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Turbulent mixing of oil droplets in a round water jet

Turbulent mixing of oil droplets in a round water jet

10510095z

10510095z [mm]90858075−30 −20 −10 0 10 20 30x [mm]Figure 4.3: The mean coflow field over 200 images at z = 75 - 105 mm. Φ co = 60 ml/s.4.1.4 Single-phase jet experimentBefore performing measurements with oil droplets inside the jet, two experiments are performedto test the PIV set-up. First, only seeding is added to the coflow while a clean jet is generated bythe micro annular gear pump (Re jet =1000). The measurement position is z = 115 - 145 mm abovethe jet nozzle. The seeding is assumed to be mixed in the jet fluid by entrainment. The resultsof this measurements are described in appendix A.1. A bias towards areas with lower velocitiesmight be expected as the high velocity jet is not seeded. However, the response of the oil dropletsto fluid accelerations is computed to be fast enough to follow the fluid motion and the (seeded)entrained flow is much larger than the jet flow rate. Moreover, the measurements are in goodagreement with the second test experiment in which the same amount of seeding is added to thecoflow and the jet (see appendix A.2). The results of both experiments agree with those of Aanen.However, in the measured region the jet seems to be not truly self-similar anymore: the virtualorigin lies about 40 jet diameters below the jet nozzle while the spreading is 30% lower than inthe experiments of Aanen. Probably, these effects are caused by the side walls. Therefore, themeasurement position should be chosen closer to the jet nozzle. However, as the exposure timemust be larger than 1.5 ms to record enough scattered light, the nearest position for which the oildroplets are not imaged as streaks is 75 mm above the jet nozzle.4.2 Velocity fieldTo minimise the effects of side walls, the measurement position is lowered to z = 75 - 105 mmabove the jet nozzle. A two-phase jet is generated as described in section 3.1.2. A typical image ofthe light scattered by the oil droplets as acquired by CamWare, is depicted in figure 4.4 (exposuretime = 1.5 ms).It is quite difficult to maintain a highly concentrated oil-water jet, as the oil droplets tend tocoalesce and rise out the initial jet mixture. Therefore, it is important to stir the mixture continuouslywhen performing the experiments. The disadvantage of this method is that it still mightchange the droplet size distribution.After 200 recordings, the computer memory is full. While the data are written do disk, a new34

50100150z [pixels]200250300350400450500100 200 300 400 500 600 700 800 900 1000x [pixels]Figure 4.4: A typical greyvalue image of an oil-water jet as recorded with CamWare.oil-water mixture is generated. In total, 5 sequential experiments are performed. The oil-wateremulsion is created by injecting the mixed fluids in 200 ml water during 5 seconds at 0.25 ml/s ofoil and 0.50 ml/s of water. The initial concentration of oil droplets is consequently around 1:150.This is larger than the 1:500 mentioned above. However, quite a significant amount of oil hasformed a liquid film on the water before the measurements are started.A typical velocity vector field as calculated by DaVis is depicted in figure 4.5.10510095z [mm]90858075−30 −20 −10 0 10 20 30(x−x0) [mm]Figure 4.5: A typical velocity vector field of an oil-water jet as calculated by DaVis.Outliers are removed and replaced by interpolating the surrounding vectors (see section 3.3.4).35

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