- Email: [email protected]
Evaluation of homogeneous droplet formation inside UCPC (TSI model 3025)
Pergamon
J. Aerosol Sci., Vol. 26, No. 6, pp. 1003-1008, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved OOZI-8502/95 $9.50 + 0.00
OOZl-8502(95)00036-4
TECHNICAL NOTE EVALUATION OF HOMOGENEOUS DROPLET FORMATION INSIDE UCPC (TSI MODEL 3025) Kaarle Hlmeri,* Jarkko Augustin, Markku Kulmala, Timo Vesala, Jyrki Makehi, Pasi Aalto and Evgenii Krissinel’+ Department of Physics, University of Helsinki, P.O. Box 9, FIN-00014 fkussian Academy of Sciences, Institute for Water and Environmental Barnaul 656099, Russia (First received 26 January 1995; and
University of Helsinki, Finland Problems, Papanintsev 105,
in final form 7 April 1995)
Abstract-The droplet formation characteristics inside an Ultrafine Condensation Particle Counter (UCPC, TSI model 3025) were tested changing working temperatures of the counter as well as flow rates and carrier gases. The onset of homogeneous nucleation was determined as a function of the temperature of the saturator and condenser. Preliminary tests were carried out using the UCPC for measurements of homogeneous nucleation rates of n-butanol vapour. The observed nucleation rates were compared with those predicted by classical nucleation theory and by self-consistent correction to the classical theory. Homogeneous droplet formation at nearly standard operation conditions of the UCPC were observed for carrier gases such as air and nitrogen. Moreover, in helium particle formation was observed already at standard operation conditions of the UCPC.
1.
INTRODUCTION
The Ultrafine Condensation Particle Counter (UCPC, TSI model 3025) is a continuous flow counter with sequential saturator and condenser (e.g. Stolzenburg, 1988; Stolzenburg and McMurry, 1991). The sample aerosol flow is divided into two parts. The filtrated sheath flow (90%) is drawn through the saturator, where it becomes nearly saturated with n-butanol at saturator temperature. The remaining flow (10%) is injected along the center of the flow just upstream of the condenser. A short vertical saturator extension allows time for the alcohol vapour to diffuse into the core flow before entering the condenser. The laminar flow is then led into the thermoelectrically cooled vertical condenser where the heat transfer to the walls causes supersaturation of the vapour. In the normal operation conditions the supersaturation maximum is high enough to activate the particles travelling with the flow. However, care is taken to avoid reaching the critical supersaturation of homogeneous nucleation to occur. _, Since the UCPC is operated not only in standard operation conditions but quite often in somehow changed conditions (e.g. Niida et al., 1988) it gives motivation to investigate the operation of the UCPC in modified conditions. In the present study we have investigated the operation conditions where the critical supersaturation of homogeneous nucleation of n-butanol vapour is reached. The parameters varied in our measurements were carrier gas, flow rate, and temperature of the saturator and condenser, The parameter values were selected to contain the default values of the flow rate (0.3 lpm) and condenser temperature (1OC) as well as the most common carrier gas (air). 2.
EXPERIMENTS
The investigations were performed for three carrier gases (bottled air, nitrogen and helium, purity 99.999%) using three flow rates at the inlet of UCPC (0.3,0.2 and 0.15 lpm).
