bubble breakthrough
On earth and in space alike, the quality of combustion depends on the quality of fuel. Bubbles that can enter the fuel for example by sloshing would significantly diminish this quality and must therefore be avoided. In space applications this is achieved by the use of filter screens as shown in Fig. 1.
Figure 1: Dutch Twilled weave with 200x1400 wires per inch and its position in our experimental setup
Such screens are woven with fine stainless steel wire which makes them flexible, highly resilient towards mechanical and thermal stresses and lightweight. Besides, the fuel ‘sticks’ to the steel weave due to its high surface energy and thus perfectly wets it. As a consequence, the fuel prevents air bubbles from passing through the weave – at least up to a critical pressure difference, the so-called bubble point. If the bubble point is reached or even trespassed, bubble breakthrough occurs (Fig. 2).
Figure 2: Breakthrough of air through the pores of the weave beyond the bubble point. Left side: view by the naked eye, right side: higher time resolution
Our experimental arrangement allows for upward and downward fluid flow through the weave.
In our upward flow experiments we measure the pressure loss over the weave as a test fluid passes through at variable flow rate. Air bubbles that are inducted into the flow become trapped at the screen and coalesce to one, see Fig. 3. Its presence reduces both the cross section for the flow and therefore the critical flow rate at which the bubble point occurs.
Figure 3: Trapped bubble below the weave
As the bubble grows in size (caused by further small bubbles that coalesce) the cross section is decreased until it is eventually blocked. We are interested in the static bubble point of the weave (no flow) which is a characteristic feature of filter weaves as well as its variable dynamic bubble point (flow present).
In our downward flow experiments we observe the behaviour of a trapped bubble below the weave which now can no longer block the flow. Under these conditions the bubble can behave very interesting, as the beauty of some observed patterns illustrates, see Fig. 4.
Figure 4: Observed patterns at a trapped bubble in downward flow. Left side: formation of a billowing bubble torus with satellite bubbles in the center, right side: a flower-like pattern with five petals and an eye.
The results of this study contribute to the understanding and reliability of fluid handling in space where porous screen applications are widespread.
The funding of the research project by the German Federal Ministry of Economics and Technology (BMWi) through the German Aerospace Center (DLR) under grant number 50 RL 0741 is gratefully acknowledged.
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