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In this section the results investigating three parameters, C0, Qt and Q1/Qt are investigated. Table 6.2 summarises the operating conditions used during each set of experiments, for example, experiment 1 lists the series ofC0values used in separate experimental runs and also the parameter values,QtandQ1/Qtmaintained at a constant value. For each set of operating conditions chosen the voltage,Vin, is also varied in steps of∼100mVpkpk, adding another parameter dimension to

Table 6.2: Operating conditions used for experimental work.

Parameter Experiment 1 Experiment 2 Experiment 3

1. 2.2e8

C0 (/ml) 2. 1.5e8 2.2e8 2.2e8

3. 9.5e7 1. 0.005 Qt(ml/s) 0.005 2. 0.007 0.005 3. 0.010 1. 0.25 Q1/Qt 0.25 0.25 2. 0.35 3. 0.45

the experimental work and investigating the majority of operating conditions represented in figure 6.5.

The following subsections present the experimentally measured and corresponding predicted data for each experiment (calculated using the particle model developed in chapter 5), accompanied by a description of the main trends observed in the data. Any explanations are reserved for the following discussion, section 6.2.4.

Experiment 1 - Particle Concentration,C0

This experiment records the influence of the inlet particle concentration,C0using three different concentration values for each run and repeated over a range of voltage levels. Figure 6.7 records data collected for several experimental runs and plots the relative concentration of fluid collected from outlet 1 and outlet 2. The relative concentration gives a simple indication of an increase or decrease in concentration, for example, atVin= 0 the relative concentration is 1 in all cases and informs us that the outlet concentrations are equal to the inlet concentration. At higher voltages, however, the relative concentration of outlet 1 fluid is lower and indicates reduction in concentra- tion, where 0.5 would represent a two-fold reduction.

Looking at the trends shown by the experimental data in figure 6.7, it can be seen that as the voltage increases, fewer particles are drawn through outlet 1, allowing the majority of particles to pass through outlet 2. This trend is expected as an increase in voltage corresponds to increases in acoustic pressure and radiation force experienced by the particles, therefore causing a greater

number of them to reach the acoustic node and be drawn off through outlet 2. The increase of concentration through outlet 2 is limited as Q2/Qt is relatively high and the particles are still highly diluted. The theoretical maximum relative concentration through outlet 2 in the case of Q1/Qt= 0.25and where all the particles pass through that outlet is 1.33.

Considering the effect of inlet concentration and the data presented by the different curves, the higher particle concentrate shows marginal improvement in separation, i.e. lower outlet 1 concen- tration and higher outlet 2 concentration. However, a clear difference in trend betweenC0= 1.5e8 and 9.5e7 is not shown, therefore it is reasonable to assume that discrepancies can be attributed to experimental error.

Assuming the data in figure 6.7 to indicate experimental error rather than a distinctive effect of the inlet concentration, figure 6.8 is based on the same data, but plots the mean values and associated error. Also shown in the figure is the predicted concentration (dotted line) which, although it

compares well up toVin = 150mVpkpk, predicts more marked separation as voltage increases.

In both data sets the degree of separation levels off at the higher voltage values. The discussion section found later in this chapter aims to explain these observations.

Experiment 2 - Total Flow Rate,Qt

The total flow rateQtis investigated and concentration results are shown in figure 6.9 for a range of flow rates. The figure adopts themeanexperimental errors for clarified and concentrated flow respectively, estimated in the preceding experiment.

The most significant influence that the flow rate has on the results is the reduction of separation at increasingly high flow rates. For example, the concentration through outlet 1 increases as flow rate increases, presumably as fewer particles reach the acoustic node to pass through outlet 2. A corresponding reduction in concentration is seen in the measured results of outlet 2.

It is also noted from these results that the concentration through each outlet (and therefore the de- gree of separation) levels off at different concentration values at high voltages, and even degrades slightly at 450mVpkpk where a small reduction in separation is recorded (the formation of bubbles due to thermal effects was occasionally observed at these higher voltage levels). In figure 6.10 the predicted concentration values are shown which indicate that, theoretically, the separation levels off to the same value irrespective of the flow rate, the flow rate influencing only the minimum voltage at which that level is reached, differing significantly from the measured data.

Figure 6.7: Influence of inlet sample particle concentration,C0, and input voltage,Vin, on particle separation. Experiment operated usingQt= 0.005ml/s andQ1/Qt= 0.25.

Figure 6.8: Influence of input voltage,Vin, on particle separation based on both experimental and predicted data. Experiment operated usingQt= 0.005ml/s andQ1/Qt= 0.25.

Figure 6.9: Influence of total flow rate,Vt, and input voltage,Vin, on particle separation. Experi- ment operated usingC0= 2.2e8 particles/ml andQ1/Qt= 0.25.

In general, the modelled data does predict increasingly poor separation for an increase in flow rate as for the experimental data. However, the model slightly underestimates the separation up to 170mVpkpk, reflected in both the outlets. Above this voltage the predicted data begins to overestimate the separation, continuing while concentrations values level off at the higher voltages.

Experiment 3 - Outlet Flow Proportions,Q1/Qt

The final experiment investigates the influence of the outlet flow proportions. Experimental data recorded for a range of outlet flow rates is shown in figure 6.11 for outlet 1 flow ranging from 25 to 45% of the total flow rate,Qt, which is held constant. Again, data is recorded for a range of voltages.

Figure 6.10: Influence of total flow rate, Vt, on particle separation based on both experimental (black) and predicted (red) data. Experiment operated usingC0= 2.2e8 particles/ml andQ1/Qt= 0.25.

that outlet also increases, and even exceeds the concentration through outlet 2 atQ1/Qt = 0.45. It would appear that as more fluid is drawn through outlet 1, a greater proportion of the parti- cle stream is drawn through the outlet, increasing concentration through outlet 1 and decreasing concentration through outlet 2 and significantly influencing the separation effect.

At high voltages there does not appear to be a small decrease in the separation effect as observed in the previous experiment where at 450mVpkpk outlet 1 concentration increased slightly.

Figure 6.12 compares the experimental data to modelled predictions. As seen in the experimental results, the model predicts that an increase in outlet 1 flow will have a detrimental effect on sepa- ration at the lower voltages, although it inaccurately predicts that good separation will be achieved at higher voltages as it assumes that a densely packed particle stream is eventually formed and passes through outlet 2.

With the vast majority of particles passing through outlet 2 (at high voltages), the model suggests that a greater outlet 2 concentration can be achieved by increasingQ1 and reducingQ2. This is because the particle stream will be less dilute as it passes through outlet 2 and within a smaller volume of water. A corresponding, although smaller, change is seen in the outlet 1 concentration.

Generally the modelled results differ significantly from the experimental data at the higher voltages where the model does not predict the movement of the particle stream switching from outlet 2 to outlet 1.