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

Introduction: Spray Pyrolysis MUSSL

Crust Formation MUSSL

Zirconia Production Zirconium nitrate  ZrO(NO 3 ) 2.xH 2 O ZrO 2 + NO 2 +H 2 O  Decomposition temperature: C Zirconium chloride  ZrOCl 2.8H 2 O ZrO 2 + HCL + H 2 O  Decomposition temperature: C Zirconium acetate  Zr(CH3COO) 4 +H 2 O ZrO 2 + CO2 + HCL Decomposition temperature: C MUSSL

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

Evaporation in low pressures  Continuum assumption is no longer valid when the pressure is relatively low.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

Objectives  Experimental investigation on the effect of operating conditions (Chamber P, T, φ, and droplet D and C in ) on the morphology of nanopowders of ZrO 2.  Experimental investigation on the single droplet evaporation in low pressures. MUSSL

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

Droplet Evaporation Characteristics Evaporation TimeTerminal VelocityEvaporation Length WaterMethanolPentaneWaterMethanolPentaneWaterMethanolPentane 30 micron 0.35 s0.056 s0.011 s0.021 m/s m/s0.013 m/s0.73 cm0.095 cm0.014 cm 200 micron 8 s1.38 s0.28 s0.93 m/s 0.74 m/s0.59 m/s700 cm102 cm18 cm 300 micron 13 s2.22 s0.47 s1.95 m/s 1.6 m/s1.33 m/s2500 cm 350 cm60 cm 400 micron 17 s2.95 s0.63 s3.15 m/s 2.6 m/s2.3 s5300 cm 7670 cm150 cm MUSSL

Evaporation of Pentane Droplets: Effect of Pressure Pressure (kPa) KnRe D U terminal Evaporation Time (symmetric) Evaporation Time Convective Evaporation Time Kinetic theory T of ambient=400 K, T of droplet=300 K, Humidity=0, Droplet initial diameter= 200 μm MUSSL

Experimental Set-up MUSSL

Vacuum System MUSSL

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

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

Droplet Generator Piezoelectric droplet generator Pneumatic droplet generator MUSSL

Pneumatic droplet generator  Air flow rate  Air pressure  Pulse width  Liquid level  Liquid properties  Orifice size MUSSL

Droplet Generator Operation t=10 x t=25 x t=40 x t=55 x t=70 x t=85 x t=100 x t=115 x t=130 x 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

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

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

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

Heating Elements  Four 1800 Watts Convective Heaters  Maximum Surface Temperature: C MUSSL

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

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

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

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

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

Concentration Equation in the Droplet )1( 8 Re)]0([, 2/1,           msl ll S ml m Y D DfkY f    MUSSL

Algorithm At any given time instant with known droplet surface temperature Ts and solvent phase species mass fraction Y ls,, the gas phase species mass fractions at the droplet surface Y gs 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