Sizing of atmospheric particulates with a high volume cascade impactor

This article describes the development of a high volume cascade impactor with a sampling rate of 20 ft³/min that can be powered by high volume blowers in common use for air sampling. With this impactor it is now possible to collect enough sample of atmospheric particulates in a 24 hour period to conveniently determine an aerodynamic size distribution of suspended particulates by gravimetric analysis. The impactor has four stages and an after-filter and separates particles from the air stream by inertial impaction. The effective cutoff diameters for the stages are 7.0, 3.3, 2.0, and 1.1 microns. The after-filter traps the submicron material.     

Experimental studies on particle impaction and bounce: effects of substrate design and material

This paper presents an experimental investigation of the effects of impaction substrate designs and material in reducing particle bounce and reentrainment. Particle collection without coating by using combinations of different impaction substrate designs and surface materials was conducted using a personal particle sampler (PPS) developed by the University of Southern California. The PPS operates at flow rate of 4 l min^-1 with a 50% cutpoint of approximately 0.9 μm in aerodynamic diameter. The laboratory results showed that the PPS collection efficiency for particles larger than 50% cutpoint is strikingly low (e.g., less than 50%) when an uncoated open cavity made of aluminum was used as an impaction substrate. The collection efficiency gradually increased when Teflon tape, Nuclepore, and glass fiber filters were used as impaction surfaces, respectively. Conical or partially enclosed cavity substrate designs increased collection efficiency of particles of 9 μm up to 80–90%. A conical cavity with glass fiber filter used as impaction surface was identified as the optimum configuration, resulting in a collection efficiency of 92% at Stokes numbers as high as 15.4 (corresponding to 9 μm in aerodynamic diameter). Particle losses were low (less than 10%) and relatively independent of particle size in any design with glass fiber filter. Losses seemed to increase slightly with particle size in all other configurations. Finally, outdoor PM₁ concentrations obtained with the PPS (in its optimum configuration) and a modified micro-orifice uniform deposit impactor (MOUDI) with coated impaction stages were in excellent agreement. The mean ratio of the PPS-to-MOUDI concentration was 1.13(±0.17) with a correlation coefficient R²=0.95. Results from this investigation can be readily applied to design particle bounce-free impaction substrates without the use of coating. This is a very important feature of impactors, especially when chemical analysis of the collected particulate matter is desirable.     

Determination of fine particulate semi-volatile organic material at three eastern U.S. sampling sites

Correct assessment of fine particulate carbonaceous material as a function of particle size is, in part, dependent on the determination of semi-volatile compounds, which can be lost from particles during sampling. This study gives results obtained for the collection of fine particulate carbonaceous material at three eastern U.S. sampling sites [Philadelphia, PA; Shenandoah National Park, VA; and Research Triangle Park (RTP), NC] using diffusion denuder technology. The diffusion denuder samplers allow for the determination of fine particulate organic material with no artifacts, due to the loss of semi-volatile organic particulate compounds, or collection of gas-phase organic compounds by the quartz filter during sampling. The results show that an average of 41, 43, and 59% of fine particulate organic material was lost as volatilized semi-volatile organic material during collection of particles on a filter at Philadelphia, RTP, and Shenandoah, respectively. The particle size distribution of carbonaceous material retained by a filter and lost from a filter during sampling was obtained for the samples collected at Philadelphia and Shenandoah. The carbonaceous material retained by the particles during sampling was found predominantly in particles smaller than 0.4 microm in aerodynamic diameter. In contrast, the semi-volatile organic material lost from the particles during sampling had a mass median diameter of approximately 0.5 microm.     

Trace element pass-through for cellulose filters when used for aerosol collection

Filter papers and impaction substrates made of cellulose fibers, such as Whatman 41 and Misco P810/252, are of considerable utility in the collection of aerosol for subsequent trace elemental analysts. This experiment evaluated the performance of Misco P810/252 in collecting trace elements, relative to a co-located standard glass fiber filter hi-vol collection. Sampling was conducted in varying meteorological conditions, so that results might be expressed in terms of environmental variables such as temperature and relative humidity. The pass-through factors presented here were derived from a series of environmental samples collected over land and over water. Overall, the Misco impactor/filter failed to collect 38% of Pb and 32% of Zn.     

Development and evaluation of an impactor for a PM2.5 speciation sampler

A conventional impactor for a particle speciation sampler was developed and validated through laboratory and field tests. The speciation sampler consists of the following components: a PM2.5 conventional impactor that removes particles larger than 2.5 microns, an all-glass, coated honeycomb diffusion denuder, and a 47-mm filter pack. The speciation sampler can operate at two different sampling rates: 10 and 16.7 L/min. An experimental characterization of the impactor's performance was conducted. The impactor's collection efficiency was examined as a function of critical design parameters such as Reynolds number, the distance from the nozzle exit to the impaction plate, and the impaction substrate coating method. The bounce of particles larger than the cut point was successfully minimized by using a greased surface as the impaction substrate. Additionally, a series of field intercomparison experiments were conducted at both 10 and 16.7 L/min airflow. PM2.5 mass and SO4(2-) concentrations were measured and compared with the Federal Reference Method (FRM) and found to be in good agreement. Results of the laboratory chamber tests also indicated that the impactor's performance was in good agreement with the FRM.     

Size distribution of black (BC) and total carbon (TC) in Vienna and Ljubljana

During two campaigns in winter 2004, size segregated impactor samples (0.1-10 microm) and filter samples were taken in two Central European cities (Vienna, Austria and Ljubljana, Slovenia). The impactor samples were analyzed for major inorganic ions and short-chain organic acids, total carbon (TC) and black carbon (BC). Maximum concentrations of total mass were 71.6 microg m(-3) in Vienna and 73.1 microg m(-3) in Ljubljana. Minimum concentrations in Vienna were only half those in Ljubljana. The BC content of the aerosol was similar (ca. 8%), but the BC/TC ratio was higher in Vienna than in Ljubljana (0.39 vs. 0.29), reflecting the different contribution of diesel traffic emissions. The mass median diameters of the submicron size distributions of all major fractions (total mass, TC, BC and SO(4)(2-)) were smaller in Vienna (0.43 microm, 0.41 microm, 0.38 microm and 0.48 microm, respectively) than in Ljubljana (0.55 microm, 0.44 microm, 0.42 microm and 0.60 microm, respectively). Impactor/filter ratios for total mass were 0.79 in Vienna and 0.82 in Ljubljana, while the ratios for BC were 0.56 in Vienna and 0.49 in Ljubljana. An estimation of the mixing state of accumulation mode BC indicated that 33% and 37% of BC, respectively, are mixed externally to the aerosol in the accumulation size range in Vienna and Ljubljana.     

Phase separation and regrowth of aerosol matter collected after size fractionation in an impactor

