Understanding the Fluid Dynamics of Boundary Layers

I had a customer who was thermal printing strip steel. The steel is washed with solvents and blown dry. He was having a problem: When the strip's speed increased, the thermo printer would catch fire. Obviously, it had to do with the solvent, but when he set a flame to a piece of the strip, he couldn't get it to burn.

I explained that the boundary layer created by the fast-moving strip was trapping the vapor between it and the sheet. When he sampled a piece of the strip, it wasn't moving. The boundary layer had collapsed, and the vapors escaped. That was why he could not replicate the fire.

So what is a boundary layer, and how is it created? It's a thin layer of compacted air against a moving surface caused by the friction between the surface and the atmospheric air bearing down on it. For those who delight in a mathematical explanation, check out your engineering handbooks. For those who work better with pictures, here's an illustration of air molecules in contact with a surface.

The circles represent air molecules, and the lines between them represent the forces between them. Atmospheric pressure (14.7 psi) is the force at which the air molecules are pressed against the surface and against one another. Like the pads on a car's disc brakes, this force creates a friction. As the surface moves, it drags along with it the layer of air molecules immediately adjacent to it. When that layer moves, it drags the layer above it, and so on.

Notice how the air molecules become compacted, forming a thin air layer. This is the boundary layer. To visualize what happens when the surface moves, think of holding a helium-filled balloon. At a steady state, it'll project straight up, like the molecules shown in the first picture. As you move forward, the balloon follows behind you and at a lower altitude, because it's reacting to resultant force vectors.

In practical applications, the boundary layer can prevent the surface of the material from being impacted by whatever surface treatment you may be trying to implement. That can be static elimination, blowoff, or even removal of some debris or combustible fume, as was the case with my customer.

If you're experiencing any of these symptoms, be aware of the boundary layer concept and the resultant problems. The way to counteract the boundary layer's effects is to break up the layer with a forceful blast of air or fluid in the direction counter to the flow of the material being treated. In machining applications, high-pressure coolant systems are effective. For flat stock or webs, such as in rolling or paper mills, a compressed air knife can generate the counter flow. For smaller applications, a pinpoint blast from an air or liquid nozzle may be more appropriate.

Joe Panfalone is an applications engineer for Exair.

Related posts: