DEVELOPMENT OF A LIGHT-WEIGHT, LOW-POWER CASCADE IMPACTOR FOR AIRCRAFT SAMPLING

Transcription

1 DEVELOPMENT OF A LIGHT-WEIGHT, LOW-POWER CASCADE IMPACTOR FOR AIRCRAFT SAMPLING Francisco J. Romay, Virgil A. Marple, Benjamin Y. H. Liu and Michael Rosen MSP Corporation, 1313 Fifth St. SE, Minneapolis, MN, INTRODUCTION Sampling of atmospheric aerosols often requires the use of research airplanes to acquire information on the spatial distribution of particle size, concentration and chemical composition of atmospheric particles. Realtime aerosol instrumentation such as optical particle counters and condensation nuclei counters have been used by several researchers. However, these instruments do not provide information on the chemical composition of the sampled particles. With this in mind MSP Corporation has developed a light-weight, low-power consumption cascade impactor for CIRPAS (Center for Interdisciplinary Remotely-Piloted Aircraft Studies). The instrument allows the collection of size-fractionated aerosol samples by using eight computer-controlled cascade impactors that can be programmed to collect sequential samples according to specified sampling strategies defined by the operator. The total weight of the instrument including the exterior pod is 30 kg, the total power consumption is 180 W (continuous), and the external dimensions of the pod are 406 mm in diameter by 1575 mm in length. The complete instrument includes the following components mounted inside an aerodynamically shaped pod constructed of a carbon-epoxy composite material: Shrouded inlet tube for near isokinetic sampling at 50 m/s aircraft cruise speed Large droplet impactor and single-stage virtual impactor with a nominal 2.5 µm cut-point Eight cascade impactors with five stages (2.4, 1.4, 0.74, 0.43 and 0.25 µm aerodynamic 100 LPM) and a final filter Instrumentation to measure temperature, pressure, and sampling flow rate Blower package with active flow/speed control An embedded computer to control the operation of the instrument and to communicate with the onboard computer and/or a laptop computer via serial ports DESCRIPTION OF THE INSTRUMENT Figure 1 shows a flow diagram of the complete instrument. The aerosol is sampled at a rate of 110 liters per minute (LPM) by a shrouded inlet tube into a large-droplet impactor followed by a virtual impactor to remove particles larger than 2.5 µm. The major flow of the virtual impactor (100 LPM) goes to the inlet manifold of the cascade impactors. Each impactor has a built-in solenoid valve at the base. During sampling the air flows through one of the eight impactors to collect six size-fractionated particle samples. From the impactor after-filter, the flow is discharged to the exhaust manifold followed by a mass flow meter and two high-speed blowers connected in series. The minor flow of the virtual impactor (10 LPM) is filtered and discharged to the suction of the second blower. The sampled volumetric flow rate is calculated from the measured mass flow rate and from the inlet temperature and pressure measured conditions. The speed of the blowers is automatically adjusted by a feedback control system to maintain a constant volumetric sampling flow rate.

2 CONTROLLER DC-DC COVERTER (8 units) SAMPLE FLOW DROPLET IMPACTOR VIRTUAL IMPACTOR DISPOSABLE FILTER P T TRANSDUCERS CASCADE IMPACTORS T FLOWMETER BLOWER 1 BLOWER 2 EXHAUST Figure 1. Flow Diagram of the Instrument Shrouded Inlet Tube The instrument samples air through a shrouded inlet tube to achieve isokinetic sampling conditions when the airplane cruising speed is 50 m/s and the sampling flow rate is 100 LPM. The 10 cm diameter shroud avoids flow separation for small angles of attack, aligns the flow and produces a uniform velocity profile at the sampling tube inlet (Murphy and Schein, 1998). Both the shroud and the sampling tube have a sharpedge tip to minimize inlet losses and oversampling errors (Huebert et al., 1990). The shroud exit to inlet area ratio and the shroud length were selected using the criteria recommended by Twohy (1998). Large droplet impactor and single stage virtual scalper The inlet flow is first sampled by a 10 µm-cut particle trap impactor (Biswas and Flagan, 1988) to remove large water droplets. A 2.5 µm-cut single-stage virtual impactor is then used to remove coarse particles. The major flow (100 LPM) with the fine particles is introduced to a manifold connected to the cascade impactors, while the minor flow (10 LPM) is filtered and exhausted. Cascade Impactors The cascade impactor has been designed for a nominal flow rate of 100 liters per minute (100 LPM) using widely accepted theoretical models for inertial impactors (Marple and Liu, 1974, Rader and Marple, 1985) and optimizing the nozzle diameter and number of nozzles to achieve low pressure drop and power consumption (Marple et al., 1991). Impactor cross-flow is avoided by maintaining the cross-flow parameter (Fang et al., 1991) below The jet to plate distance is 3 to 5 times the nozzle diameter to reduce nozzle back losses. The nozzles are uniformly spaced in four quadrants to allow the user to perform up to four different physical/chemical characterization analyses of the collected particle samples. The impaction plates are removable for easy installation of user-selected 90-mm impaction substrates before and after sampling. A 90-mm filter holder follows stage 5 to collect particles smaller than 0.25 µm. Table 1 shows critical design and operating parameters for the five stages of the impactor. The impactor base has a built-in solenoid valve to control the sequential sampling of the eight cascade impactors. The total weight of each impactor is 2.8 lb. including the built-in solenoid valve. The total pressure drop of the impactor with a 90-mm quartz after-filter is 40 inches of water at 100 LPM and standard atmospheric conditions.