Aerosol matter in the size range <2 μm was collected in a Berner impactor and subsequently analysed by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) spectrometry. Owing to the low electron beam energy of 5 keV (occasionally 10 keV), analysis was restricted to elements with atomic numbers 20 (Ca). Sub-micrometer aerosol matter was found to contain mostly S, O, and C as well as some K and Ca. Nitrogen appeared to escape detection, probably due to bombardment-induced sublimation of NO₃ and NH₄. During sampling at low to moderate relative humidity (<60%) the sulphur-rich fraction of the aerosol matter (most likely sulphates) regrew in the form of microcrystals with sizes up to 10 times larger than the mean aerodynamic diameter of the respective impactor stage. By contrast, when sampling during periods in course of which the relative humidity exceeded 70%, the aerosol matter regrew in the form of extended amorphous agglomerates. The aerosol deposits also contained large numbers of carbon nanoparticles, well separated from the regrown sulphate-rich matter. The nanoparticles were similar in size (20–40 nm), much smaller than the equivalent aerodynamic diameter of the impacting particles (63 nm–2 μm). Presumably, the carbon nanoparticles constituted the core of larger air-borne particles covered with sulphates (as well as with nitrates and organic carbon). The regrown microcrystals disappeared rapidly under electron bombardment at high current density, an observation that indicates high volatility at elevated temperatures. Aerosol matter collected in the size range between 1 and 2 μm contained large fractions of particles made of O, Si, P, K, and Ca (oxides). These particles were highly resistant to electron bombardment (hard) and showed little or no evidence for agglomeration or regrowth. After removing the soluble (acidic) material from the collected aerosol matter, only carbon nanoparticles and hard coarse particles were left behind. The observation of agglomerated or crystallized “soft” aerosol matter in combination with phase separation of carbon nanoparticles lends further support to the assertion that it is not possible to collect useful quantities of fine and ultrafine aerosol particles with as-suspended morphology. Some implications for health-related research are discussed.     

Development and Evaluation of an Impactor for a PM2.5 Speciation Sampler

ABSTRACT

A conventional impactor for a particle speciation sampler was developed and validated through laboratory and field tests. The speciation sampler consists of the following components: a PM_2.5 conventional impactor that removes particles larger than 2.5 μm, an all-glass, coated honeycomb diffusion denuder, and a 47-mm filter pack. The speciation sampler can operate at two different sampling rates: 10 and 16.7 L/min. An experimental characterization of the impactor’s performance was conducted. The impactor’s collection efficiency was examined as a function of critical design parameters such as Reynolds number, the distance from the nozzle exit to the impac-tion plate, and the impaction substrate coating method. The bounce of particles larger than the cut point was successfully minimized by using a greased surface as the im-paction substrate. Additionally, a series of field intercomparison experiments were conducted at both 10 and 16.7 L/min airflow. PM_2.5 mass and SO₄ ^2- concentrations were measured and compared with the Federal Reference Method (FRM) and found to be in good agreement. Results of the laboratory chamber tests also indicated that the impactor’s performance was in good agreement with the FRM.     

Technique for Measuring Dust Collector Efficiency As a Function of Particle Size

With a specially designed generator, producing an aerosol of concentration, chemical composition, and size distribution similar to incinerator aerosols, a scrubber was tested in the laboratory by sampling before and after the collector. Using an Andersen Sampler as a 7 stage cascade impactor the efficiency for 7 different size classes was determined. This technique provides a rapid and simple method for evaluating the performance of a collector as a function of particle size. The multistage impaction classifies the particles according to their aerodynamic behavior. Gravimetric analysis of each stage eliminates the need for tedious counting and sizing.     

Removal of ultrafine and fine particulate matter from air by a granular bed filter

The removal efficiency of granular filters packed with lava rock and sand was studied for collection of airborne particles 0.05-2.5 microm in diameter. The effects of filter depth, packing wetness, grain size, and flow rate on collection efficiency were investigated. Two packing grain sizes (0.3 and 0.15 cm) were tested for flow rates of 1.2, 2.4, and 3.6 L/min, corresponding to empty bed residence times (equal to the bulk volume of the packing divided by the airflow rate) in the granular media of 60, 30, and 20 sec, respectively. The results showed that at 1.2 L/min, dry packing with grains 0.15 cm in diameter removed more than 80% (by number) of the particles. Particle collection efficiency decreased with increasing flow rate. Diffusion was identified as the predominant collection mechanism for ultrafine particles, while the larger particles in the accumulation mode of 0.7-2.5 microm were removed primarily by gravitational settling. For all packing depths and airflow rates, particle removal efficiency was generally higher on dry packing than on wet packing for particles smaller than 0.25 microm. The results suggest that development of biological filters for fine particles is possible.     

