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Aerosol particle size determination using a photon correlation laser doppler anemometer
ePergamOD
PIT: 80021-8502(96)00338-2
J. "",,,oJ &L. Vol. 27. Suppl. I. pp. $531-SS32,1996 CoPyrillhl C 1996 Ebcvier Science LId Printed in Greal Britain. All rightl reserved 0021-8S02/96 S\S.OO + 0.00
AEROSOL PARTICLE SIZE DETERMINATION USING A PHOTON CORRELATION LASER DOPPLER ANEMOMETER P. JANI, A. NAGY, A. CZITROVSZKY Research Institute for Solid State Physics H-1525, Budapest P.O.Box 49, Hungary. KEYWORDS particle sizing, photon correlation, FFf techniques INTRODUCTION Several proposals are known for the simultaneous measurement of particle velocity and size [1,2]. These methods all make use of well conditioned continuos detector signals where accurate phase and amplitude measurement is possible. This is the case with large spherical particles (d> lOJ.1m) and forward scattering geometry. To overcome problems encountered in measuring single aerosol particle velocities and sizes (0.3-10J.1m) we propose photon correlation as the ultimate technique which takes the fullest advantage of the information contained in the photo-electron impulse train. From a recent study [3] it is known that for a wide range of refractive indices and particle sizes the visibility of back scattered light is a monotonous function of particle size. MEASUREMENT PRINCIPLE The information on the size of particles is contained in the visibility of back scattered light. On the other hand photo-electron impulse train does not easily lend itself for visibility measurement. What we are proposing here is to measure the ratio of the contents of two specific channels of the histogram containing the Fourier transform of the auto correlation function. It is possible to compute the expectation value of the ratio of the content of the first channel to the value of the first maximum on the Fourier transform of the auto correlation function corresponding to the transit of a single particle through the sensing volume of the LDA instrument. From our computations follows that this ratio is also a monotonous function of particle size. Such monotonous behaviour is expected approximately up to particle sizes equalling to the value of fringe distance . So it is possible to calibrate a given measurement set-up for different sizes. On Fig t. result of computation is shown for the dependence of the above mentioned ratio (R) on particle size (d), where the following measurement conditions were taken into account: particle velocity v=t4.8m/s, sampling time t=1O ns, correlation channel Nr=512, fringe separation in the sensing volume 8.5J.1m. 5531
5532
Abstracts of the 1996 European Aerosol Conference R
10 9 8 7 6 5 4 3
cu.)
1
2
3
4
5
Fig I. Fig. 2 shows the Fourier transform of the auto-correlation function corresponding to a single particle transit [4]. In this experiment d=2 J.lm polystyrene latex balls were used. It is seen that on the experimental curve the content of the first significant channel is somewhat over 7 e+6, the content of the channel at the first maximum is around 3.5 e+6. So their ratio is around 2.
FFT of a.c.func.
xroxo wxxm 5OOXXX)
4CXXXXX) 3CXXXXX)
200DXl
1~ ~~~~liiiiiQ;lil'iiRIRlilillllllll ........ CDU"lNO"IWMO
~(T")Ltlc..oCQc=lC'\I
Ch.N x1.66m/s ec
Fig. 2 According to Fig. 1. for particles of d=2 urn diameter this ratio should be around 2.5. This difference in results possibly can be accounted for the simplified computational model used . Beside this the proposed method seems to have solid practical significance in size determination in experiments where only photon correlation techniques can be applied.
REFERENCES I.W.D.Bachalo, M.J . Houser: Phase Doppler spray analyzer for simultaneous measurements of drop size and velocity distributions, Optical Engineering V23. N5.p583. 1984. 2.T. Bomer, Lidong Zhan: LDV signal analysis for particle velocity and size detection Optical Particle Sizing ed. G. Gouesbet, G. Grehan, Plenum Press, pI77. 3. Azar, M. and Ventrice, C.A. Size determination of aerosol particles using the LDAvisibility technique ... (1995) J. Aerosol Science. Vol. 26. No.6. pp. 1009-1017 4. P. Jani, A. Czitrovszky: Aerosol particle velocity and size measurement by photon correlation, J. Aerosol Sci. Vo1.26S. pp S799-S8oo.