3 TABLE 1 CASCADE IMPACTOR DESIGN AND OPERATING PARAMETERS Stage Nozzle Diameter (cm) Nozzle Count (#) Calculated Cut-point S/W Inlet Pressure (ATM) Nozzle Reynolds number Stage P (in wg) Cum. P (in wg) Cross-flow Parameter Cluster Diameter (cm) Instrumentation and Control A PC-104 embedded computer system is used to control the operation of the instrument. A computer program running C ++ under DOS is used to perform self-test procedures to check the condition of hardware components, to commission eight cascade impactor channels for sequential sampling, to acquire analog signals from temperature,pressure, and flow transducers, to calculate the sampling volumetric flow rate, to provide active control of the blower speed, to synchronize and time-stamp the acquired data, and to communicate via serial ports with the on-board computer and/or a laptop computer. The actual cut-point of the five impactor stages is a function of the temperature and pressure of the air sampled and of the volumetric flow rate. Therefore, the instrument has transducers to measure the inlet temperature and absolute pressure, and a mass flow meter located downstream of the impactors. With these three variables the volumetric flow rate can be calculated at the inlet of the impactors. The calculated flow is then used in a feedback control system to adjust the blower speed until the flow rate reaches the setpoint. Table 2 shows the predicted cut-points as a function of altitude with standard atmosphere values of pressure and temperature when the volumetric flow rate is maintained at 100 LPM. The last stage cutpoint varies from 0.25 µm at standard sea-level conditions to 0.13 µm at 8000 m altitude. In the default mode of operation a constant volumetric flow rate setpoint is used in the feedback control system that adjusts the blower speed to maintain the inlet flow rate. TABLE 2 IMPACTOR CUT-POINTS AS A FUNCTION OF ALTITUDE FOR CONSTANT INLET FLOW RATE H (m) T (K) p (Pa) Q (LPM) Stage 1 Stage 2 Stage 3 Stage 4 Stage

4 A second mode of operation is to control the flow rate so that the cut-point of the first stage is maintained at a constant value. Table 3 shows the volumetric flow rate required and the stages cutpoints as a function of altitude when the cut-point of stage 1 is maintained constant. The last stage cutpoint varies from 0.24 µm at standard sea-level conditions to 0.17 µm at 8000 m altitude. In this case the flow rate has a variable setpoint calculated from the actual inlet pressure and temperature and from the standard operating conditions. TABLE 3 IMPACTOR CUT-POINTS AS A FUNCTION OF ALTITUDE FOR CONSTANT CUT-POINT AT STAGE 1 H (m) T (K) p (Pa) Q (LPM) Stage 1 Stage 2 Stage 3 Stage 4 Stage Blower Package The operation of the instrument requires to select a variable-speed blower to adjust the sampling flow rate according to the atmospheric conditions at the altitude at which sampling takes place. The combined pressure drop of the particle scalper, the cascade impactor and the associated piping can be as high as 40 inches of water at a total flow of 100 LPM. This pressure drop required to use two high-rotational-speed blowers connected in series. Each blower weighs 340 g and is driven by a DC motor with a 7 to 12 VDC variable input voltage (Ametek Rotron Model R304). PRELIMINARY EVALUATION OF THE CASCADE IMPACTOR A prototype cascade impactor has been tested experimentally in the laboratory with an oleic acid polydisperse aerosol. Bare aluminum foils were used as impaction substrates. No bounce was observed since the test particles were liquid. The efficiency curves for stages 2 and 3 were obtained by sampling the oleic acid aerosol upstream and downstream of these stages with an aerodynamic particle sizer (Model 3310, TSI Inc.). Figure 2 shows the calibration efficiency curves for stages 2 and 3. The measured cutpoints for stages 2 and 3 with this calibration technique were 1.6 µm and 0.90 µm respectively, which are 15 and 20% higher than the predicted cut-points. Also, the efficiency curves are less sharp compared with efficiency curves of similar cut-point stages of the MOUDI impactor (Marple et al., 1991). This might be due to the use of larger jet to plate distances. However, it is also possible that the polydisperse aerosol calibration technique has less size resolution compared with a monodisperse aerosol calibration. Therefore, a complete calibration with monodisperse particles will be done and reported soon.