Fine and coarse particulate matter chemical characterization in a heavily industrialized city in central Mexico during Winter 2003

This paper presents the results of the first reported study on fine particulate matter (PM) chemical composition at Salamanca, a highly industrialized urban area of Central Mexico. Samples were collected at six sites within the urban area during February and March 2003. Several trace elements, organic carbon (OC), elemental carbon (EC), and six ions were analyzed to characterize aerosols. Average concentrations of PM with aerodynamic diameter of less than 10 microm (PM10) and fine PM with aerodynamic diameter of less than 2.5 microm (PM2.5) ranged from 32.2 to 76.6 [g m(-3) and 11.1 to 23.7 microg m(-3), respectively. OC (34%), SO4= (25.1%), EC (12.9%), and geological material (12.5%) were the major components of PM2.5. For PM10 geological material (57.9%), OC (17.3%), and SO4= (9.7%) were the major components. Coarse fraction (PM,, -PM2.5), geological material (81.7%), and OC (8.6%) were the dominant species, which amounted to 90.4%. Correlation analysis showed that sulfate in PM2.5 was present as ammonium sulfate. Sulfate showed a significant spatial variation with higher concentrations to the north resulting from predominantly southwesterly winds above the surface layer and by major SO2 sources that include a power plant and refinery. At the urban site of Cruz Roja it was observed that PM2.5 mass concentrations were similar to the submicron fraction concentrations. Furthermore, the correlation between EC in PM2.5 and EC measured from an aethalometer was r(2) = 0.710. Temporal variations of SO2 and nitrogen oxide were observed during a day when the maximum concentration of PM2.5 was measured, which was associated with emissions from the nearby refinery and power plant. From cascade impactor measurements, the three measured modes of airborne particles corresponded with diameters of 0.32, 1.8, and 5.6 microm.     

Particle size distributions of polycyclic aromatic hydrocarbons in rural and urban atmosphere of Tianjin, China

The size distributions of 16 polycyclic aromatic hydrocarbons (PAHs) and particle mass less than 10 microm in aerodynamic diameter (Dp) were measured using a nine-stage low-volume cascade impactor at rural and urban sites in Tianjin, China in the winter of 2003-2004. The particles exhibited the trimodal distribution with the major peaks occurring at 0.43-2.1 and 9.0-10.0 microm for both urban and rural sites. The concentrations of the total PAH (sum of 16 PAH compound) at rural site were generally less than those of urban site. Mean fraction of 76.5% and 63.9% of the total PAH were associated with particles of 0.43-2.1 microm at rural and urban sites, respectively. Precipitation, temperature, wind speed and direction were the important meteorological factors influencing the concentration of PAHs in rural and urban sites. The distributions of PAHs concentration with respect to particle size were similar for rural and urban samples. The PAHs concentrations at the height of 40 m were higher than both of 20 and 60 m at urban site, but the mass median diameter (MMD) of total PAH increased with the increasing height. The mid-high molecular weight (278 >or= MW >or= 202) PAHs were mainly associated with fine particles (Dp or=MW >or=178) PAHs were distributed in both of fine and coarse particle. The fraction of PAHs associated with coarse particles (Dp>2.1 microm) decreased with increasing molecular weight. The relatively consistent distribution of PAHs seemed to indicate the similar combustion source of PAHs at both of rural and urban sites. The fine differences of concentration and distribution of PAHs at different levels at urban site suggested that the different source and transportation path of particulate PAHs.     