*Corresponding
author. 1003
1004
K. Hlmeri et al
The carrier gas flow was carefully cleaned from particulate and gaseous impurities with an molecular sieve adsorbent and an absolute filter before entering the UCPC. The flow rate was calibrated for each of the carrier gases using a bubble flow meter. The condenser temperature was kept constant (4.0, 7.0, 10.0 or 13.O”C) and the particle concentration was measured changing the temperature of the saturator. The temperature of the optics was kept 3.O”C higher than the saturator temperature. At a sufficiently low saturator temperature in the beginning of the experiments no particles were detected. By increasing the saturator temperature particle formation started. At each experimental point the saturator temperature was allowed to stabilize for roughly 15 min. The temperature stability was controlled by monitoring the temperatures and waiting for constant particle concentrations. The vapour and temperature profiles inside the chamber were determined solving the Graetz problem for both heat and mass transfer (see e.g. Jakob, 1949; Bird et al., 1960). The profiles were evaluated using two independent computer programs: The code ACTEFF developed by Stoltzenburg (1988) and a code TUBE developed by our group. The profiles were in principle similar. A slight difference occurs since the temperature dependence of physico-chemical parameters of the carrier gas and the working fluid is taken into account in our program. The code ACTEFF assumes constant values. However, the nucleation rate is such a strong function of temperature and saturation ratio that the differences between the codes are mostly negligible. From the temperature and saturation ratio profiles the maximum experimental nucleation rate as a function of temperature and saturation ratio was determined using the method discussed by Wagner and Anisimov (1993) and Anisimov et al. (1994). Experimental nucleation rates at certain saturation ratios and temperatures were compared using: (a) the classical nucleation theory developed by Volmer and Weber (1926), Farkas (1927), Volmer (1929, 1939), and Becker and Doring (1935) and (b) self-consistent correction (SCC) to the classical theory (Girshick and Chiu, 1990; Girshick et al., 1990; Girschik, 199 1). The thermodynamic data of n-butanol, air, nitrogen and helium was taken from Landolt-Bornstein (1960), Perry (1984), Reid et al. (1987) Schmeling and Strey (1983), Strey and Schmeling (1983) and Timmermans (1965).
3. RESULTS
AND
DISCUSSION
Table 1 lists the temperatures of saturator and condenser at the conditions where the UCPC reads a concentration of 10 and lo3 cmm3, respectively, for different flow rates and carrier gases. At the UCPC detected concentration of 10 cme3 (Table 1) the corresponding nucleation rate is on the order of 104-lo5 cm-3s-1 and at the UCPC concentration of lo3 cmm3 (Table 1) the corresponding nucleation rate is on the order of 106-10’ cm-3s-1. The normal operation conditions (T, = 10°C and T, = 38°C) are seen to be close to those where particle formation is detected and in the case of helium significant concentrations are found already earlier. Due to fluctuations in the measurements the error in temperature was estimated to be at maximum + 0.5”C. The temperatures of condenser and saturator at which droplet formation starts to become significant (UCPC concentration above approximately 10-l cmd3) are plotted for different carrier gases in Fig. 1. The area below the lines in Fig. 1 represents “safe” operation conditions. The flow rate was 0.3 lpm. It can be seen that while the lines for air and nitrogen are close to each other the one for helium is clearly different. It is important to notice that when operating the UCPC with helium as a carrier gas the normal operation temperatures are above the “safe” line. This shows that the homogeneous nucleation will cause problems if care is not taken. We point out here that the temperatures of condenser ( 10°C) and saturator (41°C) in the investigations by Stolzenburg and McMurry (1991) were above the limits where homogeneous nucleation was found to occur. However, Stolzenburg and McMurry (1991) report that they observed homogeneous nucleation when the saturator temperature was 42-44°C which can be considered rather close to our results,
Evaluation
of homogeneous
droplet
formation
inside UCPC
1005
Table 1. The critical temperatures of condenser and saturator corresponding UCPC concentrations of 10 and lo3 cmm3 for different carrier gases and flow rates Carrier
gas
Air
Nitrogen
*Data
Flow rate (lpm)
T, (“C)
T, (“C) (C = 10 cm-s)
T, (“C) (C = lo3 cm-“)
0.3
4.0 7.0 10.0 13.0
35.0 39.8 42.1 46.2
37.2 41.7 44.3 47.9
0.2
4.0 7.0 10.0 13.0
* 41.4 45.1 48.4
37.5 43.5 46.9 50.1
0.15
4.0 7.0 10.0 13.0
38.8 42.5 46.0 50.1
40.8 44.5 47.9 51.7
0.3
4.0 7.0 10.0 13.0
38.5 42.0 41.7 46.3
40.2 43.7 43.8 48.0
0.2
4.0 7.0 10.0 13.0
35.6 38.9 41.8 45.9
37.4 40.4 43.6 47.6
0.15
4.0 7.0 10.0 13.0
39.1 42.9 46.0 50.0
40.5 44.3 47.7 51.7
0.3
4.0 7.0 10.0 13.0
29.1 33.0 35.9 38.8
30.2 34.0 37.2 40.0
0.2
4.0 7.0 10.0 13.0
26.0 32.8 36.5 39.5
27.3 34.0 38.0 40.8
0.15
4.0 7.0 10.0 13.0
27.9 34.5 37.8 37.2
29.5 35.6 38.9 38.5
missing.
since the counters as sensitive instruments may have some individual characteristics in their operation. The concentration of particles was read directly from the UCPC taking into account the effect of flow rate. However, more reliable data could have been obtained by calculating the concentration from the pulse output of the UCPC. The pulse height distribution in our experiments could be compared to that in normal operation (e.g. detection of atmospheric aerosols) in order to make sure that all the particles are really detected. It is assumed that due to the increased saturation ratio in our experiments, the resulting particles are large enough to be detected and no significant error is made. An example of an experimental nucleation rate as a function of saturation ratio is compared with different theories in Fig. 2. The condenser temperature was 10°C the flow rate was 0.3 lpm, the carrier gas was air. The nucleation rate was obtained using the method discussed by Wagner and Anisimov (1993) and Anisimov et al. (1994). The trend of experimental nucleation rate as a function of saturation ratio is rather similar to both theories. Even though no correct nucleation theory is known to exist, this is a typical indication of homogeneous nucleation (see also e.g. Viisanen and Strey, 1994). However, the change of slope in the lowest nucleation rate region indicates that some heterogeneous
1006
et al.
K. Hameri
45
1
40
._~.
I E,,
[..
g
_.
I-’
,”
_,,;,,/ /,,.‘.
.I----.“.
.,;;
,_-
30
I
_J
/’
,’
I’
/’
/’
/,’
/’
I’
/_,,_*,’
Fig. 1. The temperature of saturator (T) as a function of the temperature of condenser (Tc) at which the droplet formation was found to begin. The lines for carrier gases air, nitrogen and helium are plotted using the inlet flow rate of the UCPC 0.3 lpm. The area below the line can be considered as the “safe” operation area.
lo8 ?? ----
I,,,. air, 0.3 lpm Classical theory
/
XC-theory !
10:
??
/ I
’
!’ -7 10” cc
7
E 2.L w 10”
.
. lo4
/ i’ _ -L
lo3 3
??
. 4
s
5
6
5
Fig. 2. An example of experimental nucleation rate as a function of saturation ratio is compared with different theories. Here the condenser temperature of 10°C and flow rate of 0.3 Ipm were used for air as carrier gas. The nucleation temperature varied from 287.78 to 288.32 K. The theoretical curves were calculated at 288.0 K. The calculations were done using the computer code TUBE.
processes may be occurring as well. It is also possible that the real operation of UCPC differs somewhat from the ideal one (e.g. the temperatures may not be constant and the flow profile may not be laminar). It is seen that the measured values for the nucleation rate differ from those predicted by the classical theory by ca. one order of magnitude and more when compared to KC-theory. This kind of difference is quite typical in measurements of homogeneous nucleation. The experimental nucleation rates shown in Fig. 2 were obtained using the code TUBE. The use of code ACTEFF was compared to give an order of magnitude higher values compared to the code TUBE. The homogeneous nucleation rates of n-butanol vapour were measured by Viisanen and Strey (1994) using an expansion chamber. However, the temperature range in their measurements was different and an exact comparison is thus not possible. Also they used noble carrier gases such argon, helium and xenon, while our nucleation rate was evaluated in air.
Evaluation
of homogeneous
droplet
formation
inside UCPC
1007
Qualitative comparison gives, however, some reason to question the conclusion of Viisanen and Strey (1994) that classical nucleation theory consistently underestimates nucleation rates by a maximum of about two orders of magnitude since in our case the trend is opposite. One possible reason could be that the flow after the saturator in our experiments may not have been saturated. According to model calculations performed we remark that the data points in Fig. 2 will be shifted left if the conditions are undersaturated. The nucleation rate itself varies within an order of magnitude when the initial saturation ratio is changed by 50%. This kind of change in saturation ratio is high enough to explain the differences with theories or other experiments. It was also observed that the temperature of the condenser fluctuated during measurements by f O.l‘C. This could lead to profiles of temperature on the condenser wall and consequently an error in the evaluation of the nucleation rate. The experimental data points in Fig. 2 show that the change of 1°C in temperature generates a change of ca. one order of magnitude in nucleation rate. The inaccuracy in the temperature in our measurements could lead to noticeable error in experimental nucleation rate. We note finally that nucleation rate is a very steep function of saturation ratio and temperature and therefore our results are still in reasonable agreement with those by Viisanen and Strey. 4.
CONCLUSIONS
It was shown that special care has to be taken when operating the UCPC with different carrier gases. Homogeneous droplet formation at nearly standard operation conditions of UCPC was observed for carrier gases such as air and nitrogen. Furthermore, it was found that using helium as a carrier gas nucleation occurs already below the normal operation conditions. The homogeneous nucleation rates in the range of 103-lo* s-r cme3 were observed. This corresponds to UCPC concentrations of lo-‘-lo5 cmm3. The comparison between inlet flow rates was carried out. The lowest critical values for homogeneous nucleation to occur are found for flow rate 0.3 lpm. Tools for determining the saturation ratio and temperature in the nucleation volume were developed. Since the choice of the carrier gas seems to have an effect on the droplet formation, the critical conditions for homogeneous nucleation to occur are not possible to determine directly from the calculations but need also experimental investigations. Due to several uncertainties in the operation of the UCPC it is concluded that it is difficult to use as a chamber for nucleation studies. REFERENCES Anisimov, M. P., Hlmeri, K. and Kulmala, M. (1994) Construction and test of laminar flow diffusion chamber: homogeneous nucleation of DBP and n-hexanol. J. Aerosol Sci. 25, 23-32. Becker, R. and Doring, W. (1935) Kinetische Behandlung der Keimbildung in iibersiittigten Dampfen. Ann. Phys. (Leipzig) (5) 26, 719. Bird, R. B., Stewart, W. E. and Lightfoot, E. N. (1960) Transport Phenomena. Wiley, New York. Farkas, L. (1927) Keimbildungsgeschwindigkeit in iiberslttigten Dampfen. 2. Phys. Chem. 125, 236. Girshick, S. (1991) Comment on: “Self-consistency correction to homogeneous nucleation theory”. J. them. Phys. 94, 826. Girshick, S. and Chiu, C.-P. (1990) Kinetic nucleation theory: A new expression for the rate of homogeneous nucleation from an ideal supersaturated vapor. J. them. Phys. 93, 1273. Girshick, S., Chiu, C.-P. and McMurry, P. H. (1990) Time-dependent aerosol models and homogeneous nucleation rates. Aerosol Sci. Technol. 13, 465. Jakob, M. (1949) Heat Transfer. Wiley, New York. Landolt-Bornstein (1960) Zahlenwerte und Functionen aus Physik-Chemie-Astronomie-Geophysik-Technik. Springer, Berlin. Niida, T., Wen, H. Y., Udischas, R. and Kasper, G. (1988) Counting efficiency of condensation nucleus counters in N,, Ar, CO, and He. J. Aerosol Sci. 19, 1417. Perry, R. H. and Green, D. (1984) Perry’s Chemical Engineers’ Handbook, 6th Edn. McGraw-Hill, New York. Reid, R., Prausnitz, J. and Poling, B. (1987) The Properties of Gases nnd Liquids, 4th Edn. McGraw-Hill, New York. Schmeling, T. and Strey, R. (1983) Equilibrium vapor pressure measurements for the n-alcohols in the temperature range from - 30°C to + 30°C. Ser. Bunsenges. Phys. Chem. 87, 871. Stolzenburg, M. R. (1988) An ultrafine aerosol size distribution measuring system. Ph.D. thesis, Department of Mechanical Engineering, University of Minnesota, Minneapolis, U.S.A. Stolzenburg, M. R. and McMurry, P. H. (1991) An ultrafine aerosol condensation nucleus counter. Aerosol Sci. Technol. 14, 4845.
1008
K. Hameri
et al.
Strey, R. and Schmeling, T. (1983) Surface tension measurements for the n-alcohols in the temperature range from - 40°C to + 40°C. Ber. Bunsenges. Phys. Chem. 87, 324. Timmermans, J. (1965) Physico-Chemical Constants of Pure Organic Compounds. Vol. II. Elsevier, New York. Viisanen, Y. and Strey, R. (1994) Homogeneous nucleation rates for n-butanol. J. Chem. Phys. 101, 9. Volmer, M. (1939) @ret& der Phasenbildung. Verlag Theodor SteinkoptT, Dresden und Leipzig. Volmer, M. (1992) Uber Keimbildung und Keimwirkung als Spezialfalle der heterogenen Katalyse. 2. Elektrochem. 35, 555. Volmer, M. and Weber, A. (1926) Keimbildung in iiberslttigten Gebilden. Z. Phys. Chem. 119, 277. Wagner, P. E. and Anisimov, M. P. (1993) Evaluation of nucleation rates from gas flow diffusion chamber experiments. J. Aerosol Sci. 24, 1033104.
J. Aerosol Sci., Vol. 26, No. 6, pp. 1003-1008, 1995 Copyright 0 1995 Elsevier Science Ltd Printed in Great Britain. All rights reserved OOZI-8502/95 $9.50 + 0.00
OOZl-8502(95)00036-4
TECHNICAL NOTE EVALUATION OF HOMOGENEOUS DROPLET FORMATION INSIDE UCPC (TSI MODEL 3025) Kaarle Hlmeri,* Jarkko Augustin, Markku Kulmala, Timo Vesala, Jyrki Makehi, Pasi Aalto and Evgenii Krissinel’+ Department of Physics, University of Helsinki, P.O. Box 9, FIN-00014 fkussian Academy of Sciences, Institute for Water and Environmental Barnaul 656099, Russia (First received 26 January 1995; and
University of Helsinki, Finland Problems, Papanintsev 105,
in final form 7 April 1995)
Abstract-The droplet formation characteristics inside an Ultrafine Condensation Particle Counter (UCPC, TSI model 3025) were tested changing working temperatures of the counter as well as flow rates and carrier gases. The onset of homogeneous nucleation was determined as a function of the temperature of the saturator and condenser. Preliminary tests were carried out using the UCPC for measurements of homogeneous nucleation rates of n-butanol vapour. The observed nucleation rates were compared with those predicted by classical nucleation theory and by self-consistent correction to the classical theory. Homogeneous droplet formation at nearly standard operation conditions of the UCPC were observed for carrier gases such as air and nitrogen. Moreover, in helium particle formation was observed already at standard operation conditions of the UCPC.
1.
INTRODUCTION
The Ultrafine Condensation Particle Counter (UCPC, TSI model 3025) is a continuous flow counter with sequential saturator and condenser (e.g. Stolzenburg, 1988; Stolzenburg and McMurry, 1991). The sample aerosol flow is divided into two parts. The filtrated sheath flow (90%) is drawn through the saturator, where it becomes nearly saturated with n-butanol at saturator temperature. The remaining flow (10%) is injected along the center of the flow just upstream of the condenser. A short vertical saturator extension allows time for the alcohol vapour to diffuse into the core flow before entering the condenser. The laminar flow is then led into the thermoelectrically cooled vertical condenser where the heat transfer to the walls causes supersaturation of the vapour. In the normal operation conditions the supersaturation maximum is high enough to activate the particles travelling with the flow. However, care is taken to avoid reaching the critical supersaturation of homogeneous nucleation to occur. _, Since the UCPC is operated not only in standard operation conditions but quite often in somehow changed conditions (e.g. Niida et al., 1988) it gives motivation to investigate the operation of the UCPC in modified conditions. In the present study we have investigated the operation conditions where the critical supersaturation of homogeneous nucleation of n-butanol vapour is reached. The parameters varied in our measurements were carrier gas, flow rate, and temperature of the saturator and condenser, The parameter values were selected to contain the default values of the flow rate (0.3 lpm) and condenser temperature (1OC) as well as the most common carrier gas (air). 2.
EXPERIMENTS
The investigations were performed for three carrier gases (bottled air, nitrogen and helium, purity 99.999%) using three flow rates at the inlet of UCPC (0.3,0.2 and 0.15 lpm).
