, according to the viscosity, the data on performance of a n exhaust gas atomizer method illustrated above. (Table X) are interesting. Since the density of hot exhaust gas Results obtained by this sampling technique were reproducible is less and the viscosity greater than the corresponding figures for and appeared to be representative. I n the testing of each slide air a t atmospheric temperature, i t might be expected, on both a count was made of the number of drops of various sizes in a counts, that observed values of Dowould be somewhat higher series of swaths across the slide. The count was continued until than values predicted by the equation. This is seen to be true further counting led t o no appreciable change in the distribution at low liquid rates, where the first term in Equation 1 is the conof drop sizes. Normally, this point was reached at a total of trolling factor. On the other hand, at high liquid rates, where about 300 drops. For each set of operating conditions duplicate the second term in Equation 1 is important, observed values of or triplicate slides were secured and found t o be in close agreeD O are clearly much lower than the predicted values. I n all ment. Plots of the data according to the method of Nukiyama probability, the low values of D Oobserved a t high liquid rates and Tanasawa yielded substantially straight lines in all cases; are the result of reduction of liquid viscosity by heat transfer and the fact that the drop size distribution data of Nukiyama from the hot gases to the liquid being sprayed. As noted 'in and Tanasawa, Lee, Houghton, and the present writers have all Table X, the calculated values of DOwere computed on the asbeen correlated by straight-line plots of the relatively simple sumption that the average temperature of the oil during the distribution equation of Nukiyama and Tanasawa may be taken atomization process was 100" F. Apparently thc true average t o indicate that the data of all these investigators correspond to temperature of the oil is somewhat higher than 100" F. From reasonably representative samples of the sprays. However, the a practical point of view, the results suggest that the use of hot particular method of sampling employed by the present writers is exhaust gases or steam instead of compressed air will improve not recommended for drop sizes lower than 10 microns. I n the the atomization of viscous liquids when the ratio of liquid to gas range below 10 microns a tendency was observed for points to is high, but not when it is low. The data presented in Table XI fall below the straight line; this indicated incomplete collection are of interest principally because they show that the equation of of the drops. Nukiyama and Tanasawa, derived from esperiwents on labora-
OF OIL IN VENTURIATOMIZER USING TABLE IX. ATOMIZATION COMPRESSED HELIUM GAS*
:
(I
a
-
E
14
INDUSTRIAL AND ENGINEERING CHEMISTRY
tory models, is applicable t o large Venturi atomizers. The rough equivalence of measured and predicted values of Do is consistent with the fact that the ratio of liquid to gas is intermediate between the two regions evident in Table X. ACKNOWLEDGMENT
This paper is based on work done for the Office of Scientific Research and Development under Contract OEMsr-102 with the University of Illinois. The investigation was suggested by H. F. Johnstone, director of the N.D.R.C. Munitions Development Laboratory, and by E. W.Comings, project supervisor, and was carried out under their direction. P a r t of the data on exhaust Venturis was obtained by the Solar Aircraft Company of San Diego, Calif. The graphs were prepared by M. H. Roberts, research assistant in the Engineering Experiment Station. LITERATU-RE CITED
(1) Castleman, R . A , , Jr., Bur. Standards J . Research, 6 , 369 (1931). (2) Crane, H. L., W. Va. Univ. Agr. Exp. Sta., Bull. 169 (1919). (3) Houghton, H. G., in J. H. Perry’s Chemical Engineers’ Handbook, 2nd ed., pp. 1984-1993, New York, McGraw-Hill Book Co., 1941.
Vol. 40, No. 1
Houghton, H. G., and Radford, W. H., Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, Papers in Phusical Oceanographu a n d Neteorology, Vol, VI, No. 3 (1938). I b i d . , Vol. VI, No. 4 (1938). Kleinschmidt, R. V., in J. H. Perry’s Chemical Engineers Handbook, 2nd ed., pp. 1983-1984, New York, McGraw-Hili Book Co., 1941. Lee, D. W., Natl. Advisory Comm. Aeronazc!., R e p t . 425 (1932). Littayc, G., Compt. rend., 217, 99 (1943). Ibid., 217, 340 (1943). Ihid., 218, 440 (1944). Nukiyama, S., and Tanasawa, Y., T r a n s . SOC.Mech. Engrs. (Japan), 4, No. 14, 86 (1938). I b i d . , 4, No. 15, 138 (1938). Ibid., 5, No. 18, 63 (1939). Ibid., 5, No. 18, 68 (1939). I b i d . , 6, NO. 22, 11-7 (1940). Ibid., 6 , KO. 23, 11-8 (1940). (17) Sauter, J., Forsch. Gebiete Ingenieurw., No. 297 (1926).. (18)Ibid., No. 312 (1928). (I9) Schweitzer, P. H., J . Applied Phys., 8, 513 (1937). (20) Spray Engineering Co., Somerville, Mass., Bull. 604F (1944). RECEIVED -4pril 1 2 , 1947.