An efficient venturi scrubber system to remove submicron particles in exhaust gas

An efficient venturi scrubber system making use of heterogeneous nucleation and condensational growth of particles was designed and tested to remove fine particles from the exhaust of a local scrubber where residual SiH4 gas was abated and lots of fine SiO2 particles were generated. In front of the venturi scrubber, normal-temperature fine-water mist mixes with high-temperature exhaust gas to cool it to the saturation temperature, allowing submicron particles to grow into micron sizes. The grown particles are then scrubbed efficiently in the venturi scrubber. Test results show that the present venturi scrubber system is effective for removing submicron particles. For SiO2 particles greater than 0.1microm, the removal efficiency is greater than 80-90%, depending on particle concentration. The corresponding pressure drop is relatively low. For example, the pressure drop of the venturi scrubber is approximately 15.4 +/- 2.4 cm H2O when the liquid-to-gas ratio is 1.50 L/m3. A theoretical calculation has been conducted to simulate particle growth process and the removal efficiency of the venturi scrubber. The theoretical results agree with the experimental data reasonably well when SiO2 particle diameter is greater than 0.1 microm.     

An analytical method for the measurement of nonviable bioaerosols.

Exposures from indoor environments are a major issue for evaluating total long-term personal exposures to the fine fraction (<2.5 microm in aerodynamic diameter) of particulate matter (PM). It is widely accepted in the indoor air quality (IAQ) research community that biocontamination is one of the important indoor air pollutants. Major indoor air biocontaminants include mold, bacteria, dust mites, and other antigens. Once the biocontaminants or their metabolites become airborne, IAQ could be significantly deteriorated. The airborne biocontaminants or their metabolites can induce irritational, allergic, infectious, and chemical responses in exposed individuals. Biocontaminants, such as some mold spores or pollen grains, because of their size and mass, settle rapidly within the indoor environment. Over time they may become nonviable and fragmented by the process of desiccation. Desiccated nonviable fragments of organisms are common and can be toxic or allergenic, depending upon the specific organism or organism component. Once these smaller and lighter fragments of biological PM become suspended in air, they have a greater tendency to stay suspended. Although some bioaerosols have been identified, few have been quantitatively studied for their prevalence within the total indoor PM with time, or for their affinity to penetrate indoors. This paper describes a preliminary research effort to develop a methodology for the measurement of nonviable biologically based PM, analyzing for mold and ragweed antigens and endotoxins. The research objectives include the development of a set of analytical methods and the comparison of impactor media and sample size, and the quantification of the relationship between outdoor and indoor levels of bioaerosols. Indoor and outdoor air samples were passed through an Andersen nonviable cascade impactor in which particles from 0.2 to 9.0 microm were collected and analyzed. The presence of mold, ragweed, and endotoxin was found in all eight size ranges. The presence of respirable particles of mold and pollen found in the fine particle size range from 0.2 to 5.25 microm is evidence of fragmentation of larger source particles that are known allergens.     

Low Flow Rate Sharp Cut Impactors for Indoor Air Sampling: Design and Calibration

Two single round nozzle impactors have been developed for use in Harvard’s indoor air pollution health study. Both impactors operate at flow rates of 4 L/m and are nearly identical, differing only in their cut sizes of 2.5 μm and 10 μm aerodynamic diameters. Two identical cascaded stages of the same cut size are used to obtain sharp cut-off characteristics. The particles are deposited on impaction plates made of oil impregnated, porous material to reduce particle bounce and are discarded. Only the particles collected on the afterfilter are analyzed. Special care has been taken to collect the particles uniformly on the afterfilter to aid in particle analysis.

The jmpactors were calibrated with a vibrating orifice monodisperse aerosol generator. However, due to the sharp cut of the impactors, doublets and triplets in the calibration aerosols, even in small quantities, gave erroneous calibration curves. Therefore, the number of doublets and triplets in the challenging aerosols were measured and appropriate corrections made to the calibration curves.     

