As engineers pursue higher performance in thermal-fluids systems, two-phase flows, which are extremely efficient at dissipating and transporting heat, are being used in increasingly complex geometries. This trend will likely increase as ever more compact heat sinks and heat exchangers are designed for these modern systems. These two-phase flows are incredibly complex and often have non-intuitive behaviors. For example, the above high speed video obtained in our lab shows relatively large bubbles being entrained into the wake behind a cylinder in cross flow. These bubbles are effectively ‘held up’ by the cylinder wake, which creates a region with high bubble concentration just behind the cylinder. This void distribution is important to the design and implementation of two-phase heat sinks that rely on small fins or tubes in cross-flow. Avoiding concentrated bubbly regions like those shown will be essential to prevent regions of vapor dryout in these components.
The behavior of the bubbles largely depends on the bubble size compared to the relevant lengthscales of the flow. We are investigating how bubbles of varying sizes interact with these cylinder wakes so that we can then mitigate these concentration effects. As shown below with contours of average relative bubble concentration, the bubble Stokes number (St) begins to explain whether or not it will be trapped behind the cylinder, with bubbles with very low Stokes numbers not being trapped. We have found that this parameter alone does not explain the two-phase flow behavior and are investigating additional parameters to universally predict this trapping phenomena.
