Bubble Generation
The Bubble Generation Cycle
The Bubble Generator produces bubbles by mixing air and liquid (Surfactant Solution) flow at a screen. A blower moves the air into the bubble generator housing where it can only exit through the bubble screen. The liquid is sprayed on the screen by a suitable BG Nozzles positioned within the bubble generator housing. The Liquid Pump must supply the pressure and flow necessary to drive the nozzle and properly atomize the liquid into fine droplets that are sprayed evenly on the inner surface of the bubble screen.
It is important to have a uniform distribution of air pressure and flow over the entire surface area of the screen. Also, the liquid spray must wet the entire screen area evenly and completely, without producing areas that are too wet or too dry. The variations of the quantity of liquid that is wetting the screen will cause variations in the airflow that are counter productive, since dry areas of the screen will have low resistance to the passage of air and no bubbles will be made, while the wet areas become even wetter by lack of airflow since the air is escaping through the dry zones. The air from the blower (or fan in some cases) should be non-turbulent and have an even laminar flow, forcing through the wet screen and thereby blowing the bubbles. There must be no gaps in the screen or around the edges or any way for 'free' air to exit from the bubble generator with the bubbles. Large pockets of air present within the bubble flow will indicate these faults. Such air pockets will prevent the cavity space from filling completely with bubbles.
I have had the best results with open cell porous plastic foam used in sheets about 10 mm thicknesses. This material has little physical strength and should be sandwiched between layers of plastic or metal mesh. The housing should have outward turning flanges to mount the screen so that excess liquid in the housing can flow out by gravity along the lower edge of the screen. This drainage causes no pressure loss. Holes or openings of any kind in the screen housing should be avoided. The back of the belled housing has a duct that is fitted to the outlet of the blower. The housing is flared out from here to match the size and shape of the bubble screen. It is practical and economical to use sheet metal such as aluminum, painted steel or stainless steel, but a completely corrosion proof construction will assure a long, trouble free life for this equipment.
It is important that the bubbles produced from the screen cannot short circuit around the bubble generator housing and get back to the inlet side of the machine. The bubbles produced must flow all the way around the flow-path. Nor should any of the bubbles flowing into one side of the roof cavity be able to get through gaps in the partition and short circuit the flow-path. The bubbles will fill along the first sloping roof all the way to the opposite endwall where the last air will vent through the bubble generator housing at that end—unless the opposite generator is also blowing bubbles, in which case the air is pushed through the wet screen by the operation of the blower or fan. If no bubbles are being produced by this generator the air can easily vent through the dry screen over to the other side of the ridge.
As the cavity space commences to be filled with bubbles, they will fall into and fill the lower portions of the endwalls and sidewalls. The bubbles entering these areas of the cavity space will become stationary while the bubbles in the roof level will continue to move as a mass. A blower with a good air volume will make bubbles at such a rate that the entire roof zone has flowing bubbles. A bubble generator with less air/bubble flow rate will tend to move only a smaller cross-section of the roof area channeling along the upper portion of the roof.
The rate of flow depends upon the specifications of the blower component and the nozzle is selected to give the proper ratio of liquid to air. Our first system for the Tunnel should produce 40 M3 of bubbles per minute and at 400 times expansion that would require about 100 liters of liquid per minute on the screen. This rate of flow of bubbles into the roof cavity (and associated wall cavity spaces) would fill about 8 linear meters of the flow path per minute. At this rate the 20 meter long by 6 meter wide house would fill in about 6 minutes. In practice it may take about 10 minutes for the bubble generating cycle to be complete. By that time the bubbles will have filled in the entire cavity and the blower inlet will begin to take in bubbles rather then air. Placing a section of bug-screen material at the inlet and extending this cover like a snorkel, to reach the last pocket of air at the inlet side of the roof cavity will help to fill the bubbles right to the peak of the roof.
What we have described above is the ON-CYCLE for the bubble generator. The pump and the blower operate together during the entire ON-CYCLE, but it is important to give the pump an advance ON and a delayed OFF. This lets the liquid reach and wet the bubble screen before the blower begins operation. Therefore only bubbles are produced. At the end of the bubble generating cycle it takes some moments for the blower to stop rotating after it is turned off. It is important that the screen is still wet during this period or air may be blown into the roof. When the pump has a delayed OFF the screen remains wet during this shut down interval and only bubbles will produced. The advance and delay on this equipment should be set according to the actual requirement of this equipment. If the pump is some distance from the bubble generators then it may need a longer advance ON. A foot valve will help to reduce the time needed and may be required in any case to keep the pump primed.
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