PIT: 80021-8502(96)00338-2
J. "",,,oJ &L. Vol. 27. Suppl. I. pp. $531-SS32,1996 CoPyrillhl C 1996 Ebcvier Science LId Printed in Greal Britain. All rightl reserved 0021-8S02/96 S\S.OO + 0.00
AEROSOL PARTICLE SIZE DETERMINATION USING A PHOTON CORRELATION LASER DOPPLER ANEMOMETER P. JANI, A. NAGY, A. CZITROVSZKY Research Institute for Solid State Physics H-1525, Budapest P.O.Box 49, Hungary. KEYWORDS particle sizing, photon correlation, FFf techniques INTRODUCTION Several proposals are known for the simultaneous measurement of particle velocity and size [1,2]. These methods all make use of well conditioned continuos detector signals where accurate phase and amplitude measurement is possible. This is the case with large spherical particles (d> lOJ.1m) and forward scattering geometry. To overcome problems encountered in measuring single aerosol particle velocities and sizes (0.3-10J.1m) we propose photon correlation as the ultimate technique which takes the fullest advantage of the information contained in the photo-electron impulse train. From a recent study [3] it is known that for a wide range of refractive indices and particle sizes the visibility of back scattered light is a monotonous function of particle size. MEASUREMENT PRINCIPLE The information on the size of particles is contained in the visibility of back scattered light. On the other hand photo-electron impulse train does not easily lend itself for visibility measurement. What we are proposing here is to measure the ratio of the contents of two specific channels of the histogram containing the Fourier transform of the auto correlation function. It is possible to compute the expectation value of the ratio of the content of the first channel to the value of the first maximum on the Fourier transform of the auto correlation function corresponding to the transit of a single particle through the sensing volume of the LDA instrument. From our computations follows that this ratio is also a monotonous function of particle size. Such monotonous behaviour is expected approximately up to particle sizes equalling to the value of fringe distance . So it is possible to calibrate a given measurement set-up for different sizes. On Fig t. result of computation is shown for the dependence of the above mentioned ratio (R) on particle size (d), where the following measurement conditions were taken into account: particle velocity v=t4.8m/s, sampling time t=1O ns, correlation channel Nr=512, fringe separation in the sensing volume 8.5J.1m. 5531
5532
Abstracts of the 1996 European Aerosol Conference R
10 9 8 7 6 5 4 3
cu.)
1
2
3
4
5
Fig I. Fig. 2 shows the Fourier transform of the auto-correlation function corresponding to a single particle transit [4]. In this experiment d=2 J.lm polystyrene latex balls were used. It is seen that on the experimental curve the content of the first significant channel is somewhat over 7 e+6, the content of the channel at the first maximum is around 3.5 e+6. So their ratio is around 2.
FFT of a.c.func.
xroxo wxxm 5OOXXX)
4CXXXXX) 3CXXXXX)
200DXl
1~ ~~~~liiiiiQ;lil'iiRIRlilillllllll ........ CDU"lNO"IWMO
~(T")Ltlc..oCQc=lC'\I
Ch.N x1.66m/s ec
Fig. 2 According to Fig. 1. for particles of d=2 urn diameter this ratio should be around 2.5. This difference in results possibly can be accounted for the simplified computational model used . Beside this the proposed method seems to have solid practical significance in size determination in experiments where only photon correlation techniques can be applied.
REFERENCES I.W.D.Bachalo, M.J . Houser: Phase Doppler spray analyzer for simultaneous measurements of drop size and velocity distributions, Optical Engineering V23. N5.p583. 1984. 2.T. Bomer, Lidong Zhan: LDV signal analysis for particle velocity and size detection Optical Particle Sizing ed. G. Gouesbet, G. Grehan, Plenum Press, pI77. 3. Azar, M. and Ventrice, C.A. Size determination of aerosol particles using the LDAvisibility technique ... (1995) J. Aerosol Science. Vol. 26. No.6. pp. 1009-1017 4. P. Jani, A. Czitrovszky: Aerosol particle velocity and size measurement by photon correlation, J. Aerosol Sci. Vo1.26S. pp S799-S8oo.