Peculiarities of aerosol remote sampling from the gas lines. the problem of sampling points selection

~, Aeromo~ sol., Vol. 21. Suppl. 1, pp. $641-$643, 1 9 9 0 . Printed in Great Britain.

0021-8502/90 $3.00 + 0.00 Pergamon Press plc

PE~LIARZTIES OF AEROSOL R ~ D T E SARPLIN0 FR0~ THE GAS LINES. THE PROBL~4 OF SAMPLING POINTS SEL~CTION G.N. Lipatov, S.A. Grlnshpun & T.I. Semen~ak O~esza State University, Phys. Dep., 2~ Petra Velikogo St., 270000 Odessa (USSR)

Reliability of measurements of the average and loo-1 ooncentratlonz of &ePosol particles in a gem-line depends on firs% determining a OOZTOO% sampling point (or points) across the gas-line and then providing a represents%iwe sample with due account of the aspirational errors. Significance of the firs% factor depends on inhomogeneity of the flow pattern and particle c o n oeatration across the 6~s-line. Correction of sampling errors is based upon the flow oondltions at the sampling point i.e. is determined 1~ the first factor. At the same time to solve the problem of adequate account of the anplrational errors for a polydlsperse aerosol one needs information on the particle size distri~ation (~R~) together with the motion characteristics of the gas flow and particles. The above dlztri1~tion, however, is the object of measurements, which is paradoxical so far as the be found result is the b a s e of measurement or&~nisation. The c o n c e p t o f P o l y d i s p e r s e a e r o s o l t o t a l f l o w mass c o n c e n t r a t i o n ( C ~ ) i s commonly u s e d a s a g e n e r a l s y s t e m p a r a m e t e r f o r t h e e v a l u a t i o n o f a e r o s o l e f f l u e n t e f f e c t on t h e p r o d u c t i o n p r o c e s s o r e n v i r o n m e n t . The f ~ a c t i o n a l c o n centration ~ C m ~ R ~ R ÷ ~ L ~ is known to be the ratio of the mass flow rate of a given fraction particles I;o the respective flow of the carrier-gaze

Herr R is the particle radius, V~(R) and ~ are the particle and gas velocities normal to the zurface ~t% t ~1%m is the mass of the given fraction particles p e r u n i t volume~ ~ is the particles' aenstty, ~ is the total number o f p a r t i c l e s o f a l l f r a c t i o n s p e r u n i t v o l u m e . The f l o w mass conoen%r a t i o n f o r t h e whole s i z e - s p e c t r u m o f a e r o s o l p a r t i c l e s i s as f o l l o w s s Cm

=

4~?no ~u~

. ~,(,~").V~.(,~)a~ o

(2)

When aerosol moves through the E~-line the ~o and ~ parameters var~ across the oh-n-el due to a variety of physical effects (the particles' inertia, migration, sedimentation, ooa&,~lation~ interaction with the walls, etc. ). The particles' and gas velocity distributions modify in much the same wa~r. Suppose ~(~;~;~) is the radius-vector of a point within the channel, then spatial variability of the above quantities is reflected in thzir relationship to the coordinates on the oh~wel total crosS--section (8), i.e. ~quation (2) transforms into =

. _(s)

o.

'

$641

'

(~)

$642

G.N.

LIPATOV

et al.

Hence f o r calculation o f t h i s concentration the &iatril~Itiono of the paramet e r s ~(~) , no , V~(~ , and ~ ao~no the channel have to be Mao~.

However, nowadaors such distrilxLtioms are avaAlable but f o r oez~tain elememtary ohenael geometries and narrow range o f Reynolds nwRbers. ~nm, u one-point sampling procedure is, in general, inadequatet while integration of the m~Itipoint sampling results has to be aooompaaled b~ evaluation (at least, rough) of relationships in Ikluatioa (3)- The 8ise diftrlb~tlon ~(~) , for instance, is commonly presented by the lognormal di~tri~tloa with oorreetlve term~(~) , which i n l i m i t e ~ by e i ~ i c i e a o y o f m i g r a t i o n lXrooenas. ~3~zLos of the oros~vine particle migration also p e r a i t s the ratio VA(R,~)/~(~ ) being expressed i n terns o f ~ e Stokes n ~ b e r . The uA(~) speotrum f o r one-dimensional problem is u s u a l l y calculated t~ the f o l l o w i n g e q ~ i o n