Impact and growth phenomena observed with sub-micrometer atmospheric aerosol particles collected on polished silicon at low coverage

The processes occurring during the initial stages of size-selected sampling of atmospheric aerosol matter were explored by means of high-resolution scanning electron microscopy and energy dispersive X-ray spectrometry. The final four stages of a Berner impactor served to collect particles with aerodynamic diameters between 60 nm and 1 μm on polished silicon substrates at sampling times between 10 s and 16 min. In the single-particle impact regime (fractional coverage below 5%), most of the particles produced circular black tracks, of which 15 to 30% contained cores of carbonaceous nanoparticles. At a fractional coverage above about 10%, initially generated deposits became increasingly active as centres of preferred aerosol collection. As a result, the number of black spots remained almost constant, but the area per spot increased rapidly. At this stage of fractional coverage, structured aerosol matter became visible within large black areas of random shape. The deposits showed clear evidence for the onset of phase separation between carbon nanoparticles and material rich in sulphur, probably a mixture of alkaline and alkaline earth sulphates. After some time of sample storage in air the low-coverage sulphate matter tended to attain a fractal pattern. At a fractional coverage of several hundred per cent, large amorphous or crystalline objects had grown, but 30% or more of the nominal impact area still did not contain any aerosol matter. Moreover, carbon nanoparticles were found to be phase separated from the newly grown, very uniform aerosol material. The observations imply that sub-micrometer aerosol matter collected in impactors has lost memory of the original particle morphology.     

Continuous monitoring of ultrafine,fine, and coarse particles in a residence for 18 months in 1999-2000

Continuous monitors were employed for 18 months in an occupied townhouse to measure ultrafine, fine, and coarse particles; air change rates; wind speed and direction; temperature; and relative humidity (RH). A main objective was to document short-term and long-term variation in indoor air concentrations of size-resolved particles (0.01-20 microm) caused by (1) diumal and seasonal variation of outdoor air concentrations and meteorological variables, (2) indoor sources such as cooking and using candles, and (3) activities affecting air change rates such as opening windows and using fans. A second objective was to test and compare available instruments for their suitability in providing real-time estimates of particle levels and ancillary variables. Despite different measuring principles, the instruments employed in this study agreed reasonably well for particles less than 10 microm in diameter. The three instruments measuring fine and coarse particles (aerodynamic diameter between 0.3 and 20 microm) agreed to within 30% in their overall estimates of total volume. Two of these instruments employed optical scattering, and the third used an aerodynamic acceleration principle. However, several lines of evidence indicated that the instrument employing aerodynamic acceleration overestimated concentrations for particle diameters greater than 10 microm. A fourth instrument measuring ultrafine and accumulation-mode particles (0.01-1 microm) was operated with two different inlets providing somewhat different particle size ranges. The two inlets agreed in the ultrafine region (< 0.1 microm) but diverged increasingly for larger particles (up to 0.445 microm). Indoor sources affecting ultrafine particle concentrations were observed 22% of the time, and sources affecting fine and coarse particle concentrations were observed 12 and 15% of the time, respectively. When an indoor source was operating, particle concentrations for different sizes ranged from 2 to 20 times the average concentrations when no indoor source was apparent. Indoor sources, such as cooking with natural gas, and simple physical activities, such as walking, accounted for a majority (50-90%) of the ultrafine and coarse particle concentrations, whereas outdoor sources were more important for accumulation-mode particles between 0.1 and 1 microm in diameter. Averaged for the entire year and including no periods when indoor sources were apparent, the number distribution was bimodal, with a peak at approximately 10 nm (possibly smaller), a shallow minimum at approximately 14 nm, and a second broad peak at approximately 68 nm. The volume distribution was also bimodal, with a broad peak at approximately 200 nm, a minimum at approximately 1.2 microm, and then an upward slope again through the remaining size fractions. A database was created on a 5-min averaging time basis. It contains more than 90,000 measurements by two of the instruments and approximately 30,000 by the two optical scattering instruments. About 4500 hour-long average air change rates were also calculated throughout the year using a dedicated gas chromatograph with electron capture detection (GC/ECD). At high air change rates [> 0.8 air changes per hour (hr(-1))], particle concentrations were either elevated (when no source was present) or depressed (when an indoor source was operating) by factors of up to 2 compared with low air change rates.     

