In part 2 of our blog series on flow visualization, we discuss the benefits of liquid flow visualization, along with several primary methods for creating successful flow visualization.
Part 2: Liquid Flow Visualization
Flow visualization is usually easier to perform in water than air and yields results of better quality. As with smoke visualization, dye entrainment is successful mostly in laminar flow. The enhanced mixing in turbulent flow causes the smoke streaks to diffuse too rapidly to be of value as tracers. Compared to smoke visualization in air, dye entrainment in liquids is helped by the fact that the mixing between most dyes and water is less intense than between smoke and air. As a result, water-flow tunnels are frequently used to study air flows by testing scaled models at lower velocity. This often provides a better description of the flow.
A liquid flow model is scaled with a different working fluid than air using dimensional analysis. For a treatment of the principals of dimensional analysis and similitude that should be used in applying flow visualization in model experiments, reference can be made to any standard textbook on fluid mechanics. For the flow conditions around a model to be completely similar to those of the prototype, all relevant dimensionless parameters must have the same corresponding values: the model and prototype are then said to possess geometric, kinematic, dynamic and thermal similarity.
To visualize the flow, water-soluble dyes such as food coloring, potassium permanganate, methylene blue, ink and fluorescent ink may be injected using hypodermic needles or entrained from holes or slots in the walls of a test section. It is important that the velocity and density of the injected dye equal those of the surrounding fluid to maintain a stable dye filament and reduce disturbance of the surrounding flow.
In summary, fluid flow visualization is a powerful and unique technique for quickly identifying the flow distribution and approach air velocity to thermally challenging components in a complex PCB structure. By using this technique, one can attain the following:
-Examine the PCB layout at the design stage for expected thermal performance, e.g., determine flow stagnation areas.
-Make component layout recommendations that provide a thermally optimum board.
-Identify approach air velocities necessary for component thermal management and the choice of cooling system.