Research Papers

Modeling Liquid Film Evaporation in a Wetted Wall Bioaerosol Sampling Cyclone

[+] Author and Article Information
J. A. Hubbard

e-mail: jahubba@sandia.gov

J. S. Haglund

The Applied Research Laboratories,
University of Texas, Austin;
Department of Mechanical Engineering,
University of Texas, Austin

1Present address: Sandia National Laboratories, Fire and Aerosol Sciences, P.O. Box 5800 MS1135, Albuquerque, NM 87185-1135.

2Corresponding author.

Manuscript received April 19, 2012; final manuscript received November 15, 2012; published online June 24, 2013. Assoc. Editor: S. A. Sherif.

J. Thermal Sci. Eng. Appl 5(3), 031007 (Jun 24, 2013) (10 pages) Paper No: TSEA-12-1056; doi: 10.1115/1.4023309 History: Received April 19, 2012; Revised November 15, 2012

The wetted wall bioaerosol sampling cyclone (WWC) is a complex multiphase flow device which collects and concentrates dilute bioaerosols into liquid samples for biological analysis (McFarland et al., 2009, “Wetted Wall Cyclones for Bioaerosol Sampling,” Aerosol Sci. Technol., 44(4), pp. 241–252). Understanding heat and mass transfer processes occurring inside the WWC is the key to enhancing its performance through an effective coupling to lab-on-chip analysis platforms which require small volumes of liquid output. There exists a critical liquid input rate below which there is no sample collection since all liquid is lost to evaporative effects. The purpose of this study was to model critical film evaporation based on first principles and develop semi-empirical WWC performance correlations as an improvement to existing empirical correlations. A one-dimensional, coupled heat and mass transfer model was developed approximating WWC multiphase flow as cocurrent air-film flow. Governing equations were simplified and approximate solutions were used to optimize model parameters like the heat transfer coefficient based on empirical data from previous works. Optimized model parameters were then used in the full numerical solution to calculate liquid evaporation rates within the WWC over the full range of relative humidity and air temperature. Semi-empirical correlations developed in this study were compared to existing empirical models and showed much improvement: proper physical behavior at the extreme limits of temperature and relative humidity was observed, and the nonlinear dependence of evaporative effects on environmental conditions was also captured.

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Fig. 1

(a) Picture of WWC-100 cast from stainless steel and (b) schematic of WWC internal multiphase flow

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Fig. 2

Schematic of cocurrent air-film flow coupled heat and mass transfer model

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Fig. 3

Characteristic adiabatic vapor mass fraction difference ( ) for various environmental air temperature (K) and relative humidity (%) conditions calculated from equilibrium conditions described by Eq. (22).

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Fig. 4

Dimensionless vapor mass fraction difference versus dimensionless axial distance as calculated with numerical solution (lines) and approximate solution (symbols) for (a) adiabatic wall condition for selected evaporator effectiveness and (b) constant film surface temperature

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Fig. 5

Semi-empirical model predictions of the critical film evaporation rate for WWC-100, WWC-400, and WWC-1250 over a broad range of environmental air conditions

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Fig. 6

Experimental measurements, empirical model predictions, and semi-empirical model predictions of the critical film evaporation rate in the WWC-100 over a broad range of environmental air conditions. The linear dependence of the empirical model [2] leads to error at the asymptotic limits (e.g., high relative humidity) outside the range of experimental data. The semi-empirical model is nonlinear and has the proper asymptotic limits.

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Fig. 7

Difference between empirical and semi-empirical model predictions of the critical film evaporation rate (μl min−1) in the WWC-100 over a broad range of environmental air conditions. Deviations from the zero plane demonstrate regions where the empirical model is inaccurate and WWC control can be improved with the semi-empirical evaporation model.




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