Determination of Fine Particulate Semi-Volatile Organic Material at Three Eastern U.S. Sampling Sites

ABSTRACT

Correct assessment of fine particulate carbonaceous material as a function of particle size is, in part, dependent on the determination of semi-volatile compounds, which can be lost from particles during sampling. This study gives results obtained for the collection of fine particulate carbonaceous material at three eastern U.S. sampling sites [Philadelphia, PA; Shenandoah National Park, VA; and Research Triangle Park (RTP), NC] using diffusion denuder technology. The diffusion denuder samplers allow for the determination of fine particulate organic material with no artifacts, due to the loss of semi-volatile organic particulate compounds, or collection of gas-phase organic compounds by the quartz filter during sampling. The results show that an average of 41, 43, and 59% of fine particulate organic material was lost as volatilized semi-volatile organic material during collection of particles on a filter at Philadelphia, RTP, and Shenandoah, respectively. The particle size distribution of carbonaceous material retained by a filter and lost from a filter during sampling was obtained for the samples collected at Philadelphia and Shenandoah. The carbonaceous material retained by the particles during sampling was found predominantly in particles smaller than 0.4 μm in aerodynamic diameter. In contrast, the semi-volatile organic material lost from the particles during sampling had a mass median diameter of ~0.5 μm.     

典型危险废物的工程力学特性

掌握填埋废物的工程力学特性是确保危险废物填埋堆体稳定性的重要前提条件,因此,针对6种典型危险废物的工程力学特性开展了调查研究.研究结果表明,污泥类危险废物的含水率和有机质含量远高于非污泥类危险废物,而前者的比重则显著低于后者.污泥类危险废物的渗透系数相对较小,比污染土壤和飞灰炉渣混合物的渗透系数小1个数量级,比滤池填料的渗透系数小2个数量级.不同危险废物的无侧限抗压强度排序为:飞灰炉渣混合物(314.3 kPa)和滤池填料(278.9 kPa)＞污染土壤(124.9 kPa)＞杭州氧气厂污泥(68.6 kPa)、锌铬污泥(71.4 kPa)、油泥(58.4 kPa).不同危险废物的粘聚力排序为:飞灰炉渣混合物(40.8 kPa)、滤池填料(31.1 kPa)＞污染土壤(20.9 kPa)＞杭州氧气厂污泥(1.94kPa)、锌铬污泥(3.10 kPa)、油泥(1.81 kPa).此外,不同危险废物的内摩擦角差异相对较小,其变化范围在18.8°～35.0°之间.综合分析可知,飞灰炉渣混合物和滤池填料的工程力学特性较好,而污泥类危险废物的工程力学特性则较差.     

Predicting cutoff aerodynamic diameter and sharpness of single round-nozzle impactors with a finite impaction plate diameter

This investigation numerically examined the cutoff aerodynamic diameter (da50) and the sharpness (GSD) of the particle collection efficiency curve of impactors with a finite impaction plate diameter. Results revealed that the inertial impactors have a limited cutoff aerodynamic diameter at different air velocities. The extreme value of the cutoff aerodynamic diameter increases with the nozzle diameter (W)/the plate diameter (Dc). The computed da50/Dc values of the impactors increase with W/Dc at various Reynolds numbers (Re) and with the nozzle-toplate distance (S)/Dc when Re is 100. The value of GSD slightly increases with W/Dc for Re of 10 and 100, although the effect of S/Dc on GSD is not evident at various Res. The particle collection efficiency curve of the impactor with a lower Re is less sharp than that with a high Re at various W/Dc and S/Dc values. Statistical equations closely fitted the obtained numerical results for Res of 10-3000. The equations are useful for directly calculating the cutoff aerodynamic diameter and the sharpness of the particle collection efficiency curve for single round-nozzle impactors with a finite impaction Dc.