*Corresponding
author. 1003
1004
K. Hlmeri et al
The carrier gas flow was carefully cleaned from particulate and gaseous impurities with an molecular sieve adsorbent and an absolute filter before entering the UCPC. The flow rate was calibrated for each of the carrier gases using a bubble flow meter. The condenser temperature was kept constant (4.0, 7.0, 10.0 or 13.O”C) and the particle concentration was measured changing the temperature of the saturator. The temperature of the optics was kept 3.O”C higher than the saturator temperature. At a sufficiently low saturator temperature in the beginning of the experiments no particles were detected. By increasing the saturator temperature particle formation started. At each experimental point the saturator temperature was allowed to stabilize for roughly 15 min. The temperature stability was controlled by monitoring the temperatures and waiting for constant particle concentrations. The vapour and temperature profiles inside the chamber were determined solving the Graetz problem for both heat and mass transfer (see e.g. Jakob, 1949; Bird et al., 1960). The profiles were evaluated using two independent computer programs: The code ACTEFF developed by Stoltzenburg (1988) and a code TUBE developed by our group. The profiles were in principle similar. A slight difference occurs since the temperature dependence of physico-chemical parameters of the carrier gas and the working fluid is taken into account in our program. The code ACTEFF assumes constant values. However, the nucleation rate is such a strong function of temperature and saturation ratio that the differences between the codes are mostly negligible. From the temperature and saturation ratio profiles the maximum experimental nucleation rate as a function of temperature and saturation ratio was determined using the method discussed by Wagner and Anisimov (1993) and Anisimov et al. (1994). Experimental nucleation rates at certain saturation ratios and temperatures were compared using: (a) the classical nucleation theory developed by Volmer and Weber (1926), Farkas (1927), Volmer (1929, 1939), and Becker and Doring (1935) and (b) self-consistent correction (SCC) to the classical theory (Girshick and Chiu, 1990; Girshick et al., 1990; Girschik, 199 1). The thermodynamic data of n-butanol, air, nitrogen and helium was taken from Landolt-Bornstein (1960), Perry (1984), Reid et al. (1987) Schmeling and Strey (1983), Strey and Schmeling (1983) and Timmermans (1965).
3. RESULTS
AND
DISCUSSION
Table 1 lists the temperatures of saturator and condenser at the conditions where the UCPC reads a concentration of 10 and lo3 cmm3, respectively, for different flow rates and carrier gases. At the UCPC detected concentration of 10 cme3 (Table 1) the corresponding nucleation rate is on the order of 104-lo5 cm-3s-1 and at the UCPC concentration of lo3 cmm3 (Table 1) the corresponding nucleation rate is on the order of 106-10’ cm-3s-1. The normal operation conditions (T, = 10°C and T, = 38°C) are seen to be close to those where particle formation is detected and in the case of helium significant concentrations are found already earlier. Due to fluctuations in the measurements the error in temperature was estimated to be at maximum + 0.5”C. The temperatures of condenser and saturator at which droplet formation starts to become significant (UCPC concentration above approximately 10-l cmd3) are plotted for different carrier gases in Fig. 1. The area below the lines in Fig. 1 represents “safe” operation conditions. The flow rate was 0.3 lpm. It can be seen that while the lines for air and nitrogen are close to each other the one for helium is clearly different. It is important to notice that when operating the UCPC with helium as a carrier gas the normal operation temperatures are above the “safe” line. This shows that the homogeneous nucleation will cause problems if care is not taken. We point out here that the temperatures of condenser ( 10°C) and saturator (41°C) in the investigations by Stolzenburg and McMurry (1991) were above the limits where homogeneous nucleation was found to occur. However, Stolzenburg and McMurry (1991) report that they observed homogeneous nucleation when the saturator temperature was 42-44°C which can be considered rather close to our results,
Evaluation
of homogeneous
droplet
formation
inside UCPC
1005
Table 1. The critical temperatures of condenser and saturator corresponding UCPC concentrations of 10 and lo3 cmm3 for different carrier gases and flow rates Carrier
gas
Air
Nitrogen
*Data
Flow rate (lpm)
T, (“C)
T, (“C) (C = 10 cm-s)
T, (“C) (C = lo3 cm-“)
0.3
4.0 7.0 10.0 13.0
35.0 39.8 42.1 46.2
37.2 41.7 44.3 47.9
0.2
4.0 7.0 10.0 13.0
* 41.4 45.1 48.4
37.5 43.5 46.9 50.1
0.15
4.0 7.0 10.0 13.0
38.8 42.5 46.0 50.1
40.8 44.5 47.9 51.7
0.3
4.0 7.0 10.0 13.0
38.5 42.0 41.7 46.3
40.2 43.7 43.8 48.0
0.2
4.0 7.0 10.0 13.0
35.6 38.9 41.8 45.9
37.4 40.4 43.6 47.6
0.15
4.0 7.0 10.0 13.0
39.1 42.9 46.0 50.0
40.5 44.3 47.7 51.7
0.3
4.0 7.0 10.0 13.0
29.1 33.0 35.9 38.8
30.2 34.0 37.2 40.0
0.2
4.0 7.0 10.0 13.0
26.0 32.8 36.5 39.5
27.3 34.0 38.0 40.8
0.15
4.0 7.0 10.0 13.0
27.9 34.5 37.8 37.2
29.5 35.6 38.9 38.5
missing.