5 IMPACTOR CALIBRATION CURVES WITH OLEIC ACID PARTICLES Stage 2 Stage Aerodynamic Particle Diameter ( µm) The size distribution of Emery oil aerosol was measured simultaneously with a 8-stage MOUDI (Model 100, MSP Corporation) and with the high-flow 5-stage cascade impactor. Figure 3a shows the comparison of the cumulative aerosol mass distribution measured by both impactors. The measured size distributions were fitted by lognormal distributions. The agreement among the fitted parameters (mass median diameter and geometric standard deviation) is quite good, which indicates that the calculated cut-points of the 5- stage cascade impactor are very close to the actual cut-points. Figure 3b shows the Emery oil mass distribution histograms measured by both cascade impactors. No visual evidence of inter-stage particle losses was observed in any of the five stages of the high-flow cascade impactor.

6 Figure 3 Emery Oil Size Distribution Measurement 10 cum-m vs UL-M cum-f vs UL-F Plot 2 Aerodynamic Diameter Cumulative Mass (%) 0.5 dm/mdlnd dm/mdlnd-f 0.4 dm/mdlnd p Aerodynamic Diameter CONCLUSIONS

7 This paper reports the development of a sampling instrument to collect size-fractionated particle samples from the wing of an airplane. The instrument includes all the components required to introduce the samples isokinetically from the outside air into one of eight high-flow cascade impactors. An embedded computer is used to control the operation of the instrument and to communicate with the on-board computer via serial port. In addition to the cascade impactors the instrument has temperature, absolute pressure and flow rate transducers to provide real-time information on all the variables required to determine the cut-point of the impactor stages throughout a sampling campaign. The instrument in housed by an aerodynamically shaped pod that can be mounted to the wing of a Pelican or Twin Otter airplane. The high-flow cascade impactor has been evaluated experimentally with a polydisperse test aerosol. A preliminary calibration has been performed, but a more complete particle calibration with monodisperse particles will be done to completely characterize the cascade impactor. Flight-tests of the instrument will be performed during the summer of 1999 in Monterey, California. REFERENCES Biswas, P. and Flagan, R. C., The particle trap impactor, J. Aerosol Sci., Vol. 19, No1, pp , Fang, C.P., Marple, V.A., and Robow, K.L., Influence of cross-flow on particle collection characteristics of multi-nozzle impactors, J. Aerosol Sci., 22: , Huebert, B.J., Lee, G., and Warren, W.L., Airborne aerosol inlet passing efficiency measurements, J. geophys. Res., 95: Marple, V.A. and Liu, B.Y.H., Characteristics of laminar jet impactors, Envir. Sci. & Tecnol., 8: , Marple, V.A., Rubow, K.L., and Behm, S.M., A micro-orifice uniform deposit impactor (MOUDI): description, calibration, and use. Aerosol Sci. & Technol. 14: , Murphy, D.M., and Schein, M.E., Wind tunnel tests of a shrouded aircraft inlet, Aerosol Sci. & Technol. 28:33-39, Rader, D.J., and Marple, V.A., Effect of ultra-stokesian drag and particle interception on impaction characteristics, Aerosol Sci. & Technol. 4: , Twohy, C.H., Model calculations and wind tunnel testing of an isokinetic shroud for high speed sampling, Aerosol Sci. & Technol. 29: , ACKNOWLEDGEMENT This research has been funded by a phase II SBIR grant awarded by the Office of Naval Research. Contract Number N C The financial support of the sponsor is gratefully acknowledged