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One of the contributing factors to air entrainment at the beginning of the water treatment process in AguaClara plants is the waterfall effect inside the Linear Flow Orifice Meter (LFOM). This effect is composed of two basic entrainment mechanisms: 1) the surface of the jet of water becomes sufficiently turbulent to entrain air before it reaches the surface water; and 2) the penetrating jet creates a void from where which small air bubble bubbles pinch off. To minimize the effect of these entrainment mechanisms the following retrofit design options were calculated and tested in lab:
1) Pipe Inside LFOM: A pipe of a small diameter would be placed within LFOM with about a few inches of separation between the outer surface are of the pipe and the inner surface area of the LFOM.
2) Vertical/Inclined Plane: A verticle vertical or inclined plane would be placed within the LFOM that would prevent the water jets from hitting the surface of the water directly.
3) Bucket with Holes: A bucket would be placed inside of the LFOM that would catch the water jets and slow down the inflow of water by using an orifice.
4) Bucket with out without Holes: Rather than have an orifice slow down the flow of water in the LFOM, the bucket would be over flooded so that the water dribbles down the side of the bucket.
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In addition to these designs, we also tested for the three limiting parameters of foam formation from water jets:.
Figure: setup Setup testing parameters for foam formation
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Three parameters were investigated to study foam formation,:
1) Average velocity of jets
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3) Length the water travels down the incline plane
The velocity of the jets in our experiment was increased by increasing the number of pump (heads?) used during the experimentflow coming from the pump, which substituted for increasing the head in the actual plants. In the first run, the jets fall onto the accumulated water on the tank, then a plane was used to catch the water jets upon before falling to into the tank. The plane was placed at different distance distances and angle angles to observe the change in bubble formation.
Calculations
As a team, we worked in MathCAD to calculate the distance that the jets of water coming into the LFOM would travel inside the LFOM. If we had discovered that a bucket inserted into the LFOM below the orifices can could catch the jets before they hit the water, the bucket theory will would be a viable option. We determined that all but the top three jets of water would in fact hit the far wall of the LFOM before reaching any size bucket that we could place in the LFOM. Also, we used MathCAD to determine the size of the orifice needed in the bottom of the bucket to maintain the plant flow rate. We determined that the orifice would have to have a 4.5cm radius, which was far too large for our buckets. The MathCAD calculations eliminated both the teacup theory (what is the teacup theory?) and the variation on that bucket theory and the bucket with a hole theory as possible solutions. The MathCAD files are attached.
(Please document equations used and summarize results obtained on the wiki as well)
Jet Distance Calcs 7-13-09.xmcd
CDC_Retrofit_Designs.xmcd
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The experiment affirmed that the velocity of the jet, its length, and the type of surface it impacts are all limiting parameters. By observing the water jet at different flow rates it was observed determined that the length of the jet is dependent on the velocity of the jet. At low flow rates the length of the jet is shorter than at higher flow rates. As seen in Figure 1, the length of the jet refers to the part of the jet where the water surface is turbulence-free and thus appears smooth and transparent. It is when the surface of the jet becomes sufficiently turbulent that air is entrained as the symmetry of the jet breaks during free fall. When a jet hits the water surface a void forms in the water and from the tip of this void an air bubble is pinched off as demonstrated in the attached scientific paper: The entrainment of air by water jet impinging on a free surface. Therefore, when the water jet hit a solid smooth surface there was no foam formation.
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