since the counters as sensitive instruments may have some individual characteristics in their operation. The concentration of particles was read directly from the UCPC taking into account the effect of flow rate. However, more reliable data could have been obtained by calculating the concentration from the pulse output of the UCPC. The pulse height distribution in our experiments could be compared to that in normal operation (e.g. detection of atmospheric aerosols) in order to make sure that all the particles are really detected. It is assumed that due to the increased saturation ratio in our experiments, the resulting particles are large enough to be detected and no significant error is made. An example of an experimental nucleation rate as a function of saturation ratio is compared with different theories in Fig. 2. The condenser temperature was 10°C the flow rate was 0.3 lpm, the carrier gas was air. The nucleation rate was obtained using the method discussed by Wagner and Anisimov (1993) and Anisimov et al. (1994). The trend of experimental nucleation rate as a function of saturation ratio is rather similar to both theories. Even though no correct nucleation theory is known to exist, this is a typical indication of homogeneous nucleation (see also e.g. Viisanen and Strey, 1994). However, the change of slope in the lowest nucleation rate region indicates that some heterogeneous
1006
et al.
K. Hameri
45
1
40
._~.
I E,,
[..
g
_.
I-’
,”
_,,;,,/ /,,.‘.
.I----.“.
.,;;
,_-
30
I
_J
/’
,’
I’
/’
/’
/,’
/’
I’
/_,,_*,’
Fig. 1. The temperature of saturator (T) as a function of the temperature of condenser (Tc) at which the droplet formation was found to begin. The lines for carrier gases air, nitrogen and helium are plotted using the inlet flow rate of the UCPC 0.3 lpm. The area below the line can be considered as the “safe” operation area.
lo8 ?? ----
I,,,. air, 0.3 lpm Classical theory
/
XC-theory !
10:
??
/ I
’
!’ -7 10” cc
7
E 2.L w 10”
.
. lo4
/ i’ _ -L
lo3 3
??
. 4
s
5
6
5
Fig. 2. An example of experimental nucleation rate as a function of saturation ratio is compared with different theories. Here the condenser temperature of 10°C and flow rate of 0.3 Ipm were used for air as carrier gas. The nucleation temperature varied from 287.78 to 288.32 K. The theoretical curves were calculated at 288.0 K. The calculations were done using the computer code TUBE.
processes may be occurring as well. It is also possible that the real operation of UCPC differs somewhat from the ideal one (e.g. the temperatures may not be constant and the flow profile may not be laminar). It is seen that the measured values for the nucleation rate differ from those predicted by the classical theory by ca. one order of magnitude and more when compared to KC-theory. This kind of difference is quite typical in measurements of homogeneous nucleation. The experimental nucleation rates shown in Fig. 2 were obtained using the code TUBE. The use of code ACTEFF was compared to give an order of magnitude higher values compared to the code TUBE. The homogeneous nucleation rates of n-butanol vapour were measured by Viisanen and Strey (1994) using an expansion chamber. However, the temperature range in their measurements was different and an exact comparison is thus not possible. Also they used noble carrier gases such argon, helium and xenon, while our nucleation rate was evaluated in air.
