Seguro en la Edad Media: (Siglos X al XIII)

The experiments suggest that, over the ranges of air velocity and particle sizes tested, particles deposit with increasing particle size. This is consistent with the

simulation work in Chapter 2. In most cases, the model underpredicted the experimental deposition results, although the overall agreement between the modeled and measured results is reasonable. In particular, there is good agreement between the shape of the simulated and measured deposition fraction curves.

The model tends to show increasing underprediction of measured deposition with increasing velocity. The highest velocity experiments (Figure 3.9) show that the steep increase in deposition occurs at a smaller particle diameter than the simulations predict. This raises some questions about the modeling of inertial mechanisms. One possibility is that air turbulence leads to deposition of much smaller particles than the model predicts. Given the importance of impaction on tubes as a removal mechanism for larger particles at higher velocities, a deficiency with the modeling of this mechanism is a possibility. The wake from upstream tubes could lead to increased deposition on downstream aligned tubes.

Another question that the isothermal experiments can illuminate is whether the division between deposition mechanisms in Chapter 2 is valid. Over the range of 1 – 7 µm in particle diameter, deposition by fin impaction is the dominant deposition

mechanism in the simulations. Experimental extractions of the leading edge (and the first 5 mm of the fin channels) suggest that 30 - 75% of the particles that deposit do so on this part of the heat exchanger. Although these fractions have large uncertainty associated with them (and a bias towards undermeasurement because of leakage of buffer out of the coil during extraction causing less than the first 5 mm to be extracted), they suggest that all of the discrepancy is caused by deposition in the fin channels, not on the leading edge. A likely source of this error is the macroscale surface roughness elements and fin

discontinuities that occur in real heat exchangers. These irregularities occur for ease of manufacturing and to promote air turbulence and improved heat transfer. Figure 3.12 provides a depiction.

Figure 3.12: Top view of idealized (left) and real (right) fin channels.

The test heat exchanger had 14 discontinuities in each fin. There is likely additional inertial impaction on the leading edge of each discontinuity, although

modeling this phenomenon would be very difficult because of the difficulty in simulating boundary layer growth on complex surfaces.

The cooling of the heat-exchanger surfaces led to an increase in deposition by an amount that was higher in the experimental data than predicted by the simulations. The underprediction of deposition (by about 4 percentage points) can only partially be explained by the discontinuities discussed above. One possibility for the additional difference is that the model for thermophoretic penetration was assumed to hold across the whole fin channel. A two-layer model that divides transport of particles to the edge of the boundary layer and then the existing model used to describe deposition of particles across the boundary layer might result in better predictions of the thermophoretic

deposition. Such a model is beyond the scope of this dissertation, and would be

challenging to implement for real fin surfaces, but could result in better predictions. The inclusion of a solution to the energy equation would also allow a better understanding of

temperature gradients and could also decrease the modeled-measured discrepancy for non-isothermal systems.

The presence of condensed water led to a dramatic increase in the deposition fraction. This effect was not predicted particularly well with the model. In absolute terms, a 4 – 12 percentage point underprediction is observed between the modeled and the measured data for each of the tests. The experiment with the smallest discrepancy (on the order of the discrepancy associated with the cooling-only experiments described above) had a much smaller average water layer thickness. This suggests the possibility of a systematic problem with the model. One possibility is that the definition of an average water layer thickness is not valid. If the water is not distributed evenly over all surfaces, then the modeled increases in fin thickness and tube diameter are not accurate. The refrigerant tubes are the coldest part of the heat exchanger, so more condensation might occur on them. This would lead to the model further underpredicting deposition fraction because the fractional change in tube diameter is much smaller than the change in fin thickness, and the increase in fin thickness affects more deposition mechanisms. However, water drains off tubes differently than it drains off fins, particularly real heat exchanger fins (as opposed to idealized ones). Another future area of research is an investigation of the nature of condensation on the heat-exchanger surfaces. If beading, or other non-uniform water accumulation, occurs on different parts of the heat exchanger, this would change the estimates of deposition fraction. Meanwhile, the model should be considered to underestimate the actual deposition that results for condensation,

The experiment to measure pressure drop as a function of deposited material showed a progressive increase in pressure drop with deposited material over a more than doubling of the clean heat exchanger pressure drop. This is different than results

suggested by other fouling researchers (i.e. Bott, 1983; Epstein, 1988) who suggest an asymptotic fouling rate and that the relative pressure drop should level off. There are two possibilities to account for this discrepancy, the first is that the heat exchanger really doesn’t foul asymptotically. The second is that had it been possible to keep fouling the heat exchanger, the exponential curve would have developed an “S” shape and begun to asymptote. One mechanism for this to occur is through sloughing of deposited material as the heat exchanger becomes loaded. The high rate and short duration of loading might have artificially limited this loss of material. However, Bott and Bemrose (1983)

investigated this theory for a circular fin-and-tube heat exchanger and found that the same asymptotic pressure drop was reached regardless of the duration of fouling and the air concentration of fouling agent. Although academically interesting, this argument is not useful to HVAC heat exchangers: Parker et al. (1997) suggest that HVAC evaporator coils should be cleaned when the increase in pressure drop reaches 10-15% of the clean pressure drop, a much lower value than the more than doubling reported here.

The overall conclusions are that the experiments described here offer the first available particle and air velocity resolved measurements of deposition in HVAC heat exchangers. These results will be applied to typical HVAC systems to explore indoor air quality and energy consequences in Chapters 4 and 5.

CHAPTER 4: BIOAEROSOL DEPOSITION ON HVAC HEAT