ME 538: Experiment # 1 Flow measurement and particle generation instruments Suresh Dhaniyala Mechanical and Aeronautical Engineering Clarkson University
Flow rate measurement Various types of flowmeters can be used for flow measurement Variable head meters Critical orifice meter Laminar flow element Porous metals Rotameters Variable area meters For aerosol measurements, choose a flowmeter that results in no/minimal particle loss through the system.
Laminar flow element For laminar flow through a pipe, the pressure drop over a known length is directly proportional to the flow rate This technique will enable flowrate measurement with minimal/no particle loss. 4 πδpd Q = 128μL Where Q is the flowrate (cm 3 /s), ΔP is the pressure drop across the tube (dyne/cm 2 ), d is the tube diameter (cm), L is the length of the tube (cm) and μ is the fluid viscosity (poise)
Critical Orifice Under sufficient pressure difference (P 2 /P 1 < 0.528) the flow velocity in a orifice reaches a critical or sonic value. Further reduction of downstream pressure will result in no further change in mass flowrate Note that changing the upstream pressure will result in changing the gas density and hence varying the mass flowrate through a critical orifice. The mass flowrate through a choked nozzle can be calculated as: m& max m & max = 0. 6847 P A Where is the maximum mass flowrate for choked conditions and P 0 and T 0 are the absolute upstream pressure and temperature and A* is the critical orifice area. 0 * RT 0 P 1 P 2
Particle generation Nanoparticle generation Metal particles Place a small amount of the required material in an oven and evaporate the material in high temperature. Cool the resultant vapors to nucleate particles and produce monodisperse nano-particles (10-40 nm) Organic molecules Electrospray technique Spark generator and flame generator Using an electric discharge soot particles can be formed A flame generator is often used to produce combustion nanoparticles
Particle generation Sub-micron particles: Nebulizer Generates polydisperse particles in the 20-700 nm range Schematic diagram of the nebulizer (from Hinds, 1999)
Particle generation Large particle generators ( D p > 1 μm) Liquid particles Vibrating orifice aerosol generator (Berglund and Liu, 1973) Can be used to generate largely monodisperse particles Commercialized by TSI Inc. Solid particles Dust generators Fluidized bed (TSI) Powder dispenser (BGI, Topas-Gmbh) Carbon particle by spark generator (Palas)
Particle generation Two-component Fluidized-Bed generator (Marple, Liu and Rubow, 1978). Vibrating orifice aerosol generator (Berglund and Liu, 1973).
Spark Generation Can generate a range of particles Commonly Carbon Peak sizes in the nanometer range High number concentration Steady generation Roth et al., 2004
Particle generation Monodisperse particles: For sub-micron particles, typically nebulizers (atomizers) are used to generate particles in the laboratory polydisperse particles For monodisperse particles, these particles are passed through a differential mobility analyzer (DMA) to obtain particles of a selected size
Particle Generation Sheath air in HV supply 20-10,000V Differential Mobility analyzer (DMA) Sizes particles by their electrical mobility Usually very high resolution measurements are possible Downstream particle counter is required for particle size distribution measurements Usually a CNC Due to the high accuracy of this instrument it is a standard aerosol instrumen TSI 3080 DMA (Knutson and Whitby, 1975) Polydisperse aerosol in Trajectories of particles below the selected size Trajectories of particles corresponding to the selected size Trajectories of particles larger than the selected size Excess air out Monodisperse aerosol out
Particle counting Condensation nucleus counter Particles are grown by condensation Usually a high vapor pressure liquid like Butanol is used Particles are counted as they pass through a light scattering region Popular instrument to measure total aerosol concentration Can count particles of sizes > 2nm Upper limit is dependent on particle transport through the instrument
CNC (TSI; Agarwal and Sem, 1980) Aerosol Inlet (1.0 lpm) Cooled condenser Natural convection heat sink Alcohol reservoir
References Agarwal, J. K., & Sem, G. J. (1980). Continuous Fow single-particlecounting condensation nuclei counter. Journal of Aerosol Science, 11, 343 357. Berglund, R.N., Liu, B.Y.H., 1973. Generation of monodisperse aerosol standards. Environmental Science and Technology 7 (2), 147-153. Hinds, Aerosol Technology, Wiley-Interscience, 1999 Knutson, E. O., & Whitby, K. T. (1975). Aerosol classi$cation by electric mobility: Apparatus, theory, and applications. Journal of Aerosol Science, 6, 443 451. Marple, V.A.. B.Y.H. Liu and K.L. Rubow, "A Dust Generator for Laboratory Use," Am. Ind. Hyg. Assoc. J. 39:26-32, 1978. Roth C.A, et al., Generation of Ultrafine Particles by Spark Discharging, Aerosol Science and Technology, 38: 228 235, 2004.