Evaluation
of homogeneous
droplet
formation
inside UCPC
1007
Qualitative comparison gives, however, some reason to question the conclusion of Viisanen and Strey (1994) that classical nucleation theory consistently underestimates nucleation rates by a maximum of about two orders of magnitude since in our case the trend is opposite. One possible reason could be that the flow after the saturator in our experiments may not have been saturated. According to model calculations performed we remark that the data points in Fig. 2 will be shifted left if the conditions are undersaturated. The nucleation rate itself varies within an order of magnitude when the initial saturation ratio is changed by 50%. This kind of change in saturation ratio is high enough to explain the differences with theories or other experiments. It was also observed that the temperature of the condenser fluctuated during measurements by f O.l‘C. This could lead to profiles of temperature on the condenser wall and consequently an error in the evaluation of the nucleation rate. The experimental data points in Fig. 2 show that the change of 1°C in temperature generates a change of ca. one order of magnitude in nucleation rate. The inaccuracy in the temperature in our measurements could lead to noticeable error in experimental nucleation rate. We note finally that nucleation rate is a very steep function of saturation ratio and temperature and therefore our results are still in reasonable agreement with those by Viisanen and Strey. 4.
CONCLUSIONS
It was shown that special care has to be taken when operating the UCPC with different carrier gases. Homogeneous droplet formation at nearly standard operation conditions of UCPC was observed for carrier gases such as air and nitrogen. Furthermore, it was found that using helium as a carrier gas nucleation occurs already below the normal operation conditions. The homogeneous nucleation rates in the range of 103-lo* s-r cme3 were observed. This corresponds to UCPC concentrations of lo-‘-lo5 cmm3. The comparison between inlet flow rates was carried out. The lowest critical values for homogeneous nucleation to occur are found for flow rate 0.3 lpm. Tools for determining the saturation ratio and temperature in the nucleation volume were developed. Since the choice of the carrier gas seems to have an effect on the droplet formation, the critical conditions for homogeneous nucleation to occur are not possible to determine directly from the calculations but need also experimental investigations. Due to several uncertainties in the operation of the UCPC it is concluded that it is difficult to use as a chamber for nucleation studies. REFERENCES Anisimov, M. P., Hlmeri, K. and Kulmala, M. (1994) Construction and test of laminar flow diffusion chamber: homogeneous nucleation of DBP and n-hexanol. J. Aerosol Sci. 25, 23-32. Becker, R. and Doring, W. (1935) Kinetische Behandlung der Keimbildung in iibersiittigten Dampfen. Ann. Phys. (Leipzig) (5) 26, 719. Bird, R. B., Stewart, W. E. and Lightfoot, E. N. (1960) Transport Phenomena. Wiley, New York. Farkas, L. (1927) Keimbildungsgeschwindigkeit in iiberslttigten Dampfen. 2. Phys. Chem. 125, 236. Girshick, S. (1991) Comment on: “Self-consistency correction to homogeneous nucleation theory”. J. them. Phys. 94, 826. Girshick, S. and Chiu, C.-P. (1990) Kinetic nucleation theory: A new expression for the rate of homogeneous nucleation from an ideal supersaturated vapor. J. them. Phys. 93, 1273. Girshick, S., Chiu, C.-P. and McMurry, P. H. (1990) Time-dependent aerosol models and homogeneous nucleation rates. Aerosol Sci. Technol. 13, 465. Jakob, M. (1949) Heat Transfer. Wiley, New York. Landolt-Bornstein (1960) Zahlenwerte und Functionen aus Physik-Chemie-Astronomie-Geophysik-Technik. Springer, Berlin. Niida, T., Wen, H. Y., Udischas, R. and Kasper, G. (1988) Counting efficiency of condensation nucleus counters in N,, Ar, CO, and He. J. Aerosol Sci. 19, 1417. Perry, R. H. and Green, D. (1984) Perry’s Chemical Engineers’ Handbook, 6th Edn. McGraw-Hill, New York. Reid, R., Prausnitz, J. and Poling, B. (1987) The Properties of Gases nnd Liquids, 4th Edn. McGraw-Hill, New York. Schmeling, T. and Strey, R. (1983) Equilibrium vapor pressure measurements for the n-alcohols in the temperature range from - 30°C to + 30°C. Ser. Bunsenges. Phys. Chem. 87, 871. Stolzenburg, M. R. (1988) An ultrafine aerosol size distribution measuring system. Ph.D. thesis, Department of Mechanical Engineering, University of Minnesota, Minneapolis, U.S.A. Stolzenburg, M. R. and McMurry, P. H. (1991) An ultrafine aerosol condensation nucleus counter. Aerosol Sci. Technol. 14, 4845.
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