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Evaporation of Solution Droplets in Low Pressures, for Nanopowder Production by Spray Pyrolysis

Evaporation of Solution Droplets in Low Pressures, for Nanopowder Production by Spray Pyrolysis. August 2004 MUSSL. Outline. Introduction Objective Experimental set-up Future Work Theoretical Model Timetable. MUSSL. hollow particle. solution. droplets.

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Evaporation of Solution Droplets in Low Pressures, for Nanopowder Production by Spray Pyrolysis

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  1. Evaporation of Solution Droplets in Low Pressures, for Nanopowder Production by Spray Pyrolysis August 2004 MUSSL

  2. Outline • Introduction • Objective • Experimental set-up • Future Work • Theoretical Model • Timetable MUSSL

  3. hollow particle solution droplets droplets solid particle decomposition drying precipitation evaporation atomization Introduction: Spray Pyrolysis a salt + a solvent MUSSL

  4. CSS T4 T3 ES concentration(mol/l) T2 T1 T1>T2>T3>T4 0 1 r/R Crust Formation MUSSL

  5. Zirconia Production Zirconium nitrate • ZrO(NO3)2.xH2O ZrO2 + NO2+H2O • Decomposition temperature: 270 0C Zirconium chloride • ZrOCl2.8H2O ZrO2 + HCL + H2O • Decomposition temperature: 380 0C Zirconium acetate • Zr(CH3COO)4 +H2OZrO2 + CO2 + HCL Decomposition temperature: 320 0C MUSSL

  6. Modeling Nanopowder Production • Nanopowder production in the atmospheric pressure occurs in the Transition Regime: Kn~1 Actual case P=101 kPa d=100 nm Kn=1.8 Modeled case P=0.05 kPa d=200,000 nm Kn=1.8 MUSSL

  7. Evaporation in low pressures • Continuum assumption is no longer valid when the pressure is relatively low. • For low density gases in equilibrium the kinetic theory applies. • Nanopowder production occurs in the transition regime and in this region the Boltzmann equation should be solved for the velocity distribution. • Evaporation data of solution droplets for low pressures is very sparse in the literature. MUSSL

  8. Objectives • Experimental investigation on the effect of operating conditions (Chamber P, T, φ, and droplet D and Cin) on the morphology of nanopowders of ZrO2. • Experimental investigation on the single droplet evaporation in low pressures. MUSSL

  9. Important Issues • Chamber heating in low pressures • Adequate chamber height • Uniform droplet generation • Accurate imaging MUSSL

  10. Droplet Evaporation Characteristics MUSSL

  11. Evaporation of Pentane Droplets: Effect of Pressure T of ambient=400 K, T of droplet=300 K, Humidity=0, Droplet initial diameter= 200 μm MUSSL

  12. Liquid and power feedthrous Droplet generator Heaters Grooved plate View ports thermocouples Laser Source photodiode Data acquisition system Light Camera Support frame Powders To the vacuum pump Experimental Set-up MUSSL

  13. Vacuum System MUSSL

  14. Chamber Accessories • Thermocouple feedthroughs • Power feedthroughs • Liquid feedthroughs • Signal feedthroughs • Pressure gauge • Discharge Valve MUSSL

  15. Droplet Generator Requirements • Repeatable droplet generation (equal size) • Capable to operate in hot and low pressure environments • Easy to operate MUSSL

  16. Droplet Generator Piezoelectric droplet generator Pneumatic droplet generator MUSSL

  17. Pneumatic droplet generator • Air flow rate • Air pressure • Pulse width • Liquid level • Liquid properties • Orifice size MUSSL

  18. t=10 x 10-4 t=55 x 10-4 t=70 x 10-4 t=100 x 10-4 t=115 x 10-4 t=130 x 10-4 t=25 x 10-4 t=40 x 10-4 t=85 x 10-4 Droplet Generator Operation • Single Droplet Generation • Multiple Droplet Generation: A droplet with several satellites • Difficult to produce, but relatively repeatable • Droplets wander during their fall. To reduce droplet drift, a glass tube will be used around the flow path. MUSSL

  19. t=135 x 10-4 t=105 x 10-4 t=0 t=15 x 10-4 t=90 x 10-4 t=120 x 10-4 t=30 x 10-4 t=45 x 10-4 t=60 x 10-4 t=75 x 10-4 Droplet Generator Operation • Stream of droplets: Smaller droplets are produced, but not repeatable MUSSL

  20. Data Acquisition System • IEEE 488 GPIB Interface • Temperature module • Non-conditioning module • SCXI 1000 Chassis • LabView software: • Temperature measurement • Pulse generation • Trigger system • Pressure recording MUSSL

  21. Trigger System Photodiode: a semiconductor sensor Light Source: Laser DAQ Laser Camera MUSSL

  22. Heating Elements • Four 1800 Watts Convective Heaters • Maximum Surface Temperature: 325 0C MUSSL

  23. Imaging • FASTCAM-Ultima 1024 model 16K • 16000 fps • One camera will be moved to take several images at different locations MUSSL

  24. intensity 1 MICRON Future Work • XRD TEST • Reflection of x-ray beams from parallel atomic planes • Identifying crystalline phases • Crystallite size • TEM or SEM TEST • Examine microstructure • Identifying Hollow or dense particles MUSSL

  25. Theoretical Model • Inviscid free stream of gas outside its wake and flowing over the droplet • Gas-phase viscous boundary layer and near wake. • Core region within the droplet, that is rotational but nearly shear free and can be approximated as a Hill’s spherical vortex. MUSSL

  26. Gas Phase Analysis • Boundary Layer Equations of Momentum, Energy and Mass is applied to the boundary layer around the droplet. • For the stagnation point and the shoulder region (θ=π/2), where the pressure gradient is zero and the flow locally behaves like a flat-plate flow, local similarity is believed to be a very good approximation MUSSL

  27. Heat Transfer in the Droplet • With a certain coordinate transformation, the large Peclet number problem can be cast as a one-dimensional, unsteady problem (Tong and Sirignano). • In axisymmetric form of the energy equation, and in a large Peclet number situation, heat and mass transport within the droplet involve a strong convective transfer along the streamline with conduction primarily normal to the stream surface MUSSL

  28. ö ¶ - r 1 / 2 Y k [ f ( 0 )] Re D l , m ÷ = = - f ( Y 1 ) ÷ m l , ms ¶ f r 8 D ø l l S Concentration Equation in the Droplet MUSSL

  29. Algorithm • At any given time instant with known droplet surface temperature Ts and solvent phase species mass fraction Yls, ,the gas phase species mass fractions at the droplet surface Ygs can be obtained by means of Raoult’s and Clausius-Clapyron laws. • Therefore, boundary conditions of the gas phase equation will be determined. • From the solution of the gas phase, the boundary conditions of the liquid phase will be determined. • Enegy and concentration equations will be solved. The new droplet surface temperature and the new liquid phase mass fractions at the droplet surface are used for the gas phase solution for the next time step. • When the surface concentration reaches the critical super saturation (CSS), precipitation starts from the surface of the droplet • If at this moment, the concentration of the droplet center is higher than the equilibrium saturation (ES) of the solution, a solid particle will form, otherwise, the particle will be hollow. • This new model predicts that the dried particle will have two not necessarily spherical pores on account of the fluid circulation within the droplets MUSSL

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