Viabilidad de la implementación de la propuesta del modelo integral estratégico en

Deposition fraction vs. aerodynamic diameter curves for three velocities appear in Figures 3.7 – 3.9. The horizontal error bars show one standard deviation in particle aerodynamic diameter (an indication of how monodisperse the particles produced by the vibrating orifice were). The vertical error bars, which are difficult to see for some

smaller particles and lower velocities, reflect the results of a propagation-of-error analysis on the deposition fraction using the uncertainties described in Table 3.4. The uncertainty of the deposition fraction results for 5.1 m/s (Figure 3.9) are larger than for the other two velocities. This is a consequence of the resuspension of deposited particles from previous experiments. A blank experiment (no particle generation) was completed twice at all velocities and the resulting fluorescence signal (which only occurred for 5.1 m/s) was used to evaluate the contribution to uncertainty of resuspended particles. Two sets of experiments were repeated three times for 3 µm particles at 1.5 m/s and 5.5 µm particles at 5.1 m/s to test the validity of the uncertainty analysis. These repeated experiments

show an overlap of the vertical error bars from each repeated experiment and suggest good agreement between the predicted and measured uncertainty.

The results of the simulations described in Chapter 2 also appear on each plot.

Aerodynamic Diameter (µm) 1 2 3 4 5 6 7 8 10 15 20 De p osi tion F rac ti on (% ) 0 10 20 30 40 Experiment Model

Figure 3.7: Modeled and measured deposition for 1.5 m/s air velocity.

The general trend is increasing deposition with increasing diameter, with the exception of 5 – 6 µm particles at 1.5 m/s. There is also increasing deposition with increasing velocity for most particle sizes. There is very good agreement with the shape of the model, and reasonable agreement, on average, between the modeled and measured data. The modeled-measured agreement diminishes with increasing air velocity. The simulations, with a few exceptions at 5 – 6 µm at 1.5 m/s air velocity and 2 – 3 µm at 5.1 m/s air velocity, tend to underpredict the measured deposition fractions.

Aerodynamic Diameter (µm) 1 2 3 4 5 6 7 8 10 15 20 Deposition F rac tion (% ) 0 10 20 30 40 Experiment Model

Figure 3.8: Modeled and measured deposition for 2.2 m/s air velocity.

Aerodynamic Diameter (µm) 1 2 3 4 5 6 7 8 10 15 20 Deposition F rac tion (% ) 0 10 20 30 40 Experiment Model

Figure 3.9: Modeled and measured deposition for 5.2 m/s air velocity.

The difference in deposition fractions calculated with Equation (3.11) and Equation (3.13) was negligible (typically less than 5%) for particles up to 2.5 µm in diameter, significant (10 – 20%) for particles between 2.5 µm and 7.5 µm, and typically greater than 40% for particles greater than 7.5 µm. Equation (3.11) always predicted greater deposition, which is consistent with particles depositing by gravity and in the

separation region downstream of test heat exchanger. All results reported here are

calculated with Equation (3.13) with the exception of two points (3 µm at 1.5 m/s and 2.2 µm at 5.1 m/s) because of problems with the coil extraction for those experiments. The deposition fraction for those two cases was calculated with Equation (3.11) with a

correction based on the difference in the results of Equations (3.11) and (3.13) for similar particle sizes and velocities.

The results of three experiments are not reported. They were done with large particles injected 28 diameters (4.3 m) upstream of the test coil (the motivation was to limit gravitational settling in the duct of large particles at the lower velocities). These results, based on a single upstream centerline air concentration measurement, suggested very high deposition fractions, which suggested the possibility of non-uniform mixing of particles. To investigate, all subsequent experiments were performed with five filter samplers upstream of the heat exchanger as depicted in Figure 3.3. Injecting the particles so close to the heat exchanger did not allow for uniform mixing of particles: the variation between air concentrations across the duct cross section ranged from 5 to 30%.

Subsequent experiments with particles injected at the mixing box showed uniform mixing for particles less than 5 µm and slight variability for larger particles (< 5% difference between measurement points on the cross section).

The coil was extracted in two sections. The first 5 mm closest to the leading edge of the fins was extracted separately from the rest of the heat exchanger. The fraction of particles that deposited on or near the leading edge of the heat exchanger varied

considerably (from 30 to 75 %) between experiments. The fraction of particles that deposited near the leading edge for each experiment is listed in Table B.2 in Appendix B.

This measurement is imprecise because the gasket that was used when sealing the coil would compress by slightly different amounts, which would change the fraction of coil that was extracted in the initial measurements. Also, although every attempt was made to keep the heat exchanger level during extractions, this was not always possible which led to additional inaccuracies. The model predicts that more than 80 - 90% of the total deposition should occur on the leading edge for particles smaller than 8 µm in diameter (because fin edge impaction is the dominant deposition mechanism). This discrepancy suggests that most of the discrepancy between modeled and measured deposition fraction is because of additional deposition in the core of the heat exchanger.