~.(~ = K C ~ , R , ) ~ ~/"(~0

(4)

where ~(~.Re) aa~ ~ ! { ~ are the f a c t o r s aoootmting geometr~r of the ohmmel oross-seotton and degree o f the f l o v turlxaleaoo; ~ i s the d~mensionless OX~OOS O O O l ~ n t

n •

Deserves c o n s i d e r a t i o n a s t r o n g dependence o f the t o t a l mass ooncentr&tion on the percentage of' the coarse f ~ a ~ t i o n : ~C~ ~ ~ . ~ ) . F i ~ e 1 s~o~s paz~ t i o l e s i s e d i f f e r e n t i a l d i s t r i k t i o n s f o r the oowater and ~ s oonoemtrations o f coal ~nst p a r t i c l e s . The : e d i a n - e ~ e n aerod~amaioal radium (see spe~trma 1) e v i d e n t l y exceeds the raedias-oounter r a d i u s ( s p e o t r t ~ 2). T~ o t h e r woz~ls, the coarse i ~ a o t i o n s ma~ o o n t r i b ~ t e to C~ r a t h e r more than the f i n e ones even t h o u ~ "the l a t t e r have higher counter c o n c e n t r a t i o n . This fa~t i s o f p a r t i c u l a r importance f o r correction of sampling errors. So, the fractional coefficient of smisokiaetical aspiration being the ratio of oorreslxmding counter noncent ration8

A£(measured)

~(tr~e )

(5)

may he equal to unity for all fractions from the main re~ion of ~(~) measurement, excluding the "right tail" of the sise spectrum (the a~pirational errors usually manifest themselves but for particles larger than 5 - ~0 ~m). At the same time just this "tail" of particle size distribution, which represents existence of the scanty coarse fraction, may substantially distort the summary (total) mass concentration. To measure the above distortion for polydisperse aerosol the integral aspiration coefficient has been suggested previously:

~ &Cm(measured)°~

o

(6)

The aspirational errors arising upon sampling of a monodisperse aerosol have been measured in the wind tunnel with a contoured nozzle, which ~ives the plug flow, and in the industrial gas-line with channel of I.~6 m diameter. The experiments corroborated that even small percentage of the coarse fractions must be taken into account in determination of the total mass concentration of polyctisperse aerosol. They also revealed conditions per~aitting the substitution of one-point axial sampling (sampler is on the axis of the channel) for the multi-point procedure. Non-axial one-point sampling in case of its aniookinetioity rules out correction of sampling errors by the common relationships because of symmetry violation in the particle stream flowing within a sink field. Conditions even so requiring such corrections w~re d.e~ermined for the foll~wing values of characteristic parameters: ~ = ~O~kg-m ~, tArA - ~ -- ~0 m.s -, the thin-walled nozzle diameter ~ = 4 - 15 mm, %he mean aspiration velocity
Remote

sampling

from gas lines

$643

of a sample exceeds 10% (R% i s the counter median r a d i u s and ~t i s the s t a ndard ~eometrio d e v i a t i o n from the mean p a r t i c l e r a d i u s f o r losnormal d i s t r i bution). Taking into account features of the multi-point sampling the separate experiments have been conducted for investigation of aerosol aspiration in the vicinity of channel wall, and for exploration of the adjacent samplers mutual influence. The fractional aspiration coefficients were dete~mine~ cemparlson method. Fi6~res 3 and 4 show the shaded regions where influence of the wall (Fig. 3) or the second sampler (Fig. 4) is appreciable (the discrepancy between measured and ideal aspirations/ coefficients exceeds 10%). Here • ~ is the sampling nozzle axis distance from the channel wall, 6z is the spacing between the axes of the adjacent thin-walled sampling no=sles, K is th~ anisokinetical factor. The flow conditions are within the limits Re . I ~ 10~, St - 0.1 - 10.

2 -

1

100

4000

40

2000

0

lO

20

30

40

50

60

7o

80

PARTICLE RADIUS, ~um Fig. I .

lo0 1o i

1

0.1

2

~%- value

3

Fig. 2.

6

5 4

~3 2 10 K-value

Fig. 3.

100

10

K-value

Pig. 4.

100