HIGH VELOCI
RIZERS
E. W. COMINGS, C. H. ADAYIS’, AND E. D. SHIPPEE2 Cniversity of Illinois, b-rbana, I l l .
A method of Waporizing relatiFely high boiling or thermally unstable liquids into a hot gas stream is described. The liquid under low pressure is introduced as a simple jet into a high velocity hot gas stream. The liquid is thus atomized to drops less than 100 microns in diameter, and high rates of heat transfer to the liquid and vaporization of the liquid are obtained. .4Venturi tube is a useful design for obtaining the high welocity. The hot gas is cooled rapidly, and the time of exposure of the liquid to a high temperature is very short, of the order of a few milliseconds.
D
URING World War I1 there was a need for a method of vaporizing relatively high boiling and a t times thermally unstable liquids. The usual commercial methods for vaporizing such liquids would involve vacuum evaporation or evaporation in a suitable packed tower. These methods either required too large a volume for a given capacityor tended t o hold the thermally sensitive liquid a t a high temperature for too long a time. -4 simple and unique type of vaporizer was developed t o meet this need. This found application in the generation of screening smokes ( 5 ) and colored signal smokes. Many liquids were readily vaporized into a hot carrier gas with extremely short times of exposure t o the high temperatures. These liquids included high boiling h”ydrocarbons such as Diol and special smoke oils, molten paraffin wax, oleum, sulfur, tert-butyl stearate, glaurin (diethylene glvcol monolaurate), and solutions of D D T in oil ( 2 , 3, 6). I t is likely that industrial applications for this method will be found. PRINCIPLE OF OPERATION
The vaporizer takes advantage of the high rates of heat transfer t o small droplets in a turbulent hot gas stream and their accompanying rapid evaporation. The small droplets are produced by giving the hot gas stream a high velocity and then introducing the liquid at low pressure into this high velocity stream. The
* Present address, Monsanto Chemical * Present address, .Johns-Manville Ino.,
Company, Springfield, Iv1a.w. Manville, h-.J.
high velocity of the gas past the liquid atomizes the latter, and evaporation begins a t once. The rapid heating of the liquid and evaporation absorbs sensible and latent heat and quickly cools t h e hot gases. The gas velocities used have been in the subsonic range and thus avoid large pressuie losses. They have ranged from 500 to 1800 feet per second. I n the smoke generators, the gas-vapor mixture, after evaporation is complete, issues a t once into the atmosphere as a jet. This jet entrains eo01 air and is thus cooled belon its saturation temperature. A dense cloud results. With the high gps velocities and the subsequent rapid cooling the liquid is exposed to high temperatures for a time in the order of only a few thousandths of a second. This short exposure time allows the use of higher gas temperatures or more thermally sensitive liquids than would otherwise be possible. This simple method of atomizing the liquid avoids the need for pressure pumps. I n several applications the liquid is fed by the decrease in pressure associated with accelerating the gas to a high velocity. This amounts t o a few pounds per square inch pressure difference. il typical vaporizer arrangement is shown in Figure 1. The Venturi tube offers a convenient means for obtaining the high velocity. I t conserves energy by converting the kinetic energy of the high velocity gas t o flow work. This greatly reduces the upstream pressure required t o obtain the high velocity. At the same time the region about the throat is a t a lower pressure Rhich facilitates introducing the liquid.. Although the Venturi design is not essential t o the high velocity vaporizer principle, it has proved t o have marked advantages in certain applications. The curves in the lower part of Figure 1 show the variation of pressure along the Venturi vaporizer for a fixed rate of flow of hot gas. The vaporizer discharges t o atmospheric pressure, po. The lower curve is for operation without liquid injection. Here the inlet pressure is p l , and this pressure drops t o p L at the throat as the gas velocity increases. I n the diverging section the velocity decreases and the pressure rises t o pz, which is only slightly less than p,. -4lthough a relatively large pressure change was necessary t o accelerate the gas t o a high velocity, the net pressure drop across the Venturi represented by p~ - pl is small. The drop from p , t o po is across the exit nozzle. The upper curve shows the conditions during injection of liquid. The pressure change p i -pt’
